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

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Normally BB measurements are only taken in the wheel tracks. These values are in fact the
Def n values since that pavement area has been subjected to n load repetitions. If we also take
deflection measurements between the wheel tracks, then we get a good estimate of the
flexural stiffness of that part of the pavement that is not subjected to traffic loads. These
deflections can be taken as representative for Def 0 .
Assume that the deflection measured between the wheel tracks is 350 µm and that the
deflection in the wheel tracks is 390 µm. The pavement has an unbound base. Then we arrive
to:
DeltaDef n = 390 – 350 = 40
and
DeltaDef c / Def 0 = 0.4767 – 0.000299 x 350 = 0.372
so
DeltaDef c = 0.372 x 350 = 130
We also calculate:
b = 0.91
so
DeltaDef n / DeltaDef c = (n / N) b
40 / 130 = (n / N) 0.91
n / N = 0.27
Normally road authorities are not interested in a damage ratio or a remaining pavement life
expressed in a number of allowable load repetitions but much more in a remaining life in
years. This can be estimated in the following way.
Assume the traffic composition has not changed in time and for reasons of simplicity we also
assume that no growth in the number. of vehicles per day has taken place. This means that
the area indicated in figure 26 is representative for the cumulative amount of traffic n that has
passed the road during time period t.
Traffic intensity
n
N
t
T
Time
Figure 26: Procedure to estimate the remaining life in years from the n/N ratio.
In the same way the allowable number of load repetitions N is arrived after T years. From this
simple example it is clear that in this case:
t / T = n / N