Few components of any new construction or renovation project get overlooked like concrete. All of us have seen it, stepped on it and, more than likely, skinned a knee or two on it. Concrete is certainly construction’s unsung hero, but arguably one of the most important components of any project.
Concrete is a scientific blend of ingredients designed to be mixed, poured and finished following a set of specific guidelines. Unfortunately, many contractors don’t seem to adhere to these guidelines during cold weather months. And, in most cases, the drawbacks of concrete poured in cold weather months will be discovered much too late and at great expense to the homeowner.
Your best bet is to be educated about best practices on working with concrete in cold weather months.
DEFINING COLD WEATHER
First let’s define cold weather in regards to concrete. Cold weather is defined as a period when for more than three consecutive days, the following conditions exist:
- The average daily air temperature is less than 40 degrees Fahrenheit (five degree Celsius), and
- The air temperature is not greater than 50 degrees Fahrenheit (10 degrees Celsius) for more than one-half of any 24-hour period.
Typically this means fall through spring in Northwest Indiana. These conditions warrant special precautions when placing, finishing, curing and protecting concrete against the effects of cold weather.
WHAT’S THE BIG DEAL?
The two main issues for homeowners that can arise from concrete being poured in cold weather are:
- Concrete can freeze before it gains strength which breaks up the matrix, causing a loss in strength and durability. Concrete surface flaking is the most common visible sign of inadequate cold weather protection of concrete during a pour. If the surface of the new pour freezes, flakes of concrete may pop off, and if the flaking is severe, replacement may be needed.
- Concrete sets more slowly when it is cold—very slow below 50 degrees Fahrenheit and below 40 degrees Fahrenheit, the hydration reaction basically stops and the concrete doesn’t gain strength. Anything below 40 degrees Fahrenheit will slow the curing process and may even stop it altogether. It is critical that the concrete sets before it is exposed to freezing temperatures. The problem occurs when all of the water freezes because it expands, causing your concrete to crack. The key is to do what you can to make sure the concrete sets fast enough to prevent this freeze/thaw disaster.
HOW YOU SHOULD PROCEED
Necessary precautions are needed to protect the concrete until it can handle the cold on its own, but before we outline them, we must first begin with a surface that is free from snow, ice, and frost. Our concrete will only be as good as what it is underneath it. If it meets the above criteria, it is time to proceed.
The general rule is that once the concrete has gained strength to about 500 pound per square inch (psi), then it is safe to proceed. At almost the same time that the concrete achieves 500 psi compressive strength, hydration of the cement has consumed enough of the water in the original mix so that even if it does freeze, there’s not enough water left in the pores to damage the concrete. With most concrete, even at 50 degrees Fahrenheit, this happens during the second day. Typically the two ways to aid in the concrete safely achieving the desired 500 psi are usually a combination of modifying the concrete mix and protecting the concrete.
WHAT’S IN THE MIX?
In regards to the mix, most concrete suppliers have special mixes depending on conditions and the time of the year that the concrete is being poured. Common mix modifications can include adding hot water to the mix (usually around 65 degrees), chloride or non-chloride accelerators in air-entrained concrete, and or increasing the overall quantity of cement in the mix.
PROTECTING YOUR CONCRETE
As to protecting your concrete, this is usually the area with the highest percentage of error. Unfortunately, many contractors are not well versed on this subject. It is often time consuming and costly to do it properly. As a result, many contractors may err on the side of chance, rather than taking the steps to ensure a quality job.
Depending on the actual ambient temperature, protection of concrete placement may require the use of windbreaks, enclosures or supplementary heat. For the curing process, insulating blankets or insulated forms should be used, especially for thin sections.
Special care should be given to edges and corners, and often blankets must be doubled or tripled to protect from freezing. Blankets must also be secured so that the wind does not displace them. In addition, forms should not be stripped for one to seven days, depending on the setting characteristics, ambient conditions and anticipated load.
If the concrete is kept at around 50 degrees Fahrenheit, protection can typically be removed after three to seven days. If the concrete remains at 50 degrees Fahrenheit, depending on what kind of cement is used and how much accelerator, you should wait four weeks before actually putting it into service.
We realize that this may be a bit much for most homeowners. Your best bet? Make sure your contractor schedules the concrete project for a later date if the proper conditions cannot be achieved and maintained. It will save you a great deal of time and money in the future.