How long for concrete to cure?

Question

We had new concrete footings poured for a large deck. They were poured approx. four hours ago and now we have very heavy rain. Will the footings be okay or will the rain have an adverse effect on the concrete? Thanks for any answers.

Answer 1

It depends on the depth of the footing.

Concrete is a mixture of Portland cement, sand, gravel and water. It doesn't just set like a cake there is an actual physical reaction called hydrolyzation. This is why concrete is so much stronger than a cake you bake in your oven.

The thicker the concrete the longer it takes to cure. If the Hoover Dam was built in a monolithic block of poured concrete then it would still be curing to this day and the heat put off by it would crack the out concrete shell and it could even boil the water near a crack.

The industry standard for roads and housing is to give the concrete 30 days to cure before it reaches its maximum hardness. It actually continues to cure and harden, but the rate of hardening drops of quickly after 30 days.

For your deck you need at least 12-24 hours for it to cure properly. Now the presence of extra water isn't going to hurt the fresh concrete, in fact as it cures it draws in more water from around it.

You footings may be pockmarked or if they are still in a form then they could be diluted, which will require longer for it to cure. So if you just poured the footings into the ground then you should be okay. I am more concerned about the temperature. It is going to be drop near to freezing in my area tonight and that will be detrimental to the concrete, which is why it is normally only poured in the spring and summer (concrete can only be poured at temperatures between 50 to 70 degrees Fahrenheit, as near as 73°F as possible). The goal is to keep concrete cool and moist so it gains strength slowly, but efficiently. Laboratory tests show that moist-cured concrete can be twice as strong as concrete cured in a dry environment. One industry method of doing this is called ponding where the form is extended to a few inches beyond grade so that standing water can sit on top of the concrete. In very warm temperatures ice can be added to the mix, but when it gets above 80 degrees F then it is not safe to pour the concrete.

When it has to be poured in cold weather than chemicals are added to the mix to increase the exothermic (heat producing) reaction and to decrease the curing time, however the cost of this is that the concrete will be weaker. If this is done with an engineer monitoring the temperature and the slump tests then it can be safely done.

You can't do any of that though. The best thing to do is to go out tomorrow and test the concrete. If you can still poke a hole in it then you have a bad pour and it will not support your deck. In that case you will have to dig out the footings (ugh).

Answer 2

Rain after four hours might have an adverse effect on the "finish" (surface) of concrete, such as a sidewalk or driveway, but since these are footings and had four hours to cure, I do not believe the strength of the concrete has been compromised.

Answer 3

28 days is the accepted norm for the curing of concrete. it does not dry it sets up through a chemical reaction. when all the moisture present is gone the reaction stops, so this time may vary, but 99years, gimme a break...your posts should be fine since we are talking about a very small surface area...

Answer 4

Cover them now! Also complain to the contractor. They may "PIT".

I'm curious to know where number two came up with 99 years?

Concrete BTW is not Waterproof in a general sense.

Steven Wolf

Answer 5

throw something over them to get the direct rain from hitting them , they will be alright but a little pitted from the rain , it takes 99 years for all the moisture to leave concrete

Answer 6

They should have been covered up with plastic. That being said they are probably ok.