Curing Concrete
Adding water to Portland cement to form the water-cement paste that holds concrete together starts a chemical reaction that turns the paste into a bonding agent. This reaction, called hydration, produces a stone-like substance, the hardened cement paste. Both the rate and degree of hydration, and the resulting strength of the final concrete, depend on the curing process that follows placing and consolidating the plastic concrete. Hydration continues indefinitely at a decreasing rate as long as the mixture contains water and the temperature conditions are favorable. Once the water is removed, hydration ceases and cannot be restarted.
Curing is the period of time from consolidation to the point where the concrete reaches its design strength. During this period, you must take certain steps to keep the concrete moist and as near 73°F as practical. The properties of concrete; such as freeze and thaw resistance, strength, watertightness, wear resistance, and volume stability; cure or improve with age as long as you maintain the moisture and temperature conditions favorable to continued hydration.
The length of time that you must protect concrete against moisture loss depends on the type of cement used, mix proportions, required strength, size and shape of the concrete mass, weather, and future exposure conditions. The period can vary from a few days to a month or longer. For most structural use, the curing period for cast-in-place concrete is 3 days to 2 weeks. This period depends on such conditions as temperature, cement type, mix proportions, and so forth. Bridge decks and other slabs exposed to weather and chemical attack usually require longer curing periods. Figure 77 shows how moist curing affects the compressive strength of concrete.
Curing Methods
Several curing methods will keep concrete moist and, in some cases, at a favorable hydration temperature. They fall into two categories: those that supply additional moisture and those that prevent moisture loss. Table 7 lists several of these methods and their advantages and disadvantages.
Methods that supply additional moisture include sprinkling and wet covers. Both these methods add moisture to the concrete surface during the early hardening or curing period. They also provide some cooling through evaporation. This is especially important in hot weather.
Sprinkling continually with water, shown in Figure 78, is an excellent way to cureconcrete. If you sprinkle at intervals, do not allow the concrete to dry out between applications. The disadvantages of this method are the expense involved and the volume of water required.
Wet covers, such as straw, earth, burlap, cotton mats, and other moisture retaining fabrics, are used extensively in curing concrete. Lay the wet coverings as soon as the concrete hardens enough to prevent surface damage. Leave them in place and keep them moist during the entire curing period.
If practical, horizontal placements can be flooded by creating an earthen dam around the edges and submerging the entire concrete structure in water.
Methods that prevent moisture loss include laying waterproof paper, plastic film, or liquid membrane forming compounds, and simply leaving forms in place. All prevent moisture loss by sealing the surface.
A sheet of plastic, as shown in Figure 79, can be used to cure horizontal surfaces and structural concrete having relatively simple shapes. The plastic should be large enough to cover both the surfaces and the edges of the concrete. Wet the surface with a fine water spray before covering. Lap adjacent plastic 12 inches or more and weigh their edges down to form a continuous cover with closed joints. Leave the coverings in place during the entire curing period.
Plastic form materials are sometimes used to cure concrete. They provide lightweight, effective moisture barriers that are easy to apply to either simple or complex shapes.
Some thin plastic sheets may discolor hardened concrete, especially if the surface was steel troweled to a hard finish. The coverage, overlap, weighing down of edges, and surface wetting requirements of plastic film are similar to those of waterproof paper.
Curing compounds are suitable not only for curing fresh concrete, but to further cure concrete following form removal or initial moist curing. You can apply them with spray equipment, such as hand operated pressure sprayers, to odd slab widths or shapes of fresh concrete, and to exposed concrete surfaces following form removal. If there is heavy rain within 3 hours of application, you must respray the surface. You can use brushes to apply curing compound to formed surfaces, but do not use brushes on unformed concrete because of the risk of marring the surface, opening the surface to too much compound penetration, and breaking the surface film continuity. These compounds permit curing to continue for long periods while the concrete is in use.
Because curing compounds can prevent a bond from forming between hardened and fresh concrete, do not use them if a bond is necessary. Forms provide adequate protection against moisture loss if you keep the exposed concrete surfaces wet. Keep wood forms moist by sprinkling, especially during hot, dry weather.