Civil Engineering Association

The primary purpose of this work was to assess the effect of air entrainment and time of surface finishing operations on the frost durability and scaling resistance of high-strength concrete. The conditions under which entrained air is necessary to produce a frost-resistant mixture are explored, particularly in light of current ACI 318 provisions for air content. The laboratory program consisted of the production of six concrete mixtures with water/cement ratios of 0.50, 0.45, 0.40, 0.35, 0.30, and 0.25; each at three levels of air content: non-air entrained, 4%, and 6%. No supplementary cementing materials were used. Frost resistance was investigated as a bulk or interior concrete property, via modified ASTM C 666, and as a surface property, via ASTM C 672. Both tests were initiated at 28 days with the same curing conditions applied to the specimens. The influence of time of surface finishing on the scaling resistance was investigated by finishing the scaling specimens at two different times relative to the time of initial set as defined by ASTM C 403. Additionally for each mixture, compressive strength (ASTM C 39), rapid chloride permeability (ASTM C 1202), and microscopic analysis of the air void system (ASTM C 457) were performed. For the mixtures investigated here, it was possible to obtain frost resistance based on the modified ASTM C 666 without air entrainment for w/c = 0.35 or less, while entrained air was necessary for mixtures with w/c greater than 0.40. As far as scaling resistance is concerned, no air entrainment was necessary for mixtures with w/c of 0.25, while entrained air was necessary for mixtures with w/c greater than 0.25. It was observed that the ACI 318 provisions for frost durability are somewhat conservative. While ACI 318 requires air entrainment for all mixtures subject to freezing and thawing, mixtures studied here with w/c of 0.25 and no intentionally entrained air were shown to be frost resistant. Further, properly air-entrained mixtures with w/c of 0.50 were frost resistant, even though the w/c was in excess of the 0.45 required by the ACI 318 provisions
for freeze-thaw durability.