THE INFLUENCE OF SCM TYPE AND QUANTITY ON THE CRITICAL CHLORIDE CORROSION THRESHOLD

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Title: THE INFLUENCE OF SCM TYPE AND QUANTITY ON THE CRITICAL CHLORIDE CORROSION THRESHOLD

Author(s): David Trejo and Cody Tibbits

Publication: Special Publication

Volume: 308

Issue:

Appears on pages(s): 1-20

Keywords: Corrosion; Chloride transport; Diffusion coefficient; Diffusivity; Critical chloride corrosion threshold; Chloride; Fly ash; Slag; Portland cement

Date: 6/1/2016

Abstract:
Chloride–induced corrosion of embedded metals in reinforced concrete structures is dependent on the quantity of chlorides in the concrete material. Because of this, most ACI documents limit the amount of chlorides in the concrete for new reinforced concrete structures. Most documents in the United States limit the chlorides as a function of cement content, generally accepted to be the portland cement content. Significant changes have occurred in the cement and concrete industries, such as performance based specifications for cement (e.g. ASTM C1157) and the ubiquitous use of SCMs. The allowable chloride limits published by ACI Committee 222 specifically states that the allowable limits are based on portland cement content. ACI 318 specifies limits based on weight of cement but is not clear on what determines cement (portland cement only or all cementitious materials?). At issue is whether the allowable chloride limits reported by ACI and other organizations are applicable for mixtures containing SCMs and whether these limits should be based on portland cement content or total cementitious materials (cement + SCM) content. Significant research has been performed to assess the influence of SCMs on transport properties but limited research has been performed on assessing the influence of SCMs on the critical chloride corrosion threshold of steel in concrete. This paper reports on research investigating the influence of SCMs replacement types and quantities on the diffusivity and critical chloride corrosion threshold. Mortar mixtures containing fly ash had three levels of fly ash (0, 20, and 40%) and mortar mixtures containing slag had three levels of slag (0, 30, and 60%). The reinforcing steel for all specimens was conventional steel reinforcement meeting ASTM A615 specifications. Results indicate that the diffusivity of the mixtures containing SCMs was significantly lower than the mixture with portland cement as the only binder. The results also indicate that the critical chloride threshold levels for mixtures containing SCMs is significantly lower than the critical chloride threshold of mixtures containing only portland cement as the binder. An analysis of time to corrosion indicates that although the mean time to corrosion can be lower for systems containing SCMs, t-tests indicate that the null hypothesis (that the means of the time to corrosion of the systems are equal) cannot be rejected. This indicates that although there is a significant reduction (positive influence) in diffusivity when using SCMs, there is also a significant reduction (negative influence) in critical chloride threshold. The benefits of the reduction in the rate of diffusivity seems to be offset by the detrimental effects of SCMs on critical chloride threshold. Limits on allowable chlorides in concrete for new construction published by ACI 222 were based on concretes containing only portland cement as the binder. This research indicates that the ACI published limits may not be applicable to systems containing SCMs and that the limits are less conservative when basing the limits on total cementitious materials content.