In the winter months, we have to plan for more than just the cold weather. Low temperatures can be harmful to concrete in the early stages of hydration, but our attempt to raise the temperature of the environment is often a disaster. The number one unseen danger is carbonation. Concrete can become carbonated at any time, but is most vulnerable when being placed.
The process of carbonation is quite simple, and can be very costly. Heaters that burn fossil fuel produce flue gasses that contain carbon dioxide (CO2). Carbon dioxide is heavier than air. Therefore, it can settle into the bleed water of the concrete. When that happens, the carbon dioxide gas actually dissolves in the concrete's bleed water and forms carbonic acid.
Freshly placed concrete goes through two rather quick, and opposite phases. The first phase is hydrophobic, when the concrete bleeds water to the surface. The next phase is hydrophilic, when the concrete absorbs the bleed water back into the concrete. If carbonic acid is present in the bleed water, it will chemically react with calcium hydroxide in the cement paste and form a compound called calcium carbonate.
Calcium carbonate is a very soft, white powder. When this powder is found at the surface of concrete, the concrete is said to be "dusting." The carbonated surface of the concrete is soft and has low abrasion resistance. In many cases, such a concrete surface is non-serviceable.
The calcium carbonate powder can be washed away from a carbonated concrete surface, but will return in a few days. That's because the water used to wash the powder away is absorbed into the concrete. As the water evaporates, it pulls more of the powder to the surface and the cycle begins again.
If the damage is not too great, the problem can be solved by applying a chemical hardener having a silicate or siliconate base. The chemical hardener will chemically react with the calcium carbonate to form calcium silicate hydrate, which is the same compound that is formed when portland cement hardens (hydrates). Depending of the level of damage, the concrete may require several applications of chemical hardener.
Twenty degrees half and double rule:
As we all know, the temperature of concrete has a direct effect on the set and strength gain of concrete. The optimum curing temperature for concrete is 70° F. It would seem that we are always placing concrete at temperatures very much higher or lower. A good rule of thumb for determining the work time of concrete is the twenty degrees half and double rule. Most concretes will take twice at long to set at 50° F as at 70° F, and at 90° F will take half as long as at 70° F. I have found this rule to be a very useful tool in determining timing for placing and finishing of concrete.
One should be aware that as the temperature drops, the rate of strength gain also drops. As the temperature approaches 50° F, the rate of strength gain slows greatly. As the temperature approaches 40° F, the rate of strength gain is extremely slow, and at 32° F, it stops.
At about 28° F the water in the pores of the concrete starts to freeze. Early freeze thaw protection is a must in cold weather. Once proper air-entrained concrete reaches 500 psi compressive strength, it can withstand one freeze thaw cycle without damage. But play it safe: it is often difficult to determine when concrete has reached 500 psi.
In fact, it is best to play it safe even after the concrete has matured and been put into service. It is best to not apply deicing salt to the concrete until the concrete is one year old. Some DOT's are following this practice. The concrete must be allowed to mature. This is especially true of concrete placed in the late fall.
To learn more about this subject, you can acquire a copy of ACI 306, Cold Weather Concreting, at www.aci.org.
© 2005 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2005.