Chris Kestner, Senior Project Manager -- On pour day, confirm that you don’t have any frozen sub-grade and that all of your formwork is free from ice and snow. Review with your pour team the importance of a low slump mix, which also will decrease the curing time, and be sure you have enough manpower to accommodate that lower slump. Once it’s placed, it’s very important that the finishing process is delayed until all of the bleed water is gone, and that no additional water is introduced during finishing. Use a finishing agent, if necessary, but avoid water. After all, the whole idea is to accelerate curing, not slow it.
After finishing, there are several stages of protection. In the early hours, when there is still some surface and bleed water present, ensure that the concrete doesn’t freeze. Ice formation halts hydration and seriously affects your final strength – by as much as 50%. Even one freeze/thaw cycle prior to achieving 500 psi concrete strength can have serious adverse affects. Guidelines vary, but a good rule of thumb is to maintain 50 degrees for a period of at least 3-4 days. It’s probably a good idea to include your engineer when establishing the period of controlled curing, to keep him informed and on board.
On very cold days, temperature maintenance may be difficult without significant heating. Depending on the mass of the pour, just the heat of hydration, along with insulated blankets or poly sheeting (to hold the heat) may be sufficient. Thinner slabs may require an enclosure to be built, with heating provided. On elevated decks, where heat can be introduced from below, we’ve found that a very small amount of heat, with insulated blankets on the topside, can be an effective way to control concrete temperature. Whatever your approach, consider that you can always scale back on your protective measures, but if you need to increase them, by the time you recognize you have a problem, it may be too late.
While we’ve addressed the major points in cold weather concreting, there are volumes of information available. Your ready-mix supplier, admixture manufacturers, engineers and ACI 306 can all be valuable resources. At the end of the day, it comes down to communication and a plan. Be attentive and conservative – your slab, your team and your client will appreciate it.