A special technique for determining the bond strength of micro-fibres in cement matrix by pullout test

The complete relationship between the pullout load and the displacement of a fibre when it pulls out of the matrix serves as an important parameter in the design of composite materials. Many analytical models for the prediction of the behaviour of composite materials are based on the assumption of a constant friction load between the fibre and the matrix, regardless of the displacement of the fibre. Based on this assumption test techniques were developed [1, 2] in which the critical length of the fibre was determined. The bond strength was calculated from the critical length assuming that the fibre strength and diameter are known. The two techniques are quite similar, and were found by [1] as inapplicable for extremely brittle fibres such as carbon fibres. In addition, some pullout tests suggest an interface friction which depends on the amount of local slippage [3]. Such slip-dependency cannot be identified by test methods based on measuring the critical fibre length only. In the following a detailed description of the new technique used for the determination of the complete P u (pullout load-fibre displacement) curve of micro-fibres from cement matrix will be described. There is an increasing trend in recent years towards the use of micro-fibres in cement reinforcement. Micro-fibres can be defined as those with diameter equal to or smaller than cement grain size, typically in the range 10-20/~m. Problems which arise when making a direct pullout test of carbon fibre from cementitious matrix include: