Whether you pour concrete for a walkway or patio, a strong gravel base is required to prevent the concrete from cracking and shifting. Gravel is especially important in clay soil because it doesn't drain well, which results in water pooling under the concrete slab and slowly eroding the soil as it finally drains.
Because concrete is a very porous material, it will absorb any moisture that it contacts. This can cause pooling. Without crushed stone, pooling water will settle under it and erode your slab. Adding a layer of crushed stone will add proper drainage, as well as create a barrier between your slab and the ground.
If the weight of the concrete deforms the dirt base in any way, the entire concrete structure will be flawed. All of that is not to say that it is impossible to pour concrete over dirt. You can absolutely do it successfully, but you need to follow a specific process.
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.
Most concrete contractors want a mix of coarse and fine aggregate to create a compactable base that is going to be safe for settlement and drainage. Crusher run (a mix of crushed stone and stone dust) and #57 coarse aggregate are two of the best base materials for concrete slabs.
The best sub-base for a concrete slab is gravel. The soil has to be prepped first before the gravel can be laid down; pouring concrete directly onto rock is not considered a best practice. You can use different kinds of gravel for the subbase as long as it is clean and uniform in shape and size.
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. Though all outdoor fixtures are subject to natural wear and tear, concrete will break and deteriorate more quickly without a base to support it.
Most driveways require at least 6 inches of stone crushed or gravel. 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.
Standard concrete floor slab thickness in residential construction is 4 inches. Five to six inches is recommended if the concrete will receive occasional heavy loads, such as motor homes or garbage trucks. To prepare the base, cut the ground level to the proper depth to allow for the slab thickness.
Technically you can, but you really shouldn't pour concrete over grass. To maintain its tensile strength, concrete needs to be laid on a firm, even, and dry foundation. Grass and soil will naturally channel moisture into the concrete from below, which will likely result in it cracking over time.
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.
A mixture of sand, cement and water is called mortar. Mortar mix is made by combining cement, sand, and water. Mortar doesn't have very much compressive strength because it contains no gravel. So its not used as a structural material. But it's still very strong and sticky.
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.
Yes! A strong gravel base is required when building a concrete patio in order to prevent the concrete from shifting or cracking. Adding gravel under the concrete also provides a level surface for your foundation and adds proper drainage when there is pooling water.
Rebar is not necessary for every concrete project. The general rule of thumb is that if you are pouring concrete that is more than 5 inches in depth, you are probably going to want to add in some rebar to help reinforce the entire structure.
The best concrete thickness for a shed tends to be 4 inches which is the standard size used for most residential concrete patios, walkways and sidewalks. You can pour 6-8 inches if your storing something heavier but it's rare. Concrete slabs are typically poured over 4-8 inches of compacted gravel and earth.
As for thickness, non-reinforced pavement four inches thick is standard for passenger car driveways. For heavier vehicles, a thickness of five inches is recommended. To eliminate standing water, the driveway should be sloped towards the street a minimum of one percent, or 1/8 inch per foot, for proper drainage.
Gaps against a house can direct water against the foundation, leading to more problems. Once a year, go around your home and fill these gaps and joints with urethane caulk (or polyurethane caulk) to prevent problems.
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.
Concrete will stick to some plastics, wood, metal, vinyl, cloth and other porous surfaces. But that's about it. Concrete won't stick to molds, smooth metal, glue, paint, oil, mortar, other concrete or most masonry products.
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.
What is Vapor Barrier? A plastic sheet that is placed between the base and the concrete slab. Always placed beneath the rebar or wire mesh and on top of the gravel base. The sole purpose of the vapor barrier is to prevent moisture from penetrating through the concrete.