Page 326 - Planning and Design of Airports

284 Airp o r t D e sign

The FAA’s FAARFIELD pavement design program includes capa-
bilities for designing airfield pavement overlays. The four types of
overlays considered in FAARFIELD are
1. Hot mix asphalt overlay of existing flexible pavement
2. Concrete overlay of existing flexible pavement
3. Hot mix asphalt overlay of existing rigid pavement
4. Concrete overlay of existing rigid pavement
Based on the thickness and condition of the existing pavement
layers, FAARFIELD estimates the required thickness of the overlay.
Figure 7-13 provides an illustration of FAARFIELD’s output for a
flexible overlay on an existing flexible pavement.
The design of overlays over an existing rigid pavement is slightly
more complex as the condition of the existing rigid pavement plays a
significant role in the required thickness of the overlay. The condition of
the existing rigid pavement is estimated using a structural condition index
(SCI), a value which ranges from 0 to 100, in which 100 representing a
pavement with no visible structural deficiencies and 0 representing total
structural failure. Visible distresses that contribute to a lower SCI include
• Corner breaks
• Longitudinal, transverse, or diagonal cracking

AC_6E_Chapt4 Ex41 Des. Life = 20
Layer Thickness Modulus or R
material (in) (psi)

P-401/P-403 AC Overlay 7.78 200,000

P-401/P-403 AC Surface 4.00 200,000

P-209 CrAg 10.00 53,948

P-154 UnCrAg 6.00 22,766

Subgrade CBR = 10.0 15,000

N = 0; Subgrade CDF = 1.00; t = 27.78 in
FIGURE 7-13 Design example of fl exible overlay on existing fl exible
pavement.