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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