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12 CHAPTER 2 Background and Research Approach 2.1 Background stressing load, and for determining elastic deflections caused by dead and live loads, axial shortening and elongation, and Use of SCC in the construction of precast bridge members prestress losses. Literature review showed that the modulus of and bridge substructures and in the repair of bridges has elasticity of SCC could be as low as 80% of that for HPC of been limited in the United States. Properly designed SCC is normal consistency because of the lower coarse aggregate vol- expected to provide similar properties as the conventional ume of SCC [Holschemacher and Klug, 2002]. However, counterparts except for the high workability. However, changes under air-drying conditions, the elastic modulus of SCC can in mix design and fluidity of SCC can result in SCC with be higher than that of normal concrete at long term. Limited hardened properties and performance that are different from published data are available on relationships between flexural that commonly expected from conventional concrete. Proper strength and compressive strength of SCC, and applicability selection of material constituents and proper proportion- of the various code models to SCC need to be validated. ing are necessary for achieving the desired workability and Typically, SCC mixtures are proportioned with higher performance of SCC. The factors that significantly influence the design, constructability, and performance of precast, binder content, lower coarse aggregate volume, and smaller prestressed bridge elements with SCC need to be identified. MSA, which increase thermal, autogenous, and drying shrink- There is also a need to develop guidelines for the use of SCC age, and creep leading to high loss of prestress and excessive in bridge elements and to recommend changes to AASHTO deflections and elastic shortening. Therefore, creep and dry- LRFD Specifications. These guidelines will provide highway ing shrinkage characteristics of SCC need to be determined agencies with the information necessary for considering con- and considered in the design of precast, prestressed bridge crete mixtures that are expected to expedite construction and elements. According to the literature survey, there seems to yield economic and other benefits (e.g., better surface finish, be some discrepancy regarding the visco-elastic properties of lower labor cost, etc.). SCC because of differences in mix design (w/cm), type and For successful design of SCC, some factors require greater content of coarse aggregates, type of chemical admixture, and attention than is generally required for conventional concrete, testing exposure. It is reported that the creep potential of SCC including type and size and grading of coarse aggregate, com- appears to be slightly higher than that of conventional concrete position and content of binder materials, and w/cm. Proper made with the same raw materials and having the same 28-day selection of material constituents is also necessary for worka- compressive strength [Attiogbe et al., 2002; Pons et al., 2003; bility and performance of the hardened concrete. Byun et al., 1998]. Depending on the selected binder, w/cm, A number of test methods have been used to characterize and ambient temperature at the precasting plant, the use of new workability of SCC, including filling ability, passing ability, generation HRWRA may eliminate the need to use radiant and segregation resistance. However, no single test method heat or steam curing. has been found to fully characterize all relevant workability SCC used in precast, prestressed applications is typically aspects of SCC. Selection of proper combined test methods proportioned with a low w/cm (0.32 to 0.36) to enhance stabil- can facilitate workability testing protocol and provide means ity of the plastic concrete. Relatively low w/cm values, coupled for quality control of field applications. with high content of binder, lead to a greater degree of auto- Knowledge of the compressive strength, elastic modulus, genous shrinkage than in conventional concrete. Such type of and flexural strength of concrete is required for estimating shrinkage also increases with increased fineness of the binder camber of prestressed members at the release of the pre- and fillers in use. Therefore, drying shrinkage, autogeneous