Design

The major design elements in slab on grade design are thickness, expansion joint spacing, and re-bar size/grid spacing. At first, I tried getting some help by contacting some friends over email. The responses were vague, and when I asked for specifics, they stopped responding at all. Another friend recommended checking out the ACI ( American Concrete Institute ) ACI-INT.ORG. The only information on their web site was regarding things they wanted to sell, books and seminars. I wound up spending about $120 on 2 slab on grade design manuals.

Using some nasty looking design equations I calculated the amount of steel required. As with most engineering calculations, there are some values left up to the discretion of the designer. These were difficult for me because I lacked experience in slab design. My friend, Paul Gee, was able to fill in the gaps. He is a civil engineer studying for the PE exam. The variables were Young’s Modulus for concrete, the MOR (modulus of rupture) value, coefficient of sub-grade friction, maximum and minimum temperatures, expansion coefficient, steel grade, slab thickness, and control joint spacing. With Paul’s help, solving for quantity of re-bar, with 3 different methods we came up with 0.22 to 0.28 square inches of steel per linear foot of width. A #4 (1/2") bar with 12" spacing is 0.2 sq-in, so I decided to use #5 bar (5/8"). When I ordered it from Weldon, I mistakenly ordered #6 (3/4") bars. I called them later to verify what grade steel it was. I had expected and calculated for grade 60, but it was grade 40--a lower strength. The number 6 bars weren’t so big after all by the documented design method. The building inspector asked if we would be driving tanks on it. Well, more is better.

Slab prep

Before I started insulating the bottom of the slab, I had to stub some conduit and piping that had to surface just above the footing. The 1 ½" chilled water piping requires a special type of insulation for underground, called foamglass. It was special order and cost $7.50 per foot. I only needed a little for now. The electrical conduits were for the future chiller in back and for the site lighting. With that complete the insulation and the vapor barrier (6 mil polyethylene sheeting) were put down. The perimeter of the slab was formed so that it was a little thicker, 8", than the rest of the slab, which was 6".

To position the rebar in the center of the slab, bottom spacers, called re-bar chairs were used.

Rich DiBernado helping out to install the re-bar

 The re-bar comes in 20’ lengths so it had to be cut. I bought a metal cutting blade for my chop saw. To be able to pour the slab by Saturday, August 28, I needed to install all the re-bar on Wednesday. The masons, Asen and John came by to help after work. A friend of mine from work , Rich DiBernado also came by to see the house and I put him to work. I had hoped that I could take the evening off to chat with him, but to get the slab poured, the re-bar had to get done. When I got home, I tried cutting a bar with the chop saw. It cut very very slow. I needed to cut each bar at least once, adding up to 200 cuts! I went right out with Rich and rented a gas powered cut saw to do the job.

the gas powered cut-off saw was an animal

The following morning about 3:00 AM a thunderstorm filled the tarp with water between the block wall and the center rope. To prevent the 500 gallons of rain water from filling up the foundation, I lowered an electric pump into it from the block wall and pumped it out. It was still raining so I needed to prevent it from happening again. I stood the ladder up on a pallet to hold up the center. 

I stood watch until the rain stopped around 6:00 AM and then went back to bed. We got 3" of rain that night, according to my rain gauge.

Thursday, John finished wire tying the rebar. After work, my friend Ben Bertolotti and John helped install the radiant heat tubing. It had to be piped to the manifold and tied to the top of the re-bar.

On Friday, I had to take a vacation day from work to finish the preparation. I had help from John and a worker. There were a lot of small things to do. It was a long day, but we did finish all the items on my punch list.