Concrete is a composite product made by combining Portland cement, water and aggregate. It gains its strength during the hydration process, which is the chemical reaction between the cement and water. Temperatures from 50 to 85 degrees Fahrenheit are best for hydration. If the water evaporates too quickly because of warm temperatures, hydration is delayed and even prevented. In cold weather, the water in the mixture turns to ice and expands, cracking the concrete.
Curing concrete involves monitoring and controlling the conditions that affect its rate of hydration so the concrete reaches full strength and durability. Proper curing is especially vital when temperatures are outside the optimum range for the hydration process. Curing techniques can be used in two ways. Before concrete is poured, additives that delay or accelerate its setting are incorporated into the concrete mix. After concrete is poured, external measures control environmental conditions around the concrete during hydration.
Curing the concrete means controlling the surrounding conditions to protect the rate of hydration so that the concrete achieves optimum strength with reduced risk of cracking. The curing process involves minimizing evaporation in hot weather. During cold weather, the curing process focuses on retaining heat in the concrete. Curing should be conducted for at least three days, when the the concrete achieves 60 percent of its full strength. Concrete is even stronger and more durable when cured for seven days.
External Curing Practices
Moisture loss is controlled in hot weather by covering concrete with plastic sheeting Water-curing by spraying the concrete with a continuous fine mist also keeps it from drying too quickly. Concrete has to be insulated from freezing temperatures for curing in cold weather. The natural heat of hydration can be used by covering the concrete with insulating blankets or insulated forms to retain the heat over the structure. A shelter is built over the structure in very cold temperatures.
Internal Curing Compounds
Additives, or internal curing compounds, that either retard or accelerate the setting of the concrete are placed in the concrete mix according to the conditions. In hot weather, retarding additives keep the concrete more workable, inhibit water loss, improve strength and reduce shrinkage. During cold weather, accelerating admixtures help concrete develop early strength, reducing the curing period and allowing finishing to start earlier. Properties of additives should be understood well before including them in the concrete mix.
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