1. Oklahoma DOT Perpetual Pavement Test Sections at the NCAT Test Track

2. Compression
Tension

3. Perpetual Pavement
Structure and materials are designed to provide a long service life (50 years +)

4. Early Texas Perpetual Pavements
Waco
Cotulla
McAllen
1.5” PFC
2.0” SMA
3.0” 19 mm SP
10.0” 25 mm SP
4.0” 19 mm
2% air voids
6.0”
crushed stone
3.0” SMA
3.0” 19 mm SP
8.0” 25 mm SP
2.0” 12.5 mm
2% air voids
1.5” PFC
2.0” SMA
10.0”, 19 mm or
25 mm SP
3.0” 12.5 mm
2% air voids
8.0”
lime-treated
salvaged aggregate

5. Waco Cotulla McAllen 16” Total HMA 20.5” Total HMA 16.5” Total HMA
1.5” PFC
3.0” SMA
1.5” PFC
2.0” SMA
2.0” SMA
3.0” 19 mm SP
3.0” 19 mm SP
10.0”, 19 mm or
25 mm SP
8.0” 25 mm SP
10.0” 25 mm SP
3.0” 12.5 mm
2% air voids
2.0” 12.5 mm
2% air voids
4.0” 19 mm
2% air voids
8.0”
lime-treated
salvaged aggregate
16” Total HMA
6.0”
crushed stone
20.5” Total HMA
16.5” Total HMA

6. How thick is too thick? Cost of 5 Miles of Pavement
Assume 80’ width, $50 per ton
Save 1” in over-design: $650,000
Save 2” in over-design: $1,300,000
Save 4” in over-design: $2,600,000

7. How thin is too thin?
If the perpetual pavement structure is designed too thin, the risk of bottom-up cracking increases dramatically, defeating the purpose of the design and resulting in an expensive rebuild.

8. Rut Resistant Material
To design the appropriate pavement thickness, stresses and strains
To design against potential bottom-up cracking, certain strain thresholds cannot be exceeded. }
1.5-3 in. PFC, SMA, etc. 3” to 6”
High Shear
Zone
High Modulus
Rut Resistant Material
(Varies As Needed)
Max Flexural Strain
Pavement Foundation

9. Limiting Strain Theory
Based on laboratory beam fatigue data
“Small” strains will never induce cracking
Limiting strain at 75 to 125 microstrains
No bottom-up cracking below this level

10. These strains can be calculated based on total pavement thickness and material properties.
However, the calculations would be based on assumptions made from previously collected data.

11. There are large deviations in the types of environment around the country, the types of materials used and the types of pavement specified. These deviations decrease the reliability of the assumptions made to calculate strains.
Estimated Strain
If the strains could be measured directly, using Oklahoma materials and mix designs, the data would be much more reliable.

12. National Center for Asphalt Technology (NCAT) Test Track

13. ODOT’S PERPETUAL PAVEMENT STRUCTURAL SECTIONS AT NCAT TEST TRACK
PLAN VIEW
SECTION N8 – 150’
SECTION N9 – 150’
25’ TRANSITION
25’ TRANSITION
50’ TRANSITION

14. ODOT’S PERPETUAL PAVEMENT STRUCTURAL SECTIONS AT NCAT TEST TRACK
PLAN VIEW
SECTION N8 – 150’
SECTION N9 – 150’
25’ TRANSITION
25’ TRANSITION
50’ TRANSITION
PROFILE VIEW
2” SMA w/PG 76-28
3” SuperPave 19.0mm w/PG 76-28
3” SuperPave 19.0mm w/PG 64-22
2” RBL w/PG 64-22
3” SuperPave 19.0mm w/PG 64-22
3” RBL w/PG 64-22
*RBL = RICH BOTTOM LAYER

15. Structural Study Test Sections
N8 and N9 are part of an eleven-section structural experiment starting in FHWA has partnered with OK on section N9 to have additional instrumentation installed in section N9 to more precisely measure strain and temperature at various depths. There is an error on this graphic - please note that the Missouri Type 5 base is only 4 inches thick.

16. Rut Resistant Material
The maximum flexural strain is directly measured using a series of strain gauges installed at the bottom of the asphalt section.
1.5-3 in. PFC, SMA, etc.
High Modulus
Rut Resistant Material
(Varies As Needed)
Max Flexural Strain
Pavement Foundation

17. If we have determined the strain for a 10” and 14” thick pavement, the thickness at the critical strain level can be interpolated / extrapolated ?
MICROSTRAIN
10”
14”
16”

18. Instrumentation – N8
12 Asphalt Strain gauges (6 longitudinal, 6 transverse)
2 pressure plates (1 at top of base, 1 at top of subgrade)
Laser to measure wheel wander
Temperature array to measure temps at top, middle and bottom of HMA

19. Accelerated Loading Compress 10-15 years into 2 years
Heavy triples optimize safety and efficiency
Each axle loaded to federal legal bridge limit
Heavier loads induce unrealistic damage
No bridge spans to overload on NCAT track
Average gross vehicle weight 155k pounds

20. Long Term Response Measurements
Weekly - three passes of 5 trucks
Steer axle (12 kip), tandem (40 kip), 5 single axles (20 kip/axle)
45 MPH
Measure strain
Measure pressure
Measure temps
Characterize long-term trends in strain response
Graph shows transverse strain in section N9 for one truck pass. The steer, tandem and 5 single axles are clear to see.

21. Graph shows transverse strain in section N9 for one truck pass
Graph shows transverse strain in section N9 for one truck pass. The steer, tandem and 5 single axles are clear to see.

22. To design the appropriate pavement thickness, stresses and strains
The vertical compressive strain is directly measured using pressure plates installed at the top of the subgrade.
1.5-3 in. PFC, SMA, etc.
High Modulus
Rut Resistant Material
(Varies As Needed)
Max Flexural Strain
Vertical Compressive Strain

23. Moisture sensors were also placed in the subgrade to continuously monitor soil conditions

24. Temperature probes were placed in the pavement to continuously monitor temperature at various depths

25. A datalogger is used to collect information from the different pavement sensors.
In addition to the normal “slow speed” mode that continuously gathers data, a “high speed” mode can be used to collect 40,000 data points per second.

26. Additional Testing - Surface Map Cracking

27. FWD Testing Rut Testing Skid Testing Inertial Profiler –. Rutting
FWD Testing Rut Testing Skid Testing Inertial Profiler – Rutting Smoothness Surface Texture

28. N8 and N9 – Strain vs. Date
This plot shows the 95th percentile measured longitudinal strain from single axles in sections N8 and N9. In a given day’s worth of data collection, we have 15 truck passes * 5 single axles per truck = 75 measurements. The data on this plot represent the 95th percentile of the 75 measurements. As expected, N9 is significantly lower than N8. Ratios were computed by dividing N8 strain by N9 strain. The ratio varies somewhat, but on average, the strain in section N8 is about 2.6 times greater than N9.
The seasonal trend is also evident in both sections where the strains rise significantly in the warmer summer months.

29. Effect of Depth (N9)
This plot shows the effect of depth on strain within section N9. These are measured longitudinal strains under single axles.
As expected, the bottom of the HMA sees the greatest strain since it’s at the extreme point going through the most bending. As you move up in the structure, the strains are reduced since they are approaching the neutral axis where the strain is zero.
An important consideration here is that each strain plot represents different mixtures. Theoretically, they may have different strain thresholds. So, even though the strains are reduced, they may not be as fatigue resistant compared to the rich bottom.
Note: There are gauges in Lift 4, however after reviewing the data, they were not included. They were very erratic which was not unexpected since they are so shallow.

30. Temperature Normalized Response
Section N9 is the only section in the structural experiment that could be classified as perpetual based on these average temperature normalized responses; however, the analysis is ongoing.

31. Normal Operations 55% of Strains below threshold
The measured mid-depth HMA temperature was used with the speed and temperature equation to determine strains between the start of the experiment (11/10/06 and 1/14/2008). As shown in the graph, and shown previously for both N8 and N9, the seasonal trend is clearly evident. Also, strains in N9 can be quite high. For example, the highest measured temperature was on August 9, 2007 (118F) between 6 and 7 PM. During this hour, strains were on the order of 580 microstrain.
Again referring to a fatigue threshold of 100 microstrain, the graph shows that approximately 55% of the strains were below the threshold with 45% exceeding the threshold. Until more performance data are recorded, it is difficult to ascertain whether 100 microstrain is appropriate to use as the fatigue threshold. What can be said, however, is that a significant number of strain readings exceed 100 microstrain.
It is important to note the difference in how this graph was generated compared to the N8, N9 graph shown previosly. The previous graph was generated from measured data only. This graph was generated by developing a strain vs. temperature equation from which we could reasonably approximate strains during any hour of testing.

32. Supplemental FHWA Experiment
N9 Instrumentation (Multi-Depth Array)
12 strain gauges at bottom of HMA
2 pressure plates (top of base, top of subgrade)
Additional three groups of four gauges/group measuring strain on top of successive paving lifts
Laser to measure wheel wander

33. N9 Cross Section and Supplemental Instrumentation
Additional instrumentation funded by FHWA as part of strain vs. speed and temperature study
Instrumentation same as N8, plus gauges installed on successive lifts going up into the structure.
More detailed temperature readings at top, middle and bottom of each paving lift

34. Strain vs. Speed and Temperature (ODOT and FHWA)
Primary Objectives
Study a perpetual pavement under a wider set of conditions
Primarily look at speed and temperature effects

35. Data Collected 4 Speeds Tested 4 Dates Measured
15-45 mph (10 mph increments)
4 Dates
April – May 2007
Measured
Temperature
Tensile Strain

36. Investigations Studied relationships Strain Threshold Load Durations
Speed-Strain
Temperature-strain
Combined
Strain Threshold
Load Durations
Relate to Frequency

37. Relationships Speed-Strain Temperature-Strain Strain at bottom of HMA
Decrease with speed
Temperature-Strain
Strain at bottom of HMA
Increase with temperature
Trends were examined that looked first at speed effects and secondly the temperature effects in N9. The first equation shows that strain is related to the natural log of speed and tends to decrease as speed is increased. The second equation shows that strain tends to increase with temperature. A third equation was developed that incorporates both variables. The R2 on the data set was very high ( )
R2:

38. Strain Threshold Tensile strain threshold: 100με
These design curves show the effect of temperature on strain at a variety of reference speeds. For example, if the design speed were 45 mph, the critical temperature would be approximately 72F. Above this temperature, strains would exceed the commonly held fatigue threshold of 100 microstrain. At 65 mph, the critical temperature would be approximately 80F. Curves such as these can help to extrapolate the findings from the test track to those encountered on open access facilities (i.e., in Oklahoma).

39. THANK YOU!
QUESTIONS?