Effect of Different Humidity-Controlling Modes on Microstructure and Compressive Behavior of Ordinary Concrete

Abstract

Concretes at different saturation degrees are needed to be prepared in the laboratory when studying the effects of humidity on its mechanical properties. Different humidity control methods would produce different results. The present work investigates the changes in compressive behavior and microstructure of the ordinary concrete caused by two different humidity-controlling modes: (1) the isothermal drying humidity-controlling mode and (2) the rewetting humidity-controlling mode. The compressive behaviors of concrete in different humidity states are compared, and the microstructure is studied in detail using scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP). The results indicate that drying results in an increase of compressive strength, whereas rewetting results in a significant decrease. Both drying and rewetting humidity-controlling modes cause some changes in the microstructure of concrete. Drying results in calcium silicate hydrate (C-S-H) globule densification and makes cement matrix denser, increases porosity, and coarsens the pore size distribution. Rewetting makes porosity decrease and the pore size distribution slightly refined, as well as causes serious microcracking. The differences in compressive behavior of concrete is related to the shrinkage or swelling, the change of pore size distribution, and the microcracking caused by two different humidity-controlling modes. The isothermal drying humidity-controlling mode is suggested as the more reasonable mode for concrete because it is more effective in controlling saturation degree, takes less experiment time, and causes less effect on microstructure and the compressive strength of concrete.