The soil base for poured concrete must be well-drained and compact. Concrete is porous, so you need to place concrete slab where drainage is not an issue. Sub-surface water can cause the ground to flex, creating stress cracks in the cement. Properly preparing the surface helps limit the potential for cracks in the finished concrete slab. Preparation is the most important aspect of pouring a concrete slab.
Dig the ground down to the proper depth. Determine the proper depth by establishing the height of the concrete slab. Add 5 cm (2 inches) to the depth for gravel if the soil is normally moist or has a high concentration of clay. Most concrete slabs will sit 2.5 to 5 cm (1 to 2 inches) higher than the dirt and 5 to 7.5 cm (2 to 3 inches) below the surface of the dirt, making the needed depth of the hole 7.5 to 12.5 cm (3 to 5 inches).
Smooth out the ground with the flat side of a rake so that you have a level surface. Fill in any low spots with dirt.
Tamp the ground with a hand tamper or mechanical tamper. A hand tamper is a heavy pole with a flat metal base and two handles on the side. A mechanical tamper is available from most plant hire companies or DIY centres and comes with instructions for use. The tamper packs the soil down. The finished product should be so compact that walking across it barely leaves footprints.
Pour 5 cm (2 inches) of small, rounded gravel for additional drainage. Tamp the gravel down until it is smooth and flat.
To shore up the sub-surface, sprinkle a thin layer of cement sand onto the gravel and dampen it with water. Do not leave any standing water and allow the dampened sand to dry for about an hour before pouring the concrete.