360R-06 Design of Slabs-on-Ground

DESIGN OF SLABS-ON-GROUND 360R-35
quarter of the slab. Both reinforced and plain slabs, as well as
fiber-reinforced slabs, displayed this behavior.
8.4.2 Prism and slab expansion strains and stresses—
Because the reinforcement percentage varies, the ASTM C
878 restrained concrete prism test is used to verify the
expansive potential of a given mixture. 
Figure 8.2 
may then
be used to determine the amount of slab expansion (strain)
using the known prism expansion value and the percent of
reinforcement in the slab.
With the use of
 Fig. 8.2
, the amount of internal compressive
force acting on the concrete can be estimated knowing the
maximum member (slab) expansion and the percent of
internal reinforcement in the slab.
8.4.3 Expansion/isolation joints—Because a slab may be
restrained externally on one side by a previously cast slab,
the opposite side should be able to accommodate the expansive
strains. When a slab is also adjacent to a stiff wall, pit wall,
or other slab, external restraint on two opposite sides is
present. Compressive stresses as high as 45 to 172 psi (0.31
to 1.19 MPa) (Russell 1973) have been measured, and if the
external restraints are sufficiently stiff, they may prevent the
concrete from expanding and elongating the steel.
Normal asphaltic premolded fiber isolation joints are far
too stiff to provide adequate isolation and accommodate
expansion as their minimum strength requirements are in the
150 psi (1.0 MPa) range at a compression of 50% of the original
joint thickness. A material with a maximum compressive
strength of 25 psi (0.17 MPa) at 50% deformation according
to ASTM D 1621 or D 3575 should be used.
If a slab is allowed to expand only at one end during initial
expansion,
the width of the isolation joint (in inches) should
be equal to two times the anticipated slab expansion, as taken
from 
Fig. 8.3
, and multiplied by the length of the longest
dimension of the slab (in inches). For a 100 x 120 ft (30 x 37 m)
slab with expansion strain of 0.00035:
Joint width = 2 × 120 × 12 × 0.00035 (2 × 36.6 × 1000 × 0.00035)
= 1.008 in. (25.60 mm)
Use 1 in. (25 mm) thick joint material if the slab is to expand
only at one end; and
Use 1/2 in. (13 mm) thick joint material if allowed to expand
at both ends.
8.4.4 Construction joints—ACI 223 states that with the use
of shrinkage-compensating concrete, slabs may be placed in
areas as large as 16,000 ft
2
(1500 m
2
) without joints. Place-
ments of this size should only be considered in ideal conditions.
Placements of 10,000 ft
2
(930 m
2
) or less are more common
with joint spacing of 100 ft (30 m).
Slab sections should be as square as possible, and provisions
should be made to accommodate differential movement
between adjacent slabs in the direction parallel to the joint
between the two slabs. Further explanation and details are
found in ACI 223.
8.4.5 Placing sequence—For slabs-on-ground, the place-
ment sequence should allow the expansive strains to occur
against a free and unrestrained edge. The opposite end of a
slab when cast against a rigid element should be free to
move. A formed edge should have the brace stakes or pins
loosened after the final set of the concrete to accommodate
the expansive action.
The placing sequence should be organized so that the edges
of slabs are free to move for the maximum time possible before
placing adjacent slabs. At least 70% of the maximum measured
laboratory expansion per ASTM C 878 should occur before
placing adjacent slabs when a slab is not free to expand on two
opposite ends. Examples of placement patterns are shown in
ACI 223. Checkerboarded placements should not be used
unless a compressible joint material is placed between the slab
before concrete placement as per Section 8.4.3.
Before establishing the placement sequence, results of
expansion testing per ASTM C 878 should be considered. A
minimum level of prism expansion of 0.04% is recom-
mended for slabs-on-ground. Higher expansion results
would accommodate larger slab placements or slabs that
have higher amounts of reinforcements. Trial batches for the
tested mixture proportion should use materials identical to
those that will be used during construction and tested at the
proposed slump that will be used in the field.
8.4.6 Concrete overlays—Overlays are used at times to
increase the thickness of a slab during initial construction or
as a remedial measure. Improved wear performance or a new
finished floor elevation may be the most frequent reasons for
using overlays. The two types of overlays—bonded and
nonbonded—are covered in ACI 302.1R as Class 6 and
Class 7 floors.
Bonded overlays are generally a minimum of 3/4 in. (19 mm)
thick, but thicknesses of 3 in. (76 mm) or more are not
uncommon. Typical bonded overlays are used to improve
Fig. 8.4—Calculated compressive stresses induced by
expansion (from ACI 223).

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