FREEZE-THAW RESISTANCE OF CONCRETES SUBJECTED TO DRYING AT EARLY AGES

Abstract

Freeze-thaw expansion of concretes is mainly controlled by air content rather than type of cement or drying initiation ages. However with a 4.5% of air content, considerable expansion at 300 freeze-thaw cycles, leading to a fracture, may occur when subjected to drying at early ages depending the type of cement and water-cement ratios. Use of blended cement with a high water-cement ratio should be accompanied by sufficient moist curing at early ages. Difference in pore volume at the radius ranging from 1,800 to 10,000 A was found between damaged and undamaged concretes during freeze-thaw test. These pore regions increased considerably with drying at early ages. Freeze-thaw scaling increased when a drying initiation age became earlier. Scaling resistance can be increased by a moist curing immediately after concrete placement, but a decrease in water-cement ratio was still necessary when blended cement was used. Amount of scaling was highly correlated to a pore size distribution within 1 cm from the surface, especially to a pore volume more than 560 A in the radius. Pore volume of this region is likely to increase when subjected to dying at early ages. Degradation due to scaling can be evaluated by compressive strengths at drying initiation ages and a scaling degradation grade was proposed. Moist curing period necessary for assuring scaling resistance was given.