Flexural tensile behaviour of alkali-activated slag-based concrete and Portland cement-based concrete incorporating single and multiple hooked-end steel fibres

Abstract

In this study, three-point bending tests on notched beams according to EN 14651 have been performed to evaluate the flexural post-cracking behaviour of alkali-activated slag-based concrete (AASC) and Portland cement-based concrete (PCC) incorporating single (3D) and multiple (4D, 5D) hooked-end steel fibres in different volume fractions up to 0.75 %. According to the experimental results, the post-cracking residual flexural strength increases with the increase in the fibre volume fraction for each fibre and concrete matrix type. AASC mixes incorporating 3D and 4D fibres show higher values of residual flexural strength for the same crack opening than PCC mixes with the same fibre type and dosage. Only for the mixes incorporating 5D fibres, steel fibre-reinforced PCC (SFRPCC) mixes outperform steel fibre-reinforced AASC (SFRAASC) mixes in terms of post-cracking behaviour. According to EN 14651, the values of the residual strengths $f_{R1}$ and $f_{R3}$, corresponding to a crack mouth opening displacement (CMOD) of 0.5 mm and 2.5 mm, respectively, and their corresponding characteristic values $f_{R1k}$ and $f_{R3k}$, respectively, can be derived from the experimental load-CMOD curves. Following the fib Model Code 2020, each mix can then be classified according to the values of $f_{R1k}$ and the $f_{R3k}/f_{R1k}$ ratio. As a result, empirical models have been developed for SFRPCC to predict the values of $f_{R1}$ and $f_{R3}$ and the applicability of such models to SFRAASC is evaluated in this study. Once the values of $f_{R1k}$ and $f_{R3k}$ are known, tensile constitutive models can be derived according to the fib Model Code 2020 and used as input parameters for finite element modelling. In this study, the accuracy of the code-based constitutive model to predict the flexural behaviour of SFRAASC and SFRPCC is evaluated using the concrete damage plasticity (CDP) model available in ABAQUS. The numerical model based on the tensile stress-strain curve in the fib Model Code 2020 can qualitatively capture the post-cracking behaviour of SFRAASC and SFRPCC incorporating 3D, 4D and 5D at 0.25 % fibre volume fraction, despite overestimating their tensile strength. For higher fibre volume fractions, the CDP model, in conjunction with the mode I parameters derived from the fib Model Code 2020, is unable to adequately describe the post-hardening behaviour exhibited by the composites.