If there is one thing that Canadians know, it’s hockey the cold!
We know how it feels to place concrete in winter, wrestling with insulated tarps, waiting for concrete that takes forever to cure, and daydreaming of hot bathtubs. On any construction project, cold weather concreting comes with a set of additional challenges. But don’t let subfreezing temperatures get to you! We’ve created this quick guide in line with the Canadian Standard Association and the American Concrete Institute recommendations (respectively, CSA A23.1/A23.2 and ACI 306R) summarizing everything you need to know. After reading this article, you will be a step closer to pouring concrete in cold weather with confidence.
Ready?
Put on your toque, and enjoy.
Table of Contents
What is “Cold Weather Concreting,” According to ACI 306 and CSA A23.1/A23.2?
Does Cold Weather Concreting apply to you?
If you don’t think so, keep reading. Cold Weather Concreting applies to the majority of projects spanning across winter.
The American Concrete Institute under ACI 306R determines cold weather concreting conditions exist “when the air temperature has fallen to, or is expected to fall below, 40°F (4°C) during the protection period.”
In Canada, CSA A23.1/A23.2 defines the weather as cold “when there is a probability of the temperature falling below 5C within 24h of placing the concrete” as per the forecast of the nearest official meteorological office.
Convinced yet?
Take a look at the maps below. They display the average monthly temperature for January 2019 in the U.S., and the average winter temperature in Canada.
That’s right! Except for Florida, the risk for cold weather concrete affects pretty much everyone.
Why Does Cold Weather Concreting Demand Extra Protection?
If you’ve ever worked with concrete, you know as much as we do that it takes time to cure. At the micro-level, the hydration of the cement progressively increases the resistance of the mix. When the cold snaps, the cement paste takes much longer to develop its early strength, and that could cost you some valuable time before you can strip the formwork and advance the project. If you are unprepared, the internal temperature could even violate critical limits and provoke irreversible damage to the structure, possibly requiring you to demolish and repour.
In addition to air temperature, you should monitor the relative humidity and the wind speed, as both can have a significant impact on the evaporation rates and the internal temperature of the concrete.
Frozen concrete at early age
Exposing plastic concrete to sub-freezing temperatures can reduce its final strength by up to 50%. Damage occurs due to the expansion of water when it freezes, creating ice crystals that will lead to a porous microstructure. Therefore, keeping the concrete warm until it reaches a resistance of 500 psi (3.5 MPa) is best practice. But too much heat compromises the ultimate resistance. It is about finding the right balance.
Always remember that reaching strength is part of the goal but avoiding concrete damage is imperative.
Besides frozen concrete, winter aggravates the risk of thermal and shrinkage cracking.
Shrinkage cracks
Shrinkage occurs when the surface moisture evaporates faster than bleed water replaces it. Low humidity with a cold breeze is a textbook scenario for shrinkage, which will lead to thin but profound cracking through the concrete.
Thermal cracking
Thermal cracking occurs due to excessive temperature differences within the concrete. It is more frequent on larger elements, or mass concrete, when the surface is exposed to a low air temperature while the cement hydration is releasing heat, and the core is getting warmer. As a rule of thumb, 35 degrees Fahrenheit (20°C) is the maximum range you should allow through the concrete element at any given time unless you really understand the performance characteristics of the concrete mix.
Rapid variation of temperature can also result in this type of cracking.
Properly Equip Yourselves to Keep Your Construction Projects Advancing.
It’s better to invest in specialty admixtures, concrete protections, and technology that helps monitor concrete remotely than incur the costs from late delivery or failure to meet specifications. Stopping operations is always an expensive call, even if it is to await better weather conditions. Follow the Boy Scouts’ motto of “always be prepared:” have what you need on hand in materials and a work plan to combat whatever the elements throw at you.
These solutions are critical for major projects like Rio Tinto’s new LNG terminal in Kitimat, BC. A senior engineer working for the joint venture in charge of the construction explains that staying on schedule would not have been possible without the implementation of solutions like the one provided by EXACT Technology.
As we are getting buried in snow on site, I must say that it would be impossible to build on critical path throughout the winter here without the EXACT Technology tool. In addition, comparing this system of thermocouples with all the other systems available in Canada, which we demo on site in Kitimat, it was a right choice to deploy the EXACT Technology system. It is robust for civil/marine projects of this kind, and completely eliminates the necessity for field technicians to go to collect data to unsafe zones and difficult locations, where people are exposed to safety risks.
Control the Initial Concrete Temperature.
Commiting effort here can save a lot of time and money. From using accelerating admixtures in your mix to warming up the water or aggregates, you can control your concrete’s temperature at the time of discharge to assist the curing process.
Accelerating admixtures
This type of admixture is an excellent solution to increase the rate of cement hydration. Accelerating admixtures will help the cement react faster, resulting in the concrete generating heat quicker to balance the low ambient temperature and accelerate the strength development.
Make sure to get assistance when selecting an accelerating admixture as there are plenty of options, from calcium or sodium nitrate to calcium chloride.
Hot water and aggregates
Warmed-up aggregates and water are usually the most cost-effective means to control initial concrete temperature, although water and aggregate temperatures should not exceed 176°F (80°C) as a flash set might result. Crews must maintain the temperature of the mixed concrete above 50°F (10°C) . When you are using water above 176°F (80°C), it may be necessary to adjust the order in which you batch and mix the ingredients. Industry best practice requires you to add hot water and coarse aggregates ahead of the cement and stop or slow the addition of water. This procedure is based on ACI 306R.
Take the Necessary Precautions During Concrete Placement.
Frozen ground
Make sure not to place concrete on the frozen ground. Pouring concrete on a frozen subgrade could cause the concrete to freeze and crack before it cures. It also could leave a void underneath the concrete after the soil thaws and recedes, causing cracking. Utilize ground heating blankets placed on the subgrade to thaw the ground out prior to placing concrete.
Steel surfaces
If you want to be on the safe side, you could also heat the forms and rebar before the pour. Steel is a highly conductive material, and glacial surfaces could result in a temperature drop. On the other hand, Ron Kosikowski shared his research results in this ACI presentation demonstrating that cold rebar causes no significant impact on the surrounding concrete and that rebar temperatures rose above freezing within minutes of the concrete pour.
Protect Fresh Concrete From Cold.
New concrete in cold weather conditions may need extra protection to maintain the activation energy to hydrate properly. Consider insulated blankets or heated enclosures.
Insulated blankets and windbreaks
Using insulated blankets will help the concrete maintain the desired temperature and gain its correct strength. Windbreaks about six feet high should suffice to protect the concrete and crews from winds that cause quick temperature drops and swift evaporation.
Heated enclosures
Heated chambers are another solution. You can build these structures of wood, canvas tarps, polyethylene sheets, or commercial rigid-plastic sections. Simply add a heater once you’ve erected the enclosure to keep the inside warm.
Removing formwork and protection
It is very important to understand when it is appropriate to remove the formwork and thermal protection. This will generally be specified in a thermal control plan for the project based on temperature differentials and required strength. For a smooth temperature transition that prevents thermal cracking, we recommend slowly phasing out the heaters and protection.
In an ideal world, we would leave our forms on for an extended period of time to ensure adequate strength gain and proper curing has been achieved but most schedules push us to remove as soon as possible. EXACT Technology has helped many contractors determine the earliest time they can remove their forms or molds safely.
Use Heaters During the Curing Period.
Heating the concrete ensures that proper strength develops and maintains hydration, which is essential during the concrete curing process. If your slab gets too cold, then curing can stop altogether. Portable heaters deliver extra heat into the ground and directly on the concrete, assisting the curing process. But be careful, as improperly heating the concrete can compromise the structure as well.
Direct-fired heaters
We recommend electric heaters if you want to use direct-fired heaters that you can direct towards the slab within an enclosure.
Indirect-fired heaters
If you decide to go with indirect-fired heaters (fuel-burning heaters), they need to be placed outside the enclosed structure due to safety concerns. Therefore, the warm air is funneled into the enclosure via flexible ducts. You will also need to supply additional moisture. These extra precautions keep the new concrete surface from becoming carbonated (ACI 306R). Remember, we want the concrete to cure, not to dry.
Hydronic systems
Another heating tactic is to use a hydronic system that circulates a warm mixture of glycol and water throughout the enclosure via pipes or hoses. This solution is more effective than the ones above, which limits the crew and equipment downtime.
We want the concrete to cure, not to dry.
Determine In-Place Strength.
Monitoring the curing phase is just as crucial as the previous steps of this guide. Knowing how the temperature, humidity, and strength evolves in your concrete will help you to adjust your parameters continuously. There are several techniques you can use, depending on the project specifications:
Rebound hammer
This device is a spring release mechanism that activates a hammer to drive a plunger into the concrete’s surface. The rebound distance from the hammer to the concrete’s surface is given a value from 10 to 100 used to calculate its strength. It’s easy to use onsite. The downside: the device requires pre-calibration for accurate measurements, and large aggregates or rebar below the testing location could skew the results. We do not recommend this method to measure the strength of the concrete in the first few days.
Penetration resistance
A device drives a small pin or probe into the surface of the concrete. The force used to penetrate the concrete and the resulting hole’s depth are correlated to the concrete’s strength. This method is also easy to use. However, surface conditions as well as the types of form and aggregates used greatly affect data. Moreover,the pre-calibration requires multiple concrete samples to ensure accuracy. As with the rebound hammer, this method is not appropriate for monitoring early strength development.
In-place maturity testing
Embedded sensors enable you to record the internal concrete temperature at multiple depths throughout the curing process. Most systems require a technician to check the loggers periodically and write down the data, but companies like EXACT Technology now offer real-time, wireless solutions. This approach delivers better accuracy than the above methods, but do your research — not all solutions provide the same level of reliability and functionality or have the same proven track record.
Cylinder test breaks
This method tests for the compressive strength of cylindrical concrete specimens representing the in-situ concrete. It is usually performed for quality control, acceptance of concrete, and the scheduling of construction activities. But the results are not always reliable. EXACT Technology developed a portable curing box that matches the actual temperature of the concrete being placed. This way, the results are more accurate, and users avoid certain problems that arise in the cold (such as frozen specimens). Match curing enables you to verify target strengths much faster than traditional methods and can subtract days from your project schedule.
Leverage the Benefits of Real-Time Online Concrete Data.
As a result of technological advancement, sensors that you can read in real-time from any connected device are replacing old thermocouples. Adoption is snowballing as the benefits are evident:
- Savings on labour and improved safety for crews responsible for collecting temperature data from hazardous locations;
- 24/7 instant data with maturity curves already calculated, and real-time alerts via email or SMS;
- Reduced heating expenses from stopping the heating system as soon as the concrete achieves its required strength;
- Benefits associated with advancing projects as soon as concrete reaches target strength;
- Accelerated response time to any problems – like heater shutdowns or rapid temperature drops;
- Increased confidence in consistent adherence to concrete specifications.
In 2016 when we were developing the first prototype of our monitoring equipment, we installed it in parallel to the traditional system at the Eglinton LRT project. This was on a Friday in the winter and the project team was planning to place the concrete the following Monday morning so they had heaters running throughout the weekend to make sure the area was warm enough to place the concrete. Because this was new to everyone, we were all watching the data online and on the Sunday we realized that the temperature in the enclosure had dropped below freezing. We called the superintendent and he went to site and, of course, the heaters ran out of fuel. He was able to react and fix this to avoid having to delay the pour the following morning. It’s events like this one that make our client understand the full value of our online real-time technology.
Add the Extra Costs of Cold Weather Concreting to Your Bids.
There will be extra costs when pouring concrete in winter. It’s unavoidable, but it’s better to pay a little more than to accrue fees from extending deadlines or to risk major structural defects. We want you to get ahead of these costs, so here are the areas to budget extra:
- Special additives in the concrete mixes;
- Protection measures for the concrete and subgrade when applicable;
- Preparation of the subgrade for slabs poured directly on the ground;
- Reliable monitoring systems and testing equipment;
Plan to add these costs to your bids!
Conclusion
We often hear that you should not be performing concrete work below 40 degrees Fahrenheit (4°C). In reality, schedules might demand otherwise. And honestly, there are many ways to keep moving forward, whatever the weather is. Using the above vital points as a guide can significantly improve your experience pouring concrete in cold weather conditions, whether you are working on a bridge, a dam, an LRT, LNG terminal, or a high-rise. We hope you find these tips useful and that they will help you maintain operational efficiency throughout the winter.
To learn more about Cold Weather Concreting, we recommend referring to the CSA and ACI page dedicated to the topic.
