Concrete curing

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How the process works

Concrete achieves its strength through a hydraulic process known as hydration. With the addition of the correct amount of water, cement gels into a paste that glues sand and aggregates together to form hardened concrete. For our purposes, this process begins in the mixer and continues throughout the time the concrete is reaching its ultimate strength.

The climate during the concrete curing process is important for the quality of the hydration and strength gain. The ambient temperature plays an important role in determining the speed of the hydration process. The warmer the air, the warmer the concrete and the quicker the concrete strength gain. That being said, the four most important aspects of the curing climate are moisture, temperature, circulation and carbon dioxide. All four of these factors have a positive effect on the hydration process at elevated, though controlled levels. This is important because with an improper hydration reaction the surface of the concrete product being cured will be porous, possibly leading to primary efflorescence and a weakened product. backhoe attachments

Hydration and its effect on primary efflorescence backhoe bucket

Hydration, as mentioned previously, is a chemical reaction that takes place in the formation of concrete structures and is the mechanism by which Portland cement gains its strength. This reaction is caused by the mixing of cement, aggregates and water. Improper hydration on the surface of a concrete product can cause it to have rough, honeycombed surfaces and will leave you with a porous concrete structure. These porous concrete structures will then allow more calcium hydroxide to reach the product surface causing primary efflorescence and also weakening the structure. bulldozer parts

As stated by the Ohio Ready Mix Concrete Association uring concrete is one of the most important steps in concrete construction and regrettably, one of the most neglected. Effective curing is absolutely essential for surface durability. The curing of concrete involves maintaining a proper moisture vapor transmission rate (2% mvtr) immediately after concrete placement and throughout the ensuing period of approximately 28 days.

For centuries curing concrete has been accomplished by bathing green concrete with water, moist straw or burlap blankets. Actually, Water Curing concrete consistently for 14-28 days remains a sound method of curing concrete, but labor conditions have rendered this method impractical except for extremely unique projects.

During the chemical age of the 1950, in an attempt to reduce labor costs and increase consistency, curing concrete with Thin Film Curing Agents came into practice. This time also marked advent of retarders and accelerators some of which may not have a positive affect on concrete quality. Thin film curing agents, typically acrylic resins or wax, are designed to remain on top of the concrete during the 28 day curing process. Even distribution and thickness of the film layer is critical to obtain the desired mvtr - as such proper application is critical. When functioning properly these films dissipate by the conclusion of the curing process. However, variance in temperature, weather conditions and exposure to UV light make the timing of this process unpredictable. If dissipation is incomplete uneven blotchy appearance may occur. In almost all cases grinding, stripping or shot blasting is required to commence with permanent sealing or coating, which adds additional labor cost and time cost.

During the 1980 the US government and military began the use of Chemically Reactive Penetrating Curing Agents containing siliconates and organo-siliconate compounds to highly valued projects such as airport runways, bridges, tunnels and high priority applications. Tests by the American Concrete Institute (ACI) have also demonstrated the high performance curing properties of certain siliconates and organo-siliconate. Application is straightforward and typically done with a low pressure garden-style sprayer. Because the chemical reaction is predictable, the highest standards of consistency are achieved, and within 3-5 hours variables such as temperature, weather and UV exposure become negligible. Additionally, the chemical reaction is permanent, which permanently increases the strength as measured in pounds per square inch (PSI), hydrophobic resistance, and oliophobic resistance of concrete. Further, within 21 days penetrating chemically reactive cures can allow additional treatments densifier/hardeners, stains, epoxies and urethanes - to be applied without the added labor cost and time cost for removal of residual film.

All national concrete authorities, American Concrete Institute, Portland Cement Association and North American Ready Mix Concrete Association stress the importance of properly curing concrete. Chemically Reactive Penetrating Curing Agents, once considered prohibitively expensive, are gaining rapidly in popularity as they can provide cost effective, reliable curing results with the added benefits of permanently improved concrete strength, moisture resistance and dust resistance.

Carbonation and its effect on secondary efflorescence

Secondary efflorescence is another significant problem that can be averted if the appropriate action is taken. Secondary efflorescence appears at any stage after the initial hardening process has ended. This can be while the concrete is stored or after the final product has been installed. This second type of efflorescence is caused by a chemical reaction known as carbonation that takes place on the surface of the concrete. Carbonation is a natural reaction between the materials in cement, water and the atmosphere. That being said, this secondary chemical reaction can be expedited and controlled in order to form this carbonation under the surface of the concrete. This interior carbonation then remains unnoticeable and, by blocking the concrete pores, will inhibit the formation of efflorescence on the surface of the product in the future.

Accelerated curing of concrete and its effect on all efflorescence

The three most widely accepted methods of accelerated curing are by using steam boilers (provide steam at atmospheric pressure which leads to high temperatures and moisture), hot air heating combined with misting systems (produces a higher curing temperature and provides moisture), and direct fired vapor generators (provides a high temperature, moisture and carbon dioxide).

Steam boilers: These are systems that produce a steam at atmospheric pressure, which simultaneously introduces heat and moisture to the products being cured. These are two important elements to the accelerated curing of concrete products.

Hot air/misting systems: These systems provide heat for the products and they provide moisture for the products. However, the mist (water that has just been made more fine) does not undergo the change in matter from a liquid to a gas. Therefore, it has no energy (heat) meaning it is not as efficient. This misting will increase the relative humidity while at the same time it will decrease the temperature. Concrete needs this higher temperature to cure.

Direct fired vapor generator: These vapor generators produce a vapor from water that is in direct contact with the combustion gasses. This vapor has all of the qualities of steam but it is non-pressurized making it safer. Also, the by-product of this combustion process is carbon dioxide, which is one of the gasses in the carbonation reaction mentioned earlier. Therefore, this direct-fired process fills a curing enclosure with approximately 3% carbon dioxide in the air as opposed to only 0.03% that occurs in the natural environment. That expedited carbonation reaction is the key to reducing secondary efflorescence.

References

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^ http://www.understanding-cement.com/hydration.html

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