1 September 1999
d. Slab-On-Grade Reinforcement.
Slab-on-grade shrinkage reinforcement will be located 40 mm (1-1/2 inches) from the top of
the slab in order to restrain shrinkage and reduce curling. Maximum spacing of reinforcing
bars should not exceed three times the slab thickness. For plain wire reinforcement the
spacing should not be more than 350 mm (14 inches) longitudinally and 350 mm (14 inches)
transversely. The percentage of steel determined should not be less than 0.15 percent. Wire
mesh reinforcements meeting the 0.15 percent steel requirement, for various slab thickness, is
provided in Table 5-1. Deformed welded wire fabric in flat sheets, or deformed reinforcing bars
will be used. The positioning of the steel in the slab is critical for proper crack control.
Reinforcing steel will be supported on chairs and every precaution taken to assure the
reinforcing bars are positioned, as intended after construction is complete.
Table 5-1. Minimum Slab-On-Grade Reinforcement Requirements.
Wire Mesh Reinforcement
305 x 305 - MW 48.4 x MW 48.4 (12 x 12 - W 7.4 x W 7.5)
100 mm (4-inch)
305 x 305 - MW58.1 x MW 58.1
(12 x 12 - W9 x W 9)
125 mm (5-inch)
305 x 305 - MW 71.0 x MW 71.0 (12 x 12 - W11 x W 11)
150 mm (6-inch)
Suppliers because of the large bar sizes and wide spacing may not normally stock the wire
mesh reinforcement sizes indicated in Table 5-1. The large bar sizes are desirable to prevent
bending of the steel and provide adequate stiffness to keep the steel in the upper half of the
slab during concrete placement. For smaller jobs [less than 7000 square meters (75,000
square feet)] in those cases when the wire mesh sizes listed in Table 5-1 are unavailable, wire
mesh spacing with reduced bar diameters and closer spacing may be used provided minimum
steel requirements are met.
e. Vapor Barriers. High levels of moisture in the subgrade increase slab curling. If the
subgrade can become moist because of ground water an impermeable vapor barrier should be
provided. The minimum thickness of the vapor barrier should be 1.3 mm (50 mil) and be
covered with a 150 mm (6 inches) of crushed stone topped with a 13 mm (1/2 inch) thick layer
of sand. The advantage of the 165mm (6-1/2 inch) stone / sand cover over the vapor barrier is
that the vapor barrier will not be punctures nor will the fill material be easily displaced as
construction equipment is driven over the stone and sand cover (Ytterberg, 1987). If the only
purpose of the vapor barrier is to reduce friction between the slab and subgrade in order to
reduce drag, then a polyethylene slip sheet can be placed directly under the slab provided
holes are punched in the polyethylene to allow water to leave the bottom of the slab before
final set occurs in the concrete.