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Detailed Exploration of the Locking Point Concept in Mix Design

Project Information

Project ID: 
Project Abstract: 

The project will use x-ray computed tomography and mechanical performance tests to evaluate the manner in which aggregate particles pack during mix design to assess whether there is a point where aggregates no longer pack but simply move around. This will be compared to empirical definitions of locking point, which try to define the same thing. Then contemporary performance tests will be used to quantify the performance consequences for alternate mix design practices that attempt to increase binder content and durability without sacrificing permanent deformation (rutting resistance).


The objective of this research is to recommend mix design and performance test practices to increase asphalt mixture durability based on the hypothesis that current Superpave mix design compaction levels are too high and result in lower than optimal asphalt contents.

Research HUB ID: 
Project Status: 
Project Funding Amount (Contract Award Amount): 
Start Date: 
Tuesday, September 1, 2009
End Date: 
Wednesday, February 27, 2013
Public Access Plan: 
Asphalt Binder and Mixtures Laboratory (ABML)
FHWA AMRP Program: 
FHWA AMRP Activity: 
Pavement and Materials
Project Outputs: 

It is recommended that Agencies use the Asphalt Mixture Performance Tester (currently being implemented) using confined Flow Number tests and Simplified Viscoelastic Continuum Damage fatigue tests to confirm that durability is increased and rutting is not significantly affected by a change in mix design specification. A more simplistic recommendation is to increase asphalt binder contents by 0.4% if the standard design gyration compaction levels are kept. Four mixtures studied that were standard designed to 75 gyrations indicated that compaction was effectively achieved at around 60 gyrations. These mixes were redesigned at 65 gyrations and three of the four responded well to increase durability without sacrificing significantly rutting resistance. There was no general, consistent rule and essentially every mix should be checked with performance tests if modified Superpave mix design practices are used. Two additional mixes are being added to the four primary mixes to further confirm the findings.