Experimental study on bond degradation in reinforced concrete tension members under cyclic loads

Abstract

The accurate assessment of existing structures requires refined bond-slip models, which could account for the unloading and reloading of structures. Although bond has been investigated for as long as reinforced concrete exists, most bond-related design provisions rely on simplified empirical approaches. A reason for that are the technical challenges of measuring the bond. With the development of new measurement technologies, the highly complex bond behaviour can be assessed along the embedded reinforcement length and throughout the lifetime of a structure, without affecting the bond conditions. This paper presents first selected results of an extensive experimental campaign comprising tension chord specimens subjected to static, cyclic, and high-cyclic loads. The loading history and load amplitude were varied in the tests, and all reinforcing bars were instrumented with distributed fibre optical sensors. This novel measurement technique allows for a quasi continuous strain measurement along the bars. The nominal steel stress distribution can then be derived using constitutive laws, and the bond shear stress distribution follows from the latter by equilibrium. The assessment of the concrete contribution between the cracks in the tension chords shows a pronounced reduction of tension stiffening with an increasing number of load cycles, causing larger deformations. The findings contribute to a better understanding of the deterioration process due to repeated loads at the interface of concrete and steel.