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SECTION IX. CURING

IMPORTANCE OF CURING TO HYDRATION

5-70. Curing keeps concrete moist. The moisture is needed for any chemical reactions.

HYDRATION

5-71. Adding water to portland cement to form the water-cement paste that holds concrete together starts a chemical reaction that makes 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 FACTORS

5-72. Curing is the period of time from consolidation to the point where the concrete reaches its design strength. There are numerous facts which affect the curing process.

Importance of moisture and temperature. During this period, take steps to keep the concrete moist and as near to 73F as practicable. 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 moisture and temperature conditions are maintained and are favorable to continued hydration.

Length of curing period. The length of time that concrete is protected against moisture loss depends on the type of cement used; mix proportions; the required strength, size, and shape of the concrete mass; the 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 usually 3 days to 2 weeks, depending upon such conditions as temperature, cement type, mix proportions, and so forth. Bridge decks and other slabs exposed to weather and chemical attack usually require more extended curing periods. Figure 5-20 shows how moist curing affects concrete's compressive strength.

 

Figure 5-20. Moist curing effect on compressive strength of Concrete

CURING METHODS

5-73. 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 5-5 lists several of these effective curing methods and their advantages and disadvantages.

Table 5-5. Curing methods

Method Advantage Disadvantage
Sprinkle with water or
cover with wet burlap
Excellent results if constantly
kept wet.
Likelihood of drying between sprinklings.
Difficult on vertical walls
Strawing Insulator in water. Can dry out, blow away, or burn.
Moist earth Cheap, but messy. Stains concrete. Can dry out.
Removal problem.
Pounding on flat surfaces Excellent result, maintains uniform
temperature.
Requires considerable labor; undersirable
in freezing weather
Curing compounds Inexpensive, easy to apply. Sprayer needed. Inadequate coverage
allows drying out. Film can be broken
or tracked off before curing is completed.
Unless pigmented, concrete can
get too hot.
Waterproof paper Excellent protection, prevents
drying.
Heavy cost can be excessive. Must be
kept in rolls; storage and handling
problem.
Plastic film Watertight, excellent protection.
Light and easy to handle.
Pigmented for heat protection;
requires reasonable care and tear;
must be patched; must be weighted
down to prevent blowing away.

METHODS THAT SUPPLY ADDITIONAL MOISTURE

5-74. Both sprinkling and wet covers add moisture to the concrete surface during the early hardening or curing period. They also provide some cooling through evaporation, which is especially important in hot weather. Methods that gives additional moisture are--

Sprinkling continually with water. This is an excellent way to cure concrete. However, if sprinkling only at intervals, do not allow the concrete to dry out between applications. The disadvantages of this method are the expense involved and volume of water required.

Covering with wet material. This type of covering includes wet burlap, cotton mats, straw, earth, and other moisture-retaining fabrics. These are used extensively in curing concrete. Figure 5-21 shows a typical application of wet burlap. 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.

Flooding with water. If practical, horizontal placements can be flooded by creating an earth dam around the edges, and submerging the entire concrete structure in water.

 

Figure 5-21. Curing a wall with wet burlap sacks

METHODS THAT PREVENT MOISTURE LOSS

5-75. Moisture lost prevention methods include laying waterproof paper, plastic film, or liquid-membrane-forming compounds, and simply leaving forms in place. Moisture loss can be prevented by sealing the surface with--

Waterproof paper. Use waterproof paper (see Figure 5-22) to efficiently cure horizontal surfaces and structural concrete having relatively simple shapes. The paper 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 sheets 12 inches or more and weigh their edges down to form a continuous cover having completely closed joints. Leave the coverings in place during the entire curing period.

Plastic. Certain plastic film materials are used to cure concrete. They provide lightweight, effective moisture barriers that are easy to apply to either simple or complex shapes. However, 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. These are suitable not only for curing fresh concrete, but to further cure concrete following form removal or initial moist curing. 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. See TM 5-337 for application details. Respray any concrete surfaces subjected to heavy rain within 3 hours of application. Use brushes to apply curing compound to formed surfaces, but do not use brushes on unformed concrete due to 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. Do not use curing compounds if a bond is necessary because curing compounds can prevent a bond from forming between hardened and fresh concrete.

Forms. Forms provide enough protection against moisture loss if the exposed concrete surfaces are kept wet. Keep wood forms moist by sprinkling, especially during hot, dry weather.

 

Figure 5-22. Waterproof paper used for curing

David L. Heiserman, Editor

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Revised: June 06, 2015