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Inverted Pavement

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Inverted Pavement

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  1. Inverted Pavement Vulcan Materials Company’s Experience

  2. Inverted Pavement Why is the aggregate industry interested Background of inverted pavement Overview of inverted pavement Vulcan’s experience with inverted pavement

  3. Aggregate Industry’s Interest in Inverted

  4. Construction Aggregates • Probably the most basic building block for construction of all types • Every American uses about 10 tons of aggregate per year • Not like a manufacturing industry however • We can’t selectively make one type or size of aggregate • When we make coarse aggregate, we also make fines and base at the same time

  5. Construction Aggregates Like the beef industry Multiple cuts of beef are made Can’t make only tenderloin Someone has to buy the other cuts Otherwise, the tenderloin would be un-affordable

  6. Construction Aggregates • Construction aggregates are the same way • When we crush stone, we make approximately the following: • 35% coarse aggregate • 20% fine aggregate • 45% aggregate base • This varies by quarry and each particular geology

  7. Construction Aggregates • We do everything we can to optimize our plants and to match production to sales • Plant Studies • Crusher Studies • Process Improvement Reviews • There are limitations

  8. Aggregate Base & Clean Stone Sales in NC

  9. Factors Impacting Base Sales • Coarser asphalt mixes • More use of Superpave, ultrathin, OGFC, SMA • Forces us to turn base into coarse aggregate & screenings • Less new pavement construction in recent years • Not as much aggregate base being used • Some new pavement using no base at all • Full Depth Asphalt • Permeable Pavements • Flaws in the MEPDG program

  10. Factors Impacting Base Sales • Increased use of in place recycling • Full Depth Reclamation uses no aggregate base • May only use virgin aggregate in the top lift • In place recycling given unwarranted advantages over virgin aggregates • The result is this…

  11. Product Balance

  12. No End in Sight • Trend is for more clean stone compared to aggregate base and Fines • In some areas, we are buying land to store excess product • This is not a sustainable way for us to do business • Bad for the environment • Bad for future aggregate costs

  13. The Cost of Unsustainability • Excess inventory has a cost associated with it • Crushing and screening products that aren’t sold – excess fuel, electricity, time • Handling those products – excess fuel and time • Stockpiles covering reserves may need to be moved multiple times – excess fuel and time • Excess costs for the aggregate industry will end up being a cost for its customers and end users

  14. How Does Inverted Pavement Help?

  15. What is an Inverted Pavement? 2” to 3” HMA 6” to 8” Unbound Aggregate Base Compacted to 100% + Modified Proctor 6” to 10” Cement-Treated Base Prepared Subgrade

  16. Inverted Pavement History • Used successfully in South Africa since the 1970’s G1 Base Refers to Inverted Pavement System Use.

  17. Inverted Pavement History • Traffic levels increase • US and Europe relied on thicker asphalt & increased concrete use • Not economically viable in South Africa • Investigated ways to improve roads by improving the aggregate base • Instituted strict gradation limits • Limited plasticity • Required better aggregate quality

  18. Inverted Pavement History • Wanted to improve/increase the density aggregate base • Led to a cemented subbase being used as an “anvil” on which to compact the aggregate base • Enables high level of compaction Compactive Effort Aggregate Base Cement Treated Base

  19. Inverted Pavement History • Soon discovered that this pavement • Could handle the highest traffic loads • Was impervious to water ingress • Performed well even when wet • Decades of research have shown these pavements can be used on roads up to 50 to 100 million ESALs

  20. Road (Pavement) CategoriesSouth Africa Structural Design of Flexible Pavements, Draft Technical Recommendation for Highways (TRH) 4, Pretoria, South Africa, 1996

  21. Typical Pavement Structural DesignsSouth Africa 1.5” 6” 10” Structural Design of Flexible Pavements, Draft Technical Recommendation for Highways (TRH) 4, Pretoria, South Africa, 1996

  22. Mechanics of Inverted Pavement Systems d } 1 3 3 3  = 1 + 23

  23. Traditional Flexible Pavement System • Successive stiffer layers from subgrade up • Each layer “absorbs” the load as it’s distributed to subgrade • Traditional designs put unbound aggregate base on top of subgrade • Built on the idea of protecting the layer below

  24. HMA BASE SUBBASE / PREPARED SUBGRADE NATURAL SUBGRADE Flexible Pavement System

  25. Inverted Pavement • Changes the way we think about pavement • Utilizes the stress dependency of graded aggregate base

  26. Resilient Modulus Stress Dependency Granular Material are Stress Dependent: Higher Modulus with Increasing Stress State

  27. What is Bulk Stress? • Bulk stress () represents the total stress condition that a given location in the pavement experiences. •  is higher closer to the surface d } 1 3 3 3 1 = Total Vertical Stress d = Deviator Stress 3 = Confining Stress  = 1 + 23

  28. Inverted Flexible Pavement 2” to 3” HMA 6” to 8” Unbound Aggregate Base Compacted to 100% + Modified Proctor 6” to 10” Cement-Treated Base Prepared Subgrade

  29. Isn’t This Asphalt Too Thin? • The South African protocol uses very thin asphalt layers ESALs (millions) 1-3 3-10 10-30 30-100 40mm = 1.6 in 50mm = 2 in

  30. Recent Research • Research at Georgia Tech on inverted systems • Traditional pavement, asphalt acts like a beam • Inverted pavement, thin asphalt performs like a membrane Compression Tension

  31. Beam versus Membrane Compression Compression Tension Tension Source: Papadopoulos, “Performance of Unbound Aggregate Bases and Implications for Inverted Base Pavements”, May 2014

  32. Inverted Pavements • CTB used as strong foundation • Anvil on which to compact aggregate base • Aggregate base placed in the optimal position in the pavement • Near the surface • High bulk stress increases stiffness • Thin asphalt protects the aggregate base • Acts like a membrane • Reduced tension

  33. Inverted Pavements in the U.S.

  34. Morgan County Quarry Haul Road (GA) Constructed in 2003 Still performing well

  35. LaGrange Bypass, GA • Constructed in 2009

  36. Luck Stone, Bull Run Quarry, VA • Constructed in 2011 • FHWA installed pressure and strain gauges • Still gathering data

  37. I-25 in Northern New Mexico • Interstate 25, Raton, NM • 54+ inches of snow per year • Constructed in 2012 3” HMA 8” Aggregate Base 10” Cement-Treated Base

  38. Vulcan’s Inverted Pavement • Need to relocate a road in our Pineville Quarry near Charlotte, NC • Great opportunity for an inverted pavement • New road was long enough for a test section and control section • Cement silo and pugmill on site at the quarry

  39. Road Relocation Scalehouse Truck Traffic

  40. Road Relocation

  41. Road Relocation Conventional Inverted

  42. Pavement Design • Estimate 1.5 to 2 million ESALs over 20 years based on sales forecast • Used South Africa Department of Transport TRH 4 to develop a pavement design

  43. Pavement Design 1.18 in Asphalt 6 in aggregate base 7.87 in CTB

  44. Other Inverted Designs in U.S. Raton, NM Morgan, GA LaGrange, GA 3.6” HMA 3.5” HMA 3” HMA 6” UAB 6” UAB 6” UAB 10” CTB 10” CTB 8” CTB Subgrade mixed with UAB Cement Stabilized Subgrade Subgrade mixed with UAB

  45. 1" NCDOT SF9.5A with PG 64-22 Pineville Inverted Pavement Design 1.5" NCDOT S9.5B with PG 64-22 6" NCDOT Aggregate Base (102% modified proctor) 8" NCDOT Cement Treated Aggregate Base (97% modified proctor) Subgrade (100% standard proctor)

  46. Subgrade

  47. Cement Treated Base -- Design • Used a typical NCDOT Aggregate Base Course (ABC) • 2% cement • 7 day compressive strength: 550 psi

  48. Cement Treated Base