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0-6613

SWUTC Research Project Description

Evaluate Binder and Mixture Aging for Warm Mix Asphalts

University: Texas A&M University

Principal Investigator:
Charles Glover
Texas Transportation Institute
(979) 845-3389

Funding Source: SPR Program

Total Project Cost: $256,025

Project Number: 0-6613

Date Started: 9/1/12

Estimated Completion Date: 8/31/13

Project Summary

Project Abstract:
Warm mix asphalt (WMA) technologies employ reduced mixing and placement temperatures thereby allowing reduced fuel consumption, enhanced compaction, increased haul distances and an extended paving season.  Issues of concern in WMA are binder oxidation and absorption and their impact on pavement durability.  Ongoing TxDOT project 0-6009 is quantifying oxidation rates in HMA pavements and their impact on pavement durability, but does not address warm mixes or binder absorption.

The purpose of this project is to evaluate binder oxidation, binder absorption by aggregates, and the impact of these issues on mixture durability for the various common warm mix technologies and to develop a new binder specification, suitable for unmodified and modified binders, that incorporates binder oxidative aging and its impact on WMA pavement durability.

This proposed project addresses these issues by making measurements of laboratory and field warm mix materials: binders, aggregates, lab-compacted mixtures, and pavement cores. Proposed studies include measurements of fundamental absorption-related properties of binders and aggregates, absorption of warm mix and hot mix asphalts by aggregates at both warm mix and hot mix temperatures, and characterization of laboratory and field warm mixture specimens as to both mixture rheology and recovered binder oxidation and rheological hardening.

Project Objectives:
A work plan with seven tasks is proposed to meet the three objectives of the project : 1) evaluate warm mix asphalt oxidation in binders and mixtures; 2) evaluate asphalt absorption by aggregates; and 3) develop a new binder specification, suitable for unmodified and modified binders, that incorporates binder oxidative aging and its impact on WMA pavement durability.

Task Descriptions:

Task 1: Conduct Information Search

Task 2:  Finalize Experimental Design

Task 3: Evaluate Oxidation of Warm Mix Binders and Mixtures

Task 4:  Evaluate Binder Absorption by Aggregates and Its Impact on Mixture Durability

Task 5:  Develop a New Binder Specification for WMA Pavement Durability

Task 6: Meet with TxDOT Project Personnel

Task 7: Document Findings and Recommendations


Implementation of Research Outcomes:
Warm mix asphalt (WMA) technologies, through reduced mixing and placement temperatures, have reduced fuel consumption, enhanced compaction, increased haul distances and an extended paving season. However, there have been issues of concern in WMA including binder oxidation, binder absorption, and the impact of  both of these issues on pavement durability.

Researchers used a wide range of methods, both new and new to asphalt materials, for assessing warm mix binder, mixture properties, and field performance. Laboratory work included characterizing asphalt oxidation kinetics, binder absorption (with estimates of precision) as a function of time and temperature, and measurements of mixture fatigue/durability, all for several common warm mix technologies. Researchers developed guidelines for specifications, suitable for unmodified and modified binders, which incorporate binder oxidative aging and its impact on WMA pavement durability. The research team also measured binder absorption in field loose mix materials and characterization of field warm mix specimens for both mixture rheology and recovered binder oxidation and rheological hardening. Two field projects employed the use of multiple WMA technologies.

Impacts/Benefits of Implementation:
Absorption: A key finding of this research is that absorption is directly related to aggregate void fraction. Other absorption key findings are:
• There is an effect of binder grade (viscosity).
• WMA absorption is somewhat less than hot mix asphalt (HMA) absorption.
• The density gradient column provides a reliable and relatively easy measure of absorption for an aggregate/binder pair.
• Standard (ASTM) methods for measuring absorption can be problematic, depending on the level of absorption.

Oxidation Kinetics:  Binders modified using warm mix technologies were found to have similar oxidation kinetics to their base binders.

Mixture Fatigue:  The overlay tester and viscoelastic characterization (VEC) measurements were successfully used to characterize mixture fatigue. Other mixture fatigue key findings are:
• Mixture fatigue resistance declines with binder oxidation.
• The VEC modulus varies over 1.5 inches of pavement depth due to the difference between the aging rate at the surface and the aging rate 1.5 inches below the pavement surface. The magnitude of the difference depends largely on average annual daily solar radiation.
• Unaged binder properties, together with binder oxidation kinetics and climate data (primarily solar radiation), can be used to estimate pavement durability (fatigue resistance).
• From overlay tester data, it can be shown that there is an aging function that relates the Paris’ law exponent to aging, a result that is omitted entirely from typical pavement design guides (e.g., the Mechanistic Empirical Pavement Design Guide).

WMA Curing at Early Pavement Lives:  During the first summer of WMA’s service life, oxidative aging, curing, and absorption have a significant beneficial effect on the performance of warm mixes. In particular, at early times warm mixes are more susceptible to moisture damage than hot mixes.

This research, impacting the USDOT goal of state of good repair, produced two procedures which are recommended for developing into a durability specification for both worm mix and conventional hot mix binders.

Procedure 1
One of these procedures requires characterizing the oxidation kinetics and hardening susceptibility of a specific binder along with the climate for the specific pavement of interest. The approach then uses this information in a pavement oxidation model and couples the result to mixture characteristics (response to binder oxidative hardening and traffic loading) to predict pavement durability (fatigue resistance, for example) over time. Moisture damage is not included directly in the approach, although adjustments can be made to model parameters that would indirectly account for both fatigue and moisture damage.

Procedure 2
The second procedure incorporates the same factors mentioned above but is less specific to each binder type and climate and has the advantage of also including moisture susceptibility. Such a specification procedure would use a table that specifies the maximum allowable dynamic shear rate function value, according to mixture type, climate zone, and pavement target fatigue life.

Web Links:
Final Technical Report