If you are pouring concrete in cold weather, weather where the average daily temperature does not exceed forty degrees farenheit, your concrete is considered “Cold Weather Concrete.” The American Concrete Institute has specific guidelines for how to pour cold weather concrete. Their document ACI 306 spells out the things that should be done. Warming the reinforcing steel, mixing the concrete with hot water, adding accelerating agents to the mix, covering the concrete with insulating blankets, and adding more cement to the mix to strengthen the concrete and speed up the curing time are all methods for overcoming the potential problems with pouring concrete in cold weather.

Concrete does not dry in order to get solid. This is a common misconception. Concrete hardens through a chemical reaction. And water is critical to this reaction.  The curing process is often referred to as hydration. Concrete that dries out too fast will be weak and prone to failure (excessive cracking). You may have seen contractors spraying water on concrete slabs when they pour them in hot weather. That’s because the water in the concrete mixture is drying out too fast due to evaporation. Without the right amount of water, the chemical reation will slow or stop. Concrete is actually more difficult to pour in hot weather than in cold due to this quick evaporation problem.

But when pouring concrete in cold weather, care must be taken to prevent ice crystals from forming in the concrete as it cures. And colder temperatures slow down the hydration (curing) process. That’s why “accelerating agents” are used to speed up the process. Once concrete has reached a strength of 500 psi, it is no longer significantly affected by cold temperatures. Most concrete used for house foundations and slabs is called 3000 psi concrete. That is the strength it will reach after about a month. So it only takes a few days to reach 500 psi. During that time it needs to be protected from the cold. Since concrete generates a lot of heat while it cures, simply insulating it is all that needs to be done. No additional heat source is needed.

Don’t be worried about pouring concrete in cold weather as long as your builder follows the guidelines in ACI 306. If he does not know about these guidelines, point them out to him and insist on compliance. Or find a builder who knows about the proper methods of pouring concrete in cold weather.

If you would like to read more articles about homebuilding and house design, please visit my other website, www.about-home-design.com.

Concrete “flatwork,” that is, concrete poured in slabs for floors, sidewalks, driveways, and roads, is particularly prone to cracking. It will shrink as it “cures” and crack. And it will continue to swell and shrink when it gets warmer or cooler and be susceptible to developing more cracks.

There are measures you can take to reduce the chances of cracking. Control joints and expansion joints can be placed in the surface. Expansion joints are gaps purposely placed and filled with flexible material. Treated wood strips, cork strips and caulking are commonly used. The flexible material “gives” when the concrete moves and takes up the dimensional change, preventing cracking. Expansion joints are usually only placed in large expanses of concrete.

Control joints are smaller grooves placed in concrete. These are placed minimally at ten foot spaces or less. The idea is to place them to create approximately square panels so the expansion and contraction of each segment is about equal in each direction. So a four foot wide sidewalk might get control joints every four feet. Control joints actually weaken the concrete at that spot with the idea being that when a crack occurs, it will occur within the control joint and not be visible, irregular, and unsightly. Of course, we have all seen plenty of concrete slabs that have cracks running right along side a control joint, completely ignoring the groove they were supposed to follow. So this is no surefire method for avoiding cracks.

There are no good ways to fix concrete cracks. They are usually irregular, jagged lines. Filling them with caulking will keep water from filling the crack and increasing the damage as it freezes and thaws. But the crack will remain visible and even be accentuated by the color of the caulking. You can chop out the cracked portion of the driveway or walk and replace the concrete. The problem with this is the new concrete will always be a different color than the original. So the patch will stand out.

But leave it to the innovators in the world. There may be a cure, at last. Check out this video on Bio-Concrete.

This clever young man has added bacteria and food for the bacteria into concrete. When a crack occurs, the bacteria start growing and build a calcium based filler in the concrete that is strong, keeps the water out, and is much more visible discreet than any other filler you could use. I can’t wait for this to hit the market.

My son, Matt Hirsch, told me about bio-concrete. He also works with bacteria. Only his bacteria are a relatively newly discovered type that can remove nitrates from ponds, lakes, and streams. I believe him when he tells me that microbiology holds many keys to a better world.

If you would like to read more articles about house and home design, please visit my other website, www.about-home-design.com.