Placing a Small Concrete Slab
Although the amount of concrete used is small, the forming and finishing techniques for a slab such as a deck-stair landing—or in this case, a propane-tank pad—aren’t much different from those used for larger slabs. First and foremost is subgrade preparation. Get that wrong, and the slab will crack. The underlying ground needs to be compacted evenly. In most cases, slabs shouldn’t be placed next to new buildings until the backfill around them has settled for several years. After digging out the slab location, compact the soil directly below so that there’s no loose dirt.
Use a gravel base
One step that’s called for but rarely done on small jobs is to place gravel between the slab and the subgrade. The usual explanation is that the gravel provides drainage to prevent soil saturation and the resulting frost heaving. But unless you drain that gravel somewhere with pipes, where’s the water going to go?
There are two reasons to use gravel. First, concrete moves because of thermal expansion and contraction. Restricting this movement will crack the concrete. A gravel base allows the slab to move freely. Second, slabs need a flat base to ensure uniform thickness, and gravel is easier to grade than many soils.
For a slab that’s 40 sq. ft. or more (about 1/2 cu. yd. of concrete for a typical 4-in.-thick slab), it’s easiest to order truck-mixed concrete. This 3-ft.-sq. slab was small enough that mixing bagged concrete by hand made sense. At 9 sq. ft. and 4 in. thick, the project called for 3 cu. ft. of concrete. An 80-lb. bag of concrete mix makes 3/5 cu. ft., so this slab took five bags.
I used a crack-control concrete, which includes reinforcing fibers. The slab was small enough that no steel reinforcement was needed, but that little extra strength from the fibers only cost me $5. Larger slabs, say 5×5 and up, will benefit from reinforcement with rebar or wire mesh to control cracking.
Concrete needs moisture to cure. After finishing the slab, cover it with plastic and keep it damp for at least a day. A week is better, and 28 days ensures the best cure.
Online Extra: Mastered in a Minute – Concrete Slabs