CEB Foundations 101
CEB foundation and wall sketch
The sketch above of the entry door for our energy building shows you the basic design of the foundation slab, the piers under each corner and door posts with “toothed” blocks that will be encased in reinforced concrete around the door frame. The next 2 blogs will show you how that design became a reality on the ground.
Forming and piers
Forming is necessary to get a slab that is level, square and oriented exactly as it should be in the landscape. Once the elevations are determined, then the forms are set to the correct dimensions. Here we need a finished slab 16ft. long and 12 ft. wide. The initial forms shown here are laser leveled and squared. The yellow string is tied to each corner and left throughout the process to be sure the forms never get out of alignment or off-level. Because this is a slab on grade and not a big pour, the braces for the form boards are only 18 inches, cut perfectly square so they do not stick up and interfere with screeding later on. The forms are set and braced. Then we dug the footer ditches and placed that dirt behind the forms to add support against the heavy concrete.
You can see the holes dug for the piers. The steel rebar is #4 (which means it is ½ inch dia. or 4/8ths of an inch). The rebar rods come in 20ft. lengths and must be bent so as to make a corner with no ties. The reinforcement for the belled piers does get tied to the corner but that does not compromise the strength like tying ends of rebar in the corner does. Rebar is tied at least a foot away from the corner and each overlap needs to be about 2 ft. long.
To make sure that the rebar cage in the footer (deeper ditch around the exterior perimeter that will support the heavy walls) is stable and each bar is embedded in at least 2 inches of concrete, we made up some stirrups or rebar squares and tied each rebar into one corner to make a cage. To unify the entire footer and mesh system, we wired on a series of steel bars along both the short and long axis. On top of that we wired on steel mesh. To keep the rebar in place while pouring, we used plastic chairs to hold them up. These chairs are important because they allow the concrete (with a vibrator) to completely envelope the rebar. If you use a block of wood or hunk of brick, it leaves a large portion of the rod exposed. When, not if, it begins to rust, it will ruin the entire steel cage and leave the slab with no tensile strength. That’s also why you never drive a rod into the ground to use as a support for the rebar cage in the footer trench.
Making stirrups is not hard, so it should be always be undertaken. You can see a small vice attached to my tractor bucket allowed me to mark and bend #2 (1/4 in.) rebar into the desired squares. Since the trench was 12 inches deep and 14 inches wide, the stirrup needed to be 4 inches less than that on each side to assure 2 inches of concrete would surround the rebar. Rebar will be tied inside this stirrup where the red tape is.
The stirrup is not closed so it slips over the long rebar and gets tied wherever necessary.
Steel mesh unified cage
I placed 1inch insulation under the unified steel cage to dampen vibrations and noise when the generator is running later. Note the chairs under the steel grid. The concrete did not push them down into the insulation since I did not allow the operator to dump 5 tons of wet concrete in one big pour. I used 4,000psi concrete with a 5inch slump. Normal concrete is max. 3000psi, and you’re lucky if you get that. The big slump enabled an inexperienced crew to work less and still get a quick and even pour.
Note the 6 mil plastic sheet. It is not necessary to use plastic if the slab will exceed 8 inches in thickness, so you don’t usually see plastic under the footer.
I think it makes the concrete ease under the steel better so I do use it even up on the forms. Helps release them without release oil.