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Curing is key to concrete perfection

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In real-estate transactions, “location, location, location” is an industry phrase that describes the value of your purchase. In the ready-mixed concrete business, the phrase curing, curing and more curing becomes the method to providing durable concrete applications and getting the most value out of your investment.

Curing is defined as maintenance of a satisfactory moisture content and temperature in the concrete for a period of time immediately following placing and finishing so that  the desired properties may  develop. Early curing lays the groundwork for the permeability and the durability of the concrete structure.

There are three key components that affect curing. These are time, temperature and moisture.

• Maintaining the moisture allows for the hydration process to continue.

• Controlling the temperature allows for the chemical reaction to take place until completion.

• The appropriate time ensures that the necessary durability is developed under the temperature and moisture conditions.

Curing needs to be initiated as soon as the finishing operations are complete and the surface will not be damaged by the curing operation. The CSA A23.1 Table 20 standards spell out the recommended wet-curing times, but range from three to seven days. The longer the time, the better the result.

A 28-day air-drying period is recommended immediately following the 28-day curing period to provide the necessary freeze/thaw resistance for the concrete.

Curing methods that result in fully saturated concrete, which will be exposed to freeze/thaw cycles once the curing period is over, may result in premature deterioration of the concrete (even if the concrete is properly airentrained). Two basic methods exist for the curing of concrete:

1) Preventing the loss of moisture from the concrete.

2) Keeping the exposed surface continuously wet.

There are several methods to obtain either result and they must also be chosen considering the weather and site conditions. Preventing moisture loss Several strategies may be used to prevent moisture loss. One of these is the use of curing compounds. These essentially form a membrane on the top of the concrete to control the moisture loss during the hydration process. Care must be taken to ensure complete coverage of the concrete, including manufacturer’s recommendations for application rates and compatibility with potential floor coverings. The surface must be clear of excess moisture so the curing compound doesn’t trap it underneath.

Plastic sheeting is another method of preventing moisture loss. The surface must be completely covered and the edges sealed to obtain the proper coverage. Protection from mechanical damage is important during the curing process. Selection of colour of the sheeting can affect the concrete, depending on the ambientair conditions. If discolouration of the surface cannot be allowed, alternative methods should be looked at.

Leaving the formwork in place for vertical applications can assist in curing related to moisture loss. The top exposed surface must also be protected, in this case. If this method is used, releasing the forms too early could cause early moisture loss. Insulating concrete forms provide an ideal curing environment for vertical walls.

Keeping exposed surface continuously wet Several methods are available to achieve continuous surface wetting. Ponding water is one of the most effective: however, the water must completely cover the concrete surface and the temperature of the water must not be more than 12 deg. C. cooler than the concrete. Otherwise, there is a risk of thermal cracking.

Spraying water or misting over the surface is another method, but the coverage of the surface must be complete and there must be an area for water runoff. The strength of the surface of the concrete must be sufficient to avoid any damage from the spray. This method is difficult if there are substantive winds that prevent complete coverage from the sprinklering. Wet burlap may be used and covered with a plastic sheet to prevent drying out. The key to this is ensuring there is nothing to leach out of the burlap into the surface of the concrete and the burlap must be kept wet. New burlap should be rinsed to eliminate any soluble substances and to make it more absorbent. This method allows for reuse of the burlap on other stages of the project.

Another method is the use of sand or straw for footings and slabs. The rules are the same for this technique, requiring full coverage and continual wetting. The disadvantage is the potential for discolouration of the concrete.

Weather and site conditions Site conditions and weather conditions can play havoc with your method of curing. High winds can impact the surface drying, depending on the curing technique, as well as high and low humidity and direct sunlight (temperature control). If special admixtures are used in the concrete mix, the contractor should be aware of the impact on his curing practices. In general, hot-weather curing raises some concerns that need to be addressed. If you are covering the concrete surface using water curing, the temperature of the water must not be excessively cooler than the concrete in order to avoid thermal cracking. Moist curing is the preferred method but curing compounds are highly successful in flatwork situations, as long as the compound completely covers the concrete area.

Cold-weather concreting In cold-weather concreting, the grade temperatures are a concern. The temperature difference will draw off the heat of hydration and create a challenge for keeping the concrete in the proper temperature regime for curing. The definition of cold weather is when the air temperature falls below five deg. C. or will fall below five deg. C. within the next 24 hours of placement of the concrete. This all must be taken into consideration when choosing a method of curing. Remember: curing, curing, curing is the best way to protect your investment.

by Ross Monsour, 2006

Published with permission of On-Site Magazine

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