Introduction
Fiber-Reinforced Cement-based Materials (FRCM) have excellent toughness and impermeability, and have a broad application in the repair of deteriorated concrete structures and the reinforcement of fractured rock mass. The bonding performance between the cement-based material and the substrate is affected by many factors, including the mix ratio of cement-based materials, the saturation of substrates, and the surface roughness of substrates. The initial water content of substrates affects the water exchange between substrates and fresh cement-based repair materials, further affects the water-binder ratio of repair materials, the hydration of cementitious materials and the shrinkage deformation of materials, etc., and ultimately affects the mechanical properties of the interface. Further research is needed on the effect of water saturation of different types of substrates on the interfacial bonding properties between them and FRCM with different mixing ratios. Appropriately increasing the surface roughness of old cement-based materials and rock substrates is an effective method to improve the bonding performance between them and cement-based materials. Compared with ordinary mortar, the content of fly ash and other auxiliary cementitious materials in FRCM is higher, and the toughening effect of fiber is remarkable. The bonding performance between FRCM and mortar substrates with different surface roughness is worthy of further study. Cement-based repair materials are generally poured on the surface of existing substrates, and the impermeability is crucial to the long-term effectiveness of repair and reinforcement. The water transport properties of FRCM bonded with different types of substrates with various initial water content are worthy of further study. In this work, the effects of fiber type, fly ash content, initial water saturation of substrates, and grooving form of the substrate surface on the interfacial shear strength between FRCM and ordinary cement mortar, concrete, granite, and sandstone were systematically studied through mechanical performance tests. The capillary water absorption properties of FRCM bonded to different types of substrates with different initial water saturations were studied by weighing method.
Methods
The dynamic quality monitoring method was used to control the saturation of the substrate to 0%, 50% and 100%, and an angle grinder was used to cut different forms of grooves on the surface of mortar substrates. FRCM was prepared with cement, fly ash, quartz sand, water, superplasticizer and fibers. The fiber types included PVA fiber and basalt fiber, and the fly ash content was 1.2 times and 1.8 times of the cement mass, respectively. After reaching the curing age of 28 d, three specimens were selected for interfacial shear strength test. Three 28 d old bonding composites in each group were subjected to capillary water absorption test.
Results and discussion
The increase in the saturation of the mortar substrate was not conducive to its adhesion to basalt or PVA FRCM. It’s speculated that the above phenomenon was related to the water exchange between repair materials and substrates. When the substrate was dry, the water in the fresh repair material can carry cement particles to the substrate under the action of capillary suction. The water exchange across the interface was beneficial to the adhesion of repair materials to substrates, and at the same time the decrease of water-binder ratio of the repair material at the interface improved the interfacial shear strength. The increase of fly ash content enhanced the interfacial shear strength between PVA FRCM and substrates, but lead to the decrease of interfacial shear strength between basalt FRCM and substrates. Due to the pozzolanic effect and filling effect of fly ash, the increase of fly ash content in FRCM was conducive to the formation of denser microstructure, which was beneficial to its bonding with substrates. However, the improvement of fly ash on the toughness of cement-based materials can be affected by its synergistic effect with fibers. When the mechanical properties and content of fibers were low, excessive fly ash can lead to the reduction of material toughness. In the case of dry substrates, the increase of the number of grooves on the substrate surface has a more significant effect on the interfacial shear strength between PVA FRCM than that of basalt FRCM, which may be related to the excellent mechanical properties of PVA FRCM.The increase of fly ash content can significantly improve the capillary water absorption properties of basalt or PVA FRCM bonded to mortar substrates, while the change of substrate saturation has insignificant effect on the capillary water absorption properties of basalt FRCM bonded to mortar substrates. When the mass ratio of fly ash to cement in the repair material was 1.2 and the substrate saturation was 0%, 50% and 100%, the capillary water absorption coefficient of basalt FRCM was significantly lower than that of PVA FRCM, which may be related to the higher porosity of PVA fiber.
Conclusions
The interfacial shear strength between PVA and mortar substrates, or basalt FRCM and mortar substrates decreased with the increase of substrates saturation. When the saturation of mortar substrates was constant, the increase of fly ash content enhanced the interfacial shear strength between PVA FRCM and substrates, but reduced the interfacial shear strength between basalt FRCM and substrates. The more the number of grooves on the substrate surface, the higher the interfacial shear strength between it and PVA or basalt FRCM. This phenomenon was more obvious for PVA FRCM. The effect of the grooves on the surface of dry substrates on the improvement of the interfacial shear strength between dry substrates and the basalt or PVA FRCM was more obvious than that of saturated substrate. The interfacial shear strength between PVA FRCM and concrete and sandstone substrates decreased with the increase of substrates saturation. When the substrates were dry, the interfacial shear strength between PVA FRCM and concrete was higher than that of mortar, sandstone and granite. When the substrates were partially saturated, the interfacial shear strength between PVA FRCM and concrete and mortar was equivalent or higher than that of sandstone. When the substrates were saturated, the interfacial shear strength between PVA FRCM and mortar was higher than that of concrete and sandstone. The capillary water absorption coefficient of FRCM bonded to dry mortar, concrete and sandstone substrates was generally lower than that of the same type of FRCM bonded to pre-wetted substrates. There was no significant difference in the capillary water absorption coefficient between PVA FRCM with the same mix ratio bonded to different types of substrates. The increase of fly ash content can significantly increase the capillary water absorption coefficient of FRCM bonded to substrates. When the content of fly ash was low, the capillary water absorption coefficient of PVA FRCM was significantly higher than that of basalt FRCM bonded to mortar substrate with same saturation.