Comparative Flexural Toughness Evaluation of Steel Fibre-Reinforced Concrete

A comparative appraisal of the protocols recommended in the ASTM and Japanese standards for evaluation of flexural toughness of three concrete grades reinforced with steel hooked-end fibres is presented. Target 28-day cube compressive strengths of the three concrete grades were 25 MPa, 35 MPa and 45 MPa and the fibres having an aspect ratio of 80 were used at volume fractions of 0%, 0.13%, 0.19%, 0.25%, 0.31% and 0.38%. Testing for toughness was carried out in the form of displacement-controlled four-point bend tests on prismatic specimens of size 150 mm × 150 mm × 700 mm and stable load–deflection behaviour was obtained only when fibre dosage was at least 0.25%. For fibre volume fractions of 0.25% and more, the toughness parameters obtained from the ASTM and the Japanese standard were comparable. From the results of the comparative study, it is recommended that for an Indian standard, if the toughness characterisation is to be done, then equivalent flexural strength should be considered as the parameter over residual strength. In addition, ‘equivalent flexural strength ratio is also to be considered, since it represents the load retaining capacity of the specimen after the peak load. Nevertheless, equivalent flexural strength is the average flexural strength up to the specified deflection limit which does not specify any post-peak behaviour, hence need not be a characterising parameter for understanding fibre performance in concrete.