View Full Version : Concrete over concrete

08-27-2003, 11:29 AM
Through bad planning by previous owners our house has an 8'x10' concrete slab, adjacent to the foundation, that slopes back towards the house. I am considering options for grading this slab away from the house. The area needs to stay concrete/paved as it provides access to a storage area for our garden tools. Note that I'm not considering the idea of demo and repour of the slab. The reason for this is that the slab is (1) in good shape (except for the grading problem) and (2) sits above both the water and gas feed lines into the house. Based on the photos from SuperRy during his patio work, I'd rather not go there.

Therefore, I have a few scenarios that I'm considering and would like to get your feedback on these (or maybe you have other ideas?):

1) pour new, thin concrete over the existing slab. I've never done this, but I'm handy with diy concrete. If there is a product that can lay on in layers from 1 to 4 inches, this would be an option I would consider. Is there such a diy concrete product? Would it last?

2) lay a bed of either mortar or compacted builders sand over the concrete with the appropriate grade...with pavers over the top. Trick here is that I would need to throw down up to 4 inches of bed at the thickest point. Would sand beat out mortar or vice versa? Would either one invite settling or cracking of the sub-bed?

Thanks in advance,

Bob Crosley
08-27-2003, 01:21 PM
Quickcrete makes a concrete "resurfacer" that can be troweled to, I think, a 1/16 of an inch thin. A friend used it to even out a basement floor in his old house.

The other option would be to use a concrete levelling service, sometimes called a mudjack. They pump a solution under the existing slab to move it into a better grade. A service here in southeastern Mich. uses a limestone solution. They can change the grade of the existing slab without the risk of cracking it.


Super Ry
08-27-2003, 01:27 PM
I got to add a second slab ontop of the existing - for that wonderful patio project :) Looking back on it I would have preferred to drill some holes in the existing one to add vertical rebar to keep both attached to one another. I'm NO mason but it seems like this would hold em together better than my way - which was to just pour on top of the old (I only had 3' by 4' - so prolly no biggie for mine) but larger might need some grabbin power to keep both together. Couse by doin that you limit your expansion/contraction of the new vs old - they have to move as one.

So basically I just confused myself :)

Lets hear what the pros have to say

08-27-2003, 02:20 PM
Thanks Bob. I'll look into the quikrete.

Although the leveling service sounds enticing, I'm not sure that I want to go that route. Reason being: the current is at the least well bonded to the foundation wall, and at the worst permanently attached. Seeing as the portion of the slab next to the house it what will be raised, I don't see that being a viable alternative.

However, that leveling service might do me wonders for the walkway along the side of my house, which is detached.


08-27-2003, 02:22 PM

I understood your point. I will say that I don't know if that would encourage or discourage cracking in the top slab though...we'll see what others have to say.


08-27-2003, 02:53 PM
Ry, I am with you. If I were to pour a slab over an existing slab, I would first drill some holes for rebar re-enforcement, then pour over the top. To keep both slabs moving in the same directions, I would make sure to place all of my expansion cracks/joints in the same location as the expansion cracks/joints on the existing slab. This would, in my opinion, minimize the potential for cracking.

08-27-2003, 07:45 PM
Concrete does not stick to old concrete very well. There is a glue that is used on the existing concrete to provide a bond. The stuff looks like regular white glue, but tastes different :9 . Using rebar "pegs" won't hurt any.

08-27-2003, 09:56 PM
Hi, Tim.

One huge important question right off the bat. Is there ROOM above the slab for the new pour? Would that rise obstruct the bottom of a doorway, for example?

I'd be awfully tempted to pour a separate slab atop the old, isolated from both the old slab and the house with a layer of Celotex and reinforced with remesh - with "stools" in the higher areas to lift the remesh above the Celotex. In the spots where it gets thin, that approach might get a little dicey, though. Hate to snap off a "shingle" of 'crete.

I kinda' like the idea of using sand & pavers, too. You'd want to compact the sand with a vibratory tamper to get it to settle nicely before you lay down the pavers. Only problem with THAT approach is that it won't do a very good job of redirecting runoff away from the house, owing to sand's porosity. It'd more likely give you a subterranean pool. A fix to that issue would be to lay down MOST of the sand, then a layer of plastic sloped to tha appropriate grade, then a thin layer of sand to bed the pavers.

You CAN pour new 'crete directly on top of old, provided you etch the old 'crete with (I think it's) muriatic acid. That'll provide a good surface for the new 'crete to bond to, but it stinks like crazy & you don't want to breathe any more of it than you have to. If you take that approach, it'd be best to bore & bar like SR was suggesting. You'd end up with a pretty-much homogenous slab that way, and you could skip the remesh 'cause the reinforcing in the old slab'll do it all for you.

Then again, you could jack up the entire house, including the attached edge of the slab... :) :) :) Never mind me, my brain just wanders around sometimes, humming merrily to itself...

-- Tim --

An intelligent man always seeks greener grass.
A wise man grows it under his feet.

08-27-2003, 10:16 PM
Where do you live, in general anyway, I hafta know what kind of frost depth do you have in your area?

How long has the "old slab" been there?

Steve Cox

"A man may not care for golf and still be human, but the man who does not like to see, hunt, photograph, or otherwise outwit birds or animals is hardly normal He is supercivilized, and I do not know how to deal with him" Aldo Leopold

08-27-2003, 11:57 PM
I know, I know...

IMHO, it will probably be less costly in the long run to remove and repour.

Based upon what you've said...
Pour by previous owner
Slopes back to foundation (Basement?)
Over utilities

The slab probably wasn't poured by a professional.
The slab is probably on top of or slightly below existing w/o sub base.
It is against code to anchor a slab to the foundation.
Even here in California, utilities (water) are burried at least 18" below grade with gas much deeper.

Honestly, if it were my house, a repour would be the logical choice.

08-28-2003, 08:58 AM
First off, the slab is not anywhere near a door and has no clearance issues. The storage area that it meets up with is under a porch and the gate to it has 6 inches of clearance. Therefore, regrading is doable.

By the way, we're in Maryland...'tween Baltimore and DC.

About the slab...it's really in very good shape. It's the best piece of concrete on the property. It does not slope dramatically towards the house, just enough to be a problem once a year when snow melts occur and runoff starts to overwhelm our property. It is difficult to explain, but you might care to know that this house is a bungalow with long eave overhangs. At one point the front porch had stairs that landed on this slab and that, apparently, this slab's primary purpose was a foundation for the stairs. Therefore, it is possible that (1) this might be an original slab, though I doubt it and (2) that when the wide porch stair case was in place that the slab saw almost no water near the house under the eaves. By the way, the subtle slab grade could be from 50 to 80 years of normal settling.

Hearing more about the problems that I may encounter pouring new concrete over old, this leads me to believe that the paver idea may be a nicer way to go.

Tim H. mentioned that with pavers you would have issues with water infiltration. I'm curious about this, as I've read and been told that properly installed pavers will shed runoff. I gather that it is important to use jointing sand, to build concrete edgers to hold the paver shape, and to compact the pavers once down. Am I being misled?

I am very thankful for all of your great input. I'm learning a lot.


08-28-2003, 10:21 AM
My experience says remove it and redo it. Your utilities should be out of the way as previously expressed. However, be a little careful around 'em. I've poured thousands of yards of concrete in my lifetime, and a crack free slab, with good finsh, good strength, and bad profile/grade is useless, just as it would be if it was a beat up old sand mix slab.

I'll bet you spend more time an money with inferior results, especially down the road. The "bonding" that folks are suggesting via rebar can be the worst thing depending upon the existing base slab. This can cause refelective cracking or at least make it worse. Bonding is important is thin overlays and patching, but not on the scale you're talking about. Also, the mixes they advertise for overlays are generally formulated with high cement content and to aid the cure process as much if not more than to generate final strength. With high cement content you also have high cost and greater shrinkage which means greater opportunity for cracking.

Whether you remove your existing slab or not you need to determine the quality of the subgrade, especailly near the house. You need to see if it's been settling and thats why your current slab has it's current inadequate profile. Otherwise, you'll be doing this all over again.

Steve Cox

"A man may not care for golf and still be human, but the man who does not like to see, hunt, photograph, or otherwise outwit birds or animals is hardly normal He is supercivilized, and I do not know how to deal with him" Aldo Leopold

08-28-2003, 04:09 PM
If you're not worried about the grade that much since it's going to be for a garden toolshed, why bother? Why not make a berm or swale of concrete or something like it next to the house? It'd be a whole lot easier and cheaper.

On the other hand, if you do put some concrete down on the existing pad just get a good masonry blade for your grinder or saw and score the surface about a quarter inch deep in several places. It will be quicker than drilling holes and cutting rebar. The other benefit is you don't have to worry about the utilities under the pad.

I've poured just enough concrete to justify a 4' magnesium bullfloat but not enough to consider myself proficient.


Cody Colston
08-29-2003, 09:11 AM
I put concrete over concrete on my front sidewalk.

I did the 42" x 35' sidewalk myself, getting the concrete from a local rental place that had those 1 yard concrete trailers for transporting the stuff. The trailers actually look like mini-concrete trucks; the tanks revolve, tilt, etc.

Anyway, it was summer with temps in the high 90's and about 10 miles to the rental place. When I got it home, I could not back the 4-wheel trailer up the slight incline to where I wanted to pour the sidewalk and had to wheelbarrow it in place. I got it poured and screeded but when I began troweling, it was too stiff to work. Although level, the surface was very rough.

Well, I had to go back to work offshore for 2 weeks. When I got back home, I moved my form boards up 1 1/2 inches, got another 1/2 yard of concrete and re-surfaced the sidewalk. This time, I was able to back the lighter trailer all the way to the forms and the
temperature was probably 10 degrees cooler. I was able to put a smooth troweled surface on the sidewalk, removing the forms the next day. There is a "line" where the 1.5" cap meets the 3.5" base, but the grass now obscures it.

That was 3 years ago. So far, no problems with the sidewalk except for my girls leaving their sidewalk chalk scattered around on it. There are no cracks and none of the surface has broken off either.
BTW, this was also a solid walk, with no expansion joints in the

Just my $.02


When you get to the end of your rope...tie a knot and hold on!

Danford C Jennings
08-29-2003, 09:54 AM

FWIW, I believe you should follow Steve's advice; "capping" an existing slab is never a good approach. "If you don't have the time to do it right, how will you find the time to do it over?"


08-30-2003, 06:51 PM
Tear it out and start over. I may be doing it soon too. My wife and I poured a pea gravel patio at the back of our house when the house was finished. The house is brick and we may have covered up one of the little slots that the masons leave so the wall can breathe. Anyways, water may be getting into this hole because we used the tar expansion joint between the brick and the pea gravel. We find out soon. I may just tear out the concrete to about 6" from the house and place a nice flagstone covered drain under it.


08-30-2003, 09:21 PM
>place. I got it poured and screeded but when I began
>troweling, it was too stiff to work. Although level, the
>surface was very rough.
>Well, I had to go back to work offshore for 2 weeks. When I
>got back home, I moved my form boards up 1 1/2 inches, got
>another 1/2 yard of concrete and re-surfaced the sidewalk.
>This time, I was able to back the lighter trailer all the

With the concrete sitting just 2 weeks it was probably still damp inside, so I see no problem pouring a "cap" as you did, it should and looks like it did- bond just fine.

If someone were pouring new concrete over old dried out concrete and didn't dampen the old stuff well first, and keep the whole thing covered 7 days with plastic after trowelling, then I'd absolutely expect to see a problem.

I think one of the issues with new on top of old not adhering well is most people likely don't SOAK the old thoroughly, not spritz a little water from the hose on for 30 seconds but SOAKING it so it absorbs the water all the way to the center like a sponge.
Not doing that will mean the old will suck out some of the water the new needs to cure with.

To give an idea how much water concrete will absorb...

When I was building my block wall in one part of the new basement recently, I soaked the blocks in a wheelbarrow full of water for at least a half hour or longer as I worked, took breaks etc took a couple out, added two more.
Even the blocks sitting immersed in water for a half hour when pulled out and stacked to start using, within 5 minutes all the surface water on them was gone and soon they were starting to turn dry- whitish in spots and if wet again would absorb that.
The blocks would be noticeably heavier when wet but I couldn't say how much actual volume of water they took.

I had the pallet of block on the driveway sitting in the heavy rain overnight, they STILL absorbed water the next morning.

Obviously a 30 second spritz with a garden hose ain't gonna cut it but many DIY probably do only that and call it good.

Can just imagine how much water a 5 year old 6" thick slab would absorb if SOAKED. SO I think this is at least part of the reason some folks have problems with "capping" or patching.

Mark F
09-03-2003, 09:21 AM
As someone suggested earlier, I think this may be a perfect application for concrete jacking. I talked to a guy about this at a home show but have never tried it. I bet they would come out and give a free estimate. It is in the yellow pages here under "mud jacking contractors". There is a website at www.raiseconcrete.com which estimates the cost at $2.50 - 6.50 per square foot. I don't know what it would cost to pour a new slab, but removal and disposal of the old is not fun.

Good Luck
Mark F

09-09-2003, 01:50 PM
Personally I would NOT do it myself.
There is a product (out about 5 yrs) that appears epoxy like with granuals/aggragate that can be of many colors. This is poured over old concrete and then leveled out -- you could just pour the area closer to the house a little thicker ? This would help the slope issue.
Also, there is a concrete refurb. service you might want to look up in your area. It appears to be a thin layer (1/4" ?) poured over your existing slab.
I am with you about not digging up the existing slab, but pouring another 3" of concrete onto old concrete might be a issue during frost time. RESEARCH this for a few months first before you do anything.
Good luck and get back to us, Randy from Oceanside, CA

09-09-2003, 08:12 PM
I was just out mowing the lawn and thinking about concrete over concrete. (Don't ask me why because I haven't got a clue.) My thought was, What would happen in a frost zone? All it would take is a little moisture and the seal between the two slabs would start to open up and allow more moisture between the slabs, etc. In a few years would you be ripping up two slabs?

09-10-2003, 01:51 PM
Well all, in case you were interested in how this was progressing, here's a little update.

After seriously considering all your good advice, I contacted two separate concrete/demo companies that recently poured driveways in my neighborhood. They both came with good references. Each came out to look. Only one showed interest in the job. The other balked immediately upon seeing the gas and water line entry to the house below the slab. The interested party gave an estimate that I thought exceeded my expectations by a good margin. The estimate for demo and new slab was around $1000. I had expected, based on the prices from my neighbor's jobs something around $400 to $500. When I inquired as to why it was $1000, the guy basically said that the demo job would be especially tricky.

Well, sensing my disappointment in the $1000 price tag, the guy made an interesting offer. I'll attempt to describe it, but before I do, note that I did not ask him about "capping" the original slab. I contacted both companies for estimates to demo then pour and that's all that came up in the brief conversations...

The proposal he made was to excavate a 4 to 6 inch wide trench on the three sides of the slab away from the house. The trench would be approximately 8 inches deep with a base of 2 inches of gravel. He would then build a form on the outside of the trench. He then suggested that he cover the existing slab (top and sides) with thick landscaping plastic and pour a new slab over this. The new slab would be at the thinnest point just over 2" thick. The rest of the slab would be thicker...approaching 5" nearest the house. He would stop the slab with a piece of thin waterproof foam at the foundation and then apply thick butyl caulk over this for expansion. The slab would extend beyond the original and would essentially have "curbs" into the trenches. Finally, it would be up to me to regrade the flagstone walkways adjacent to this and the company would demo and remove the very small remnant of sidewalk that is near to the slab. I would replace the side walk (that does not meet the house) with pavers/flagstones...whatever I chose.

The total price for this job was almost $500 (which is just for the company work). I said I'd get back to him, but really thanked him for the flexibility he showed. I'm mulling this idea over and am considering getting another bid or two to check the competitiveness of his price.

We're in Maryland, 'tween DC and B-more. It's an expensive place to live, generally. So bear that in mind.

What do you all think?

{edit} I forgot to mention that the contractor intended to reinforce the slab and extend that to the "curbs". {edit}


09-10-2003, 08:46 PM
I think they're trying to get away with something. It's a small job and they're looking for ways to make more, in my opinion. I guess I work around utilities like this all the time and don't have much trouble. They evidently have plenty of work.

If you didn't live 1000 miles away, we'd start tear out Friday night at five. Maybe get it formed before dark, if not the next morning, and certainly be drinking beer by noon Saturday either way.

Steve Cox

"A man may not care for golf and still be human, but the man who does not like to see, hunt, photograph, or otherwise outwit birds or animals is hardly normal He is supercivilized, and I do not know how to deal with him" Aldo Leopold

09-10-2003, 10:24 PM
A few years ago I had to remove a sidewalk. I used a 12 pound sledge and was amazed at how easy the thing broke up. I wasn't doing much more than dropping the sledge on the sidewalk.

I think that if I were going to attack your slab I would use a hose with a pipe on the end to hollow out under a bit of the slab. Then a quick hit with the sledge will crack off part of the slab. It may take a some time to get the slab all broken up but the work shouldn't be that hard.

I think that most contractors are afraid of the labor aspect of removing the slab. However, if you already have the slab broken up the contractor may have different thoughts.

09-11-2003, 07:47 AM
Good points.

I don't see why I don't just do this myself. I guess that I'll have to do it one weekend and send my wife away unawares. I think she has corrupted me into thinking I'll blow up the house doing this.

Anywho, the modified cap was an interested idea the guy came up with. I still wonder if that would work at all.


09-11-2003, 08:12 AM
If the utilities are just under the slab, have at it with you sledge. If the lines are actuall running in the slab, ie go up to your gas meter, start being careful with the sledge when you get with a foot or so. The big mistake you can make if the gas line runs vertically through the slab is using a heavy iron bar/pick and raising an edge of the slab to make it easier to break off with the sledge. This action then crimps/bends/breaks the line. Also, once you get with in a couple feet of the pipe try to note whether the slab is supported by the fill underneath. If it is floating anymore than an 1/2" or so, try stuffing boards or anything underneath it to avoid that same twisting action as the slab is struck with the sledge which could damage your gas pipe(s) running vertically through the slab. However, if your drawing is as I remember it you should be alright. The gas meter wasn't over the slab, and it's line should be buried relatively deep for obvious reasons, and the water line need to be a couple feet deep to stay out of the frost.

If the slab is reinforced with rebar it'll take quite a few swings to get her broke up, esspecailly without a quickie saw. but it's only 8x10 so it can be done. If it's reiforced with wire make sure you have a pair of bolt cutters handy and an iron bar to help with opening up the cracks. It will go alot faster. Once you get it out. Make sure you have a good base so you don't have to do this again!

Steve Cox

"A man may not care for golf and still be human, but the man who does not like to see, hunt, photograph, or otherwise outwit birds or animals is hardly normal He is supercivilized, and I do not know how to deal with him" Aldo Leopold

09-11-2003, 02:29 PM
For woodworking questions I go to Woodworking.com, For concrete questions I go to WWW.Concrete.com.

It's a useful site, aimed more for the professional than the concrete hobiest, but it does have forums for posting questions and seem very willing to help the DIY homeowner.

09-12-2003, 06:24 PM
Just a quick thought.
Looked these up at Lowes [link:www.lowes.com/lkn?action=productDetail&productId=76776-1188-155931|MK Diamond 7" Bronze Continuous Rim Turbo Saw Blade $37.58] and HF [link:www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=47577|
7 in 30 GRIT CUT-OFF WHEELS FOR MASONRY PACKAGE OF 10 $11.99] I don't know how well they would work, but it might be easier to do some cutting and then break the slab down farther with a sledge.

Just throwing $0.02 at idea pot.

06-02-2010, 09:18 AM

I think I may have misled you in a few of my follow up posts.

1) I care about the grade very much. That is sort of the impetus for this whole post.

2) The slab does not have a storage shed on it.

The attached picture may clear things up.