Concrete Bridge Deck Performance (2004)

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.

5 CHAPTER ONE INTRODUCTION BACKGROUND Concrete bridge deck deterioration, in the form of concrete distress and reinforcement corrosion, is one of the leading causes of structural deficiency listed in the National Bridge Inventory. Transportation agencies are investing significant resources to solve the problem. These agencies often spec- ify material properties, mix designs, and construction methods in their efforts to address concrete bridge deck distress. To reduce corrosion, alternative reinforcement, appropriate slab design practices, protective barrier meth- ods, electrochemical methods, and corrosion inhibitors may be used. The success and performance of these efforts has not yet been compiled in a document widely available to transportation agencies. In a 1955 survey to ascertain the principal problems faced by bridge maintenance engineers, concrete deteriora- tion was rated fourth, although the specific deteriorating components were not described (McGovern 1955). In a 1967 survey, however, concrete bridge decks were rated first in the type of structure requiring the greatest structural maintenance (NCHRP Synthesis of Highway Practice 4 1970). Deicing salts were not commonly used until the 1950s. Their use increased as more and more states instituted a “bare pavements“ policy in response to public demand. Salt can have a pronounced deleterious effect on concrete. First, the potential for freeze-thawing damage leading to surface scaling is greater when deicing salts are used. Sec- ond, the presence of chlorides at the level of the reinforce- ment intensifies corrosion of the reinforcement leading to spalling. Although scaling and spalling can occur without the presence of deicing salts, their presence accelerates the process (Guide to Durable Concrete 1992). The first NCHRP synthesis report on bridge deck dura- bility, NCHRP Synthesis of Highway Practice 4: Concrete Bridge Deck Durability, was published in 1970. It reported that bridge deck deterioration was a major maintenance item, with the most commonly reported conditions being cracking, scaling, and spalling. Cracking was not consid- ered to be serious and scaling could be virtually eliminated by the use of high-quality, air-entrained concrete assisted, when necessary, by periodic applications of linseed oil. Spalling was considered to be the most serious defect with its cause attributed to corrosion of the reinforcing steel. Cracks provided ready access for salt and moisture to reach the steel, although porous concrete without cracks was also considered as a means for chlorides and moisture to reach the reinforcement. In 1972, the FHWA introduced a policy that required application of a deck protective system to all structures on the federal-aid system likely to be subjected to potentially damaging applications of deicing salts (Manning 1995). The market for waterproofing systems expanded as new products were introduced and put to use. In addition, increased cover over reinforcing steel, increased efforts at crack control, and the use of less porous concrete were implemented. A second NCHRP synthesis dealing with durability of concrete bridge decks, NCHRP Synthesis of Highway Practice 57: Durability of Concrete Bridge Decks, was published in 1979. This synthesis reported that concrete bridge deck durability continued to be a problem because of corrosion of the steel reinforcement. It reported that de- sign practices that improve durability included lesser skews, better drainage, thicker slabs, and greater cover to the reinforcement. Beneficial construction practices in- cluded achievement of the specified cover, use of concrete with the lowest possible water–cement ratio, and good con- solidation. The most effective coating to reduce the suscep- tibility of steel reinforcement to corrosion was identified as fusion-bonded epoxy powder. The 1979 synthesis also reported that sealants, impreg- nants, overlays, membranes, or cathodic protection had been used to improve durability. Sealants were reported to not be effective in preventing corrosion; polymer impreg- nators showed promise; overlays were low-slump concrete, latex-modified concrete, or internally sealed concrete; membranes were available in a variety of systems, however, field experience had been highly variable leading to doubt about their long-term performance; and cathodic protection was described as the only practical method to stop active corrosion. This synthesis also reported that for many years the pre- vailing attitude was that if the requirements for specified concrete strength were satisfied, the deck would perform adequately. The most important factors for the durability of concrete were identified as selection of good quality mate- rials and provision of a low water–cement ratio and air- entrained concrete.

6 NCHRP Report 297: Evaluation of Bridge Deck Protec- tive Strategies (Babaie and Hawkins 1987) reported the re- sults of an investigation of the following five strategies for preventing corrosion in new bridge decks: 1. Concrete cover, 75 mm (3 in.) or more; 2. Low-slump concrete overlay; 3. Latex-modified concrete overlay; 4. Waterproof membrane and asphalt overlay; and 5. Epoxy-coated reinforcing steel. The performance of these strategies was examined through a literature review, survey of transportation de- partments, and visual inspection of selected bridge decks. Concrete protective systems using increased concrete cover, low-slump concrete overlays, and latex-modified concrete overlays were found to be resistant, but not im- permeable, to salt penetration. Waterproof membranes with asphalt overlays were found to be effective in preventing salt intrusion into the underlying deck. Nevertheless, after 15 years of service, membranes had deteriorated as the result of aging and traffic. Epoxy coating of reinforcing steel prevented cor- rosion; however, breaks in the coating provided potential sites of accelerated corrosion. The long-term durability of epoxy coating in chloride-contaminated concrete was stated to be unknown, but concern was expressed about the presence of pinholes and the coating’s adhesion to the rein- forcement. A November 2002 multistate survey for the Michigan Department of Transportation (DOT) showed that 21 or 68% of the 31 responding states believed that the concrete deck service life would meet their expectations (Aktan and Fu 2003). When asked how long they believed their rein- forced concrete deck would last under average traffic, the overwhelming response was 30 to 40 years. Thirty states responded that they have taken action to improve the durability of reinforced concrete bridge decks. At least 23 or 74% of the responding states indicated that they have in- creased concrete cover, changed the mix design, or changed curing procedures. SCOPE This synthesis provides information on previous and cur- rent design and construction practices that have been used with the goal of improving the performance of concrete bridge decks. The primary focus is North American prac- tices for cast-in-place (CIP), reinforced concrete bridge decks on steel beams, concrete I- and T-beams, or concrete box beams. Full-depth CIP slabs and partial-depth CIP slabs on precast panels are included. Post-tensioned con- crete bridge decks are not included in this report. The in- formation was obtained from a literature review and from the 45 responses to a survey questionnaire sent to 64 highway agencies in the United States and Canada. The objective of the questionnaire was to obtain infor- mation on the following topics: • Factors that contribute to the durability of concrete bridge decks; • Performance of various types of deck protection strategies; • Lessons learned and the current state of the practice in design, construction, and maintenance of concrete bridge decks; • Available comparative analyses of the effects of using different methods and materials; • Specific reports of successes and failures; • Sample design and construction specifications; • Available life-cycle cost information; • Research in progress; and • Suggestions for future study. The remaining text of this synthesis is organized as fol- lows: • Chapter two reports on the effects of concrete constituent materials and concrete mix proportions on the durability of concrete and its effectiveness in protecting steel reinforcement from corrosion. • Chapter three summarizes different reinforcement systems that have been used as alternatives to non- coated steel reinforcement. These systems either pro- vide a barrier for the corrosive agent or use a noncor- rosive material. • Chapter four deals with barrier systems that are de- signed to protect the primary concrete and reinforce- ment from conditions that will cause their deterioration. The barrier systems include overlays, membranes, seal- ers, and cathodic protection systems. • Chapter five provides information about design and construction practices that are related to bridge deck performance, as well as limited information about costs. • Chapter six presents a discussion about cracking in concrete bridge decks. • Chapter seven contains the conclusions from this synthesis and suggestions for future study. Appendices provide the questionnaire survey (Appendix A), a list of responding agencies (Appendix B), a summary of the results (Appendix C), and a summary of research in progress (Appendix D). Full details of the responses of each agency are available on-line at http://www4.trb.org/ trb/onlinepubs.nsf, under National Cooperative Highway Research Program (NCHRP), NCHRP Synthesis Reports, Synthesis 333.

7 TYPES OF DETERIORATION The types of deterioration that generally appear in concrete bridge decks are scaling, mortar flaking, spalling, abrasion damage, alkali-aggregate reactivity, and cracking Scaling is a general loss of surface mortar usually associ- ated with freeze-thaw damage and aggravated by the presence of deicer chemicals. Scaling is primarily a physical action caused by pressure from water freezing within the concrete (Concrete Slab Surface Defects . . . 2001). It may occur in small areas or be widespread, as shown in Figure 1. FIGURE 2 Spalling caused by corroded reinforcement. FIGURE 1 Surface scaling caused by freeze-thaw cycles. Mortar flaking is similar to scaling, but occurs over coarse aggregate particles. Early drying out of the surface mortar over the aggregate results in insufficient moisture for cement hydration, leading to a mortar layer of lower strength. Upon freezing in a saturated condition, the thin layer of mortar breaks away. Whereas scaling occurs over a general area, mortar flaking only occurs above coarse ag- gregate particles. FIGURE 3 Abrasion damage caused by chain wear. Alkali-aggregate reactivity is a chemical reaction in concrete between alkalies from portland cement or other sources and certain constituents of some aggregates. Under certain conditions, the reaction may cause abnormal expan- sion and cracking of concrete in service (Cement and Con- crete Terminology 2000). The causes and remedies have been extensively researched and are not included in this synthe- sis (Stark et al. 1993; State-of-the-Art . . . 1998). Spalling is a larger surface defect than scaling or mortar flaking and is generally caused by internal pressure or ex- pansion within the concrete. The two common causes of spalling are corrosion of the reinforcement and improperly constructed or maintained joints (Guide for Concrete High- way . . . 1997; Concrete Slab Surface Defects . . . 2001). When spalling is caused by corrosion of the reinforcement, the depth of the spall extends to the level of the reinforce- ment, as shown in Figure 2. If not treated when it first ap- pears, spalling can lead to large-scale delaminations. Cracking is a characteristic of concrete because of its low tensile strength. The significance of cracks and their effect on the durability of a concrete deck are dependent on their cause, width, depth of penetration, and the concrete age when they occur. The effects of cracks on bridge deck performance are discussed in more detail in chapter six. Abrasion damage in wheel tracks can be caused by studded tires and chain wear as shown in Figure 3. Such damage can also be caused by the blades of snow ploughs, particularly on the corners of grooved surfaces. In addition, abrasion damage manifests itself as polishing of the aggre- gates, which can lead to a slippery surface. DESIRED DECK PERFORMANCE A high-quality concrete bridge deck has at a minimum the following characteristics: • Low chloride permeability,

8 • A top surface that does not deteriorate from freeze- thaw or abrasion damage, • Cracking that is limited to fine flexural cracks asso- ciated with the structural behavior, and • Smooth rideability with adequate skid resistance. All of these characteristics in a bridge deck should lead to a long service life with minimum maintenance.