The factors and considerations concrete contractors should be aware of for placing concrete in cold weather
While there have been countless articles written about the challenges involving ready-mixed concrete when faced with cold weather, this article will focus on cold weather concreting tips and best practices for those smaller projects utilizing bagged concrete or related concrete repair materials. This can help concrete contractors to employ the right cold-weather concreting precautions and practices from the fall into the spring of any given year. Concrete Accelerators Types
ACI 306R defines cold weather as a period when the average daily temperature falls below 40° F for more than 3 consecutive days and the air temperature is not greater than 50° F for more than one-half of any 24-hour cycle during the period.
When water and portland (hydraulic) cement are mixed, a chemical reaction occurs which releases heat, known as the “heat of hydration”. This exothermic reaction puts the setting of concrete in motion as well as the development of concrete strength, known as maturity. Lower temperatures slow down the cement hydration process, resulting in delayed set times and reduced compressive strength of concrete at its earlier ages.
The approximate setting time for concrete increases as the temperature drops. Plan ahead. Sakrete
The optimal temperature for concrete is 70° F. We can see from this chart that when the temperature drops to 40° F, the set time more than doubles from placement to final finishing. Bear in mind that these studies and reports, such as the chart shown above, reflect the placement of ready-mixed concrete during colder temperatures. There is a lot more mass in 1 cubic yard of concrete as compared to the 0.6 cubic feet yield from an 80-pound bag of concrete mix. This means there is a lot less heat generated when mixing bagged concrete as compared to that poured from a ready-mixed concrete truck. Due to the smaller mass being placed, one should expect the set times for bagged concrete placement to actually take longer than the “Concrete Set Time and Temperature” chart indicates in cold weather.
As air temperature drops, so does the ground temperature. In the case of a footing or post hole, you have the soil surrounding the concrete to consider. When pouring a slab, the forms and the stone base come into play. In these scenarios, the job-site factors all absorb heat from the freshly mixed and placed concrete.
There are three ways to optimize your “winter mix” to help maximize heat of hydration in colder temperatures. Either use a concrete bagged mix with a higher cement content, change to a mix with a more reactive, faster-setting cement, or both. To accomplish this, consider upgrading to Sakrete 5000 Plus Concrete Mix for the higher cement content or using Sakrete Fast Setting Concrete Mix for a speedier set.
For those heavier commercial or industrial projects, or you want the best of both worlds, take advantage of the higher cement contents and special cement blends like Sakrete Pro-X 180 Concrete Mix or Sakrete Pro-Mix Accelerated Concrete Mix.
Choosing the right mix for cold weather will help the concrete to perform better in lower temperatures, but it does not change the fact that it may still be too cold to “jumpstart” the heat of hydration. Here are strategies to help generate heat to begin the setting process:
ACI 306.1-90 recommends protecting fresh concrete from the elements for at least 3 days after placement, maintained at 50°F. The point is concrete can freeze after placement. Concrete needs to be protected until it reaches a minimum compressive strength of 500 PSI [3.5 MPa]. Plastic concrete that freezes may suffer reduced strength by 50% and its durability will be adversely impacted.
Do not count on heat generated by cement hydration to protect fresh concrete from freezing. A calcium chloride accelerator is not a concrete anti-freeze.
Although cold weather concrete placement poses its challenges, if done properly, concrete placed at lower temperatures will benefit from the slower curing times. This results in a concrete that will develop higher ultimate strength and increased durability.
If you are newer to concrete or simply wish to know more about cold weather and the challenges it poses for concrete, a good place to start is by reading the American Concrete Institute Standard Recommended Practice for Cold Weather Concreting (ACI 306R).
Concrete Accelerator Admixture Dirk Tharpe, Sakrete Concrete Expert Sakrete Dirk Tharpe has spent his professional life in the ready-mixed concrete, masonry, and concrete repair goods industries. He currently works with Oldcastle APG representing the Sakrete, Amerimix, and Belgard brands. Sakrete concrete, concrete repair, and mortars have the right Pro-Grade products for any project and a dedicated team of Technical Services Experts to help. Oldcastle APG, a CRH company, is one of the leading providers of outdoor living and building products. Oldcastle APG belongs to CRH’s Building Products division. CRH is a leading global diversified building materials group, employing approximately 77,600 people at over 3,100 operating locations in 31 countries worldwide.