Suspeded concrete slab


Old 01-11-00, 12:23 PM
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I am busy building an outdoor fireplace with under-counter storage space. This requires a suspended (on brick pillars) concrete slab with largest unsupported section 1500mm x 600mm. The slab will act as working top (light duty) and in one part carry a mild steel built-in fireplace.

Any suggestions on:
1) thickness of the slab
2) how to avoid (if possible) steel re-inforcement
3) concrete mixture
4) curing time
5) tips?

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Old 01-11-00, 04:55 PM
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About ten years ago I built something similar, only larger. It was a 17 foot span of concrete counter, with the largest free span of about 9 1/2 feet, supported on cored
concrete block that was eventually covered with stucco. On the counter was a Orno (bread oven), a built in steel barbacue, built in sink, and the counter top was tiled. To edge the top and tile it was formed with masonite, then ground flat with a diamond plane. Afterward, I made custom wood moulding, used pins for alignment, and bonded it to the concrete with a fast setting high strength acrylic adhesive.

The slab thickness I used was 2 3/4" (engineered by my brother, a Licensed Engineer). I don't think you'll be able to use anything less than 2 1/2", and that's a guess, not an engineering opinion. (Concrete countertops are 1 3/4" thick with a maximum span of 3'. A 4' sink base is pushing things).

Why on earth would you want to avoid using reinforcing steel? I don't see the problem, and I can't guess what your point is. Someone is bound to absue it, so why not plan on the abuse in the first place. 3/8" bar, 12" O.C., 1.5" from the edges, will do.

A 6-6 mix can be used for reinforced work. A 6-6 mix assumes moderate wear or a tile covering. Ratio by volume, typical mix:
6 gallons water, 6 1/4 sacks Portland Cement Type I, 14 Cu. ft. Sand, 19 Rock (1 1/2 max. size). Suggested maximum slump: 4"

Adjusting the former for a thin slab:
Trial batch weights with 1/2" max aggregate size and "wet sand":
Use a bath room scale. (By weight is more accurate than by volume).

Water 5, Portland 14.5, Sand 23, Rock 28. Max. Slump 4".

If the sand is very wet, increase sand by 1/2 pound, decrease water by 1/2 pound. If the sand is damp only, do the opposite.

The slab can be formed in place or be poured in a protected area and be lifted into place later. If it's formed in place the cribbing must be strong, and everything aligned perfectly.

If it's formed on a flat garage floor, use 3/4 Plywood for the base, laid on the floor.
It will weigh about 300 pounds or so. The plywood allows finger holds or a means to insert a pry bar without slab damage.

Minimum cure time is 14 days. 28 days is better.

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