Effect of Mild Confinement on Flexural Behavior of Reinforced Concrete Beams

Abstract

This thesis focuses on study of effect of confining the concrete compression region on flexural behavior of reinforced concrete beams. The idea is based on the concept that uniaxial stress – strain relation of concrete subjected to bending stresses does not remain valid at ultimate limit state and there exists multiaxial state of stress in the form of complementary transverse or lateral stresses. Stress – strain behavior of concrete subjected to uniaxial compression loading also suggests the presence of these transverse stresses, wherein an abrupt increase in volumetric strain/stresses can be observed beyond critical stress. These lateral or transverse stresses induce internal tensile or splitting stresses through dilation of concrete and reduce compressive strength of concrete. Thus internal cracking developed due to lateral stresses in compression concrete minimizes the ability of concrete to resist compressive stresses. Confinement through steel stirrups of concrete compression region is believed to restrain these lateral or transverse stresses, delay disintegration of concrete and result in improved behavior of structural members. Beneficial effects of confining concrete compression regions are widely recognized in columns. The idea of confining the compression concrete in beams is recently developed. This study attempts to explore the possibility of improvement in flexural behavior of reinforced concrete beams by confining compression concrete zone through the introduction of stirrups. An experimental investigation has been carried out by testing ten reinforced concrete beams which included four traditionally designed and six confined beams. Confinement was introduced in the form of short confining stirrups in the flexural span. The specimens included both under-reinforced and over reinforced beams. Test results show that ductility of under-reinforced and over reinforced beams improves while flexural capacity is not affected significantly due to confinement. The results also suggest that limit on maximum allowable longitudinal steel (ρmax) may be relaxed when adequate confinement is provided.