Proper Curing of Concrete in Cold Weather
Concrete cures slower during cold weather. In extreme conditions, it can even stop curing entirely or suffer damage via the freeze-thaw cycle that will affect its strength when cured.
Delays on site are irritating and expensive. So, how can you avoid them? This article will discuss properly curing concrete in cold weather without waiting a week or more. Read on to find out everything you need to know about curing concrete when temperatures are below 50F.
What Is Concrete Curing?Concrete sets through a chemical reaction between cement and water. This is called 'hydration' and involves the formation of crystals within the concrete that strengthen it.
The American Concrete Association defines curing as: 'action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.
So, curing means keeping moisture and temperature stable at a level that allows hydration and crystallization to occur within your concrete. Curing concrete in cold weather, however, can be a difficult process.
In temperatures over 50f, concrete takes about 24-48 hours to reach 500psi (which is usually the point at which it is considered adequately cured and suitable for walking or driving). As a rule of thumb, concrete reaches its full effective strength in four weeks.
Curing is not the same as drying. Drying concrete means allowing it to achieve a moisture level appropriate for use. Conversely, curing considers other factors such as temperature, humidity, and hydration.
Is Curing Concrete the Same as Drying Concrete?
Why is Curing Concrete in Cold Weather Difficult?Cold weather slows curing time. A slab that would take 24-48 hours to cure when it is 70F could take twice as long in 50F weather. This causes problems with project timings and can also cost a considerable amount of money (keeping finishing teams on site, for example) on large construction sites.
Very cold weather is even more of an issue. Temperatures under 40F stop the hydration process completely and can also freeze concrete in a plastic state damaging its potential for strength.
What Other Factors Affect Concrete Curing TimeA few different factors influence concrete curing time. When planning your concrete placement, you need to consider:
- Concrete design
- Strength needed
- Actual (or predicted) weather and ambient temperature
- Shape and size of concrete slab
- Specified strength
- Size and shape of the slab
Technically, concrete is permanently curing. Barring damage, it gets stronger with time. That being said, concrete is considered cured enough to walk on and work on when it reaches 500 psi.
How Long Does Concrete Take To Cure
Concrete is ready for car traffic, heavy machinery, and ordinary use when it reaches 70% of the required psi for a project. Usually, this takes around a week but times vary based on the mix used and weather conditions. As curing concrete in cold weather takes longer, these steps may come later when the ambient temperature is low.
Concrete Mixes For Cold WeatherYour concrete mix can mean the difference between properly cured concrete and concrete that takes a week to cure or never reaches its required strength. If you need help designing concrete for a project in the cold, you can talk to your ready-mix concrete supplier or call a White Cap expert to discuss concrete mix options. In the meantime, here are a few tips for mixes that will cure in the cold:
- Use Warm Water: A minimum placement temp can be hit by using warm water in the mixing process. You can test the temperature of your concrete using a temperature gun.
- Consider Slump: A slump below 4 inches will stop any excess bleeding. As concrete curing takes longer in the cold, there is more time for bleeding, and more moisture will be lost relative to the same slump in warmer temperatures.
- Avoid Fly Ash and Slag: Both will make curing time longer. They also bring down the heat of the reaction, so internal temperatures stay lower.
- Use Accelerators: To keep setting times predictable, think about using accelerators. Calcium chloride speeds up hydration, but it can also erode steel. If you have used rebar or wire mesh within your concrete, you should use a non-chloride accelerator.
- Add cement: additional cement in your mix speeds up curing
Tips For Curing Concrete in Cold Weather
Beyond the concrete design/mix you choose, you can do a few things to ensure your concrete cures properly when it's below chilly. Take precautions if air temperatures are expected to be under 40F or between 40 and 50F consistently. Plan and be ready to use some or all of these techniques when placing and curing your concrete.
It may seem counterintuitive, but concrete must be cured carefully in cold weather. The surface can dry out even quicker than in cold weather if the concrete mixture is warmer than the air around it.
You must also wait for bleed water to evaporate before finishing or curing. Bleeding lasts longer in cold weather, so there will be more water than there would be if it was warm.
- Keep the ground warm: When placing and curing concrete in cold weather, you should cover all freshly excavated ground with insulated curing blankets to keep the heat of the subsoil in. This is particularly important with flatwork. In some cases, you may also be able to warm the ground with heated blankets.
- Check temperatures with an infrared temperature gun: This takes the guesswork out of ground temperature, concrete temperature, and more. Check the temperature of your concrete to ensure it is warm enough to cure at a decent speed and achieve full strength.
- Keep tools and embeds warm: Everything that touches your concrete when mixing, placing, and finishing it may change its temperature. Ensuring everything is over 50F will help achieve a strong, quick curing process. A heated enclosure can help, but it may also dry out your concrete.
- Use curing blankets: If you haven't already read our White Cap resources on curing blankets, have a look when you're done with learning about curing concrete in cold weather. Blankets should be laid on top of your concrete directly after finishing, and the R-value should be suitable for keeping the concrete around 50F. Properly used curing blankets helps keep curing times to 72 hours