18 SPRING/SUMMER 2022 WHAT IF BMD IS A FAILURE? initial inception in the early 2000s, there has not been widespread adoption by highway agencies or the contracting industry. One drawback to the AMPT is the time required to achieve results. During the mix design process, the turnaround time from specimen preparation to final results is less critical (typically 4 to 5 days). However, if adjustments to the design are needed then the time to final approval can be extensive. Likewise, during the production of asphalt, test results are needed in “real-time” to make adjustments. If the mix design approval is based on fundamental engineering properties and not standard properties such as asphalt content, aggregate gradations and volumetrics, then a contractor will not be able to adjust their mix during production in order to ensure conformance with the mix design. Many days will pass from the time the sample is retrieved to final test results. Recognizing other challenges exist with the production use of AMPT, research moved to focus on Level 2 mix design using lab performance tests, generally referred to as BMD. Many extensive efforts have been underway for the past decade at numerous universities to develop lab performance tests that correlate to in-service performance of asphalt mixes. The most notable early tests developed have been the Illinois I-FIT, the Texas Asphalt Overlay Cracking Test, the Disc Shaped Compact Tension and the Semi-Circular Bend test. Since the development of these procedures and use by state agencies, the Indirect Tensile (IDT) has gained broader acceptance for assessing cracking potential; the Cantabro test for evaluating asphalt mix and durability, and either the Asphalt Pavement Analyzer (APA) or Hamburg wheel tracking device to assess mixture rutting potential. Based on a recent survey of state agencies, 10 states are implementing while others are in the process of either implementing or evaluating BMD tests.2 The Basic Premise As stated earlier, state agencies such as VDOT are investigating or implementing BMD in the hopes that new asphalt mixes perform better than current asphalt mixes. Better performing mixes, while potentially more expensive initially, will have lower life cycle costs due to the better performance and longer service life. Asphalt mix testing. An asphalt sample in IDT test equipment. continues on page 20 △ △ continued from page 17
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