Freezing conditions occur when the air temperature falls, or is expected to fall, below 5ºC during the protection period. If you expect low temperatures during your next concrete pour, you must plan and ensure all materials, workforce and equipment are on-site and ready before pouring concrete.
You will need to:
Concrete gains very little strength at low temperatures because the hydration process is delayed.
You must protect freshly mixed concrete until its degree of saturation has been sufficiently reduced by the process of hydration. This corresponds roughly to the time required to obtain a compressive strength of 5 MPa.
Significant ultimate strength reductions, up to about 50%, can occur if the concrete is frozen within a few hours after placement or before it attains a compressive strength of 5 MPa.
However, concrete placed during cold weather, protected against freezing, and properly cured for a sufficient time, can develop higher ultimate strength and greater durability than concrete placed at higher temperatures.
The temperature of fresh concrete as mixed should be in the range of values shown in Table 1 (adapted from Table 5.1, ACI 306-R16).
Make sure mixing water is available at a consistent temperature for every batch.
If you use water above 80ºC, it might be necessary to change the order in which ingredients are blended. It might help to add the hot water and coarse aggregate before the cement, and to slow the addition of water as the cement and aggregate are loaded.
You can use boiling water (100ºC) if the fresh concrete mix temperature is within limits shown in Table 1 and no flash setting occurs.
You also need to heat the aggregates if the air temperature is consistently below -4ºC, at least to eliminate ice, snow and frozen lumps. If the mixing water is heated to 60ºC, you won’t probably need to heat the aggregates above 15ºC.
You usually don’t need to heat the sand above 40ºC if the mixing water reaches 60ºC.
Avoid overheating so that spot temperatures do not exceed 100ºC and the average temperature does not exceed 65ºC when adding aggregates to the batch.
The concrete mix will lose temperature during transport. You can use these equations from ACI 306R-16 to calculate the approximate temperature drop for a 1 h delivery:
Where:
Adjust the values proportionally for delivery times greater or less than 1 h.
Remove snow and ice from all surfaces before pouring concrete. This may require a temperature increase of the massive metallic embedments, formwork, and supporting materials.
Heat formwork surfaces above -12ºC. The metal forms will usually be at the air’s temperature, so it is more effective to maintain the air temperature above -12ºC than heating the formwork.
Fresh concrete can freeze when it contacts metal embedments, such as steel structural members. Most embedments and reinforcement bars do not need heating unless the air temperature is below -12°C.
If the embedments have a cross-sectional area greater than 2580 mm2, heat them above 0°C.
You should limit the differential temperature between the subgrade materials and the concrete to less than 11 ºC to avoid inconsistent setting, rapid moisture loss, delaminations, and plastic shrinkage cracking.
BS EN 206 requires that the temperature of fresh concrete shall not be less than 5ºC at the time of delivery.
You should cast concrete at a temperature in the range of values shown in Table 2 (adapted from Table 5.1, ACI 306-R16).
BS EN 13670 requires that no part of the concrete surface falls below 0ºC during the curing period, until the concrete has attained a strength of 5 MPa.
Protect concrete from freezing as soon as possible after placement, consolidation and finishing. This will allow the concrete to gain strength and prevent the concrete from early-age damage by freezing the mixing water.
The temperature of concrete placed during cold weather should be maintained at the recommended temperature given in Table 2, until the concrete has reached the strength required to remove formwork and protection.
Embed expendable thermistors or thermocouples in the fresh concrete so you can monitor the internal temperature over time and ensure it stays in the range recommended in Table 2.
Thermal gradients within the concrete can cause cracking and curling. Monitoring the temperature will let you adjust the heating and protection methods as needed to obtain an optimum concrete structure.
You must measure temperatures near the surfaces and in the interior. The corners and edges of concrete are most vulnerable to freezing, so check the temperature at these locations more often.
Monitoring the temperature can help you to predict the concrete’s developed strength.
Converge’s Signal Sensor + with single-probe or multi-probe thermal Tails allows you to measure the temperature continuously without visiting the construction site.
The Signal Sensor uses Bluetooth to transmit data to the Converge app on your smart device or the cloud automatically. No wires, no thermocouples or routes for water ingress. The sensor is compatible with most rebar sizes and is easy to install.
Converge predicts the concrete strength with 95% accuracy using an AI model (Concrete DNA) trained on vast concrete data sets.
Several techniques are available for estimating the in-place strength of concrete. When these have been correlated to standard-cured cylinders, they can be used to determine the concrete strength.
One technique is maturity testing. Concrete maturity is based on the concept that the combination of curing time and temperature of the concrete yields a specific strength for a given concrete mixture. The maturity method develops a relationship between time-temperature history and concrete compressive strength.
Electronic instruments known as maturity meters permit direct and continuous determination of the maturity index at a particular location in the structure.
Converge’s ConcreteDNA provides real-time concrete curing data and accurate AI predictions thanks to the network of wireless sensors. This allows building up to 30% faster.
The app notifies you when the concrete has reached critical strength and it’s time to strike. You don’t need to test the concrete strength yourself anymore.
ConcreteDNA also avoids the risk of thermal gradients in concrete and helps you to deliver the project on time, while adapting to seasonal weather changes.
Protection removal
At the end of the protection period, protection removal should result in the gradual cooling of concrete surfaces not exceeding the rates indicated in Table 3 below (adapted from Table 5.1, ACI 306-R16).
This can be accomplished by slowly eliminating heat sources, or leaving insulation in place until the concrete has reached equilibrium with the mean ambient temperatures.
After removing the temperature protection, you won’t usually need to provide measures to prevent surface desiccation if the air temperature remains below 10ºC and the relative humidity is above 40%.
You may remove sections temporarily to pour additional concrete, but make sure that the concrete you placed previously doesn’t freeze.
You may not need external heating if you install insulation: it will maintain the heat of hydration and avoid concrete freezing.
Keep the insulating material dry and in close contact with the concrete or form surface. Some common materials are:
The longer the protection period, the more insulation you need to maintain the concrete’s temperature.
The corners and edges are particularly vulnerable during freezing weather. The insulation thickness for these areas should be approximately three times the thickness recommended for walls or slabs.
The most common types of heaters are:
1) Direct-fired heaters
2) Indirect-fired heaters
3) Hydronic heating systems.
Direct-fired and indirect-fired heaters discharge hot air into an enclosed space. Hydronic heaters circulate a heated glycol/water heat transfer liquid through a system of heat transfer hoses placed on the surface.
Don’t use direct-fired heaters in an enclosed space: they typically burn fuel oil, kerosene, propane, gasoline, or natural gas and produce large amounts of carbon dioxide (CO2).
Carbon dioxide combines with calcium hydroxide (Ca(OH)2) on the surface of fresh concrete to form a layer of calcium carbonate (CaCO3). This layer interferes with the hydration reaction and produces a soft, chalky surface that will continue to dust during the concrete’s life.
You can use indirect-fired heaters because they only discharge clean air in the enclosure. The exhaust is separate from the hot air and will vent outdoors, avoiding the reaction of carbon dioxide with the fresh concrete.
Install the hydronic heaters after the concrete placement reaches its final set, and cover it with polyethylene film or other suitable material to serve as a vapour barrier.
The insulation blankets confine heat to the concrete surface instead of heating the air, so you won’t need to build enclosures.
Another advantage of hydronic heaters is their homogeneous heat distribution, practically eliminating curling and cracking caused by temperature gradients within concrete.
Enclosures block the wind, keep out cold air, and conserve heat. They are made with materials such as wood, canvas, building board, or plastic sheeting.
Enclosures can be the most effective protection method, but they can also be the most expensive.
You will need an enclosure if the air temperature is below -20ºC. You may also need it for higher temperatures, depending on the structure to protect and the weather conditions.
Freshly cast concrete will freeze during cold weather unless you maintain the correct internal temperature.
Therefore, it is critical that you monitor the concrete’s temperature continuously. This allows a proper cure and avoids thermal gradients that can produce cracking and curling.
In addition, you will be able to estimate the strength and remove the protection at the right time.
ConcreteDNA measures the temperature continuously and allows you to build faster during low temperatures:
Talk to sales before pouring concrete in freezing conditions and save time, money and headaches.
ACI Committee 306: Guide to cold weather concreting ACI 306R-16. American Concrete Institute, Farmington Hills (MI), 2016.
BS EN 206:2013+A2:2021 Concrete. Specification, performance, production and conformity
BS EN 13670:2009 Execution of concrete structures
Concrete on site 11: Winter working. The Concrete Society, Blackwater (England), 2015.
Liu, Zhuangzhuang et al. (2017): Portland cement hydration behavior at low temperatures: Views from calculation and experimental study. Advances in Materials Science and Engineering, vol. 2017.
National structural concrete specification for building construction (NSCS), 4th edition. The Concrete Centre, Blackwater (England), 2010.
Rubene, Sanita; Vilnitis, Martins (2017): Impact of low temperatures on compressive strength of concrete. International Journal of Theoretical and Applied Mechanics, 2, 97-101.
Wilson, Michelle L.; Tennis, Paul D.: Design and Control of Concrete Mixtures, 17th Edition. Portland Cement Association, Skokie (IL), 2021.
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