Page 323 - Planning and Design of Airports

Structural Design of Airport Pavements 281

Thickness of Slab Diameter Length Spacing
∗
6–7 in (150–180 mm) ¾ in (20 mm) 18 in (460 mm) 12 in (305 mm)
∗
8–12 in (210–305 mm) 1 in (25 mm) 19 in (480 mm) 12 in (305 mm)
∗
13–16 in (330–405 mm) 1¼ in (30 mm) 20 in (510 mm) 15 in (380 mm)
∗
17–20 in (430–510 mm) 1½ in (40 mm) 20 in (510 mm) 18 in (460 m)
∗
21–24 in (535–610 mm) 2 in (50 mm) 24 in (610 mm) 18 in (460 mm)
∗ Dowels noted may be solid bar or high-strength pipe. High-strength pipe must be
plugged on each end with a tight-fitting plastic cap or mortar mix.
TABLE 7-7 Dimensions and Spacing of Steel Dowels in Rigid Pavement

The amount of reinforcing steel required to control volume changes
is dependent primarily on the slab thickness, concrete tensile strength,
and yield strength of the steel. While several procedures have been
proposed for estimating the required amount of steel, experience indi-
cates that it should be approximately 0.6 percent of the gross cross-
sectional area and that the yield strength should be at least 60,000 lb/
2
in . The minimum amount may be determined by Eq. (7-6).
f t
.
13
P (%) = ( . − 02 F) (7-6)
s
f
s
where P = percentage of embedded steel
s
f = tensile strength of concrete, lb/in 2
t
f = allowable working stress in steel, lb/in 2
s
F = coefficient of subgrade friction
The FAA recommends that the cross-sectional area of the reinforc-
ing steel A be obtained by Eq. (7-7).
s
(. ) LLt
37
A = (7-7)
s
f
s
where
A = area of steel per foot of width or length, in 2
s
L= length or width of slab, ft

T = thickness of slab, in
f = allowable tensile stress in steel, lb/in 2
s
Note: To determine the area of steel in metric units:
Lshould be expressed in meters

t should be expressed in millimeters
f should be expressed in meganewtons per square meter
s
The constant 3.7 should be changed to 0.64.
f will then be in terms of square centimeters per meter.
s