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Thread: OT - New concrete over old slab
05-21-2007, 06:42 PM #1
I'm demolishing a 12'x20' outbuilding and building a 15'x24' one in its place. The old slab is 8 inches thick. I need to raise the floor of the new building by at least 6 inches for drainage reasons.
So I want to pour the new slab over the old. The inspector will let me do it. My question is: do I try to tie the two slabs together as much as possible (bolts / concrete paint in between) or do I try and let them float as much as possble (sand / rock / foam in between). The old slab, 60 years old now, is pretty cracked, but the soil isn't bad.
If it makes a difference, the new slab isn't centered on the old. The SE corner will be the same as the old, and the new slab will extend more to the N and W.
Also, I'll put up a tarp on tent stakes (instead of a building) before I'm willing to break up and haul out the eight inches of existing concrete, so that's not really the advice I'm looking for.
Anybody have any experience with this?
05-21-2007, 06:54 PM #2
Don't have direct experience, but would suggest contacting a paving contractor with experience in fabric underlayments. These separate a top pavement surface from movement underneath. I'd guess that if you throughly compact the new areas (dig up the earth, gravel, sand, membrane); use enough rebar, and protect the whole thing from water intrusion into the surrounding soil that you'd be OK.
05-21-2007, 10:08 PM #3
Really need more info to give a decent answer. What type of building are you erecting? Will it be heated? What type of loads are you going to put on this slab in the worst case scenario?
05-21-2007, 10:33 PM #4
It will be an unheated garage, basically. Wood frame construction. Max loads would be a few machines and/or a vehicle, so no more than 10 or 12,000 pounds at the absolute max. Probably much less right now.
I do plan on adding compacted fill and a thickened edge to the new part, and I'm leaning toward rebar instead of wire mesh in the slab itself. Got a good price on rebar recently, so it comes out about the same cost.
05-21-2007, 11:00 PM #5
You and I are in the same climate. I take it, it will be a floating slab with no foundation. I would go with atleast a 6" slab and rebar at 2' on center and I think you will be fine. I wouldn't go with bar smaller than #4.
05-22-2007, 12:28 AM #6
I would not bother to tie the two slabs together or apply a separator. Simply reinforce the new stuff as described above, and put extra in your thickened edge. As long as the earth under the new slab is properly compacted, you'll be fine.
05-22-2007, 01:25 AM #7
If you have a concrete batching plant in your neighbourhood you might ask them what they do with surplus concrete in their trucks when they return from a job.
Over here the normal practice is that they have a pond dump out the back where the trucks drop the material and wash out the agitator bowls. A lot of the cement is washed out of the gravel.The idea is that they reuse the gravel in a new mix but often this doesn't happen.
If you can get some of this material and use it where the old slab doesn't go then if it was like my experience it sets fairly hard. It might be the cheapest approximation to concrete available to you.
Failing that then several layers of well compacted roadbase, which is a mixture of fines and coarser crushed rock, should be laid no more than maybe 6" thick with a suitable water content.
It might be necessary to excavate down to subsoil before importing roadbase or if you go deeper than say 18" you might be able to use a cheaper fill material.
As mentioned in another thread if you want to bind the new slab with the old one you need to scabble the surface of the old slab. This involves removing the surface and exposing the aggregate in the old slab sweeping the debris away and wetting the old slab well some hours before and again before pouring the new slab on top.
An electric jackhammer with a scutching comb is useful in scabbling the old slab.
05-22-2007, 02:01 AM #8
The new slab will crack to follow any cracks present in the old slab. I suggest you dowel the old cracks via 12" lengths of re-bar let in and grouted across the old cracks on 12" centers. Separate the old from the new with a vapor barrier (you did mention drainage meaning to me a high water table at least seasonally). Lumps and bumps in the old slab will key to the new.
Before you start anything, be sure there is no subsidence under the old slab. Maybe a survey and a slab pumping job would be the first order of business. A short length of heavy chain dragged across the old floor with a stick on a quiet night will tell your ears whether the slab is well supported.
05-22-2007, 09:05 AM #9
Forest is correct about the old slab cracks telegraphing threw the new slab. You sure as hell don't want to bond the two together either. Separate the new from old with a layer of new compacted fill around 4" thick.
05-22-2007, 01:08 PM #10
I should not even post here
you guys are way out of my league
but i have done some concrete slabs for myself
like a basement floor and outdors
and common sense tells me that you cannot pour
concrete over the old slab and make it larger than
the previous slab
without either making the underlying slab larger first or removing the old slab entirely.
We had a new development for a shopping center here
in Islip, NY.
and they poured about 5-10 acres or more of concrete
for a parking lot
then the development failed and someone else took
over or the same owner had a new idea:
they actually tore up the entire 5-10 acre or larger
parking lot and trucked it away.
Then they built new additional buildings
and then they made a new parking lot
The cost of this staggers the imagination.
The previous parking lot was about 2 years old
before they tore it up again.
05-22-2007, 04:41 PM #11
Pour Over Top
Think of the old concrete as a really solid base for one area of the new building.
For the new area excavate to subsoil and fill with gravel. Compact it fully and then do it again. You want it at 100% not 99%. Your old concrete is at 100%.
Your new concrete should not crack. I assume the movement in the old concrete was from loads not frost. The new concrete will distribute the load on the old ocncrete so you shouldn't have any motion.
Use 1/2" rebar on 2" centers with 6" of concrete and look forward to all the fun in the new building.
If you have any lingering doubts buy a grade or two up in specs on the ocncrete.
05-22-2007, 05:42 PM #12
I would sure put control joints in the new conc. in line with the edges of the existing slab. If it does crack, and it most likely will - (look at your exist. slab) make sure it cracks where YOU want it to, not where IT wants to. Simple as a tooled joint in the new concrete.
05-22-2007, 09:13 PM #13
Several years ago I was involved in putting 18" of new concrete over 24"+ of old concrete. The building was 70' wide by 120' long. Yes, this was a lot of concrete! We were doing it for the very same as CCC is doing it, we needed to go up above the new grade level. Our luck was that we were near a concrete mixing plant and there was a huge construction project going on nearby, so we were able to get the loads that were rejected by the construction project. We used those loads for the 1st 12-14" then bought high strength concrete for the final pour. We laid two layers of #4 rebar on 2' centers with the two layers offset to prevent parallel strings of rebar.
I am not involved with the group any longer, but do visit from time to time and have yet to see any cracks.
BTW we poured the new concrete directly on top of the old concrete, it was really beat up so we did not figure we needed to do any preperation other than a good hosing off.
Oh yeah I forgot to mention, this floor now supports two 80 ton steam locomotives.
05-23-2007, 12:51 AM #14
Texas is rebuilding I-10 and they are building the roadbed with concrete and then poring 12" of concrete on top of that,i think if its good enuff for a hiway with that much traffic & weight then it's bound to hold for a shop.
05-23-2007, 01:29 AM #15
This one kinda hits me. My own garage floor is about 25 years old. Block around it, backfilled, compacted, poured, now, I think the floor is settling. Hey, a whole bunch a cracks should be an indicator.
Hip roof, ONE post for support.Don't know HOW in the hell I could remove all that I have in there to pour another 4 inches on top of it. Damned near have to move to take all that stuff out of there.
05-23-2007, 02:20 AM #16
There is a saying that goes something like "where two or more Psychiatrists are in conversation you will get at least three opinions".
It is my opinion that the new slab is likely to crack where the existing slab finishes. As the earlier poster says assume the new slab wants to crack there and put in a construction joint.
05-23-2007, 04:52 AM #17
Hi, you don't know me.
If this is a project you have a little time to research.
Before the days of concrete, there was lumber.
The Cutty Sark comes to mind, noting the recent fire, then all the ships come to mind. Wood decks
Study the ribs and joints.
Think about the USS Constitution or HMS Victory.
The Star of India has been moored in the water for more than 50 years, Iron hull.
Consider the many bridges...
Steel, Wood and Concrete and Stone.
Consider the Goat Canyon Bridge
Trains are still crossing this bridge.
Notice the stones? California.... home of earthquakes and pancakes.
It is almost as old as the Titanic.
I think of fortresses.
Now I could take you many..............MANY... other places. But I won't... and you know why??
You probably have already been there.
But if not...oh well.
After all, today's Rome is built upon ancient Rome. On the top thereof.
Consider the Great Pyramids of Geezer in Egypt.
Build your garage likewise, on a rock and you will be pleased.
Well, consider the advice of others,
my opinion, it's free and worth every cent.
05-23-2007, 11:40 AM #18
Thanks for all the advice. I'm hoping to do grading and forms this weekend. I think I'm going to go with the general consensus here: well compacted fill around the old slab, pour new right on top of old with #4 rebar both ways, a few control joints where the old slab has edges or major striaght cracks.
Maybe I'll let you know in 5 years how it holds up. [img]smile.gif[/img]