iv) Conventional post-tensioned cast-in-place flat slabs. One-way
slabs range from 4-1/2 inches (113 mm) thick with No. 4 (12.7-mm-diameter)
rebar at 36 inches (900 mm) on-center and No. 5 (15.9-mm-diameter) rebar at
12 inches (300 mm) on-center up to 9 inches (225 mm) thick with No. 4
(12.7-mm-diameter) rebar at 24 inches (600 mm) on-center, and No. 6 (19 mm)
at 12 inches (300 mm) on-center.
Two-way slabs range from 7 inches (175 mm) thick with No. 4
(12.7-mm-diameter) rebar at 36 inches (900 mm) on-center up to 10-1/2 inches
(265 mm) thick with No. 4 (12.7-mm-diameter) rebar at 24 inches (600 mm)
on-center and No. 5 (15.9-mm-diameter) rebar at 12 inches (300 mm) on-center.
v) Slabs-on-grade. In general, it is not practical to attack
slabs-on-grade. Slabs-on-grade are used for floors only.
The thickness may
be as low as 4 inches (100 mm) to as high as 12 inches (300 mm). The
corresponding reinforcement may be as low as a single layer of No. 3 (9.5-
mm-diameter) rebar at 12 inches (300 mm) on-center each way to as high as No.
7 (22-mm-diameter) rebar at 6 inches (150 mm) on-center each way and on each
face, or perhaps wire mesh. Concrete with compressive strengths between
3,000 and 6,000 psi (21 and 42 MPa) are typically used.
b) Penetration times for existing construction. Table 42
sumarizes the maximum penetration times for existing concrete construction of
roofs and floors. Estimated penetration times for both upward and downward
attacks on representative major conventional construction types can be
estimated using Figure 33 and Table 22, and Figure 35 and Table 23. A review
of the data in Figures 33 and 34 shows that a wide range of penetration times
are possible, depending primarily upon the thickness and type of slab, size
and spacing of the reinforcement, and the direction of the attack (typical
ceiling, roof, and floor covering materials contribute very little to
penetration times). The lower bound is less than 2 minutes for very thin,
nominally reinforced slabs to more than 60 minutes for very thick slabs [12
inches (300 mm)] with heavy reinforcements. For a downward attack on roof or
floor slabs made of up to 12-inch (300-mm)-thick reinforced slab, Figure 33
and Table 22 can be used to estimate penetration times for various thickness
and rebar combinations. For an upward attack on floors of various thickness,
the conventional concrete family of curves, shown on Figure 35 and cross-
referenced to Table 23 can be used. For floors, an upward attack is more
difficult and requires a different combination of tools than a downward
attack on the same cross section. The result is increased penetration times
for the same cross section. The difference is typically 5 to 10 minutes.
For upward attacks on floor slabs less than 11 inches (275 mm) thick, the
primary factor influencing penetration time is the thickness of the slab.
Beyond 11 inches (275 mm), the type, size, and spacing of reinforcing also
becomes important. This is shown in Figure 35 for slabs up to 12 inches (300
mm) thick reinforced with rebar. For floors reinforced with mesh rather than
rebar (used Curve B in Figures 33 or 35), the mesh spacing or size of wire
mesh has a small effect on penetration times. In general, reinforced
concrete roofs and floors