Shear transfer across a crack in ordinary and alkali activated concrete reinforced by different fibre types

Fibre reinforced concrete (FRC) increases shear capacity mainly by providing post-cracking residual strengths and by improving the aggregate interlock mechanism on the two crack faces. Hence, direct shear tests can be adopted to study the shear transfer mechanisms across a crack. Several researches studied the behaviour of steel fibre reinforced concrete by means of shear tests initially developed for plain concrete (PC). Due to an increased heterogeneity of material (caused by a random fibre distribution) and the need to carry out the test up to a higher crack width (Mode I) and slip (Mode II), tests on FRC are more difficult as compared to PC and the issue related to the rotation of the cracking plane is more likely to develop. In addition, other fibre types or materials different than ordinary concrete have not been studied in depth so far. In this context, the present study firstly evaluates the influence of rigid (steel) and non-rigid (polypropylene) fibres on the direct shear behaviour of ordinary concrete (considering a broad range of FRC toughness). To do this, the modified JSCE SF6 test was improved by avoiding friction and by providing a steel system to control rotations. Secondly, the direct shear behaviour of alkali activated concrete (AAC) reinforced by steel fibres was compared against ordinary FRC in order to underline possible differences. Experimental results showed that, under direct shear tests, the fibre influence on the shear stresses transferred across a crack is only related to FRC toughness and not to fibre type (rigid or non-rigid). AAC also showed to have a shear behaviour comparable to ordinary concrete.