1. PAVEMENT DESIGN for DUMMIES
Boulder County Resource Conservation – Glasphalt Research Project
prepared by LBA Associates, Inc. – November 2006 (draft)
Draft 11/2/06

2. U.S. Highways (2001 data) 8.21M lane miles in place
Truck traffic increasing faster than passenger vehicle traffic
93% asphalt pavement
7% concrete pavement
Note: pavement discussions in this presentation are limited to asphalt pavement as glasphalt research is the ultimate goal of Boulder County’s project.
Draft 11/2/06

3. (Paved) Roadway Functions
Carries vehicle loads that unpaved roads cannot
Provides drainage away from driving surface
Add smoothness to allow higher vehicle speeds and greater “drivability”
Provide skid resistance for vehicles
Provides a relatively dust-free surface
High-speed surface
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4. Will have samples to circulate to audience
Definitions
Aggregate = natural sand, gravel & stone
Asphalt = petroleum residue (naturally occurring or from distillation of crude oil)
Asphalt pavement = aggregate + asphalt + air
Concrete pavement = aggregate + cement + water + air
Will have samples to circulate to audience
Draft 11/2/06

5. Part I: Road Construction
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6. Subgrade
Existing materials quality based on strength, gradation & drainage capacity – “good” soils will retain most of their load-bearing capacity when wet
If “poor” soil (e.g., swelling soils);
Remove to reach better soils below
Replace with better material
Stabilize to increase stiffness
Compaction of subgrade materials is key
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7. Base Courses Provides load distribution, drainage & frost resistance
Materials are moderately stiff to help carry traffic & minimize deflection of pavement materials
Subbase Course is optional - typically includes relatively fine aggregate (minus ¾”) & 12” thick
Base Course - typically includes moderately coarse aggregate (minus 1-1/2”) & 6-12” thick
Can be aggregate only (loose, unbound) OR
Aggregate bound with asphalt (pavement)
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8. Load Bearing by Flexible v. Rigid Pavement
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9. Surface Course (in contact with traffic)
Surface Course = aggregate + asphalt
Includes highest quality & stiffest materials to absorb loads
Aggregate is typically
minus 3/8”
Placed in 8” layers
prior to compaction
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10. Asphalt Pavement
Also known as blacktop, flexible pavement or bituminous concrete
Typically used in lower load applications
Less construction costs than concrete pavement – may have shorter life & greater maintenance
Support loads by flexing & distribution through bottom layers
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11. HMA Hot Mix Asphalt (HMA) is the most common type of asphalt pavement
HMA is made principally from aggregate and asphalt binder at elevated temper-atures at plant
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12. Part II: Materials
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13. Aggregate
Found in floodplains, stream terraces and alluvial fans – also excavated from quarries
Described in terms of size gradation
“Well graded” aggregate (preferred) = wide range of sizes
Fine aggregate = minus 3/16”
Coarse aggregate = > 3/16”
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14. Aggregate, con’t Other Characteristics
Toughness – resists crushing or disintegration when tested, stockpiled, transported or made into HMA
Durability – resists damage from wetting & drying
Particle Shape/Texture – cubic & angular with rough surfaces
Clean – free of contaminants (vegetation, clay lumps, soft particles, dust)
U.S. produced more than 3B tons in 2001 at value of $14.2B – highways consumed 40% of this material
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15. Preparing Aggregate Excavated – dug or blasted
Size-reduced – crushed & screened
Washed - to remove undesired small particles
Stockpiled
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16. Asphalt Also called asphalt binder or bitumen Natural Characteristics
Waterproof
Thermoplastic
Elastic
Adhesive
U.S. produced more than 35M tons in 2001 at value of $6B – highways consumed 85% of this material
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17. Asphalt, con’t Other Characteristics Durable - ages well with time
“Rheology” or deformation with temperature
Too warm/too much flow – can cause rutting
Too cold/too little flow – can cause cracking
Purity – pure bitumen
Most current asphalt pavement design approach is based on specifying asphalt in terms of maximum & minimum temperatures it will be exposed to
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18. Asphalt Modifiers May be added to:
Lower viscosity (thickness) & increase workability
Increase viscosity & decrease rutting
Increase adhesion between aggregate and binder (especially in presence of moisture) – decrease stripping
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19. Part III: Pavement Design
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20. Pavement Design Considerations
Loads – penetrate 2-3’
Quantity/repetition of traffic
Type of traffic – passenger, trailer, construction
Type of vehicles – tire pressure, load, wheel configuration
Vehicle speed
Road configuration (curvatures)
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21. Pavement Design, con’t Environment
Temperature – impacts on binder rheology (extremes lead to rutting & cracking)
Frost action – subgrade heaves, thaw weakening
Moisture
Design mitigation = increase pavement to frost depth, replace frost-susceptible & weak (expansive) subgrade soils, increase drainage
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22. Pavement Design, con’t Drainage
Surface HMA relatively impermeable if well compacted
Need cross slopes of 2%
Subsurface – facilitate good permeability with subgrade soils and base course design
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23. 30% of cost of HMA pavement
Aggregate + Asphalt
Aggregate
Asphalt Binder
92-96% by weight
4-8% by weight
30% of cost of HMA pavement
25-30% cost of HMA pavement
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24. HMA Manufacturing
Manufacturing = blending & heating components to job specifications
Batch plant (older technology)
Continuous drum plant (can product tph)
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25. HMA Placement Equipment – transfer vehicles & asphalt paver;
Paver is self-propelled unit
Includes tractor, hopper & floating “screed”
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26. Placement, con’t Compaction – most important factor for performance
75-85% maximum density achieved by screed
Remainder achieved by rollers – steel or pneumatic tires, may use vibration
Must compact prior to cooling (rutting)
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27. Part IV: Other Asphalt Pavement Applications
Pavement Placement – improve placement success
Leveling layer
Tack coat
Surface Treatment – increase smoothness, appearance, safety; reduce noise; correct defects
Fog & slurry seals
Overlays
Repair – address specific defects
Crack & slurry seals (crack seals often contain crumb rubber)
Patching
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28. Other Applications, con’t
Rehabilitation – improve strength or salvage stressed pavement
Overlays
Hot In Place Recycling – old pavement is heated, scarified/removed, modified, placed & compacted
Cold in Place Recycling – old pavement removed, pulverized, modified, placed & compacted
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29. Recycled Asphalt Pavement (RAP)
The most recycled materials in U.S. - approximately 80M tpy
Can be used to make new HMA (10-30% by weight), in cold mix, as fill or other
Recycled in place – used to resurface existing pavement or pulverized for base material
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30. Part V: The Glass Alternative
Colorado Aggregate
80% of Colorado’s aggregate is sand & gravel found in drainage areas along the Front Range
20% is crushed rock mined from quarries in the mountains
Haul costs significant where source is not located near aggregate plant or project
How long before land use & environmental constraints cause an aggregate shortage?
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31. Glass Blended w/ Aggregate
Characteristics similar to natural aggregate (hard, durable, drains well & dries fast)
Higher heat retention (good with frost penetration but may take longer to “cure” during placement)
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32. Glass in HMA Reflective properties (good & bad)
Decreased skid resistance if gradation is too large
Does not stay bound to asphalt well without anti-stripping agent (“raveling”)
Real & perceived issues with glass in surface
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33. Glass Preparation Requirements
Blended w/ Aggregate
Crushing - most states require all glass at minus 3/8” (but < 8% passing No. 200 sieve)
Cleaning – debris threshold may be < 5%
Transporting - to aggregate production site
Blending with aggregate
Used in HMA
All of above – except that gradation & debris threshold may be tighter
Mix glass/aggregate w/ binder (also req’d w/o glass)
Add anti-stripping agent
May require adjustments during pavement placement
Note: health & safety issues associated with crushing glass
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34. Glasphalt Economics
Many pilot project in 1970s to 1990s - earliest projects findings were promising
Several states allowed glass to be used with roadway aggregate (a few also allowed in HMA)
As alternative glass markets developed economics were less favorable
Today very few state/federal roads utilize glass in roadways
Feasibility still exists in areas where public agencies are building roads, limited local glass markets exist and/or regional climates require additional road base fill
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35. Questions to Ask
Are there any obstacles to using natural aggregate in the region? (availability, cost, engineering properties or other)
Are there significant advantages to using a new material? (same reasons, plus the ability to develop an end market for glass)
Are there net benefits with using glass?
Are enough quantities of glass produced to make the investigation worthwhile for both recyclers & aggregate/HMA producers?
Draft 11/2/06

36. Thanks To
The following references were used to develop this presentation:
The Washington Asphalt Pavement Association’s Asphalt Pavement Guide
USGS’ Front Range Infrastructure Resources Project
Bicycling Magazine (September, 2006)
MidAmerica Recycling & Strategic Materials websites
Draft 11/2/06