Air void system and freezing-thawing resistance of concrete composite with the incorporation of thermo-expansive polymeric microspheres

Air-entraining agent (AEA) was first used in concrete in 1930's to improve freezing - thawing resistance of concrete. After about 90 years of experience, many debates have been raised regarding the benefit of durability improvement using AEA in concrete. Under this circumstance, polymeric microspheres show promising ability to replace the AEA with the purpose of entraining air in concrete. In this study, thermo-expansive polymeric microspheres were added to concrete. As a comparison, concrete specimens with the addition of AEA were also prepared. The air contents of freshly mixed concrete and hardened concrete were characterized. In addition, compressive strength development was measured. Moreover, the microstructure and air-void system of concrete was characterized by multiple methods. Finally, the dynamic modulus of elasticity and scaling of concrete beams after the cyclic freezing-thawing attack were evaluated. It was found that compared with the addition of AEA, adding polymeric microspheres entrained comparable air content in concrete. The void size distribution of polymeric microspheres mixed concrete is finer than that of AEA mixed concrete, which provides comparable freezing-thawing resistance of concrete in the viewpoint of dynamic modulus. However, due to the potential "internal curing" and nucleation effects with the presence of microspheres, the strength of microsphere-mixed concrete is slightly higher than that of plain concrete, and freezing-thawing caused surface scaling of concrete mixed with microspheres is much less severe than that incorporated with AEA.