Table 4.2—Factors of safety used in design of

DESIGN OF SLABS-ON-GROUND
360R-21
the static tensile strength that can be supported for a given
number of load repetitions. As the ratio of the actual flexural
stress to the modulus of rupture decreases, the slab can with-
stand more load repetitions before failure. For stress ratios
less than 0.45, concrete can be subjected to unlimited load
repetitions according to PCA (2001). Table 4.3 shows
various load repetitions for a range of stress ratios. The
safety factor is the inverse of the stress ratio.
CHAPTER 5—JOINTS
5.1—Introduction
Joints are used in slab-on-ground construction to limit the
frequency and width of random cracks caused by volume
changes. Generally, if limiting the number of joints or
increasing the joint spacing can be accomplished without
increasing the number of random cracks, floor maintenance
will be reduced. The designer should provide the layout of
joints and joint details. If the joint layout is not provided, the
contractor should submit a detailed joint layout and placing
sequence for approval by the designer before proceeding
with construction.
Every effort should be made to avoid tying the slab to any
other element of the structure. Restraint from any source,
whether internal or external, will increase the potential for
random cracking.
Three types of joints are commonly used in concrete slabs-
on-ground: isolation joints, sawcut contraction joints, and
construction joints. Appropriate locations for isolation joints
and sawcut contraction joints are shown in Fig. 5.1. With the
designer’s approval, construction joint and sawcut contrac-
tion joint details can be interchanged. Joints in topping slabs
should be located directly over joints in the base slab and, if
the topping is bonded, no additional joints are required. The
bonded topping slab should be designed for the shrinkage
restraint due to the bond to the existing slab, and the bond
should be sufficient to resist the upward tension force due to
curling. For a thin, unreinforced, unbonded topping slab,
additional joints should be considered between the existing
joints in the bottom slab to help minimize the curling stress
in the topping slab. The topping slab can have high curling
stress due to the bottom slab being a hard base for the
topping slab. Also, any cracks in the base slab that are not
stable should be repaired to ensure they will not reflect
through into an unreinforced topping slab.
5.1.1 Isolation joints—Isolation joints should be used
wherever complete freedom of vertical and horizontal
movement is required between the floor and adjoining
building elements. Isolation joints should be used at junc-
tions with walls (not requiring lateral restraint from the slab),
columns, equipment foundations, footings, or other points of
restraint such as drains, manholes, sumps, and stairways.
Isolation joints are formed by inserting preformed joint
filler between the floor and the adjacent element. The joint

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