When Is It Too Cold To Pour Concrete?

Concrete is one of the most versatile materials used in modern construction, and it can be considered one of the most important as well. It forms the foundation of buildings, can be used to make walls, sidewalks, patios, roads and bridges and is resistant to the damage and gradual decay that can affect stone, wood and metal. In addition, because concrete is usually applied by pouring rather than by using planks, sheets or panels, it can be used with molds and frames to form complete structures quickly at a relatively low cost.

However, concrete is not perfect. Like any other building material, it has traits and quirks around which you must plan. In addition to being incredibly heavy once set, freshly poured concrete is vulnerable to extreme temperatures. While concrete can be poured at any time of year and in nearly any location, there ​is​ an ideal range of temperature in which to pour concrete, and if it's too cold or too hot outside, you will need to account for that while using the material.

Concrete is a material made up of two core components: ​an aggregate,​ made up of sand and gravel, and ​a paste,​ made from a mixture of clean water and cement – a fine powder made from a combination of finely ground and heated stone, minerals, ash and other products. When the paste and aggregate are blended, it creates a malleable, semi-liquid material that can be poured and shaped, but when left alone for some time, it hardens into a heavy, hardy, rock-like mass.

Concrete gains its strength through a process known as ​hydration,​ which is commonly referred to as "curing." During this process, the compounds in the cement react with water, crystallizing and chemically binding together individual particles of cement, resulting in a solid slab.

The initial hardening process takes place over the first few days after the concrete is poured, but the curing process continues in some form for years after that. This is why concrete is said to grow stronger over time. However, because the curing process involves water, high and low temperatures present problems.

The first three days after concrete is poured are considered the most important. During this time, the semi-liquid concrete hardens and undergoes the majority of its curing as the cement binds with water. Because the presence of water is so critical to the curing process, ​pouring concrete in winter and summer presents problems.​

In seasons where temperatures are liable to fall ​below 40 degrees Fahrenheit​ (4.4 degrees Celsius) ​or exceed 90 degrees Fahrenheit​ (32 degrees Celsius), the ratio of water, cement and aggregate can shift, changing the end result. In high temperatures, water can evaporate and result in a misshapen and weaker final product, but in low temperatures, the ​freezing​ of water in the concrete mixture can lead to flaws that can be significantly more dangerous. At and below these temperatures, concrete cannot be poured without preparation and extra equipment.

The curing process that causes concrete to harden involves a chemical reaction that is affected by heat. While the excess heat presented by summer can negatively impact concrete curing, the low temperatures seen in winter can do dramatically more damage. ​At temperatures below 40 degrees Fahrenheit,​ the initial curing process can take anywhere from two to 24 hours longer – and when cold enough, it can be stopped entirely.

At the same time, concrete poured in freezing temperatures can develop cracks as water in the mixture freezes and expands. This can lead to the top layer of newly laid concrete popping off like a lid or can cause foundations to shatter under the weight of a building in extreme situations.

Concrete laid on frozen ground can also fail to harden properly, leading to unseen flaws and making constructions more vulnerable to shifting. For these reasons, concrete pouring in winter is usually done by professionals who know what precautions to take.

Despite the problems that can arise during the process, pouring concrete in cold weather can still produce a solid end product. It just requires precautionary measures. First, the ground where the concrete will be poured must be prepared. Builders will often use heated mats to warm the ground to a more stable temperature and will then use a special concrete blanket (or straw, in some cases) to cover and keep the curing mixture warm for the first few days.

Chemical accelerators that speed up the curing process can also be added to the concrete mix prior to the pour if the area is expected to be incredibly cold. Heated concrete mixes and cement blends intended for use in frigid environments also exist and are used in specialized cases. These can allow concrete to be poured at temperatures under 30 degrees Fahrenheit.