Flow number as a discriminating HMA mixture property : [project brief].
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Flow number as a discriminating HMA mixture property : [project brief].

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    • Abstract:
      T he flow number test is used to measure the rutting potential of asphalt concrete mixtures. It is a variation of the repeated load, permanent deformation test that has been used by researchers since the 1970s. The flow number is one of three tests that were identified in National Cooperative Highway Research Program (NCHRP) Project 9-19 as simple performance tests related to the rutting resistance of hot mix asphalt (HMA) mixtures; the others are dynamic modulus and flow time. Flow number is defined as the number of load pulses when the minimum rate of change in permanent strain occurs during the repeated load test and is determined by differentiation of the permanent strain versus number of load cycle curve. In NCHRP Project 9-33, tentative flow number criteria for mixture design were developed based on evaluation of a limited number of mixtures. These tentative criteria were included in the Mix Design Manual that was developed in that project.

      What is the Problem?

      The flow number criteria developed in NCHRP Project 9-33 were based on traffic moving at normal highway speeds. However, mixtures placed at intersections are subjected to the effects of slow or standing traffic resulting in greater potential for rutting. Current mixture design methods provide designers considerable freedom in selecting the composition of a mixture. With limited information available on the effects of mixture composition on the flow number, it may be difficult to develop mixtures that meet the flow number criteria. Additionally, acceptance criteria permit deviation from the design job mix formula during construction, which may result in a change in the flow number and rutting resistance of the mixture. There is a need to investigate the effect of changes in mixture composition on the flow number and rutting resistance of hot mix asphalt (HMA) and to develop flow number criteria for mixtures used at intersections.

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