Experts agree that the best temperature to pour concrete is between 50-60 °F. The necessary chemical reactions that set and strengthen concrete slow significantly below 50 °F and are almost non-existent below 40 °F.
The Issue with Pouring Concrete in Cold Weather
When temperatures dip below 40 °F, the chemical reactions that strengthen concrete slow down and can lead to weaker concrete. If concrete curing temperatures are below freezing, the water inside the concrete can freeze and expand, resulting in cracks in your surface.
If heated concrete is not available it is advisable to delay concreting until the ambient temperature has risen above 2ºC. If young concrete is allowed to cool below freezing point it may be damaged to such an extent that it will be unfit for use.
Concrete needs warm temperatures (minimum 10˚C) to cure and reach its desired compressive strength. And it will freeze at -4˚C. If it's too cold, the concrete won't cure at all. So be sure to keep the concrete warm with heaters while curing in winter.
The definition of cold weather – for concreting
The Australian Standard 1379: Specification and Supply of Concrete stipulates that concrete temperatures should be between 5°C and 35°C at the point of delivery. Precautions are recommended once the air temperature falls below 10°C.
The design code BS 8110 required that concrete should not be exposed to temperatures of below 5 degrees Celsius until the strength of the mixture has reached at least 5 MPa.
At a minimum, concrete should be cured at temperatures above 7.2 degrees Celsius (45 degrees Fahrenheit). Below this temperature, concrete's chemical reaction completely stops – which can result in costly delays and rework.
If the ground is frozen or covered with ice or snow, you should not place your concrete. Frozen ground will settle as it thaws, which leaves your concrete susceptible to cracking. In addition, when wet concrete is placed on a cold surface, the concrete will set more slowly.
Answer. As the water in the concrete freezes, it expands and then thaws as temperatures fluctuate. This process stresses the concrete and causes cracking. The result of the repeated freeze- thaw cycles can be surface spalling (flaking) and widening cracks that usually don't emerge until the spring.
In no case should concrete be allowed to freeze during the first 24 hours after it has been placed. Since cement hydration is an exothermic reaction, the concrete mixture produces some heat on its own.
Concrete shall not be placed in cold weather unless suitable precautions are taken. The minimum temperature of ready-mixed concrete when delivered should be 5°C, in accordance with BS EN 206.
Many people ask whether you can safely pour ready mix concrete in the rain? The answer is yes, it is absolutely possible to pour in the rain, but ideally you should avoid. Rain presents challenges, so doesn't always mean that your schedule is out of the window, as long as you take proper action.
Concrete can successfully placed, finished, and cured in cold weather or during the winter, but it requires an understanding of the impact of cold weather on the process of creating long-lasting concrete. Fresh and newly-hardened concrete both lose moisture and heat rapidly in cold-weather conditions.
Your concrete should be solid enough to walk on, without leaving footprints, after anything from 24 to 48 hours. By seven days, your concrete should be cured to at least 70 percent of its full strength.
The heat produced by concrete during curing is called heat of hydration. This exothermic reaction occurs when water and cement react. The amount of heat produced during the reaction is largely related to the composition and fineness of the cement.
The general rule of thumb is that concrete takes about 28 days to dry for every inch of slab thickness. Within 24 to 48 hours, the concrete will be ready for foot traffic.
Because concrete throws off heat during the chemical reactions of curing, insulated blankets and insulated formwork can be sufficient to maintain proper curing temperatures. They must be secured so that wind cannot blow them off.
Explosive spalling is of special concern to tunnel engineers. If a fire breaks out in a tunnel, heat has limited escape routes and can increase temperatures to over 1,000°C (1,832°F) in a short time, causing concrete to explode like popcorn.
Plan to pour concrete before 10 a.m. or after 8 p.m. in order to avoid the hottest part of the day. If you are concerned about the temperature while working on a summer project, consider pouring concrete at night to give the mixture the best chance to harden and cure before the sun comes up.
The most efficient way to cure concrete is to use a plastic cover. The covers are placed on freshly poured concrete until the concrete has a chance to dry. Plastic concrete curing covers seal off concrete and prevents water from evaporating.
Cold Weather Concrete Curing
Ideal temperatures (50-60°F) should be maintained for about 48 hours for the concrete to reach optimal strength as it sets.
Yes, you can pour fresh concrete over existing concrete. As long as your concrete is in mint condition, this task is doable. If you decide to pour a new mixture over an old slab, be sure it is at least 2 inches thick.
Fortunately, whether you plan to make a patio slab or a driveway, working in small areas divided by concrete forms and using bagged concrete mix can make the job easier for a DIYer.
Many construction specialists recommend watering it down five to ten times per day, for the first week, starting 2-4 hours after it has been poured. The first two to four hours lets the concrete “finish”, which refers to the setting process.