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2014FallShotcreteEMag

Providing Sulfate Resistance in Severe Exposure Conditions By Mark R. Lukkarila NStates and Canada, especially the western do not have the silo capacity to accommodate anreplace the Type I or Type II in one of their siloswith a Type V for their concrete mixtures if theyaturally occurring sulfates are common insoils within certain regions of the United United States and prairie provinces of Canada. extra cement. The severity of the damage caused by sulfate In 2008, ACI 318 was revised to allow alterna- attack is dependent on the exposure conditions; tives to specifying or using Type V portland concrete mixture proportions (water-cementitious cement when Exposure Class S3 conditions are material ratio is very important); and quantity of encountered on a project. This involves using sulfates in the soil, rock, or water. Shotcrete is blended hydraulic cement or a Type II, Type III, often in contact with the ground (for example, or Type V portland cement in combination with slope stabilization) or rock formations in mining an effective SCM to mitigate sulfate attack. and tunneling applications. When sulfates are in The provisions in ACI 318-11 list the types of the soils, water, or rock with which shotcrete will cementitious materials allowed for the different be in contact, it is important to know at what level exposure classes. For the most severe class (S3), of sulfate is present. Once the level of sulfate is Table 4.4.1 lists ASTM C150 Type V plus a poz- known, a strategy to mitigate sulfate attack must zolan or slag, ASTM C595 Type IP (high sulfate be developed. Without mitigation, sulfate attack resistance) or Type IS (HS) plus a pozzolan or will deteriorate shotcrete over time and may slag, or an ASTM C1157 HS blended cement plus ultimately result in failure. pozzolan or slag. However, Table 4.5.1 allows for The primary mechanisms involved with alternative combinations of cementitious mate- sulfate attack are the formation of ettringite rials in lieu of Type V for sulfate exposures in (CaO•Al2O3•3CaSO4•32H2O) and the formation Section 4.5 (Fig. 1). A suitable alternative to Type of gypsum (CaSO4•2H2O). The formation of V combined with an SCM for Exposure Class S3 ettringite can lead to expansion and the formation is determined by performing an ASTM C1012/ of gypsum can lead to softening of the concrete C1012M test for a period of 18 months. If the paste and loss of strength. expansion after 18 months is less than 0.1%, the In specifications, the ASTM C150 cement type cementitious system is considered a suitable that is specified depends on the severity of the alternative to Type V with a pozzolan for Expo- sulfate exposure. Soils are classified as Exposure sure Class S3. Classes S0, S1, S2, or S3. Exposure Class S0 is The use of pozzolans in the concrete mixture the least severe and there are no limitations on as a partial replacement of portland cement has cement type. Exposure Class S3 is the most several benefits. One of the most important is severe. Many specifications still require the use the reduction in permeability of the hydrated of Type V portland cement or Type V combined cementitious paste. Sulfates can attack concrete with a supplementary cementitious material through physical and chemical mechanisms. The (SCM) for concrete in the most severe sulfate ability of hardened concrete to resist the ingress environments (S3). Prior to 2008, Type V portland of aggressive solutions such as sulfates will cement combined with a pozzolan was the only greatly increase the service life of the concrete in option in ACI 318, “Building Code Requirements severe environments. Pozzolans react with calcium for Structural Concrete and Commentary,”1 for hydroxide, a product of cement hydration, and Exposure Class S3. form calcium silicate hydrate. The calcium silicate Type V portland cement will always be an hydrate produced during the reaction between the excellent option for mitigating sulfate exposure. pozzolan and calcium hydroxide continues to fill There are instances, however, when concrete pore space in the hydrated cement paste over suppliers have limited silo space or Type V portland time. This greatly reduces the permeability of cement is not readily available. Ready mix the concrete. suppliers that use Type I or Type II portland The partial replacement of Type II portland cement for the bulk of their work would need to cement with a pozzolan also reduces the C3A 28 Shotcrete • Fall 2014


2014FallShotcreteEMag
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