1
Development of the AASHTO Performance
Equation for Rigid Pavement
In the AASHO Road Test, the performance of the
continuously reinforced concrete pavements was
found to be not significantly different from that of
the plain concrete pavements.
The structural design of the test sections could be
characterized by the thickness of the slab, D.
The relationship between pavement design (in terms of D)
and traffic and performance can be expressed by:
Log Wt= 5.85 + 7.35 Log (D + 1) - 4.62 Log (L1+ L2)
+ 3.28 Log L2+ Gt/$
W = axle load applications of end of time t
D = concrete slab thickness in inches
L1= load on one single or one tandem axle set, kips
L2= axle code (1 for single and 2 for tandem axle)
Gt= a function of the change in PSI from initial to final PSI.
$= a function of design and load variables.
For Rigid Pavements:
Gt= log [(c0–p
t)/(c0– 1.5)]
= log [(4.5 – pt)/(4.5 – 1.5)]
Where pt= terminal PSI
β= 1.0 + 3.63 (L1+ L2)5.2 / (D + 1 )8.46 L23.52
The above equation relates the pavement design (in
terms of D) to expected performance (in terms of the
allowable load applications (Wt) to cause a certain
reduction in serviceability of the pavement).
However, this equation applies to only one type of
traffic at a time.
The derivation of the Traffic Equivalency Factor for
rigid pavements is similar to that for flexible
pavement and is shown next.
For 18-kip single axle loads, L1= 18 & L2= 1
Log Wt18 = 5.85 + 7.35 Log (D + 1)
- 4.62 Log (18 + 1) + Gt/$18
For any axle load equal to x,
Log Wtx = 5.85 + 7.35 Log (D + 1)
- 4.62 Log (Lx+ L2) + 3.28 Log L2+G
t/$x
For single axle loads, L2= 1, the equation becomes:
Log Wtx = 5.85 + 7.35 Log (D + 1)
- 4.62 Log (Lx+ 1) + Gt/$x
For Single axle loads, ratio of Wtx to Wt18 becomes:
Log Wtx / Wt18 = 4.62 Log (18 + 1) - 4.62 Log (Lx+ 1)
+G
t/$x -G
t/$18
= C
Wtx / Wt18 = 10C
Wt18 =W
tx / 10C
Wtx applications of Lxsingle axle loads are equivalent to
(Wtx / 10C) applications of 18-kip single axle loads.
(1 / 10C) is the traffic equivalence factor for Lxsingle axle load.
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