Staff Writer| The Columbus Dispatch
Q: I’m renovating a basem*nt bath during winter while things are slow at work. When I removed the old shower insert, I discovered a large area of crumbling and missing concrete. Because of the high groundwater table this time of year, there’s standing water in the ground. I’ve discovered conflicting information online as to whether I can pour concrete into the water to fill the hole. How would you do this job?
A: The good news is that you can pour the concrete in water. Professionals do this all the time.
The biggest issue with pouring concrete under water is movement. If the water is moving, it can wash away the cement paste that holds the sand and gravel together. If the water is calm, then it’s not a problem
If 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 three-quarters-inch PVC pipe at the bottom of the hole. And there’s a good chance that I’d dig a small hole at the bottom of the place where 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 one-eighth-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 in mud. Then I’d 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 that the gravel around the pipe has no sand that could clog the holes in the side of the pipe.
I’d extend this pipe up above where the top of the slab is going and attach the hose of a wet-dry vacuum to it. Just before pouring the slab, I’d use the vacuum to suck out all water from the sump and around where I’m pouring.
Then I’d pour the concrete. Every 10 minutes or so I would again use 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 thought there was any chance of the water moving under the slab and eroding the cement paste, I’d take this precaution.
After about an hour — two at the most — and assuming the temperature in the basem*nt where I’m pouring is 60 degrees or higher, I wouldn’t worry anymore. The concrete, by then, has transitioned from the plastic state to the solid state. The cement paste would be hard to erode unless the water was moving exceptionally quickly.
Once concrete is hard enough to stand on, water is its friend. If you can keep fresh concrete wet for days after pouring, that helps the curing process. The last thing you want is for the water you used in mixing 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 continues to happen for days, weeks and months after concrete is poured. Water is needed to complete the reaction.
Tim Carter writes for Tribune Content Agency. Visit his website at www.askthebuilder.com.
