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Specifications for concrete to be used in bridge structures have traditionally been prescriptive in nature, meaning that the constituent materials of the concrete and their relative propor- tions are stated like a recipe for a cake. In the 1990s, FHWA began to introduce the concept of performance specifications, which designate required results rather than means. FHWA suggested eight performance characteristics that could be quantified using standard tests, four related to concrete durability and four related to concrete strength for structural design. The durability characteristics were intended to increase the service life of the concrete, in particular, bridge decks, which have always been the Achilles heel of bridges. The structural design characteristics related to the applications of high-strength concrete, which allows the use of longer span lengths; wider girder spacings and fewer girders; shallower girders; or a combination of these, resulting in more economical structures. This material was called high performance concrete (HPC). State transportation agencies adopted these characteristics for concrete in bridges to varying degrees. State documents for bridges generally include standard specifications, supplemental speci- fications, and special provisions. The standard specifications document is used for all projects. It may be updated periodically through the use of supplemental specifications or through the issuance of a revised document. Special provisions are issued to define the requirements for specific projects. This last type was used by those states that initially moved forward with the HPC concept. Since then, some states have begun to require HPC in their standard specifica- tions, while others have changed their prescriptive specifications as a result of experience with HPC (although the concrete may not be called HPC). The primary objectives of this synthesis are to document the types of specifications and practices used by state agencies to produce HPC, and to identify specifications and practices reported as having improved concrete performance and those that have not. Information for the synthesis was gathered from a survey sent to the departments of transportation (DOTs) of all U.S. states and Washington, D.C. The survey achieved an 82% response rate (42 responses). Six agencies were contacted after the survey for additional details about their specifications and practices. Information was also obtained from specifications for bridges published by AASHTO and state DOTs and from a literature review. The synthesis is intended to help bridge owners and designers determine the appropriate specifications for HPC in bridges. Information collected for this synthesis indicates that specifications for concrete to be used in bridges remain largely prescriptive. The use of performance specifications is generally limited to special provisions for individual projects. Nevertheless, numerous changes have been made to the specifications, and states report that the performance of HPC is better than that of conventional concrete. However, despite the use of HPC, states still express concern about the amount of cracking in concrete bridge decks. All states now permit the use of supplementary cementitious materials in the form of fly ash, silica fume, or slag cement in the concrete. These materials reduce the permeability of concrete, which makes the concrete more resistant to the penetration of water and deicing salts, and thus reduces the likelihood of bridge girder and deck deterioration from freeze-thaw damage and reinforcement corrosion. Summary HIGH PErFOrmaNCE CONCrETE SPECIFICaTIONS aND PraCTICES FOr BrIDGES
2 Statesâ prescriptive specifications limit the total amount of cementitious materials (cement plus supplemental cementitious materials) that may be used in the concrete, along with the percentages of supplementary cementitious materials that may be included. These specifications also address fine aggregate, coarse aggregate, chemical admixtures, air entraining admixtures, and other additives. The survey of the state DOTs provided information about practices that were successful and unsuccessful in reducing bridge deck cracking. The successful practices generally related to reducing drying shrinkage of the concrete mixâfor example, limiting the amounts of cementitious materials and water in the concrete and using the largest practical size of aggregateâin combination with construction practices to minimize deck cracking. These included avoiding high compressive strength concrete; applying wet curing immediately after finishing the concrete surface and continuing wet curing for at least seven days; and applying a curing compound after the wet curing to slow the moisture loss from the concrete. Overall, no single best practice that can be used to enhance concrete bridge deck performance was identified. Several practices were identified by individual respondents as having no effect in improving concrete bridge deck performance in their state. These included specifying maximum slump; using prescriptive mix designs; specifying maximum and minimum concrete temperatures; using curing membranes and evaporation retardants; and omitting the casting of a trial or test slab before casting the deck. Specifications for precast, prestressed concrete are less prescriptive than for cast-in-place concrete, with greater reliance placed on the supplier to develop the mix proportions. Com- pressive strength is the dominant property specified for precast, prestressed concrete. Precast, prestressed concrete beams appear to be performing satisfactorily using the existing specifi- cations, with only a few performance issues reported in the survey. In response to the survey, seven states suggested topics for future research, all but one of which were related to cracking in concrete bridge decks. The scope of these included identifying causes of cracking, evaluating the effect of different constituent materials, and identifying cost-effective methods of sealing cracks. The one exception asked for more effective means to ensure that concrete meets the performance criteria for the intended application.
