The case for air-entrainment in high-performance concrete

摘要

Freezing-thawing cycles are a major cause of degradation of concrete structures in cold climates. The use of air-entraining admixtures to induce an adequate network of closely spaced air bubbles in concrete has been proven to dramatically decrease its susceptibility to frost action. However, with the advent of high-strength high-performance concrete (HPC), the need for air-entrainment in such concrete has become a controversial issue since the 1980s. Some researchers advocate using air-entrainment in HPC, while others argue that this practice is unnecessary owing to the very low porosity and amount of freezable water in HPC. In this paper, an extensive review of the available literature indicates that there is substantial scatter and irreproducibility of data regarding this issue. There is also evidence of vulnerability of HPC to both micro and macrocracking in service and a demonstrated reduction in performance under combined ageing mechanisms. This could make non-air-entrained HPC vulnerable to freezing-thawing degradation even though such vulnerability may not be detected using classical frost durability tests. Moreover, there are practical limitations to the use of very low water/cement ratio concrete in field conditions, which make the production of HPC with negligible freezable water content difficult to achieve in full-scale structures. This paper presents evidence that supports the argument that the use of air-entrainment in HPC is a practice that provides enhanced safety, reliability and serviceability, and is therefore recommended for concrete practitioners.