The warm winters of recent years have brought with them many a temptation. One of them is concreting in the cold season. However, this is only possible with special measures.
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Concreting in winter is about ensuring that the quality of the concrete is not negatively affected by frost, snow, rain and wind. Effects of low temperatures on young concrete include:
- Delay in setting and strength development (low concrete temperatures)
- Permanent structural damage due to freezing through too early
- Impairment of strength, freeze resistance, impermeability and durability due to increased w/c ratios
- Reduction of the adhesive bond between reinforcement and concrete
- Danger of frost damage if the ground is frozen (freezing through before frost resistance is reached)
- Cracking due to higher temperature differences (e.g. after removing the heat-insulating cover
- Cracking from cold, dry air and/or wind (increased dehydration)
Air temperature and minimum temperature of the fresh concrete
The magic limit for concreting is an ambient temperature of plus five degrees Celsius. If the thermometer falls below, certain precautions must be taken. Nevertheless, you should not concrete in the freezing cold. Because the lower limit is minus five degrees Celsius. In addition to the air temperature, you must also observe the minimum temperature of the fresh concrete during installation. According to DIN EN 13670 and DIN 1045-3, the following temperatures are required:
An air temperature of plus five to minus three degrees Celsius is the minimum temperature of the fresh concrete during installation
- plus five degrees Celsius in general
- plus ten degrees Celsius for cement weighing less than 240 kilograms per cubic meter and for LH cements
If the air temperature is below minus three degrees Celsius, the minimum temperature of the fresh concrete of ten degrees Celsius should be maintained for at least three days during installation.
Solution: winter recipes
So-called winter recipes have a higher hydration heat development. In addition, these concrete compositions set faster. You can achieve these properties through
- increasing the cement content to at least 300 kilograms per cubic meter
- reducing the w/c ratio, for example with concrete liquefiers or superplasticizers
Measures when concreting
To ensure that the concreting process runs smoothly, you should
- keep all work surfaces and scaffolding free of ice and snow
- Limit waiting times to a maximum of 15 minutes (minimum temperature)
- pour the concrete quickly
- Remove snow and ice from concrete and floor surfaces
- do not concrete against frozen ground
protection and aftercare
Measures that serve to protect the concrete must be carried out until freeze resistance is achieved. You should not drop below a temperature of plus five degrees Celsius.
heat protection
If the frost lasts only a short time, you can protect the floor slab with heat-insulating covers, such as:
- Thermofolien
- board formwork
- dry mats of reed or straw
- lightweight panels
- plastic panels
In the case of prolonged or severe frost, one cover is no longer sufficient. In this case, you need to warm the layer of air surrounding the concrete. are for heating
- infrared lamps or
- dry air
suitable. However, make sure that the concrete surface does not dry out.
moisture loss
In addition to heat loss, concrete must also be protected against moisture loss in winter. In cold and/or dry weather, the moisture content of the air is very low. This causes too much moisture in the concrete to evaporate. In the course of post-treatment, you compensate for this excessive evaporation. The starting point for this is the minimum curing time for the respective concrete. Add to this the time when the surface temperature is below plus five degrees Celsius.
frequently asked Questions
The compressive strength must be at least 5 N/mm so that young concrete can withstand a single freezing through without damage.
No, you should definitely not do this.
No, you must remove the frost damaged concrete. Only then can you continue concreting if the air and minimum temperature are correct.
