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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 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

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  1. Lesson 4: Hot Mix Asphalt – Mix Design CEE 595 Construction Materials Winter 2008

  2. Reading • Virtual Superpave Laboratory • WSDOT Pavement Guide Interactive

  3. Lesson Outline • Hot mix asphalt (HMA) description • Superpave mixture design procedure

  4. 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

  5. Section 1 HMA by the Numbers Europe and Japan numbers from the European Asphalt Pavement Association(http://www.eapa.org)

  6. Section 1 HMA Constituents

  7. Section 1 Types of HMA

  8. Section 1 Hot Mix Asphalt • Described by: • Maximum aggregate size • Aggregate gradation • Asphalt binder • Asphalt binder content

  9. 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

  10. Section 1

  11. 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

  12. Section 1

  13. 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

  14. 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

  15. Section 2 Superpave Mix DesignProcedure

  16. Section 2 Concepts • We design volume but measure mass • Specific gravity is very important • Converts mass to volume

  17. Section 2 Variables • Aggregate • Asphalt binder • Ratio of aggregate to asphalt binder

  18. Section 2 Goals • Stability (deformation resistance) • Durability • Fatigue resistance • Ductility (low temp. cracking resistance) • Moisture damage resistance • Skid resistance • Workability

  19. Section 2 Basic Procedure • Select aggregate • Select asphalt binder • Design aggregate structure • Select asphalt binder content • Conduct performance tests • Develop job mix formula (JMF)

  20. Section 2 Select Aggregate • Must meet: • Consensus requirements • Source properties

  21. 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

  22. Design Aggregate Structure • Not many formal procedures to do this • Typical Superpave aggregate structures create quarry waste

  23. Select Asphalt Binder Content • Level of sample compaction is critical Bleeding Raveling

  24. Conduct Performance Tests • There are none for Superpave right now • Agencies typically use • Moisture susceptibility test • Laboratory wheel tracking devices • Hveem stabilometer

  25. 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)

  26. Mix Design Equations • Air voids • VMA • VFA Pb = % asphalt binder by weight of mix, Ps = % aggregate by weight of mix Gsb = aggregate specific gravity

  27. 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

  28. Mix Design Job Mix Formula (JMF)

  29. Section 2 Other Mix Design Methods • Marshall Method • Hveem Method • All use the same basic procedure

  30. Marshall Hammer • Compacts samples in Marshall method • Analogous to Superpave gyratory compactor

  31. California Kneading Compactor • Compacts samples in Hveem method • Analogous to Superpave gyratory compactor

  32. Mix Design Marshall Stability Test • Squeezes sample diametrically

  33. Height4.5 inches Height2.5 inches Diameter6 inches Diameter4 inches Compacted Samples Marshall & Hveem Superpave

  34. 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?

  35. 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.

  36. 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.

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