Concrete Curing and Building in Cold Weather

Concrete does not dry, it hardens through a chemical reaction that depends on temperature and moisture. Curing in hot and cold weather and rules for winter work.

On building sites you often hear "let the concrete dry". In fact concrete does not gain strength simply by drying out. Cement reacts chemically with water, and this reaction, hydration, is what gives concrete its strength. The reaction needs both water and a suitable temperature. If concrete loses its water too early or freezes, it may never reach the strength assumed in the design. That is why the days after a pour matter as much as the day of the pour itself.

How concrete gains strength

Hydration starts shortly after pouring and continues for a long time. Strength rises quickly in the first days and then more slowly. Design strength is usually defined at 28 days under normal conditions; this does not mean concrete stops gaining strength after 28 days, only that it is the accepted reference point.

The rate of strength gain depends directly on temperature. Heat speeds the reaction up, cold slows it down, and below freezing it practically stops. The cement type and mix composition also affect the rate.

For this reason, striking formwork, applying loads and moving to the next stage should be decided by the actual condition of the concrete, not by the calendar. The same element reaches the same strength in a different time in summer and in winter.

Curing in hot weather

In Azerbaijan's summer months, the main danger is rapid evaporation of water from the concrete surface. Sun and wind dry the surface, causing fine cracks and leaving the top layer weak. Large exposed surfaces such as floor slabs are especially vulnerable.

The main measures in hot weather are:

• scheduling pours for the cooler hours, early morning or evening;
• dampening the formwork and base before pouring;
• covering the surface after finishing with plastic sheeting, damp material or a curing compound;
• keeping the surface moist regularly during the first days;
• shortening the time the concrete spends in transit.

Adding water to concrete on site is a frequent but unacceptable mistake in hot weather. It makes the mix easier to place but reduces strength and increases cracking.

The risks of cold weather

In cold weather the main danger is the water inside fresh concrete freezing. Water expands as it freezes and damages the structure of concrete that has not yet gained strength. Even if such concrete later thaws and continues to harden, its strength and durability are reduced.

Even above freezing, cold weather slows hardening significantly. In construction practice, periods when the average daily temperature falls below a certain threshold are treated as winter conditions and require special measures. The threshold and the measures are set out in the standards and the project documents.

Rules for concrete work in winter

Pouring concrete in cold weather is possible but requires preparation. There must be no snow or ice on the formwork and reinforcement, and concrete must not be placed on frozen ground. The mix design and, where needed, admixtures suited to cold weather are agreed with the concrete plant.

After pouring, the concrete is protected with heat-retaining covers. For small volumes insulation materials are used; for larger structures, heating under temporary enclosures or electrical heating methods are applied. Heating the concrete too quickly and drying it out is also harmful, so the temperature is kept under control.

Measuring and recording the concrete temperature is an important part of winter work. These records show when formwork can be struck and whether the concrete is past the risk of freezing.

Striking formwork and loading

Removing formwork early is tempting to speed up construction, but a hasty decision can cause a slab to sag or crack. Side forms come off once the concrete is strong enough to hold its shape; soffit forms and props come out only when the element can carry its own weight and the loads on it.

In cold weather these periods are longer. Stacking heavy piles of masonry materials on a fresh slab should also be restricted until curing is complete.

What the client should ask

• In what weather and at what time is the pour planned?
• How will curing be organised, and who is responsible?
• What protection method will be used in cold weather?
• Will sample cubes be taken and the results provided?
• Who will decide when formwork is struck, and on what basis?

Zaferoğlu İnşaat works with a team of professional tradespeople experienced in modern construction technologies. You are welcome to discuss with us how to plan your project around the season.