National Academies Press: OpenBook

Design Guide for Bridges for Service Life (2013)

Chapter: 3 Materials

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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

123 3.1 introduction This chapter provides essential information on materials used for constructing du- rable bridge structures. It focuses on durability and service life issues related mainly to concrete and steel, materials widely used in bridge construction, and provides limited information on other material types used in bridge construction. Information for enhancing the service life of materials used in bridge systems, sub- systems, and components is summarized. Figure 3.1 identifies the materials-enhance- ment selection process developed in this chapter, which begins with developing an understanding of the types of materials. These viable materials are further evaluated for the factors that adversely affect their service life, and individual strategies are devel- oped to mitigate these adverse effects. The overall strategy selection is then developed, blending these individual strategies that are sometimes in conflict with one another. The components of an overall strategy should • Identify appropriate design methodologies; • Select durable material types, considering life-cycle costs; • Consider additional protective measures, such as cathodic protection and electro- chemical chloride extraction; • Specify best practices for construction; and • Develop an effective maintenance plan. Sections 3.2 and 3.3 provide a general description of material types, primarily concrete, reinforcement, and structural steel, used for bridge systems, subsystems, and components. Section 3.4 addresses factors known to adversely affect the service life of these materials and presents a fault tree approach for considering these factors. 3 MATERiALS

124 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Section 3.5 provides available solutions, methods, technologies, and other informa- tion helpful in developing individual strategies to mitigate factors adversely affecting service life. Section 3.6 identifies a process to select the overall material selection and protection strategy for providing materials with enhanced service life; however, much of this selection process is highly dependent on the application and is addressed in subsequent chapters. 3.2 deScriPtion oF mAteriAL tyPeS This section describes concrete, reinforcement, and structural steel, the materials pri- marily used for bridge systems, subsystems, and components. Different concretes for bridge elements require different degrees of durability, depending on the exposure environment and the properties desired. There are many examples of longevity of concrete from ancient times; Rome’s Pantheon, for example, was built around 126 CE and remains intact. The Confederation Bridge, which joins the eastern Canadian provinces of Prince Edward Island and New Brunswick, was constructed in 1997 and is a good example of a modern concrete bridge designed to resist a harsh marine environment for at least 100 years (Figure 3.2). Concrete has high compressive strength but low tensile strength, which makes it prone to cracking. Early concrete structures were designed to be subjected only to compression in order to avoid tensile failures. Arch shapes were used to span distances. Figure 3.1. Materials enhancement selection process. Figure 3.1. Materials enhancement selection process. Material Types (Sections Factors A ffecting Service Life (Section 3.4) Evaluate Strategies for Enhanced Service Life (Section 3.5) Mitigate Develop Strategy Selection (Section 3.6) Design methodology, material selection, protective measures, best practices for construction, maintenance plan. 3.2 and 3.3) Figure 3.2. Confederation Bridge.

125 Chapter 3. MATERiALS In modern structures, reinforcement is common for providing tensile capacity, crack control, and ductility. The service life of reinforced concrete structures depends on the durability of the concrete and the durability of the reinforcement. The major distress in reinforced concrete is due to the corrosion of the reinforce- ment. Reinforcement must be protected from aggressive environments, either through measures such as the use of the low-permeability concrete, adequate cover, and corro- sion inhibitors, or the use of noncorrosive reinforcement. Preventive measures could also be employed to prolong the service life of concrete structures, such as incorporat- ing cathodic protection systems. Structural steel is another common material used for bridge systems, subsystems, and components. It provides high compressive and tensile strengths with consider- able ductility, which makes it particularly suitable for long-span bridges. The most common steel bridge systems used today are composite multigirder deck systems that use either rolled beams, plate girders, or tub girders. These systems can be single or multispan, and either straight or curved. Simple-span systems were often used in the past, but most multispan systems today are continuous. Rolled beam bridges using W-shapes are used in shorter spans up to about 100 ft for simple spans and up to about 120 ft for continuous spans. Welded deck plate girders are most often used for spans over 120 ft. 3.2.1 Concrete Concrete consists of cementitious material, aggregate, water, and admixtures. Con- crete may also include fibers. The following sections provide general descriptions of each concrete ingredient. 3.2.1.1 Cementitious Material Cementitious materials include portland cements, blended cements, other hydraulic cements, specialty cements for repairs, and supplementary cementitious materials (SCMs). These cementitious materials have different chemical and physical properties that affect the durability of concrete. 3.2.1.1.1 Portland Cement Portland cement is produced from a combination of calcium, silica, aluminum, and iron (Kosmatka and Wilson 2011). During processing, the raw materials reach tem- peratures of 2,600ºF to 3,000ºF, forming clinker. Clinker and gypsum are ground to a fine powder such that nearly all the material passes a No. 200 mesh (75-μm sieve). Cement has four main compounds: tricalcium silicate (3CaO·SiO2 = C3S), dicalcium silicate (2CaO·SiO2 = C2S), tricalcium aluminate (3CaO·Al2O3 = C3A), and tetracal- cium aluminaferrite (4CaO·Al2O3Fe2O3 = C4AF). These compounds form the different types of portland cements conforming to the requirements of ASTM C150. Ten types of portland cement are summarized in Table 3.1. Some cements are designated with a combined classification, such as Type I/II, indicating that the cement meets all the requirements of the specified types.

126 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.2.1.1.2 Blended Cement Blended cements are produced by intergrinding or blending two or more types of fine material such as portland cement, ground granulated blast furnace slag, fly ash, silica fume, calcined clay, other pozzolans, hydrated lime, and preblended combinations of these materials (Kosmatka and Wilson 2011). ASTM C595 includes three classes of blended cement: 1. Type IS (X)—portland blast furnace slag cement; 2. Type IP (X)—portland–pozzolan cement; and 3. Type IT(AX)(BY)—ternary blended cement. The letters X and Y stand for the percentage of SCM included in the blended cement, and A and B are the types of SCMs (S for slag and P for pozzolan). Type IS(X) can include up to 95% slag cement. Type IP(X) can include up to 40% pozzolans. Spe- cial properties are added after the percentages and are designated in the following list: 1. A—air entrainment; 2. MS—moderate sulfate resistance; 3. HS—high sulfate resistance; 4. MH—moderate heat of hydration; and 5. LH—low heat of hydration. For example, IP(25)(HS) indicates 25% pozzolans and high sulfate resistance. As an example for ternary cements, Type IT(S25)(P15) contains 25% slag and 15% pozzolans. Type IT can meet MS, HS, and LH options. tABLE 3.1. cement tyPeS And uSeS Type of Cement Use Type I General purpose Type IA Type I + air entraining Type II General use, moderate sulfate resistance Type IIA Type II + air entraining Type II(MH) General use, moderate heat of hydration, moderate sulfate resistance Type II(MH)A Type II(MH) + air entraining Type III High early strength Type IIIA Type III + air entraining Type IV Low heat of hydration Type V High sulfate resistance

127 Chapter 3. MATERiALS 3.2.1.1.3 Other Hydraulic Cement All portland and blended cements are hydraulic cements. ASTM C1157 is a perfor- mance specification that includes portland cement, modified portland cement ( specialty cement providing characteristics of more than one type of portland cement), and blended cements. ASTM C1157 recognizes six types of hydraulic cements: 1. Type GU—general use; 2. Type HE—high early strength; 3. Type MS—moderate sulfate resistance; 4. Type HS—high sulfate resistance; 5. Type MH—moderate heat of hydration; and 6. Type LH—low heat of hydration. 3.2.1.1.4 Expansive Cements Expansive cements are modified hydraulic cements that expand slightly during the early hardening period after setting. They are used to compensate for volume decreases attributable to drying shrinkage, to induce tensile stress in reinforcement, and to sta- bilize the long-term dimensions of posttensioned concrete. ASTM C845 designates E-1 with three varieties (K, M, and S), and only K is available in the United States. Type E-1 (K) contains portland cement, anhydrous tetracalcium trialuminosulfate, cal- cium sulfate, and uncombined calcium oxide (lime). It has been demonstrated that using lightweight aggregate (LWA) in concrete with expansive cements is very beneficial for achieving the expansion from expansive cements (Russell 1978). This effect is attributed to the continued presence of internal moisture that has been absorbed by the LWA before batching. This internal moisture provides a prolonged reaction time for the expansive cement, allowing it to achieve greater expansion. It is suggested that LWA be used in conjunction with expansive cements. 3.2.1.1.5 Specialty Cements for Repairs Specialty cements for repairs are needed to achieve high early strengths, and cements other than portland cements may be used. They may be rapid-setting hydraulic cement, gypsum-based cement, magnesium phosphate cement, or high-alumina cement for use in partial depth concrete repairs (Caltrans 2008; ACPA 1998). Gypsum-based cement mixtures contain calcium sulfates and gain strength rapidly. They are placed easily, can be used at cold temperatures above freezing, and are toler- ant to high ambient temperatures (Good-Mojab et al. 1993; NCHRP 1977). However, they exhibit poor performance when placed in rainy and freezing weather conditions (NCHRP 1977), and the presence of free sulfates has been found to promote corrosion (Smith et al. 1991).

128 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Magnesium phosphate cement mixtures have rapid setting time, high early strength, low permeability, and good bonding to clean, dry surfaces. They are extremely sensi- tive to water content, and extra water reduces their strength (NCHRP 1977). Also, they are not used to repair concrete with limestone aggregates because the carbon dioxide that forms weakens the bond (ACI 546, 2004; Smith et al. 1991). High-alumina cement mixtures provide rapid strength gain, good bonding to dry surfaces, and very low shrinkage (Smith et al. 1991). However, they experience strength loss due to chemical conversions of some of their calcium aluminate hydrate components (ACPA 1998). Other rapid-setting materials are available but should be used with caution to avoid adverse chemical reactions. For example, some rapid-hardening repair materials contain high-alkaline–bearing materials, which may react with certain siliceous aggre- gates to cause alkali-silica reactivity (ASR) (ACPA 1998). 3.2.1.1.6 Supplementary Cementitious Material SCMs generally consist of by-products from other processes or natural materials. They exhibit hydraulic or pozzolanic activity and contribute to the properties of concrete. Pozzolanic materials do not possess cementitious properties, but when used with port- land cement they form cementitious compounds. SCMs modify the microstructure of concrete and reduce its permeability. They can reduce internal expansion caused by chemical reactions. They can also reduce the heat of hydration, which can cause ther- mal cracking. Typical examples are natural pozzolans, fly ash, slag cement, and silica fume. They are used individually with portland or blended cements or in different combinations. Fly ash is a finely divided residue that results from the combustion of ground or powdered coal and is transported by flue gases. It is a by-product of power- generating stations. Natural pozzolans found in nature may be calcined to induce sat- isfactory properties. Fly ash and natural pozzolans are covered by ASTM C618. Slag cement is a by-product of iron blast furnaces and consists essentially of silicates and alumina silicates of calcium and other bases. Slag cement conforms to ASTM C989. It is classified by performance in the slag activity test into three grades: Grade 80, Grade 100, and Grade 120. Silica fume is a finely divided residue resulting from the produc- tion of elemental silicon or ferro-silicon alloys that is carried from the furnace by the exhaust gases. It conforms to ASTM C1240. ASTM C1697 covers blended SCMs that result from the blending or intergrinding of two or three ASTM-compliant SCMs. SCMs are summarized in Table 3.2. SCMs improve durability by reducing permeabil- ity and improving chemical resistance; however, the level of durability improvement depends on the type, chemical and physical characteristics, and amount used. For ex- ample, concrete with Class F fly ash is expected to have better sulfate resistance than concrete with Class C fly ash. In some areas, a superfine (ultrafine) grade of fly ash is available that exhibits char- acteristics midway between normal fly ash and silica fume in cost, effectiveness, and desirable dose rate. Unlike normal fly ash, it does not require large-volume batching facilities, and it is not as difficult a material to handle and disperse effectively as silica fume (Day 2006).

129 Chapter 3. MATERiALS Superfine fly ash is generally processed from a Class F fly ash by passing the parent ash through a classifier in which the coarse and fine particles are separated. The aver- age particle size of raw or unprocessed fly ash is around 20 to 30 μm, and the largest size is about 100 μm; however, an ultrafine fly ash has a maximum size less than 10 μm and an average particle size of 2 to 4 μm. The finer size provides additional reactive surface area, which contributes to the high-early-strength and low-permeability con- crete. Early strengths and durability measures similar to those of silica fume concrete were observed when a slightly higher dosage of ultrafine Class F fly ash was used with a small reduction (10%) in water content (Obla et al. 2003). 3.2.1.2 Aggregates Aggregates are granular material, such as sand, gravel, crushed stone, or iron blast furnace slag, used in a cementing medium to form hydraulic-cement concrete. Ag- gregates are divided into two categories: coarse and fine. Coarse aggregates are those predominantly retained on a No. 4 sieve. Fine aggregates pass the No. 4 sieve and are predominantly retained on the No. 200 sieve. However, the combination of coarse and fine aggregates is important in concrete. The combined grading indicates the amount of paste needed to affect the water demand and cement content. In comparison to aggregates, paste is more porous, enabling easier transport of water and solutions, which can be detrimental to durability. Normal-density fine and coarse aggregates meet the requirements of ASTM C33. Structural low-density aggregates conform to the requirements of ASTM C330. The aggregate characteristics that affect the proper- ties of concrete are the grading, durability, particle shape and surface texture, abrasion and skid resistance, density, absorption, and surface moisture. Angular, elongated, and rough-textured aggregates have a high water demand that can lead to a high water- cementitious materials ratio (w/cm). The absorption and porosity of the aggregate may affect the resistance to freezing and thawing. Aggregates should be free of potentially deleterious materials such as clay lumps, shales, or other friable particles that can affect the water demand and bond. The chemical composition of the aggregates is important because of the possibility of expansive reactions. tABLE 3.2. ScmS: AStm StAndArd SPeciFicAtionS And uSe SCM ASTM Standard Use Fly ash, Class F C618 Low permeability, chemical resistance Fly ash, Class C C618 Low permeability, chemical resistance Fly ash, Class N C618 Low permeability, chemical resistance Slag cement C989 Low permeability, chemical resistance Silica fume C1240 Low permeability, chemical resistance Blended SCMs C1697 Low permeability, chemical resistance

130 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.2.1.2.1 Normal-Weight Aggregates Most commonly used normal-weight aggregates (NWAs)—sand, gravel, and crushed stone—produce concrete with a density of 140 to 150 lb/ft3. Aggregates must be clean, hard, strong, durable particles, free of absorbed chemicals, coatings, and other fine material that can adversely affect hydration and bond of the cement paste (Kosmatka and Wilson 2011). The grading, shape, and texture of aggregates affect the water de- mand. Concretes with angular and poorly graded aggregates are also difficult to pump. Aggregates may also be reactive, causing ASR or alkali-carbonate reactivity (ACR). If reactive aggregates are used, pozzolans and lithium-based admixtures can be added to minimize the expansion and provide resistance to ASR. The space in the aggregate also provides a place for the reaction products, if any expansion occurs as a result of the other aggregates in the concrete. In ACR, diluting the aggregates or changing the source, or selective quarrying, could minimize the expansion. In selective quarrying, the layers that contain reactive aggregate are avoided (Ozol 2006). Reactivity is mea- sured using ASTM C1105. 3.2.1.2.2 Lightweight Aggregates In the United States, LWA is typically manufactured by expanding shale, clay, or slate by firing the materials at high temperatures in a rotary kiln. At high temperatures, gases are evolved within the pyroplastic mass, forming bubbles that remain after cool- ing (ACI 213R, 2003b). The cellular structure of LWA results in a density that is lower than that of NWA; when used in concrete, LWA reduces the density of concrete. The porous structure of LWA absorbs more water than NWA. Pores close to the surface are readily permeable, but interior pores are hard to fill. Many of the interior pores are not connected to the surface at all. Water absorbed in the surface pores by prewetting before batching is released into the paste during hydration of the cement, which provides internal curing (Holm and Ries 2006), which in turn results in improved properties and reduced cracking within the concrete. LWA used in structural lightweight concrete (LWC) must be capable of producing concrete with a minimum 28-day compressive strength of 2,500 psi with an equilib- rium density between 70 and 120 lb/ft3 (ACI 213R, 2003b). The strength of LWA varies with type and source and will affect the strength of LWC that can be achieved. The density of LWA varies with particle size, increasing in density for the smaller particles. Because of this density variation, the grading requirements for LWA (ASTM C330) deviate from those of NWA (ASTM C33) by requiring a larger mass of the LWAs to pass through the finer sieve sizes. This modification yields the same volumet- ric distribution of aggregates retained on a series of sieves for both LWA and NWA. 3.2.1.3 Water ASTM C1602 covers mixing water used in the production of hydraulic-cement con- crete. Mixing water consists of batch water, ice, water added by truck operator, free moisture on the aggregates, and water introduced in the form of admixtures. Potable and nonpotable water are permitted to be used as mixing water in concrete. Nonpotable

131 Chapter 3. MATERiALS water, including treated wash water and slurry water, is not used in concrete unless it produces 28-day concrete strengths equal to at least 90% of a control mixture using 100% potable water or distilled water and time of set meets the limits in C1602. Some excessive impurities may cause durability problems; therefore, optional chemical lim- its for combined mixing water are given for chloride, sulfate, alkalis, and total solids. Small solid particles increase the water demand as a result of large surface area. 3.2.1.4 Admixtures Chemical admixtures are the ingredients in concrete other than portland cement, wa- ter, and aggregate that are added to the mix immediately before or during mixing. Ad- mixtures are primarily used to achieve the following objectives (Kosmatka and Wilson 2011): • To reduce cost of concrete construction; • To achieve the properties of hardened concrete effectively; • To maintain the quality of concrete during mixing, transporting, placing, and cur- ing in adverse weather conditions; and • To overcome certain emergencies during concrete operations. Chemical admixtures must conform to the requirements of ASTM C494 or, when flowing concrete is applicable, to C1017. ASTM 494 covers the materials and the test methods for use in chemical admixtures to be added to hydraulic-cement con- crete mixtures in the field. The admixtures given in ASTM C494 and C1017 are summarized in Table 3.3. The terms superplasticizer and high-range water-reducing admixture are used interchangeably. Other admixtures, such as shrinkage-reducing or viscosity- modifying admixtures, are covered by Type S, specific-performance admix- ture, in ASTM C494. Corrosion-inhibiting admixtures are covered by ASTM C1582, and the cold-weather admixtures by ASTM C1622. tABLE 3.3. AdmixtureS: AStm StAndArd SPeciFicAtionS And uSe Type ASTM Use A C494 Water reducing B C494 Retarding C C494 Accelerating D C494 Water reducing and retarding E C494 Water reducing and accelerating F C494 High-range water reducing G C494 High-range water reducing and retarding S C494 Specific-performance admixtures I C1017 Plasticizing II C1017 Plasticizing and retarding

132 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Admixtures help in achieving workable, low w/cm, and low-permeability con- cretes. Entrained air voids provide resistance to freezing and thawing, improved work- ability, and reduced bleeding and segregation. Shrinkage-reducing admixtures reduce shrinkage and cracking potential. Viscosity-modifying admixtures improve stability of the mixture by minimizing segregation. Corrosion-inhibiting admixtures increase the resistance to corrosion. They can form a protective layer at the steel surface as a result of chemical reaction with ferrous ions (as with inhibitors containing calcium nitrite) or they can provide a protective layer and reduce chloride ion ingress (as with inhibi- tors containing amine/ester). ASTM specifications are available to provide guidance in using admixtures in concrete, as summarized in Table 3.3. 3.2.1.5 Fibers Fibers are added to concrete to control cracking. Steel fibers conform to ASTM A820, and synthetic fibers meet the requirements of ASTM C1116, 4.1.3, Type III. Fibers, gen- erally synthetic, are added to concrete at a low-volume dosage, about 0.1%, to reduce plastic shrinkage cracking. At high-volume percentages, up to 2%, they can increase resistance to cracking in hardened concrete and decrease crack width ( Kosmatka and Wilson 2011). Plastic shrinkage cracks are addressed in bridge structures by proper curing. Crack control in hardened concrete has been attempted in decks and overlays (Ozyildirim 2005; Sprinkel and Ozyildirim 2000; Baun 1993). The cost and handling of fibers have limited their use in bridge structures. Synthetic fibers offer one other advantage: they reduce spalling during fires, which has been a concern, especially in tunnels (Parsons et al. 2006). In a fire, spalling of concrete can occur as a result of high vapor pressure. Spalling becomes more severe with an increase in strength. Synthetic fibers such as polypropylene fibers reduce the risk of spalling. At high temperatures these fibers melt, leaving pores or channels in the concrete for the vapor to escape. 3.2.1.6 Concrete Types Types of concrete used in bridge structures include normal-weight concrete (NWC); high-performance concrete, including self-consolidating concrete (SCC); ultrahigh- performance concrete (UHPC); fiber-reinforced concrete (FRC); and LWC. In repairs, overlay concretes (latex-modified concrete, low-slump concrete, silica fume, polymer concrete, and very-early-strength concretes), shotcrete, and grouts have been used. The following sections describe different types of concretes and provide information on their characteristics that can be considered in selecting the proper type of concrete for a given application. 3.2.1.6.1 Normal-Weight Concrete NWCs have a wide range of ingredients and performance characteristics including air content, slump, and temperature. The density (unit weight) of NWC is approximately 145 lb/ft3. The density of concrete varies, depending on the amount and density of the

133 Chapter 3. MATERiALS aggregate and the air, water, and cement contents. Generally, strength is the selected parameter. Concrete ingredients, proportions, handling, placing, curing practices, and the ser- vice environment determine the ultimate durability and life of concrete (Ozyildirim 2007). The durability of concrete depends largely on its ability to resist the infiltration of water and aggressive solutions. Concretes that are protected from the environment can provide a long service life. For longevity, concretes with low permeability are needed. Concrete may deteriorate when exposed to cycles of freezing and thawing, especially in the presence of deicing chemicals, and become critically saturated. The addition of an air-entraining admixture to critically saturated concrete can improve the resistance to freezing and thawing. Concrete can resist most natural environments and many chemicals. However, certain chemicals can attack concrete and cause deterioration. For example, sulfate attack, ASR, ACR, acid attack, corrosion, and wear can damage concrete and reduce its service life. However, in such environments proper material selection, proportion- ing, and construction practices can protect concrete from unwanted attack and distress. For example, if reactive aggregates are used, pozzolans and lithium-based admixtures can be added to minimize the expansion caused by ASR. 3.2.1.6.2 High-Performance Concrete High-performance concretes exhibit high workability, strength, and/or durability. High-performance concrete has been used in decks, superstructures, and substruc- tures to extend the service life. FHWA proposed defining high-performance concrete by using long-term performance criteria (Goodspeed et al. 1996). The goal was to stimulate the use of higher-quality concrete in highway structures. The definition of high- performance concrete developed for FHWA (Goodspeed et al. 1996) had four performance parameters related to durability (see Table 3.4): • Resistance to freezing and thawing; • Resistance to scaling; • Resistance to abrasion; and • Resistance to chloride ion penetration. The four structural design characteristics were compressive strength, modulus of elasticity, shrinkage, and creep. The tensile strength, which is related to compressive strength, modulus of elasticity, shrinkage, and creep, is an important factor affecting the cracking potential. For each characteristic, standard laboratory tests, specimen preparation procedures, and grades of performance were presented. Later additions to this definition and changes to the grade limits were recommended (Russell and Ozyildirim 2006) and are summarized in Table 3.5. These additions included resis- tance to ASR and sulfate. Workability was also added as a characteristic; workability affects durability because concrete should be well consolidated (either through vibra- tion or self consolidation) in order to achieve the desired hardened concrete properties.

134 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 3.4. grAdeS oF PerFormAnce chArActeriSticS For high-PerFormAnce StructurAL concrete Performance Characteristic Standard Test Method FHWA High-Performance Concrete Performance Grade 1 2 3 4 Freeze–thaw durability (x = relative dynamic modulus of elasticity after 300 cycles) AASHTO T 161 ASTM C666 Procedure A 60% ≤ x ≤ 80% 80% ≤ x Scaling resistance (x = visual rating of the surface after 50 cycles) ASTM C672 x = 4, 5 x = 2, 3 x = 0, 1 Abrasion resistance (x = average depth of wear in mm) ASTM C944 2.0 > x ≥ 1.0 1.0 > x ≥ 0.5 0.5 > x Chloride permeability (x = Coulombs) AASHTO T 277 ASTM C1202 3,000 ≥ x > 2,000 2,000 ≥ x > 800 800 ≥ x Strength (x = compressive strength) AASHTO T 22 ASTM C39 41 ≤ x < 55 MPa (6 ≤ x < 8 ksi) 55 ≤ x < 69 MPa (8 ≤ x < 10 ksi) 69 ≤ x < 97 MPa (10 ≤ x < 14 ksi) x ≥ 97 MPa (x ≥ 14 ksi) Elasticity (x = modulus of elasticity) ASTM C469 24 ≤ x < 40 GPa (4 ≤ x < 6 × 106 psi) 40 ≤ x 50 GPa (6 ≤ x < 7.5 × 106 psi) x ≥ 50 GPa (x ≥ 7.5 × 106 psi) Shrinkage (x = microstrain) ASTM C157 800 > x ≥ 600 600 > x ≥ 400 400 > x Creep (x = microstrain/ pressure unit) ASTM C512 0.52 ≥ x > 0.41 0.41 ≥ x > 0.31 0.31 ≥ x > 0.21 0.21 ≥ x Source: Goodspeed et al. 1996 (available at http://www.fhwa.dot.gov/bridge/HPCdef.htm). tABLE 3.5. AdditionAL grAdeS oF PerFormAnce chArActeriSticS Performance Characteristic Standard Test Method FHWA High-Performance Concrete Performance Grade 1 2 3 Alkali-silica reactivity (ASR = expansion at 56 days, %) ASTM C441 0.20 ≥ ASR > 0.15 0.15 ≥ ASR > 0.10 0.10 ≥ ASR Sulfate resistance (SR = expansion, %) ASTM C1012 SR ≤ 0.10 at 6 months SR ≤ 0.10 at 12 months SR ≤ 0.10 at 18 months Workability (SL = slump, SF = slump flow) AASHTO T 119 ASTM C143 ASTM C1611 SL > 7½ in. SF < 20 in. 20 ≤ SF ≤ 24 in. 24 in. < SF Source: Russell and Ozyildirim 2006.

135 Chapter 3. MATERiALS Another characteristic to be considered is density. Concretes with densities varying from light weight to normal weight can be prepared to address the dead load in span- ning long distances. 3.2.1.6.3 Self-Consolidating Concrete SCC is a highly flowable, nonsegregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation (ACI 237R, 2007b). SCC contains a large amount of fine material to obtain a stable mixture; sometimes a viscosity-modifying admixture is used to provide the stability instead of, or in combination with, the fine material. The high-range water-reducing admixtures used are generally based on polycarboxylate ethers; they provide large water reductions and slow slump loss. Eliminating the consolidation problem would enhance the strength and reduce the permeability of concretes, essential characteristics for longevity. SCC has been used in Japan since the late 1980s (ACI 237R, 2007b) and is used widely in the precast industry in North America, but its use in the ready-mixed concrete industry has been slower. Some of the benefits of SCC are decreased labor requirements, increased construction speed, improved mechanical properties and durability characteristics, the ability to be used in heavily reinforced and congested areas, consolidation without vibration, and a reduced noise level at manufacturing plants and construction sites (Okamura and Ouchi 1999). The flow characteristic of SCC is measured using the slump flow test (ASTM C1611). The stability of the mix- ture can be qualitatively assessed in accordance with ASTM C1611 by using the visual stability index. A visual stability index value of 0 indicates a highly stable mix with no evidence of segregation or bleeding; 1 is a stable mix with no evidence of segregation but with slight bleeding; and values of 2 and 3 indicate unstable mixtures. To deter- mine the ability of SCC to pass through the reinforcement, a J-ring test is used (ASTM C1621). ASTM C1610 addresses the determination of static segregation of SCC by measuring the coarse aggregate content in the top and bottom portions of a column. SCC is placed in a column separated into three sections, and the coarse aggregate in the top and bottom sections is removed by washing. Another ASTM test method, C1712, covers the rapid assessment of static segregation resistance of normal-weight SCC. This test does not measure static segregation resistance directly, but rather pro- vides an assessment of whether static segregation is likely to occur. There are some concerns about the use of SCC, among them segregation due to a high flow rate; a poor air void system and form pressure and tightness due to the high fluidity; and high amounts of high-range water-reducing admixture resulting in coarse air bubbles, shrinkage due to smaller maximum size aggregate, and lower amount of coarse aggregate (ACI 237R, 2007b). SCC has been used successfully in beams (Ozyildirim 2008), drilled shafts (Schindler and Brown 2006), substructure repairs, and tunnel sections (ACI 237R, 2007b).

136 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.2.1.6.4 Ultrahigh-Performance Concrete UHPC has high strength and high ductility. A proprietary UHPC that is commonly available is discussed in this section; this UHPC is formulated by combining portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water, and steel or organic fibers. Its superior durability and negligible permeability is expected to reduce maintenance and extend the service life. Current applications (as of 2012), including beams and connections, are explained in this section. UHPC is expected to achieve compressive strengths greater than 21.7 ksi (150 MPa), and it contains fiber reinforcement for improved ductile behavior (AFGC 2002). Small brass-coated steel fibers with a diameter of 0.007 in. (0.185 mm) and a length of 0.55 in. (14 mm) are commonly used as fiber reinforcement in UHPC. A syn- thetic fiber (polyvinyl alcohol) has also been used (Parsekian et al. 2008). To achieve very high strengths, exceeding 30 ksi, UHPC is steam cured. UHPC exhibits strain hardening that results in numerous tight cracks rather than one large crack prior to failure. It has a very low w/cm (about 0.2) and a very dense matrix, leading to negli- gible permeability. The high amount of binder and very low w/cm in UHPC make it very cohesive; however, it flows within the forms without the need for vibration. The high strength and low permeability of UHPC are attributed to a very dense packing of fine material and a low w/cm (Graybeal 2006). UHPC contains no coarse aggregate. Fine sand, typically between 0.006 and 0.024 in. (150 and 600 μm) is the largest particle, followed by crushed quartz, cement, and silica fume. The resulting gradation allows for tight packing of these particles. The unit weight of UHPC is approximately 155 lb/ft3. The coefficient of thermal expansion is about 50% higher than that of con- ventional concrete (Graybeal 2006). Whether steam cured or not, commonly available UHPC exhibits enhanced dura- bility compared with NWC (Graybeal 2006; Graybeal and Tanesi 2007). Graybeal and Tanesi (2007) studied UHPC characteristics under four curing regimes: steaming at 60°C (194°F) and 95% relative humidity for 48 h (recommended by the manufac- turer), untreated (no steam curing, specimens kept in the standard laboratory environ- ment from demolding until testing), tempered steam treated (temperature in steam chamber limited to 60°C [140°F]), and delayed steam treated (steam treatment initiated after 15 days of casting). Regardless of the curing treatment applied, UHPC exhibited enhanced durability properties over normal concretes. Thus, field casting and curing can provide the desired properties without the need for steam curing. Steam curing can improve UHPC properties even more, but are relevant to precast operations. UHPC also exhibits strong freeze–thaw resistance. The relative dynamic modulus of UHPC was at least 96% after being subjected to 690 cycles of freezing and thaw- ing (more than two times the normal number of 300 cycles indicated in ASTM C666). The concrete was innocuous to ASTM C1260 ASR deterioration, to ASTM C672 scaling deterioration, and to AASHTO T259 chloride penetration. The ASTM C1202 Coulomb test result was negligible, less than 40°C, if any steam-based curing treat- ment was applied, and ranged from very low (averaging 360°C) at 28 days to negligi- ble (averaging 76°C) at 56 days in the absence of any steam curing. The ASTM C1202

137 Chapter 3. MATERiALS test is based on electrical conductance of concrete, and the presence of steel fibers affects the charge passed; however, the very dense matrix of UHPC isolates the steel fibers and provides very high resistance to current flow. Ponding test results ( AASHTO T259) have shown that the volume of chlorides that penetrate is extremely low. No scaling was observed when tested in accordance with ASTM C672. The abrasion resis- tance (ASTM C944) of steam-treated UHPC was very low; it abraded 0.1 to 0.3 g. The untreated UHPC lost 10 times more (1 to 3 g) in the abrasion test. Although air-entraining admixture is not added during casting, UHPC exhibits satisfactory resis- tance to cycles of freezing and thawing. The first bridge with UHPC beams was constructed in Wapello County, Iowa (Moore and Bierwagen 2006). The three 110-ft beams were modified 45-in. Iowa bulb-tee beams. To save material in the beam section, the web width was reduced by 2 in., the top flange by 1 in., and the bottom flange by 2 in. The Virginia Department of Transportation (DOT) used five 45-in.-tall bulb-tee beams with UHPC in the bridge on Route 624 over Cat Point Creek (Ozyildirim 2011). The bridge has ten 81.5-ft spans, one of which contained UHPC beams. The steel fibers provided adequate shear resistance, so the UHPC beams did not contain the conventional stirrups normally used as shear reinforcement; however, confinement steel was included at the beam ends (Ozyildirim 2011). Other applications in bridge structures have been accomplished or recommended. Garcia (2007) detailed UHPC flexural behavior and offered a design methodology for two-way ribbed, precast bridge decks. Under the FHWA Highways for LIFE Program, a two-lane bridge on a secondary road in Wapello County, Iowa, was constructed using prestressed concrete girders and 14 UHPC waffle deck bridge panels (Heimann and Schuler 2010). UHPC has very high bond strength. At the Virginia project, the extra UHPC flowing to the top of the steel form had bonded strongly to the form after hardening, making the removal of the form very difficult (Ozyildirim 2011). The high bond strength, low permeability, and crack resistance make UHPC highly desirable in joints and connections, and such applications have been successfully completed (Perry and Royce 2010). 3.2.1.6.5 Fiber-Reinforced Concrete FRC is expected to improve tensile strength; provide crack control; and increase dura- bility, fatigue life, resistance to impact and abrasion, shrinkage, and fire resistance (ACI 544.1R, 1996). Crack control is critical for longevity in reinforced concrete structures, and one effective solution is to use fiber-reinforced concrete. FRCs containing steel and synthetic fibers are common. Two or more fiber types can also be used to produce hybrid FRC to obtain improved properties or cost reduction. For example, steel and/ or macrosynthetic fibers that enhance toughness and postcrack load-carrying capacity can be combined with microfibers that help control plastic shrinkage cracking. The aspect ratio of fibers, which is the ratio of length to diameter, affects the work- ability and the hardened concrete properties. Typical aspect ratios range from about 20 to 100 (ACI 544.1R, 1996). Fibers with high aspect ratios can improve the ductile behavior of concrete and also increase strength and stiffness. However, fibers with high ratios tend to interlock to form a mat, or ball, which is very difficult to separate by

138 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE vibration alone. In contrast, short fibers with ratios less than 50 are not able to inter- lock and can easily be dispersed by vibration. FRC can reduce the amount of cracks in concrete; however, wide cracks (>0.004 in. [0.1 mm]) still exist. Cracks less than 0.004 in. (0.1 mm) wide inhibit the intrusion of corrosive chemicals and are expected to hinder the intrusion of harmful solutions (Wang et al. 1997; Lawler et al. 2002). To obtain tight cracks, high-performance FRC is needed. High-performance FRC undergoes large deflection and exhibits deflection or strain hardening, causing multiple microcracks instead of one large localized crack. As deflection occurs, strain hardening is exhibited after the first crack is initiated, and an increase in stress occurs with further deformation. One such concrete is SIFCON (slurry infiltrated fiber concrete) (Naaman and Homrich 1989), which is produced by filling an empty mold with loose steel fibers (about 10% by volume) and filling the voids with high-strength cement-based slurry. The resulting composite exhibits high strength and ductility. UHPC with steel or polyvinyl alcohol fibers and engineered cementitious composite with polyvinyl alcohol fibers also exhibit tight cracks (Li 2002, 2003). Both the UHPC and engineered cementitious composite are mortar mixtures without the coarse aggregate. High-performance FRC with hybrid fibers (steel and polyvinyl alcohol) containing #8 coarse aggregate with a nominal maximum size of 3/8 in. was developed and exhibits deflection hardening needed for tight crack forma- tion (Blunt and Ostertag 2009). FRC can be used in bridge decks, deck repairs, and overlays. For example, the Ohio DOT has used steel fibers in bridge-deck overlays containing silica fume or dense concrete (Baun 1993). High-performance FRC can be used in joints, connections, and link slabs. 3.2.1.6.6 Lightweight Concrete LWC is typically used to reduce the dead load of a structure in order to improve structural efficiency, thus allowing reduced element sizes, less reinforcement, increased span lengths, fewer piers, or reduced foundation elements (ACI 213R, 2003b). As prefabrication becomes more widely used for bridge elements, LWC can save money by reducing handling, shipping, and erection costs. It has also been shown that LWC provides enhanced durability by reducing the permeability and cracking tendency of concrete and has a higher fire resistance than conventional concrete. LWC consists of LWA or a blend of lightweight and normal-density aggregates. Standard procedures and admixtures are used to proportion LWC mixtures. Standard batching and transporting equipment are also used for LWC. LWC can have an equi- librium density between 90 and 125 lb/ft3, but values from 110 to 120 lb/ft3 are most common. Equilibrium density is defined in ASTM C567 as the density reached after exposure to relative humidity of 50% + 5% and a temperature of 73.5°F + 3.5°F for a period of time sufficient to reach constant mass. The fresh density is used for quality control in the field and should also be used to compute precast element weights for handling and shipping.

139 Chapter 3. MATERiALS LWA is sometimes used in combination with NWA to create structural concretes with densities between 120 and 145 lb/ft3. The properties such as strength and modu- lus of elasticity would vary and should be addressed by performance requirements. Although design compressive strengths of 3,000 to 5,000 psi are common for LWC, design strengths up to 10,000 psi have been used in bridge beams (Liles 2010). The maximum strength that can be achieved using an LWA source may be increased by reducing the maximum aggregate size. As is typical with NWC, the use of a high-range water-reducing admixture enables w/cm reduction, and with the addition of SCMs, LWC with high workability, strength, and durability can be achieved. LWA costs more than NWA because of the thermal processing used to manufac- ture it. The impact of the increased cost of LWA on the difference in cost between LWC and NWC depends on the cost of the LWA and the cost of the NWA that it is replacing. As sources of good NWA dwindle, the use of LWA will become more cost-effective because NWAs will also be shipped greater distances. In spite of the increased cost of LWC, the benefits that can be achieved using LWC can make it a cost-effective solution for concrete structures. LWC can be placed and finished using conventional equipment. It exhibits a lower slump than NWC with the same workability as a result of the reduced aggregate density. In high-workability mixtures, lower-density LWA particles may rise to the surface, unlike NWA, in which aggregates segregate by settling to the bottom. There- fore, LWC mixtures should be designed to be cohesive, and excess vibration should be avoided. Slump, air content, and temperature requirements for LWC are similar to those for NWC. LWA should be prewetted before use in concrete that will be delivered by pumping. Without adequate moisture, the aggregate may absorb mixing water and cause slump loss during pumping. With proper preparation, LWC has been success- fully pumped for long horizontal and vertical distances (Valum and Nilsskog 1999). LWA suppliers can provide additional guidance in preparing for pumping LWC. Experience shows that LWC with a proper air void system can be durable and exhibit the satisfactory performance expected of NWCs (Holm and Ries 2006). Resis- tance to freezing and thawing of LWC in the presence of deicing salts is similar to that of NWC (ACI 213R, 2003b). Because LWC contains more absorbed water than NWC, LWC should be allowed to dry before it is subjected to freezing and thaw- ing. ASTM C330 requires 14 days of drying for the LWC tested in accordance with ASTM C666. Because of the porous structure of LWA particles, the resistance to wearing forces may be less than that of a solid particle. However, in many cases, LWC bridge decks have exhibited wearing performance similar to that of NWC (ACI 213R, 2003b) because the aggregate is a vitrified material with hardness comparable to quartz. LWA is nonpolishing, so it has excellent skid resistance. The hardened properties of LWC are equal to or somewhat lower than NWC in many cases. The modulus of elasticity of LWC is reduced from NWC. The splitting tensile strength and the modulus of rupture values are lower for LWC, about 60% to 85% of NWC (ACI 213R, 2003b). Poisson’s ratio may be assumed as 0.20, which is similar to NWC. The creep and shrinkage of LWC are similar to or a little higher than

140 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE that of NWC (Davis 2008). In general, the creep and shrinkage are higher at low- strength LWC (i.e., for a bridge deck). However, it has been found that conventional methods can be used to estimate prestress losses for LWC bridge girders (Kahn and Lopez 2005). Although values for some properties may be lower for LWC than for NWC, designs can usually be adjusted to account for the differences. In some cases, differences may be offset by the reduced dead load in the structure. The increased modulus of elasticity of LWC results in a high ultimate strain capac- ity, which is beneficial in reducing the cracking tendency of concrete. The low level of microcracking observed in LWC provides high resistance to weathering and corrosion. ASR is not expected in concretes with LWAs, as the surface of the aggregate acts as a source of silica. Silica reacts with the alkalis at an early stage to help counteract any potential long-term disruptive expansion. The space in the aggregate also provides a place for the reaction products if any expansion occurs as a result of other aggregates in the concrete. LWC is more fire resistant than NWC because of its lower thermal conductivity, lower coefficient of thermal expansion, and the fire stability of the aggregate, which is already exposed to high temperatures (over 2,000ºF) during processing (ACI 213R, 2003b). 3.2.1.6.7 Overlay Concrete The use of overlays is described in additional detail in the bridge-deck section. Five types of concretes that have been used in overlays are briefly described here. Latex-modified concrete consists of a conventional concrete supplemented by a polymeric latex emulsion (styrene–butadine–latex) (Russell 2004). The water in the emulsion contributes to hydrating the cement. It has low permeability and, conse- quently, good durability, and also has good bonding characteristics. It outperforms conventional and low-slump dense concrete overlays and can be expected to last up to 25 years. Latex-modified concrete requires special mobile mixers, and proper curing is needed. It is typically applied in thicknesses of 1.5 to 2 in. Low-slump dense concrete has a moderate to high cement content and low w/cm ratio (ACI 546R, 2004; Russell 2004). It has increased resistance to chloride ion pen- etration. The main problems are that it is difficult to place, expensive, and prone to surface cracking. It requires special equipment for proper consolidation, and proper curing is critical. The standard has been to apply a 2-in. overlay. Silica fume concrete is widely used to produce concrete with greater resistance to chloride penetration (ACI 234R, 2006). It can be used effectively in thin overlays, in similar thickness as the latex-modified concrete, and like latex-modified concrete, it provides resistance to the penetration of chlorides. It is mixed in stationary mixers at the plant or in ready-mix concrete trucks. Proper curing is necessary. Polymer concrete is a composite material in which the aggregate is bound together in a dense matrix with a polymer binder (ACI 546R, 2004). It provides low permeabil- ity to water and aggressive solutions. Performance is highly dependent on the strength of the bond between the overlay and the concrete underneath, which is also dependent on surface preparation, cleanliness, and field application techniques. Most failures are

141 Chapter 3. MATERiALS attributed to workmanship or improper handling of materials. Epoxy polymers have a minimum thickness of 0.25 in. and are expected to last 10 to 15 years. Polymer con- crete has been used in applications up to 1.5 in. Certain types have an expected life of 20 years, depending on the thickness of treatment. Very-high-early-strength concrete is achieved using special blended cement with high fineness and high aluminum oxide (Al2O3) and sulfur trioxide (SO3) (Sprinkel 1998). Very-early latex-modified concrete provides a reliable driving surface within a few hours and reduces traffic interruption. 3.2.1.6.8 Shotcrete Shotcrete is mortar or concrete pneumatically projected at high velocity onto a surface (ACI 506R, 2005b). The high velocity of the material striking the surface provides the compactive effort necessary to consolidate the material and develop a bond to the existing surface. It contains coarse and fine aggregates, water, admixtures, and fibers. The use of an air-entraining admixture improves the resistance of shotcrete to freezing and thawing. The use of fibers improves toughness and gives load-bearing capacity after cracking. Fibers also help in minimizing plastic shrinkage cracking. Fibers used in shotcrete are generally divided into two groups by their diameter (ACI 506.1R, 2008b). Fibers with equivalent diameters greater than 0.012 in. (0.3 mm) are known as macrofibers; they are either steel or synthetic fibers. Macrofibers reduce crack prop- agation, increase flexural toughness, and improve ductility and impact resistance. They can provide resistance to drying shrinkage cracking and control crack widths at dos- ages as low as 0.25% by volume. Fibers with diameters less than 0.012 in. (0.3 mm) are known as microfibers. Microfibers used in shotcrete are mainly polypropylene or nylon. They can provide resistance to plastic shrinkage cracking due to excessive mois- ture loss at early ages at volume percentages as low as 0.1%. Shotcrete is capable of being placed in vertical and overhead applications without the use of forms (ACI 546R, 2004). There are two basic shotcrete processes: wet mix and dry mix. In wet-mix shotcrete, ingredients are mixed and pumped through a hose to a nozzle where air is added to project the material onto the surface. In dry-mix shotcrete, cementitious material and aggregate are premixed and pumped and water is added at the nozzle and projected onto a surface. Shotcrete is frequently used for repairing deteriorated concrete bridge substructures. It is also used for reinforcing structures by encasing additional reinforcing steel added to beams, placing bonded structural linings on walls, and placing additional concrete cover on existing concrete structures (ACI 546R, 2004). The success of shotcrete depends on the material used and the skill of the nozzle operator. In repairs with irregular shapes, shotcrete may be preferred because formwork is not needed. Shotcrete failures occur mainly due to inadequate preparation of the existing surface, poor workmanship, and not accounting for the relatively impermeable nature of shotcrete. The material may trap moisture and contribute to critical saturation, which is harmful during cycles of freez- ing and thawing. Wet-mix shotcrete is generally used where high production rates are needed. Concrete trucks usually supply concrete for wet-mix shotcreting. In substructure repairs, with small quantities of material, dry mix is commonly used.

142 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.2.1.6.9 Grout Grout is a mixture of cementitious material and water, with or without aggregate, pro- portioned to produce a pourable consistency without segregation of the constituents. Grout is a common material used in repairs to fill cracks, honeycombed areas, and interior voids and as a bonding agent. In new construction, it is used in open joints and to fill tendon ducts (ACI 546R, 2004). It can be a hydraulic-cement grout or other chemical grout such as a polymer–cement slurry, epoxy, urethane, or high-molecular- weight methacrylate. Grouting can strengthen a structure, arrest water movement, or both. Grout can be injected into an opening from the surface of a structure or through holes drilled to intersect the opening in the interior. When injected from the surface, short entry holes (ports), a minimum of 1 in. in diameter and a minimum of 2 in. deep, are drilled into the opening. The surface of the opening is sealed between ports, and grout is injected under pressure. Grouting is usually started with a relatively thin grout, but is thickened when possible. Narrow cracks would be filled by injection under pressure; however, wider cracks can be filled by gravity. Even though grouts are used as a bonding agent, research has shown that concrete bonds well to existing concrete, provided that the surface has been properly prepared. Selection of type of grout depends on the magnitude of stress at the location, the movement of the crack, the presence of solutions, crack width, required internal grout pressure, setting characteristic, heat liberation (high for epoxy types) cost, compatibil- ity with the existing concrete, penetrability, and bonding in the presence of moisture (ACI 546R, 2004). Chemical grouts have different mechanical properties and are more expensive than cement grout, and a high degree of skill is needed for satisfactory use of chemical grouts. Some epoxy systems do not bond in the presence of moisture. Chemical grouts can fill cracks as narrow as 0.02 in. (0.5 mm); for cement grouts the minimum crack width is 3 mm. ASTM C1107 covers three grades of packaged, dry, hydraulic-cement (nonshrinkable) grouts intended for use under applied load, such as to support a structure or a machine, where change in thickness below initial placement thickness is to be minimized. Rigid chemical grouts, such as epoxies, exhibit excellent bond to clean, dry substrates, and some bond to wet concrete. These grouts can restore the full strength of a cracked concrete member. ASTM C881 covers two-component, epoxy–resin bonding systems for application to concrete that are able to cure under humid conditions and bond to damp surfaces. Grouts in tendon ducts hinder the penetration of chlorides to reach the steel and bond the internal strands to the structure (Corven and Moreton 2004). Complete fill- ing of the duct is essential for proper protection. The primary constituent of grout is ordinary portland cement (Type I or II). SCMs are added to lower the permeability. Prepackaged materials are preferred because a more uniform product can be obtained. Total chlorides in grouts should be less than the specified limit of 0.08% by weight of cementitious material as specified in Table 10.9.3-2 of Construction Specifications in the LRFD Bridge Construction Specifications (AASHTO 2010a). Chlorides are limited to ensure that the protective layer over the strand is not compromised. The bleeding and resulting voids in grouts have been a concern. Usually the voids are

143 Chapter 3. MATERiALS interconnected and facilitate the intrusion of harmful solutions. Similarly, shrinkage should be controlled by using nonshrink grouts, so that cracks that can facilitate the intrusion of chlorides are eliminated or minimized. Grout properties are given in Con- struction Specifications, Table 10.9.3-2, along with the maximum total chloride ions (AASHTO 2010a). Grouts can be placed by pumping and vacuum injection. Vacuum injection is gen- erally used after initial grouting and requires that the duct system be sealed. To ensure that the duct is filled completely during construction, thixotropic grouts are used. These grouts have very low viscosity after agitation, making them easy to pump. They stay fluid when mechanically agitated or moved during pumping, but stiffen when at rest. 3.2.2 Reinforcement material Carbon steel (black steel) bars are commonly used as reinforcement in concrete. In the high-alkaline environment of concrete (pH ≈ 13.0 to 13.8), an oxide layer forms on the steel that protects the reinforcement from corrosion (Hartt et al. 2009). Chlorides pen- etrating the concrete break down this protective layer and initiate corrosion. Carbon- ation can lower the pH, increasing the corrosion rate. The iron corrosion products that form on steel have much greater volume than the metal that is consumed in the corro- sion reaction. This increase in volume causes tensile stresses in the concrete. When the stresses exceed the strength of the concrete, cracks, spalls, and delaminations occur. Alternatives to black steel are introduced to minimize the corrosion distress. These corrosion-resistant reinforcements are expected to extend the service life of structures. They include epoxy-coated steel, galvanized steel, low-carbon chromium steel, stain- less steel (solid or clad), nickel-clad reinforcement and copper-clad reinforcement, tita- nium, and fiber-reinforced polymers (FRPs). 3.2.2.1 Carbon Steel Reinforcing steel is available in different grades and specifications. The grades vary in yield strength, ultimate tensile strength, chemical composition, and percentage elonga- tion. The grade designation is equal to the minimum yield strength of the bar in kips per square inch (1 ksi = 1,000 psi). For example, Grade 60 rebar has a minimum yield strength of 60 ksi. Carbon steels are covered by ASTM specifications as follows: ASTM A615/A615M, deformed and plain carbon steel bars for concrete reinforcement (covers Grades 40 and 60); ASTM A996 (replaces ASTM A616), rail steel (covers Grades 50 and 60) and ASTM A617, axle steel (covers Grades 40 and 60); and ASTM A706 (Grade 60) for enhanced weldability. A706 includes Grade 80; however, its use may be outside of consideration of consensus design codes and specifications as noted in the ASTM specification. 3.2.2.2 Epoxy-Coated Reinforcement Organic coatings were first investigated as a possibility for rebar protection in the 1970s, when FHWA began searching for an organic coating that would be best suited for that purpose (Ramniceanu et al. 2008). After looking at 47 coatings, researchers

144 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE determined that the best candidates were four fusion-bonded epoxy powders (Kepler et al. 2000). 3.2.2.3 Galvanized Reinforcement Hot-dipped zinc-coated, or galvanized, steel reinforcement may provide superior per- formance to that of uncoated steel (Kepler et al. 2000; Xi et al. 2004; Virmani and Clemeña 1998). 3.2.2.4 Low-Carbon Chromium Steel Typical carbon steels form a matrix of chemically dissimilar materials: carbide at the grain boundaries and ferrite. In a moist environment, a microgalvanic cell forms that initiates corrosion. Low-carbon chromium steel matrix is almost carbide free, mini- mizing the galvanic action. Low-carbon chromium steel has low carbon content, less than 0.15%, and contains from 8% to 10.9% chrome (ASTM A1035). It exhibits strength and toughness (not brittle), and it is significantly more corrosion resistant than conventional steel. The chloride corrosion threshold for initiation of corrosion was found to be four times that of black bar (Hartt et al. 2009). 3.2.2.5 Stainless Steel Stainless steels are chromium-containing steel alloys with a minimum chromium con- tent of 10.5% (Markeset et al. 2006). Corrosion-resistant stainless steel contains a minimum of 12% chromium (Scully and Hurley 2007). An iron-chromium-rich oxide layer protects the stainless steel. Additional alloying elements such as nickel, molybde- num, and titanium are added for improved corrosion resistance. ASTM Specification A955 covers three grades of stainless steel bars with minimum yield strengths of 40, 60, and 75 ksi. Another type of stainless steel reinforcement, stainless steel–clad bars, is available. The cladding is a barrier coating used to prevent corrosive agents from coming in contact with the core of the bar. The improved cor- rosion resistance of stainless steel is due to a thin chromium oxide film that is formed on the steel surface. Oxygen is required for the film formation. Other typical alloying elements are molybdenum, nickel, and nitrogen. Nickel is mostly alloyed to improve the formability and ductility of stainless steel. The four major types of stainless steel (Markeset et al. 2006) are as follows: • Martensitic (not for reinforcement); • Ferritic. This has properties similar to mild steel but with better corrosion resis- tance, even though in the lower range of corrosion resistance for reinforcement; • Austenitic. This is the most widely used type of stainless steel and is rated in the higher range of corrosion resistance for reinforcement; and • Austenitic-ferritic (duplex). The duplex structure delivers both strength and ductil- ity, and is rated in the very high range of corrosion resistance.

145 Chapter 3. MATERiALS It is apparent that stainless steel has varying properties and corrosion-resisting potential, and studies are continuing to identify them for use as reinforcement (Hartt et al. 2009). 3.2.2.6 Nickel-Clad Reinforcing Bars Nickel coatings at least 0.025 mm (0.001 in.) thick provide good corrosion resistance (Virmani and Clemeña 1998). Even if the coating is damaged, the underlying diffusion zone of alloyed nickel and iron provides additional corrosion protection. A concern is the high cost of nickel. 3.2.2.7 Stainless Steel–Clad Reinforcing Bars Stainless steel–clad bars are an attractive alternative to solid stainless steel from the standpoint of both corrosion mitigation and cost. One would ideally gain the resis- tance to corrosion of solid stainless steel at a fraction of the cost of solid single-phase stainless steel (Scully and Hurley 2007). Concerns about using stainless steel–clad rein- forcement include (1) the difficulty encountered bonding the cladding to the bars and (2) the possibility that if areas of carbon steel are exposed because of mechanical dam- age (e.g., construction site handling or unsealed cut ends), those localized areas will corrode (Darwin et al. 1999; Scully and Hurley 2007). The corrosion resistance of clad bars is dependent on any defects that expose the carbon steel core. The performance is similar to solid stainless steel when intact, and when defective, it is similar to that of carbon steel rebar (Scully and Hurley 2007). 3.2.2.8 Copper-Clad Reinforcement Copper coatings of about 0.5 mm (0.02 in.) provide good corrosion resistance (Virmani and Clemeña 1998). However, copper can retard the hydration of cement. This retar- dation can lead to unhydrated cement particles around the bars, even though the paste has hardened. Copper-clad bars are expected to be cost-effective. 3.2.2.9 Titanium Titanium is highly resistant to corrosion; however, the cost can be five times that of stainless steel (MacDonald 1995). 3.2.2.10 Fiber-Reinforced Polymer FRP composite bars and strands are commercially available and have been used in structures; their use in highway infrastructure is discussed in NCHRP Report 503 (Mertz et al. 2003). FRP composite materials are light in weight and easy to construct; provide excellent strength-to-weight characteristics; and can be fabricated for made-to-order strength, stiffness, geometry, and other properties (ACI E2, 2000; Mertz et al. 2003). In addi- tion, FRP is nonmagnetic. FRP composite materials may be the most cost-effective

146 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE solution for repair, rehabilitation, and construction of portions of the highway infra- structure (Mertz et al. 2003). FRP is composed of a polymer matrix, either thermoset or thermoplastic, rein- forced with fiber or other reinforcing material (ACI 440R, 2007a). An anisotropic material, its most favorable properties are in the direction of the fibers. FRP perfor- mance depends on the fiber, the matrix, and the interaction of the two. The resin or the polymer holds the fibers in place, the fibers provide the mechanical strength, and the fillers and additives aid in processing and performance. Principal types of fibers used in structural applications are glass, carbon, and aramid. FRP bars are generally made of glass fibers embedded in a matrix (thermoset or thermoplastic resins). The properties of the FRP bars, such as high-temperature perfor- mance, corrosion resistance, dielectric properties, flammability, and thermal conduc- tivity, are mainly derived from the properties of the matrix. Depending on the matrix used, the mechanical properties of FRP bars (e.g., tensile strength, ultimate strain, and Young’s modulus of elasticity) might degrade under specific environmental conditions such as alkaline environment and moisture (water and salt added to water) as follows: • Effects of the alkaline environment. The alkaline water contained in concrete pores may cause degradation, which might cause damage to the polymeric matrix; and • Effects of moisture (water and salt added to water). The main effects of moisture absorption by the matrix may result in strength reduction and stiffness reduction (less pronounced) in the FRP. The absorption of moisture depends on the type of polymeric matrix, the matrix interface, and the quality of the bars. Glass fibers are available as E-glass (electrical grade), high-strength (S-2) glass, and improved acid resistance (epoxy-coated reinforcement (ECR)) and alkali-resistant (AR) glass (ACI 440R, 2007a). These glass fibers generally range in diameter from 9 to 23 μm. Fibers are drawn in at high speed through small holes in electrically heated bushings to form the individual filaments. The filaments are coated with a chemical binder or sizing for protection and to enhance the composite properties at the fiber–matrix interface. The filaments are gathered into groups or bundles called strands or tows. E-glass fibers are the most susceptible to degradation caused by alkalinity and moisture and must be protected by the appropriate resin. Carbon fibers are inert to the environment. Different fiber systems and resin systems provide different levels of resis- tance to environmental conditions such as moisture, alkaline solution, UV radiation, or extreme temperatures. Selection of the proper reinforcement and the proper resin is needed for longevity. The coefficient of thermal expansion of the FRP composites with glass fibers is higher than that of the concrete. The difference should be considered when FRP is in direct contact with concrete. At low temperatures and exposure to cycles of freezing and thawing fibers are not affected, but the resin and the fiber–resin interface are affected. Cycles of freezing and thawing and the presence of road salts may result in microcracks and gradual degradation. Polymer resins exhibit high creep and relaxation behavior; addition of fibers increases the creep resistance. Thermosetting resins are more resistant to creep than the thermoplastic resins. FRP generally has good fatigue performance because the fibers

147 Chapter 3. MATERiALS have minimal defects and the matrix resists the propagation of cracks. Several draw- backs of FRP are that it does not exhibit ductile failure mode as steel, it is the lower elastic modulus, and the strands are difficult to grab, making anchorage details critical. 3.2.3 Structural Steel This section describes the various steel types, including currently used types and older types found in older bridges, and describes various characteristics that are important for durability and long-term service life. 3.2.3.1 Current Steel Grades Currently, there are seven structural steel grades for bridges covered by ASTM A709 (AASHTO M-270) specifications with four yield strength levels (ASTM A709-11). Table 3.6 identifies these current types. Included are ASTM reference standard grades, along with the steel descriptions and yield and tensile strengths. The letter W is at- tached to the grade number to designate steel that has weathering capability. The letter S is attached to designate special steel grade for structural shapes. Grades 36, 50, 50W, HPS 50W, and HPS 70W are available with the properties shown in plate thicknesses up to 4 in. Properties for HPS 100W will vary depending on plate thickness, as shown in Table 3.6. Grade 36 steel, which has been a highly used grade over the years, includes basic carbon steel shapes, plates, and bars for use in riveted, bolted, or welded construction. Grade 50 and 50S steel includes high-strength, low-alloy columbium-vanadium structural steel for shapes, plates, bars, and sheet piling and is intended for riveted, bolted, or welded construction. tABLE 3.6. current AStm A709 SteeL grAdeS (AStm A709-11) Grade Additional Reference Standard Name or Designation Yield Strength (ksi) Tensile Strength (ksi) 36 A36 Structural carbon steel 36 (min) 58 to 80 50 A572 High-strength low-alloy columbium-vanadium steel 50 (min) 65 ksi (min) 50S A992 Structural steel shapes 50 to 65 65 (min) 50W A588 High-strength low-alloy weathering steel 50 (min) 70 (min) HPS 50W High-performance steel 50 (min) 70 HPS 70W High-performance steel 70 (min) 85 to 110 HPS 100W (to 2.5 in. thick) High-performance steel 100 (min) 110 to 130 HPS 100W (over 2.5 in. to 4 in.) High-performance steel 90 (min) 100 to 130 Note: min = minimum.

148 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Grade 50W includes high-strength, low-alloy weathering steel shapes, plates, and bars for welded, riveted, or bolted construction, but it is intended primarily for use in welded bridges in which savings in weight and added durability are important. New high-performance steels have been added with three grades as shown in Table 3.6. These new steels are further described in Section 3.2.3.6. Table 3.7 identifies older steel grades that are still available, but AASHTO and A709 have replaced them with HPS 70W and HPS 100W grades because of improved properties. 3.2.3.2 Steel Properties Steel’s mechanical properties are those that characterize its elastic and inelastic behav- ior under stress and strain. Such properties include parameters that are related to the material’s strength, ductility, and toughness. Other parameters, such as weldability, machinability, and weathering, are also important in regard to the ease with which the material can be fabricated and achieve long-term durability (Barson 1994). The following discussion of various steel properties is adapted from Barson (1994) and Errera (1964). Strength is represented by the material’s yield strength and ultimate tensile strength. Ductility is the ability of a material to undergo large plastic deformations without fracture. It is represented by the amount of elongation that the material can experience after yielding and before reaching its ultimate strength. Ductility is an important mate- rial property because it allows redistribution of high local stresses that occur in welded connections and at regions of stress concentration such as holes or geometric changes. Toughness is the capacity of a material to absorb large amounts of energy before fracture. It is related to the area under the stress–strain curve and is dependent on strength and ductility. The larger the area under the stress–strain curve, the tougher the material. Weldability is the material’s capability to withstand welding without seriously impairing its mechanical properties. Weldability varies considerably for different types of steels and different welding processes. Machinability is the ease with which a material can be sawed, drilled, or otherwise shaped without seriously impairing its mechanical properties. Weathering is the material’s capability to resist corrosion in a given environment. tABLE 3.7. oLder StructurAL SteeL grAdeS StiLL AvAiLAbLe Grade Additional Reference Standard Name or Designation Yield Strength (ksi) Tensile Strength (ksi) 70W A852 High-strength low-alloy quenched and tempered steel 70 (min) 90 (min) 100/100W A514 High-strength quenched and tempered alloy steel 100 (min) 110 to 130 Note: min = minimum.

149 Chapter 3. MATERiALS The steel’s mechanical properties are affected by the following three factors: 1. Chemical composition, 2. Processes used to transform the base metal into the final plate or structural shape product, and 3. Heat treatment. The following sections describe these three factors. 3.2.3.3 Chemical Composition Chemical composition is a primary factor in determining the properties of a steel type. Structural steels are made up of iron and carbon with varying amounts of other ele- ments, primarily manganese, phosphorus, sulfur, and silicon. Specific properties are achieved by the addition or presence of these and other elements in various combina- tions and proportions (Barson 1994) In carbon steels, the elements carbon and manganese have a controlling influ- ence on strength, ductility, and weldability. Grade 36 carbon steel is more than 98% iron, with about 0.25% carbon and about 1% manganese by weight. These percent- ages change dramatically among the various grades. Carbon increases the hardness or tensile strength, but it has adverse effects on ductility and weldability. To counter- act these adverse effects, small amounts of various alloying elements are sometimes used to increase the ability of a steel to achieve higher strength while maintaining a lower percentage of carbon content. Phosphorus and sulfur typically have a harmful effect, especially on toughness, and the fractional percentages of these elements must be kept low. Small amounts of copper increase corrosion resistance and help develop the weathering steel grades; fractional percentages of silicon are used mainly to elimi- nate unwanted gases from the molten metal; and nickel and vanadium have a generally beneficial effect on steel behavior (Errera 1964). Table 3.8 lists advantages and disadvantages of commonly used alloy elements and includes approximate chemical composition used in plate steel grades for comparison. It is interesting to note how each element percentage changes in progressing from Grade 36 to 50 to 50W to HPS 50W and finally to HPS 100W. The progression in steel grades increases in strength, weathering characteristics, and ultimately in toughness and weldability, and the various chemical percentages illustrate how these properties are partially achieved. The chemical composition of Grade HPS 70W is same as HPS 50W. Grade 50S is not shown. 3.2.3.4 Rolling and Shaping Processes The process used to transform the base metal into the final plate or structural shape product is another factor affecting steel’s mechanical properties. Hot rolling is typi- cally the process used to shape steel. It consists of passing the material between two rolls revolving in opposite directions, with the distance between the rolls less than the thickness of the original entering material. The rolls grip the piece and reduce its cross-sectional area, increasing its length and to a lesser extent, its width. Because of

150 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 3.8. eFFectS And comPoSition oF ALLoying eLementS Alloy Element Advantage Disadvantage Steel Grade Typical Composition Carbon Principal hardening element in steel. Increases strength and hardness. Decreases ductility, toughness, and weldability. 36 50 50W HPS 50W HPS 100W 0.25% 0.23% 0.19% 0.11% 0.08% Manganese Increases hardness and strength. Controls harmful effects of sulfur. High amounts can cause embrittlement and reduce weldability. 36 50 50W HPS 50W HPS 100W 0.8% to 1.2% 1.35% 0.8% to 1.25% 1.1% to 1.35% 0.95% to 1.5% Phosphorus Increases strength and hardness. Improves corrosion resistance. Generally considered an impurity. Decreases ductility and toughness. 36 50 50W HPS 50W HPS 100W 0.04% 0.04% 0.04% 0.02% 0.015% Sulfur Improves machinability. Generally undesirable. Decreases ductility, toughness, and weldability. Adversely affects surface quality. 36 50 50W HPS 50W HPS 100W 0.05% 0.05% 0.05% 0.006% 0.006% Silicon Used to deoxidize molten steel. Increases strength and hardness. Decreases weldability. 36 50 50W HPS 50W HPS 100W 0.4% 0.4% 0.3% to 0.65% 0.3% to 0.5% 0.15% to 0.35% Aluminum Used to deoxidize molten steel. Refines grain size, thus increasing strength and toughness. None. HPS 50W HPS 100W 0.01% to 0.04% 0.02% to 0.05% Vanadium Used to refine grain size. Small additions increase strength and toughness. High amounts reduce hardness. At high finishing temperature may be detrimental to finishing. 50 50W HPS 50W HPS 100W 0.01% to 0.15% 0.02% to 0.1% 0.04% to 0.08% 0.04% to 0.08% Columbium Small additions produce finer grain size, which increases strength and toughness. None. 50 HPS 100W 0.005% to 0.05% 0.01% to 0.03% Nickel Increases strength and toughness and improves corrosion resistance. Cost. 50W HPS 50W HPS 100W 0.4% 0.25% to 0.4% 0.65% to 0.9% Chromium Increases strength and corrosion resistance. Used in weathering steel. Cost. 50W HPS 50W HPS 100W 0.4% to 0.65% 0.45% to 0.7% 0.4% to 0.65% (continued)

151 Chapter 3. MATERiALS Alloy Element Advantage Disadvantage Steel Grade Typical Composition Molybdenum Increases strength, weldability, toughness, and corrosion resistance. Cost. Delays rather than eliminates temper embrittlement. HPS 50W HPS 100W 0.02% to 0.08% 0.4% to 0.65% Copper Increases corrosion resistance. Used in weathering steel. May adversely affect notch toughness. 50W HPS 50W HPS 100W 0.25% to 0.4% 0.25% to 0.4% 0.90% to 1.2% Nitrogen Increases strength and hardness. Decreases ductility and toughness. HPS 50W HPS 100W 0.015% 0.015% Note: Advantages and disadvantages are from the Highway Structures Design Handbook (Vol. 1, Chapter 3, Properties of Bridge Steels, Barson 1994) and the Metals Handbook (ASM 1998); percent composition is from ASTM A709-11. tABLE 3.8. eFFectS And comPoSition oF ALLoying eLementS (continued) the rolling, the steel fracture toughness is greater in the direction of rolling than in the direction perpendicular to it. Further, a greater reduction of cross section during the normal hot-rolling process will produce a greater yield and tensile strength (Barson 1994). Some steels are purposely cold rolled to obtain higher strength levels. The cold working strain hardens the material. 3.2.3.5 Heat Treatment A variety of heat treatments used in the steel-making process develop certain desirable characteristics in steel. These heat treatments can be divided into two groups: slow cooling and rapid cooling. Slow-cooling treatments include annealing, normalizing, and stress relieving, which decrease hardness, promote uniformity of microstructure, and improve ductility and fracture toughness. They also improve machinability or facili- tate cold forming, and they relieve internal stresses. Rapid-cooling treatments, such as quenching and tempering, increase strength, hardness, and toughness (Barson 1994). Quenching and tempering consist of heating steel to a high temperature (about 1,650°F) long enough to produce a desired microstructural change and then quenching by immersion in water. After quenching, the steel is tempered by reheating to a specific temperature (usually between 800°F and 1,250°F), holding for a specified time, and then cooling under suitable conditions to obtain the desired properties. The rapid cooling by quenching increases strength but reduces ductility. Tempering restores part of the duc- tility, but gives up some of the strength gained by the quenching. This process permits attainment of higher strengths while retaining relatively good ductility (Barson 1994). 3.2.3.6 High-Performance Steel A cooperative research program between FHWA, the American Iron and Steel In- stitute, and the U.S. Navy was launched in 1994 to develop new high-performance steels for bridges. The driving force was the need to develop improved higher-strength,

152 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE higher-toughness steels with improved weldability. Improvements in these character- istics would improve the overall quality and ease of fabrication of bridge steels used in the United States. Further, the steel would have weathering characteristics, with the added designation “W.” Three strength grades were developed and are now available for general use: HPS 50W, HPS 70W, and HPS 100W. Grades HPS 70W and HPS 100W have now replaced the older high-strength, low-alloy quenched and tempered steel (AASHTO M270 Grade 70W) and the high-strength quenched and tempered alloy steels (Grades 100 and 100W). It is the intent that the newer high-performance steels should be used at these higher strength levels because of their enhanced proper- ties. The older steels are still available, but their use is discouraged (FHWA 2002). HPS 70W is the most widely used grade in the HPS group because its reduced weight can make it more economical than conventional Grade 50W steel. HPS 70W is produced by quenching and tempering or thermal-mechanical controlled processing. Because quenching and tempering processing limits plate lengths to 50 ft in the United States, thermal-mechanical controlled processing practices have been developed to produce HPS 70W up to 2 in. thick and up to 125 ft long, depending on the weight. HPS 50W steel, which has the same chemistry as HPS 70W, is produced using con- ventional hot rolling or controlled rolling up to 4 in. thick in lengths similar to Grade 50W steel (FHWA 2002). A major advantage of high-performance steel is its increased fracture toughness, which is much higher than that of conventional bridge steels. This is evident from Figure 3.3, which shows the Charpy V-notch transition curves for HPS 70W and con- ventional Grade 50W steel. The brittle-ductile transition of HPS 70W occurs at a much lower temperature than conventional Grade 50W steel. This means that HPS 70W remains fully ductile at lower temperatures, while conventional Grade 50W steel begins to show brittle behavior (FHWA 2002). The current AASHTO Charpy V-notch toughness requirements are specified to avoid brittle failure in steel bridges above the lowest anticipated service tempera- ture. The HPS 70W steels tested so far show ductile behavior at the extreme service Figure 3.3. Charpy V-notch transition curves for 50W and HPS 70W steels. Source: FHWA 2002.

153 Chapter 3. MATERiALS temperature of –60°F, which corresponds to Zone 3, with a minimum service tempera- ture of below –30°F to –60°F (FHWA 2002). With higher fracture toughness, high-performance steels have much higher crack tolerance than conventional-grade steels. Full-scale laboratory fatigue and fracture tests of precracked I-girders made of HPS 70W steel showed that the girders were able to resist the full design overload without fracturing even when the initial crack was large enough to cause a 50% loss in tension flange net section. This is a major feature that can increase service life of bridges using HPS steels because a large crack tolerance increases the time for detecting and repairing fatigue cracks before the bridge becomes unsafe (FHWA 2002). 3.2.3.7 Weathering Steels Uncoated weathering-grade steels have been used for bridge applications since the mid- 1960s. When used in the right environment, these steels result in both initial and life- cycle cost savings as they eliminate the need for shop and field painting and can provide over 100 years of service life with minimal maintenance. Weathering-grade steels are currently supplied under AASHTO Specification M270, ASTM A709 Grade 50W, and in high-performance steel Grades HPS 50W, 70W, and 100W. These steels have a special alloy composition with small amounts of copper, phos- phorus, chromium, nickel, and silicon that makes them able to resist corrosion with- out any applied coating. During initial exposure to the elements, these steels form a dense, tightly adhering patina that is essentially a natural oxide coating, about the same thickness as a heavy coat of paint. It is this patina that protects the steel and enables it to resist further atmospheric corrosion (McEleney 2005). Weathering steel bridges initially look orange-brown in color; the color darkens as the patina forms. In 2 to 5 years, depending on the climate, the steel will attain a dark, rich, purple-brown color that many think is attractive (McEleney 2005). Guidelines for proper application of weathering-grade steels in highway structures and recommendations for maintenance to ensure continued successful performance were issued by FHWA in Technical Advisory T-5140.22 (FHWA 1989). Despite its many advantages, there are certain conditions where the use of uncoated weathering-grade steel should be considered with caution. Proper weathering behavior of these steels depends on alternating cycles of wet and dry conditions that allow a proper patina or protective layer to form (McEleney 2005). In environments of contin- ual wetting, such as areas with frequent high rainfall, high humidity, or persistent fog or with low-level water crossings, the patina development can be adversely affected, resulting in continuing corrosion (FHWA 1989). Exposure to salt is also detrimental to proper weathering. This can occur in marine coastal areas where the steel is subject to salt-laden air or spray. It can also occur in certain grade separation conditions where a tunnel effect is created by the combina- tion of a bridge with minimum clearance over a depressed roadway section between vertical approach retaining walls and deep abutments that are directly adjacent to shoulders. In these conditions, roadway spray cannot be dissipated by air currents, and where heavy deicing salt is used, it can result in excessive salt deposits on the overhead

154 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE bridge steel (FHWA 1989). These conditions are often found in urban grade separation settings. Exposure to roadway drainage, especially when combined with roadway deicing salts, is also detrimental. This often occurs below open or leaking expansion joints, and FHWA recommends that the ends of weathering steel girders below roadway joints be painted within a length equal to 1.5 times the depth of the girder for protec- tion (FHWA 1989). This girder end painting also helps prevent the staining of concrete piers and/or abutments below (McEleney 2005). Heavy industrial environments where steel may be subject to concentrated chemical fumes (especially from sulfur dioxide) that may drift directly onto the structure can also be detrimental. These environments should be evaluated before making decisions about using uncoated weathering steel in them. Moderate industrial environments are not det- rimental and can actually speed up the weathering process, however (McEleney 2005). 3.2.3.8 Structural Stainless Steel A new steel was developed in recent years, ASTM A1010, which is a 12% chromium structural steel with corrosion resistance that is superior to conventional weathering steels’. Currently, A1010 is widely used in aggressive structural applications such as coal rail cars and coal processing equipment, but because of A1010’s superior corrosion resistance, it is also being considered for bridge applications in corrosive environments. A1010 steel is available in typical plate sizes in 50- and 70-ksi strength levels, and data on thicknesses up to 4 in. are now available (Fletcher et al. 2003; Wilson 2005). Laboratory corrosion testing to the SAE 52334 standard has shown that A1010 performs in a superior fashion in a wet–dry saltwater environment when compared with weathering or galvanized specimens. Figure 3.4 summarizes test results of A1010 compared with carbon steel, A588 weathering steel, and galvanized carbon steel in a salt, wet–dry 8-week test. In addition, long-term exposure in seaside locations has shown that A1010 performs significantly better than a variety of weathering steels. Although this material is more expensive than conventional steels, its ability to per- form in highly corrosive environments may give it an advantage when considering life- cycle cost over a long-term service life (Fletcher et. al. 2003; Wilson 2005). Precautions must be taken, however, when using stainless steel to avoid galvanic corrosion when in contact with carbon steel, zinc, or aluminum. 3.3 concrete And SteeL diStreSSeS And SoLutionS The common distress factors that affect durability and the relevant solutions are given in Section 3.3.1, and methods for protecting reinforcement against corrosion are given in Section 3.3.2. 3.3.1 Common Concrete Distress factors and Solutions Concrete is subject to deterioration due to physical factors (moisture, temperature, freeze and thaw); chemical factors (ASR, ACR, carbonation, chlorides, sulfates, and acids) and functional factors (vibrations, impact, concrete consolidation, concrete

155 Chapter 3. MATERiALS curing, and concrete placement). Stresses induced in concrete caused by volumetric changes resulting from these chemical, physical, and/or functional factors can exceed the strength of the material, leading to cracks, delaminations, and spalling. Proper steps should be taken to protect concrete from cracking. 3.3.1.1 Factors The most common outcome of a distress is the formation of cracks in concrete. Volu- metric changes that occur in concrete; the restraint in the bridge system; and the elastic modulus, creep, and tensile strength characteristics lead to cracking (TRB 2006). The lower the modulus of elasticity, the lower will be the amount of the induced elastic tensile stress for a given magnitude of volumetric change, thus reducing the possibility of cracking. In contrast, high elastic modulus is associated with brittle concretes that are prone to cracking and crack propagation. In concretes with low elastic modulus, such as lightweight concrete (LWC), stresses are less for a given strain compared with conventional normal-weight concrete (NWC) with higher elastic modulus. Reduced stresses minimize the occurrence and severity of cracks (number and width). Cracks facilitate the intrusion of aggressive solutions that adversely affect the durability of concrete. In columns and prestressed beams reduced modulus of elasticity would in- dicate higher deformation and prestress losses. Concrete can be engineered to have tight cracks (Sahmaran and Li 2001). In such concrete cracks formed are numerous and very tight, less than 0.004 in. (0.1 mm), in width. It is difficult for harmful solu- tions to penetrate such tight cracks. Research has shown that concrete with crack widths less than 0.004 in. (0.1 mm) performs like sound concrete and does not allow water to penetrate the cracks easily (Wang et al. 1997; Lawler et al. 2002). ACI 224R Source: Wilson (1999). Figure 3.4. Corrosion resistance of A1010 steel. [Composition: Please del te “(1) salt, wet/dry, 8-week test” at the lower left of the figure, if possible.] Figure 3.4. Corrosion resistance of A1010 steel. Source: Wilson 1999.

156 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE (ACI 2001a) provides guidance on tolerable crack widths for reinforced concrete ex- posed to different exposure conditions. For example, the reasonable crack width for reinforced concrete exposed to seawater and seawater spray wetting and drying is 0.006 in. (0.15 mm). A note to the table in ACI 224R (ACI 2001a) indicates that a portion of the cracks in such a structure are expected to exceed these values; with time, a significant portion can exceed these values. The provisions contained in the LRFD Bridge Design Specifications (AASHTO 2012) are based on a maximum crack width of 0.004 in. (0.1 mm) for reinforced concrete exposed to a marine environment. Volumetric changes can occur because of moisture loss, as in plastic shrinkage or drying shrinkage, or in response to high temperatures and high temperature dif- ferentials. As cement and water react, heat is generated; this phenomenon is known as heat of hydration (Kosmatka and Wilson 2011). The rate of heat generation is greatest at early concrete ages. For thin concrete elements heat generation is gener- ally not a concern because the heat is dissipated quickly. However, in mass concrete (Figure 3.5), heat is not easily dissipated, and a significant rise in temperature occurs. As a general rule, any placement of structural concrete with a minimum dimension equal to or greater than 3 ft should be considered mass concrete (Gajda 2007). Simi- lar considerations should be given to other concrete placements that do not meet this minimum dimension, but contain ASTM C150 Type III or HE (high-early-strength) cement, accelerating admixtures, or cementitious materials in excess of 600 lb/yd3 of concrete (Gajda 2007). Mass concrete specifications, generally, limit the temperature difference between the center and surface of the concrete to 35°F. This is a conserva- tive limit that has been effective (Gajda 2007). A performance-based temperature dif- ference limit tailored to the particular job can indicate temperature differences higher than 35°F without adverse effects. Finite element modeling and detailed calculations are often used to develop temperature difference limits. High temperatures exceeding 160°F during the first few hours following place- ment can lead to delayed ettringite formation at later ages. Ettringite forms when gyp- sum and other sulfate compounds react with calcium aluminate in cement in the first Figure 3.5. Mass concrete: footings and columns. Source: Courtesy Virginia DOT.

157 Chapter 3. MATERiALS few hours after mixing with water (Kosmatka and Wilson 2011). However, at high temperatures the normal formation of ettringite during the first few hours is impeded, and delayed ettringite formation occurs in hardened concrete, which can crack the concrete. Heat-related cracking can also occur in bridge decks, which are typically not considered to be mass concrete (Figure 3.6). In this case the bridge beams restrain temperature-related movement of the deck. The heat of hydration causes the deck to expand and then contract during cooling, and the beams restrain the movement. To prevent the strains and resulting tensile stresses, a maximum temperature differential of 22°F between the beams and the deck is recommended for at least 24 hours follow- ing concrete placement (Babaei and Fouladgar 1997). As with mass concrete, higher temperatures may be justified, but they require detailed analysis to justify. To minimize the potential for temperature-related cracking, the amount of portland cement should be minimized, concrete delivery temperature reduced, and pozzolans and slag included in the mixture. With proper temperature management thermal cracks can be mini- mized and controlled. Volumetric changes also occur as a result of freezing and thawing cycles and chem- ical (corrosion, ASR, and ACR) reactions. 3.3.1.2 Prevention of Distress 3.3.1.2.1 Surface Treatments Surface treatments, membranes, sealers, and overlays are widely used to reduce so- lution infiltration. They protect the deck concrete from deterioration induced by freeze–thaw cycles and protect the reinforcement from corrosion (Kepler et al. 2000). Currently, there are three types of waterproofing membranes used in North America: preformed sheets, liquid membranes, and built-up systems (Russell 2012). Preformed systems are labor intensive and require good workmanship. Liquid systems are easier to place. Built-up systems are generally labor intensive and expensive (Manning 1995). Figure 3.6. Thermal cracks in bridge deck. Source: Courtesy Virginia DOT.

158 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Sealers can be a pore blocker, forming a microscopically thin (up to 2 mm) protec- tive layer on the concrete surface, or a penetrating liquid that acts as a hydrophobic agent (Zemajtis and Weyers 1996). When used as a pore blocker on bridge decks, the effect on skid resistance should be considered (Sherman et al. 1993). Sealers should be breathable, enabling escape of moisture to prevent high vapor pressures that can cause blistering and peeling (Sherman et al. 1993). 3.3.1.2.2 Control of Volumetric Changes Caused by Moisture and Temperature To minimize volumetric changes caused by shrinkage and temperature, low cementi- tious material, low water content, and low paste content are desirable. This can be achieved by using well-graded aggregates, reducing mix temperature, and selecting the appropriate admixtures. 3.3.1.2.3 Control of Wear and Abrasion Traffic on bridge decks causes abrasion. Moving water also carries objects that can cause abrasion. Studded tires are very damaging to the surface of the concrete and are not permitted in some states, and many other states have seasonal restrictions. Chains also have a damaging effect, although not as much as studs. Chains are encouraged in winter in many areas, and some states require chains for commercial trucks. Abrasion resistance of concrete is a function of the water-cementitious materials ratio (w/cm) at the surface and the aggregate quantity and quality. The Los Angeles abrasion test indicates the abrasion resistance of aggregates; in general, siliceous aggregates provide satisfactory abrasion resistance. Proper finishing and curing are also factors that influ- ence abrasion resistance. 3.3.1.2.4 Control of Freeze–Thaw Damage Concrete that can become critically saturated and exposed to cycles of freezing and thawing must be properly air entrained, have sound aggregates, and have the maturity to develop a compressive strength of about 4,000 psi to avoid cracking and scaling (Mather 1990). Air voids must be small in size, closely spaced, and uniformly distrib- uted to ensure adequate resistance to freezing and thawing and satisfactory strength. A spacing factor less than 0.008 in. is needed for adequate protection during freezing and thawing. However, with the use of high-range water-reducing admixtures and low-permeability concretes, higher values may be acceptable and should be verified by relevant tests, such as ASTM C666 Procedure A. 3.3.1.2.5 Control of Chemical Reactions Disruptive chemical reactions can occur in concrete that adversely affect durability. The chemical composition of the cementitious material affects the rate of hydration and pozzolanic reaction and heat generation and contributes to the formation of dis- ruptive products. The fineness of the cementitious material affects the rate of reac- tions and water demand. At high water w/cm, permeability is increased and durability is reduced. The presence of certain chemicals in sufficient amounts in the cements

159 Chapter 3. MATERiALS contributes to the expansion. Chemical reactions involving alkalis with aggregates, sil- icates, and carbonates; carbonation; chlorides; sulfates; acids; and salts are explained in the following subsections. 3.3.1.2.5a Alkali-Aggregate Reaction Alkali-aggregate reactions are the reactions between the hydroxide ions in the pore fluid of concrete, usually associated with alkalis from the cement or from outside sources such as deicing salts, and the reactive constituents of the aggregates (ACI 221.1R, 1998). This reaction results in expansion and cracking. 3.3.1.2.5b Alkali-Silica Reaction A chemical reaction between aggregates containing reactive silica and the alkalis in concrete can produce an alkali-silica gel that swells when water is absorbed. The high pressures generated within the concrete lead to cracking (Figure 3.7). To prevent ASR, nonreactive aggregates, or pozzolanic materials or slag, lithium nitrate, low-permea- bility concrete, and cements with low alkali contents are used. 3.3.1.2.5c Alkali-Carbonate Reaction Some argillaceous, dolomitic aggregates can react with alkalis, causing the aggregates to expand. Figure 3.8 shows a joint closing due to ACR expansion. A common preven- tion for ACR is to avoid using reactive aggregate or to dilute the aggregate by blending with nonreactive aggregate. 3.3.1.2.5d Carbonation Carbon dioxide (CO2) produced by plants penetrates concrete and reacts with the hydroxides, such as calcium hydroxide (lime), to form carbonates. In this carbonation process the pH is reduced to less than nine and influences the protective layer over the steel (Neville 1995). Figure 3.7. Alkali-silica reaction. Source: Courtesy Virginia DOT.

160 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.3.1.2.5e Chlorides Chlorides that penetrate the concrete and reach the steel surface destroy the protec- tive oxide layer, making reinforcement prone to corrosion. Without the protective layer, steel will corrode rapidly in the presence of water and oxygen. The corrosion of steel is accompanied by expansive pressures, which lead to cracking (Figure 3.9). Figure 3.8. Closing of joints as a result of ACR. Source: Courtesy Virginia DOT. Figure 3.9. Corrosion. Source: Courtesy Virginia DOT.

161 Chapter 3. MATERiALS Corrosion- inhibiting admixtures can be used to stabilize the passive oxide layer on the reinforcement, or viscosity-modifying admixtures can be added to improve the stability of the mix. 3.3.1.2.5f Sulfates Sulfates react with the calcium hydroxide [Ca(OH)2] and the calcium aluminate hy- drates, causing expansive reactions that can affect the cement paste (Neville 1995). Figure 3.10 shows the loss of material after sulfate attack. To increase the resistance of concrete to sulfate attack, a low tricalcium aluminate (C3A) content and low quanti- ties of Ca(OH)2 in the cement paste and low-permeability concrete are needed. These results can be obtained by using sulfate-resistant cements and pozzolans. 3.3.1.2.5g Acids Pollutants cause acid rain that can cause deterioration (Neville 1995). In damp condi- tions, SO2, CO2, SO3, and other acid forms that are present in the atmosphere may attack concrete by dissolving in water and removing parts of the cement paste. These acids will leave a soft and mushy mass (Eglinton 1975). To minimize acid attack, low- permeability concrete and barrier coatings may be used. A barrier material separates the concrete surface from the environment. 3.3.1.2.5h Salts Chloride-bearing deicing chemicals initiate and accelerate corrosion. Magnesium salts are very damaging to concrete, causing crumbling (Lee et al. 2000). Calcium mag- nesium acetate has been suggested to prevent corrosion, but it was found to be very damaging to concrete. Deicing chemicals can also aggravate freeze–thaw deteriora- tion. Osmotic pressure occurs because moisture tends to move toward zones with higher salt concentrations. In addition, the salts increase the rate of cooling, causing an increase in the potential for freeze–thaw deterioration at the concrete surface. Proper air entrainment and maturity are needed to provide the necessary protection. Figure 3.10. Sulfate attack. Source: Courtesy Virginia DOT.

162 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.3.1.2.6 Functional Considerations Functional considerations include vibrations, impact, concrete consolidation, concrete curing, and concrete placement. 3.3.1.2.6a Vibrations Vibration of fresh concrete using vibrators may cause loss of air in mixtures with a high sand content and could result in freeze–thaw damage. In these mixtures, the frequency and duration of vibration should be reduced to prevent segregation (loss of stability) and loss of air. A recommended practice is to prepare mixtures with a large amount of coarse aggregate content. However, in some regions where D-cracking (Fig- ure 3.11) is an issue, the size and amount of coarse aggregate is reduced, which leads to mixtures with an excessive sand content. In D-cracking, the aggregate has a pore structure that hinders the expulsion of water from the aggregate pores during freezing, which results in the cracking of the aggregate and concrete. Concrete can exhibit fatigue behavior when subjected to cyclic loading of a given level (below its short-term static strength) and will eventually fail. 3.3.1.2.6b Impact Concrete can be subjected to extreme loads from the impact of falling rocks, snow avalanches, landslides, or vehicle crashes. The impact resistance is related to the com- pressive strength and aggregate type (Kosmatka and Wilson 2011). Fiber-reinforced concretes are used to minimize the effect of impact in certain applications. 3.3.1.2.6c Concrete Consolidation Proper consolidation ensures that undesirable entrapped air voids are eliminated. Voids in concrete reduce strength, and when interconnected or in large amounts, they can increase permeability. Currently, workable concretes are made using water- reducing admixtures. It is also possible to make stable concretes that have high flow rates and are self-consolidating. Figure 3.11. D-cracking. Source: Courtesy Portland Cement Association, Skokie, Illinois.

163 Chapter 3. MATERiALS 3.3.1.2.6d Concrete Curing Curing ensures that reactions occur and volumetric changes are minimized. Curing should continue until a certain level of the desired properties is achieved. Concrete should stay wet during the curing period, and the temperature should be managed to eliminate large differentials. ACI 308R provides recommendations for adequate cur- ing. ACI 305R and ACI 306R provide information on curing of concrete in hot and cold weather, respectively. 3.3.1.2.6e Concrete Placement Concrete should be placed without causing segregation and loss of moisture. Forms should be adequately set, clean, tight, and adequately braced. Forms should be oiled or treated with a form-release agent. Reinforcing steel should be clean and free of rust or mill scale. In cold weather, concrete should not be placed in contact with metal forms and embedments, such as steel structural members or reinforcement, which can freeze the concrete (ACI 306R, 2010). If the frozen concrete does not thaw before the bulk of the concrete sets, bond may be significantly reduced, and concrete quality adjacent to cold metal would be poor. Ideally, the adjacent metal should be heated to the tem- perature of the concrete immediately before concrete placement (ACI 306R, 2010). Concrete should be deposited continuously and as near as possible to its final posi- tion without objectionable segregation (Kosmatka and Wilson 2011). Concrete should be deposited in areas free of standing water. However, in some applications, such as drilled shafts, standing water may be present. In such applications, concrete should be placed in a manner that it displaces the water ahead of the concrete without mixing the water with the concrete. Pumps and tremies with ends buried in the fresh concrete can be used to maintain a seal below the rising surface. Pumps are widely used to place concrete in bridge structures. Care should be exercised in delivering with the pump because free fall within the pump hose can result in loss of slump and air. 3.3.2 Protection of Reinforcing Steel Against Corrosion Methods for protecting reinforcing steel elements from corrosion include the use of corrosion-resistant reinforcing steel and the use of nonintrinsic protection of the re- inforcement (such as admixtures, cathodic protection systems, and electrochemical chloride extraction [ECE] techniques) as described in the following section. Chapter 5 covers the cathodic protection system and ECE in additional detail. 3.3.2.1 Corrosion-Resistant Reinforcement Corrosion-resistant reinforcement allows chlorides to penetrate around the reinforce- ment without causing significant damage to the reinforcing steel. These systems in- clude epoxy-coated reinforcement (ECR), galvanized reinforcement, titanium rein- forcement, stainless steel reinforcement, stainless steel–clad reinforcement, nickel-clad reinforcement, and copper-clad reinforcement and are described in Section 3.2.2.

164 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.3.2.2 Nonintrinsic Corrosion Protection of Reinforcement 3.3.2.2.1 Admixtures for Corrosion Protection Chemical admixtures that are added to concrete during batching to protect against corrosion of embedded steel reinforcement due to chlorides are available. There are two main types: corrosion inhibitors and physical barrier admixtures. Some corrosion inhibitors also act as physical barrier admixtures (ACI 222.3R, 2003a). Corrosion inhibitors, although known for many years, are only now beginning to be actively marketed as a preventative treatment in a concrete repair program ( Macdonald 2003). Calcium nitrite admixtures are the most researched inorganic inhibitor and the most widely used (Berke and Rosenberg 1989). Inhibitors do not create a physical barrier to chloride ion ingress. Rather, they modify the steel surface, either electrochemically (anodic, cathodic, or mixed inhibitor) or chemically (chemical barrier) to inhibit chloride-induced corrosion above the accepted chloride corrosion threshold level. They are added to the concrete at the time of batching (ACI 201.2R, 2008a). Calcium nitrite is also an accelerating admixture. If the accelerating effect is undesirable, a retarding admixture can be added. Admixtures with organic compounds protect steel from chloride-induced corro- sion. They include alkanolamines and an aqueous mixture of amines and fatty acid esters (Nmai et al. 1992; Nmai and Krauss 1994). They are claimed to both reduce ingress of chlorides (physical barrier) and enhance the passivating layer on the steel surface (corrosion inhibitor). Other similar amine products are claimed to migrate through concrete in the vapor phase to provide protection to embedded steel. Cor- rosion inhibitors are attractive from a conservation point of view as they are almost invisible on application, although their long-term visual effect is unknown ( Macdonald 2003). Physical barrier admixtures reduce the rate of ingress of corrosive agents (chlo- rides, oxygen, and water) into the concrete. These admixtures belong to one of two groups. One group comprises waterproofing and damp-proofing compounds. The second group consists of agents that create an organic film around the reinforcing steel, supplementing the passivating layer. They typically contain bitumen, silicates, and water-based organic admixtures consisting of fatty acids, such as oleic acid; stearic acid; salts of calcium oleate; and esters, such as butyloleate (ACI 222R, 2001b). A liquid admixture containing a silicate copolymer in the form of a complex, inorganic, alkaline earth may also be effective in reducing the permeability of concrete and pro- viding protection against corrosion of reinforcing steel (Miller 1995). 3.3.2.2.2 Cathodic Protection Cathodic protection can effectively stop corrosion in contaminated reinforced con- crete structures and can reduce the concentration of chloride ions at the steel surface of protected reinforcement (Kepler et al. 2000). Cathodic protection is applicable to areas that are contaminated chlorides but are still sound (Polder 1994).

165 Chapter 3. MATERiALS The wide use of cathodic protection has been hampered by the high cost and the maintenance of the power source or the protective material. Impressed current can be used on bridge decks and substructures; galvanic anodes are limited to substructures. 3.3.2.2.3 Electrochemical Chloride Extraction ECE is applied to concrete structures containing reinforcement in order to extract chlo- rides from the concrete (Clemeña and Jackson 2000) and can be used for decks and sub- structures. The steel acts as a cathode and is connected to the negative pole of the power source. The anode, which is either steel or a titanium mesh, is temporarily placed on the concrete cover and is connected to the positive pole of the power source. The electrolyte is placed on the concrete cover and allows the current flow. Due to the electric field, the negative ions, like chlorides, migrate from the rebar to the concrete cover. During ECE, hydroxyl ions are generated at the reinforcement, increasing the alka- linity and making the concrete susceptible to ASR. ASR can be mitigated by lithium salts (Velivasakis et al. 1997). 3.3.2.2.4 Other Protective Methods Other protective methods include the use of drainage design and stay-in-place metal forms. Posttensioning of members would also eliminate the formation of cracks. 3.4 FAuLt tree AnALySiS oF FActorS inFLuencing Service LiFe 3.4.1 Service Life of Concrete Concrete deterioration is caused by deficiencies in one or more factors, listed as ma- terials, design, and workmanship; the effect of external factors; and the occurrence of cracks. These factors are shown in the main fault tree (Figure 3.12) and are explained in additional detail in the fault trees for each factor shown in Figures 3.13 to 3.18. Figure 3.12. Main fault tree for reduced service life of concrete.   Figure 3.12. Main fault tree for reduced service life of concrete.

166 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.4.1.1 Materials Material-related factors are shown in Figure 3.13. They include the ingredients of the concrete: the cementitious material, aggregates, water, admixtures, and fibers. The w/cm is also included in this module. Information on materials is given in Section 3.2 under description of material types. The cementitious materials have different chemi- cal and physical properties that affect the hydration reaction, harmful chemical re- actions, and volumetric changes that relate to the durability of concrete. The type, quality, grading, and texture of the aggregates affect the water content and durability. Aggregates may cause D-cracking, ASR, and ACR; precautions are needed to inhibit such occurrences. Water, whether it is potable or recycled, may have some excessive impurities that may cause durability problems as a result of expansive chemical re- actions and increased w/cm. Small solid particles in water have large surface area; they increase the water demand and the w/cm if the cement content is kept the same. Admixtures help in achieving workable, low w/cm, and low-permeability concretes, leading to improved durability. Fibers are used to control cracking. Figure 3.13. Materials fault tree. Figure 3.13. Materials fault tree.

167 Chapter 3. MATERiALS 3.4.1.2 Design The design of the structure through the selection of the geometry, detailing, and flex- ibility affects the performance of the concrete. The design fault tree is shown in Fig- ure 3.14. In the geometry, the cover depth over the reinforcement has a large influence on salt intrusion to the level of the steel. Thicker decks provide more rigidity and less cracking potential. Long-span length causes more flexibility, increasing the possibility of cracking. Design details that minimize saturation with water would lead to improved dura- bility. Eliminating joints is desirable because joints cause leakage problems. 3.4.1.3 Workmanship Workmanship is important in achieving the desired durability; the materials have to be fabricated properly under adequate inspection to achieve the desired performance. Frequent maintenance to correct deficiencies is also needed to eliminate premature failure of the elements. Fabrication of components includes mixing, consolidation, finishing, and curing, as shown in the workmanship fault tree in Figure 3.15. ! Figure 3.14. Design fault tree. Figure 3.14. Design fault tree.

168 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The concrete mixer should be in good working order, the capacity given by the manufacturer should not be exceeded, and enough mixing time at the specified mixing rate should be maintained. Following the proper mixing guidelines will ensure a uni- form, consistent concrete mixture. Consolidation, which is achieved by proper internal or external vibration, eliminates the large air voids that adversely affect the strength and durability of concrete. Bridge decks require minimal finishing operations. The vibratory screed with proper speed, vibration, and setup of the auger and the roller can provide adequate vibration if the workability of the concrete is adequate. The sides and ends that the Figure 3.15. Workmanship fault tree.   ig r . . r anship fault tree.

169 Chapter 3. MATERiALS vibratory screed cannot reach are finished by hand. Extra hand finishing is detrimental and may delay the curing operation and cause loss of entrained air voids near the top surface. Curing is essential for the continuation of the hydration reactions and the control of cracking due to volumetric changes. The best curing process is a water cure that enables moisture retention and temperature management. Curing compounds are also used to maintain satisfactory moisture and temperature. The Virginia DOT uses curing compounds after a 7-day wet curing of decks. 3.4.1.4 External Factors External factors that affect the service life of concrete are loads and the environment. Loads are illustrated in Figure 3.16, and environment factors are shown in Figure 3.17. 3.4.1.4.1 Loads Loads can be traffic induced, age dependent, or system dependent. Traffic-induced loads are a result of vehicle loads, the frequency of traffic imparting fatigue stresses, and overloads due to unexpected high loads. The adverse effects of loads are wide and frequent cracks that facilitate the intrusion of harmful solutions. The duration of loading has to be considered, as loads can be instantaneous or time dependent. Time- dependent loads cause additional distress and result in additional cracks. System- dependent loads are a result of moisture and temperature variation and the available restraint. When restrained, deformations lead to stresses that can exceed the strength of the material, leading to cracking. When cement reacts with water, an exothermic reaction takes place: temperature rises, heat is given off, and concrete expands. As the reaction slows, cooling takes place and thermal contraction occurs, subjecting the restrained concrete to tensile stresses. When the stresses exceed the strength of the ma- terial, cracks occur. This thermal effect is more pronounced in mass concrete because the dissipation of heat is difficult in a large mass. High heat can also result in delayed ettringite reaction, which can cause cracks in hardened concrete. In hot weather, ther- mal effects can be detrimental; however, in a cold environment, the heat of hydration can provide the favorable temperature needed for the hydration reactions. 3.4.1.4.2 Environment The environment can provoke damage in structures caused by physical and/or chemi- cal factors. The physical factors are the result of freezing and thawing of critically satu- rated concrete, scaling of surfaces due to salt concentrations or excessive wear, seismic activity, settlement, volumetric changes due to moisture and temperature variation, or stresses due to wind velocity. Chemical factors involve corrosion; carbonation, which reduces pH and makes steel vulnerable to corrosion; sulfate attack; or expansion due to alkali-aggregate reactions or ASR.

170 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.4.1.5 Cracking Cracking of concrete can cause serious and costly damage to concrete structures. The cracking fault tree is shown in Figure 3.18. The factors affecting cracking can be due to design, materials, and/or construction. The design factors of interest include the restraint, span type, deck thickness, girder type, and the steel alignment and location. When concrete is restrained, deformations result in stresses. Flexible structures and low rigidity lead to additional cracks. For example, decks on steel beams exhibit addi- tional cracking compared with decks on rigid concrete beams. High modulus and low creep, which can be beneficial in reducing prestress losses and deflections in beams, can lead to additional cracking in decks. High modulus leads to brittle structures, and low creep does not enable relaxation that reduces stresses. It can be expected that low w/cm, high paste contents, and high heat of hydration can cause additional cracking. Concretes with low w/cm are brittle and more sensitive to curing. Concretes with high Age System-DependentTraffic-Induced Loads Figure 3.16. Load fault tree.

171 Chapter 3. MATERiALS water, cement, and paste content exhibit additional shrinkage and additional tempera- ture rise. During construction and afterward, the weather conditions, curing, time of setting, consolidation, and curing sequence and length affect the cracking pattern and severity. 3.4.2 Service Life of Reinforcement Reduced service life of reinforcement can be attributed to three causes: load-induced, man-made, or natural hazards; causes resulting from production defects in construc- tion processes and/or design details; or operational procedures. These deficiencies are illustrated in the main fault tree shown in Figure 3.19. Environment Physical Chemical Figure 3.17. Environment fault tree.

172 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 3.18. Cracking fault tree. Figure 3.19. Main fault tree for reduced service life of reinforcement. Load-Induced Natural or Man-Made Hazards Production/ Operation Defects Reduced Service Life of Reinforcement   Figure 3.18. Cracking fault tree.

173 Chapter 3. MATERiALS 3.4.2.1 Load-Induced Factors Load-induced bridge-deck deterioration can be attributed to fatigue, strength and brit- tleness, or thermal incompatibility. These load-induced factors are listed in the fault tree in Figure 3.20. 3.4.2.1.1 Fatigue Fatigue is caused by the repetition of applied loads that result in a degradation of the strength resistance of the reinforcement. Information on reinforcement material is summarized in Section 3.2.2. Corrosion-resistant reinforcement has fatigue properties similar to that of carbon steel reinforcement when tested in the atmosphere. How- ever, in a corrosive environment, corrosion-resistant reinforcement performs better than the carbon steel because carbon steel is expected to corrode, lose material area, and develop corrosion pits. The fatigue limit is related to the tensile strength of the steel; hence corrosion-resistant reinforcement with increased strength has an increased fatigue limit. 3.4.2.1.2 Strength and Brittleness Strength and brittleness affect cracking and failure. In general, corrosion-resistant rein- forcement has higher tensile strength and ductility than the carbon steels. Cold-formed austenitic reinforcement has a combination of high strength and good ductility; a yield stress level of 70 ksi or higher and elongation at maximum force higher than 15% is achieved (Bourgin et al. 2006). Duplex grades exhibit strengths exceeding 70 ksi in hot-rolled and 90 ksi in cold-rolled bars. At the higher strengths, ductility is at least as good as the carbon steels. Load- Induced Fatigue Strength and brittleness Thermal incompatibility Figure 3.20. Load-induced deficiency fault tree.

174 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.4.2.1.3 Thermal Compatibility Thermal compatibility affects cracking potential. Temperature changes in a material result in deformations that can cause significant stress when restrained by the sur- rounding material. However, in reinforced concrete the carbon steel and concrete have similar coefficients of thermal expansion; this similarity means that stresses caused by temperature changes in the structure are negligible. Carbon steels have a coefficient of thermal expansion of about 5.5 × 106/ºF; corrosion-resistant reinforcements with austenitic steels have a coefficient of thermal expansion of approximately 8.9 × 106/ºF; and austenitic-ferritic duplex steels have a coefficient of thermal expansion of about 7.2 × 106/ºF (Markeset et al. 2006). No problems stemming from these differences have been reported. 3.4.2.2 Natural or Man-Made Hazards Natural or man-made hazards include effects from areas with adverse thermal cli- mates, coastal climates, and chemical climates, and fire. These natural and man-made hazards are listed in the fault tree provided in Figure 3.21. 3.4.2.2.1 Thermal Climate The thermal climate affects corrosion activity. In cold climates, chloride-bearing deicing salts are commonly used to prevent ice buildup on roads. Chlorides destroy the protective iron oxide layer over the carbon steels, exposing the reinforcement to corrosion. Corrosion of prestressing steel is generally a greater concern than corrosion of nonprestressed reinforcement because of the possibility that corrosion may cause a local reduction in cross section and failure of the prestressing steel (ACI 222R, 2001b). The typical higher stresses in the prestressing steel also render it more vulnerable to stress-corrosion cracking and to corrosion fatigue. Because of the potentially greater vulnerability and the consequences of corrosion of prestressing steel, chloride limits for prestressed concrete are lower than those for reinforced concrete (ACI 222R, 2001b). Corrosion rate is dependent on the temperature, humidity, and chloride content. An increase in temperature in dry environments results in reduced corrosion activity and an increase in expected life (Lopez et al. 1993). However, in environments with high humidity, increasing temperatures result in increased corrosion activity, leading to reduced expected life. Low-temperature, cryogenic applications can cause brittle failure. Carbon steel reinforcement exhibits brittle behavior below 0°F when exposed to sudden loading and seismic actions. Austenitic stainless steels do not present such a transition and can be used in cryogenic applications; their toughness remains very high at temperatures as low as –320°F. Duplex stainless steel may not be used below –60°F (Markeset et al. 2006).

175 Chapter 3. MATERiALS 3.4.2.2.2 Coastal Climate Coastal climates introduce salt spray and high humidity, and salt and moisture accel- erate the corrosion rate. Salt spray in coastal climates provides high chloride buildup that can destroy the protective iron oxide coating over the steel reinforcement. Humidity affects corrosion activity. In the absence of chloride ions, little corrosion activity takes place when the relative humidity is under 60% (Jung et al. 2003). The corrosion activity increases as the relative humidity is increased up to a fully saturated state (>95% relative humidity) and then begins to decrease again. When concrete is fully saturated, the corrosion rate is reduced because the oxygen level in the concrete pores is too low (Qian et al. 2002). When chlorides are present at relative humidity below 60%, corrosion activity may still develop. Figure 3.21. Natural or man-made hazard fault tree. Figure 3.21. Natural or man-made hazard fault tree.

176 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 3.4.2.2.3 Chemical Climate Chemical climates influence the performance of reinforcement. The main effect can be attributed to corrosion-inducing chemicals that occur naturally and can be man-made. Chlorides are the main chemical substance that adversely affects the corrosion process. 3.4.2.2.4 Fire Fire generates heat that affects mechanical properties. Cold-worked steel subjected to temperatures of less than 850°F typically recovers all of its yield strength after cooling (Suprenant 1996). Hot-rolled steel can be exposed to temperatures as high as 1,100°F and recover its yield strength. Higher temperatures may cause rapid strength loss in reinforcing steel and lead to excessive deflections in reinforced members. The effect of fire is more critical on prestressing steel; at temperatures of 750°F, the strength of prestressing steel can be reduced by more than 50% (Suprenant 1996). Austenitic stainless steels maintain their strengths at considerably higher tempera- tures than carbon steel and thus are more resistant and robust under fire loading than carbon steel (Markeset et al. 2006). Heating also adversely affects the bond between concrete and reinforcement (Suprenant 1996). At 570°F the bond strength is no greater than 85% and at 930°F no greater than 50% of the strength at ambient temperatures. 3.4.2.3 Production and Operational Defects Production and operational defects are shown in the fault tree in Figure 3.22 They include design and detailing, construction, inspection, and maintenance issues. 3.4.2.3.1 Design and Detailing Design and detailing factors are listed in the fault tree shown in Figure 3.23. They include the design philosophy, mix design, and drainage. The design philosophy of providing proper concrete cover, eliminating joints, min- imizing cracking through geometry (e.g., large skews exhibit additional cracking), and the selection of corrosion-resistant reinforcement affect the corrosion potential. The redundancy and ductility design aspects in structures should be improved to confine the damage to a small area in the event a major supporting element is damaged or an abnormal loading event has occurred (ACI 318-11). The following considerations and relationships should be adopted in the concrete mixture design: • Low-permeability concretes hinder the penetration of aggressive solutions to the level of reinforcement; • High alkalinity of the concrete passivates the steel through its protective cover; • Cracking resistance of concretes can be improved by optimizing the strength (high- strength concrete exhibits brittle behavior), minimizing paste, and adding latex modifiers;

177 Chapter 3. MATERiALS Design/Detailing Construction Inspection Production/ Operation Defects Figure 3.22. Production and operation defects fault tree. • The use of corrosion-inhibiting admixtures increases the passivation state of the reinforcement, extends the time to corrosion, and reduces the corrosion rate of embedded metal; • Increased creep and reduced elastic modulus and reduced shrinkage are helpful in reducing the cracking; and • Cracks facilitate the intrusion of aggressive solutions into concrete; chlorides initi- ate and accelerate the corrosion process. 3.4.2.3.2 Construction Construction-related parameters affect the performance of structures. It is critical that the correct amount of reinforcement is placed in the right location within the specified tolerances. Reinforcement during concrete placement should be free from mud, oil,

178 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE or other nonmetallic coatings that decrease bond (ACI 318-11). A normal amount of rust or mill scale that is not loose on the reinforcement is not detrimental to the bond between the concrete and the bars. In the field, bending to proper bend diameters is needed to ensure that there is no breakage and no crushing of the concrete inside the bend (ACI 318-11). To minimize the corrosion potential, dissimilar metals should not be in contact. Further, reinforcement should be protected from the weather to minimize contamina- tion and corrosion. If there is a coating over the reinforcement, special care is needed to avoid damage to the coating during handling and placement. Design Philosophy Mix Design Design/Detailing Drainage Figure 3.23. Design and detailing defects fault tree.

179 Chapter 3. MATERiALS 3.4.2.3.3 Inspection Visual inspection can indicate the condition of the reinforcement and determine if there are any gross mistakes in the reinforcement selection and placement. The avail- ability of a large number of reinforcement types makes it difficult to identify the rein- forcement visually; nondestructive evaluation is beneficial in this respect. Rebar corrosion in existing structures can be assessed by different methods, such as open circuit potential and surface potential, concrete resistivity, linear polarization resistance, Tafel extrapolation, galvanostatic pulse transient method, electrochemical impedance spectroscopy, harmonic analysis, noise analysis, embeddable corrosion- monitoring sensor, cover thickness, ultrasonic pulse velocity technique, X-ray, gamma radiography, infrared thermograph, electrochemical method, and visual inspection (Song and Saraswathy 2007). 3.4.3 Service Life of Structural Steel The primary factors affecting service life of structural steel are fatigue and fracture and corrosion. These factors are covered in Chapter 7, Fatigue and Fracture of Steel Structures; and Chapter 6, Corrosion Prevention of Steel Bridges. 3.5 individuAL StrAtegieS to mitigAte FActorS AFFecting Service LiFe 3.5.1 Concrete The durability of concrete depends largely on its permeability. For longevity, concrete must be designed, proportioned, and constructed properly. In addition to the concrete material issues, the structural design must also be performed properly to avoid high stresses and load-related cracking. Section 3.3.1 provides additional information on distresses due to physical (volumetric changes, freezing and thawing), chemical ( alkali– aggregate reaction, ASR, ACR, carbonation, chlorides, sulfates, acids, and salts), and functional (impact, concrete consolidation, curing, and placement) factors. Table 3.9 is a technology table that informs the designer of the most common types of service life issues and distresses related to concrete materials. For each service life issue, solutions are identified, along with their advantages and disadvantages. The technology table for concrete durability also includes concrete-related issues involved in resisting the corrosion of reinforcement, such as low permeability, w/cm, aggregates, chemical admixtures, shrinkage, modulus of elasticity, cover, overlays, and corrosion inhibitors. 3.5.2 Steel Reinforcement Corrosion of reinforcement is a major problem requiring costly repairs. Methods for protecting reinforcing steel elements from corrosion include as the use of corrosion- resistant reinforcing steel, admixtures, cathodic protection systems, and ECE techniques.

180 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 3.9. technoLogy tAbLe For concrete durAbiLity Service Life Issue Solution Advantage Disadvantage Freeze and thaw Good air void system High resistance to freezing and thawing Reduction in strength due to extra air Sound aggregates Durable aggregates Availability Strength of 4,000 psi and up Used to overcome stresses Increased strength makes concrete more brittle Drainage design Minimizes saturation Ingress of water Low w/cm Reduces infiltration of water Can produce high-strength concrete that is brittle Abrasion and wear Hard aggregates Attain high concrete strengths and increased resistance to abrasion and wear Hard to obtain in some areas High-strength concrete Reduces wearing Concrete more brittle Add cover Provides new surface Extra weight Chemical reactions (ASR) Nonreactive siliceous aggregates Reduces ASR Hard to obtain in many areas SCMs Reduces permeability, reduces ASR, limits alkalis from outside Quality fly ash or slag missing in many areas Low w/cm Reduces infiltration of solutions, limits alkalis from outside Can produce high-strength concrete that is brittle Chemical admixtures Improved properties Cost, incompatibility, side effects Lithium-based admixtures Inhibits ASR Cost Limestone sweetening (blending with limestone) Limits expansion Reduced skid resistance Chemical reactions (ACR) Nonreactive carbonate aggregates Reduces ACR Hard to obtain in some areas Reduce infiltration of solutions, limit alkalis from outside Can produce high-strength concrete that is brittle Cracking Blend aggregate Limits expansion Hard to obtain in some areas Limit aggregate size to smallest practical Limits expansion Rich mixes with high paste content Sulfate attack Low C3A contents Reduces sulfate attack na SCMs Reduces permeability, reduces sulfate attack, limits sulfates from outside Quality fly ash or slag missing in many areas Low w/cm Reduces infiltration of solutions, limits sulfates from outside Can produce high-strength concrete that is brittle (continued)

181 Chapter 3. MATERiALS Service Life Issue Solution Advantage Disadvantage Corrosion of reinforcement Low permeability Reduces infiltration of aggressive solutions Can produce high-strength concrete that is brittle Membranes and coatings Reduces infiltration of aggressive solutions Difficult to apply in the field, wear of traffic Sealers for pore lining and blocking Reduces infiltration of aggressive solutions Difficult to apply in the field, concrete may be difficult to penetrate Low w/cm High strength, low permeability Excessive cracking, shrinkage Low shrinkage Minimizes cracking Low water content may adversely affect workability Low modulus of elasticity High deformation, minimizes deck cracking Reduces stiffness SCMs Reduces permeability Quality fly ash or slag missing in many areas Large maximum aggregate size Less surface area, less water, cement, and paste Less bond Well-graded aggregates Less paste Problem when good shape is missing Chemical admixtures Reduced permeability Cost, incompatibility, side effects Cover More resistance to penetration of solutions Wider cracks, extra weight and cost Overlays Creates a low-permeability protective layer over the conventional concrete Difficult to place, expensive, and prone to cracking; proper curing is critical. Corrosion inhibitors Stable protective layer on the steel Cost Note: SCM = supplementary cementitious material; na = not applicable. tABLE 3.9. technoLogy tAbLe For concrete durAbiLity (continued)

182 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Table 3.10 is a technology table that summarizes the solutions to reinforcement corrosion. This table includes other protective methods mentioned previously in this chapter, such as epoxy-coated, Z-bar, low-carbon chromium steel, and stainless steel. tABLE 3.10. technoLogy tAbLe For corroSion oF reinForcement Service Life Issue Solution Advantage Disadvantage Corrosion of reinforcement Electrochemical chloride extraction Extracts chlorides from the concrete, or used in new structures to increase corrosion threshold Extraction depends on the depth and location, risk of embrittlement (prestressed). Difficult to predict service life. Cathodic protection Prevents corrosion from initiating, advantage as a repair method High cost involved in maintaining the power source and sacrificial mesh anode. Embrittlement of strand and softening of concrete (prestressed structures). Sealers Prevents solutions from penetrating the concrete, easy to apply either during or after construction Difficult to ensure adequate coverage. Varying performance and cost. Short service life. Abrasion, sunlight, and environment affect the sealer’s efficiency. Membrane Prevents moisture infiltration Varying performance. Difficult to install on curved or rough decks and to maintain quality and thickness during field installation. Stay-in-place metal form for marines structures Prevents infiltration of aggressive solutions Cost. Stainless steel High resistance to corrosion Initial cost. Fiber-reinforced polymer High resistance to corrosion Fiber-reinforced polymer prone to degradation from environmental factors. Z-bars (galvanizing over epoxy coating) High resistance to corrosion Cost. Epoxy-coated steel Creates protective layer over the steel and increases the electrical resistance Epoxy coating can be damaged during handling, shipping; and storage, and corrosion can initiate under the coating. Low-carbon chromium steel High resistance to corrosion High strength, no yield point. Drainage design Minimize saturation Continuous maintenance. Posttension Puts the concrete in compression, minimizing cracks that facilitate the penetration of chlorides Posttensioning ducts and grout are concerns in resisting corrosion.

183 Chapter 3. MATERiALS 3.5.3 Structural Steel Individual strategies to mitigate factors affecting service life of structural steel are dis- cussed in Chapter 6. 3.6 overALL StrAtegieS For enhAnced mAteriAL Service LiFe The introduction to this chapter describes a process for developing a strategy selection to enhance material service life. This process is summarized in Figure 3.1. Providing materials with enhanced service life requires a complete understanding of the potential deterioration mechanisms. These mechanisms, described in Section 3.3, are associated with load-induced conditions, local environmental hazards, pro- duction-created deficiencies, and lack of effective operational procedures. Mitigation of these deterioration mechanisms through the selection of enhancement techniques, described in Section 3.5, requires a thought process that combines the individual strat- egies to define a single family of symbiotic strategies. This process will produce the best approach to providing materials with enhanced service life. This chapter provides guidelines for selecting the most appropriate individual strategy to achieve the desired service life. Although the individual strategies provide solutions to many of the material durability issues, the majority of the strategies must be developed in conjunction with the material’s application, such as bridge decks. Sub- sequent chapters will refer to this chapter as needed. 3.6.1 Design methodology With limited funds available for bridge construction, cost is often an overriding factor in critical material selection decisions. However, to take advantage of the long-term advantages of durable materials, service life enhancement strategies must be applied to a cascading series of economic, design, construction, and maintenance measures. Suc- cess of the strategy selection process is dependent on the ability to predict service life and the incorporation of best practices to enhance service life. 3.6.2 material Selection and Protection Strategies The selection of the type of concrete for a particular application depends on many factors, including the design of the structure (span length and slenderness of columns), availability of the type of concrete, subsurface conditions, and the environmental con- ditions (temperature and chemical exposures). Examples of factors to consider in the selection of concrete include the following: • If poor soil conditions exist and longer spans are planned, or if the substructure is to be kept but additional or wider lanes and shoulders are planned, lightweight concrete would be the material of choice. • If there is severe exposure to salts or marine spray, high-performance concretes with low permeability would be appropriate.

184 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE • In areas with congested reinforcement or intricate formwork, high-performance con- crete with high workability, such as self-consolidating concrete, would be preferred. • In bridge decks, self-consolidating concrete can lead to difficulty in maintaining the grade or the cross slope due to high flow rates; normal-weight concrete may be preferable unless durability or weight is of concern. • Ultrahigh-performance concrete can be used when very small cross sections or height restrictions exist or if high bond strengths and low permeabilities are needed, as in connections. Care should be exercised to select or specify only the necessary criteria for the subject application. Additional criteria can cause undesirable distresses that adversely affect performance and also increase the cost of construction. For example, for a bridge deck, if a low w/cm (less than 0.40) is specified to achieve lower permeability, high strengths will be obtained that would make the bridge-deck concrete more prone to cracking. High strengths are accompanied by high stiffness (elastic modulus) and low creep, which are instrumental in increased cracking potential. Cracks will facili- tate the intrusion of chlorides, negating the benefits obtained by low w/cm. A better approach would be to use moderate w/cm (0.40 to 0.45) with pozzolanic material to reduce permeability. In addition, to achieve a low w/cm, high cement factors are used that would increase the cementitious material and paste contents, thus making the concrete more vulnerable to shrinkage and thermal problems. Table 3.11 summarizes durability strategies for concrete materials. The strategies for each potential deterioration mode must be compared for conflicts in order to estab- lish the overall strategy to be deployed. For example, a designer faced with a bridge deck having the potential for deterioration from wear and abrasion and differential shrinkage should not specify concrete with both high strength and a low modulus. In this case, using an overlay or membrane would be more appropriate. Because the selection of appropriate material and protection strategies is highly dependent on the application of the materials, material selection and protection strate- gies considering the overall structure (not only materials) are provided in subsequent chapters. 3.6.3 Construction Practice Specifications Once the materials are selected, a proper set of specifications must be developed to en- sure that the highest standard of care is used during construction. These specifications and procedures are fairly well established and documented by FHWA and various state agencies. 3.6.4 maintenance Plan An effective maintenance plan should be developed to ensure the maintenance assump- tions regarding upkeep made in the material selection process are properly identified for staff and budget requirements. If the bridge owner cannot commit to such a pro- gram, then strategies for low-maintenance life-cycle costs should be recommended.

185 Chapter 3. MATERiALS tABLE 3.11. concrete durAbiLity StrAtegieS Potential Deterioration Mode Material Selection and Protective Measures Selection Maintenance Mode Life-Cycle Costs Initial Long Term Freeze and thaw Minimum 6% air entrainment Sound aggregates Strength >3.5 ksi None Low Low Proper drainage and cover None Low Low Membrane or overlay Continual overlay Replacement every 20 years Medium Medium ASR Nonreactive aggregates None Medium Low Low-alkali portland cement None Medium Low Blended aggregates Low-alkali portland cement SCMs (e.g., fly ash, slag) None Medium Low Blended aggregates Low-alkali portland cement Lithium nitrate None Medium Low Proper drainage None Low Low Membrane or overlay Continual overlay Replacement every 20 years Medium Medium ACR Nonreactive aggregates None Medium Low Blended aggregates None Medium Low Proper drainage None Low Low Sulfate attack Cement with low C3A content, early curing temperature <160°F None Low Low Pozzolans, low w/cm, proper drainage None Low Low Delayed ettringite formation Cement with low C3A content, early curing temperature <160°F None Low Low Pozzolans, low w/cm, proper drainage None Low Low

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TRB’s second Strategic Highway Research Program (SHRP 2) Report S2-R19A-RW-2: Design Guide for Bridges for Service Life provides information and defines procedures to systematically design new and existing bridges for service life and durability.

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