Effect of lightweight aggregate pre-wetting on frost-resistance of combined aggregate concrete

The frost-resistance of combined aggregate concrete prepared with different pre-wetting degrees of lightweight aggregate (LWA) and water-binder ratios at 28 d and 90 d was investigated. The microstructure of LWA and the interfacial zone were examined, and the effect of LWA pre-wetting on frost-resistance of concrete was analyzed. The results show that the porous air-dry LWA can play the role of "air-entraining", and the strong water absorbing effect can make the thickness of interfacial zone of about two times of that of prewetted LWA, as well as the densification of the interfacial paste. The frost-resistance of concrete prepared with air-dry LWA was superior. The air-entraining and water-absorbing effect of LWA both decreased with the increase of LWA pre-wetting degree, thus leading to the gradual decrease of concrete frost-resistance. However, the strong water-releasing effect of pre-wetted LWA in later age could improve the interfacial structure that decreased the volume content of harmful pores and increased the micro-hardness of the interfacial paste, which could favor the long term frost-resistance of concrete, especially for concrete with a lower water binder ratio. For concrete with a higher water-binder ratio, the high water content of pre-wetted LWA had a negative effect on the concrete frost-resistance, which may be due to the loose cement paste structure and frost-heave force. Even for the saturation pre-wetted LWA, the frost-resistance of combined aggregate concrete was better than that of ordinary concrete at a lower water binder ratio.