Direct shear behaviour of steel fibres in a cementitious matrix.

An experimental investigation into the direct shear behaviour of steel fibre reinforced composites utilising discrete fibres at pre-defined angles and fibres randomly distributed is described. The direct shear tests encompassed the complete range of loading from its initial application to failure of the double L-shaped push-off specimens. Hooked-ended and straight steel fibres were used in the tests with the fibres oriented at angles of ±75°, ±60°, ±45°, ±30°, ±15° and 0° with respect to a plane normal to the loading direction. The embedment lengths of the fibres, related to the total fibre length lf , each side of the shear plane were 0.5lf :0.5lf and 0.25lf :0.75lf . In addition to the single fibre tests, tests were conducted on randomly oriented steel fibre reinforced composites with fibre volumes of 0.005, 0.010, 0.015, and 0.020 with hooked-ended and straight steel fibres.In addition to the tests outlined above, a series of non-destructive tests employing radiographic techniques was carried out to produce photographic images of events taking place of fibres pulling out from a cementitious element. The tests consisted of hooked-ended steel fibres oriented at angles of -60°, -30°, 0°, +30° and +60° to the cracking plane and straight fibres oriented at angles of -60°, 0° and +60°. The non destructive technique allowed the internal behaviour occurring within the specimen along the shear plane to be investigated without impacting on the direct shear tests.The angle of the fibre to the interface plane is an important parameter in determining the behaviour of the fibres under load and for the mode of failure; viz fibre pullout or fibre fracture. The effect of the end hook on behavioural aspects becomes increasingly less significant for more acute fibre angles where bending and snubbing effects become increasingly influential on the load versus displacement behaviour and mode of failure.Contrary to expectations, the fibre embedment length had little influence over the peak loads attained for the discrete fibre tests and, in a number of specimens, fibres pulled out from the longer embedded side. This observation is contrary to the generally accepted assumption that a fibre remains rigidly embedded on the long side and pulls out from short side. The traditional role that uniform bond stresses along a fibre length and friction have played in the description of fibre behaviour is not as significant as previously reported, other effects such as snubbing are more important in anchoring a fibre. Various models need to be revised with this observation in mind.The experimental results and observations from the discrete hooked-ended and straight steel fibres investigation are incorporated in the development of a behavioural model, the Variable Engagement Model II (VEMII). The VEMII describes the behaviour of randomly oriented discontinuous steel fibre reinforced composites loaded in shear. The model is verified against a series of randomly distributed fibre reinforced mortar specimens carried out in this study. Two forms of models are analysed: 1) a model based on the observation of lumped shear stresses at the fibre hook and in the snubbing zone; and 2) a uniform fibre bond stress applied along the embedded part of the fibre. The lumped bond stress approach and the uniform approach were found to give reasonable comparisons with the test data for the hooked-ended fibres but were conservative for the straight fibres. The VEMII confirms the applicability of the uniform bond approach adopted by previous researchers even though it does not correspond to the observations of fibre pullout behaviour of single fibres. The VEMII model provides a versatile approach that can also be applied to hybrid fibre combinations