Concrete is one of Earth's most versatile and widely used construction materials. It’s strong, durable, low maintenance, fire resistant, simple to use, and can be made to fit any size or shape, from massive skyscrapers to humble stepping stones.
Yet, whether you're a concrete contractor or a homeowner wanting to pour your own concrete slab, you know that getting concrete poured, set, and cured can be a more difficult process than many think.
Why?
The weather.
Here at Gra-Rock, we've been in the ready-mix concrete business for years, and we understand the frustration of working with concrete in different weather conditions. While concrete is fairly simple to pour, its durability and longevity are often dependent on heat, cold, humidity, or rain.
If you are planning a concrete pour or have experienced unsatisfactory results from previous pours, this article is for you! We’ll give you tips and tricks for pouring concrete in any weather and help you understand how concrete works so you can become a pro at pouring concrete.
Ready to dive in? Let’s get started!
How Concrete Sets and Cures
Before we get into practical tips for pouring concrete in any weather, let’s make sure we have a clear understanding of what concrete is and how it works.
First of all, concrete is not just cement! However, cement, which is crushed limestone and clay heated to high temperatures in a kiln, is one of the key ingredients in concrete. To make concrete, you need a combination of the following:
Cement
Sand (fine aggregate)
Water
(If you want to know more about concrete, read our Beginner's Guide to Ready-Mix Concrete.)
If you’ve ever used a bag of ready-mix concrete, you’ve received the first three ingredients already measured and mixed. Your job is simply to add the last ingredient: water. Mix, and voila! You have concrete you can use for any small project.
Understanding concrete’s relationship with water is extremely important in mastering the art of pouring concrete in inclement weather.
It's a common misconception that concrete "dries." Concrete does not actually “dry” as the water evaporates. Instead, concrete sets and cures through a chemical reaction called hydration. The water actually becomes an essential part of the cured concrete. You don't want all the water to evaporate from your pour, or you've got problems!
But wait—what’s actually happening in the chemical reaction of hydration?
What is Hydration?
When cement's chemical properties interact with water, compounds like tobermorite gel and calcium hydroxide begin forming on the outside of the cement particles. This growth results in a strong, interlocking rod structure.
Although the primary work of hydration happens within 5 hours of contact with water, the process of hydration will continue slowly for a very long time.
This is why you shouldn’t let the concrete dry out while it's curing. Lack of water can prematurely stop the hydration process, keeping the concrete from reaching its full strength. In fact, as long as you avoid washing out the cement, concrete can be successfully placed and cured completely underwater!
How the Weather Affects Concrete
This doesn’t mean that more water is always better in pouring concrete. Extra water creates even bigger problems than water deficiency.
When there is too much water in your concrete mix, the chemical bonds will be separated from each other by large areas of water. This also reduces the strength of cured concrete.
More water results in increased porosity, meaning that the hardened concrete will have many canals, which can affect any color or finish that may be applied to the surface.
Once cured, concrete with too much water will also be much more prone to shrinkage and cracks, especially in freeze-and-thaw climates, meaning that it will be particularly weak.
Pro tip: Contact your local ready-mix concrete supplier to ensure that your concrete has the proper ratio of cement, aggregate, and water!
It’s likely they have a mix that fits your needs, and the concrete will be delivered straight to you, making the whole process much less of a headache.
But now that we understand how concrete works, let's look into how different weather affects concrete.
Pouring Concrete in Cold Weather
The first question to ask about pouring concrete in cold weather is, “How cold is too cold?”
Here's how the American Concrete Institute defines cold weather:
The average daily temperature is less than 40° Fahrenheit (5° Celsius)
The air temperature is not greater than 50° Fahrenheit (10° Celsius) for more than 12 of the 24-day hours.
If you are pouring concrete in conditions cooler than those listed above, you should take extra precautions to make sure your concrete will set and cure properly.
Here are a few tips to ensure you have a successful pour in cold weather:
Remove snow and ice from the work area to prepare the pour site. Never pour concrete over frozen ground!
Start your pour in the early morning so the concrete can absorb the full sun's effect throughout the day.
Use a low slump and minimal water-to-cement ratio. This will reduce water bleeding and the set time, which allows less time for the water in the concrete to freeze.
Use a product to speed up the set time of the concrete, like SpeedSet or calcium chloride.
Use concrete curing blankets to prevent freezing and keep the concrete at an optimal curing temperature. Make sure to keep the concrete from freezing for a minimum of 3 days. You can use these same products to protect the ground from freezing that you will pour the concrete on.
The temperature of any items to be embedded in the concrete (rebar, wire mesh, etc.) needs to be above freezing before coming into contact with fresh concrete.
Do not begin final finishing operations while bleed water is present.
Request a heated mix from your concrete supplier.
Set up a heated enclosure. If combustion heaters are used, remember to vent outside to prevent carbonation!
Use a waterproofing concrete sealer as a curing compound instead of water curing.
Once the concrete is hard, don’t use any de-icing salts. These corrode the surface and allow water to permeate, freeze, and eventually crack your fine work.
By following these tips when pouring concrete in cold weather, you up your chances of a quality pour!
Pouring Concrete in Hot Weather
So, if you live in a climate that doesn’t experience cold or freezing temperatures, you can safely pour concrete all year long, right?
Wrong.
Remember, hydration is the most important aspect of achieving strong, durable concrete. Hot temperatures cause concrete to have a deficit of water and prevent it from setting correctly.
Concrete cures best in a range between 70° to 80° F. So, while higher temperatures are nice, you’ll start running into problems as they approach 90° F.
These issues are most severe on days with low relative humidity and high wind speeds.
Remember, maintaining an appropriate level of moisture in the concrete is paramount. Warm temperatures, low humidity, and high wind speeds increase the rate of evaporation, making it more challenging to retain moisture in the concrete.
If outdoor temperatures are nearing 90° F, or if the day is hot, dry, and windy, you should take precautions to correct for the high temperatures.
Here are a few tips to ensure a good concrete pour in hot weather:
Use a large size and amount of coarse aggregate particles if hot weather is likely to occur during the concrete placement. Larger aggregates will minimize the probability of having concrete shrink due to environmental conditions.
If possible, avoid pouring concrete at noon or during the afternoon.
Consider using hot weather admixtures, such as recycled plastic additives. This will make your mix easier to work with and increase its setting time so it can cure to greater strength.
Request chilled water from your concrete supplier, or if mixing your own concrete, use an industrial chiller.
Use sunshades or windbreaks to reduce possible harsh conditions.
Once water has been added to the mix, reduce your mixing time. Over-mixing will create more water loss through evaporation.
All necessary equipment should remain covered until the last moment before use. If possible, keep chutes, conveyors, and accessories under a roof and spray some water over them regularly.
When placing concrete for a slab, first dampen the subgrade.
Use cool water to dampen side forms for slabs or walls.
Call for all hands on deck. Due to faster set times, you will need as much manpower as possible to get the job done quickly.
Pouring concrete in the summer heat can be challenging for both our bodies and the concrete! Using the tips and tricks listed above will help concrete set and cure properly and ensure that you feel great about the job you did!
NOTE: Make sure to have plenty of liquid on hand for your crew, too! Pouring concrete in hot weather is hard work.
Pouring Concrete in Rainy Weather
Probably the biggest challenge you'll face when pouring concrete is rain.
Whenever possible, avoid pouring concrete in the rain! Rain causes all sorts of issues. If there is torrential rain or prolonged periods of rain, reschedule the project for another day.
However, if you experience a long period of damp weather and you’re facing a project deadline, or if you get stuck in an unexpected downpour, here are a few steps you can take to help your cause:
Make sure you have a great drainage system. This ensures that rainwater does not pool in trenches dug for footers, foundations, and slabs.
If possible, tent the pour with tarps to keep rain from mixing with the concrete. The more water that mixes with semi-solid concrete, the more likely it is to fail.
Don’t work the rainwater into the concrete surface!
Don’t soak up rainwater with dry cement. This will impair the finish and further weaken the top layer of concrete.
Once the rain passes, use a float to push the water off the edge of the slab before you start finishing.
Conclusion
Sometimes, the weather makes concrete pouring difficult. That's why it's best to be informed about how to pour concrete in all weather conditions. Focus on the elements you can control, such as finding a reliable ready-mix concrete supplier.
At Gra-Rock, we deliver quality ready-mix concrete and strive to be a stable and reliable provider.
Whether it's our proven mixes and designs, our guaranteed delivery, or our helpful team of truck drivers, we aim to ensure that you will be happy and confident in your concrete.
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Contact us today to get the concrete you need and the customer service you deserve!