Curing concrete is critical to concrete’s long-term durability. Concrete should 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. Without proper curing, concrete may only achieve 50% of its potential design strength. The wisdom 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 and sealers
- Water ponding or spraying a mist over the concrete
- Coverings such as wet burlap, polyethylene, insulating blankets, or similar materials
- 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. A spray or roller is used to apply curing agents.
According to CSA A23.1, 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. Suitable precautions should be taken to maintain strength development.
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
Local Lehigh Hanson Canada companies have experience in cold weather concrete technique. So ask if you have questions. For a complete review of cold weather concrete practices, please refer to CSA A23.