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Lesson 4: Hot Mix Asphalt – Mix Design CEE 595 Construction Materials Winter 2008 Reading Virtual Superpave Laboratory WSDOT Pavement Guide Interactive Lesson Outline Hot mix asphalt (HMA) description Superpave mixture design procedure Section 1 Hot Mix Asphalt (HMA)

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lesson 4 hot mix asphalt mix design

Lesson 4: Hot Mix Asphalt – Mix Design

CEE 595 Construction Materials

Winter 2008

reading
Reading
  • Virtual Superpave Laboratory
  • WSDOT Pavement Guide Interactive
lesson outline
Lesson Outline
  • Hot mix asphalt (HMA) description
  • Superpave mixture design procedure
hot mix asphalt hma

Section 1

Hot Mix Asphalt (HMA)
  • Bituminous concrete
  • Distinguished by:
    • Materials (asphalt binder and aggregate)
    • Mix design methods
    • Elevated mixing temperature
  • Other names:
    • Asphalt concrete (AC)
    • Blacktop
    • Bitumen
hma by the numbers

Section 1

HMA by the Numbers

Europe and Japan numbers from the European Asphalt Pavement Association(http://www.eapa.org)

hot mix asphalt

Section 1

Hot Mix Asphalt
  • Described by:
    • Maximum aggregate size
    • Aggregate gradation
    • Asphalt binder
    • Asphalt binder content
maximum aggregate size

Section 1

Maximum Aggregate Size
  • Largest aggregate “diameter” as defined by:
    • Maximum SizeThe smallest sieve through which 100 percent of the aggregate sample particles pass
    • Nominal Maximum SizeThe largest sieve that retains some of the aggregate particles but generally not more than 10 percent by weight
gradation

Section 1

Gradation
  • Dense graded
    • Most common
  • Gap graded
    • Stone matrix asphalt (SMA)
  • Open graded
    • Used as a surface course in many areas
  • Uniformly graded
    • Not used as a construction material
asphalt binder

Section 1

Asphalt Binder
  • Choose asphalt binder grade
    • Select a grade based on:
      • Design pavement high temperature
      • Design pavement low temperature
    • Adjust based on reliability
    • Adjust based on traffic speed & loading
  • Add modifiers to achieve certain properties
asphalt binder content

Section 1

Asphalt Binder Content
  • Ratio of asphalt binder to aggregate
  • Determining this is a key component of mix design
  • Typical binder contents
    • Usually expressed as a percentage by weight of total mixture
    • Dense graded HMA = 4.5 to 6.0 percent
    • SMA = 6.0 – 8.0 percent
concepts

Section 2

Concepts
  • We design volume but measure mass
  • Specific gravity is very important
    • Converts mass to volume
variables

Section 2

Variables
  • Aggregate
  • Asphalt binder
  • Ratio of aggregate to asphalt binder
goals

Section 2

Goals
  • Stability (deformation resistance)
  • Durability
  • Fatigue resistance
  • Ductility (low temp. cracking resistance)
  • Moisture damage resistance
  • Skid resistance
  • Workability
basic procedure

Section 2

Basic Procedure
  • Select aggregate
  • Select asphalt binder
  • Design aggregate structure
  • Select asphalt binder content
  • Conduct performance tests
  • Develop job mix formula (JMF)
select aggregate

Section 2

Select Aggregate
  • Must meet:
    • Consensus requirements
    • Source properties
select asphalt binder
Select Asphalt Binder
  • Almost always done to local standard
    • Eliminates the calculations associated with LTPP Bind software
    • Easier to follow in specifications
    • Easier for suppliers
design aggregate structure
Design Aggregate Structure
  • Not many formal procedures to do this
  • Typical Superpave aggregate structures create quarry waste
select asphalt binder content
Select Asphalt Binder Content
  • Level of sample compaction is critical

Bleeding

Raveling

conduct performance tests
Conduct Performance Tests
  • There are none for Superpave right now
  • Agencies typically use
    • Moisture susceptibility test
    • Laboratory wheel tracking devices
    • Hveem stabilometer
important volumetric properties
Important Volumetric Properties
  • Tests run on compacted samples
    • Mixture specific gravity (Gmb)
    • Aggregate specific gravity (gives Gsb, Gsa, Gse)
  • Test run on loose mix (uncompacted)
    • Theoretical maximum specific gravity (Gmm)
    • Often called “Theoretical maximum density”
    • Often called “Rice density”
  • Calculated volumes (based on above tests)
    • Air voids (Va)
    • Voids in the mineral aggregate (VMA)
    • Voids filled with asphalt (VFA)
equations

Mix Design

Equations
  • Air voids
  • VMA
  • VFA

Pb = % asphalt binder by weight of mix, Ps = % aggregate by weight of mix Gsb = aggregate specific gravity

develop job mix formula jmf
Develop Job Mix Formula (JMF)
  • End result of mixture design
  • A specification (along with tolerances) used for plant production of HMA
  • Cannot be significantly altered without another mix design
other mix design methods

Section 2

Other Mix Design Methods
  • Marshall Method
  • Hveem Method
  • All use the same basic procedure
marshall hammer
Marshall Hammer
  • Compacts samples in Marshall method
    • Analogous to Superpave gyratory compactor
california kneading compactor
California Kneading Compactor
  • Compacts samples in Hveem method
    • Analogous to Superpave gyratory compactor
marshall stability test

Mix Design

Marshall Stability Test
  • Squeezes sample diametrically
compacted samples

Height4.5 inches

Height2.5 inches

Diameter6 inches

Diameter4 inches

Compacted Samples

Marshall & Hveem

Superpave

study questions
Study Questions
  • Draw a volumetric diagram of HMA and label the following volumes: asphalt binder, air, VMA, effective asphalt binder, absorbed asphalt binder and aggregate.
  • Discuss the three main types of HMA (in regards to their gradation) and when each might be appropriate for use.
  • What is the difference between “maximum aggregate size” and “nominal maximum aggregate size”?
  • Explain what a 0.45 power curve is and why it is used for viewing HMA gradation.
  • Why are the specific gravity calculations so important in mix design?
  • Describe the basic mix design process including each step, how it is done, why it is done and any special considerations.
  • What are the differences between Superpave, Marshall and Hveem mix design procedures?
discussion forum
Discussion Forum

The SGC has an issue with internal angle of gyration (see SGC – Background). Basically, it’s a dimension that is not specified in instrument manufacture because it is assumed to be directly related to another specified dimension. Since this internal angle of gyration can vary between brands of SGCs and even between SGCs of the same brand, the industry is noticing that calculated bulk specific gravity (and thus the calculated air voids) of the same mixture can vary between SGCs even though the same number of gyrations is specified.

Given that measurements of laboratory bulk density are often used by owners for acceptance testing and by contractors for process control, how does one reconcile this problem so that an SGC can be used for quality assurance testing (in other words, if each machine measures differently, how can the owner and contractor compare numbers)?

For instance, a contractor’s machine may produce samples that indicate 4% air voids, while the agency’s machine may indicate 6% air voids, while a third party machine may indicate 5% air voids – all on the same mix.

references
References
  • European Asphalt Pavement Association website. http://www.eapa.org
  • Muench, S.T. et al. (2005). Virtual Superpave Laboratory. http://guides.ce.washington.edu/UW/VSL/
  • Muench, S.T.; Mahoney, J.P. and Pierce, L.M. (2003). WSDOT Pavement Guide Interactive. http://guides.ce.washington.edu/UW/WSDOT
  • Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.