One of the first questions homeowners ask after having a new driveway installed is how long they should wait before using it. A new concrete path can be walked on after about a day, but cars weigh a lot more than people. Driveways also tend to be a fair bit thicker than patios, and using more concrete means more prolonged drying and curing times. Concrete begins to dry and cure as soon as it is poured, but drying and curing are not the same things.
The Difference between Curing and Drying
Curing is a chemical reaction that takes place inside concrete. This is the process you will be most interested in if you want to park on a new driveway since this is when concrete gets its strength.
While several factors can influence how long this takes, a general rule of thumb is that it takes about 30 days for an inch of concrete to cure. However, a new concrete driveway should be strong enough to support an automobile after about ten days.
Drying takes a lot longer than curing. This is because only a part of the water added to concrete during mixing is used in the curing process. The rest of the water remains inside the concrete and slowly makes its way to the surface. How long this water takes to evaporate depends on the air temperature and relative humidity.
Curing Times for Concrete
Adding or reducing the amount of water used to mix concrete will lengthen or shorten its curing time, but getting the ratios just right takes a trained eye and a careful hand.
Too much water will result in concrete that is porous and prone to chipping and cracking, but not enough water makes mixing very difficult. Using concrete mixes that contain accelerators like calcium chloride is one way to shorten curing times, but these substances can also corrode rebar.
Most concrete mixes reach 70% of their compressive strength after about seven days. They harden very quickly for the first few days, and then they continue to strengthen for another few weeks.
Curing Concrete-Colder Temps
Concrete cures the best at temperatures higher than 50 degrees Fahrenheit. Hydration slows down significantly at even slightly cooler temperatures and almost stops entirely at about 40 degrees Fahrenheit. Concrete poured in these conditions can lose as much as half of its 28-day strength.
To prevent this from happening, it is crucial to stop new concrete from freezing until it has reached a strength of approximately 500 psi. This generally takes about two days. Things that can be done to keep concrete warm during this time include:
- Using hot materials: Using hot water and aggregates to mix concrete can really take the edge off cold weather. If the temperatures are not too cold, this may be all that is needed to achieve the desired strength.
- Adding accelerators: Accelerators like calcium chloride, calcium nitrate and sodium nitrate speed up curing times, but they should be used as one step of a larger plan because they do not make concrete more resistant to freezing.
- Mixing air-entrained concrete: This special kind of concrete fills with air bubbles when it cures. These bubbles then give water a place to go when it freezes and expands. This allows air-entrained concrete to freeze and thaw without cracking.
- Covering concrete: There are now heated blankets available that can keep recently poured concrete warm enough to ensure that it cures properly. These coverings run on electricity like a heated bed blanket, and they can cure concrete about three times faster than tarps or regular blankets.
- Using a heater: Heaters can both raise the temperature and lower the humidity of the air near concrete.
The Dos and Don’ts of Curing Concrete
Time is the magic ingredient that allows something that starts with the consistency of oatmeal to harden into a substance that can hold buildings up. Of course, all of that strength is built up in just a few weeks, as long as moisture levels and temperatures remain where they should. Here are some dos and don’ts that you can use on your next concrete job.
Do Wet Down Your New Concrete
Concrete that is kept wet for a few days after being poured can be up to 50% stronger than concrete that is allowed to dry naturally. This is usually done by spraying new concrete with a garden hose up to 10 times a day. Another option is building berms around concrete and then flooding the area inside them. This method is known as pond curing, and it offers the same benefits as spraying without all of the work.
Don’t Forget to Use Control Joints
Control joints are cut into concrete less than 24 hours after it has been poured. They prevent random cracking by giving concrete a predefined area of weakness. When cracking does occur, it is in a straight line and below the surface.
Do Keep Your Concrete Warm
The temperature of your concrete should be 50 degrees Fahrenheit or hotter for at least two days. If you have to work in cooler temperatures, heat your water and aggregate before mixing and cover your concrete with a blanket or plastic sheet.
Do Use a Curing Compound
Curing compounds are solutions that can be sprayed onto concrete to form a protective film. This barrier prevents evaporation and ensures concrete cures at a consistent rate. Check the label before starting, as some curing compounds disintegrate naturally, while others must be removed by scrubbing.
Don’t Paint or Stain Concrete for at Least a Month
While your new concrete driveway will be strong enough to support a car after about a week, it will take a month before the hydration process is over. If you add a layer of paint or stain while moisture is still rising inside the concrete, it will probably not adhere very well and start peeling or flaking.
Factors That Affect Concrete Setting
The factors that affect setting times the most are the materials used to make concrete and the weather conditions it is poured in. Using hot materials, along with less water and accelerants, speed up setting times, but it will still take about a week for poured concrete to reach about 70% of its final strength.
Controlling humidity is especially important as moist air does not draw water out of concrete as well as dry air will. This problem can be avoided by using desiccants indoors and heaters or condensation dehumidifiers outdoors.
Tips on Curing Concrete
You can do a few things to reduce the amount of time it will take new concrete to cure. Here are a few final tips.
How to Speed Up Concrete Drying Time
The best way to speed up the time it takes concrete to dry is to use just enough water to make a mix that is easy to work with. Chemical admixtures like silica fume promote desiccation and speed up drying times, and putting a vapor retarder under concrete prevents ground moisture from entering the slab.
DON’T let new concrete get too cold
Pouring concrete in cold weather takes experience and skill. If a temperature of at least 50 degrees Fahrenheit is maintained for two days, the concrete will lose up to half of its strength. Fortunately, you can avoid losing concrete integrity by using hot water to mix concrete, adding accelerants to the mix and using blankets or heaters.
DON’T subject new concrete to excessive weight
The waiting is the hardest part of laying a concrete path, driveway or patio. Here is a rough timetable of how strong your new driveway will be during the various stages of the curing cycle:
- The first 24 hours: This is the time when your concrete driveway is at its most vulnerable. Cordon the area off, and do not allow anybody to walk on it or drag objects over it.
- Between one day and nine days: Concrete is tough enough to walk on after about 24 hours, but it is still curing under the surface
- After 10 days: This is the point when concrete has gained enough strength to support a car.
- After 28 days: Concrete is usually fully cured after about 28 days. After that, it is strong enough to support large trucks.
A Business Built on Service
If you would prefer to let professionals worry about aggregate, humidity and curing times, you should think about calling Nashville Concrete Contractors for your next project or repair. We always go the extra mile for our customers, which is why our business has been built on referrals and recommendations. If you would like to learn more about our concrete installation and repair services, you can fill out our online form or contact us at (615) 505-3595.