Prestress loss and flexural behavior of precracked RC beams strengthened with FRP/SMA composites

Abstract

This paper explored a kind of fiber reinforced polymer (FRP)/shape memory alloy (SMA) composite prestressed strengthening system, and studied the prestress loss and flexural behavior of precracked reinforced concrete (RC) beams strengthened with FRP/SMA composites by experiments. In the experiment, the length ratio of SMA wires to FRP sheets, the type of FRP sheets, and the area ratio of SMA wires to FRP sheets were selected as the main parameters. The prestress loss test results indicated that the elongation of FRP sheets in the strengthening zone, relative slip between SMA wires and FRP sheets, and crack closure are the main causes of prestress loss in beams strengthened with FRP/SMA composites. Among these, the prestress loss caused by the elongation of FRP sheets in the strengthening zone accounted for a significant proportion of the total loss, while the remaining two losses accounted for a minor proportion. The increase of the length ratio of SMA wires to FRP sheets could effectively reduce the prestress loss due the decrease of SMA shrinkage strain. The type of FRP sheets in the strengthening zone affected the prestress loss mainly by affecting the elongation of FRP sheets. The area ratio of SMA wires to FRP sheets influenced the total recovery force level. The prestress loss mainly occurred within 12 h (initial curing of epoxy resin) after thermal activation of SMA wires, and then the prestress loss was small in the following 60 h (complete curing of epoxy resin). The flexural test results of beams strengthened with FRP/SMA composites showed that the flexural capacity was significantly improved, and the yield and ultimate load significantly increased. At the same time, the service performance was also improved due to the introduction of SMA wire recovery stress. In addition, the introduction of prestress to FRP sheets in the strengthening zone delayed the debonding failure induced by the crack. Lastly, the sectional analysis method was also applied to predict the load-deflection responses of the strengthened beams, which was validated through comparisons with the test results.