ASPHALT (Materials 2)

Asphalt Concrete

a mixture of asphalt (glue/matrix) and 95% mineral aggregates (stones, sand, gravel which are the strong particles that carry the load and makes it stiffer/stronger)

Asphaltenes (Composition)

• large, dark solids that make asphalt thick and firm

• brittle when cold

• high MW

Resin (Composition)

• helps hold asphalt together and keep asphaltenes mixed

• soften when heated

• medium MW

Oils (Composition)

• Thin liquids that make asphalt softer and help it flow easily

• low MW

Components

• more liquid = lower melting point

• more solid = higher melting point

Layers

1. asphalt concrete surface layer

2. granular base

3. granular subbase

4. prepared subgrade (compacted soil/ground)

1. Making Pavements

asphalt cement and aggregates are mixed at high temperatures and compacted on road while still hot

• haul via dump truck

• placement - self propelled paver

2. Mix Design

addresses performance of asphalt concrete under working conditions:

• climate

• loading

• time

  • compaction

Compaction - Vibratory Roller

• prevent further compaction

• provide shear strength and resistance to rutting (grooves and dips)

• ensure mix is waterproof

• prevent excessive oxidation of binder

Climate (Design Criteria)

behavior is temperature dependent

• when heated:

  • asphaltenes dissolve in resin

  • resin dissolve in oils

  • less viscous/sticky

• hot climate: use harder grades to avoid rutting

• when cooled:

  • opposite

• cold climate: use soft grades to avoid low temperature cracking

Loading (Design Criteria)

repeated traffic loads over time cause fatigue cracking

Time (Design Criteria)

• aging/oxidation and moisture can damage

• lead to rutting/cracking/aggregate debonding

Superpave

super performing asphalt pavements

• performance-based mix design

• choose asphalt binder not based on its components but potential for rutting/cracking

Superpave concrete design

asphalt binder grade determined based on performance in:

• compactibility (bulk specific gravity)

• shear

• tensile strength and fracture

• creep

• bending beam rheometer

Asphalt binder grading

performance grading “PG X-Y”

• x and y are max and min pavement design temperatures

Recycling asphalt concrete

• surface recycling: heat the top of pavement to repair minor cracks

• central plant recycling: remove old asphalt, take it to a plant, mix with new materials

• in place recycling: break up old asphalt, add aggregate and mix at the site

Rubber tires added to asphalt concrete

• increase elasticity/stiffness

• increase cement-aggregate bond

• reduce thermal cracking/freeze-thaw damage

• added with binder or with aggregate