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Segregation in Transport

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“Segregation” is a term often used in the HMA industry to describe a number of different phenomena. It’s most general definition comes from Stroup-Gardiner and Brown (2000[1]):

“Segregation is a lack of homogeneity in the hot mix asphalt constituents of the in-place mat of such a magnitude that there is a reasonable expectation of accelerated pavement distress(es).”

They point out that “constituents” should be interpreted to mean asphalt binder, aggregates, and air voids. This would describe a range of segregation found in conventional dense-graded mixtures, SMA, OGFC and large stone mixtures, as well as any other mixtures with unique proportioning or compositional factors (Stroup-Gardiner and Brown, 2000[1]). This definition includes both aggregate segregation and construction-related asphalt concrete temperature differentials that can lead to higher than desirable HMA air voids.

 

Video 1. Segregation Effects.

Aggregate Segregation

Based on several articles (Kennedy et. al., 1987[2]; Brown and Brownfield, 1988[3]; Williams et. al., 1996a[4] and 1996b[5]; Khedaywi and White, 1996[6]; AASHTO, 1997[7]) the commonly accepted qualitative definition of aggregate segregation is “the non-uniform distribution of coarse and fine aggregate components within the asphalt mixture.” There are two basic types of aggregate segregation:

  1. Coarse segregation. Occurs when gradation is shifted to include too much coarse aggregate and not enough fine aggregate. Coarse segregation is characterized by low asphalt content, low density, high air voids, rough surface texture, and accelerated rutting and fatigue failure (Williams et. al., 1996b[5]). Typically, coarse segregation is considered the most prevalent and damaging type of segregation; thus segregation research has typically focused on coarse segregation. The term “segregation” by itself is usually taken to mean “coarse segregation.”
  2. Fine segregation. Occurs when gradation is shifted to include too much fine aggregate and not enough course aggregate. High asphalt content, low density, smooth surface texture, accelerated rutting, and better fatigue performance characterize fine segregation (Williams, Duncan and White, 1996[8]).

A quantitative aggregate segregation definition is less certain. Since coarse segregation is generally accepted as most destructive, a general quantitative definition is a sample ³ 10% coarser than the JMF on the 2.36-mm or the 4.75-mm sieve (Brown and Brownfield, 1988[3]; Cross and Brown, 1993[9]; Williams, Duncan and White, 1996[8]).

The Joint Task Force on Segregation from the American Association of State Highway and Transportation Officials (AASHTO) and NAPA (1997[7]) identified five major segregation sources:

  1. Mix design. The key mix design parameters in determining segregation susceptibility are gradation and asphalt content (AASHTO, 1997[7]). Specifically, open-graded mixes and low asphalt binder content mixes are particularly prone to segregation.
  2. Stockpiling. If aggregate is loaded on top of a conical stockpile then larger aggregate sizes are more likely to roll to the outside and bottom.
  3. Asphalt plants. Key portions of the plant generally concerned are (Brock, 1986[10]; Kennedy et. al., 1987[2]; AASHTO, 1997[7]):
    • Cold feed bins. Bins may not feed uniformly.
    • Batch plant hot bins. Dust may collect on the wall and discharge in large concentrated slugs.
    • Drum mixers. Large particles will generally flow through a drum mixer more quickly than smaller particles.
    • Surge and storage bins. Situations such as an off-set discharge chute, angled material flow from the discharge chute, and excessively large bin openings can lead to segregation.
  4. Truck loading and unloading. Loading trucks in one large dump may cause the larger aggregate sizes to roll to the outside and bottom.
  5. The asphalt paver. Segregation occurs as a result of hopper operation, auger speed, and disruptions in paver movement.

The chief detrimental effects of segregation on HMA performance are: reduced fatigue life and increased rutting, raveling, and moisture susceptibility. These effects can cause a severe reduction in pavement life. More information on segregation causes and cures can be found in Segregation Causes and Cures for Hot Mix Asphalt (QIP-110) by AASHTO and NAPA.

Finally, aggregate segregation and construction-related temperature differentials display the same symptoms and result in the same types of damage, which can cause them to be confused with one another. However, the ultimate damage mechanism, excessive air voids (often expressed as “inadequate density”), is the same in both cases.



Footnotes    (↵ returns to text)
  1. Stroup-Gardiner, M. and Brown, E.R.  (2000).  National Cooperative Highway Research Program Report 441: Segregation in Hot Mix Asphalt Pavements.  Transportation Research Board, National Research Council.  Washington, D.C.
  2. Kennedy, Tomas W.; McGennis, Robert B.; and Holmgreen, Richard J.  (1987).  “Asphalt Mixture Segregation; Diagnostics and Remedies.”  Proceedings of the Association of Asphalt Paving Technologists, Vol. 56.  pp. 304-329.
  3. Brown, E.R.; and Brownfield, J.R.  (1988).  Investigation of Rutting and Segregation of Asphalt Mixtures in the State of Georgia.  Georgia Department of Transportation.  Atlanta, GA.
  4. Williams, R. Christopher; Duncan, Gary Jr.; and White, Thomas D.  (1996a).  Sources, Measurement, and Effects of Segregated Hot Mix Asphalt Pavement.  Report for the Joint Highway Research Project, Project No. C-36-36MM, File No. 2-4-39.  Purdue University, West Lafayette, IN.
  5. Williams, R. Christopher, Duncan, Gary Jr., and White, Thomas D.  (1996b).  “Hot-Mix Asphalt Segregation:  Measurement and Effects.”  In Transportation Research Record 1543.  Transportation Research Board National Research Council, Washington, D.C.  pp. 97-105.
  6. Khedaywi, Taisir S.; and White, Thomas D.  (1996).  “Effects of Segregation on Fatigue Performance of Asphalt Paving Mixtures.”  In Transportation Research Record 1543.  Transportation Research Board National Research Council, Washington, D.C.  pp. 63-70.
  7. American Association of State Highway and Transportation Officials (AASHTO).  (1997).  Segregation: Causes and Cures for Hot Mix Asphalt.  Publication by the Joint Task Force on Segregation of AASHTO Subcommittee on Construction; AASHTO Subcommittee on Materials; and National Asphalt Pavement Association.  AASHTO.  Washington, D.C.
  8. Williams, R. Christopher; Duncan, Gary Jr.; and White, Thomas D.  (1996a).  Sources, Measurement, and Effects of Segregated Hot Mix Asphalt Pavement.  Report for the Joint Highway Research Project, Project No. C-36-36MM, File No. 2-4-39.  Purdue University, West Lafayette, IN.
  9. Cross, Stephen A.; and Brown, E. R.  (1993).  Effect of Segregation on Performance of Hot-Mix Asphalt.  Transportation Research Record 1417.  Transportation Research Board, National Research Council, Washington, D.C.  pp. 117-126.
  10. Brock, J. Don.  (1986).  “Segregation of Asphaltic Mixtures.  Proceedings of the Association of Asphalt Paving Technologists, Vol. 55.  pp. 269-277.

 

 

 

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