Concreting at low outside temperatures
Published
03/2023 by Thomas Hartwig
The fresh and young concrete reacts sensitively to low temperatures and is damaged by frost.
The reasons for this are:
Strength development slows down the lower the ambient temperature. Below freezing point, it even comes to a standstill.
The freezing of water in the concrete damages its structure.
To be noted:
Freeze-resistant concretes continue to harden normally after a single freeze-through, but even such concretes do not survive multiple freeze-throughs.
Air entraining agents do not contribute to frost resistance.
The addition of accelerators may lead to faster hardening, but this is difficult to control.
Concrete can be poured in cold weather, provided the necessary precautions are taken:
Increasing the cement content ≥ 300 kg/m³ and/or the use of cement with higher heat development with otherwise the same raw materials.
Reducing the w/c ratio ≥ 0.55 by using a concrete plasticiser or superplasticiser.
Extension of the formwork removal times and the curing time.
Use of materials with increased thermal insulation properties for the formwork and for curing (e.g. thermal mats).
Raising the fresh concrete temperature through targeted heating of the addition water and/or heating of the aggregate.
Protect component or entire building from heat loss and draught.
Concrete may not be poured on frozen ground, nor on frozen building components. on frozen structural elements.
Keep formwork surfaces and reinforcements free of ice and snow, but never with water, but by heat treatment.
The preheated concrete must be placed quickly in the formwork, which must be compacted immediately.
If possible, protect the young concrete from heat extraction during transport and on site.
Precautions must be taken in the poured concrete to measure the concrete temperature continuously.
When placing and during processing, the fresh concrete must not be colder than + 5 °C without special measures. For concrete surfaces with increased requirements, it is recommended that the fresh concrete temperature be increased to + 10 °C. For cement contents below 240 kg/m³ and when using cements with low heat of hydration, the fresh concrete temperature must not be lower than + 10 °C.
At air temperatures below -3 °C, a fresh concrete temperature ≥ + 10 °C must additionally be maintained for at least three days.
In frosty conditions, curing with water is not permitted.
The freeze resistance of the young concrete is reached when it has a compressive strength of 5 N/mm².
