Repair and strengthening of Pre-1970 reinforced concrete corner beam-column joints using CFRP composites

Abstract

The results of an experimental investigation are presented which examine the seismic adequacy of pre-1970 reinforced concrete (RC) corner beam-column joints and the efficacy of carbon fiber-reinforced polymer (CFRP) composites for both pre- and post-earthquake retrofit of such joints. Four full-scale corner beam-column-slab subassemblages built with identical dimensions and pre-1970 reinforcement details were subjected to a reverse-cycle bidirectional displacement history consisting of alternate and simultaneous cycles in the two primary frame directions before and/or after retrofit. Two of the specimens were first subjected to severe and moderate levels of damage, respectively, then repaired by epoxy injection, and strengthened by adding a #7 reinforcing bar within the clear cover at the column inside corner and by externally bonding multiple layers of carbon fabric to form a carbon-epoxy retrofit system. Two other specimens, one of which had a significantly lower concrete compressive strength, were strengthened in their as-built condition. The CFRP scheme was improved in light of the findings as the experimental program progressed. Pre-1970 RC corner beam-column joints were found to be severely inadequate in meeting seismic demands because of column bar yielding, joint shear failure, loss of anchorage of beam bottom bars, failure of column lap-splices, and the resulting loss of stiffness and strength that dominate their behavior even at relatively low interstory drift levels. Bidirectional loading played a significant role in such response. It was shown, however, that such joints can be strengthened easily both before and after earthquake damage by using CFRP composite schemes. Regardless of the level of existing damage and concrete strength, a "rigid" joint behavior up to interstory drift ratios of at least 2.4% and joint shear strength factors ranging from 1.06 to 1.41√MPa were achieved; such shear strength factors are larger than the value of 1.00√MPa recommended for use with seismically designed, code-conforming corner beam-column joints. A ductile beam hinging mechanism was achieved and energy dissipation capacity was improved efficiently for joints with concrete strengths ranging from 26 to 34 MPa. The subassemblage with significantly low-strength concrete (15 MPa) had low overall lateral stiffness and reduced reinforcement anchorages which prevented the formation of beam hinging. In cases of such low-strength concrete, more invasive operations may be required so that the improved joint shear strength can be mobilized. It is recommended that bidirectional loading be always considered in both pre- and post-retrofit evaluation of corner joints.