Concrete Slab - Proper Thickness and Placement of Steel Reinforcement
Concrete Slab Thickness
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.
Building Codes - It's the Law!
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). And every state (except Michigan) has adopted the IBC as their base
building standard.
IBC 1901.2 states "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, snow loads, FL - Hurricanes &
Winds, Tidal Surge Areas, etc.).
Supporting the reinforcement
Contractors are NOT ALLOWED to place the reinforcement on the grade and
"hook" it (lift the reinforcement into the slab with a hay hook) as they
place the cement. After they lift the reinforcement to an arbitrary level, 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 worthless.
Concrete blocks or "chairs" must not be space so far apart, that
they allow 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."
Vapor Barriers
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 ï½µm)
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."
Proper Grading
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.
Concrete Strength
Though the codes specify 2,500 PSI as the minimum strength for concrete
foundations, there are significant benefits and little added cost from using 5000+ PSI concrete. Added strength, durability, wear resistance and reduced permeability are all achieved by merely increasing the strength. Larger aggregates also increase the concrete strength. Most contractors utilize 3/8" aggregate in concrete,
because the cost of pumping is a little lower. 3/4" rock is preferred (and is
specified by most State Highway Departments), as it yields higher
compressive strengths. A
3/4" aggregate and 5,000+ PSI mix and a 3/4" concrete pump is worth the return on your
investment. There is a significant return on the investment of a few thousand dollars.
Concrete Mix Design - Minimizing Water
Most contractors, pump operators and cement truck drivers do not understand "concrete
science." Concrete only needs enough
water to hydrate the cement particles.
Water makes concrete weaker, by creating microscopic voids in
the matrix of the concrete. When the water evaporates, it leaves microscopic
honeycombs behind - weakening the concrete.
There are "water reducers," super plasticizers and
other chemical admixes can reduce the ACTUAL water content required. They increase the strength, while maintaining the flowability, pumpability 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. By adding water they just
weaken 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.
Your structural engineer or ready mix plant "mix master" should specify the mix design, based upon strength required, truck travel time, weather conditions, distance to pump & pump type, etc. They can provide an "allowance" for the addition of some extra water, to be specified in measured gallons.
Paolo Benedetti
Aquatic Artist, Watershape Consultant, Expert Witness
"Creating water as art."™
Aquatic Technology Pool and Spa
©www.aquatictechnology.com
