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What if you’ve had lots of rain & humidity. Our area is slightly sloped. The ground is clay, rocks & some sand. I’m concerned about having concrete poured on a wet ground. We need to have the area graded and sloped slightly in the opposite direction, away from the house. If it’s too wet, will the new patio be unstable? I’m worried about cracking & flaking if the ground isn’t dry enough.enter image description here

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Civil engineer here. The concrete needs to stay wet in order to cure properly. As noted before, concrete doesn't dry out to harden, it cures through a chemical reaction that needs water to facilitate the reaction. If the ground is dry, the ground will absorb moisture from the concrete and it won't cure properly. The ground should be very moist and compacted as well as you can manage.

Also, wet clay is notoriously weak and you may end up with uneven support under the concrete, which will then lead to it cracking and settling after it's cured. Hard to say without seeing it but if you can remove as much clay from beneath the concrete slab, you'll have less chance of it settling and cracking. This is especially true if the clay is expansive. It will expand when wet and shrink when dry and your concrete slab will be a disaster. Best to get as much clay removed as possible, compact the sub-grade evenly, and make sure to water the ground before placing the concrete.

Have you thought about proper joints? Concrete will crack. If you can get it to crack along a joint, no one will see it. Look at some professionally done concrete flatwork and you'll see what I mean. Good luck.

  • I agree but there are 2 types of concrete, one that has cracks and the other will in the future.+ – Ed Beal Jul 12 '18 at 18:54
  • If the ground starts out dry and absorbs some of the water from the concrete, will that water eventually be replaced from another source, like rain? In other words, will it still cure properly if given enough time and occasional exposure to water? – mrog Jul 12 '18 at 19:55
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Damp ground will not be a problem unless is can not support the weight of the concrete. Cement/concrete does not "dry" , it cures , a chemical reaction. It absorbs water to cure, as noted , some concrete applications are sprayed with water to aid the cure.

  • Actually, concrete does “dry” in the curing process. In fact, it continues drying its entire life. Water sprayed on concrete to help it cure is because you want to “slow” the curing (drying) process. Also, it doesn’t “absorb water to cure”. – Lee Sam Jun 3 '18 at 15:22
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    You're splitting hairs and confusing the issue. It's a common mistake to allow concrete to dry while it's curing. This results in a weaker initial cure, and can result in damage. Also, concrete is sprayed with water to help it cure. This is done to counteract drying due to sun exposure. – isherwood Jun 3 '18 at 22:08
  • @isherwood Chalking has nothing to do with “weaker initial cure and can result in damage.” It has to do with the finish or wearing surface, as I stated before. Water is sprayed on the surface to, 1) keep the very top of the slab surface (1/16” or so) from drying out (and powdering) and 2) to slow the top portion (top half) from drying out during the curing process and shrinking, compared to the lower portion of the slab. I added a link below to explain that condition. – Lee Sam Jun 4 '18 at 7:31
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    I am going to stick with what a doctor of cement ( ceramics) told me ; Cement absorbs CO 2 and water for years as the chemical reactions continue. Calcium silicates comprise an exceptionally large complicated group of materials. – blacksmith37 Jun 5 '18 at 12:05
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I would wait until it dries up a bit. The concrete has a lot of moisture that needs to go somewhere. If the ground is too wet (clay doesn't absorb water) and the air is humid, you are probably asking for trouble. I see your problem all the runoff heads toward your house at worst or toward the lower right side at best. I suggest waiting till it's pretty dry. We had a similar problem with building a shed in our backyard. The best place was low and rain kept that area really wet. We put an 8" foundation in when it finally dried up. That foundation did the job. We also built up the sides of the foundation with fresh soil. The wettest area became normal.

  • I’m not an expert and will leave writing the answer to those that do this more often, however I disagree with your answer. With proper grading and a proper gravel bed there shouldn’t be issue. Concrete cures via chemical reaction that also produces heat—it’s not simply wait for the water contained in the mix to dry. In fact, during hot weather the surface is often spayed with water and kept wet to slow drying and allowing the concrete to cure evenly, that is not dry to quickly at the surface—which is what causes chalking and chipping. – Tyson Jun 3 '18 at 13:51
  • @Tyson Chalking has nothing to do with the drying too quickly. It has to do with the finishing process. – Lee Sam Jun 3 '18 at 15:38
  • Awesome thanks for clarifying.. as I said I’m not a subject matter expert I just know this answer is not correct. – Tyson Jun 3 '18 at 16:40
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    I also disagree with this answer. The water in concrete mix goes into the finished concrete, for the most part. It doesn't need to drain out or evaporate. – isherwood Jun 3 '18 at 22:12
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    I disagree with this answer also, did you know you can pour concrete in water and it will work? – Ed Beal Jul 12 '18 at 18:56

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