Effect of shrinkage reducing admixture on new-to-old concrete interface

New-to-old concrete interfaces can be widely seen in the process of repairing or strengthening concrete structures, such as concrete jacketing and concrete overlay or repair mortar (high performance fiber reinforced one included etc.) or self-consolidated concrete or self-consolidated mortar. One major difference between new and old concrete is that the shrinkage of new concrete is much higher than that of old concrete during its hardening process. This difference in volume change results in the incompatibility between old concrete and new concrete (as strengthening and repair material). Moreover, prior research studies in this area indicate that such incompatibility between new and old concrete can result in the development of stress concentration at the interface, leading to cracks and premature failure of the repair overlay. The cracks provide access for free water carrying chloride ions or carbon dioxide, which further reduces the durability of a new-to-old concrete system. The usage of shrinkage reducing admixture (SRA) in new concrete design can reduce the shrinkage of new concrete by reducing the surface tension of water. Moreover, it is reported that SRA can reduce the water diffusivity in concrete in order to achieve an enhanced durability. To study the effect of SRA on the interfacial integrity of a new-to-old concrete system, an experimental study was first conducted to investigate the interfacial fracture toughness of the new-to-old concrete system. It was found that the interfacial fracture toughness could be increased by adopting SRA in the design mix of new concrete. Moreover, when the samples were exposed to moist conditions, the decreasing rate of interfacial fracture toughness was lower when SRA was adopted in the design mix, which indicates that the new-to-old system is more durable in moist condition with SRA. To explore water transition behavior at the new-to-old concrete interface, molecular dynamics simulations have been performed. Based on the results from molecular dynamics simulations, it is revealed that the use of SRA can reduce the water diffusion coefficient at pores around new-to-old interface, so that a more durable performance can be achieved when the system is under moisture attack. The finding provides insightful information on the role of SRA in the interfacial integrity and durability of the new-to-old concrete system and may cover strengthening jackets and repair mortars.