With our recent mild winters, sometimes it’s hard to remember how cold it can get during the months of January and February. Even though we are currently experiencing more temperate weather, a review of how cold temperatures can impact certain aspects of construction is always beneficial. This is especially true here in Oklahoma where it can be 70F in the morning and 20F by nightfall!
One of the challenges construction teams typically face during winter months is placing concrete during periods of low ambient temperatures. This is because the ability for concrete to strengthen and cure is dependent on the temperature of the day it is placed. Specifically, the American Concrete Institute (ACI) advises that when the average daily temperature falls below 40F for more than three successive days, these conditions may warrant special precautions when placing, finishing, curing, and protecting concrete against the effects of cold weather. For example, concrete must be protected from freezing shortly after being placed, as it takes longer to develop the required strength in the cold. Additionally, being careful to not remove formwork when concrete is too cold or hasn’t reached the desired strength is critical, as this could reduce its strength and cause surfaces to collapse.
Ross Group encountered these conditions in 2009 during construction of a new Maintenance Hangar located at the Tulsa International Airport. The project was to construct an 82,000 sq. ft. fabric membrane structure to serve as a maintenance facility for American Airlines. At the time the concrete for the project was scheduled for placement, Tulsa experienced several periods of weather around 30F for multiple days in a row. This cold weather had the potential to impact the foundations for the hangar and the parking aprons. Having strong concrete is particularly important when vehicles the size and weight of aircraft will be resting on it.
Ross Group’s superintendent for the project, Ken Baker, spoke to us about the steps he implemented to safeguard the concrete placements. “The ACI manual is the “bible” for all concrete work and there is a section of the manual solely devoted to the proper placement techniques for concrete in cold weather. We knew the conditions we could be faced with and relied on the manual,” Ken informed us. His team used insulated concrete blankets to cover and protect the slab and footing subgrade from freezing prior to the placement of the concrete. This allowed the subgrade to warm to ACI specifications. After the concrete was placed, the entire concrete surface was covered with insulated blankets. This held in the heat generated from the concrete curing process and prevented the surface top from freezing. The team also used insulated formwork to protect the concrete during the first 48 hours, when concrete strength development is critical. Finally, they used hot water when mixing the concrete at the batch plant to speed up the setup period of the concrete being placed. These proper curing conditions prevented cracking and provided the intended serviceability of the structure.
The team considered several other options, such as portable heaters to introduce warm air under the blankets and ensure proper subgrade temperatures prior to concrete placement. In extreme conditions, chemical additives can be introduced into the concrete mix design to hasten the initial set of the concrete and counteract the effects of low ambient temperatures to the curing process. These admixtures come at a high price and are sometimes cost prohibitive to the project. Fortunately, neither of these steps were required on our hangar project.
The team kept the project on schedule despite the potential cold weather delays. Our project team knew that proper planning was the cornerstone to a successful concrete placement. For most projects, this planning takes place weeks ahead of a scheduled concrete placement and weather is always a determining factor when deciding whether to proceed with this activity. In this case, our project team knew that there could be weather risks leading up to the placement, so they began implementing a cold weather concrete placement plan early to be prepared for any weather type. As a result, Ross Group was able to proactively address delays and provide a critical facility that serves an important function for our client.
So, what does this mean to you? Cold weather construction can impact you whether you are placing concrete for a home improvement project, such as a patio or driveway, or for a large project for your company. Monitoring the weather and preparing for its potential impacts are lessons we can all apply to complete our projects successfully. #coldweatherconstruction #coldweatherconcrete