Direct Tensile Tests of Supercritical Steel Fibre Reinforced Concrete

Steel fibre reinforced concrete (SFRC) becomes increasingly interesting for structural design and application. However, to reinforce structures just with steel fibres – not including any rebar – supercritical fibre contents are essential to ensure hardening behaviour in the post-cracking domain. Material properties are usually determined from experiments conducting three- or four-point bending tests. Specific conversion factors capture the softening behaviour and enable to transform flexural into tensile strengths. Own experiments prove that fibre contents of 1.8 Vol.-% yield flexural strengths of about 8 MPa. To get definite and reliable tensile strengths, direct tensile tests on optimised bone-shaped specimens made of supercritical SFRC are proposed here. As a specimen a slab (w × h × l = 200 × 100 × 720 mm3) is casted horizontally. That way, fibre orientation and distribution representative for practically relevant slabs with 10 cm thickness are simulated. To eliminate the so-called wall-effect that occurs during casting, the edges are cut off by water jet cutting before testing. Two pairs of displacement transducers on each face of the slab record the crack opening over a measuring length of 100 mm on the top and 400 mm on the bottom face. A new test set-up is introduced. Loading is applied to the specimen by friction using pre-tensioned threaded steel rods. Coating with an epoxy resin and corundum guarantees the required coefficient of friction. Displacement transducers on the top and bottom of the specimen record the relative displacement between the specimen and the test station. Axial loading is induced by a triangular steel structure (framework). Strain gauges on the outer faces of the diagonal struts control inevitable eccentricities of the load transfer. Consequently, highly accurate measurements are recorded. During testing, the crack flanks are slowly pulled apart from another (up to 4 mm) but without complete separation. On average a maximum tensile strength of 3 MPa and a coefficient of variation of 11% for maximum force is recorded what indicates a small scatter and highly accurate strengths.