Measurement of bar strain during pull-out tests: use of electrical resistance gauge methods under large displacement

Abstract

Two issues make measuring the local bond stress–slip relationship difficult: first, the bar is embedded within the concrete, making measurement of local strain values complex; second, yielding of the steel bar influences the bond stress–slip behaviour. Global bond stress–slip behaviour is generally obtained by measuring the slip that occurs at the free end of the bar during pull-out testing. Several distinct mounting methods have been used to install electrical resistance strain gauges on embedded bars in concrete to obtain local strain data. These methods are reviewed in relation to their ability to deliver reliable and robust data at both small and large values of slip. The use of such strain gauges is found to provide a viable method for assessing the variation in strain along embedded steel bars during pull-out testing. The use of surface-mounted strain gauges leads to high failure rates due to physical damage to either the gauges or their connections. The use of near-surface- and central-groove-mounted strain gauges enables measurement of local bar strain over large displacements. Using data from near-surface-mounted strain gauges, it is shown that in the post-peak region the bond stress increases linearly from the free end of the bar.