Floor Slab on Ground
Design Information Sheet
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Design Discussion
imix XS® Technical Discussion
Lightly Reinforced Floor & Pavement Slabs-on-Ground

Most floor and pavement slabs-on-ground are designed using plain concrete to carry the applied loads. In addition, it is well known that all concrete cracks. The challenge facing designers and builders of concrete floors is controlling the appearance of random cracks, which are unsightly and require long-term maintenance. Cracks appear because forces develop in the concrete that exceed its tensile capacity. These tensile forces develop from the restraint of slab movement by the sub-grade, racks, walls or other building components, from volume changes due to water reduction within the concrete matrix, from settlement of the sub-grade, and from temperature effects.

For this reason a minimum stiffness of the sub-grade is specified and steel rebar or mesh is usually added to the slab which to limit the width of random cracks that form between control joints. This reinforcement does not prevent cracking, nor does it add to the load-carrying capacity of the slab. In reality, this reinforcement is usually not installed where it will do the most good; in the top third of the slab thickness. These slabs are referred to as lightly reinforced slabs.

Steel fibers are often overlooked as a means to minimize random cracking in concrete slabs. When steel fibers are added to plain concrete, an inherently brittle material is transformed into a ductile composite. This steel fiber reinforced concrete, or SFRC, initially inhibits the propagation of cracks then maintains a measure of control after visible cracks appear.

Steel fibers can provide the same level of crack control as steel rebar or mesh. This guide describes how to select the correct amount of steel fiber reinforcement to equal steel rebar or mesh used as reinforcement to minimize random cracking.

 

 

 

 

 

 

 

References:
1. McKee, D.C., —The Properties of an Expansive Cement Mortar Reinforced with Random Wire Fibers“, Ph.D. Thesis, University of Illinois, Urbana 1969.
2. Soroushian & Lee, —Distribution and Orientation of Fibers in Steel Fiber Reinforcement Concrete“, ACI Materials Journal 87-M44, 1990.
3. Technical Report 34 (TR 34), —Concrete Industrial Ground Floors - A Guide to their Design and Construction“, The Concrete Society, 1994.

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Methods
Three methods are used to determine the appropriate equivalent steel fiber dose.

Method 1: A minimum quantity of steel fibers is established using the spacing theory [1]. See ICC-ES Report ESR 3226 available at www.icc-es.org
ESR 3226 Fibrous Reinforcement

Method 2: A quantity of steel fibers is calculated by equating the area of steel provided by rebar or mesh to the area of steel provided by steel fibers. The equivalent cross sectional steel area is based on a method by Soroushian and Lee [2] and determines the number of fibers crossing a plane per unit area using the following formula:
SF dose = As x 13200 ó (a x t x 12)

Where,           As = conventional steel area
t = slab thickness
a = fiber orientation factor

Method 3: A quantity of steel fibers is calculated by equating the moment capacity of a steel fiber reinforced concrete section to the moment capacity of a conventional rebar or mesh reinforced section. The method uses yield line analysis and is described in TR-34 [3]. The quantity of steel fibers is calculated using the following formula:
Mo = Mn+Mp = [1+R10,50] x fr x S

Where,           fr = plain concrete modulus of rupture
S = section modulus
R10,50 = SFRC residual strength factor
Mn = negative moment resistance of slab
Mp = positive moment resistance of slab
Mo = limit moment resistance of slab

The residual strength factor is directly related to the dose of a specific steel fiber type and the concrete compressive strength. This relationship is determined from laboratory scale beam tests performed in accordance with ASTM C 1018.

All three methods are evaluated to obtain the quantity of steel fibers to equal certain configurations of steel rebar and mesh. The spacing theory, method 1, establishes the minimum dosage. The equivalent area of steel, method 2, is used when the dosage of steel fibers is not sufficient to produce a residual strength factor greater than 30%. The equivalent moment capacity, method 3, is used when the dosage of steel fibers is sufficient to produce a residual strength factor greater than 30%. A complete explanation and development of the equivalent steel fiber dosage is beyond the scope of this document, but can be provided upon request.

It must be emphasized that when using SFRC for lightly reinforced slabs, control joint dimensions must be chosen using PCA and ACI guidelines or other local code requirements assuming no reinforcement is present.