Pour Concrete In Water
DEAR TIM: I'm renovating a basement bath during the winter while I'm slow at work. When I removed the old shower insert, I discovered a large area of crumbling and missing concrete. There's standing water in the ground because of the high ground water table this time of year. I've discovered conflicting information online as to whether or not I can pour concrete with this water in the hole. Can I pour concrete in water and if so what do you have to do to make sure it doesn't fall apart? How would you do this if it were your job and you're on a tight timetable? Paul F., Bloomfield, NJ
DEAR PAUL: I love the Internet and it's become a core part of my business, but as we all know with many things, you get the good with the bad. The biggest problem I can see with the Internet is that if you can fog a mirror, you can become a publisher and proclaim your opinions to the world whether or not they're true.
If you also have some design skills, you can create a website that rivals some of the best looking websites out there. It gets worse. Millions of pages of content at some very fancy home improvement websites have been created by people who've never had a hammer in their hand. This makes the Internet a very scary place for folks like you to get information.
My advice is for you to only trust content created by a person that can prove he / she worked in the field for at least twenty years at paying customers' houses. That's the sign of a true professional - one who gets paid in full for services rendered and who has a proven track record of staying in the construction business.
Let's now talk concrete! The good news for you is that you can pour the concrete in water. Professionals do this all the time. You just have a small standing puddle in your basement and that won't be a problem.
The biggest issue with pouring concrete under water is movement. If the water is moving, it can wash away the cement paste that's holding the sand and gravel together. But if the water is calm, then it's not a problem at all.
You can prove this to yourself by doing a small experiment. Take an old five-gallon bucket and put about 2 inches of clear water in it. Then get a bag of regular concrete from a hardware store or home center. Mix it up per the instructions so it's all wet, but quite stiff. You want the concrete consistency to resemble very thick oatmeal. Just mix up about one gallon of concrete for your test.
Carefully transfer the concrete from your mixing pan into the bucket. Drop it into the bucket of water slowly. Tamp it only once or twice very gently to try to level it out. The water in the bucket will be on top of the concrete and will probably turn cloudy. That's okay.
Let the bucket sit overnight and then dump it over to pour out the water. The concrete will be rock hard in the bottom of the bucket.
If I was the contractor on your job and I had to pour this slab, I might do the following if there was even more water in the hole. Before the pour, I'd install a 3/4-inch PVC pipe that would be at the bottom of the hole. There's a good chance I'd dig a small hole at the bottom of the place I'm pouring the slab. This small hole or sump would be about 6 inches deep below the bottom of where the slab is supposed to be.
I'd drill several 1/8-inch diameter holes in the side of the pipe at different heights. These are places where water can get into the pipe. I'd put the end of the pipe at the bottom of this sump making sure it's not clogged or not in mud. I'd then fill the small sump with rounded gravel about the size of peas or marbles to the bottom of where the slab is going to be. It's important the gravel around the pipe has no sand that could clog the holes in the side of the pipe.
Then I'd extend this pipe up above where the top of the slab is going to be and attach the hose of a wet-dry vacuum to it. Just before pouring the slab I'd turn on the vacuum to suck out all water from the sump and around where I'm pouring.
I'd then pour the concrete and every ten minutes or so I'd turn on the vacuum to get rid of any water that came back under the slab. I don't have to go to all this trouble, but if I felt there was any chance the water would be moving under the slab eroding the cement paste, I'd take this precaution.
After about an hour, two at the most, and assuming the temperature in the basement where I'm pouring is 60 F or higher, I'd not worry anymore as the concrete by then has transitioned from the plastic state to the solid state and the cement paste would be quite hard to erode unless the water was really moving fast.
Once concrete is hard enough to stand on after it's finished, water is its friend. If you can keep fresh concrete wet for days after you pour it, it helps the curing process. The last thing you want to happen is for the water you used to mix the concrete to leave the slab too fast.
The chemical reaction of hydration that allows the microscopic crystals of Portland cement to grow and interlock all the sand and gravel together continues to happen for days, weeks and months after you pour the concrete and it needs water to complete the chemical reaction.