Fatigue and fracture of cement mortars containing fly ash

Doctoral Thesis


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University of Cape Town

The aim of the work described in this thesis was two-fold; to investigate the effects of fly ash on the fatigue resistance of cement mortars when included as a partial cement replacement, and to seek to improve the understanding of cyclic fatigue crack growth mechanisms in cementitious materials. Mortar mixes were prepared with similar compressive cube strengths using a range of three fly ash contents from 0% to 25% (by mass of cement). Samples prepared using these mixes were tested in a double torsion facility under cyclic loading, and the rates of crack growth measured and recorded. These crack velocities were plotted against the applied stress intensities on log-log scales in so called V-K diagrams. An advantage of using the double torsion system was that the applied stress intensity was constant for constant load conditions and changing crack length, unlike many other configurations. However, the amount of scatter inherent in the system, and in testing cementitious materials, is large. This has meant that comparison between sets of data has had to be carried out on the basis of comparing the positions of clouds of data, rather than comparing the slopes of best fit lines. Another advantage of the OT system is that some of the parameters pertaining to the test can be changed whilst the test is in progress. This means that the effects of changing, say, load amplitude or cyclic frequency can be observed on the same specimen. Parameters that were considered in the test matrix. included the following: fly ash content, sample age, cyclic frequency and amplitude, maximum applied stress intensity, relative humidity and temperature of the environment, drying preparation of the sample and the type of fluid in which the samples were tested. The relative effects of all of these variables were compared in a series of V-K plots and trends were noted.