Tips for a 10-inch double mat garage slab that nobody talks about

I finished a similar pour last October for a commercial storage unit using 4,500 psi mix with a retarder because the sun was brutal that day. Those double #6 mats are no joke; we used the heavy-duty Castle Chairs to keep everything from sagging under the weight of the crew. Even with the chairs, we had a guy dedicated just to checking the clearance as the pump hose moved around. If you don't have someone watching those standees, the top mat will definitely dip while you're focused on the screed.

One thing I did differently was using a pencil vibrator every few feet. With a 10-inch slab and that much steel, it’s easy to get honeycombing near the bottom mat if the cream doesn't settle right. We almost missed a few anchor bolts near the corners because the slurry covered our marks on the 2x10 forms. I started using bright orange spray paint on the form tops for the bolt locations instead of just markers. It saved our tails when the light started fading toward the end of the finish.

I ran a 10-inch slab last summer and found that standard bull floats just weren't cutting it with that much aggregate. I switched to a big channel float to get the surface flatter before the sun baked the top.

Also, since you've got those double mats, watch your feet. I tripped on a tie-wire loop and almost buried my boot in the mud while screeding. I ended up zip-tying some scrap PVC over the corner rebar so we wouldn't snag our pants or skin our shins while wrestling the hose. It's a trip hazard nightmare in there once the mud starts covering the bottom bars.

What exactly is a 'pencil vibrator'? I'm pretty new to this and haven't heard that term yet. Does it just shake the air bubbles out? I'm also curious about the 'double decker' part—does that mean a whole second floor is going on top of this 10-inch slab? It sounds like so much weight. How do you even know if the bottom mat stayed in place once the mud is poured over it if you can't see it anymore?

Grab an Oztec with a 1-inch head—that’s your pencil vibrator. It’s the only way to get that 5,000 PSI mix to flow around double #6 mats without leaving honeycombs. For a double decker garage, you’re likely dealing with a structural suspended slab or a heavy-duty lift area, so consolidation is everything.

To ensure the mat stays put, I use Dayton Superior slab bolsters. If you don't feel the vibration thru the rebar when you're dipping the head, you’re missing the mark. Also, keep a Marshalltown magnesium float ready; that much mud generates some serious heat and you'll want to close up the surface before it gets away from you.