Experimental research on preventing debonding problem in retrofitted RC beam - column joints using composite materials

Abstract

Beam-column joints in existing reinforced concrete (RC) structures that were designed according to pre-1970s national design codes pose serious danger in moderate to severe earthquakes due to the fact that they were designed for gravity loads and lateral force levels that were much lower than those specified by the current codes. Common deficiencies of these joints could be summarized as; widely spaced column ties, no transverse reinforcement in the joint region, inadequate lap splicing in the column and insufficient embedment length of beam bottom reinforcement in the joints. This lack of seismic detailing may cause non-ductile behavior and excessive lateral deformation demands which cannot be accommodated, and therefore, failure mechanisms resulting in total collapse may easily develop. Thus, the shear capacity and the effective confinement of joints must be improved. This research investigates the actual behavior of such joints and debonding problem in Carbon Fiber Reinforced Polymer (CFRP) wrapping methodologies that were developed to improve the seismic behavior of such deficient joints. In order to investigate the effect of CFRP retrofitting on the behavior of exterior beam-column joints, nearly three full-scale reinforced concrete beam-column subassemblies were produced and tested under constant axial load and reversed cyclic loading. One of the specimens was detailed according to ACI 318-09 code requirements and the remaining two specimens that are deficient in shear detailing requirements were retrofitted using CFRP sheets according to previous studies conducted by Altay‘s PhD dissertation using new anchorage techniques for preventing debonding effect on CFRP from concrete surface. Experimental and analytical results showed that the CFRP-retrofitted specimens exhibited significant behavior in terms of load carrying capacity. The desired failure mechanism, which is the hinge formation in the beam, was reached and debonding problem of the CFRP was prevented, by using increased number and improved quality of anchorages.