Cold Weather Concrete Tips
by Doug Bannister from The Stamp Store
Oh, if only temperatures always ranged from 60 to 80 degrees, it never rained during daylight hours, all my jobs went off without a hitch and I had an eight shot handicap.The reality is temperatures drop below freezing for many of us during the winter months, and I’m a bogey golfer, at best.
I think of implementing cold weather practices when nighttime temperatures start to fall below freezing and daytime temperatures, hopefully, rise above freezing. Here are some of the “do’s and don’ts” for cold weather stamping:
Protecting the subgrade:
Never place concrete on frozen ground because the concrete will settle as the subgrade thaws. Likewise, placing and compacting fill material on a frozen subgrade doesn’t solve the basic issue of the expanded soil underneath. Make sure the base is thawed out before compacting, fine grading and placing!
Cover the subgrade before the pour to raise the ground temperature. Use insulated blankets, straw or fiberglass insulation sandwiched between two layers of 6 mil poly. The poly alone will suffice if the temperatures don’t drop below the mid 20s overnight, but frost can penetrate several inches overnight with temperatures in the teens or lower. We have sprinkled calcium chloride flakes and pellets on a frozen subgrade to generate more heat under the blankets and help thaw out frozen ground.
Our friends way up north – where frost depths may reach 5 and 6 feet – have to build heated enclosures and wait days before they are able to get the frost out. Make sure the exhaust is vented to the outside as the fumes can blister the top layer of the fresh concrete, not to mention harming your personnel.
Making adjustments in the mix design are necessary to compensate for cold weather conditions. Here are several options we exercise for accelerating the mix:
- Ask your ready mix producer to use hot water; increase the cement content half a bag or more; use a non chloride accelerator; and even place the concrete at a lower slump. Uggh!
- Ask if high early Type III cement is available to be used in place of regular Type I. Make sure your air content is up to snuff. We use 5 – 6 percent here in Oklahoma City. You want your concrete to 55 degrees or warmer at placement, and you want to maintain at least 50 degrees for two days if possible.
- Calcium chloride tends to discolor and produces more efflorescence, so avoid using it. One downside of the non-chloride is that it does not accelerate quite as fast. We hope to have non- chloride in disposable bags soon so you can accelerate it at the job site just as you do with Mini-Delay set to slow down the setting time in the summer.
Protecting the pour:
After placement, it is important to maintain the heat generated by the hydrating concrete so the curing process can continue. Hydration slows drastically when the concrete temperature drops below 50 degrees. Concrete must reach at least 500 psi before being subjected to freezing temperatures. Heat coming off the fresh pour may ward off temperatures dropping slightly below freezing the first night, but why take the risk?
Cover with blankets or place the unbatted side of fiberglass insulation down and cover with poly or blankets as soon as you can walk out. Putting the fuzzy side of the insulation on the fresh surface prevents any staining that might occur when the poly contacts the fresh surface. The fiberglass insulation doesn’t have to provide 100 percent coverage, just close enough to keep the blankets or the 6 mil. poly up off the surface of the concrete. Pay special attention to the edges where less heat is generated by the concrete.
The protection you have placed can be removed long enough for saw cutting, but recover and leave covered until you are confident that the strengths are up. One way to tell if you have at least 500 psi is when the saw blade doesn’t cause the edges to ravel when you are cutting the control joints.
Implementing these suggestions allows us to produce work comparable in quality to the idyllic warm weather days.