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Detailed Exploration of the Locking Point Concept in Mix Design

Project Information

Project ID: 
Project Status: 
Start Date: 
Tuesday, September 1, 2009
End Date: 
Wednesday, February 27, 2013
FHWA Program: 
FHWA Subprogram: 
Pavement and Materials
FHWA Topics: 
Roads and Bridges--Pavement and Materials
TRT Terms: 
Mix Design; Hot Mix Asphalt; Aggregates; Durability; Asphalt; Infrastructure; Pavements
FHWA Discipline: 
Pavement and Materials
TRB Subject Area: 
Materials, Pavements, Research

Contact Information

First Name: 
Last Name: 
(202) 493-3090
Email Address: 
Infrastructure Materials Team
Office of Infrastructure Research and Development
Office Code: 

Project Details

Project Description: 

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.

Project Findings: 

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.


Deliverable Name: 
Mix design guidance with explicit examples
Deliverable Type: 
Research report or guidelines
Deliverable Description: 
The first phase of the study illustrated a proof of concept that aggregate packing and locking likely can be achieved less than the established design gyration levels, but confirmation is needed. The second phase of the study then illustrated the sensitivity of rutting and cracking performance as well as compact ability when a moderate reduction in gyration level was pursued. The third phase of the study expands the number of mixtures and aggregate types evaluated in the third phase.