Cold Weather Concrete
Albuquerque is unlike a lot of places in this country. It has large temperature swings. The temperature may be 10 below zero in the winter and 100 degrees in the summer.
If it’s cold outside what do I need to do to pour and protect my concrete? Frozen concrete is worthless. It just crumbles. Here are some tips to avoid the crumbles.
In the winter we are searching ways to keep the concrete’s heat until it reaches 80% of its designed strength.
Make sure your sub-grade is NOT Frozen. This will undoubtedly knock that heat out of your concrete and may freeze your fresh concrete. If temperatures are going to be less than freezing the night before you pour you should “blanket the sub-grade”. This will help maintain the ground temperature and allow you to pour the next morning.
Do not pre-wet the sub-grade before you pour because the extra free water may freeze and again “knock” the heat out of that concrete.
Exterior concrete should be air entrained, I will address why later. As ambient temperatures drop Duke City begins heating its’ mixing water. Concrete temperatures must be at least 50 degrees per ASTM for the cement to start the hydration process. We are usually between 65 and 70 degrees in the winter months and we achieve that by increasing our mixing water temperatures. Heat is needed for the cement to hydrate, you lose that heat and the hydration process comes to a grinding halt. That being said, you may still need to add an accelerator to your mix. These admixes accelerate that hydration process therefore speeding up the set time. There are two types of accelerators, Calcium Chloride based and a Non-Chloride type. The first is less expensive but can cause corrosion to any steel used in the concrete, since it is a corrosive the maximum allowable is a 2% of calcium to cement ratio. Anything higher than that may actually decrease the strength properties of the concrete. Non-chloride accelerator is twice as expensive and could be used in heavier dosages without any corrosive properties. Non-chloride accelerator is sold by set levels since it can be dosed at greater quantities as needed. A set level 2 is comparable to a 1% dosage of calcium chloride Accelerators are used to speed up the set time. It has been my experience that if you do not reach initial set by the time you lose any heat generation (ie. the sun) the set times will be exponentially greater as ambient temperatures drop. Sometimes a trowel finish cannot be completed until the next day. Sun exposure is vital to set times in the winter, so consider this when ordering. Time of day, whether it is on the south or north side of a structure, tree, etc.. Ask your representative what he or she would recommend for the conditions on the day you are pouring. Because of the slower set times you may want to order a lower slump. It is a tradeoff between working harder to lay it down and waiting for it to set.
Wow, that was hard work wasn’t it? But wait, now we need to make sure it doesn’t freeze after it’s been putdown. If ambient temperatures are expected to be freezing or below you will need to blanket the concrete for the first seven days. Concrete will hold its’ temperature pretty well but we need to ensure too much doesn’t escape. It will help to pull off those blankets during the day if you have sun to accelerate the curing time, but remember to put those blankets back on before night fall.
Exterior concrete needs some attention in the cold weather after it is put into use. I mentioned earlier to order air-entrained concrete for exterior concrete. Air entrained concrete has up to 8% miniscule air bubbles in it. These bubbles are designed to give permeating water a place to expand and NOT break up the hardened concrete. These mixes are designed and tested to 300 freeze thaw cycles without fail. If excess water is present it can seep into the concrete and freeze. That is a freeze thaw cycle. To reduce those cycles it is a good idea to shovel off fresh snow and not allow it to melt, permeate into the concrete and freeze again.
DO NOT USE DEICING SALTS as this will corrode the surface and allow water to permeate deeper into the concrete.
I hope this helps you when considering pouring concrete in the cold temperatures of Albuquerque in the winter. Feel free to call and talk to a Duke City representative about any concrete pour you are contemplating.