Cold weather is defined as a period when the average daily temperature falls below 4?C for more than three successive days. These conditions warrant special precautions when placing, finishing, curing and protecting concrete.
Concrete production always faces a problem during cold weather. There are two main problems associated with cold weather. The general rule is that the concrete has to gain strength till about 500 psi (35 kg per cm2).
Concrete can freeze before it gains strength which breaks up the matrix;
Concrete sets more slowly when it is cold - very slow below 6? C; below 4? C, the hydration reaction basically stops and the concrete doesn't gain strength.
But these are concrete temperatures, not air temperatures. So when it's cold, the concrete has to be protected until it can handle the cold on its own. The general rule is that the concrete has to gain strength till about 500 psi (35 kg per cm2). Almost the same time that the concrete achieves 500 psi compressive strength, hydration of the cement consumes enough of the water in the original mix so that even if it does freeze, there's not enough water left in the pores to damage the concrete. With most concrete, even at 6¦C, this happens during the second day.
To help it reach that 500 psi strength, the mix can then be changed to get it to set more quickly or protect the concrete from the cold - or more likely do both.
Changes to Concrete Mix during Cold Weather
Many of the problems with cold weather can be overcome by the ready mix producer. Here are a few things to keep in mind:
Hot water: A ready mixed concrete producer will usually have, and use, hot water in the concrete when the weather turns cold. Most producers will try to have the concrete be at least 180C when it leaves the plant, which is generally good enough depending on air temperature and thickness of the concrete element. The slump has to be specified at less than 100 mm and air-entrained concrete should be used to reduce bleeding.
Accelerators: Since colder weather leads to colder concrete, the set time can be delayed. Accelerators added to the concrete can keep it on schedule. Addition of 2 per cent (by weight of cement) of calcium chloride is the traditional way to accelerate the hydration reaction - it is very effective and reasonably cheap. But - a big but -that much chloride can lead to corrosion of any steel embedded in the concrete (like rebar) and can lead to a mottled surface appearance with coloured concrete.
Non-chloride accelerators are also widely available and are very effective. They won't discolour the concrete, but they are a bit expensive. Accelerators are not anti-freeze agents - they simply increase the rate of the hydration reaction.
Fly ash: Producers should typically stay away from using fly ash or slag cement in cold weather, since those materials set more slowly and generate less internal heat; slag can cause the same effect.
To make the reaction a bit hotter, the ready mix producer can add some extra cement (typically 60 kg per cubic metre) or can use Type III (high-early strength) cement, which hydrates more rapidly.
Producers have to be careful with water reducers in cold weather, since they can slow the set time. Besides, cooler concrete seldom needs water reduction since the cooler temperatures prevent slump loss. For admixtures added at the job site, they shouldn't be used if they have frozen. The chemicals may have separated.
Precautions before Placing Concrete in the Cold
When cold weather comes on unexpectedly, the following tips will help:
Frozen ground: Concrete should be never placed on frozen ground or onto ice or snow. There are a couple of problems with this. First, frozen ground will settle when it thaws, cracking the concrete. Second, when the ground is cold, the concrete in contact with it will be cold and will set more slowly. Crusting might also occur, with the top part of the concrete set and the bottom still soft. If the ground is frozen, it can be thawed using hydronic heat pipes and blankets (such as those from ground heaters), or electric blankets.
Anything that will come in contact with the concrete should be warmed up, including forms and any embedment, to at least 00C. If it's not too cold and everything is covered with tarps the day before the pour, it will stay dry and warm enough.