Crack morphology tailoring and permeability prediction of polyvinyl alcohol -steel hybrid fiber engineered cementitious composites

The intrinsic tensile strain-hardening properties of engineered cementitious composites (ECC) determines its multiple cracking and crack control characteristics. The relatively low permeability of cracked ECC is of special significance for more efficient use of cementitious materials, which contributes to the mitigation of carbon emissions. The crack morphology of ECC plays an important role in tailoring the permeability of cracked ECC, as well as the durability of structures made or strengthened by ECC. In the present study, the characterization of crack morphology is conducted by digital image processing (DIP) method, which mainly involves the grey processing, background elimination, binarization and pixel column labeling of digital image. Crack morphologies of ECC are modified by blending PVA fibers with a volume content of 2.0% and steel fibers with volume contents of 0.0%, 0.3%, and 0.6%. A permeability model for cracked hybrid fiber ECC (HyECC) has been proposed based on the Weibull distribution of crack width and crack number. The results indicate that the addition of steel fibers in ECC optimizes the crack morphology, yielding more tortuous and finer cracks. The average crack width decreases from 67.3-115.8 μm in mono fiber system to 46.0-67.6 μm in hybrid system with steel fiber content of 0.6%. The average crack width and crack number increase with the tensile strain, while the average crack spacing behaves in an opposite manner. The evolution of crack morphology could be described by Weibull distribution as a function of tensile strain. The permeability model derived from the crack morphology and the modified Hagen-Poiseuille law well predicts the permeability of cracked HyECC. It reveals that the addition of steel fiber enhances the impermeability of cracked HyECC, especially at large strains. The flow rate at tensile strain of 1.5% is reduced from 7.75 × 10−7 m3/s to 4.82 × 10−9 m3/s as the steel fiber content increases from 0.0% to 0.6%. The findings are expected to support the material-efficient design, crack control and durability improvement of ECC in structural applications.