An intentionally entrained air void system is a well-known and widely used strategy to improve the durability of conventionally vibrated concrete (CVC) exposed to freeze-thaw cycles. But what about using air-entrainment in roller-compacted concrete (RCC)?
Conventionally vibrated concrete is typically batched and mixed by mechanical methods. RCC is commonly made in a pug mill which mixes much faster than typical CVC batch plants. The RCC passes through a pug mill mixing chamber in a matter of seconds. Is that long enough to establish a quality network of entrained air bubbles?
Chemical admixtures entrain air in concrete using surfactant technology. Air-entraining admixtures are added to the concrete mixture immediately before or during the mixing process and the mechanical action during mixing causes the generation and dispersion of microscopic air bubbles throughout the concrete. Entrained air is differentiated from entrapped air by size and spacing. Entrained air voids are much smaller (10 to 1000 μm versus 1000 μm or larger), are not interconnected, and are well dispersed and randomly distributed in hardened concrete. Entrapped air does not provide freeze-thaw resistance.
In the early days of RCC dam construction, low cementitious RCC (LCRCC) with high (or no) vebe times were common. The stiff nature and low paste content of LCRCC made it virtually impossible to use an air-entraining admixture to improve freeze thaw performance. However, current trends of RCC construction tend to use high cementitious RCC (HCRCC) with lower vebe times and higher paste contents where air entrainment with admixtures appears to be possible.
Of the over 350 RCC projects completed in the United States, only a few have used an air entrainment admixture. Two recently completed projects were constructed using air-entrained internally vibrated RCC (IVRCC) which were HCRCC mixtures with Vebe times under 10 seconds capable of being compacted with vibratory rollers or consolidated with internal vibratory equipment.
This presentation will review the history of the use of RCC in freeze-thaw environments and examine how well the RCC has performed. Research performed on recent projects that used air-entrained RCC will be presented that compares air-entrained RCC test results to that of air-entrained CVC.
