Usually, granular fill or road base is used as a subbase, although in some rare cases, soil itself can be used if it is extremely compacted and stable. The soil underneath your subbase is called your subgrade, and your concrete is only going to be as strong as your subgrade.
A base of gravel will help prevent erosion and keep the slab from settling. Pour concrete sand to fill the gaps in the gravel base. Use a tamper or plate compactor to flatten the gravel base. If needed, add more gravel and compact until you have a 4-inch base.
Laying Down the Base
Before you can put down a concrete slab, you have to lay down a base. A solid base protects the concrete from settling dirt, water runoff, and other natural hazards.
The base course material, according to ACI 302, "Concrete Floor and Slab Construction," should be "compactible, easy to trim, granular fill that will remain stable and support construction traffic." ACI 302 recommends material with 10 to 30% fines (passing the No. 100 sieve) with no clay, silt, or organic materials.
You should not pour concrete directly on the ground before preparing a proper concrete base. The minimum best practice is to thoroughly prepare the ground by leveling and compacting the subgrade. Then add a subbase of gravel for additional load support of the final slab if needed.
Medium gravel, which is 1.8 cm (3/4 inch) in diameter, is the best material for a concrete base. To cover a 4 inch thick concrete slab, you'll need 3 inches of gravel. Because gravel surface is level, you can easily estimate the number of yards of concrete required.
It also depends on the size of the loads placed on the slab, such as a patio (people and furniture) versus a driveway (cars). As a general rule, stay away from using sand as a subbase. You're better off using gravel. Your concrete will last longer and have less of a chance of cracking over time.
To prevent cracking and crumbling, concrete patios need to be built on solid, well-drained soil. If you live in a region with lots of clay in the soil, you will need to dig down further and backfill with layers of compacted sand and gravel.
Instead, use a layer of gravel underneath your concrete slab. Because concrete is porous, it absorbs water. If you place concrete directly onto dirt, water will collect under the concrete and slowly erode the bottom of the pad. Gravel prevents this erosion by allowing water to drain away from the concrete.
Subbase for Concrete Slab
The subbase placement is optional and it serves as a work platform for construction of the slab and provide more uniform support for the slab. As the thickness of subbase increases, the ultimate load carrying capacity of slab increases.
Make sure the material under the slab is properly compacted. If you believe it needs more sand or gravel for better drainage or to level the area, then that added material has to be compacted before placing the concrete. Sand or road-gravel compacts better than loose stone.
The principal ingredients that make up the concrete mix are: cement, fine aggregate, coarse aggregate, water, chemical admixtures, and mineral admixtures. Concrete used in construction may also contain reinforcing bars, welded wire fabric (wire mesh), and various reinforcing fibers.
Working With Ready-Mix Concrete
And, if you mix it yourself, it's the cheapest way to make your slab. For most do-it-yourselfers, the best material for building a concrete slab is a ready-mix, crack-resistant concrete product. The wet mix is poured into a prepared wood form, then left to cure.
Properly curing your concrete improves strength, durability, water tightness, and resistance for many years. The first 7 days after installation you should spray the slab with water 5-10 times per day, or as often as possible. Once the concrete is poured the curing process begins immediately.
How long does four to six inches of concrete take to cure? As stated previously, concrete takes approximately 26-30 days to reach its full strength. If the concrete is professionally poured and floated, the curing process should be sound and ensure proper hardening of the concrete base.
A Level Surface
Simply pouring concrete onto the ground will expose it to elemental erosion that will cause cracking and sinking. Similarly, if there are roots or plants under your slab, they may rot and cause unevenness. Pouring concrete over crushed stone, however, will eliminate these worries.
To compact soil ready for a concrete slab, a mechanical compactor should be used. A plate compactor is the most common tool used to compact soil for a concrete slab. For small areas, a hand tamp may do the trick, but a mechanical compactor is better. Strong metal plates and lots of force compact soil best.