Investigation on the mechanical and shrinkage performance of UHP-ECC for existing brick masonry wall reinforcement

Ultra-high performance engineered cementitious composite (UHP-ECC) has shown great promise as a reinforcement material for existing brick masonry structures due to its superior mechanical properties and crack resistance. However, the extremely low water-binder ratio and high cementitious content in UHP-ECC lead to significant shrinkage, potentially compromising durability and structural integrity. In this paper, the mechanical properties and shrinkage performance of UHP-ECC prepared with Polyoxymethylene (POM) fiber for the reinforcement of brick masonry walls were explored through the mechanical test, as well as the autogenous and restrained shrinkage measurements. The results show that reducing the water-binder ratio can enhance the mechanical properties of UHP-ECC, while the superplasticizer dosage has minimal effect. The optimal limestone powder and POM fiber contents are 35 % and 2.5 %, respectively. The shrinkage strain of UHP-ECC decreases with higher water-binder ratios, limestone powder content, and POM fiber volume, but is less affected by superplasticizer. Specially, the water-binder ratio most significantly affects UHP-ECC shrinkage, with specimens at 0.18 showing 62 % lower shrinkage than those at 0.15. Internal temperature and humidity also influence shrinkage, which exhibits a quadratic polynomial relationship with compressive strength. Under masonry constraint, with the number, spacing and depth of masonry mortar joints focused on, the shrinkage of UHP-ECC decreases with fewer mortar joints, larger joint spacing, and greater joint depth. Conversely, the interfacial stress exhibits the opposite trend, but remains below the tensile strength of UHP-ECC, indicating low cracking risk. These findings provide practical guidance for optimizing UHP-ECC mixtures and joint designs in masonry wall reinforcement applications. • The shrinkage development of UHP-ECC can be effectively inhibited by increasing the water-binder ratio, limestone powder content, POM fiber content, and UHP-ECC layer thickness. • A significant correlation between shrinkage and compressive strength is accurately represented by a quadratic polynomial model well-aligned with experimental results. • The shrinkage degree diminishes with fewer mortar joints, increased spacing between joints, and greater joint depth in the brick masonry • The analyzed shrinkage-induced constrained interfacial stresses between UHP-ECC and bricks increase with greater joint depth, larger joint spacing, and fewer joints number, yet remain below the tensile strength of UHP-ECC.