Mixed Class I/Class II post-peak curves of mortar models of rock samples

A comparative analysis of post-peak tests in uniaxial compression under different methods of load control is presented. The majority of the samples showed mixed Class I / II curves: the post-peak curves start with Class I curve which then turns into Class II. This mixed curve is observed under all types of loading control we tested. However the extent of the initial, Class I curve depends upon the pre-defined lateral strain rate in lateral strain control. The Class II curve obtained under lateral strain control is attributed to the unloading of the loading frame attempting to maintain the pre-defined lateral strain rate, but is not the real Class II behaviour. The sample failure pattern depends upon the mortar compositions and reveals the presence of either sub-vertical fractures (the loading direction is vertical) or inclined (oblique) fractures of vertical fractures with inclined segments. In many samples spallation was also observed. The type of loading control affects the growth of internal cracks/fractures and the failure pattern. Yet, both types of loading control (the lateral strain and failure control) stabilise the crack growth to the extent sufficient for investigating the mechanics of fracturing of mortar/rock samples in uniaxial compression.