The role of rheology control in achieving consistent and improved flexural performance of engineered cementitious composites (ECC)

Achieving reliable flexural performance and consistency in engineered cementitious composites (ECC) remains challenging due to the complex interplay among material composition, rheology, and fiber dispersion. This study investigates within-batch and batch-to-batch variations in the flexural behavior of two ECC mixtures with fly ash-to-cement ratios of 1.2 and 2.2 by mass to assess the influence of fly ash content on performance reproducibility. Building on improved consistency observed in the higher fly ash mixture, rheological control was further explored across flowability levels represented by Marsh cone flow times from 30 s to 4 min, used as indicators of plastic viscosity. The results show that flowability strongly influences flexural response and reproducibility. Highly flowable mixtures (≈30 s) achieved large deflections but exhibited greater variability among specimens, whereas mixtures with longer flow times (≈4 min) demonstrated enhanced flexural performance and consistency. The findings suggest that proper fiber incorporation is crucial for achieving consistent ECC performance. In highly flowable mortars, sufficient mixing shear and fiber pre-dispersion promote uniform fiber distribution, whereas lower-flowability mixtures benefit from internal shear during mixing, reducing the need for pre-dispersion. Overall, this study provides guidance for optimizing rheology to promote uniform fiber dispersion and enhance flexural reliability in ECC systems.