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Control of ConCrete CraCking in Bridges Many advances in bridge design, concrete technology, and reinforcement materials have been made over the years. Nevertheless, cracking of concrete in bridges continues to be a concern for bridge owners, particularly in bridge decks exposed to severe environments. The presence of cracks pro- vides a direct path for water and chlorides to penetrate the concrete and reach the reinforcement, which can lead to corrosion of steel reinforcement or degradation of the concrete. The AASHTO LRFD (load and resistance factor design) Bridge Design Specifications provide requirements for minimum amounts of reinforcement and maximum spacing of reinforcement to control crack widths. Some requirements are based on in-depth research; others are based on experience. Nevertheless, bridge owners find the need to supplement the AASHTO specifications with their own requirements. The control of cracking for aesthetic, durability, and structural reasons becomes increasingly important as service-life goals are extended and higher strength concrete, higher strength reinforcement, and different types of reinforcement are used in bridge construction. The primary objectives of this synthesis are to provide a compilation and discussion of methods used to control concrete cracking in bridge superstructures and substructures and to present informa- tion on the influence of cracking on long-term durability. Superstructure components discussed in this synthesis include full-depth, cast-in-place concrete decks; partial-depth, precast concrete panels with a cast-in-place topping; full-depth, precast concrete deck panels; and prestressed and nonprestressed concrete beams. Information for concrete decks on both steel and concrete beams is included. Sub- structure components include pier caps, columns, abutments, and pile caps. Information for this synthesis was obtained from a literature review, surveys of state departments of transportation, and surveys of provincial and territorial agencies in Canada. The survey achieved a 78% response rate from U.S. agencies (39 responses). Information gathered in this synthesis pro- vides a basis to understand the causes of concrete cracking in bridges and helps to establish the most practical and efficient methods for reducing the occurrence of cracking and controlling cracking when it occurs. The survey of the state departments of transportation and Canadian agencies provided information about the frequency of concrete cracking, materials being used in concrete mixes, types of reinforce- ment being used, and practices that were successful and unsuccessful in reducing bridge deck cracking. Concrete cracking occurs most often in full-depth, cast-in-place, concrete bridge decks. The successful practices to reduce bridge deck cracking generally relate to reducing drying shrink- age of the concrete mix and reducing temperature differences. Drying shrinkage can be reduced by limiting the amounts of cementitious materials and water in the concrete and using the largest practi- cal size of aggregate in combination with appropriate construction practices. These practices include avoiding high compressive strength concrete, applying wet curing immediately after finishing the concrete surface, continuing wet curing for at least 7 days, and applying a curing compound after the wet curing to slow moisture loss from the concrete. Temperature differences can be controlled by limiting the temperature of the concrete at time of placement and ensuring that the concrete tem- perature does not increase too much as the concrete hydrates. Nighttime concrete placements also can be beneficial, particularly in hot climates. Overall, no single most effective practice can be used to enhance concrete bridge deck performance. summary
2 Practices that show potential for reduction of shrinkage and shrinkage cracking include the use of supplementary cementitious materials, internal curing, shrinkage-reducing admixtures, or shrinkage- compensating concrete. End zone cracking, which occurs in prestressed concrete beams, is an uncommon occurrence and often can be prevented by revising the detensioning sequence. Cracks that occur can be controlled through the use of appropriate reinforcement. Concrete used in substructures also cracks but far less frequently than that used in concrete bridge decks. Consequently, there appears to be much less concern about substructure cracks. In most cases, the substructure is protected from rain and deicing salts by the superstructure and thus is not subject to the same harsh exposure. Based on the survey results, most U.S. highway agencies use epoxy-coated reinforcement for cor- rosion resistance. Some states have used zinc-coated, stainless-steelâcoated, or solid stainless steel reinforcement. They reported that the use of these materials had no noticeable effect on deck crack- ing. However, the use of fiber-reinforced polymer reinforcement has a major effect on crack widths because the reinforcement has a lower stiffness. Consequently, the amount of fiber-reinforced polymer reinforcement often is determined on the basis of controlling crack widths rather than satisfying the need for structural strength. Four case examples are included describing the steps taken by the states of California, Kansas, Pennsylvania, and Washington to reduce the severity of cracking in their bridge decks. The response to the survey resulted in eight topics suggested for research. A research problem statement incorporating several of the topics related to the use of reinforcement to control cracking was developed.
