Interfacial debonding and fibre pull-out stresses. III: Interfacial properties of cement matrix composites

Following the development of an improved theoretical analysis of fibre pull-out on the basis of the concept of fracture mechanics in Part II, the theory has been successfully used to characterize the debonding and frictional pull-out behaviour in cement mortar matrix composites reinforced with steel and glass fibres. It is shown from the plots of partial debond stress, σdp, versus debond length, l, that these composites are typical of mechanical bonds at the interface. For the steel fibre-cement matrix composites, the theory overestimates the post-debond frictional pull-out stress, σfr, particularly for long embedded fibre length, L, otherwise the prediction agrees well with the experiments for the maximum debond stress, σd*. This seems to be a direct result of decay of frictional bonds at the interface region after debonding due mainly to compaction of the porous cement mortar surrounding the fibre, effectively reducing the residual clamping stress, q0, arising from shrinkage of the cement matrix. Therefore, a correct theoretical prediction is made for σfr using a lower value of q0 while other parameters are kept constant, which gives good agreement with experimental results. For glass fibre-cement matrix composites, an accelerated cure condition promotes rapid hydration of cement and densification of the matrix. This effectively improves the chemical as well as mechanical bonds at the fibre-matrix interface through the formation of CH crystals and large fibre-solid matrix contact area of the interface, and consequently ameliorating the interfacial properties, interfacial fracture toughness, Gic and q0 in particular. Predictions of σd* and σfr taking into account these changes due to cure condition, results in good agreement with experimental results.