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Perpetual Pavements Concept and History Iowa Open House

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Presentation on theme: "Perpetual Pavements Concept and History Iowa Open House"— Presentation transcript:

1 Perpetual Pavements Concept and History Iowa Open House
October 5, 2005 In this presentation, the idea of perpetual asphalt pavements will be introduced. These pavements are designed and constructed from the bottom up to provide a structure having more than 50 years of useable life with a renewable asphalt surface. A durable, strong asphalt pavement will ensure minimal disruption of traffic and reduced maintenance costs over its life.

2 40-75 mm SMA, OGFC or Superpave
} 40-75 mm SMA, OGFC or Superpave 100 mm to 150 mm Zone Of High Compression High Modulus Rut Resistant Material (Varies As Needed) Flexible Fatigue Resistant Material mm Max Tensile Strain This is the concept underlying the perpetual pavement. Traffic stresses are highest near the surface of the pavement, so materials in the upper pavement layers must be resistant to rutting. The intermediate or binder courses should be comprised of rut-resistant material. Materials which have worked well include large-stone mixtures or others which provide a strong stone skeleton. Fatigue resistance is important in the lowest HMA layer or base layer. It is well documented that the most costly form of distress to fix in a pavement is bottom-up fatigue cracking. The pavement foundation serves as the ultimate support for the structure during construction and service, so it must be considered an integral part of the pavement. Pavement Foundation

3 Value Quality is what the customer gets out [of a product] and is willing to pay for. A product is not quality because it is hard to make and costs a lot of money This is incompetence. Customers pay only for what is of use to them and gives them value. Nothing else constitutes quality. Peter Drucker

4 Economics Total Costs Alternative Perpetual Pavement Time

5 Why are Perpetual Pavements Important?
Lower Life Cycle Cost Better Use of Resources Low Incremental Costs for Surface Renewal Lower User Delay Cost Shorter Work Zone Periods Off-Peak Period Construction

6 Design

7 Mechanistic Performance Criteria
Under ESAL Thick HMA (> 200 mm) Limit Bending to < 65me (Monismith, Von Quintus, Nunn, Thompson) The thickness of the HMA pavement will be determined according to the response of the pavement structure to load. (mouse click) A standard single axle load of 18,000 lbs will be used in modeling these responses. According to a number of researchers, the amount of bending strain in the bottom layer of HMA should be less than 65 microstrain and the amount of compressive strain at the top of the subgrade should be limited to 200 microstrain. A design approach is needed whereby a traffic analysis shows the probability that these strain levels will be exceeded in a given situation. In this way, the concept could be applied at all traffic levels. Base (as required) Subgrade Limit Vertical Compression to < 200me (Monismith, Nunn)

8 Fatigue Resistant Asphalt Base
Minimize Tensile Strain with Pavement Thickness Thicker Asphalt Pavement = Lower Strain Strain Below Fatigue Limit = Indefinite Life Compressive Strain Indefinite Fatigue Life Strain A thick asphalt layer will dramatically reduce the tensile strain at the bottom of the HMA. The thickness should be such that the strain is lower than the fatigue limit, and the fatigue limit is the strain below which fatigue damage in the pavement does not accumulate. Using a sufficiently thick asphalt layer will effectively prevent bottom-up cracking in the pavement structure. Fatigue Life Tensile Strain

9 PerRoad

Thickness of bituminous layer (mm) Rate of rutting (mm/msa) 100 200 300 400 0.1 1 10 1000 At TRL in England, Nunn and his colleagues found that high rates of rutting were associated with thin asphalt pavement sections. They noted that those over about 200 mm or 8 inches tended to show a relatively slow rate of rutting. TRL

11 Thickness of asphalt layers
500 TRL Design Chart DBM DBM50 HDM 400 300 Thickness of asphalt layers (mm) 200 The result of the TRL work was a design chart to determine the thickness of the asphalt structure above the subbase layer. This varies according to mix type as shown here. A thinner structure is allowed for a heavy-duty macadam mixture, designated as HDM, than for a conventional dense-graded mixture with a stiff asphalt (DBM50) or a conventional dense-graded mixture with a normal asphalt (DBM). In any case, the minimum thickness of HMA is about 8 inches (200 mm) and the thickest HMA structure is about 16 inches (400 mm). 100 TRL 10 100 Design life (msa)

12 Perpetual Pavement versus Conventional Design

13 Construction

14 Foundation - Illinois 5 10 15 20 25 1 2 3 4 6 7 8 9 CBR
5 10 15 20 25 1 2 3 4 6 7 8 9 CBR Required Thickness Above Subgrade, inches No remedial procedure required Remedial procedure required Remedial This is Illinois’ requirement for foundation stiffness. If the CBR of the subgrade is below 6, then an improvement must be made by means of lime modification or a thickness of granular material. procedure optional

15 Mass Rod Reference

16 Leads to Poor Compaction!
Compaction Support Weak Support Leads to Poor Compaction! Weak!!!

17 Compaction Support Strong Support Helps Compaction! Strong!!!

18 30OF Surface, 40OF Air Temperature, 15 mph wind

19 Performance

20 Time from Original Construction to First Resurfacing
Performance of Washington Interstate Flexible Pavements (based on 284 km) 12.4 230 (9.2) 31.6 Average 2 to 25 100 to 345 23 to 39 Range Time from Original Construction to First Resurfacing (years) Thickness of Original AC (mm (in.)) Time Since Original Construction (years) Statistic The ability to obtain long lasting pavement structures from HMA is shown by the performance of asphalt interstate highways in the state of Washington. These pavements range in age from 23 to 39 years, with an average of about 32 years since their original construction. They have a little more than 9 inches of HMA thickness on average, although the thickest is about 14 inches. Another interesting point is that the first resurfacing of these pre-Superpave HMA pavements occurs at about 12.5 years. With recent changes in technology, this resurfacing time should increase dramatically.

21 Ohio Study of Flexible Pavements
Examined Performance on 4 Interstate Routes HMA Pavements - Up to 34 Years without Rehabilitation or Reconstruction “No significant quantity of work for structural repair or to maintain drainage of the flexible pavements.” Only small incremental increases in Present Cost for HMA pavements. A study funded by Flexible Pavements, Inc. of Ohio surveyed the performance of four interstate pavements. This work proves the point that Perpetual Pavements have been in use for some time. It was found that the HMA pavements provided up to 34 years of service without the need for expensive rehabilitation or reconstruction. The report from the study states that, “No significant quantity of work was done for structural repair or to maintain drainage of the flexible pavements.” An examination of life-cycle costs for these roads showed that HMA pavements produced only small incremental increases in present worth as overlays were added. This underscores the principles of the Perpetual Pavement: Design: build it for the traffic, soil and climate, and the only cost thereafter should be associated with periodic overlays.

22 FHWA - Data from Long-Term Pavement Performance Study
Data from GPS-6 (FHWA-RD ) Conclusions Most AC Overlays > 15 years before Rehab Many AC Overlays > 20 years before Significant Distress Thicker overlays mean less: Fatigue Cracking Transverse Cracking Longitudinal Cracking An FHWA report documenting the performance of rehabilitated asphalt pavements from the Long-Term Pavement Performance study found that most asphalt overlays last 15 years or more before further rehabilitation is needed and that many HMA overlays last beyond 20 years. Further, it was concluded that thicker HMA overlays had less cracking type distresses than thinner overlays. Again, the point is made that if the pavement structure remains in tact, then the surface can be periodically rehabilitated. While the pavements in the LTPP GPS-6 study performed well, it should be remembered that these represent pre-Superpave mixtures.

23 Structure Remains Intact
Rehabilitation Possible Distresses Top-Down Fatigue Thermal Cracking Raveling Solutions Mill & Fill Thin Overlay { High Quality SMA, OGFC or Superpave mm 20+ Years Later Structure Remains Intact What makes the Perpetual Pavement perpetual is that, while the surface will need periodic replacement, the bulk of the pavement structure will remain intact. Depending on the type of surface material used and the environmental conditions present, the types of distress which may arise could include top-down fatigue cracking, thermal cracking and raveling.(Click mouse to activate animated cracks.) Although surface layer lives may vary, current technology allows for achieving over 20 years, if so desired. Solutions for these distresses could include milling off the old layer and replacing it with a new surface (Click to show overlay) or simply overlaying the pavement. One advantage to this approach is that new innovations in technology can be applied to the pavement surface as they become available, possibly further extending the surface life.

24 Should break costs into
Important to consider Initial Costs Rehabilitation and Maintenance Costs Reconstruction costs Should break costs into Agency costs User Costs Life Cycle Costs Time

25 Design Comparison – Low Volume
50 year design 1 mile, 2 Lane, 12 ft lanes Traffic: ADT (Rural Setting) 6” HMA Perpetual ($360,000) Conventional ($230,000) 12” Granular Base 3” HMA 6” Granular Base Subgrade Muench, et al., 2004

26 Rehabilitation Schedule
Conventional Design (3” HMA/6” GB) 1.75” Overlay in years 6, 15, 31 and 40 Rebuild at 25 years Perpetual Design (6” HMA/12” GB) 1.75” overlay every 13 years Values based on WSDOT Experience Muench, et al., 2004

27 Considered variability of inputs User delay not significant
Cost Analysis Used LCCA Software Considered variability of inputs User delay not significant Muench, et al., 2004

28 What About User Delay Costs?

29 WZ User Cost Comparison
4-lane 40,000 AADT LCCA Software Delay calculations based on Highway Capacity Manual Compare work-zone alternatives 24 hour closure 20 hour closure 11 hour closure

30 85% of cost is waiting to move through WZ
Option 1 – 24 Hr Closure Cost/mile/day $116,000 85% of cost is waiting to move through WZ

31 Option 2 – 20 Hr Closure Cost/mile/day $42,000

32 Option 3 – 11 Hr Closure No stopping! Cost/mile/day $12,000

33 Perpetual Pavement Structure Lasts 50+ years.
Bottom-Up Design and Construction Indefinite Fatigue Life Renewable Pavement Surface. High Rutting Resistance Tailored for Specific Application Consistent, Smooth and Safe Driving Surface. Environmentally Friendly Avoids Costly Reconstruction. The Perpetual Pavement requires that the structure be designed and constructed from the bottom-up and that it have a combination of thickness and HMA characteristics which preclude fatigue cracking and durability problems. The renewable pavement surface must be designed to resist rutting and tailored for specific applications. In the end, the Perpetual Pavement will provide a uniform, smooth and safe driving surface, and it will avoid the huge expense associated with reconstruction.

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