Detailing a thin radiant slab
My garage is used primarily for antique-auto restoration, but its blacktop floor is so soft that on a hot day a bicycle kickstand will sink into the floor and the bike will fall over. This floor is far from adequate for equipment such as floor jacks and engine hoists. The floor is composed of 2 in. of blacktop over 6 in. of concrete. I would like to shave off the blacktop and replace it with concrete. In the process, I plan to install plastic tubing for a radiant-heating system; however, I wish to maintain the 2-in. depth with the new concrete so that I don’t have to reset the three garage doors and to maintain vehicle-roof clearance. I also do not want a lip at the garage door because it makes it a lot harder to move a dead vehicle into the garage. Is this new 2-in. slab adequate protection for the tubing? Will the 2 in. be dense enough for vehicular traffic? If so, what type of mix should I order?
Steven F. Lenahan, Point Pleasant Beach, NJ
Michael Luttrell of Warm Floors at Napa, California, replies: Two inches of concrete provides adequate protection for hydronic tubes, and the concrete can support cars without problems. Your biggest concerns include minimizing cracks, preventing gasoline and other petroleum distillates from getting to the tubes and providing freeze protection for the tubes in the slab.
I would recommend that you remove the asphalt and all traces of emulsion, which is a petroleum distillate and therefore potentially corrosive to any plastic tubing. Consider installing some 1/4-in. rigid-foam insulation before laying out the tubes to provide insulation and protect the new slab from any residual emulsion.
Install tubes by shooting powder-actuated concrete nails into conduit clips that hold the tubes in place. Put the tubes 6 in. o. c.
Cover the tubes with 6-in. wire mesh. Get the flat type, not rolls, which can protrude through the wet concrete. To avoid damaging the tubes, pressurize them to 100 psi while placing the slab. Use regular concrete with high compressive strength and fibrous reinforcement for further crack prevention. Place the concrete as dry as possible. Try not to pump the concrete because pumped concrete has high water content and, therefore, less overall strength than drier mixes.
Given that automotive restoration is messy work, consider a hard-trowel finish on the slab surface and paint with an epoxy sealer to prevent the penetration of petroleum products.
If any cracks develop in the radiant slab, fill them immediately with an epoxy filler before petroleum distillates get to the tubes. And to prevent pipes from freezing, run the system with an antifreeze mixture designed for heating systems, as opposed to automobile engines. Use a small water heater, a small pump, a pump relay and a setback thermostat because you’ll be turning the system on and off a lot and you probably won’t need the system to run more than a few hours at a time.