Adding new poured concrete slabs around your home during the winter is a tricky job. All concrete contains some amount of moisture no matter how long it has dried, but freshly poured concrete is full of water. Serious complications arise when this moisture freezes. Concrete that freezes during the curing process may need repair or even replacement.
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When the water trapped in freshly poured concrete freezes, it drastically lowers the finished strength and durability of the slab. Strength drops by up to 50 percent, according to the National Ready Mixed Concrete Association. The curing process allows concrete to bond tightly together through the formation of crystal structures. When the water freezes instead of evaporates, it forces the concrete apart as the ice expands. This leaves tiny cracks and gaps throughout the slab, reducing its ability to withstand high pressure and heavy loads.
Once the concrete you've poured has frozen and thawed, you can determine that the slab was comprised by looking for some physical damage. The surface of the concrete will chip off in flakes under little pressure if the mixture froze too early in the curing process, says Inspectapedia. Spalling also occurs, which involves larger chunks of the surface breaking off and leaving pits or craters surrounded by webs of fine cracks. If your new concrete begins to show these signs after a freeze, the interior of the slab is likely damaged too.
Keeping the concrete warm during the curing process is one way to prevent freeze damage. Professionals often use electric heating blankets or space heaters, but other insulating materials work. Keeping up the wooden forms you're using to support the fresh concrete helps protect the sides and center of the slab from freezing, according to Power Blanket. Adding at least one layer of burlap or plastic sheeting will also keep the concrete warmer than leaving it exposed to the air.
Concrete usually needs to stay wet for a certain amount of time to cure properly. If accelerating mixtures are adding to the concrete, the curing process speeds up and you will have to keep the concrete warm for fewer days, according to Concrete Contractor. An alternative to adding calcium chloride or another mineral additive is adding hot water when mixing the concrete. This speeds up the curing process and keeps the concrete above freezing for longer. Adding cement works in the same way, but it affects the strength of the concrete and the ratio should be modified by a professional if you are pouring a structural support or a foundation.
- National Ready Mixed Concrete Association: Cold Weather Concreting
- Inspectapedia: How to Identify & Evaluate Freezing, Water & Other Damage to New Concrete Slabs or Foundations
- Power Blanket; Tips For Pouring Concrete in Cold Weather; January 2008.
- Concrete Contractor; Cold Weather Pouring; John Kulczycki; January 2011.