Evaluation of SuperpaveConstruction Quality on Low

Volume Roads

Scott Schram

Magdy Abdelrahman

1/30/2008

BackgroundObjectiveDataSummary FindingsConclusion

Common to have TWO Mix Design Methodologies• Marshall (Low Volume)• Superpave (High Volume)

Requires more resources• 2 Specification Systems• 2 QA/QC Systems

Traffic Distribution• 35,000 Local Government

Agencies

• 2 Million Total Miles of Flexible Pavement

• 70% Low Volume

*U.S. Roads

Superpave

Marshall

30% Other

70% Low

Volume

Superpave had no significant increase in costMore Efficient!IMPROVED PERFORMANCE• Rutting• Transverse Cracking

What about Quality?

Superpave originally developed for hightraffic loadingsRestrictive Specifications for high quality materials (crushed aggregates)$$$ Not Cost Effective on Low Volume RoadsExperts agreed these requirements were not appropriate for all mixesHow to meet specifications with local materials?• Relax Specifications

How does quality change with traffic volume in Superpave Mixes?Define Quality with• Compliance (Percent Within Limits)• Variability (Coefficient of Variation)

High Compliance ≠ Low Variability

Analysis• Effect of traffic on quality• How these effects affect performance• Effect of traffic on QA/QC agreement

Nebraska Department of Roads550 Superpave Projects (1999-2005)7 MixesQA/QCPerformance

Performance• Nebraska Serviceability Index (NSI) (0 to 100)• IRI• Rutting• PSI• Thermal Cracking• Transverse Cracking

QA/QC• CAA• FAA• VMA• VFA• Density

Compliance• 26 states use PWL

Variability• Average CV (CAA, FAA, VMA, VFA, Density)

Angularity• Promotes aggregate interlock and stability• Negatively affects workability and compaction

effort

Insufficient VMA• Little relief during natural compaction causing

material flow• Causes flushing followed by raveling

Effects durability and flexibilityLow VFA• Cracking• Raveling• Binder Hardening

High VFA• Rutting• Flushing

Low Binder Content• Can lead to raveling

Excess asphalt• Foremost contributor to rutting• Flushing and other major distresses• Increased workability• Easily Heated• Arguably common in low volume HMA

construction

Adequate Compaction• Ensures required air voids• Provides high shear strength• Resistance to rutting

Compliance• Differed among traffic levels (p< 0.05)

Variability• CV significantly different among mixes

Consequences of lower quality?60 ProjectsPerformance restricted to first 2 yearsQuality averaged over (FAA, CAA, etc.)NSIRutting

When PWL < 85%, early NSI ratings decreasedWhen PWL < 85%, early Rutting increased 78%

No significant difference in performance was found in the 60 projects between the variability of high volume mixes and low volume mixes.Study by Abdelrahman (2003)• Early pavement failures• Concluded that variability played a significant

role

Ongoing topicDoes traffic effect agreement?• 76 High Traffic Projects• 258 Low Traffic Projects• Number of Data Points

• 2 sample t-test

What does this tell us?The quality of construction on low volume roads is in disagreement between contractors and owners

Quality was significantly lower for Low Volume RoadsAverage Overall PWL• High Volume Roads = 90%• Low Volume Roads = 78%

NSI 4 pointsRutting 78%Low Volume roads had excess asphalt on averageQA vs. QC• Disagreements for Low Volume Roads only

Reduce variability = ☺!!