Structural Behavior of Strand Reinforced Concrete Members without Prestressing

Abstract

One way to reduce the large amount of steel present in reinforced concrete members that are subjected to high loads is to use high strength materials. In this study, use of high strength steel, seven-wire strands with tensile strength of 1860 MPa instead of mild steel rebars, is explored as longitudinal reinforcement in concrete beams and columns without prestressing. The use of such reinforcement is not addressed in design codes since the idea is new. Thus, appropriate strength reduction factors with high strength reinforcement is not established. The stress-strain relationship for the high strength steel strands does not have a clear yield point and lacks adequate ductility compared with conventional mild steel with yield strength less than 520 MPa. The study addresses the issue through both experimental tests and computational studies. The experimental program considers eight flexural tests of 2.2 m long beams subjected to 2-point loading, another six flexural tests of 1.7 m short beams subjected to a single point load, and four axial compression tests of 1 m tall columns. The variables considered in the study are the steel reinforcement ratio, compressive strength of the concrete, and loading condition. The experimental results showed that the load-deflection relationship for strand reinforced beams is much different than that for rebar reinforced beams. Both the stiffness prior to steel yielding and the ductility in the high strength steel beams are lower than in the mild steel beams. These findings were closely predicted by the theoretical part of the study that involved modeling the concrete by the Thornfeldt model and the steel by an elasto-plastic relationship. The correlation between steel strain and curvature ductility for strand reinforced sections is used to predict new strain limits for compression-controlled and tension-controlled regions to achieve similar ductility as the mild steel reinforced sections. The new strain limits for Grade 1860 MPa steel for compression-controlled and tension-controlled sections are 0.01 and 0.02, respectively. The limited tests conducted on columns showed that the strength of strand reinforced columns is about 10% lower than the theoretical prediction for ideal conditions by the code, with no appreciable effect on ductility.