Effectiveness of treated flax fibers in improving the early age behavior of high-performance concrete

In recent years, ecoefficiency incentives have recently attracted growing research attention towards the valorization of natural fibers in cement systems. Specific attractive features of natural fibers include cost-effectiveness, abundance, renewability, and non-toxicity while providing several functionalities to concrete such as volumetric stability, strength enhancement, and crack control. This study is aimed at leveraging the hydrophilicity, hygroscopicity, and bridging capacity of flax fibers to design high-performance concrete (HPC) with controlled autogenous shrinkage. The effect of raw fibers and that of alkali-treated fibers on autogenous shrinkage of HPC and its fresh and mechanical properties were investigated and compared to the effect of shrinkage reducing admixture (SRA) and expanded shale lightweight aggregate (LWA). Results show that while SRA was found efficient in reducing autogenous shrinkage at 7 days (64% lower), this option adversely affected mixture flowability and mechanical performance (particularly flexural capacity). Similarly, LWA reduced autogenous shrinkage (by 36%) but without a noticeable effect on mechanical performance. Interestingly, the use of alkali-treated fibers allowed obtaining an HPC mixture with reduced autogenous shrinkage (26% lower) while maintaining improved flowability, compressive strength, and flexural capacity (up to 17% increase). Therefore, alkali-treated flax fibers can be viewed as a viable tool for mitigating autogenous shrinkage, thereby offering a green alternative promoting the sustainability of the built environment.