EFFECTS OF DEICING SALTS ON THE DURABILITY OF CONCRETE INCORPORATING SUPPLEMENTARY CEMENTITIOUS MATERIALS

The durability of concrete mixtures incorporating one of two supplementary cementitious materials (SCMs), slag cement or Class C fly ash, exposed to sodium chloride (NaCl), calcium chloride (CaCl2), or magnesium chloride (MgCl2) is evaluated based on damage due to wetting and drying and scaling, with the goal of determining appropriate replacement levels of portland cement using these SCMs. The mixtures had water-to-cementitious material ratios of 0.38 and 0.44 and SCM percentage replacements of portland cement of 0%, 20%, 35%, and 50% by volume. Fourteen concrete mixtures (204 specimens) were cast to evaluate the damage to concrete subjected to 300 cycles of wetting and drying while exposed to solutions of one of the three deicing salts or deionized water in which the temperature of the specimens ranged from 39.2 ºF (4 ºC) during the wetting cycle to 73 ± 3 ºF (23 ± 2 ºC) during the drying cycle. Of special interest were the effects of CaCl2 and MgCl2, which result in the formation of calcium oxychloride. Durability was evaluated based on the average relative dynamic modulus of elasticity and the nature of physical damage, if any. Mixtures subjected to CaCl2 or MgCl2 that exhibit no spalling at test completion are considered to be durable. Ten concrete mixtures (156 specimens) were cast with curing periods of 14 or 28 days to investigate scaling over 56 cycles in accordance with Quebec test BNQ NQ 2621-900 using NaCl or CaCl2. Mixtures are considered durable if the average cumulative mass losses are less than the BNQ NQ 2621-900 failure limit of 0.10 lb/ft2. The results show that wetting and drying with deionized water or an NaCl solution does not cause deterioration of concrete. Exposure to CaCl2 and MgCl2, however, both of which result in the formation of calcium oxychloride, causes physical damage and a reduction in the dynamic modulus of concrete with portland cement as the only binder, with CaCl2 being the more deleterious of the two. A partial replacement of portland cement with either slag cement or Class C fly ash is effective in producing durable concrete exposed to CaCl2 or MgCl2. Using a 20% replacement of portland cement with an SCM, however, is not sufficient to produce durable concrete under conditions that result in the formation of calcium oxychloride, while replacing 35% or 50% of the portland cement with one of the SCMs used in this study is. The results, also, show that using slag cement or Class C fly ash as a replacement of portland cement results in an increase in scaling compared to concrete with portland cement as the only binder. For mixtures with portland cement as the only binder and with a 20% slag cement replacement of portland cement, CaCl2 causes somewhat more scaling than NaCl. For mixtures with a replacement percentage using either SCM of 35%, NaCl causes more scaling than CaCl2. In all cases, however, the scaling mass losses for mixtures with 35% SCM replacements of portland cement were below the BNQ NQ 2621-900 failure limit. At 50% volume replacements, the increase in scaling is noticeably higher than with 20% and 35% replacements, especially for mixtures exposed to NaCl. The mass losses for mixtures with a 50% SCM replacement of portland cement exposed to NaCl exceeded the BNQ NQ 2621-900 failure limit. Extending the curing period from 14 to 28 days has no measurable effect on the scaling for most concrete mixtures in the study. Based on the findings of the wetting and drying and scaling tests, a partial replacement of portland cement with either slag cement or Class C fly ash is essential to produce durable concrete that will be subjected to the deicing salts CaCl2 or MgCl2 that cause the formation of calcium oxychloride. Using a 20% volume replacement of portland cement is not adequate, while a 35% volume replacement is. Replacement percentages above 35%, however, are not recommended when the deicing salt NaCl may be used because of increasingly poor scaling resistance with increasing slag cement and Class C fly ash replacement levels.

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