Cold weather means football season, your in-laws are in Florida, and you are not spending your weekends cutting grass or painting the outside of the house. And it's time to finish up those summer projects before real winter sets in.
While some activities, like painting and caulking, have to wait until better weather returns, concrete work can usually continue provided you plan appropriately and apply the extra care that's needed. With that in mind, here are a few reminders about the perils—and pluses—associated with cold-weather concreting, and how to cope with them.PREPARATION
Check the forecast and use your local experience. An overnight freeze might not be predicted, but clear skies lead to cold nights. Get to know the folks on the Weather Channel.
Have the blankets, insulation, tools, and equipment onsite. Don't count on being able to get what you need if the weather suddenly turns colder than expected.This column shows the signs of localized freezing at the corners. In humans we call this “frostbite” and the way to prevent it is to pay special attention to edges and corners of concrete, as well as fingers, toes, noses, and ears of humans.
Have a snow or ice plan just like local schools do. Know who will talk to the batch plant to decide if the pour is on in bad weather. You might be able to pour if the trucks can get to the site, but if the drivers can't get to the plant, or if your placing and finishing crews can't get to the site, you'll have to cancel the pour.
Cold weather usually means shorter, darker days. If you need an early start or if the finishers will be working late, you may need floodlights. Later sunrise may mean a later start to the pour, and more traffic congestion for ready-mix trucks.
Check your equipment! Nobody likes to do routine maintenance when it gets colder outside. Even changing oil can be a nasty job on a cold, wet, windy day. Watch for freeze-ups in water-cooled engines. Check radiator fluid. The last time your cold weather stuff was used may have been last year. Remember what the first use of the snowblower is like each year, and how you swore that you would do some maintenance on it before the weather got cold? Can you find your snow shovel on the first storm of the season? Plan extra time to get equipment cranked up in cold weather.ONSITE PREP OF FORMS AND SHORES
Slower hydration of cool concrete increases lateral form pressure. Form-ties that were strong enough or spaced adequately in warm weather could pop in the cold. Slower strength gain in cold weather usually requires a longer time before form and shore removal, and may make you reconsider shoring, placing, and reshoring cycle times.
When snow or freezing rain is in the forecast, cover the tops of your forms with tarps the day before the pour. Snow and ice on the forms and rebar is dangerous, and if not removed before pouring, can result in voids in the hardened concrete. Removing snow and ice from inside forms and around rebar is a royal pain, and using the trusty cutting torch to melt ice on rebar is not a good way to start a winter's day.BATCHING, TRANSPORTING, AND PLACING
The as-delivered temperature of concrete depends on both the temperature and batch weights of the raw materials. The temperature of the cement in the silo will depend on its delivery temperature from the cement tanker (usually lower in winter than in summer), and how long it has been stored in the cold silo. The aggregates are often far less protected than the cement, and the water in the aggregate piles may even have turned to ice. Aggregates can be heated, and need to be when frozen, but the most manageable way to adjust concrete temperature is to use heated water. By adjusting the temperature and quantity of heated versus nonheated water, the concrete mix temperature can be accurately adjusted if the temperatures of all of the other ingredients are known. The total water content has to remain at the level in the approved mix design, and note that the lower the water content of the mix the hotter the water must be to provide the same heating capacity. There are plenty of resources to help you make these calculations, but they are only as accurate as your knowledge of the temperatures of all the other ingredients. If you don't compensate for varying cement or aggregate temperature, or for the presence of ice, mix temperatures can vary widely from batch to batch, causing some trucks to be rejected for being too cold and others for being too hot!