Gravel, or crushed stone, provides a stable surface for your concrete slab. Even the most solid, compacted soil can change dramatically with weather changes, undermining the slab and causing cracking. With above-grade slab foundations, an alternative to native soil material can be used to restore grade. These imported fillers can include granular materials such as sand, gravel, or crushed stone.
These high-strength, permeable materials offer low compressibility, which is ideal for a slab support system. Because concrete is a very porous material, it will absorb any moisture it comes in contact with. Without crushed stone, pool water will settle underneath and erode the slab. Adding a layer of crushed stone will add proper drainage and create a barrier between the slab and the ground.
For added protection, consider adding a vapor barrier as well. Concrete Network explains that the two levels below the access roads include the native land subgrade covered with a gravel subbase under the concrete slab. The best subbase for a concrete slab is gravel. The soil must first be prepared before gravel can be laid; pouring concrete directly onto the rock is not considered good practice.
Maximum compaction is possible before the material has dried out too much or has been exposed to periods of rain where water can accumulate. A flat, smooth surface is good for flat work because concrete can contract as it cures without being restricted by the base material. Some claim that any crushed rock is fine, even mixing gravel with old concrete or other solid material. The tensile strength of concrete is typically taken to be 10 to 15% of the compressive strength, i.e., only about 400 or 500 psi.
Leveling, rolling and compacting a properly graded subbase material will virtually eliminate the upward bulge of angular stone particles that can cause many types of generic vapor retarder materials to pierce when heavy traffic crosses the wheels. Gravel larger than 1 can be difficult to compact and may be too porous for some types of concrete. Some infill materials also provide good drainage properties and can be used to prevent moisture concentrations under the slab. If that happens, it could lift some of the insulation and cause it to protrude over the top of the slab, causing cracks as the concrete settles and compromising the ability of the board to insulate the slab completely.
I've been watching some of my neighbors pour small concrete slabs into their backyards for various projects. But what if there is a delay at this point before the concrete is laid? If the subbase rains or freezes before placing the concrete, it can go from being ready to being too soft. The top 6 to 12 inches of under-slab fill should be filled and compacted with well-graded gravel or crushed material. When using rigid foam, make sure the vapor barrier goes above the rigid insulation so that concrete does not go under the insulation during pouring.
Concrete strength also comes into play, but most concrete slabs are around 3000 to 4000 psi, so it's not a major factor. Too often, builders don't properly prepare subgrade for concrete foundations, sidewalks, driveways, or patios.