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No. 18 of 19 Geosynthetics in Asphalt Pavements by Prof. S.F. Brown FEng University of Nottingham

No. 18 of 19 Geosynthetics in Asphalt Pavements by Prof. S.F. Brown FEng University of Nottingham

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No. 18 of 19 Geosynthetics in Asphalt Pavements by Prof. S.F. Brown FEng University of Nottingham

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  1. No. 18 of 19 Geosynthetics in Asphalt Pavements by Prof. S.F. Brown FEng University of Nottingham The information presented in this document has been reviewed by the Education Committee of the International Geosynthetics Society and is believed to fairly represent the current state of practice. However, the International Geosynthetics Society does not accept any liability arising in any way from use of the information presented.

  2. Lecture Outline • The pavement design problem • Reinforcement • Design methods • Fatigue/failure • Construction requirements • Background research

  3. The Pavement Design Problem

  4. Asphalt Pavement Mechanics • Failure mechanisms • New roads • Maintenance of existing roads

  5. Failure Mechanisms • Fatigue cracking • Rutting • Importance of repeated loading

  6. Design Requirements for Asphalt Pavements

  7. Typical Fatigue Cracking

  8. Fatigue Cracking Principles • Crack initiation depends on tensile strain • Crack propagation depends on tensile stress • Interaction of asphalt layer and foundation

  9. Rutting Mechanism for Asphalt Layer • Permanent shear strains near surface • High temperatures • Heavy wheel loads • High traffic volume

  10. Typical Wheel-track Rutting

  11. Foundation Rutting • Lack of adequate load spreading • Granular layer or soil • Problems of water

  12. Roles for Geosynthetics • Separation/Filtration • Reinforcement

  13. Separation & Filtration • Interface between soil & aggregate • Prevent contamination • Avoid build-up of pore pressure

  14. Mechanism of Separation • Appropriate pore size for geosynthetic • Advantages of interlock • Construction expedient or long term effect

  15. Reinforcement • Only deals with potential failure mechanisms • No effect on transient strains • No reduction in surface deflection • Reduction in permanent strain level • Restriction to crack propagation

  16. Foundation Reinforcement • Reduce rutting from construction traffic • Haul roads or foundations to permanent roads

  17. Sub-base Rutting from Construction traffic

  18. Locations for Reinforcement of Foundations

  19. Design Methods • Oxford/Jewell analytical method • Based on increase of subgrade bearing capacity • Charts available • Geosynthetic at bottom of granular layer

  20. Oxford/Jewell Design Principle

  21. Oxford/Jewell Method • Input parameters • Undrained shear strength and unit weight of subgrade • Applied contact pressure & wheel load • Shear strength of granular material • Load spreading angle • No of load applications • Output • Layer thickness; reinforced & unreinforced • Tensile force in geosynthetic

  22. Design Chart 1 for Unpaved Roads (After Jewell)

  23. Design Chart 2 for Unpaved roads (after Jewell)

  24. Asphalt Reinforcement • Increase in fatigue life • Reduction in rutting • New construction • Strengthening overlays • Generally requires stiff geogrids

  25. Fatigue Cracking • Locate where tensile stress/strain is maximum • Restricts crack growth

  26. Design for Fatigue Cracking • Fatigue life increased by 10 times • Grid located at bottom of layer • Use in mechanistic design method

  27. Approximate Design for Rutting • Life for given strain level increased by factor of three • Use with mechanistic design method • Grid in top half of layer

  28. Grid Locations for Asphalt reinforcement

  29. Typical Design Chart for UK conditions

  30. Typical Design Chart for USA : New York State Asphalt Layer Thickness (mm) Unreinforced 400 1 layer 200 2 layers 0 1 10 0.1 100 Design Life (msa)

  31. Rutting Reflection cracking Composite construction Reinforcement of Asphalt Overlays

  32. Rutting in Overlays • Locate where strains are largest; mid-depth • Increase in life to critical rut of three times • Requires stiff geogrid

  33. Reflection Cracking • Locate near to existing discontinuity • Mechanistic design method still evolving • Stiff geogrid best • Geotextile can waterproof

  34. Parameters for Reflection Crack Reinforcement • Stiffness of geogrid • Shear strength of interface • Location and installation of geogrid

  35. Construction Requirements • Good site practice • Geosynthetic must be firmly fixed and correctly located • Attention to interface conditions • Resistance to damage from hot asphalt

  36. Background Research • Granular layers • Asphalt layers

  37. Geotextiles in Granular Layers

  38. Importance of Interlock

  39. Reflection Cracking Reduction

  40. Reflection Cracking Comparisons(Caltabiano & Brunton, 1991)

  41. Rutting of Asphalt (Brown et al,1985)

  42. Rutting from Wheel Tracking Tests on Asphalt Slabs

  43. Reinforcement of Asphalt

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