PAVEMENT DESIGN for DUMMIES -...
Transcript of PAVEMENT DESIGN for DUMMIES -...
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PAVEMENT DESIGN
for DUMMIES
Boulder County Resource Conservation – Glasphalt Research Project
prepared by LBA Associates, Inc. – November 2006 (draft)
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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.
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(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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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
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Part I: Road Construction
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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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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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Load Bearing by
Flexible v. Rigid Pavement
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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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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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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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Part II: Materials
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Aggregate
1. Found in floodplains, stream terraces and alluvial fans
– also excavated from quarries
2. 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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Aggregate, con’t
3. 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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Preparing Aggregate
• Excavated – dug or
blasted
• Size-reduced –
crushed & screened
• Washed - to remove
undesired small
particles
• Stockpiled
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Asphalt
1. Also called asphalt
binder or bitumen
2. 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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Asphalt, con’t
3. 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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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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Part III: Pavement Design
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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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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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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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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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HMA Manufacturing
Manufacturing = blending & heating components to job specifications •Batch plant (older technology)
•Continuous drum plant (can product 100-900 tph)
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HMA Placement
• Equipment – transfer vehicles & asphalt
paver;
– Paver is self-propelled unit
– Includes tractor, hopper & floating “screed”
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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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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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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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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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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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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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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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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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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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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?
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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