Placing a thin floor slab over an existing slab on grade.

[steve111]

We have a 50,000 sq.ft. warehouse that needs a new floor slab. The existing facility was destroyed by fire and the slab on grade is in poor condition. The contractor would like to place a 3" slab/topping over the existing slab. Looking for suggestions wrt. mix design, curing, jointing, srfc. prep., etc. Thanks.

 

[ishvaaag]

This looks a task for a specialty house. Thin overlays are sometimes added to bridge decks having an allowance for it in design, so it seems reasonable to look for the providers of such overlays (Grace?) even if what here needed is a bit less exacting.

 

[Ron]

Steve111...several cautions about this process.

If you have forklift traffic on the slab, stress from the wheel loads will be quite high at the interface between the slabs. This should be evaluated, based on the loads of the forklifts.

The next issue is that of reflective cracking. The joint pattern of the old floor will reflect through the new floor unless the patterns match exactly. Further, the old floor might have a joint pattern that is not acceptable under current standard practice, so your new floor will have to have additional joints plus the reflected joint pattern. Otherwise, you will still get random reflection of the old joints.

The next choice is to decide whether you want the overlay to be bonded or unbonded. If you have fire damage to the old floor, bonding will be difficult (due to carbonation layer in the existing floor). If you decide unbonded, you must make sure it will be truly unbonded (use heavy bond-breaker)and then accommodate the increased interface stresses.

Good luck.
Ron

 

[Jim6758]

Ron is right about the reflective cracking, especially if you go unbonded. A technique used in this area to ensure good bonding is "shot-peening". It allows some pretty thin over-lays to be applied (much thinner than 3 inches). Some fork-lift wheel traffic can be pretty hard on a warehouse floor.

 

[steve111]

thanks for the replies.

 

[Ron]

steve111...one thing I didn't think about before is to use a "crack and seat" approach. In this you will crack the existing concrete even more, making sure the "chunks" are not larger than 1 or 2 square feet. Then "roll" them in with a large compactor (for seating), then place a topping on the cracked and seated old floor. Control joints will have to be closely spaced for this procedure ( 8 to 10 feet on center, each way)

 

[RiBeneke]

Ron,
I think the concept of your last suggestion is good, but how would you actually do it ?
How would you crack the old slab, assuming it was originally designed for forklift and similar heavy traffic ?
Saw cut to the rebar depth would be expensive, vibro roller would probably not do much to crack it, multi-sided impact roller might work, but would be hard on the machine.

 

[jheidt2543]

Steve111

Contact Antigo Construction, 2520 N. Clermont St., Antigo, Wisconsin, 54409 (sorry I don't have their phone number handy but, they do have a website). Their specialty is pulverizing concrete pavements, mainly highway pavements. I had them on an industrial floor removal, 6" - 8" thick concrete in various areas of an existing building and they crushed to the size of a baseballs and smaller. They did in three days what would have taken a hydraulic hammer three weeks! Little dust and much less noise.

We then picked out the old rebar, compacted and graded the base and poured a new floor. I think you'll have a much better finished product if you take out the old and put down a new slab.

Good luck!


P.S. I have no connection to Antigo other than using them as a subcontractor 1-1/2 years ago.

 

[steve111]

Seems the possibilities are heading more towards removal of existing slab and placing a new SOG. This of course is one solution. I guess we are looking more towards mix design suggestions to control plastic/temp shrinkage cracking, curling at control joints etc., re thin slabs on grade. W/C ratios, cement content and type, reducing heat of hydration, coarse aggregate size and content, admixtures, temperature and shrinkage reinforcing (fibers), surface prep., curing, and some practicle aspects of placing and finishing.
We will have the opportunity to place the new slab sometime over the next few months in relatively favourable ambient air conditions, +/- 50 to 60 deg. F., and the building will be up and enclosed. Thank-you.

 

[jheidt2543]

Steve111:

Here are a few things to consider when pouring the new floor slab:

1. Have a well compacted subgrade.

2. I always use a 6 mil. poly vapor barrier, although there is some controversy about this in the industry.

3. Concrete mix design should be based on maximum of 5 bags of Type I cement, more cement means more shrinkage. In almost all instances, 5 bags gives plenty of strength.

4. Don't use fly ash in a floor slab. It is fine in foundations and mass concrete but, it gives too grainy a mix when trying to finish a floor.

5. Pour with as low a slump as can be worked. It is best if you can truck dump it in place (if you don't have to pump or convey it in place). You want it as stiff as possible, the concrete crew and cement finishers don't like it but, you get better results.

6. Saw control joints ASAP on maximum 20' centers, we've even gone to 12' centers (half bays) on some jobs. Using a "soft-cut" saw is the best if you have one.

7. Moist cure the floor for at least 3 days.

8. Use a liquid floor hardiner like "Lapidolith".

9. Use a semi-rigid joint sealant like MM-80 in all the joints. The only problem is timing, the sealant shouldn't go in for 20 - 30 days after curing is complete. Since it is semi-rigid, when the floor shrinks, the joints open up and the sealant is not effective. The longer you can wait after curing the better.

That's my nine cents! Ok, what does anyone else think?

Good luck.

 

[Rob44]

steve111: If your existing slab doesn't contain a lot of cracking caused by soil expansion, use spaced mechanical fasteners anchored to the existing slab to form a composite section. The new concrete overlay must be thick enough so the neutral axis of the combined sections falls in the original section. This will produce total compression in the overlay. Concrete will not crack when in compression.
I suspect you may need only a one or two inch overlay.