How to fill a hole in concrete slab roof

[waldreps]

I have a root cellar that is made of 6" concrete walls and roof. It is 3/4 in the ground but the roof and 1/4 of the walls stick out of the ground. There used to be a gas water heater in there so there's an 8" diameter hole through the concrete slab roof. I don't use that hole anymore as I have an electric water heater and want to fill that hole to keep out cold air and moisture. How do I do that? Is that too big to use spray foam? Should I use a board to block off the bottom and pour concrete to fill the hole? Please give me some suggestions and recommendations. Thanks.

[HayZee518]

take a 5/16 inch carbide drill bit and drill several holes around the inside circumference of the hole. insert 5/16 inch rod or 6 inch bolts in the holes. fasten a square piece of plywood to the underside of the slab and pour your concrete mix. float out the top so its smooth. when it cures after about three days, remove the plywood.

[waldreps]

Something like that is what I figured I should do. I just wanted to get some input and see if that was the easiest and/or best way to do it. Thanks for the input.

[waldreps]

I filled in the hole in my slab roof and am ready to remove my plywood from the bottom. I'm just giving it a little extra time since the temps have been on the cool side. I'm just wondering if the new concrete bonds good enough with the old concrete to create a watertight seal. I wonder if any moisture will seep through where the old and new concrete meet.

[pushkins]

Seldom will it create a waterproof seal, get yourself some asphalt roof sealer (roofing section of the hardware store sold in tubes) and add a layer over the repaired area making sure to spread it around about 3" bigger than your repair, smooth it out with a putty knife.

[HayZee518]

new concrete to old concrete never makes a water tight seal. they make a bonding agent that helps the new bond to the old but it'll NEVER seal it.

[mrcaptainbob]

Plugging the hole...

Too bad there isn't a way of tapering the hole sides. Although those pins all around are good idea, I'd be concerned with them rusting with time and letting that concrete plug fail. Would stainless rods or bolts work better? I know the chemicals in the 'crete are not friendly to metals. An angle grinder with a wheel for grinding stone could be used to at least chamfer the top ledge. The larger the chamfer, the better. A lot of grinding, but it would go quick on concrete. Face mask, ear plugs, eye protection, breathing mask....

[HayZee518]

I agree with the rusting, but standard re-bar is coated with a material that's pretty well dinged up before it is used in a pour. even in a rusted condition, re-bar and galvanized bolts will hold.

[waldreps]

Thanks guys....asphalt roof sealer it is. I was trying to avoid that as this thing only sticks out of the ground about 2' and I wanted to paint the whole thing. Oh well, being watertight is more important than looks. I wonder if the asphalt roof sealer will hold some paint. As far as the rebar rusting and the plug falling, I'm thinking that would take many, many years if it did happen. The hole wasn't perfectly smooth on the sides anyway so I'm guessing that the little indentations and such would hold it there. We'll see as it's already poured now...lol.

[pushkins]

If you seal over the repaired area with roofing asphalt the rebar will never rust away.