Influence of the colloidal nano silica on the bonding of new-to-old concrete interface

The quality of the interface between new and existing concrete is a crucial factor that significantly affects the effectiveness of concrete patching and rehabilitation. This transitional zone between concrete layers exhibits inherent porosity and diminished structural integrity, which can potentially serve as the origin of cracks and fractures. The presented study was conducted to evaluate the quality of the new-to-old concrete interface when colloidal nano silica (CNS) is used. Apart from directly blending CNS with concrete materials, this study uniquely employed CNS as a surface agent to enhance the quality of the interface. The microstructure, mechanical bonding performance and chloride resistance property were evaluated on samples with new-to-old interface. By microstructural analysis, it was found that the porosity at the interface region was reduced from 24% to less than 16% when CNS was directly added into the concrete. Due to the improved hydration and the formation of a denser and uniform interface, a higher bonding strength was therefore achieved. Improvement in bonding strength was also found when CNS was used as an agent on the surface of the old layer, due to the precipitation and pozzolanic reaction of the nano silica particles. There were no apparent signs of increased porosity at the interface region as the precipitation of nano silica particles densified the cement matrix. Additionally, the solid precipitates generated frictional force, increasing the bonding strength under shearing loading. However, due to the lack of cohesion, the nano silica precipitates couldn't sustain the tension under flexural test. Durability evaluation showed that the chloride resistance of the double-layer sample was effectively enhanced as the quality of the interface was improved with the use of CNS, especially when the surface method was used. The outcome of this study provides insight into the use of low CNS dosage to improve the new-to-old concrete interface and sheds light on the use of CNS as an interface agent.