The development and application of high strength concrete in structures is increasing faster than the development of appropriate prediction models and design recommendations. Thus it is necessary to examine the applicability of previous design equations based on experimental data of concrete with compressive strengths less than 41MPa for predicting properties of high strength concrete members. The objective of this study is to contribute to the understanding of structural behaviors of high strength concrete members. For this purpose, the shear behavior and the flexural behavior of high strength concrete beam have been investigated experimentally, and the confinement effect of single spirals and interlocking spirals for high strength concrete specimen has been investigated experimentally and theoretically.
The effect of concrete strength on the stirrup effectiveness was mainly investigated in the study of shear behavior of beam. Test beams consisted of 24 rectangular beams, 4 without and 20 with stirrups. Selected test variables were concrete strength and stirrup ratio. Using the ultimate shear strength of beam without stirrup, the stirrup effectiveness factor was calculated. The test results showed that the safety factor of ACI Code equation decreased with increasing the concrete strength in beams without stirrup. However it was shown that the stirrup effectiveness factor tends to increase with increasing the concrete strength. As a result of this, the safety factor of high strength concrete beams with high stirrup ratio for ACI Code equation was ensured more than that of normal strength concrete beams.
For the investigation of flexural behavior of beam, 8 singly reinforced high strength concrete beams with 4 different tensile reinforcement ratio were tested. From the test results, it was shown that ACI Code equation for prediction of the flexural strength of beam exactly estimated the flexural strength of beam tested by this study. The measured ultimate strain of extreme compression fiber of concrete was greater than 0.003. Consequently, the equivalent rectangular stress block in the ACI Code could be used for the high strength concrete beams. Based on the previous test results and this study, a model equation for estimation of the ductility index of singly under-reinforced high strength concrete beam was proposed.
In the study of the confinement effect by spirals, 36 specimens with single spirals and 108 specimens with interlocking spirals subjected to concentric axial loads were tested to investigate strength, ductility and interlocking effect. Main test variables were concrete strength, spacing of spirals, yield strength of spirals, and interlocking length of spirals. In order to predict the modified confining stress in the case of single spirals, the spiral effectiveness coefficient was derived, and the reduction coefficient of yield strength based on the experimental data and analysis was proposed. This modified confining stress was extended for calculation of the confining stress by interlocking spirals. On the basis of the proposed confining stress, model equations for the predictions of the strength and the axial strain of columns with single and interlocking spirals were proposed.