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Pouring a concrete riser on a pre-existing concrete floor
September 7, 2010 12:42 PM   Subscribe

What's the best way to pour a raised concrete slab on top of a pre-existing concrete floor?

We have a weird raised slab (24" by 30", and 2" tall) in our basement. I'd like to extend the slab further along the wall so it will be long enough to hold both our washer and dryer. Is it possible / advisable to pour concrete over concrete? How can I make a form that won't spill out from the bottom? I thought this would be a fairly trivial task, but googling for answers only yields people wanting to skim an entire new layer of concrete over a pre-existing slab.

To rehash, I simply need to pour a 2" concrete riser on top of an existing concrete basement floor. Looking for tips and/or advice. Thanks!
posted by bjork24 to Home & Garden (12 answers total)
 
2" is really thin for a concrete overpour like you are describing. I would worry that it would crack and crumble, especially with vibrating machinery on top of it. You definitely would want to put some reinforcing mesh in it, and probably it would help to roughen up the slab below -- bash at it with a cold chisel and baby sledge or something (wear eye protection!). As afar as formwork, you will want to scribe your form board to the uneven floor, and probably close the gap with some weatherstripping or some other foam-like material.

You might be better off pouring a new slab over the entire area -- say 6" thick over the stepless area, and 4" over the top of your existing stepped slab? This has the side benefit (if you have a front-loading washer/dryer at least) of getting everything at a more accessible height.
posted by misterbrandt at 1:10 PM on September 7, 2010


I'm an architect with a design/build firm...(for whatever that's worth)

This should be pretty easy. 2" is about as thin a slab as you would want to pour, but it should be ok. Be sure to use some 6-6-10 welded wire mesh (available at Home Depot) in the slab and make sure you suspend it about mid-depth in the concrete as you are pouring. Just be sure it is not laying on the bottom or within 1/2" of the top. If you are having concrete delivered, and have a choice, go with a fairly stiff (low slump) mix with a strengthening additive. Don't pour over your existing slab, as it will be too thin and crack or spall.

If I was doing it (without knowing the size of your slab or the configuration of your basement) I would probably use a 4x4 as a form that you can pin to the existing slab with a ramset nailer. You can rent these at Home Depot. If your floor is uneven, you can staple some foam "sill sealer" to the bottom of the 4x4 before attaching it to the floor. You can also get this at HD. It is usually blue and comes in rolls 4" wide. If you use a 4x4, you may consider leaving it in place so you don't wear too heavily on the leading edge of the new slab over time.

Be sure to cut some control joints in your slab the day after you pour it. You can do this with a circular saw and a concrete/diamond blade (also rent-able). Cut them about 1/4" -3/8" deep. If you can, 24"x24" squares are good. Your slab will crack, but it should be fine and will crack in these joints that you cut.

Be sure to give the concrete plenty of time to cure (1 week min) before you put a "moving" weight on it like a washer/dryer. If your area is super dry, you may even want to put some plastic over the slab while it cures so it doesn't cure too quickly. That should do it. Good luck.
posted by nickjadlowe at 1:14 PM on September 7, 2010 [2 favorites]


try a google search for "equipment pad" or similar. To get the formwork to stay put, I'd put heavy weights on/against the formwork. a 2" deep pad shouldn't put too much lateral force on the forms.
posted by Chris4d at 1:25 PM on September 7, 2010


I did this under our furnace. If I were you I'd probably break apart the existing pad and pour a new one the size you need. I like Nickjadlowe's instructions -you want a nice rigid wooden form that isn't going to shift or bulge in the middle. As long as you can hold the form tightly to the floor, the concrete won't come out the bottom. It might seep a bit but you can wipe that up. Instead of nailing it to the floor, you might consider using a few 2x4s running down from to the ceiling to hold it firm. Covering it with plastic is a good idea and splashing some water on it every few days would be good because the concrete below it may wick away the moisture too quickly. If you can keep it moist for a couple of weeks, it'll cure a lot stronger.
posted by bonobothegreat at 2:18 PM on September 7, 2010


Is the rest of the floor concrete? If you just want to raise a portion of the floor above a good concrete slab you could use a dry pack mortar like is used in tile showers.

I wouldn't place any metal reinforcement in a 2" pour. Metal should be protected by more concrete than that and the reinforcement can act as a shear plane in the concrete.

If you elect for concrete once you place the forms some simple masking tape between the form and existing floor would be sufficient to prevent the concrete from leaking past the form. Or a bead of silicone.
posted by Mitheral at 3:30 PM on September 7, 2010


When I used expanded metal in a mortar shower pan of that thickness , it didn't end well (it acted as a shear plane, as mentioned above). Some kind of mortar bed would be my bet if you wanted to go with a cement.

But, why not just build a wooden subfloor? Me and concrete products don't seem to get along well, so I would just get some (maybe pressure-treated) 3/4 plywood and some sleepers (wood to lay the plywood to). You'd have to cut the wood to the right thickness, and you'd have to check if the floor is level (which you can correct with self leveling compound if needed). You could even bolt it to to the concrete if you wanted.

From what I've read, adding concrete to concrete can be tricky, as they don't like to stick. What is usually recommended is a layer of slightly thinned thinset between the old and new, to help with adhesion.

Oh, and finally:

You might be better off pouring a new slab over the entire area -- say 6" thick over the stepless area, and 4" over the top of your existing stepped slab? This has the side benefit (if you have a front-loading washer/dryer at least) of getting everything at a more accessible height.

I'd cry over that much wasted space. You could build a wooden platform that everything sat on, and then use the space underneath for whatever you wanted.
posted by RikiTikiTavi at 4:23 PM on September 7, 2010


For your form,could you use something like the technique described in this video?

This is for making forms for concrete countertops, but I think the principle could be very useful for what you need. Basically, it uses tough foam pieces - in your case 2" x 2" x however long you need - as the walls of the form. They are attached to the ground surface with double stick tape, which doesn't sound like much, but 30 inches of doublestick (given a smooth, clean surface to stick to) will provide a surprising amount of resistance against any lateral force. And finally, a bead of silicone to complete the seal to the floor.

Finally (off the top of my head, not coming from any area of expertise) I think that investing in a cheapie hammer* drill (this or, if you think you'd have a use for it again, this) that would allow you to put bolts into the old concrete one inch deep, that protrude halfway up into the new pour might really help bond it to the old slab.


Also, this.


*Gotta be "hammer" or "impact"... regular drills, even heavy duty ones, do nuzzing!, but with the addition of the hammering motion, they go through concrete like butter.
posted by BleachBypass at 5:01 PM on September 7, 2010


BTW, if you end up using any volatile chemicals to chemically bond the old slab to the new (suggested as one possibility in the Ask-the-Builder link) make sure any pilot lights are extinguished.

Friends of mine were sealing concrete basement floors, the vapors crawled along at floor level until they found the water heater and ignited. No damage done, but scary as hell.

I often roll my eyes at the "I will assume you have no common sense and will explode/electrocute/impale yourself based on my advice" thing, hereafter referred to as Do Not Operate Hairdryer While Lounging In Tub (DNOHWLIT), but I guess obvious means different things to different people. And that one wasn't immediately obvious to me until my friend related her experience.
posted by BleachBypass at 5:17 PM on September 7, 2010


RikiTikiTavi, I figure if they can charge a couple hundred bucks for these, a few extra inches of concrete wouldn't hurt anything. What space is being wasted, BTW? The air above the washer/dryer?
posted by misterbrandt at 6:56 PM on September 7, 2010


I can't give specific advice because I don't have the knowledge, but I'd think you'd want something almost like a mortar or grout. Something that is an adhesive cement that will stick to the existing floor and to itself a bit better than standard utility grade concrete.

A thought: try and search for the material they use to make outdoor air conditioner concrete pads. Those handle the kind of abuse that a washer dryer would give it, and seem to hold together just fine.
posted by gjc at 5:16 AM on September 8, 2010


Is the rest of the floor concrete? If you just want to raise a portion of the floor above a good concrete slab you could use a dry pack mortar like is used in tile showers.

I wouldn't place any metal reinforcement in a 2" pour. Metal should be protected by more concrete than that and the reinforcement can act as a shear plane in the concrete.
posted by Mitheral at 3:30 PM

When I used expanded metal in a mortar shower pan of that thickness , it didn't end well (it acted as a shear plane, as mentioned above). Some kind of mortar bed would be my bet if you wanted to go with a cement.
posted by RikiTikiTavi at 4:23 PM


bjork24 - You may have just entered that whirlpool-like situation where you are getting a bunch of us guys standing around in a circle all saying "Yeah, but you could also do this...Or maybe you could do this other thing." There are a million ways to skin any cat so to speak, and for builder types, half the fun is trying to come with your own, unique, particularly brilliant way to get the job done. That said, I will have to respectively disagree with a couple of the suggestions here.

It's true, expanded metal is an awful thing to use as reinforcement in a slab. 6-6-10 Welded Wire mesh is made for exactly this purpose...reinforcing thin, non-prestressed concrete slabs. 6-6 refers to a 6" by 6" grid and 10 refers to the gauge of wire. Its welded at all connection points. It's designed to provide reinforcement that lets all the concrete act as "one" slab, but not to degrade the strength of it. It is the industry standard for this purpose. It's true that 2" is minimal for a slab. But concrete (not gypcrete) floors used to house hydronic heating are done fairly commonly, and as long as you control the expansion/cracking, they are OK. As was mentioned above, if you were up to busting up the existing slab you have and then pouring a new, thicker slab, that would definitely be better. But that is a lot of work, and to be honest, your situation doesn't really require it.

I would strongly recommend against using a mortar grout or material used for a shower pan. This stuff is made to be stiff and dry enough to let you work it to create a slope (for a draining shower floor). They are fine in compression, but it will take more effort than you want to get it smooth and level. It's not even a little self-leveling like regular concrete is. It has a rough-ish finished top surface which is not idea for cleaning up after water or soap spills. (The rough texture is meant to increase adherence of the base and grout of the tile that usually goes on it...but that same rough texture doesn't provide any better adhesion to the smooth concrete below it.). If you want to beef up the adhesion, there are simple liquid products that you can spread on the existing slab before your pour that are made for that purpose.

The grout adhesion idea is fine if you are using an Epoxy grout...but man, that's way overkill for a small slab...and harder to work with.

Again, your project is simple enough that you don't have too many things to guard against. You can build the form a hundred different ways. It's true that that thin a slab will cause minimal pressure against your form. Just do what seems easiest for your situation. I agree that all the suggestions here are fine. Perhaps the most important thing you may want to keep in mind is to not let the concrete cure to fast, or to put anything on it too soon. Full structural cure for almost all concrete is 28 days, but you don't need to wait that long. A week or 10 days should be fine. Good luck.

(And no disrespect meant to the other posters.)
posted by nickjadlowe at 10:18 AM on September 8, 2010


RikiTikiTavi, I figure if they can charge a couple hundred bucks for these, a few extra inches of concrete wouldn't hurt anything. What space is being wasted, BTW? The air above the washer/dryer?

Right, I wasn't clear. Those pedestals were what I had in mind. The space *below* the washers would be wasted if you pour a new slab over everything; the volume would be occupied by concrete when it could have clothes baskets, supplies, or whatever.

That's why I was advocating a wooden structure that could built to accommodate the unevenness of the surface it was resting on, and could have drawers or simply open space to stash clothes baskets. Also, of course, it's easier for many people to work wood than concrete, and the supplies are easier to come by. Less risk of problems caused by unfamiliarity with the medium.

But you can't beat the durability of concrete properly done. I wonder if there are cement products specialized to work in that thickness? I was thinking mortar, but I guess that's incorrect because of its other properties. I had thought concrete generally likes to be a bit thicker, and it's really too thick for thinset.

(And no disrespect meant to the other posters.)

None taken, here.
posted by RikiTikiTavi at 12:22 PM on September 8, 2010


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