minimum slab thickness
The garage/workshop monolithic slab is supposed to have a 12″ thickened edge, 6″ thick for the rest of it; there’s gravel fill inside the thickened edge. In the process of stubbing up some drains I discovered that some spots are going to be barely 4″ The gravel is already under poly and 6×6 wire mesh, so redistributing the gravel away from the high spots will be very time consuming. Will some areas of only 4″ thickness present a significant problem, or should I just leave it alone? It’s already been a nightmare trying to get this subcontractor to prepare for the concrete. It’s taken him 4 months so far. Any fixing is probably up to me to try to get done in the next couple of days, unless I want him to take another two months to rectify it.
An idea I had for a simple fix is to rip some 3/4″ plywood and add strips to the top of the form. That would bring the thin spots up to almost 5″. The downside is of course it will add some concrete cost, about 4 cubic yards.
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Trevor,
FWIW, My garage slab is 5" and my shop slab is 4". Both have the same 6x6 grid mesh to which I tied pex tubing for future heating if called for. I asked for fiber to be added to the garage slab in case a future owner has truly heavy vehicles. My largest is about 6500 lbs and nothing has happened to the slab. My shop did not have fiber added and again nothing has happened. The heaviest stationary tool is about 600 lbs with a 19x24 footprint. Unless you are doing smithing with a wheel hammer or some kind of impacting machinery I can't see why you would need the full 5".
If the garage portion of your garage/workshop is a separately defined area you might consider thickening that part of the slab. Just adding fiber would be simpler, but I do not know what kind of up-charge you might face. Helix fibers are actual metal and tout good strength. Poly fibers are also available with different ratings. Not sure if your current contractor could handle leveling two areas or adding to the form height given he didn't seem to level the gravel bed.
Do have a plan for rain once the slab is finish troweled. I counted on good weather and got a surprise rain burst that left me with texture that could be worked by the time we found it. Misting the slab while it cures might be helpful as well. Or I may just be behind the times on modern concrete work. Work with the correct slump specified by the engineer and don't let them make a soupy mix.