A study on the fracture mechanical properties and propagation mechanism of coarse aggregate-engineered cementitious composites under preloading

Although Engineered Cementitious Composite (ECC) materials exhibit high ductility, their engineering application is restricted by high shrinkage rates and costs; moreover, under the action of historical loads (e.g., initial static loads), ECC may experience damage before reaching their ultimate bearing capacities. To improve the safety and economic performance of ECC, Coarse Aggregate-ECC (CA-ECC) specimens with 4 different CA contents were prepared in this study, and the effects of 4 different preloading ratios were investigated in order to evaluate the fracture properties of CA-ECC under preloading and to clarify the underlying mechanism from both macro- and micro-perspectives. The results showed that ECC containing CA maintained excellent mechanical properties and deformability, and appropriate preloading can effectively mitigate the negative effects caused by CA. When the CA content was 30 % and the preloading ratio was 60 %, the Crack Mouth Opening Displacement (CMOD) of the specimen retained approximately 35.5 % of that of the original ECC specimen, which was about 30 times that of ordinary concrete. Then, combined with Digital Image Correlation (DIC) and microscopic testing methods, the interface layer between each phase medium and the matrix of CA-ECC was analyzed, and the crack initiation and propagation trends were revealed. Further, by comparing macroscopic properties and microstructural characteristics, the unique “fiber-wrapping-CA” effect in CA-ECC and the influence of preloading on the action mechanism of this effect were clarified. Overall, the findings provide theoretical support for the design and engineering application of ECC materials.