Salt-frost damage characterization of simulant multiple ITZs in nano-silica modified recycled aggregate concrete

The salt-frost damage characterization of multiple interfacial transition zones (ITZs) is one of the major concerns for further exploring the durability-related properties of recycled aggregate concrete (RAC) exposed to cold region. The actual service state of RAC structures is often coupled with multiple environments, leading to more complex freeze-thaw damage mechanisms in ITZs. This study adopted the simulant multiple ITZs of RAC specimens to investigate the damage evolution behavior of ITZs performance under the coupling actions of salt-frost cycles (SFT) and sustained compression loads. Effect of nano-silica (NS) and air-entraining agent (AE) on the salt-frost resistance strengthening for ITZs inside RAC was further explored. The apparent morphology and the relative dynamic elastic modulus (RDEM) of modified simulant multiple ITZs of RAC were measured to characterize the salt frost damage, and further the microstructural analysis including microhardness, laser scanning microscope (LSM) and SEM observation was conducted, aiming at revealing the salt-frost damage mechanisms of simulant modified ITZs inside RAC. The results indicate that the appropriate dosage of NS and AE could effectively reduce the salt-frost scaling of RAC by strengthening the internal microstructures. The RDEM of NS-modified RAC samples incorporating with AE was always higher than NS-modified RAC and non-modified RAC. After NS-modified, the minimum microhardness of old ITZs was increased by 10.2 % and the thickness was averagely reduced by about 10μm. The microhardness loss of modified RAC after SFT was relatively smaller. On the contrary, applying loading with higher compressive stress level markedly accelerates the salt-frost damage process of non-modified and modified ITZs. The results of SEM and LSM tests showed there is less salt-frost damage under sustained compression loads due to the denser microstructures of ITZs after synergistic modification by NS and AE. This is demonstrated by stronger bonding properties between original aggregate and old mortar and less elevation differences at old/new ITZs. Above all, this study explores the evolving trends of salt-frost damage and microstructure for simulant ITZs inside RAC under the coupling actions of sustained compression loads and SFT, which can provide some basis for the durability assessment of RAC structures exposed to cold region.