Although the amount of con- crete 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. Choosing concrete 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.
Thanks so much to the Orlando Fence and Deck Contractors for sponsoring my blog.