Inelastic behaviour in steel wire pull-out from Portland cement mortar

Abstract

This paper describes a study of the factors influencing the pull-out force of stainless steel wires embedded in Portland cement mortar. The first part provides a theoretical elastic analysis of the pull-out force in terms of the misfit between wire and matrix, the coefficient of friction at the wire-matrix interface and the elastic constants of the materials. The resulting equation shows that the behaviour is greatly influenced by the Poisson contraction of the wire during pull-out. Since the elastic modulus of the mortar is only five or six times smaller than that of the wire this can lead to a large reduction in stress-transfer across the wire-matrix interface and a corresponding reduction in pull-out force. In general the pull-out force is extremely sensitive to the wire-matrix misfit. The second part deals with an experimental study of the effect of surface finish of the wire and the effect of an externally applied confining pressure. The results show that when a pressure is applied the pull-out force increases, as expected, due to the increase in stress-transfer across the interface. However, a very small amount of movement of the wire leads to a large reduction in pull-out force. This is not due to wear of the matrix; it is due to densification of the cement mortar near the wire surface and is produced by the combined effect of the normal pressure and the tangential traction. This compaction in turn leads to a non-reversible reduction in the fibre-matrix misfit and a fall in pull-out force. The paper concludes with a discussion of the significance of this non-elastic behaviour in the practical performance of fibre reinforced mortars and the role of mechanical deformations of the fibres.