Concrete curing

Concrete curing is a process yielding accelerated early strength gain of concrete products for more efficient use of cement, additives and space. This process increases ultimate strength to reduce cement and additive content and is done so by providing a curing environment for optimal concrete quality, consistency, strength and durability.

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 hardening in the plant.

The climate during concrete hardening is of enormous importance 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 three (3) most important aspects of the curing climate are moisture, temperature and carbon dioxide. All three of these factors have a positive effect on the hydration process at elevated, though controlled levels. This is of utmost importance 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.

Hydration and its effect on primary efflorescence

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’s surface causing primary efflorescence and also weakening the structure.

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

es:curado de concreto