OVERCOMING THE CHALLENGES OF CONCRETING IN WINTER CONDITIONS

Low temperatures introduce a series of significant challenges that directly affect the chemical and physical processes involved in concrete hardening. Ignoring these factors can lead to serious deviations in the quality and durability of the structure.

Fortunately, with proper planning, correct selection of component materials, and the application of appropriate technology, these challenges can be successfully overcome. The goal is clear: to construct high-quality and durable concrete structures whose performance will not fall behind those built under ideal conditions.

Why are low temperatures critical for concrete?

The two global concrete standards, the European Standard EN 206 and the American Standard ACI 306R-16, provide similar definitions of what constitutes “winter conditions.” According to these standards, “winter conditions” are defined as a period of more than three consecutive days during which the average daily air temperature is below 4°C, and the temperature does not exceed 10°C for more than half of any 24-hour period. Under such conditions, several primary mechanisms of concrete damage may occur:

Slowed hydration: Low temperatures significantly slow down the chemical reaction between cement and water, known as hydration. This process, which is responsible for the binding and hardening of concrete, practically stops when the temperature of the concrete falls below 5°C.

Damage from early freezing: The greatest risk for fresh concrete is the freezing of the water inside it before reaching a minimum compressive strength of 3.5 MPa. When the water in the concrete pores freezes, it expands and forms ice crystals. This expansion generates internal stresses that result in microcracks and permanently reduce the final strength of the concrete.

Thermal shock and cracking: Large temperature differences between the interior of the concrete element (which warms up due to hydration) and the cold ambient air can cause thermal stresses. This effect is particularly pronounced when protective coverings or formwork are removed suddenly, leading to rapid cooling of the surface and the formation of cracks.

BASIC PRINCIPLES FOR CONCRETING IN WINTER CONDITIONS

In order to eliminate the risks mentioned above, four fundamental principles for concreting in winter conditions are generally established.

Preventing early-age freezing of concrete

The primary goal is to protect the fresh concrete from freezing until it develops sufficient compressive strength to resist the internal stresses caused by ice formation.

Ensuring proper strength development

Since low temperatures slow down hydration, it is necessary to maintain an adequate environment that will allow the development of strength characteristics. ACI recommends that the temperature of the concrete be maintained at or above 10°C during the first 48 hours after placement.

Minimizing thermal shock and cracking

Controlling the rate of temperature change is essential to prevent the formation of cracks caused by thermal stresses. This measure directly relates to the occurrence of “thermal shock and cracking” when transitioning from a heated environment to cold ambient air, where the temperature must be reduced gradually.

Maintaining uniform concrete temperature

Temperature consistency throughout the entire volume of the concrete element is essential to avoid internal stresses, shrinkage, and uneven hardening. The outer parts of the cross-section cool faster than the interior, which can lead to cracking. Therefore, measures such as insulating the formwork are required to ensure uniform curing.

Strategic planning and concrete mix design

Planning and careful selection of materials are the most important tools for proper concreting in winter conditions.

OPTIMIZATION OF THE CONCRETE MIX FOR WINTER CONCRETING

Type and amount of cement: Cements with faster early strength development are used, such as CEM I 52.5R or CEM II 42.5R (or ASTM Type III). These cements enable a quicker reaction with water and a more intense release of hydration heat, creating an internal source of warmth that protects the concrete. Increasing the cement content further enhances this effect.

Water-cement ratio (W/C): The water-cement ratio should be kept as low as possible. This is crucial because it reduces the amount of water that can freeze and damage the structure, while simultaneously accelerating strength development. To maintain the required workability, the use of superplasticizer admixtures is mandatory (e.g., Superfluid 21M1M).

The role of admixtures for winter concreting: Introduction of Hidrozim T

The use of chemical admixtures, especially setting accelerators, plays an important role in winter concreting.

Hidrozim T is an admixture for accelerating setting and preventing concrete from freezing, in accordance with standard EN 934-2: T6. In combination with other protective measures, it enables proper concrete placement even at temperatures below –10°C.

For achieving optimal results, the use of Hidrozim T in combination with plasticizers or superplasticizers is recommended.

PREPARATION OF CONCRETE BATCHING PLANTS FOR WINTER CONDITIONS

 A key stage after designing the concrete mix is its production at the batching plant. Here, one of the fundamental strategies is applied: ensuring that the fresh concrete mix has the required initial temperature at the moment of mixing and during transport to the site.

The EN 206 standard requires this temperature to be at least +5°C, as below this threshold the hydration process slows down drastically and nearly stops. For this purpose, at the concrete production facility it is essential to actively control, and most often heat, certain components prior to mixing.

Heating the water

Heating the mixing water is by far the most effective and most economical method for increasing the temperature of fresh concrete. However, there is an important limitation: the temperature of the water must not be excessively high (generally not above 60–70°C). If very hot water comes into direct contact with the cement, proper mixing and placement of the concrete become impossible.

Heating the aggregates

When ambient temperatures are very low and the aggregates are stored outdoors and become frozen, heated water alone is not sufficient to reach the required temperature of the concrete mix. Aggregates (both fine and coarse) make up the largest portion of the concrete mass (around 70–80%). If they are frozen, they will “absorb” all the heat from the water, and the final temperature of the concrete will again be lower than required.

In such extremely cold conditions, concrete batching plants must also apply methods for heating the aggregates. This is a significantly more complex and costly process, which may include applying hot air over the aggregate stockpiles or using steam pipes inside the storage silos. The primary goal is for the aggregates to be at a temperature above 0°C, meaning they must not be frozen. The presence of ice within the aggregates is a critical problem, because when it melts it introduces uncontrolled excess water into the mix, directly disrupting the designed water-cement ratio and reducing the final strength.

PRACTICES FOR PLACING AND PROTECTING FRESH CONCRETE IN WINTER CONDITIONS

Preparation of the formwork

Before concreting begins, all surfaces that will come into contact with the concrete, including the base, the formwork, and the reinforcement, must be completely cleaned of snow and ice.

Proper placement

The concrete should be placed and vibrated as quickly as possible after delivery to the site, in order to minimize heat loss. Rapid placement ensures that the internal heat within the concrete mix is preserved, which helps maintain the necessary temperature for setting and for the hydration process to proceed effectively.

Methods for protecting and curing fresh concrete

Immediately after placement and finishing, all new concrete surfaces must be protected and cured. This is a critical step to prevent freezing and to ensure proper curing. Protective and curing measures include the use of insulating blankets, creating temporary enclosed spaces with indirect heating, and using insulated formwork.

Monitoring and controlling the temperature of fresh concrete

During the setting of the concrete, it is necessary to regularly monitor both the ambient air temperature and the internal temperature of the concrete. This makes it possible to confirm that the concrete remains within the recommended temperature limits throughout the critical curing period. For this purpose, it is advisable to use instruments such as embedded temperature sensors, data-logging devices, or methods for real-time monitoring of concrete strength (maturity methods).

Removal of formwork

The time required for removing the formwork must be extended compared to normal conditions. The formwork should be removed only after it is confirmed that the concrete has achieved the required strength. This confirmation must be based on methods such as calculating concrete strength at the actual curing temperature or testing field-cured specimens kept under the same conditions as the structure itself.

Relying on laboratory-cured standard specimens is common but represents a critical error, as they do not reflect the real conditions on the construction site during winter.

Conclusion

Successful concreting in winter conditions is not a matter of chance. With proper planning and execution, it is possible to construct high-quality  concrete structures even in the most unfavorable weather conditions.