It is critical to concrete’s long-term durability that it be kept as moist as possible for the first seven days after placement. In addition, it must be allowed sufficient drying time before being subjected to freezing temperatures or to de-icing salts. Curing concrete properly requires the correct control of moisture and temperature and without proper curing, concrete may only achieve 50% of its potential design strength. The logic behind the practice of a seven-day curing period rests on two facts:
- Cement, the “active” ingredient in concrete requires constant moisture to gain strength.
- If the concrete is kept moist for the seven-day period, it will not only gain strength, but will also shrink less and produce fewer cracks.
Curing can be assisted in a number of ways:
- Spray-on liquid curing agents.
- Water ponding or spraying a mist over the concrete.
- Coverings such as wet burlap, polyethylene, insulating blankets, etc.
- Leave forms in place.
A liquid curing agent is the most effective and convenient method. The curing agent should be applied as soon as finishing is complete. Curing agents form a membrane on the surface of the concrete in order to retain moisture. Once the curing agent has been applied, no further working of the concrete can be done. Use a spray or roller to apply curing agents.
According to CSA A23.1-94, curing is a mandatory part of concrete construction and requires moisture to maintain the concrete at 100% relative humidity and a minimum temperature of 10°C for a period of at least 3 days. This period is extended to a minimum of 7 days for the durability requirements of exposure classes F1, C1, C2, S1, and S2.
Cold Weather Curing
Concrete can be placed safely throughout the colder months if precautions are taken. During colder weather, the mix should be adapted to the ambient temperature by heating the concrete, adding accelerators, and providing protection.
Concrete generates heat during hydration, the chemical process by which cement reacts with water to form a hard stable paste. Hydration is affected by initial concrete temperature, ambient air temperature, the dimensions of the concrete and mix design. The temperature of the concrete during and after placement is critical since concrete sets more slowly as the temperature drops. Finishing and form removal may be delayed in cool weather and strength development can be impaired if appropriate precautions are not taken.
Do not place concrete on frozen ground, on snow, or in freezing weather. If concrete freezes while in a saturated condition, surface problems like scaling, spalling or cracking can arise due to the expansion and contraction of frozen water inside the concrete. During freezing weather, water curing of concrete should be terminated 12 hours before the end of the protection period. Do not use a curing agent if there is any chance that the concrete will freeze during the curing period.
- Protect concrete from freezing temperatures for 3 to 7 days after placing.
- Leave forms in place as long as possible. Corners and edges are most vulnerable (cover and heat if necessary).
- Protect flatwork by covering and heating, or using insulated blankets, or covering with plastic and straw.
This information has been provided to assist you with your cold weather construction. For a complete review of cold weather concrete practices, please refer to CSA A23.1 94 Section 21.
After the concrete is fully cured, the application of a concrete sealer is recommended. The advantages of sealing concrete include:
- Provides protection against de-icing salts
- Resists mildew and surface staining
- Improves appearance
- Makes surfaces easier to clean
- Inhibits efflorescence (a whitish powder on the surface)
Due to the porous finish of exposed aggregate concrete, a surface sealer is highly recommended.
Superplasticizers are admixtures that increase slump to make concrete easier to place. Superplasticizers disperse particles of cement throughout a concrete mix. The effect is the same as adding water, but without the detrimental side effects. In addition, they reduce water in the mixture to gain higher, earlier strengths.
As a contractor, superplasticizers can help you to build more economically and to produce higher quality concrete at the same time, since moderately flowing concrete can be placed more easily providing good consolidation around rebar and tight forms with minimal vibration. Combine superplasticizers with low initial slump concrete to produce a workable mix with higher earlier strength.
Use superplasticized concrete for floors, suspended slabs, toppings, walls, columns, and anywhere that higher slump concrete is required.