Effect of Supplementary Cementitious Materials on Chloride Threshold and Corrosion Rate of Reinforcement

Supplementary cementitious materials (SCMs) are commonly used as a means of reducing cost, reducing environmental impact, or reducing permeability of concrete, but the current field of research has found mixed results in terms of the resulting time to corrosion initiation and corrosion rate of concrete containing SCMs. This paper examines the time to corrosion initiation, the water-soluble critical chloride corrosion threshold, and the corrosion rate after initiation for uncracked concrete specimens containing cementitious material consisting of 100% portland cement, mixtures with volume replacements of cement by 20% and 40% Class C fly ash, 20% and 40% Grade 100 slag cement, and 5% and 10% silica fume. Specimens had 1 in. (25 mm) concrete cover and a water-cementitious materials ratio (w/cm) of 0.45. Test results show that many specimens containing SCMs exhibited repassivation of the reinforcement after a “first” corrosion initiation. This “first” initiation occurred at chloride thresholds comparable to or lower than the chloride threshold for reinforcement in 100% portland-cement concrete. The reinforcement remained passive for varying lengths of time (from 3 to 50 weeks) before reinitiating. At reinitiation (“final” initiation), specimens with concrete containing SCMs exhibited times to corrosion initiation two to seven times that observed in specimens containing 100% portland cement and corrosion rates after initiation approximately an order of magnitude lower than that observed in specimens containing 100% portland cement. Increasing the amount of SCM generally lowered the corrosion rate after initiation. Chloride thresholds at final initiation for specimens containing fly ash or slag were 66 to 200% higher than that observed for specimens containing 100% portland cement. Chloride thresholds at final initiation for specimens containing silica fume were 40 to 60% higher those observed for specimens containing 100% portland cement