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.
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.
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.
For it to set correctly, you must protect fresh concrete from freezing during the first 24 hours. If it freezes, it will lose much of its strength. However, with proper safety measures and planning, you can pour concrete successfully even in cold weather conditions.
If Concrete Freezes Too Soon
Concrete that freezes when it is fresh, or before it has cured to a strength that can resist the expansion associated with the freezing water, will suffer a permanent loss of strength. Early freezing can reduce the final strength of the concrete by up to 50 percent.
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.
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.
The ideal concrete temperature range before concrete pouring is between 27-35 degrees Celsius. This range allows for the hydration process to occur properly, which is essential for the strength and durability of the concrete.
The threshold of significant degradation of concrete is around 65-93°C (150-200°F). For this reason, current codes and industry standards dealing with reinforced concrete structures specify a maximum temperature limit of about 65-93°C (15O-2OO°F) to ensure predictable concrete behavior.
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.
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.
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.
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.
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.
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.
At 25 to 27 degrees Fahrenheit, the freezing of pore water in curing concrete will halt hydration – and the curing process.
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.
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.
Freshly placed concrete should be protected from rain for at least 12 hours from its placement. After that, the rainfall rarely influences concrete properties.
Concrete that is not moist-cured at all dries too rapidly, and reaches less than half its potential design strength. It will also have a greater number of shrinkage cracks.
If water evaporates too quickly, it will weaken the finished product with stresses and cracking. To put it simply, the goal is to keep the concrete saturated during the first 28 days. The first 7 days after installation you should spray the slab with water 5-10 times per day, or as often as possible.
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.
Formed concrete surfaces sometimes show ice-crystal imprints, indicating that fresh concrete was frozen. The absence of ice-crystal imprints is one indication that concrete hasn't been damaged by freezing.
How can you tell if concrete is frozen? Imprints of ice crystals can be seen on the concrete surface if it is frozen at an early age.