Long-Term Deflection of High-Strength Concrete Beams

A large body of experimental evidence is available confirming that the creep coefficient of high-strength concrete under sustained axial compression is significantly less than that of ordinary concrete. Thus, the ratio of time-dependent deflection to immediate elastic deflection of high-strength concrete beams under sustained loads should likewise be lower. However, long-term deflection multipliers of the 1989 ACI Building Code do not account for concrete strength as a variable. Experimental results are reported for 9 beams with nominal concrete compressive strengths over a reange to 123,000 psi, loaded over a 12 month period. These tests confirm the significant differences between beams using high-strength and normal strength concrete. Based on these and other tests, modifications to the present ACI Building Code method for predicting long-term deflections are suggested.