Effects of Different Fiber Dosages of PVA and Glass Fibers on the Interfacial Properties of Lightweight Concrete with Engineered Cementitious Composite

The bond strength at the interface zone between two concrete sections plays a critical role in enhancing long-term durability, ensuring that both materials perform homogenously. Ensuring compatibility at the interfaces between repair and concrete materials is one of the most challenging aspects of constructing composite systems. Despite various studies, a comprehensive understanding of the engineered cementitious composite (ECC) bonding mechanism at the repair interface is still limited. The objective of this research is to identify the interfacial properties between lightweight concrete (LWC) and engineered cementitious composite (ECC) with varying fiber dosages of polyvinyl alcohol (PVA) and glass fibers under different surface roughness conditions. The study tested LWC-ECC specimens in direct shear using slant shear and bi-surface shear tests, recording the maximum shear stress at failure. Two grades of LWC—normal-strength lightweight concrete (NSLW) and high-strength lightweight concrete (HSLW)—were used as substrates, while the ECC overlays contained varying fiber dosages: 2% PVA, 1.5% PVA with 0.5% glass, 1.0% PVA with 1.0% glass, and 0.5% PVA with 1.5% glass. The surface conditions considered included grooved and as-cast substrates. The results indicated that the highest bond strength was achieved by specimens with 1.5% PVA and 0.5% glass fiber, with a maximum shear strength of 24.05 MPa for grooved HSLW substrates. Interface roughness had minimal impact on shear strength for NSLW substrates but significantly affected HSLW substrates, with bond strengths varying from 13.81 MPa to 24.05 MPa for grooved surfaces. This study demonstrates the critical role of fiber dosage and surface roughness in enhancing the bond performance of composite materials.