Extreme weather conditions (extreme heat or cold; windy condition, and humidity variations) can significantly alter the quality of concrete. In cold weather concreting, one should make sure that all the negative impacts of low ambient temperature are appropriately alleviated by taking the necessary precautions. In this article, we will review important steps that can ensure you will get the quality you are looking for. But first, let’s see what cold temperature is for concrete, and why it is critical.
What is cold for concrete?
American Concrete Institute definition of cold weather concreting, ACI 306, is:
1- A period when for more than three successive days the average daily air temperature drops below 40 ˚F (~ 4.5 °C) , and
2- Temperature stays below 50 ˚F (10 °C) for more than one-half of any 24 hour period.
In Canada, where temperatures tend to be much lower during the cold season, the following criteria is used by CSA A23.1:
1- When the air temperature is ≤ 5°C, and
2- When there is a probability that the temperature may fall below 5°C within 24 hours of placing the concrete.
Why Cold Temperature is critical
The hydration of cement is a chemical reaction. Extremely low temperatures as well as freezing can significantly slow down the reactions, thus, affecting the strength growth. In fact, freezing temperatures within the first 24 hours (or when concrete is still in plastic state), can reduce the strength by more than 50%. The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.
Necessary Precautions for Cold Weather Concreting
1- Removing Ice + Snow from the surface of formwork
2- Ordering concrete with temperature between 10 °C – 25 °C
Note: Heating water and/or aggregate is one way of achieving the objective; however, heating cement is not considered as effective.
3- Avoid using Calcium chloride or other de-icing salts
4- Slab formwork temperature
Slab thickness < 1.0 m : 10 °C
Slab thickness > 1.0 m : 5 °C
5- CSA A23.1 specified that protection shall be provided by means of:
Note: The heat generated from hydration process should suffice in most cases, if appropriate insulating blankets of polyethylene sheets are used. Additional source of heat might be required based on area and temperature.
6- Avoid wet curing should you expect the temperature to fall to freezing point
7- The temperature gradient of concrete surface and ambient environment should not exceed those specified in standards, such as CSA A23.1
On-Site Strength Prediction
Maturity method is an effective method for on-site prediction of concrete strength. The method is standardized by ASTM C 1074, and both ACI 318 and CSA A23.3 approve it for the purpose of on-site strength prediction. For more information on maturity method, visit the link below: