CoatingsPro Magazine

SEP 2018

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Page 43 of 83

44 SEPTEMBER 2018 COATINGSPROMAG.COM Science Behind It Exothermic Reaction in Concrete By American Concrete Institute Committee E-701 C oncrete is made from a properly proportioned mixture of hydraulic cement, water, fine and coarse aggregates, and, often, chemical admix- tures and supplementary cementitious materials (SCMs). e most common hydraulic cement used in construction today is Portland cement. A lthough other types exist, Portland cement is the most widely manufactured and the focus of this document. e successful use of concrete in construction depends on the correct selection of the appro- priate materials necessary and the proper proportioning of those materials. is requires knowledge of the material properties and the tests used to measure those properties. W hen wet concrete cures, it's not merely "drying"; chemical reactions are occurring. One of the results of those reactions is heat. is is what's called an exothermic reaction. e chemical reaction between the compounds can affect how and when a coating is applied to the surface of the slab because the slab has to be ready to receive the topping. Other characteristics of the curing concrete, such as moisture, can also affect this timing, as it did with the Wexford Contracting crew on the Beyond Storage jobs. e crew used infrared temperature gages to test the floor for these charac- teristics throughout the project. W here does the heat come from? How It Works According to the "Cementitious Materials for Concrete" bulletin, heat is liberated during the hydration reactions of the cement compounds. e amount of temperature rise in a concrete mass will depend on the cement content; the rate of hydration (determined by cement fineness, ambient tempera- ture, and admixtures); the cement composition; the volume of aggregate; the thickness of the concrete member; and the rate at which heat is lost by the concrete to the surroundings. e heat of hydration may cause a considerable rise in concrete temperatures in the first few days of hydration, and the temperature will continue to rise in mass concrete over longer periods. In cold weather, the heat of hydration may be used to help protect the concrete from freezing by trapping the heat. Fly ash and other pozzolans or slag cement can be used as a partial replacement for Portland cement to control the heat gain of mixtures during hot weather or in mass concrete. Cement composition and fineness affect the rate of heat generation. C3A (tricalcium aluminate) and C3S (tricalcium silicate) are primarily responsible for high early heat genera- tion. e rate of hydration will be faster as the fineness of the cement grind increases, although the total heat of hydration over a long time will not be particularly affected. Concrete proportions influence the amount of generated heat primar- ily as a result of cement content, with higher cement contents raising the heat of hydration. Before Coating Heat is created when the different compounds are combined to create the wet concrete. erefore, as the concrete is curing, that heat is released. is exothermic reaction is something that contractors must be aware of before install- ing any high-performance coatings to the slab to ensure a successful application. CP Authorized reprint from ACI Education Bulletin E3-I3 "Cementitious Materials for Concrete."

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