Most structural engineers, building inspectors and contractors do not comply with the published building codes that require minimum slab thicknesses & specific placement of the reinforcing steel. These standards apply to "structural slabs" which are defined as slabs that contain reinforcing steel AND support or transfer loads. Therefore, slabs that bear traffic or weight (e.g. driveways, garage floors, warehouse floors, sidewalk/drive approaches, ANY building slab, etc.) are structural slabs.
The American Concrete Institute (ACI) 318-11 (2011), section 7.7.1 specifies certain slab thickness (dependent upon the thickness of the reinforcement used). It specifies 3 INCHES of concrete between the earth side (regardless if it is soil, sand, gravel, rock or protected by plastic) and 1.5 inches of top coverage - #5 bars (5/8 inch) and smaller, and 2 inches of coverage for #6 - #18 bars (3/4 inch - 2.25 inch).
Remember when determining slab thickness that the bars cross each other - resulting in steel TWICE as thick as a single bar.
Therefore, if you are using #4 bars as reinforcement in your slab, you would have a minimum 5.5 inch slab (3 inches + 0.5 inch steel + 0.5 inch steel + 1.5 inches). Using wire mush is difficult, as you cannot maintain the required placement of the reinforcement within the slab - it bends & deforms too much.
Contractors are NOT ALLOWED to place the reinforcement on the grade and "hook" (lift the reinforcement into the slab with a hay hook) as they place the cement. They walk through the wet cement, pushing the reinforcement back to the bottom where it does absolutely NOTHING. Additionally, hooking frequently pierces the vapor barrier, rendering it useless.
Don't say that the ACI standards do not apply to your locale - the ACI Standards are incorporated DIRECTLY into the Uniform Building Code (IBC). Every state (except Michigan) has adopted the IBC as their base building standard (IBC 1901.2 "Plain and reinforced concrete. Structural concrete shall be designed and constructed in accordance with the requirements of this chapter and ACI 318..."). California has the CA Building Code, which is based on the IBC ( as usual, CA just made their code more stringent). A number of other states have done the same to meet regional issues (CO - wild fire protection, FL - Hurricanes & Winds, etc.).
The concrete blocks or "chairs" must not be space so far apart, that they allow for the reinforcing steel to deflect (bow or bend) during concrete placement or as the finishers walk on it. ACI 318-11 7.5.1 states "Reinforcing steel shall be accurately placed and adequately supported before concrete is placed, and shall be secured against displacement within tolerances..." IBC R506.2.4 states "Reinforcement support. Where provided in slabs on ground, reinforcement shall be supported to remain in place from the center to upper one third of the slab for the duration of the concrete placement.".
The plastic placed under a slab is known as a VAPOR RETARDER. It's sole purpose is to prevent capillary action (wicking of moisture) through the slab if it was allowed to remain in direct contact with the soil. Care must be taken to adequately overlap the plastic sheets, seal the seams with waterproof tape and prevent the plastic from punctures. Penetrations (pipes, conduits, vents, etc.) through the plastic sheet must be sealed with special cone shaped plastic sheets similar to roof jacks. Notice that the code said "retarder," not "preventer" or "barrier." Some will allow moisture transmission as they may decay over time.
Sand or soil is sometimes placed atop the plastic to absorb the bleed water on the bottom of the slab. The surface of slabs poured directly on plastic will bleed a lot surface water. The perimeter slabs must be elevated to account for the thickness of this material. The support chairs should sit on this material or be of an extended height to allow for it. In some areas of the country, these membranes perform multiple tasks, and are installed to also prevent methane or radon gas transmission into the living space. These membranes are made of special materials, require special seam treatments and incorporate under slab venting. Some systems utilize multiple layers of barriers to achieve the required protection.
IBC 2012 R506.2.3 states "Vapor retarder. A 6-mil (0.006 inch; 152 ｵm) polyethylene or approved vapor retarder with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists.
Exception: The vapor retarder may be omitted:
1. From garages, utility buildings and other unheated accessory structures.
2. For unheated storage rooms having an area of less than 70 square feet (6.5 m2) and carports.
3. From driveways, walks, patios and other flatwork not likely to be enclosed and heated at a later date.
4. Where approved by the building official, based on local site conditions."
The IBC also requires that the earth around the foundation fall away 6" in the first 10 feet, to promote positive site drainage away from the foundation. This also helps prevent water from flowing back under the slab, saturating the base material and causing wicking.
"Turndown" footings can also be added to a flat slab to reinforce the edges, transfer loads and to act as a "cut off wall" for ground water intrusion. These are already incorporated into building foundations as part of the designed load transfer.
Though the codes specify 2,500 PSI as the minimum strength for concrete foundations, there is little added cost and great benefits from increased, strength, durability, wear and reduced permeability in utilizing 5,000+ PSI concrete. Remember, larger aggregate equates to stronger concrete. Most contractors utilize 3/8" aggregate in concrete, because the cost of pumping is lower. 3/4" rock is preferred (and is specified by most State Highway Departments) and yields higher compressive strengths as well. The few thousand dollars extra for a 3/4" 5,000+ PSI mix and a 3/4" concrete pump is worth the return on your investment.
The last point, is that most contractors do not understand "concrete science." Water makes concrete weaker, by creating microscopic voids in the matrix of the concrete (when the water evaporates, it leaves a honeycomb behind - weakening the concrete). Concrete only needs enough water to hydrate the cement particles. There are "water reducers" and other admixes that can reduce the ACTUAL water content required, INCREASING THE STRENGTH and maintaining the flowability and workability of the plastic concrete. If your concrete contractor or ready-mix supplier does not know about these, find someone else (they have been around for decades!).
And above all, DO NOT let the concrete pump operator or cement truck driver add water when the truck arrives at the job site - they just weakened your cement, adultering the formulation and mix design! They are truck drivers and pump operators for a reason... because they are not engineers or concrete/building experts.
