Page 324 - Planning and Design of Airports

282 Airp o r t D e sign

The longitudinal embedded steel must also be capable of with-
standing the forces generated by the expansion and contraction of the
pavement due to temperature changes. Equation (7-8) determines the
amount of steel required as a function of temperature.

50 f
P = t (7-8)
s f − 195 T
s
where
P = embedded steel in percent
s
f = tensile strength of concrete, 67 percent of the flexural
t
strength is recommended
f = working stress for steel usually taken as 75 percent of
t
specified minimum yield strength
T = maximum seasonal temperature differential for pave-
ment in degrees Fahrenheit
Longitudinal embedded steel is located at mid-depth or slightly
above mid-depth of the slab.
Transverse embedded steel is recommended for CPRP airport
pavements to control random longitudinal cracking. Equation (7-9) is
used to determine the amount of transverse steel, as a percentage of
the total slab area.

WF
P (%) = s 100 (7-9)
s
f 2
s
where
P = embedded steel in percent
s
W = width of slab, in ft
s
F = friction factor of subgrade
t
2
f = allowable working stress in steel, in lb/in . Yield strength
s
of 0.75 is recommended
Transverse steel is designed in the same way as tie bars.
Design of Overlay Pavements
Overlay pavements are required when existing pavements are no
longer serviceable due to either deterioration in structural capabili-
ties of a loss in riding quality. They are also required when pavements
must be strengthened to carry greater loads or increased repetitions
of existing aircraft beyond those anticipated in the original design.
Overlays also provide a solution for increased safety. An example
would be to provide improved skid resistance and reduced risk of
hydroplaning.
There are several types of overlay pavements. A concrete pavement
can be overlaid with additional concrete, a bituminous surfacing, or a