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Concrete Curing

February 22, 2017

Working with concrete is hard, messy work, but even after everything is set in place, the job is not done. Completion of a concrete structure is not yet finished until it has completely cured.

Concrete curing is an important step in influencing the properties of the structure. It is the process of maintaining adequate temperature and humidity within the concrete to ensure hydration of the cement component. Proper curing can increase durability, strength, water tightness, abrasive resistance, volume stability and resistance to freeze/thaw and deicers. All of these characteristics are important, even critical, to the long term performance of a concrete structure whether it is an enormous skyscraper, gigantic tank holding hazardous chemicals, or a simple driveway slab for a residential property.

In concrete, cement and water mix to form a paste. This paste reacts with water and hardens to bind the aggregate materials into a rock-like structure. In any concrete mix there is normally more water than required, but loss of water through evaporation can reduce the humidity within the concrete such that there is a significant reduction in concrete properties. Water loss can cause a significant reduction in strength at the surface causing poor abrasion resistance. Also, water loss causes volumetric changes, creating internal tensile loads that can lead to cracks. The rate of hydration is greatest shortly after the concrete has been mixed and is relatively complete after 28 days. However, concrete continues to hydrate for a long time after, making concrete curing most critical from right after placing to a few days later.

There are several methods used for properly curing concrete, and determining which one to use and for how long depends on the materials, the type of structure, the surrounding environment and the intended resulting properties. Curing methods can be put into three (3) general categories: methods that maintain the presence of water, methods that prevent the loss of water, and methods that accelerate strength gain before significant water loss can occur.

Methods that maintain the presence of water include ponding, fogging or sprinkling, and wet covering. Ponding is accomplished simply by the use of berms or forms that allow water to flood the surface of the concrete. Wet covering is similar except that a moisture retaining fabric, such as burlap, is used over the surface of the concrete and kept saturated with water. Fogging and sprinkling introduce a slow but constant supply of water and is often used to cool concrete through evaporation when ambient temperatures are high. Care must be taken when using these methods to ensure that the water and other materials used do not contain substances that can stain or even degrade the concrete. Further, a constant supply of moisture or wetness must be maintained because an improper cycle of wet and dry surface can cause defects known as crazing or cracking. While curing concrete using methods maintaining the presence of water are effective, they can be challenging and expensive to maintain through the duration of curing.

Methods that prevent the loss of water include covering, applying a curing compound, and leaving forms in place. Covering involves the use of a vapor tight barrier, such as impervious paper or plastic sheets, to cover the surface of the concrete thus preventing evaporated water vapor from leaving. This can be especially important when high winds are present during curing. White or black covering can be used to absorb or reflect sunlight as desired. Covering can be used in combination with wet covering fabrics making it easier to maintain a saturated state. Curing compounds are chemicals, such as waxes or chlorinated rubber, that are sprayed on the surface retarding evaporation. The compound is applied to a moist surface soon after placing and finishing in order to be most effective. Curing compounds are commonly used on slabs in residential construction as continued maintenance is not required. However, the compounds may prevent bonding with paint or adhesives used in flooring. Further, it is important to note that there is a difference between curing compounds and sealants. Sealants are to be applied a minimum of 28 days after placing concrete and consist of different chemistry that specialize in protecting concrete from long term chemical attack, not water loss. For structures other than slabs, such as columns or walls, leaving forms in place will prevent water loss. Forms may still be wetted to cool the concrete through evaporation. There is a risk with any covering that uneven application or folded sheets or dirty forms can cause staining of the concrete. Once again, care and skill are required.

Rapid concrete curing involves curing at elevated temperatures. It is critical that concrete temperature do not change too quickly or get too high. For this reason, rapid curing is often done in a controlled environment, such as at a precast facility. Steam and heated metal forms are commonly used to control heat and maintain heat in the concrete. As temperature and humidity have such a strong influence in curing concrete, it is important that both are considered and monitored during curing.
Ambient temperatures can be difficult to plan for especially in northern climates. CSA A23.1 is the standard governing concrete construction in Canada and provides requirements for concrete curing. It is recommended that concrete temperatures are maintained above 10°C for the duration of curing. Temperatures below 10°C can cause a significant reduction in strength as hydration rates are reduced. Insulated blankets and hording are common methods for maintaining adequate concrete temperatures when ambient temperatures are low. Conversely, it is recommended that cooling methods, such as fogging, are used when ambient temperatures exceed 27°C. Concrete curing can occur at temperatures as high as 60°C, but again, only in controlled environments.

In addition to determining the appropriate curing method to use, the duration of curing must also be determined. The duration is largely dependent on the required properties of the end result. For basic curing CSA A23.1 recommends that the curing period be considered 3 days at temperatures above 10°C or until 40% of the desired compressive strength is reached. Addition curing requires 4 more days or until 70% of the desired compressive strength is reached. In residential construction, the required compressive strength is significantly less than the strength that the mix design is capable of producing and tight controls are not required. For larger projects, curing duration may be determined with testing involving samples tested for compressive strength.

Concrete may be a formable and cost effective building material, but it is not always the easiest to work with. Neglect during the critical curing window can lead to excessive water loss and cause deficiencies that are aesthetically undesirable or result in an unexpected early failure. There are many methods available that help to maintain concrete temperatures and humidity during concrete curing that greatly benefit the long term performance of a concrete structure.


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