Sustained Load Deflection of High Strength Concrete Beams

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Rapid growth interest in use of high strength concrete (compressive strength to about 12,000 psi) has caused concern that current design methods and codified procedures for design may not be appropriate. Many design equations found in the 1983 Building Code of the American Concrete Institute, governing most US practice, are based on tests of members using concrete strength not greater than 6000 psi. A nine-year program of research on high strength concrete at Cornell had three objectives: (a) to provide an understanding of the basic differences in behavior of the material, (b) to establish the engineering properties of the material, and (c) to study the behavior of reinforced concrete members made using the material. One of the more significant observations is that the creep coefficient for high strength concrete. Present Code procedures for design do not account for this. The point is of particular importance, because short- and long-term deflections may prove to be a governing aspect of design for slender members made using the material. This project is for further testing of beams under sustained loading, to study immediate and time-dependent deflections, crack growth, and internal force redistributions. Important variables include time, compressive strength, age at first loading, and the amount of tensile and compressive reinforcing steel. Results of the proposed research should help improve the implementation and increased use of high strength concrete in structures such as buildings and bridges.