The protection of freshly placed concrete, especially in the first 12-24 hours after its placement, is critical. This is because of the initial setting phase of the concrete.
If the concrete is not protected, environmental factors like winds, rains, mist, frost, etc. can easily disrupt proper concrete curing and endanger the durability of concrete.
The ordinary concrete should be protected and adequately cured for a minimum of seven days; rapid hardening concrete shall be cured for three days. The strength, durability, and aesthetics of concrete are greatly dependent on the curing process.
Successful concrete curing is based mainly on providing proper protection to allow concrete to gain adequate strength. The protection of concrete ensures that rapid moisture loss from concrete is avoided in hot weather environments, and maintains the suitable temperature to ensure the continuity of cement hydration in cold weather conditions.
How to Protect Freshly Placed Concrete During Curing?
1. Protect Concrete from Wind
Concrete loses moisture quickly on a windy day. So, it is necessary to set temporary windbreaks around the area where concrete is placed. The concrete should be wetted after it is set to make sure that it is cured and achieves the designated strength.
2. Protect Concrete from Rain
If too much rainwater falls over recently placed concrete before it is set, it can weaken the concrete strength. Therefore, it is necessary to cover and protect freshly placed concrete during rains.
It may be possible to postpone the concreting process to avoid rains, but unforeseen rains can happen, and contractors should be prepared for such unexpected conditions. Plastic sheets and tarpaulin should be kept available on-site to cover concrete and protect it from rainwater.
The cover should not be placed directly on the surface of the concrete. Install timber logs or place bricks around the cover's perimeter to create a space between the concrete and sheet surfaces.
3. Protect Concrete from Cold Weather
As per ACI 306, the temperature below 4.5℃ signifies cold weather. It is necessary to protect concrete in cold weather to avoid freezing of concrete at early ages, ensure the continuity of the curing process, develop adequate strength, and prevent thermal shock and subsequent cracking at the end of the protection period.
If freshly placed concrete freezes, it can suffer both temporary and permanent damages. After that, the curing process will not help regain the desired properties of concrete.
Therefore, the best possible solution would be to cover concrete to protect it from freezing and maintain the required temperature for proper curing. The following techniques can be used to protect newly placed concrete in a cold environment:
Insulation is a common and economical technique to protect concrete and maintain the required temperature to ensure adequate curing. The method takes advantage of the heat of hydration, i.e., the heat generated due to chemical reactions that occur in concrete. Commonly, insulation can retain the heat of hydration which is enough for the curing process.
The concrete should be covered after the pouring operation to prevent the escape of the initial heat of hydration, which is critical for maintaining cure temperature and helps to encourage hydration. The corners and edges of the area over which the concrete is laid are susceptible to freezing before other concrete parts, so they should be covered adequately.
3.2 Electric Concrete Blanket
It is a novel solution that maintains concrete curing temperature. This protection method is considered in severe winter conditions where the heat of hydration is inadequate to maintain sufficient cure temperature. As a result, supplementary heat is required. The concrete blanket is claimed to help cure concrete 2.8 times faster than a typically insulated blanket.
3.3 Hydronic Heaters
Hydronic heaters are used in freezing weather conditions. It circulates a heated glycol-water liquid through heat transfer hoses installed on forms or the concrete. A concrete insulation blanket is required to cover the hydronic heaters and prevent the loss of heat.
3.4 Heated Enclosure
A heated enclosure is another method of concrete protection in a cold environment. Combustion heaters are used to heat enclosures. The heated enclosures should not be placed directly on the concrete to avoid direct heating or drying out of the concrete.
If a surface of fresh concrete is exposed to carbon dioxide from unvented combustion heaters, it can deteriorate due to carbonation. One should know that these supplementary heating sources, namely electric heating blankets, hydronic heaters, and heated enclosure, will increase the cost of concrete protection and curing in cold weather conditions.
The removal of these protection equipment from concrete surfaces should be carried out gradually to avoid thermal shocks and subsequent concrete cracking. This can be done by initially reducing the heat from the sources and then removing the heating equipment and blankets.
If the heat is removed quickly, the concrete will cool down suddenly. Consequently, the thermal gradient between the interior portion of concrete and its surfaces generates thermal stress that can initiate surface cracking.
4. Protect Concrete from High Temperature
The freshly placed concrete can lose moisture quickly and subsequently crack due to high temperature. If concrete placement on a sunny day is unavoidable, try to set a temporary tent or shelter to prevent the sun from heating the concrete. Then, start the curing process soon after the concrete sets.
Spraying water over concrete is a suitable curing technique which should be carried out several times daily. If it is not possible to visit the construction site frequently, covering concrete with wet sheets is a good curing option. The sheet should be wetted daily for a minimum of seven days to ensure that concrete cures properly and develops designated strength.
Finally, it is possible to use ice or chilled water to decrease the temperature of fresh concrete to reduce the detrimental effects of high temperature during a sunny day. The temperature of the constituent materials or mixed concrete can be brought down. This method will reduce the loss of moisture from concrete until the curing process begins.
Successful concrete curing is based mainly on providing proper protection to allow concrete to gain adequate strength. Concrete protection ensures that rapid moisture loss from concrete is avoided in hot weather environments and maintains the suitable temperature to ensure the continuity of cement hydration in cold weather conditions.
As per ACI 306, the temperature below 4.5℃ signifies cold weather.
The following methods can be used to protect freshly placed concrete:
2. Electric concrete blanket
3. Hydronic heater
4. Enclosure heater
5. Combination of the above
The insulation sheet will prevent the escape of the heat of hydration and maintain curing temperature to make sure concrete develops the required strength.
The concrete should be cured at a temperature ranging from 10 degrees Celsius to 32 degrees celsius.