Size effect and mechanism-based multiphase modeling of strain-hardening behavior of Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC)

In order to facilitate structural applications, a systematic and rational assessment of the size effect on the tensile performance of Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) is of crucial significance. To achieve this goal, direct tension tests were conducted on dumbbell-shaped UHS-ECC specimens of three different thicknesses (i.e., 10, 20, and 30 mm). Three different lengths (i.e., 6, 12, and 18 mm) of polyethylene (PE) fibers were also studied. As the specimen thickness increased, the tensile strength of UHS-ECC decreased, while the tensile strain capacity was unaffected. Increasing the fiber length from 6 mm to 12 mm led to significant enhancements of both the tensile strength and strain capacity of UHS-ECC, but further increasing the fiber length from 12 mm to 18 mm only slightly improved the tensile strain capacity with the tensile strength nearly unchanged. Mechanism-based finite element (MMFE) modeling was conducted to interpret experimental observations and elucidate the underlying physical mechanisms. The numerical analysis demonstrated that the experimental results could be accurately predicted by appropriately selecting fiber orientation distributions, indicating that the size effect and fiber length effect were strongly, if not fully, associated with the fiber orientation distributions. Specifically, thicker specimens were related to higher fiber orientations, which was consistent with the wall effect. Longer fibers were correlated with higher fiber orientations, probably due to the tendency of longer PE fibers to become folded during the mixing process. These folded fibers exhibited a smaller projected length in the tensile direction and reduced crack bridging capacity, resulting in effects similar to increased fiber orientation. The findings in this study provide a fundamental basis for optimizing fiber length and specimen thickness to mitigate size effects and to enhance the tensile performance of UHS-ECC in structural applications. © 2025 Published by Elsevier Ltd.