Pavement Preservation and the Role of Bituminous Surface Treatments A Washington State View. Minnesota Pavement Conference February 14, 2008

Transcription

1 Pavement Preservation and the Role of Bituminous Surface Treatments A Washington State View Minnesota Pavement Conference February 14,

2 The Situation 2 WSDOT policy, in essence, mandated use of BSTs for routes with an ADT < 2,000. Pavement preservation funding level (real dollars) is down. Pavement preservation funding is a WSDOT account used to pay for contract HMA and BST projects. Questions What is a reasonable ADT upper limit for application of BSTs? What kinds of pavement performance can we expect if more BSTs are done and fewer HMA overlays?

3 WSDOT Pavements by Surface Type PCC 11% BST 28% Surface Lane Miles BST 5,000 HMA 11,000 PCC 2,000 Total 18,000 HMA 61% 3

4 Preservation Policies and Practices Surface Common Preservation Comments Hot Mix Asphalt 45-mm overlay year intervals most common All traffic levels All ESAL levels Bituminous Surface Treatment Single shot (application) BST 5-10 year intervals most common Lower traffic pavements AADT < 4,000 ESALs < 50,000/yr Portland Cement Concrete Either HMA overlay or dowel bar retrofit plus grinding Most over 30 years old Limited rehabilitation to date Most prevalent on NHS routes 4

5 WSDOT Lane-Miles by ADT AADT BST HMA Lane-miles Flexible (BST+HMA) All Types (BST+HMA+PCC) ,157 1,834 4,991 4,993 (28%) ,645 2,464 2,486 (14%) ,423 1,613 1,631 (9%) (5%) (4%) ,094 2,098 2,572 (15%) ,610 1,610 2,029 (11%) ,032 1,032 1,360 (8%) (4%) >= (2%) 42% 5

6 FHWA IRI Thresholds for Interstate Highways Description PSR Rating IRI NHS Ride Quality Very Good 4.0 <1.0 m/km Good m/km Fair m/km Mediocre m/km Acceptable (0-2.7 m/km) Poor 2.5 >2.7 m/km Less than Acceptable (>2.7 m/km) 6

7 Existing IRI Sorted by ADT ADT (2002) Average IRI (m/km) >=

8 WSDOT Pavement Preservation Funds Biennium Total Funds by Pavement Type 1,2 Funds 1,2 HMA BST PCC Other Average Note 1: Includes project engineering, construction engineering, safety, taxes. Note 2: Funds shown unadjusted for inflation. 8

9 Percentages of Pavement Preservation Funding by Type Biennium HMA BST Other % 6% 31% % 6% 17% % 9% 14% % 7% 7% % 10% 7% % 18% 12% % 19% 8% 9

10 Preservation Pinch 250% Percent Difference from 1995 Value 200% 150% 100% 50% 1995 Baseline = 100% HMA Price per Ton in Place WSDOT Preservation Funding 0% Year 10

11 Preservation Funds by Lane-Mile per Year Biennium Overall Funding Per Lane Mile Per Year 1,2 Preservation Funds by Pavement Type $/Lane-Mile/Year 1,2 HMA BST PCCP ,200 7,600 1,700 6, ,500 10,900 1,900 8, ,300 9,200 2,500 6, ,900 9,900 1,800 1, ,200 8,500 2, ,800 6,700 3,900 4, ,800 8,200 4,800 3,700 Averages ( ) 7,000 8,700 2,700 4,300 Note 1: Funding shown includes project engineering, construction engineering, safety, and taxes. Note 2: Amounts shown not adjusted for inflation. 11

12 Initial Bottom Line Not enough funding to fully preserve the route system. Increasing use of BSTs in lieu of 45mm thick HMA overlays however overlays are still and will continue to be the preservation treatment of choice. How did we examine the efficacy of increased BST use? First, let s quickly look at HMA performance. 12

13 Quick look at HMA Overlay Performance Performance for more recent (post Superpave) projects Rutting IRI Life by western and eastern Washington Projects placed between 1996 and 2001 Evaluated with 2006 pavement condition survey Pavement age: 5 to 10 years ½inch HMA only 26 projects 13

14 Average Performance IRI PSC Rut IRI (in/mi) or PSC Rut (mm) PG PG PG PG PG Age (yrs) PG PG PG 76-28

15 Rutting Rut (mm) PG PG PG PG PG PG PG PG

16 16 Ride

17 HMA Overlay Life Pavement life is also a function of Construction practices Specifications Material selection Average Overlay Life (years) East West All

18 $21,000 per lane-mile $240,000 per lane-mile 18

19 Key Question Could we save money by using BSTs on some roads that are currently surfaced with HMA? What is the effect on the WSDOT preservation budget? What is the effect on long-term network performance? What is the effect on the traveling public? Further What is a reasonable upper level of AADT, at which BST resurfacings can be used? What is a reasonable upper level of annual ESALs at which BST resurfacings can be used? What combination of BST resurfacings and HMA overlays produce a cost effective rehabilitation strategy? 19

20 HDM-4 (v2.03) The Highway Development and Management System software Integrates multiple models in a unified analysis Economic Material Structural Condition UW and WSDOT has experience with HMD-4 20

21 Preservation Strategies Modeled 21 HMA overlay only Only apply 45-mm overlays to all sections Triggers: 10% cracking, 10 mm rutting, 3.5 m/km IRI BST applications only Only apply BST surfaces to all sections Triggers: 10% cracking, 3.5 m/km IRI (no rutting trigger) BST and HMA treatment combination Apply a combination of BSTs and HMA overlays to all sections Triggers: BST: 10% cracking HMA: 3.5 m/km IRI For instance: 2 cycles of BSTs then 1 cycle of HMA overlay Open selection with any of the above three strategies HDM-4 is free to choose for each individual section, which of the 3 above strategies to use

22 HDM-4 Modeling Observations Model may be biased towards existing trends The treatment strategy selected is highly dependent on the initial roughness The BST/HMA combination strategy is often selected regardless Not able to fine-tune the BST effects enough The HDM-4 model schedules a large number of treatments in the first year HDM-4 is difficult to master HDM-4 software support is almost non-existent. 22

23 Preservation Strategy Comparison ADT less than 8,000 and annual design lane ESALs less than 40,000 Scenario a Equilibrium IRI c (m/km) Net Present Value b HMA overlay only 1.90 $17,758 BST application only 3.68 $17,856 BSTs interspersed with HMA overlays triggered at 3.5 m/km IRI 2.72 $18,170 Open selection 2.96 $18,187 Notes: a. Budget limitations are not used in triggering a resurfacing. b. All costs are in millions of present-day dollars. c. The IRI that a given scenario can maintain over time. 23

24 HDM-4 Estimated Roughness Conditions AADT less than 8,000 and annual design lane ESALs less than 40,000 4 BST applictaion only 3 Open selection IRI (m/km) 2 BSTs interspersed w ith HMA overlays triggered at 3.5 m/km IRI HMA overlay only Year 50 24

25 HDM-4 Results The NPV of all strategies are the same Increasing the use of BSTs appears to be economically feasible Agency expense and user costs increase over time for all strategies except the HMA overlay only option Reduced agency expenses will result in rougher pavements and higher user costs Higher BST use shifts cost from the agency to the user unless 25

26 HDM-4 Conclusions HDM-4 is of limited but some use. Routes with AADT > 2,000 are viable for BSTs. Considering more BSTs is not a mistake. The cost of maintaining a road network is largely a zero-sum game. 26

27 Overall Assessment (1) Analyses via HDM-4 showed that more lane-miles of BST OK from a NPV view. ADT criterion increases from 2,000 to 4,000-5,000. WSDOT Pavement Policy has been changed to reflect this. And there physically is no ADT limit for BSTs. More BSTs will likely increase the IRI on the route system but by a limited amount (cycles of BST/HMA overlays, level-up quantities, etc, limit IRI increases). BSTs will rarely be used through towns and cities or at major intersections. 27

28 Overall Assessment (2) Performance in Washington State BSTs apparently exceed typical national stats. HMA at or above national stats. Differences between western and eastern Washington need to be reduced. Does WSDOT need more pavement preservation funds? Of course but how much? Likely something close to $10,000/lane-mile/year. 28

29 Overall Assessment (3) Is WSDOT likely to get that much pavement preservation funding anytime soon NO. So what can be done? Tweak the mix of BST and HMA overlays (in progress). Maximize quality of both hence performance life. Monitor carefully via PMS. Don t do anymore HDM-4 analyses! 29

30 Thanks for you attention Questions? Contributors include Steve Muench, UW and Linda Pierce, WSDOT 30