Effects of fly ash on fatigue and fracture properties of hardened cement mortar

The aim of the work described in this paper was to investigate the effects of fly ash on the fatigue resistance of cement mortars. Mortar mixes were prepared with similar compressive cube strengths using a range of fly ash contents up to 25%. Samples from these mixes were tested in a double torsion facility under cyclic loading and the rates of crack growth measured, recorded and plotted against the applied stress intensities on log-log scales in V-K diagrams. At later ages the fly ash was found to increase fatigue resistance and toughness. This was thought to be due to the spherical fly ash particles having a blunting effect. At greater ages the mixes containing fly ash were slightly less resistant to fatigue crack growth. Bond between the gel and the large fly ash particles was observed to be poor, resulting in their effectively acting as flaws that were now large in relation to other flaws in the matrix which had reduced with continued hydration. (C) 1999 Elsevier Science Ltd. All rights reserved.