Concrete Slabs on Grade

The first thing you must do is make sure all the topsoil is removed, this is layer of soil is where the grass grows and it is full of organic material. It can vary in depth from a couple of inches to well over a foot, depending on what part of the country you live in.
Once you have removed all of the topsoil you want to use a plate tamper or roller (for a large area) to make sure the subgrade is highly compacted. By compacting the soil you are minimizing the chance that the soil will move in the future, and that is a source of cracking known as movement cracks or heaving.

This is also the time you want to make sure the subgrade is far enough below the eventual floor this is known as subgrade elevation and contour. For example, if you are planning on putting down four inches of concrete over four inches of stone, the subgrade should be at a fairly level 8 inches below where the eventual floor will be. If an excavating company is doing the work, specifications usually require them to be within 2/10ths of a foot (+/-), that's about 2 1/2 inches either way, but most good contractors using laser leveling equipment will be within 1/10th of a foot (better than 1 1/2 inches) of the desired grade.

This is also when plumbing and electrical rough-ins and anything else that needs to come up through the concrete is placed in the soil. It is extremely important that these items are backfilled and compacted as they are installed. Placement of concrete over poorly compacted pipe runs will almost surely end up cracking as the soil settles because it will create a stress point for the concrete. It is not recommended that these pipes be placed above the base as it will act as a "reverse" sawcut and will assuredly crack along the pipes path. With that said, pipes should be adequately buried and bedded to protect them from frost damage and soil or slab shifting. Slab leaks can be difficult to pinpoint and a little proactive installation can save expensive repairs in the future.

It is also advisable to get a geotechnical engineer involved to ensure soil stability and subgrade compaction.

Once the subgrade is compacted, you can place stone to act as a cushion and to help keep water from migrating up through the slab. I recommend 3/4 - 1" stone because you don't have to compact it after it is spread. If you use a smaller stone, sand, caliche, or similar base material, you will have to use a mechanical method of compaction such as a jumping jack, vibratory plate, or roller to compact the material to approximately 95% compaction. Use a laser or string-line when you are spreading it to maintain grade. To ensure correct concrete depth you can use rakes and shovels to smooth out the stone/aggregate. This is known as fine grading and when you are done the base should be within 1/4" at any given spot.

After the stone is graded the vapor retarder or vapor barrier is installed. Please note that there is a distinct difference between vapor retarders and vapor barriers. Vapor Retarders are most common and are used to create a membrane barrier between the concrete and the ground to minimize the concrete's ability to "wick" moisture up through the concrete. Vapor Barriers are designed to eliminate ground moisture and generally are only used in areas with high ground water, poor ground percolation, highly acidic or alkaline soils, and "clean room" applications.

For most applications polyethylene (plastic) sheeting is adequate as a vapor retarder. It is available in various dimensions and thicknesses (mils). Common thicknesses that are used for Slab-on-Grade (SOG) construction are 4, 6, and 10 mil. The thicker the poly the stronger it is but don't automatically grab the 10-mil poly, it is generally only used for the heaviest applications. Generally as a rule of thumb 4-mil is used for residential, 6-mil is used for light to medium commercial and 10-mil is used for heavy commercial and industrial applications. To get the best results from the vapor retarder, the joints or seams should overlap at least 6" to 12". After lapping the joints they should be taped to prevent water seepage.

When the specifications require a more robust vapor barrier they will call out a specific product or product family. Examples of vapor barriers include StegoWrap by Stego Industries and Flopruf by Grace Construction Products.

Now that the stone is down, use 2x4's or whatever lumber you need to get the concrete thickness you are looking for. Every four or five feet you want to brace the form so that the pressure from the concrete doesn't push the boards over. This edge form serves two purposes, it holds the concrete in, and acts as a guide for placing and finishing. Because it is acting as a guide, you want these edge form boards to be very level and straight. Use a string-line and level to make sure they are level and plumb, I can not overemphasize this step, check it, then recheck it and just before you place the concrete - check it again!

After the edges are formed, there are three popular methods of reinforcing the slab, they are reinforcing steel (rebar), welded wire mesh (WWF) and fibre. Rebar is generally not needed for a light duty slab, but if you are planning on having heavy items sit on the slab for a long time or will have heavy vehicles on the slab (driveway?) you will want to use at least #4 rebar placed every 12" or so in each direction.
For a lighter duty slab you can opt for welded wire fabric which comes in various guages (wire diameter) and spacing, typically we use 1.4 or 2.1 6x6 WWF for most medium duty slabs.
The third option is fibre mesh, which is installed at the concrete plant. It is composed of thousands of tiny fibers that are suspended in the concrete and take the place of WWF, it is not recommended to substitute fibre for rebar, but if added to the mix it can give added strength to the concrete. The biggest drawback with fibre is that sometimes after finishing there will be tiny hair-like fibers protruding through the concrete. These protruding fibres can be burnt off with a propane torch, or just left - they will eventually rub off. Though fibres are still a bit controversial they are gaining acceptance in various circles. If you go with the fibre, you definetly want the approval of the owner and input from both the structural engineer and concrete mix designer before proceeding.

If you don't have experience here - STOP - and seek out a qualified concrete finisher!

Check to make sure everything is ready before the concrete arrives, the equipment should have all been started to make sure it is running and everything should be in good repair.

Did you remember:
Subgrade Compaction?
Plumbing Rough-In?
Electric Rough-In?
Embedments?
Stone?
Vapor Barrier?
Edge Forms/Box-outs?
Hand tools (Concrete rakes, bull float, edgers, floats, knee boards, hand trowels, etc.)?
Power tools (trowel machine, vibrator, georgia buggy, etc.)?

Ok, so start by placing the concrete and raking it level, this will take a couple of people, behind the placers someone will be vibrating the concrete and "striking off" the concrete to knock down any high spots.
After the concrete is leveled off, a bullfloat is floated accross the top to make everything semi smooth.
Once the concrete has been floated off, the edges are started with an edger tool and hand floats while the concrete is "setting up" (starting to get hard).
After the concrete can support the weight of a person the trowel machine is used to repeatedly trowel off and smooth the top of the concrete until it reaches the desired finish - this is called "burning in" the concrete.

Even after the concrete is placed and finished, you still aren't done, you must decide how you are going to cure the concrete - the theory is that the concrete will dry on top before it does down inside the slab, if the top dries to fast it will flake up this is known as delamination - to help prevent this and other potential problems, the concrete is cured. The simplest way to cure the concrete is to use a curing compound. This is usually a liquid chemical that forms a membrane over the top of the concrete - there are numerous products and their application varies, use the manufacturers recommended application procedure. Another, more expensive method for curing is wet curing, which is usually accomplished by laying wet burlap on the finished concrete and then covering it with polyethlyene (plastic) sheeting to hold the moisture in.

Regardless of the method used for curing, you should let the slab "cure" for at least 7 days before you use it. Concrete never completely cures, the hydration process continues for years, but by 7 days it should be between 75-80% of design strength.