The winter months bring their share of obstacles when it comes to cold weather concrete. Try using these precautionary measures to ensure the concrete properly sets.
Remember that concrete must be protected from freezing until it has reached a minimum strength of 500 pounds per square inch (psi). If concrete freezes before it gains strength, it will be unable to resist expansive forces which will result in the disruption of the cement paste matrix. Early freezing reduces ultimate strength by 50%. If concrete has been able to obtain a compression strength of 500 psi, it will have sufficient strength to resist expansion and damage if frozen.
Cold weather will result in the concrete taking considerably longer to set, so prepare for crews to be on site for longer periods of time. To help keep temperatures stable, you can use windbreaks, enclosures, or heaters to protect the concrete placement. Concrete mixtures can also be adjusted to accelerate setting time.
In the winter, water and aggregates are to be kept warm to provide proper concrete temperatures upon delivery. Windbreaks, typically around 6 ft tall, are often set up to protect the concrete from drastic temperature drops due to biting winds. Sometimes heated enclosures can be set up to keep temperatures stable as well. If an area is too large to be practical for an enclosure, a hydronic heating system can be put in place. These systems circulate a glycol/water solution in a closed system of pipes to circulate heat.
You can also use accelerators to add to the concrete to keep it on schedule. This is often done by adding calcium chloride which accelerates the hydration reaction, but this must be used with caution as additional chloride can lead to corrosion of any rebar embedded within the concrete. There are non-chloride accelerator options, but these are generally much higher in price. See here for a non-chloride accelerator.
Forms can be used to assist in cold weather concrete finishing. Many are unaware that forms actually retain heat and prevent concrete from drying too quickly. So, try to keep your forms in place for as long as possible.
Before finishing concrete, it’s important to ensure all bleed water has evaporated. You will notice more bleed water than is typical and it will start later than expected due to the extended setting time of cold weather concrete pouring.
Active heating is often used to keep temperatures steady when pouring in cold weather, but it’s crucial that the concrete doesn’t cool too rapidly when active heating is discontinued. Removing active heating quickly can cause temperature differentiation to build between the outside and middle of the concrete causing cracks. To prevent this, gradually lower temperatures within an enclosure or cover the concrete with insulating blankets.
Throughout the curing period, concrete temperature should be checked regularly using an infrared temperature gun to ensure it is maintaining a temperature of at least 40 degrees Fahrenheit.
All concrete must cure for a specific period of time depending on the type of cement used. The ACI Committee 308 recommends the following minimum curing periods:
Cement Type: | Days: |
ASTM C 150 Type I cement | 7 Days |
ASTM C 150 Type II cement | 10 Days |
ASTM C 150 Type III cement | 3 Days |
ASTM C 150 Type IV or V cement | 14 Days |
ASTM C 595, C 845, C 1157 cements | variable |
All in all, there is no standard mix or method for cold weather concrete pouring. Each cold weather scenario should be analyzed individually to decide the optimal way to pour and best mix to use. So take the time to decide which methods are best for your situation!
The winter months bring their share of obstacles when it comes to cold weather concrete. Try using these precautionary measures to ensure the concrete properly sets.
Remember that concrete must be protected from freezing until it has reached a minimum strength of 500 pounds per square inch (psi). If concrete freezes before it gains strength, it will be unable to resist expansive forces which will result in the disruption of the cement paste matrix. Early freezing reduces ultimate strength by 50%. If concrete has been able to obtain a compression strength of 500 psi, it will have sufficient strength to resist expansion and damage if frozen.
Cold weather will result in the concrete taking considerably longer to set, so prepare for crews to be on site for longer periods of time. To help keep temperatures stable, you can use windbreaks, enclosures, or heaters to protect the concrete placement. Concrete mixtures can also be adjusted to accelerate setting time.
In the winter, water and aggregates are to be kept warm to provide proper concrete temperatures upon delivery. Windbreaks, typically around 6 ft tall, are often set up to protect the concrete from drastic temperature drops due to biting winds. Sometimes heated enclosures can be set up to keep temperatures stable as well. If an area is too large to be practical for an enclosure, a hydronic heating system can be put in place. These systems circulate a glycol/water solution in a closed system of pipes to circulate heat.
You can also use accelerators to add to the concrete to keep it on schedule. This is often done by adding calcium chloride which accelerates the hydration reaction, but this must be used with caution as additional chloride can lead to corrosion of any rebar embedded within the concrete. There are non-chloride accelerator options, but these are generally much higher in price. See here for a non-chloride accelerator.
Forms can be used to assist in cold weather concrete finishing. Many are unaware that forms actually retain heat and prevent concrete from drying too quickly. So, try to keep your forms in place for as long as possible.
Before finishing concrete, it’s important to ensure all bleed water has evaporated. You will notice more bleed water than is typical and it will start later than expected due to the extended setting time of cold weather concrete pouring.
Active heating is often used to keep temperatures steady when pouring in cold weather, but it’s crucial that the concrete doesn’t cool too rapidly when active heating is discontinued. Removing active heating quickly can cause temperature differentiation to build between the outside and middle of the concrete causing cracks. To prevent this, gradually lower temperatures within an enclosure or cover the concrete with insulating blankets.
Throughout the curing period, concrete temperature should be checked regularly using an infrared temperature gun to ensure it is maintaining a temperature of at least 40 degrees Fahrenheit.
All concrete must cure for a specific period of time depending on the type of cement used. The ACI Committee 308 recommends the following minimum curing periods:
Cement Type: | Days: |
ASTM C 150 Type I cement | 7 Days |
ASTM C 150 Type II cement | 10 Days |
ASTM C 150 Type III cement | 3 Days |
ASTM C 150 Type IV or V cement | 14 Days |
ASTM C 595, C 845, C 1157 cements | variable |
All in all, there is no standard mix or method for cold weather concrete pouring. Each cold weather scenario should be analyzed individually to decide the optimal way to pour and best mix to use. So take the time to decide which methods are best for your situation!
Summer has arrived, and the process of pouring concrete must change with the hotter temperatures. The heat that comes with summer affects more than just the concrete workers themselves; heat also impacts the quality of your concrete.
Hot weather can have significant impacts to the long-term strength of your concrete. Yes, the initial strength will be stronger with faster than usual set times, but it does not give enough time for the crystals surrounding the aggregate to grow and reach the max strength. A rise of 10 degrees Celsius in temperature can cause a 10% loss in strength after 28 days.
Another impact of heat is that it causes the water to evaporate, thereby decreasing the slump and causing the concrete to become unworkable. This in turn typically leads to the addition of more water being added to the mix resulting in greater strength losses up to an additional 10%.
Concrete surface drying is another impact of the summer pours. The water evaporates and is sucked to the surface. The remaining water still rises but is left on the top later of the surface. This causes weakening of the top layer which results in the surface cracking, splitting and blistering.
To prevent these negative effects on your concrete, try these changes to ensure top quality concrete:
Simply protecting your pour from the sun is an effective way of limiting the evaporation and slowing the set time.
You can slow your set time by adding retarders to the concrete. Retarders slow the set time to allow concrete to receive an appropriate amount of time to complete the chemical reaction and maintain a higher slump.
When dealing with floor work and pad work, keeping your forms and ground wet with cold water can ensure that the bottom layer maintains hydration through the evaporation stage.
Using curing blankets can keep sunlight off your pour and control the evaporation. Curing compound is a good way of sealing the top of the pour to prevent water from escaping. Different types of compound will have different long-term reactions. Chlorinated rubber and wax concrete compounds eventually wear out over time, whereas some compounds will need to be washed off.
The mix is already being affected by the sun and heat; why hamper your summer pour abilities by allowing more time for the concrete to set in the delivery stage? Producing on site with a volumetric mixer or having a ready-mix plant nearby is especially important.
Summer concrete is a challenge but using these tips can ensure that your concrete strength is maintained. And most importantly, don’t forget to keep both your concrete and your bodies hydrated.
Summer has arrived, and the process of pouring concrete must change with the hotter temperatures. The heat that comes with summer affects more than just the concrete workers themselves; heat also impacts the quality of your concrete.
Hot weather can have significant impacts to the long-term strength of your concrete. Yes, the initial strength will be stronger with faster than usual set times, but it does not give enough time for the crystals surrounding the aggregate to grow and reach the max strength. A rise of 10 degrees Celsius in temperature can cause a 10% loss in strength after 28 days.
Another impact of heat is that it causes the water to evaporate, thereby decreasing the slump and causing the concrete to become unworkable. This in turn typically leads to the addition of more water being added to the mix resulting in greater strength losses up to an additional 10%.
Concrete surface drying is another impact of the summer pours. The water evaporates and is sucked to the surface. The remaining water still rises but is left on the top later of the surface. This causes weakening of the top layer which results in the surface cracking, splitting and blistering.
To prevent these negative effects on your concrete, try these changes to ensure top quality concrete:
Simply protecting your pour from the sun is an effective way of limiting the evaporation and slowing the set time.
You can slow your set time by adding retarders to the concrete. Retarders slow the set time to allow concrete to receive an appropriate amount of time to complete the chemical reaction and maintain a higher slump.
When dealing with floor work and pad work, keeping your forms and ground wet with cold water can ensure that the bottom layer maintains hydration through the evaporation stage.
Using curing blankets can keep sunlight off your pour and control the evaporation. Curing compound is a good way of sealing the top of the pour to prevent water from escaping. Different types of compound will have different long-term reactions. Chlorinated rubber and wax concrete compounds eventually wear out over time, whereas some compounds will need to be washed off.
The mix is already being affected by the sun and heat; why hamper your summer pour abilities by allowing more time for the concrete to set in the delivery stage? Producing on site with a volumetric mixer or having a ready-mix plant nearby is especially important.
Summer concrete is a challenge but using these tips can ensure that your concrete strength is maintained. And most importantly, don’t forget to keep both your concrete and your bodies hydrated.