Q: What is the difference between a structural concrete slab and a plain concrete structural slab?
November 17, 2009
A: A structurally reinforced slab-on-ground uses a composite of concrete and structural steel to support the design load. Structural steel may be rebar or WWF. The cross-sectional area of the steel is inserted into engineering formulae found in ACI 318 to determine the load carrying capacity for a given slab design. In a structural concrete slab, the thickness of the slab is not a factor in determining the load carrying capacity of that slab. The cross-sectional area of the steel, the spacing of the steel and tensile properties of the steel are the parameters of the steel used in the calculations.
To emphasize, the load carrying capacity of the structurally reinforced concrete slab is determined by the properties of the structural steel reinforcement specified. ACI 301- “Standard Specifications for Structural Concrete” and 318 are the sources for selecting the design approach of the slab. Westergaard and/or Myerhof design methods are used to calculate the slab properties.
A plain structural concrete slab-on-ground uses the properties of the concrete to support the design loads. Here the thickness of the slab and the compression and flexural strength properties of the concrete based on 28-day tests are the controlling parameters. By definition, secondary/temperature-shrinkage reinforcement is used to control cracks after they have formed in the concrete cross-section. Secondary reinforcement is not considered in determining the load carrying capacity of the slab.
The thickness of the plain concrete slab is determined by the properties of the concrete used in the slab. ACI 302 Guild for Concrete Floor and Slab Construction and ACI 360 Design of Slab on Grade provides the design methodology for this type of slab. There are additional ACI plain concrete design protocols, such as ACI 330 for parking lots.
Typically, highways and parking lots along with most industrial, warehouse and commercial floor slabs are designed with plain structural concrete. Plain concrete slabs will be thicker than structural slabs yet in most cases cost effective compared to structural slabs. The use of Fiber Reinforced Concrete versus conventional steel as secondary reinforcement is in most cases very cost-effective since there are no on-site costs assignable to the fibers. We can compress the project timeline by eliminating the need to pre-place the wire mesh. We can also reduce costs by eliminating the need for a concrete pump when fibers are used in lieu of the wire mesh in slabs-on-ground. Here the use of fibers allows for the ready-mix truck to discharge directly on the slab base at the point of use.
The dosage level for Microsynthetic Fibers, as secondary reinforcement in residential slabs-on-ground, can range from 1.0 pound per cubic yard for monofilament fibers and up to 1.5 pounds per cubic yard for fibrillated polypropylene fiber. Lower dosage levels for each material may be used when the sole responsibility is plastic shrinkage cracking and plastic settlement. For example, the engineered dose for high fiber count monofilament polypropylene fibers as a plastic shrinkage reinforcement is ½ lb/cy.
Macrosynthetic Fibers are used in the construction of commercial, industrial and warehouse floor slabs. Here the average residual strength as determined by ASTM C1399 can be used to establish the minimum dosage requirement of the Macrosynthetic Fibers.
Several quantifiable durability properties of the concrete can be enhanced when using fibers. The test methods utilized to produce these data are found in ASTM, ACI or other consensus group or government agencies documents. ICC ES AC32 provides an excellent source of Synthetic Fiber Reinforced Concrete testing methods for both plastic shrinkage crack reinforcement and temperature-shrinkage reinforcement. ICC ES AC208 is available for Steel Fiber Reinforced Concrete.
Secondary reinforcement as defined in several the ACI documents, including 302, 318 and 330, limit the responsibility to ‘holding the concrete together after it cracks’. Furthermore, the amount of conventional secondary reinforcement is determined by one of 5 empirical formulae. Parallel reading should include a paper written for WRI by Robert Anderson, PE discussing the application of these formulae. There is a table in Mr. Anderson’s paper that relates the quantity of secondary reinforcement that each formula will provide. None of the five formulae produce the same answer.
The major issue with the use of wire mesh or #3 or #4 rebar as secondary reinforcement is the need to have this reinforcement at the proper height within the cross-section of the concrete to perform. Unless chairs/supports are specified and used the WWF is typically a non-performer. Fibers, on the other hand, can be found throughout the mass of the concrete, 3-dimensionally distributed, and have been proven to provide secondary/temperature shrinkage reinforcement as well as several other quantifiable durability benefits, which will extend the service life of the concrete.
R.C. Zellers, PE/PLS
Director, Engineering Services