Pouring concrete over concrete

[Garage5.9]

So I have a preexisting slab that is about 10x10 and really old but still in good condition considering its age. I want to use this slab to build a shack for my bbq/outdoor kitchen but it is uneven as it was a foundation from a old structure. My question is can new concrete be poured over exsisting to level it out ? Roughly 4 inches would have to be poured. What should be done to make it stick ?

 

[Colin HD]

You really need to scarify the top surface, air compressor & a needle gun to roughen up the existing concrete.
remove all: oils, dust & contaminants to make it stick.
4 inches should be thick enough.

 

[Know Wosad]

It's very do-able. Use fibercrete and consult the cement plant near you for advice and a recommended contractor. The dudes at the cement plants are awesome people. I ALWAYS go to the source.
It seems people, especially on this forum, have some stupid fear of contacting the company that makes the item in question. Goofy.

 

[Pinhook]

Use a 2 part bonding agent between the pours - follow the instructions to a "T".
- install wire mesh or #4 rebar at 12" centers in your new slab. This will help control shrinkage cracking.
- cut control joints ! - - slab that thin will crack.

 

[MG44]

Just went through tearing up an floor that had a "bonding" agent in it.

Short answer - Yes, you can pour new concrete 4 inches thick on top of old concrete.

Will it stick together? More than likely not.

Is that a problem? Nope.

What you have is a good base at this point to pour new concrete on. All the bonding agents in the world will not make it "stick" together. You can drill rebar into the old one and that will help, but it will never be one contious slab. It shouldn't be a problem for a outdoor kitchen.

 

[Leaflessshadetree]

Just went through tearing up an floor that had a "bonding" agent in it.

Short answer - Yes, you can pour new concrete 4 inches thick on top of old concrete.

Will it stick together? More than likely not.

Is that a problem? Nope.

What you have is a good base at this point to pour new concrete on. All the bonding agents in the world will not make it "stick" together. You can drill rebar into the old one and that will help, but it will never be one contious slab. It shouldn't be a problem for a outdoor kitchen.

Yep, I'd treat the existing concrete as a compacted base. If it's uneven or sunken/heaved in places those areas may continue to sink or heave. If its just cracked or spalled, no big deal.

 

[nh_yota]

Remove the old concrete.

 

[Hilltopmasonry]

I think the biggest issue is making sure that your new slab is thick enough to prevent cracking. 4 inches should be sufficient. Of course this is assuming that the old existing slab is already fully settled so it doesn't sink anymore. Personally I would be concerned if your new slab was thinner than 4 inches

A concrete contractor that I do work for is doing this exact same thing. He purchased an older warehouse for his shop and the mechanic area he is going to pour a new slab on top of the old one because the concrete is old pitted and uneven however it is structurally sound


Sent from my iPhone using Tapatalk

 

[James E]

IMHO, 4" over the top of the existing slab is enough concrete that bonding shouldn't be necessary. The problem I can foresee is cracking in the new slab at the transition where it goes from 4" to thicker concrete around the perimeter of the old slab. It would only take a tiny shift in either the new or old slab for a crack to form.

10x10 is pretty small. I'd say you'd be better off breaking up the old slab and hauling it away, or breaking it up ALOT and using it for the base of the new slab.

 

[stupid name]

Is the new slab going to be larger or just 10x10? Would make the difference whether I poured on top or demo'ed the old one. Bigger, get rid of the old. Same size, pour it on top and level it out. Do you have a slab leveling company in the area? Might be better just to have them come in and level the old one.

 

[joe49]

....breaking it up ALOT and using it for the base of the new slab.

Basically what my father taught me. Breaking it turns it into base material and removes any stress in original slab.

 

[theoldwizard1]

Slightly off topic. What about gypcrete over concrete ? Big benefit is that it is self-leveling.

 

[tthornto]

Your new slab would be highly susceptible to cracking anywhere the old slab is cracked as well as anywhere there is an elevation change in the old slab. I guess you could plan for that and make lots of control joints/cuts in a pattern that covers all the areas likely to crack.

Or

Since it is for an outdoor kitchen, why not level it out with a thin layer of sand, then lay down some pavers?

 

[James-W]

How about this idea. Using a hammer drill, make several holes in the old concrete large enough to take a half inch bolt. Drive an anchorment into each hole and screw the half inch bolt with a large washer tacked (welded) by the bolt head into the anchorment. Now you have a whole bunch of these bolts with a large washer on top sticking up all over the old concrete pad. When you pour the new concrete on top of the old concrete, the bolts will keep it from moving around.

 

[MoonRise]

4" thick new concrete?

That's a slab all by itself.

Right on top of the old slab, same exact footprint and all? And the old slab is 'stable' and in decent condition, except for the top being uneven?

Plastic sheet (thick vapor barrier stuff) to completely decouple the new concrete slab from the old slab underneath. Wrap the plastic past the old slab edges and down the corners/edges. Form the new slab with 2x lumber, right to the edges of the old slab, thus trapping the plastic sheet in place (think fitted bed sheet on a mattress). Pour new concrete slab on top of the old slab.

method = "unbonded concrete overlay"

http://www.cement.org/docs/default-...-07t-resurfacing-concrete-floors.pdf?sfvrsn=4

Note that local building codes may dictate different requirements, especially if there are seismic requirements involved.

Or break up existing slab, do proper sub-base prep, do form work, pour new slab as desired.

Your call. Check local code requirements first.

 

[Garage5.9]

Thanks for all the input I'll take everything in to consideration is using using pavers. Here is a picture of the slab in question.

 

[bgarrett]

Control joints in a 10X10?? Do you people even read the OP?

 

[Dr Stan]

Thanks for all the input I'll take everything in to consideration is using using pavers. Here is a picture of the slab in question.

What about building a pressure treated floor over the existing concrete? Looks like it would be a good foundation.

 

[rustyjames]

On a small slab like that I would just pour on top, no need for any adhesion to the other slab. No need for rebar, although WWF might not hurt. But I think the paver idea is the way to go.