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

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The remaining life ratio equals:
1 – n/N = 0.36
Assume that another 500000 standard axles should be carried by the pavement. This means
that:
∆n = 500000
The tensile strain at the bottom of the asphalt layer should be decreased to a level where N +
∆N load repetitions can be taken. This value is calculated from:
N + ∆N = ∆n / (1 – n/N) = 500000 / 0.36 = 1.39 . 10 6
By using the fatigue relation we calculate that this new number of allowable load repetitions
can be obtained if the strain is reduced to ε = 1.48 . 10 -4 [m / m]. This means that the overlay
needs such a thickness that the strain at the bottom of the existing asphalt layer is reduced to
this value.
The approach described here gives rise to some comments. It is quite clear that a very large
overlay thickness is needed when the ratio n/N approaches 1. The reason is that the fatigue
relation is based on beam fatigue tests. This implies that failure means that the specimen is in
two parts if the allowable number of load repetitions is reached (at least in load controlled
fatigue tests) which implies that the beam lost its functionality. In reality however the cracked
asphalt slab is still supported by the base and other layers; the cracked slab is still functional.
All this indicates that the procedure results in unrealistic designs in case of high values of the
damage ratio.
Furthermore the example indicates that in general fairly small strain reductions are needed
which results in rather thin overlays.
Because the overlay design is only based on the reduction of the strain level in the existing
pavement, only the thickness and the stiffness of the overlay are of importance. From practice
one knows that this cannot be true. The existing pavement normally exhibits a certain amount
of cracking when an overlay is applied and these cracks tend to propagate through the
overlay. This means that reduction of the strain level in the existing pavement cannot be the
only design criterion for overlays; also the resistance to crack reflection of the overlay should
be considered. This aspect will be discussed later in these lecture notes.
Finally the procedure described above doesn’t take into account the large amount of variation
in deflections and material characteristics that can occur in pavements.
9.2 Extension of the basic principles:
In this section an extension of the basic principles presented in the previous section will be
given. The extension is dealing with the fact that in case the n/N ratio reaches 1, realistic
values for the overlay thickness should still be obtained. Furthermore the extension takes into
account the variation in deflection level and material characteristics that occur in practice.
If there was no variation in deflection level along the section under consideration, and if there
was no variation in the thickness of the pavement layers, then there would be no variation in
the elastic modulus of the layers and there would be no variation in strain level. If there also
would be no variation in the fatigue characteristics, then the pavement would fail precisely at
the number of load repetitions predicted and the pavement would fail from one moment to the
other. This particular behaviour is illustrated in figure 28a. Such a performance however is
never observed, pavements don’t collapse in the way indicated by this figure. In reality a more
gradual deterioration is observed as is indicated by figure 28b.
If we use the mean strain level of figure 28b as design criterion and we use this strain value
together with the mean fatigue characteristic (the solid fatigue line in figure 28b) then we
determine the mean number of load repetitions. At that number of load repetitions there is a
50% chance that the pavement is failed. It can easily be shown that this means that 50% of
the trafficked pavement surface shows cracking. Because of the variation in the fatigue