Nonlinear finite element modelling of the bond behavior of near-surface mounted Fe-SMA bars

Abstract

Strengthening of concrete members with near-surface-mounted (NSM) iron-based shape memory alloy (Fe-SMA) bars can increase the load-carrying capacity and reduce deflections/crack widths owing to the unique self-prestressing ability of the Fe-SMA. However, the bond behavior of Fe-SMA bars is critical for the effectiveness of the NSM strengthening method, as inadequate bond strength can result in premature bond failure, rendering the strengthening technique ineffective. This study presents the results of a non-linear finite element (FE) model that was developed and validated to simulate the bond behavior and failure patterns of NSM Fe-SMA bars grouted into concrete. The variable parameters of the study included the effect of the compressive strength of the grout and the concrete block, the effect of cover depth and the activation of the Fe-SMA bar on the bond behavior. Investigations showed that the bond resistance and failure patterns of the NSM bars were significantly affected by the compressive strength of the concrete block and grout. The results showed that increasing the compressive strength of the grout by two times resulted in a 35 % increase in bond strength while increasing the compressive strength of the concrete block by three times resulted in a 30 % increase in bond strength. Additionally, the load transfer between the grout and concrete block was strongly influenced by the grout's compressive strength, where a higher strength of the grout resulted in a stiffer bond and increased contribution of the concrete block to resisting loads. Similarly, increasing the cover depth by four times from 5 mm to 20 mm increased the bond strength by more than twice and ultimately led to a change in failure mode from a splitting crack in the grout to a splitting crack in the concrete block. The results of the study also show that prestressing the short NSM bond with Fe-SMA can induce an initial bar slip in the range of 0.025 to 0.14 mm for recovery stresses of 100 MPa to 350 MPa.