Concrete Curing in So. CA

gnet158 · April 11, 2003

I'm having a small area of my back yard concreted (25 feet long x 8 feet wide) on Saturday and have a few questions.

1.How long does concrete take to cure?

2.It's a little cold here in CA daytime highs 63 with nighttime lows of 52. Will this effect the curing time?

3.It's supposed to rain on Sunday. Is there anything I need to do?

Thanks for your help!

CMNTMXR81 · April 11, 2003

Despite my screen name, I do not deal in concrete. But from all the concrete I've layed (keep your minds out of the gutter).

- Hard enough to at least walk on is withing 12 - 24 hours.

- Hard enough to drive a vehicle on, I'd wait at least 48 hours, maybe more. I think when I did our supplemental driveway, I waited a week just for safeties sake. But you don't need to.

- Temps, those will be fine. You don't want to do it when it's too hot either, otherwise it cures too fast and after which the top layer will flake off in spots possibly. Old mixed CMNT can also do this same thing (say if it were mixed 5 hours or so before being put down, formed, and cured).

What I've seen them do here in Chicago with the new Wacker Dr. construction (which is concrete), after they get done shaping/forming it and ready to let it cure, they put a white plastic tarp over it to protect against the elements. I think it was a breathable plastic though that let's air and some moisture from rain through on purpose.

gnet158 · April 11, 2003

Thanks for the reply. So I think your saying that as long as they (I'm defiantly not skilled enough to pour concrete so I've paid for someone to do it for me) cover it with the mentioned plastic tarp then I don't need to worry about the rain? Thanks!

CMNTMXR81 · April 11, 2003

Nope. Heck, they poured it in winter, covered it with the tarp, and it snowed on it. So I wouldn't worry much.

If they do it this way, they may not.

BlueRado · April 12, 2003

There's a whole multitude of factors involved in concrete curing and strength, etc (my Dad was a contractor and I got a lot of time on a bull float), so I'm not gonna go into details.

The best way for concrete to cure is slowly - that way the water fully hydrates the concrete. You slow down this process by covering w/ plastic, tarps, even straw as was mentioned, or even watering lightly at first, and also there are curing compounds that are sprayed on after the concrete has been completely finished (You don't cover and wet until it's completely finished, either). You usually see this as a white powdery looking substance sprayed on new concrete roadway surfaces.

24 hr is certainly fine for walking traffic, but I'd really say stay off it w/ cars, etc for 1 week. Even though concrete may be firm enough to walk on in less than 12 hrs, depending on the local conditions, etc, there are a whole world of chemical reactions still going on, and it's in this time that the concrete is fully hardening, this is really the curing process, as opposed to the setting process, which starts as soon as the wet concrete mix is poured into the forms. I'm sure the contractor will know how to do all this.

I can definitely say that if you're not sure what to do, let the pros do it - this is one area that is a killer to re-do as you only get one chance when the concrete is first placed wet into the forms - after that come he** or high water, it's gonna set up.

That's my 2 cents.

BTW, the reason they can cover concrete in Winter like CMNT mentioned, is they had probably added calcium or another agent to keep the liquid water from freezing while it hydrated the mix.

Kania2k1 · April 12, 2003

Concrete reaches it's maximum compressive strength at about 28 days. It keeps getting stronger as time goes on, but the curve drops off substantially. You can usually place concrete down to about 30 degrees without havig to cover it. The heat generated by the cemical reaction of Portland Cement and Water generates enough heat to keep it warm enough. Good luck, and make sure you put a date or your kids hand prints in it somewhere inconspicuous.

timk49 · April 15, 2003

My dad was a concrete contractor in southern Ohio. I worked on his crew many summers. What BlueRado says is word. One of the hardest jobs I have ever had to do was demoing a 60 year old 6" thick concrete porch with a 16 lb sledge hammer. It was hard as diamonds.

Wingnut · April 15, 2003

I helped an old neighbor break-up, and haul off a poured concrete in-ground pool, as well as the surrounding walkways.

To add insult to injury, after we finally got it all out....We then had to fill it up with dirt....The same way we got the concrete out. One wheelbarrow at a time. No backyard access at all.

Yup, one of the most backbreaking weeks of my life.