| Concrete has been widely used in various engineering constructions due to its excellent mechanical properties and good economy since its inception.However,under the action of loads,environment,and other factors,concrete will deteriorate to varying degrees after a certain number of years of service,seriously affecting the safety and service life of the structure.Placing fresh cement-based repair materials on existing substrates is a commonly used repair and reinforcement measure for engineering structures.When there is a capillary pressure difference between the fresh cement-based repair material and the substrate,water exchange across the interface occurs between them,which can change the mix ratio of the cement-based repair material,and affect its hydration process,microstructure,permeability,and bonding performance between the repair material and the substrate.Based on the above considerations,the specific work and main research conclusions of this article are as follows:(1)The interfacial bond strength between ordinary mortar,fibre-reinforced cementitious materials and mortar substrates with different saturation levels was investigated using direct shear tests.The interfacial microstructure was also characterized with the aid of backscattered scanning electron microscopy.The results showed that when ordinary mortar was used as the repair material,the interfacial shear strength gradually decreased as the water saturation of the old mortar increased.When Strain Hardening Cementitious Composites(SHCC)were used as repair materials,the interfacial shear strength decreased as the water saturation of the old mortar varied in the order of 0%,30%,100% and 70%,and the incorporation of silane reduced the interfacial shear strength between the SHCC and the existing substrates.When Ultra-High Performance Concrete(UHPC)was used as the repair material,the interfacial shear strength gradually increased as the water saturation of the old mortar increased.The replacement rate of lightweight aggregate for quartz sand affected the interfacial shear strength between UHPC and substrates.The interfacial shear strength tended to increase and then decreased as the replacement rate of lightweight aggregate gradually increased from 0% to 30%.It was clear from the backscattered electron microscopy images that the bonding interface microstructure was denser when ordinary mortar and SHCC were used as repair materials with and water saturation of the substrate was 0%.When UHPC was used as the repair material and the water saturation of the substrate was 0%,severe dry shrinkage cracking occurred at the interface and an increase in the lightweight aggregate replacement rate mitigate the dry shrinkage cracking at the interface.(2)The capillary water absorption of repair materials bonded to old mortar substrates with different water saturations was investigated using the weighing method.The results showed that as the water saturation of the substrate increased,the capillary absorption coefficient of the bonded ordinary mortar gradually increased,the capillary absorption coefficient of bonded SHCC varied less,and the capillary absorption coefficient of bonded UHPC gradually increased.The incorporation of silane reduced the capillary water absorption rate of SHCC bonded to old mortar.The capillary absorption coefficient of UHPC bonded to old mortar gradually increased as the lightweight aggregate replacement rate increased.In addition,the pore structure of repair materials bonded to old mortars with different water saturation levels was analyzed with the aid of low-field nuclear magnetic resonance techniques and mercury-intrusion method.The results showed that when ordinary mortar was used as a repair material and the water saturation of the substrates increased,the porosity and small pore volume fraction gradually increased and the volume fraction of medium and large pores gradually decreased,and the capillary absorption coefficient of mortar was positively correlated with its small pore volume fraction.When SHCC was used as a repair material,the porosity and the volume fraction of mesopores and macropores gradually decreased as the water saturation of the substrates increased,with smaller changes in the volume fraction of small pores.Silane incorporation has a significant effect on the pore structure of SHCC bonded to the substrates.When the substrates was dry,silane incorporation increased the volume percentage of macropores in SHCC.When the substrates was saturated,silane incorporation increased the porosity of SHCC as well as the volume percentage of capillary volume.For UHPC,the porosity gradually increased as the water saturation of the substrates increases.(3)The water exchange process between fresh repair materials and substrates with different water saturation levels was investigated using the weighing method.Significant water exchange occurred when the fresh repair material was bonded to the mortar substrates.The unsaturated substrates can absorb water from the bonded fresh repair material,and the mass of water absorbed per unit area of the substrates mortar was linearly related to the square root of time.The rate of water absorption increased as the saturation of the substrates mortar decreased.For ordinary mortars,the absorption of water by the substrates from the fresh mortar increased the shear strength of the composite and the impermeability of the repair mortar after standard curing,but also increased the proportion of large pores in the repair mortar.For SHCC,the absorption of water from the fresh mortar by the substrates increased the shear strength of the composite and the percentage of large pores in SHCC after standard curing,but the water exchange had less effect on its impermeability.For UHPC,when the substrates was dry,it can absorb a small amount of water from the bonded fresh UHPC,leading to micro-cracks at the interface of the composite after standard curing,which in turn reduced the bond strength.When the substrates was saturated,a small amount of water in the substrates can flow into the fresh UHPC,facilitating its full hydration and increasing the shear strength of the composite.In addition,the incorporation of pre-wetting lightweight aggregates increased the amount of water exchange between the fresh UHPC and the substrate as well as the water content in the UHPC slurry,which was beneficial for the bond strength of the UHPC to the substrate within certain limits. |
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