Protecting Concrete in Cold Weather

Cold weather and concrete curing

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Protecting concrete in cold weather is an ongoing challenge for concrete contractors and site supervisors. Placing concrete in the cold weather requires additional preparation and protection. All necessary precautions should be taken in order to alleviate the negative impacts of cold weather which can damage both the look and strength of concrete.  Special curing and protection is required to ensure that the temperature of the curing concrete stays as close as possible to the optimum curing temp for concrete: 50-60F. This article provides a quick review of some of the widely used protection techniques, strategies, and challenges in protecting concrete in cold weather.

Before we tackle those concerns, let’s see what cold means for concrete.   The hydration of cement is a chemical reaction. Extremely low temperatures, as well as freezing, can significantly slow down the reaction, affecting the strength growth of the concrete.  In fact, freezing temperatures within the first 24 hours (or when concrete is still in the plastic state) can reduce the strength by more than 50%.

Protecting Concrete in Cold Weather

The Canadians are very familiar with protecting concrete in cold weather.  Their CSA standards provide three primary way to protect concrete in cold weather conditions:

  1. Coverings
  2. Insulation
  3. External Heating Sources

Protection is required during the entire curing process, not just through the initial hardening phase of concrete.  Protection is needed until the required structural properties such as strength are achieved.  Before concrete is considered safe for exposure to freezing the two primary properties that need to be observed are:

  • minimum strength of 500 psi, 3.5 MPa
  • compressive strength of 7.0 MPa

Traditionally, cast-in-place punch out cylinders are used to estimate the strength at certain intervals. Recent advancements in wireless sensor technology have provided the maturity method with significant increases in popularity.  The maturity method predicts the early age strength since that strength is directly related to the hydration temperature history of cement paste.   There are a number of tools that can measure concrete structural process and standard measurements that need to be met.

Wet curing during this period should be avoided since adding water will keep the concrete saturated so that freezing will damage it even after it reaches 500 psi compressive strength.

1. Covering Concrete

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Curing blankets Without Insulation

Covering with curing blankets is widely used in construction sites during the cold season. The heat generated from the hydration of cement is normally sufficient for many cases as long as  the blankets are used properly. Blankets should remain in place for a couple of days. Determining coverage requirements, including additional insulating R-Values,  depends on the:

  • thickness of concrete
  • amount of cement
  • anticipated cold temperature.

When covering with blankets, special attention should be given to the corners and edges of the slabs.

Adding Insulation

If conditions are warrent it, insulating blankets of polyethylene sheets can be used to aid heat retention.  In addition, foam-filled insulating blankets with specified R-Values are also available for concrete curing.

3. External Heating Sources

If covering alone can not keep the concrete temperature at desired levels, external heat source such as electric heating blankets, or hydronic heating pipes should be used.   If those measures are not enough or if the weather is extremely cold (even prior to placing concrete), then heated enclosures should be used. This technique includes enclosing the construction site (for example the storey under construction), and heating the space.  Again the requirements depends on the the amount/area of cement and the anticipated cold temperature. There are certain challenges should be addressed to make sure that concrete cures properly.

  1. Carbon Dioxide-Carbonation.  One common challenge with heated enclosures is the problem of carbonation. The carbon-dioxide produced by some of commercially available heaters increases the chance of carbonation of freshly placed concrete. This can lead to formation of a weak concrete layer which is often unacceptable. Because of this, it is recommended to use heating systems that exhaust to the outside of the enclosure.
  2. Rapid Drying / Uneven Heating.  The use of heaters can result in very rapid drying of concrete.  Rapid drying increases the chance of plastic shrinkage and might lead to poor quality concrete.  The latter is particularly true if the water required for the hydration process evaporates. It is recommended to move the location and direction of heat source for a more uniform heating pattern.
  3. Fire. Special attention should be paid to the heaters that use propane gas as they pose a fire hazzard.

Monitoring the Curing Process

Site managers, and engineers are responsible for assessing the concrete to make sure it reaches the desired strength. Temperature monitoring using infrared thermography from concrete surface or maturity method can be used for better decision making.

Additional Information