Long-Term Performance of Steel Reinforcing Bars in Portland Cement Concrete Incorporating Moderate and High Volumes of ASTM Class F Fly Ash

Abstract

This paper reports the long-term performance of steel reinforcement in portland cement concrete and concrete incorporating moderate and high volumes of American Society for Testing and Materials (ASTM) Class F fly ash. In 1989, three fly ash concrete mixtures and three control portland cement concrete mixtures were made in Halifax, Canada, and concrete slabs 1,220-by-1,220-by-150 mm thick were cast. Steel reinforcing bars were placed in the slabs with concrete covers of 20 mm, 40 mm, 60 mm, and 80 mm. After 7 days of moist curing and 21 days of air drying, the slabs were ponded with 4% calcium chloride solution. In 1997, 8 years after ponding, half-cell potential measurements were taken to determine the probability of corrosion of the embedded reinforcing bars. Cores were drilled from the slabs for the determination of the compressive strength, resistance of the concrete to chloride ion penetration in accordance with ASTM C 1202, and chloride content at various depths in the slabs. The reinforcing bars were then removed from the slabs for visual inspection. Notwithstanding the results of various tests performed and the relative performance of concrete with or without fly ash after 8 years of exposure to 4% chloride solution, there were no visible signs of any corrosion products on those portions of the steel reinforcing bars that were embedded in the concrete. The only exceptions were the bars with a cover of 20 mm in Slab 5 (25% fly ash, water-cementitious materials ratio = 0.57), and with covers of 20 mm and 40 mm in control concrete Slab 6 (water-cement ratio = 0.57) that showed some evidence of corrosion. In general, the concrete incorporating moderate and high volumes of fly ash showed superior resistance to the penetration of chloride ions compared to the control concrete.