Concrete Bridge Deck Performance (2004)

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32 CHAPTER SIX CRACKING IN CONCRETE BRIDGE DECKS Cracks in bridge decks are generally characterized by their orientation with respect to the longitudinal axis of the bridge. The major types, as illustrated in Figure 8, are transverse, longitudinal, diagonal, map, and random (Schmitt and Darwin 1995). In fresh concrete, cracks may be caused by rapid loss of moisture or by settlement around reinforcing bars (Babaie and Fouladgar 1997; Causes, Evaluation, and Repair . . . 1998). In hardened concrete, cracks form whenever the tensile stress in the concrete ex- ceeds the tensile strength of the concrete. Tensile stresses are caused by applied loads such as vehicles or restraint to the length changes caused by shrinkage or temperature changes. Tensile strength of concrete is dependent on the concrete constituent materials and curing environment, and generally increases with concrete age. In a typical slab-on-beam bridge, the deck slab spans between the longitudinal girders and the primary deck rein- forcement runs in the transverse direction. Small size bars as distribution reinforcement run in the longitudinal direc- tion below the top transverse bars and above the bottom transverse bars (Guide for Concrete Highway . . . 1997). In continuous structures, larger bars run longitudinally over the piers. Reinforcement that runs parallel to the direction of concrete tensile stress functions as the tensile rein- forcement and controls crack widths after the crack forms. Reinforcement that runs perpendicular to the direction of the concrete tensile stress acts as a stress raiser and crack former by reducing the concrete cross section. A larger di- ameter bar reduces the cross section more than a smaller bar. Transverse Longitudinal Diagonal Map FIGURE 8 Examples of crack patterns.

33 CAUSES OF CRACKING IN CONCRETE BRIDGE DECKS In 1961, the Portland Cement Association began a study of concrete bridge deck durability (Durability of Concrete Bridge Decks 1970). The study included a survey of 1,000 bridges selected at random in 8 states, plus a detailed sur- vey of 70 bridges in 4 states. The study concluded that transverse cracking was the predominant type of cracking. The cracks were typically located above transverse rein- forcement. Based on the study, the use of the largest practi- cal maximum size of coarse aggregate was recommended to minimize the water content. In addition, it was recom- mended to use the lowest reasonable slump and to keep the maximum slump within a range of 50 to 75 mm (2 to 3 in.). Several other studies have identified that longitudinal and transverse cracks tend to form directly above rein- forcement in the top layer of bars because the presence of the reinforcement acts as a stress raiser (Cheng and Johns- ton 1985; Perfetti et al. 1985; Kochanski et al. 1990). This effect can be reduced by using smaller diameter bars in combination with a thicker deck (Kochanski et al. 1990). It has been reported that the incidence of cracking in- creases with span length (Larson et al. 1968; Axon et al. 1969; Durability of Concrete Bridge Decks 1970), angle of skew (Larson et al. 1968), and the use of continuous struc- tures (Axon et al. 1969; Durability of Concrete Bridge Decks 1970). A California study (Poppe 1981) showed that air content had no effect on cracking; however, a study by North Caro- lina State University found that low slump and air content increased cracking (Cheng and Johnson 1985). A study of premature cracking in concrete bridge decks for the Wis- consin DOT resulted in several recommendations to reduce cracking of bridge decks (Kochanski et al. 1990). These in- cluded limiting the water–cement ratio to 0.40 and using coarse aggregate with a maximum size greater than 19 mm (0.75 in.). In Kansas, 40 bridge decks were investigated to identify factors that contribute to cracking (Schmitt and Darwin 1995, 1999; Miller and Darwin 2000). The investigations showed that cracking increased with increasing values of slump, percent volume of water and cement, water content, cement content, and compressive strength. Based on these trends, they concluded that concrete shrinkage or restraint of concrete shrinkage was a major contributor to bridge deck cracking. Decreases in cracking were noted with in- creases in air content. No conclusions were made about the effect of water–cement ratio because the values only varied from 0.42 to 0.44 with one exception. Schmitt and Darwin (1995) reported that transverse crack density, in terms of crack length per unit area, for bridge decks using 19-mm diameter (No. 6) bars was higher than for bridge decks using 16-mm diameter (No. 5) bars or a combination of 13- and 16-mm diameter (No. 4 and No. 5) bars as transverse reinforcement. Miller and Darwin (2000) also reported that, in general, a larger trans- verse bar size and spacing tends to increase levels of crack- ing. Schmitt and Darwin (1995) also reported that transverse crack density in decks with bonded overlays was considera- bly less when transverse bar spacing was less than or equal to 150 mm (6 in.). However, the authors also pointed out that smaller spacing is associated with the use of smaller bar sizes. In addition, the authors found that steel girder bridges with integral abutments had more cracking near the abut- ments than bridges with girders on bearings. The magni- tude of the cracking increased as the length of bridge deck along the abutment increased above 14 m (45 ft). Smaller size and closer spacing for the transverse bars resulted in less cracking in two-layer bridge decks. Krauss and Rogalla (1996) examined the effects of con- crete materials, design practices, and construction practices on transverse deck cracking. They concluded that concrete material factors important in reducing early cracking in- cluded low shrinkage, low modulus of elasticity, high creep, low heat of hydration, and selection of aggregates and concrete that provided a low cracking tendency. Other material factors helpful in reducing cracking included re- ducing the cement content, increasing the water–cement ra- tio, using shrinkage-compensating cement, and avoiding materials that produced very high early compressive strengths and modulus of elasticity values. The type of cement also had a large effect on deck cracking. Decks constructed with Type II cement cracked less than those constructed with Type I cement. Type III cement gains strength more rapidly than other cement types and may increase the risk of cracking. Krauss and Rogalla also pointed out that the general chemistry and fineness of cements have changed over time. The end re- sult is that today’s cements and, therefore, the concretes made with the cements, gain strength more rapidly than previous cements. As a result, modern concretes with a high early compressive strength and modulus of elasticity have an increased risk of cracking because of the higher stresses that develop as a result of early shrinkage and thermal strains. Krauss and Rogalla (1996) identified that the major de- sign factors affecting transverse cracking in bridge decks were related to restraint, specifically bridge type, girder type, and girder size. Multispan continuous composite large steel girder bridges were most susceptible to bridge deck cracking. CIP, post-tensioned bridges were the least likely to have transverse deck cracking because the girders and the deck shrink together and post-tensioning intro- duces compressive stresses in the deck. Other design fac- tors that moderately contributed to early cracking were

34 continuous spans, alignment of top and bottom transverse bars, and the use of stay-in-place forms (Krauss and Ro- galla 1996). Silica fume concrete is very susceptible to plastic shrinkage cracking owing to its lack of bleeding. There- fore, immediate application of fog sprays or misting after placement is essential to avoid formation of plastic shrink- age cracks in silica fume concrete (Ozyildirim 1991). According to laboratory tests by Whiting and Detwiler (1998), the cracking tendency of concrete was influenced by the addition of silica fume only when the concrete was improperly cured. When concrete containing silica fume was cured for 7 days under continuously moist conditions, there was no statistically significant effect of silica fume on the tendency of the concrete to exhibit early age crack- ing. They recommended that specifications for silica fume concretes in bridge deck construction include a provision for 7-day continuous moist curing of exposed surfaces. A survey of 72 bridges for transverse deck cracking in the Minneapolis/St. Paul metropolitan area was reported by French et al. (1999). The survey included 34 simply sup- ported prestressed concrete girder bridges; 34 continuous steel girder bridges; and 4 continuous rolled steel, wide- flange girder bridges. The dominant material-related pa- rameters associated with transverse deck cracking included cement content, aggregate type and quality, air content, rate of shrinkage, and deck concrete modulus of elasticity (French et al. 1999). Overall, the decks of bridges with simply-supported prestressed concrete girders were ob- served to be in better condition than decks on continuous steel girder bridges. This was attributed to reduced end re- straint and the beneficial creep and shrinkage characteris- tics of the prestressed concrete girders. The few prestressed concrete girder bridge decks that consistently performed poorly were either bridges with reconstructed or reover- layed decks or bridges that had decks placed during ex- treme temperature conditions. Cracking as a result of deck reconstruction was attributed to shrinkage of the deck be- ing restrained by the aged prestressed concrete girders. For steel girder bridges, end restraint and shrinkage were the most significant factors contributing to deck cracking. The steel girder bridges exhibited more cracking on interior spans than end spans, more cracking in curved bridges compared with straight bridges, more cracking with 19-mm diameter (No. 6) bars than 16-mm diameter (No. 5) bars as transverse reinforcement, and more crack- ing with increased restraint owing to steel configuration, girder depth, or close girder spacing. Hadidi and Saadeghraziri (2003) summarized material and mix design factors that contribute to transverse deck cracking. Based on a comprehensive literature search, they made the following recommendations as positive steps to reduce the potential for deck cracking: • • • • • Reduce cement content to 385 to 390 kg/m3 (650 to 660 lb/yd3), Consider using low early strength concrete when early opening of the deck is not required, Limit the water–cement ratio to 0.40 to 0.45 or lower with the use of water reducers, Use the largest maximum aggregate size with the maximum aggregate content, and Do not use concrete mixes that have a high tendency for cracking. EFFECT OF CRACKS ON BRIDGE DECK PERFORMANCE It is generally recognized that cracks perpendicular to rein- forcing bars hasten corrosion of the intersected reinforce- ment by facilitating the ingress of moisture, oxygen, and chloride ions to the reinforcement at the crack location. Studies have shown that crack widths of less than 0.3 mm (0.01 in.) have little effect on the overall corrosion of the reinforcing steel (Houston et al. 1972; Ryell and Richard- son 1972). Although wider cracks accelerate the onset of corrosion over several years, crack width has little effect on the rate of corrosion (Beeby 1978). Cracks that follow the line of a reinforcing bar are much more serious because the length of the bar equal to the length of the crack is exposed to the ingress of moisture, oxygen, and chlorides. In addi- tion, the presence of the cracks reduces the resistance of the concrete to spalling as the reinforcement corrodes. Miller and Darwin (2000) reported chloride levels in bridge decks at both cracked and uncracked locations. Their results showed significantly higher chloride contents at the locations of the cracks. At the level of the transverse reinforcement, the chloride contents exceeded the thresh- old level for corrosion in as little as 1,000 days. CURRENT PRACTICES RELATED TO BRIDGE DECK CRACKING Responses to the Michigan DOT survey showed that 30 or 97% of the 31 responding states had detected early age cracking in reinforced concrete bridge decks and 25 or 81% of the states reported that this cracking was observed in the first few months (Aktan and Fu 2003). Almost all states reported that transverse cracking was the most preva- lent. In the questionnaire for this synthesis, agencies were asked to identify which strategies they currently use to minimize cracking in bridge decks. Their responses, to-

35 gether with the number and percentage of responses from the 45 agencies, were as follows: • Specify minimum curing time (42 or 93%), • Specify maximum slump (40 or 89%), • Specify maximum concrete temperature (36 or 80%), • Require fogging during and immediately after place- ment (30 or 67%), • Specify maximum cementitious materials content (15 or 33%), • Require evaporation retardants (13 or 29%), • Require wind breaks during concrete placement (10 or 22%), and • Specify maximum concrete compressive strength (2 or 4%). Other strategies that were listed included the use of wet mats, nighttime casting, and controlling the evaporation rate. The most effective strategies listed by the respondents were fogging and adequate curing. Responses to the questionnaire for this synthesis indi- cated that the maximum size bar used for deck reinforce- ment was a 16-mm diameter (No. 5) bar for 13 or 29% of the respondents and a 19-mm diameter (No. 6) bar for 23 or 51% of the respondents. A maximum spacing of 305 mm (12 in.) or less was used by 29 or 64% of the respon- dents for longitudinal reinforcement and by 43 or 96% for transverse reinforcement. For 21 or 47% of the respon- dents, the minimum deck thickness was 200 mm (8 in.). The cracking tendency of restrained concrete specimens can be determined using AASHTO Designation PP34— Standard Practice for Estimating the Cracking Tendency of Concrete. In this method, the strain in a steel ring is meas- ured as a surrounding concrete ring shrinks. The time-to- cracking of the concrete ring is determined. The test can be used to determine the effect of variations in concrete con- stituent materials or curing regimes on cracking tendency. The procedure is comparative and is not intended to deter- mine the time of initial cracking of concrete cast in a spe- cific type of structure. SUMMARY OF PRACTICES TO REDUCE CRACKING IN CONCRETE BRIDGE DECKS Practices that can reduce cracking in bridge decks are as follows: • Minimize potential shrinkage by decreasing the vol- ume of water and cement paste in the concrete mix consistent with achieving other required properties; • Use the largest practical maximum size aggregate to reduce water content; • Use minimum transverse bar size and spacing that are practical; • Avoid high concrete compressive strengths; • Use windbreaks and fogging equipment, when neces- sary, to minimize surface evaporation from fresh concrete; • Apply wet curing immediately after finishing the sur- face and cure for at least 7 days; and • Apply a curing compound after the wet curing period to slow down the shrinkage and enhance the concrete properties.