CEMP-E
TI 809-02
1 September 1999
Thickness
Maximum
of Slab
Uniform
Design Live Load
100 mm (4" )
7 Kpa (150 psf)
150 mm (5")
12 Kpa (250 psf)
200 mm (6")
20 Kpa (400 psf)
Unless otherwise specified above, the correct slab thickness will be determined in accordance
with the Portland Cement Association (PCA) Publication entitled, "Slab Thickness Design for
Industrial Concrete Floors on Grade." In the PCA design process compressive strength is
converted to modulus of rupture, which is then reduced by a factor of safety to obtain the
maximum allowable flexural tensile stress. The maximum allowable flexural tensile stress is
then used to find the required slab thickness. For typical values of modulus of subgrade
reaction refer to TI 809-27, "Concrete Floor Slabs on Grade Subjected to Heavy Loads." When
wall loads exceed 15 kPa (300 lb./foot) slabs-on grade will be thickened in accordance with
the provisions of TI 809-27.
5-3. SERVICEABILITY.
a. General. Cracking, warping, and curling can impair slab-on-grade serviceability. These
problems are directly attributable to drying shrinkage. Cracking can be controlled by
minimizing drying shrinkage, by providing adequate crack control and isolation joints, and
through the use of reinforcing steel. Water penetrating the slab is a common serviceability
problem that can be cured by proper drainage and by the use of vapor barriers.
b. Minimizing Drying Shrinkage. Cracking in slabs generally results from drying
shrinkage and restraint caused by friction between the slab and subgrade. Curling and
warping occur due to differential shrinkage when the top of the slab dries to lower moisture
content than the bottom of the slab. Drying shrinkage, curling, and warping can be reduced by
using less water in the concrete. Ways to reduce water content include using the largest
maximum sized aggregate (MSA), using an MSA equal to 1/3 the slab thickness, and by using
coarse sand. Water content can also be reduced by using coarser ground cement and cement
with a low C3A content. On large and critical slab-on-grade projects the designer should
request by specification that shrinkage tests be made of several concrete mixes to obtain a mix
with the lowest drying shrinkage potential.
c. Controlling Cracking through Control and Isolation Joints. Control and isolation joints
can be used to minimize cracking and to force cracking to occur at joint locations. Designers
should attempt to minimize the number of joints occurring in the slab. However, in most
instances, the maximum slab area bound by crack control joints should not exceed 60 square
meters (625 square feet), and distance between crack control joints should not exceed 7.5
meters (25 feet). The length/width ratio of panels bounded by joints should be as near 1.0 as
possible and should not exceed 1.25. In localities where extreme conditions of heat or dryness
tend to produce excessive shrinkage, the maximum area and joint spacing may need to be
decreased. Crack control joints may also be construction joints. Joints in the vicinity of column
pedestals will be placed at column centerline, with diamond shaped or circular isolation joints
provided at columns or square shape isolation joints provided at column pedestals. When
5-2
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