The harmful effects of low temperatures on newly executed concrete are basically twofold. Pure water freezes at 0°C and this causes its volume to increase by approximately 9%. If this occurs, and the concrete still does not have enough tensile strength to withstand the stress generated by the freezing of internal water, there will be irreversible damage that will limit the mechanical performance and durability of the concrete. In this sense, concrete is considered to be resistant to the effects of cold weather (by expansion of frozen water) when it has reached approximately 3.5 N/mm2.

The second problem is related to the speed of hydration of cement and is closely linked to the first problem. The low temperatures act as a retarder of the setting, slowing down the entire process of developing resistances (to reach the minimum value from which the concrete can already face a frost). The relevance of this problem will be even greater if cements evolve according to the logic of trends (to reduce emissions), resulting in cements more sensitive to the retarding effect of low temperatures.

All regulations of all advanced countries related to the execution of concrete structures include conditions and regulations for concreting in cold weather. In all of them, a series of mandatory measures and limit conditions are listed to guarantee the quality of the structures when concreted in cold weather, which include the use of antifreeze additives and setting accelerators.

However, in addition to the indications of the respective standards for concreting in cold weather, there are other very important considerations that are not always taken into account. And all of them aim that the concrete has developed enough resistance at the time the frost arrives.

The most important additive when concreted in cold weather is the water-reducing additive

When concreting in cold weather, the first thing that comes to mind is antifreeze additives. However, the importance and role played by the use of plasticizing additives is rarely known, due to their setting retardant character. This point is especially relevant in the case of concrete prepared in the plant and supplied by truck, where this type of additives are used on a regular basis.

The retarding effect of conventional plasticizing additives increases exponentially with the decrease in the temperature of fresh concrete, so in cold weather conditions they should be used cautiously. In low temperature situations, it is highly recommended to minimize the dosage of plasticizing additive to the maximum (<0.4% spc) and increase that of superplasticizing additive until the desired consistency is obtained. With this practice, the beginning of concrete setting is advanced, and the development of initial resistances is accelerated (with hardly any practical consequences on the maintenance of consistency during transport), a fact that is very beneficial so that the concrete can develop maximum resistance at the moment that negative temperatures arrive.

The key is to perform the concreting at the optimal time of day

On a day of cold weather, we could say that minimum temperatures are recorded from sunset until the late hours of the morning. During this period of time it is possible that the temperature is below 0°C for many hours, leaving the young concrete exposed to possible damage. Consequently, if at the time of these conditions the concrete has not been able to generate enough resistance to resist the effects of frost, damage will be inevitable and irreversible.

The commissioning must always be carried out at the earliest time of the day after the minimum temperature allowed for concreting is exceeded. It is that the fresh concrete can be the maximum of hours in the range of maximum temperatures so that it fragüe as soon as possible and can develop resistance for as long as possible (below +4 °C hydration hardly progresses). This is probably the most effective measure for concreting in cold weather with guarantees.

Antifreeze additives are used in cold weather situations to accelerate the start of setting and the development of initial resistances during the time when, after commissioning, temperatures are favorable for cement hydration to progress. These additives allow concreting to be carried out in conditions in which, without its use, young concrete could not develop the minimum sufficient resistance to face the moment of frost with guarantees. This is the case of winter days where the maximum temperature does not exceed 5-6°C, for example, or when low temperature is combined with very high humidity. But it is important to note that these are not miracle products, so the indications of current regulations must always be respected.

If at the time of the first frost the concrete has been able to develop enough resistance to tolerate the stress generated by the freezing of water, it does not matter what the minimum ambient temperature that can be reached, since the physical effects are the same at -3°C as at -10°C.

You might think that the optimal time for concreting in cold weather would be coinciding with the maximum temperatures of the day, but this is not the case. For example, a concreting at 16:00 h would be reckless despite being in the time slot of maximum temperature, since the concrete would not have enough time to develop the minimum resistance sufficient for the moment when the temperature begins to show negative values.

Take advantage of the internal heat of fresh concrete as a setting accelerator

When the climatic conditions of cold weather occur (for the purposes of concrete), without a doubt the moment when the fresh concrete is at its maximum temperature is just after the kneading. The formation of etringite in the immediate phase of the hydration of the cement and the energy of the kneading itself generate heat that accumulates the freshly mixed concrete. In addition, the concrete plant has means to ensure the minimum temperature of fresh concrete required in cold weather conditions.

From here, the internal heat accumulated in the freshly kneaded concrete will dissipate over time. Therefore, it is important to minimize the time between the kneading of the concrete and the end of the commissioning. Running a concrete with a few degrees more temperature is critical to accelerate the development of initial strength. To do this, in cold weather conditions, a good preparation on site with the aim of applying the concrete as quickly as possible reduces the typical risks of cold weather on young concrete. In the same vein, it is preferable that the transport time between the plant and the work be as short as possible.

Not all structures are the same in cold weather

In line with the previous point, the dissipation of the internal heat of the freshly kneaded concrete continues once it is already executed (until the beginning of setting, when internal heat will develop). As a general rule, the greater the volume of concrete, the lower the risks against cold weather conditions, but also the loss of internal heat is closely linked to the geometry and the exposed surface where the internal heat of the particular structure can be dissipated.

Slender structures or structures with a large exposed surface, such as a pavement, will lose the accumulated heat more quickly than a structure that accumulates the largest volume with the minimum exposure surface or directly protected (buried structures, formwork).

Always when concreted in cold weather, it is convenient to isolate and protect the newly concreted structures to the maximum to preserve the internal heat, but this measure is especially critical when unfavorable structures in terms of heat dissipation must be executed.

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