Bond Modelling of Prestressing Strand

From pull-out and push-in tests on specimens with short embedment length an empirical relation has been derived, which describes the local bond stress as a function of the local slip and steel stress change. With the help of this bond model the transfer length and the bi-linear relationship for the development length of a pretensioned strand (ACI Building Code 1989, CEB-FIP Model Code 1990) is simulated. It is also used to indicate the influence of strand yielding on the development length. For the estimation of the concrete cover and strand spacing required to prevent the occurrence of visible bond splitting cracks the response of the concrete to the radial displacement of the strand-to-concrete interface is analyzed by means of a so-called thick-walled-cylinder model. The radial interface displacement consists of transverse deformation of the strand coupled with steel stress change (Poisson effect) and wedging action caused by the shape of the strand (lack-of-fit effect) and surface roughness. Besides the section geometry, this model takes into account the softening behaviour of concrete loaded in tension, It is used to explain the influence of various parameters such as concrete cover, strand spacing, strand diameter and concrete strength on the bond properties of strand.