Structural Design of Pavements PART VI Structural ... - TU Delft

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From the equations given above it becomes clear that the quality of the predictions increases
when S logN decreases. This means that S logSCI and S lof should be as low as possible. A low
S logSCI stresses the need to pay ample attention to the discrimination of homogeneous subsections.
The only factor that cannot be easily assessed is the variation in fatigue characteristics.
Although this value can be estimated (see e.g. lecture notes CT4850 part III
Asphaltic Materials) if mixture composition data are available, extensive fatigue testing has
shown that S lof = 0.25 is a reasonable first estimate.
Overlay calculations based on the confidence level or probability of survival level P are made
in the following way. As is shown above, the number of load repetitions until a certain
probability of survival level P 1 is reached can be calculated using:
log N P1 = A 0 + A 1 C 0 + A 1 C 1 log SCI 1 – u 1 S logN
If the pavement life has to be extended to N + ∆N load repetitions and after that number of
load repetitions, the probability of survival should be P 2 , the needed SCI level to achieve this
can be calculated using:
Log (N + ∆N) P2 = A 0 + A 1 C 0 + A 1 C 1 log SCI 2 – u 2 S log(N+∆N)
After subtracting of both equations one obtains:
Log {N P1 / (N + ∆N) P2 } = A 1 C 1 log {SCI 1 / SCI 2 } – u 1 S logN + u 2 S log(N+∆N)
By writing
N P1 / (N + ∆N) P2 = 1 / X
I 1 = 10**(u 1 S logN )
I 2
= 10**(u 2 S log(N+∆N)
We arrive to
Log {1 / X} = A 1 C 1 log {SCI 1 / SCI 2 } – log I 1 + log I 2
This can be written as:
SCI 2
= SCI 1 (X I 2 / I 1 ) 1/A1C1
In these equations SCI 1 can be considered as the SCI before the overlay is placed and SCI 2
as the SCI after overlaying. In the same way S logN is valid before overlaying and S log(N+∆N) is
valid after the overlay is placed.
We still need equations to predict the SCI 2 in relation to the overlay thickness and stiffness as
well as the SCI 1 . Furthermore an equation is needed to predict S logSCI2 because from this
value S log(N+∆N) can be calculated. These equations are given below:
Log SCI 2 = b 0 + b 1 E o + b 2 h o + b 3 log SCI 1 + b 4 E o log SCI 1 + b 5 h o log SCI 1
+ b 6 h o log E o log SCI 1
S 2 logSCI2
= {b 1 + b 4 log SCI 1 + b 6 h o log SCI 1 / E o } 2 S 2 Eo
+ {b 2 + b 5 log SCI 1 + b 6 log E o log SCI 1 } 2 S 2 ho
+ {b 3 + b 4 E o + b 5 h o + b 6 h o log E o } 2 S 2 logSCI1
Where: SCI 1
SCI 2
h o
E o
= surface curvature index (d 0 – d 300 ) before overlaying [µm]
= surface curvature index (d 0 – d 300 ) after overlaying [µm]
= overlay thickness [mm]
= elastic modulus of the overlay [Mpa]
b o = -0.0506
b 1 = 1.178 10 -5
b 2 = 0.0094