Properly Curing Concrete

Image by LDS

One of the more common questions we receive on the site is in regards to the proper curing of concrete. People starting out in the concrete casting hobby are often in a rush to get their items out of the mold and on display. They want to know how fast they can “dry” their concrete.

It is important to understand though that you should not confuse drying with curing. They are actually two separate processes. Concrete doesn’t dry - it cures. And one needs to be more concerned with properly curing their concrete rather than how dry the surface appears.

Drying involves the removal of water from an item. This is usually caused by allowing the surface to be exposed so that excess water evaporates. Typically the surface of a concrete item will go from dark grey to light grey as the surface water is removed. Allowing the concrete to dry prematurely will stop the curing process and can lead to a weak concrete.

Curing is the chemical formation of hydraulic bonds within the concrete. This is what holds the concrete together, and gives the concrete its strength. The formation of these bonds requires the presence of water. Don’t be too anxious to accelerate the process. Simply put: It takes time. Usually up to 28 days to reach its full rated strength.

I cannot stress this point enough. Just because the surface is “dry” DOESN’T mean it’s finished “curing.”

A bit of trivia.... Concrete will continue to cure after the 28 day period as long as water (at the microscopic level) remains within the item. The ruins of ancient Rome contain some of the hardest concrete on the face of the planet. They’ve been curing for over 2000 years. And they continue to get harder each year!

I’ll let you in on a secret. You can even soak the concrete. The water will add to the hydration process.

This odd photograph is of my attached greenhouse foundation. The floor is sloped toward a sump in the middle.(The whole room was designed to be soaked on a regular basis.)

Note that the foundation looked like a reflecting pond. (In fact, that’s what the neighbors were thinking.) I took a hose and filled the entire foundation to the brim - and refilled daily for a week. (It helped to have a raised edge.) And that’s how you get hard cured concrete.

So... what do you think? Please leave me a comment.

5 Comments:

  • Vale: Thanks for this tip! We are making concrete pavers as a present from the grandkids to the grandparents, it’s about 3.5lbs of concrete per paver, how long do you think they need to cure properly?
  • BackYardBuilder: As a rule of thumb, it should be fully cured after about 28 days. For best results, keep it covered with plastic and moist for about a week.
  • Vale: Oh wow, that’s a lot longer than I expected! I will make sure that we cover them thanks!
  • BackYardBuilder: Typically 75-85% of the final strength is gained within the first week of curing. If your pavers are a minimum of 2" thick, and ONLY being used for walking, you may be able to put them in place after about 2 weeks. Contact with the ground will usually supply a sufficient source of dampness to continue the curing process, so they should continue to cure while in place.
  • Vale: Thanks! That’s great to know!

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