Set Acceleration/Cold Weather Concrete

Set Acceleration/Cold Weather Concrete

The concrete should be protected from rain and frost during handling. Concreting is only possible in freezing temperatures if special protective measures are taken. They must be in place from the start of concrete production to the end of curing.

They depend on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the concrete pour.

The fresh concrete must not be colder than +5°C during placement and installation without additional protective measures. The mixing water and aggregates should be preheated if necessary.

Problem

Low temperatures retard the cement setting. At temperatures below –10°C the chemical processes of the cement stop (but continue after warming). Dangerous situations arise if concrete freezes during setting, i.e. without having a certain minimum strength. Structural loosening occurs, with a corresponding loss of strength and quality.  The minimum strength at which concrete can survive one freezing process without damage is the so-called freezing strength of 10 N/mm². The main objective must be to reach this freezing strength as quickly as possible.

The temperature t of fresh concrete can be estimated by the following equation:

t_concrete= 0.7 × t_aggregate+ 0.2 × t_water+ 0.1 × t_cement

Measures

1. Minimum temperature

The fresh concrete temperature on delivery must not be below +5°C. (For thin, fine structured elements and ambient temperatures of –3°C or below, EN requires a fresh concrete temperature of +10°C, which must be maintained for 3 days!) These minimum temperatures are important for setting to take place at all. The concrete should be protected from heat loss during handling and after placing (see Protetive measures).

2. Reduction in w/c ratio

The lowest possible water content gives a rapid increase in initial strength. Generally there is also less moisture available to freeze. Superplasticizers allow a low w/c ratio while retaining good workability.

3. Hardening acceleration

Gives maximum hardening acceleration when there are high initial strength requirements.