It is well known that the compressive strength of concrete cylinder decreases as the height increases. This phenomenon can be explained by fracture mechanics. According to fracture mechanics, the larger the size of specimen with initial crack is, the smaller the strength is due to stress concentration. In the case of uniaxial compression, the height of specimen, h, can be regarded as characteristic length of size effect law, which was based on fracture mechanics. It is, therefore, true that the confinements of cylinder specimen increase the compressive strength due to the reduction of characteristic length.
This behavior of cylinder specimen can be similarly applied to the beam which has uniform effective depth but different span under pure bending. Twenty beams are tested involving the following variables; the amount of longitudinal reinforcement, with or without confinements in compression zone, and the width of beam. The behavior of test beams is observed by comparing the load-deflection curves obtained by theoretical process. As was expected, the confinements of compression zone in reinforced concrete beams show increasing of strength and ductility. Especially, in over-reinforced concrete beams, load-deflection curves show very ductile behavior and moment of resistance fairly increase. And, it is observed that the stress-strain curve in confined concrete differs from that of unconfined concrete.
