We had a building that had an existing concrete floor that was poured directly on the ground. Over the years it had cracked. Normally you would jackhammer the old concrete out and do it right. However the building supports were placed directly on top of the concrete pad.
This presented a problem of how to remove the concrete pad and still hold up the roof! After much deliberation, we decided to top the cracked concrete with a new "floating" slab. The idea being, let the original concrete slab crack and do what it wants because the upper slab will float on top, undisturbed.
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Step 1: Patch the Existing Concrete Slab.
We used Quikcrete's Vinyl Concrete Patcher to fill the cracks and to smooth the floor as much as possible.
Step 2: Creating the "Floating" Layer
We covered the floor with Home Depot's HDX 6 mil Black plastic sheet and then laid wire re-mesh sheet (also from Home Depot) over top of the plastic. The re-mesh strengthens the cement that you will lay in the next step.
Note: Keep the re-mesh sheet up about an inch above the black plastic so the re-mesh is centered in the 2" pour to strengthen the cement.
Step 3: Mixing the Concrete
You have to decide just how you intend to mix the concrete. You can use the Black plastic mixing tub or use a mixer. With 28 bags to mix, we chose to rent a mixer.
Some may ask why not just order up a truck of cement? With the new slab only being a proposed 2" thick, you can't use conventional cement. It will crack. Conventional cement is designed for 4" thick or thicker.
Step 4: Pour the New "Floating" Concrete Slab
You need a specialty concrete mix for this "Floating" concrete floor to work properly. We used Sand/Topping Mix from Quikcrete in 60 lb. bags. This mix is pre-mixed, just add water. This Sand/Topping Mix is specially designed for 2" or less thickness. We poured a 2" slab.
Step 5: Finishing the Slab
There are two methods for finishing a concrete slab, floating it smooth or brooming. We chose brooming because we wanted a non-slip floor, even in the wintertime. A wet coarse broom is lightly dragged over the partially set up cement.
Step 6: Ready for Use
After a few days, the slab was dry and hard.
Note, this "Floating" concrete slab is now 5 yrs old and has withstood -35 deg F below zero and heaving in the spring thaw with zero cracks!