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.
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.
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.
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.
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.
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.
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.
Why Do Builders Need an Under Concrete Slab Vapor Barrier Plastic? An under-slab vapor barrier protects the subfloor and, subsequently, the main floor from moisture by adding a protective layer underneath the concrete foundation before it is poured on wet.
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.
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.
A concrete mix of 1 part cement : 2 parts sand : 4 parts coarse aggregate should be used for a concrete slab.
Use a proper underlayment between the concrete slab and the flooring if necessary. This is especially true when using wood flooring. Installing a plywood underlayment can add extra protection, but it must also be tested to ensure it's not bringing in new moisture.
In terms of the ratio for concrete, it depends on what strength you are trying to achieve, but as a general guide a standard concrete mix would be 1 part cement to 2 parts sand to 4 parts aggregates. For foundations, a mix of 1 part cement to 3 parts sand to 6 parts aggregates can be used.
You don't “need” a vapour membrane under a shed concrete slab but it is still a good idea to install one. It is still a good idea to have protection against rising damp even though you don't strictly need it.
This means that in addition to groundwater working its way up, you also have a moisture-laden flooring surface slowly releasing water vapor for years. Concrete needs to breathe; it's the nature of the beast.
The American Concrete Institute's Guide for Concrete Floor and Slab Construction recommends that the thickness of the vapor retarder be at least 10 mils.
Using synthetics like plastic in concrete generally weakens the material because they do not bond to the cement mix as well as sand. Properties such as the type of plastic, particle size and shape, and the rheology of the wet mix all have an influence on the strength of the finished concrete.
Choosing the Right Vapor Barrier
The most commonly used vapor barrier under concrete slabs is polyethylene (poly) plastic sheeting with a thickness of 10 mil or 15 mil.
No, you cannot make concrete without gravel. The ingredient mix of 1 part cement, 2 parts sand, and 3 parts gravel (stone) is what makes up concrete. Without gravel, concrete would not be strong or be able to be used as a foundation or base that could withstand heavy loads.
Concrete will cure underground just as easily as it does above ground. The chemical reaction that occurs between water and cement, called hydration, will occur even if the concrete is covered in dirt.
This is the most common application for DIY concrete pouring. Be sure you never make your slab thinner than four inches for any application. Six inches is the minimum thickness for a concrete slab that may see any kind of heavy vehicle traffic.