Effects of Reheating and Mixture Composition on Balanced Mix Design Test Results

About the report

Since 2019, the Virginia Department of Transportation (VDOT) has conducted several field trials with balanced mix design (BMD) mixtures. In 2024, these efforts culminated with the statewide application of VDOT’s BMD specification to all 9.5- and 12.5-mm surface mixtures with A and D designations in maintenance resurfacing contracts. Several Virginia Transportation Research Council (VTRC) research projects helped address challenges during the specification development and implementation. These projects revealed significant and practical differences in BMD results between lots or samples of the same mixture and across producers and VDOT districts, highlighting the influence of mixture handling practices and component materials on BMD laboratory performance. A better understanding of how factors such as reheating and mixture composition affect BMD test results is essential to ensure consistent asphalt mixture production and robust quality measurement practices.

This study’s objective was to quantify the effect of reheating and mixture composition on VDOT’s BMD test results and to identify the volumetrics and gradation parameters that can significantly influence Cantabro, indirect tensile cracking test (IDT-CT), Asphalt Pavement Analyzer (APA), and indirect tensile at high temperature (IDT-HT) results. The scope of work included a literature review, statistical analysis of BMD datasets from past VTRC studies and VDOT’s 2024 BMD statewide production, a statewide stakeholder survey to gather insights regarding typical production-level practices for BMD testing, and BMD testing of four plant-produced asphalt mixtures to verify the findings.

The ratio in cracking tolerance index (CT_Index) performance criteria between 100 (reheated) and 130 (non-reheated) was validated. Significant differences in Cantabro mass loss and IDT-HT strength between reheated and non-reheated conditions were observed, and preliminary non-reheated criteria were proposed. Statistical analysis revealed that asphalt content, gradation parameters (percent passing sieves No. 8, No. 30, and No. 200), aggregate bulk specific gravity, voids in total mixture, and absorbed asphalt content are significant factors influencing BMD results, with subsets affecting individual tests. A CT_Index predictive equation framework was developed, and the case study showed its potential as a tool to support decisions in requesting or waiving additional testing during production, especially under reduced testing frequency. Laboratory testing of four plant-produced mixtures showed no significant differences in BMD tests between lag times of 1 and 4 days, which the survey revealed to be typical lag times in Virginia. The laboratory testing also confirmed the adequacy of the proposed non-reheated performance criteria for Cantabro and IDT-HT and validated the significant volumetrics and gradation parameters, given the agreement between measured and predicted values.

This study recommends that VDOT should (1) continue specifying CT_Index criteria of 100 and 130 for reheated and non-reheated mixtures during production, respectively; (2) benchmark Cantabro, IDT-CT, and IDT-HT production results during future construction seasons to evaluate the feasibility of the proposed non-reheated performance criteria for Cantabro and IDT-HT tests; (3) consider incorporating the proposed CT_Index predictive equation framework into a guidance document to support the current BMD specification when reduced testing frequency is applied during production, with refinement as additional data become available; and (4) continue monitoring and evaluating the effect of mixture composition on BMD test results. 

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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