A new concept for the early age shrinkage effect on diagonal cracking strength of reinforced HSC beams

The effect of early age shrinkage on the diagonal cracking strength of reinforced high-strength concrete beams is investigated, using thirty two reinforced concrete beams with a distance from the compressive fiber to the centroid of the reinforcing bars (effective depth) of 250 mm, 500 mm and 1000 mm, made of conventional high-shrinkage and low-shrinkage high-strength concretes with a water-to-binder ratio of 0.23. The loading test results show that the shear strength at diagonal cracking of reinforced conventional high-shrinkage high-strength concrete beams is 5% to 18% lower depending on the effective depth compared with that of reinforced low-shrinkage high-strength concrete beams, and the dependence of diagonal cracking strength on the effective depth is apparently different between both. The ultimate shear strength of the former is also 23% to 45% smaller than that of the latter on average. Moreover, a new concept based on the equivalent tension reinforcement ratio for the evaluation of the shrinkage effect on the shear strength at diagonal cracking, consisting of a tension reinforcement ratio modified taking into consideration the effect of the tension reinforcement strain produced by deformation of concrete at early ages, is proposed. This concept shows succesfully the linear relationship between the shear strength at diagonal cracking and the effective depth to -2/5 power independent of the magnitude of the early age deformation of concrete, and a design equation for the shear strength at diagonal cracking applicable to concrete compressive strengths from 90 to 130 N/mm(2) is proposed.