Residual flexural strength of SFRC: a multivariate perspective

The main contribution of steel fibres to the hardened state performance of steel fibre-reinforced concrete (SFRC) is the residual flexural strength the material exhibits, which is commonly characterised by the residual flexural strength parameters (fR1, fR2, fR3, and fR4) as defined by EN 14651. A database of values of residual strength parameters corresponding to hundreds of prismatic specimens from different SFRC mix designs has been put together from previously published papers. Multiple linear regression has been applied to derive a model which relates these parameters to the steel fibres aspect ratio, length and volume fraction as well as the relative amounts of the SFRC mix constituents. The model obtained presents a very good fit to the data collected, and its relatively simple specification makes it a promising tool to optimise SFRC mix designs from the point of view of residual flexural strength. The effect of fibre dosage and dimensions and that of their interactions with other mix design parameters such as water, cement, or aggregate contents are analysed by means of response surface plots representing the average trends reproduced by the model. These modelling and analysis efforts are part of an ongoing study, and this paper focuses on the residual flexural strength parameters fR1 and fR3. In relation to the dimensions of the fibres, the effect of fibre length on residual flexural strength has been found to be comparable to that of fibre volume fraction. This, together with the sensitivity of residual flexural strength to the fibre aspect ratio, leads to the conclusion that it is not necessary to use steel fibres in high dosages to proportion SFRC mixes with better-than-average levels of residual flexural strength. The key points emerging from the interpretation of the proposed model are presented and discussed in the context of the wide range of SFRC mixes represented by the database it is based upon.