PerRoad: Output & Design
After the structural and traffic
inputs have been entered into PerRoad, proceed to the Output and Design Module
window shown below.
The top left portion of the window (Thickness Design) allows the designer to change layer thicknesses, as needed, based on interpretation of the results in the lower two output windows. Note that when this window is first opened, the two lower output windows will remain empty until the “Perform Analysis” button has been pushed.
The top right portion of
the window (Reliability Analysis) allows the designer to change the number of
Monte Carlo simulations and execute the analysis.
The number of simulations has been set to 5,000.
More may be needed to achieve a stable solution, but 5,000 is a good
starting point. Whenever the
designer leaves this output window and comes back, the number of Monte Carlo
cycles is reset to 5,000. The
“Perform Analysis” button will start the Monte Carlo simulation and tabulate the
results in the lower two output windows as shown in the figure below.
The upper output is labeled “Conventional Perpetual Pavement Design
Results” while the lower output is labeled “Strain Distribution Perpetual
Pavement Design Results.” Each is
described in further detail below.
Evaluating Design Criteria
(Horizontal Strain Distribution or Single Threshold)
If you have selected a horizontal strain distribution, or a single threshold value with a target control percentile, you will see data in the bottom output window as illustrated below.
The outputs in the above window are defined as follows:
Layer: Layer number
where the threshold was set.
Location: Where in the
layer (top, middle or bottom) that the threshold was set.
Units: The units
corresponding to the threshold.
Target Value:
Designer-specified target pavement response value. Negative is tension,
positive is compression.
Target Percentile:
Fixed cumulative percentage value corresponding to target response value.
Actual Percentile:
Predicted cumulative percentage value corresponding target response value.
Pass/Fail?: If the actual percentile exceeds the target percentile, this criteria passes. If not, it fails. If any percentile fails, the thickness(es) should be adjusted and the design re-executed.
For all criteria
except Horizontal Strain Distribution, you may have entered a target
percentile value. If the actual percentile from the simulation exceeds the
target percentile, the pavement passes on that criterion. If you
selected a Horizontal Strain Distribution, then there are more control points to
be considered as explained below.
Recent research at the National Center for Asphalt Technology has supported
the use of strain distributions for controlling bottom-up fatigue cracking.
The main idea of this design approach is to control the range of strain values
experienced by the pavement below a pre-defined range. The range of values
are quantified by their magnitudes and corresponding percentiles. In the
program, the design will be controlled by the 95th, 85th, 75th, 65th and 55th
percentiles, respectively.
The figure below shows the results
of a sample design where strain distributions were used. The x-axis
represents the tensile microstrain. The values are negative to indicate
tension. The y-axis represents the cumulative percentile. The red
line labeled “Target” indicates the control strain
distribution. Points above this line indicate a greater chance that the
strain levels are below the target values. Points below this line indicate
a poorer chance that the strain levels are below the target values.
Therefore, the goal of the design is to select thicknesses such that the
resulting strain distribution percentiles fall entirely above the target line.
If this is the case, then fatigue cracking should not occur because the pavement
is experiencing lower strain levels than those expected to crack the pavement.
In the example
above, Trial 1 resulted in a failing distribution. At
the higher percentile and greater tensile strain levels, the results were okay
since the percentiles were above the target line.
However, at about the 78th percentile (about -160 microstrain), the percentiles
fell below the target line indicating excessive strain levels with the potential
to develop fatigue cracking. Trial 2 was then executed with
additional asphalt concrete thickness and the resulting percentiles, entirely
above the target line, correspond to an acceptable design since the strain
levels are below the point where cracking is expected.
Additional trials could be executed to bring the predicted strain distribution
closer, without falling below the target.
Evaluating Design Criteria
(Conventional Design with Transfer Functions)
If you have selected transfer functions to enable an M-E design, then you will see data in the upper output window as illustrated below.
The outputs in the above window are defined as follows:
Layer:
Layer number where the threshold was set.
Location:
Where in the layer (top, middle or bottom) that the threshold was set.
Units:
The units corresponding to the threshold.
Percent Below
Critical: The probability that a
pavement response will not exceed the threshold.
Damage/Million Axle:
If a transfer function has been defined, this parameter indicates the damage
accumulation rate as calculated by Miner’s hypothesis. The units are
damage per million axles.
Years to D = 0.1:
If a transfer function has been defined, this value estimates the amount of
time, given the current traffic volume, growth and damage accumulation rate,
before the damage number (as calculated by Miner’s Hypothesis) will reach 0.1.
It must be noted that this is a very conservative damage value. In
traditional M-E design, damage is usually set to 1.0 for failure.
Years to D = 1.0:
If a transfer function has been defined, this value estimates the amount of
time, given the current traffic volume, growth and damage accumulation rate,
before the damage number (as calculated by Miner’s Hypothesis) will reach 1.0.
This value is consistent with traditional M-E methods that designed for a
terminal level of distress. Comparing the years between time to 0.1 and
time to 1.0 gives and indication of the difference between conventional M-E and
perpetual design.
