New Slab Over Old Slab.....Pinning?

Scott.ID

Member
Howdy all,

Been a member here for 10 years or so, but usually get my answers from searching the archives. Still check in almost daily.....

Anyway, bought an old farm with a wood barn, center section is 15x40 with a REALLY rough slab poured in there probably 50 years ago. Didn't float or screed any of it, and I can't roll around toolboxes or anything else with wheels. Had several people mention pouring a new 4" slab over the old one, existing concrete "walls" are about 6" tall all the way around so we could pour right up to it.

Questions are, is it a good idea? Can't cut out the old one, and some of the old timers say I'll have to pin the new to the old. They say epoxy some 4" rebar "ells" 2" down in the old slab, then they will be up 2" into the new 4" slab.

Whatcha think? Any and all opinions appreciated!

Scott
 
When I built a 2 car garage on to my house, I left the concrete drive, covered it with about 6 inches of P-gravel to get the elevation I needed. Put a 1/2 fiber board next to block foundation so the slab would float. Used wire instead of re bar. It worked fine. the P gravel even acts like an insulator. Did this job 25 years ago.
 
That is a unique one, going over an existing slab, would raise some questions for me as well. I don't know about doweling into the existing, do you need to or not. It's typically done in new from the footing on walls and piers or similar structural components. I just wonder on a slab that size, will it have other effects, can it project or fracture upward from forces below. Sounds good at a glance as it will tie the new one into the old. Horizontally, sure, 6"x6' woven wire mesh or #4 rebar, spaced_____ each way depending on what you'll put onto it. Doweling in is easy enough to do with a hammer drill, deformed metal bar and the epoxy, and I've used the powder type that you mix for this purpose, works well. I've doweled into existing and old concrete before, small section, to place a small slab. If so, I would about agree with the depth or edge distance of the #4 bar as you mentioned. This will be interesting to hear what others think or have experienced about a detail like this, definitely unique to old barns. I have a few old barn slabs here like that, better finish, but really old concrete.

One consideration is water, if it were to get under the new slab, between them and freeze, same with anything that can react with the rebar, concrete has capillaries and does absorb moisture, so if you park a vehicle on it that just came out of the winter slush and is laden with salty accumulation, dripping down, that could be a problem, with rebar both ways, more so if the embedment is not deep enough from top of slab.

I have seen new concrete placed over existing on curbing and odd circumstances where the existing was cleaned and coated with epoxy for the new concrete to adhere to.
 
Helped convert an old steel span dairy barn into a plastic injection molding factory. Poured 6 inches over a portion of the old floor that was rough and uneven. Nothing was done over the old floor. No idea how thick the old cement floor was. At least half of the non cemented floor was dug down below the manure and gravel was graded and packed even with the old cement floor. No problems in over 40 years.
 
Just remembered, the plastic factorys I worked at in 1959 thru 1967 molding room had a rough floor. The formed around the 10 molding machines next to the basses with 2"X 4" and poured directly over the old floor. The old machines were oil leakers and the oil needed to be pumped out as a regular job but the floor was in good shape when they closed the shop.
 
Your quiry about a new slab has left me with all kinds of questions that can't be answered with out addressing the list of issues that I have, as a former builder.
1-- where do you live climate wise, frost prone or southern.
2-- What type of soil is the barn situated on,(well drained gravel or sogy clay).
3-- Is the barn atop of a knob, or on a downslope with lots of uncontrolled water running beneith the buildings floor.
4-- Is the existing slab underminded with decayed topsoil, water erosion, and vermit undermining.
You need to adress all these issues.
1-2-3 If in frost prone area perimiter drainage is a must.
4-- I would rent a Compaction tool such as a "jumping jack" or bigger if posible, and run it over the whole existing slab. Where it sounds hollow, and undermined I would break the existing floor with the jj, or other means and then fill voids with stone dust or sand, and compact it well.
Once all the voids have been remidied, I would spread a layer of stone dust or compactible crushed gravel about 2" thick to provide a release layer, much the same as a good tile floor mason would do. (forget the rebar ties to the old concrete). Make sure your compaction process is thourough on the substrath.
Buy 5x10' sheets of 6x6" mesh, not rolls, and install it on 2" plastic chairs, and tie it all together securely with wire twists.
At the sides form in 1/2" foam material for enpansion between the walls and then after concrete has cured about 12 hrs. cut in control joints, to prevent rogue cracks in the new floor. The perimiter expansion joint can be cut in a bit, and there are comercially produced seals than can be installed to prevent moisture from penetrating the expansion joint.
Loren, the Acg.
 
One of my shops had a 4" cap poured over a rough concrete floor over 30 years ago and there has been no problems with it. It is not pinned but does have heavy mesh in it. I wouldnt do it if I were working on D-8 Cats but for cars and small tractors it works OK. I also would not do it if the old floor is sagging due to animals undermining, water problems etc.
 
I poured a new floor over a slab that had been a dog pen and built a shop on it. So far after about ten years I have had no issues. For a 4" thick slab I would pour it like over any other surface. It really doesn't need to bond. If you wanted to do a skim coat that would be another story.
 
You will get varied opinions as I did many years ago when I wanted to have someone pour a 4 inch slab inside a pole barn I had that i used as a workshop/garage. I lived inside city limits, and found out the city would never let me do it unless I dug out and poured a footing around the perimeter, rebar on 2 foot centers, etc, which made the job way too expensive for me to afford, and in my opinion a waste of time and money. I contacted a friend who had worked in concrete business on his own, and he recommended we prepare the building, and bring in concrete trucks on Saturday mornings (half at a time), and close the doors when finished. We used the wire mesh for reinforcement, and the underlying stone base had been stable for years prior. Ended up having to bring concrete trucks from almost an hour away because the local plant would not work on Saturdays, supposedly only for their big customers, etc, but we finished the job, and the slab was still in good shape years later when I moved, and actually I improved the property values in the process even though the building inspector would have never stood for it, had they known. Sometimes you have to do what you have to do, within reason of course. Sure I would love to have did it their way, but it was not possible with the finances I had at the time, and no one could tell the difference once it was finished, and I was not planning on driving Caterpillars or Semi trucks on it.
 
If your old floor is solid but just rough, go rent a concrete grinder, or hire someone else to do the job. When done seal the floor and you will save a bunch of money and have a smooth floor. BTDT
 
I designed a few concrete bridge deck overlays--most of them were 2 inches thick. we jack hammered any bad spots out of the old deck, sandblasted the deck, used a bonding agent for the new overlay---never tied the two slabs together
if you thoroughly clean your old slab and remove any bad concrete, you can make a mortar/water paste as a bonding agent
and scrub it in and pour the new slab. be sure to keep it moist and above 40 degrees for 6 days
 
I build a shop 3 years ago on the place of an 70' x 46' old burned down hog barn, the foundation was still intact but had a rough concrete floor that was cracked in many places.
I cleaned the old floor and laid the rebar and pored the new 6 to 8" floor right on top.
Now 3 years later there are a few crack here and there but otherwise there are no problems.
 
They make a latex paint, for that express purpose, which we used at the Boeing Co. The old concrete needs to be clean first. Can't remember the name or brand, but it is not a secret, in construction circles, and should be easy to find out.
 
What I often hear is the exact opposite, put down 2 inches of sand, then pour the new floor on that, so there is some give between old and new, helps for expansion and contraction issues.

Pouring right on the old slab will mirror whatever problems the old floor has - cracks, lifting, etc.

In your case if your old floor is very stable and minor cracks, a solid pour over the top to level it might work out well. If the old floor is in good shape no other issues.

With a rough surface, I can't see the need for the pins, I'd use the epoxy instead as others mention, the new concrete should grab the rough old surface pretty good as it is?

Paul
 
You can pour the new floor right on top of the old one. Just pop a chalk line on your walls and pour. The old floor will be a good solid base. And as far as cracking, concrete does 2 things. Gets hard and cracks. You can help that by adding control joints. I've been pouring concrete for 35 years and I'd do it at my house or for a customer.
 
I think it was 1993 we poured over an old floor. 2" in one corner, 7" in the opposite corner! No steel reinforcing of any kind, just fiber in the concrete. It has been my shop ever since, without any problem. This is on a side hill, where drainage is not an issue.

Another contractor did the other half just last summer. His idea was to lay down plastic, because he didn't want the floors to bond to one another. I don't know that I like that idea, as it seems that any moisture that might show up could get trapped by the plastic.

Probably as many opinions as there are people.
 
What is the actual condition of the old slab? Salt from livestock manure will cause cement to spall away, leaving only the larger aggregates on the top surface of an otherwise good slab.

If the slab is still solid with no heaving splits, I would look for a concrete filler to smooth over the top rather than pour a new slab. If you have plenty of ventilation, a couple of coats of a better grade of asphalt driveway sealer might even do the trick (water based). Check with a local building center or a building contractor for the best product to use.
 
Disclaimer: I'm not a concrete guy, and anyone who has seen my work will agree.

If you install vertical dowels in your old slab, you're going to get cracks along the dowel lines. Fresh concrete contracts a bunch, but fifty year old cement isn't going to contract at all. Something has to give, and it's going to be your new floor. It's common to dowel when you pour fresh concrete next to an old pour, but concrete doesn't shrink much in thickness, and the dowels are in the midpoint of the slab.

I think you need to be more concerned about allowing the new slab to float over the old. If the floor is as rough as you say, the wet concrete is going to stick to it and you'll get new cracks above the cracks in the old concrete. Consider bringing in an inch or so of sand then pouring on top of that. Saw your control joints as soon as possible; if you do it within a couple of days of the pour you can use a "green saw" which is much smaller and easier to use than a wet saw. I'd use 4000 psi limestone mix with fiber mesh.
 
Two approaches:

If the old floor is solid and not settling with just surface cracks I'd wash it clean and pour the new concrete directly over it using wire mesh for reinforcement. No need for dowels as the concrete will adher to the rough surface.

If the old floor has cracked due to settling and continues to settle I'd go in with a jack hammer and break it up - and the areas with the settling would get the concrete broke up the most. Fix the drainage issue (if that is causing the settling) and then pour over the rubble using rebar for reinforcement.
 

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