The Need for Inducing Shear Instability to Obtain Relevant Parameters for HMA Rut-Resistance

• Instability rutting is a major distress mode in hot mix asphalt pavements, which occurs when the structural properties of the compacted mix cannot resist near surface critical stress conditions caused by traffic loads. The critical stress conditions for rutting instability include high near surface shear stresses and low confinement. On the mixture level, instability rutting is manifested in a rearrangement of the aggregate structure. This paper illustrates that it may be necessary to induce instability in mixtures to determine parameters that are relevant to mixture rut resistance. The Superpave(TM) gyratory compactor with shear measurements and the option of changing the compaction angle during compaction was used in this study. Mixtures were compacted to a density that is consistent with the air voids of field pavements immediately after construction, namely 7 percent (+/- 0.5) percent air voids. The subsequent application of high shear stresses by increasing the gyratory angle from 1.25 degrees to 2.5 degrees results in the rearrangement of the aggregate structure during which parameters can be measured that define clear differences in mixtures. Based on the testing of 31 mixtures with different void structure and aggregate characteristics, three distinctive responses were observed: a) brittle response, b) plastic response, under which once the mixtures loose strength due to rearrangement they never regain another stable rearrangement, and c) optimal response, which is bracketed by the plastic and brittle types of responses. Key parameters were identified that are relevant to the rut-resistance of mixtures. Using these new parameters, a framework for the evaluation of mixture rutting potential is developed. Based on a comparison of predicted rutting potential to APA rut depth measurements, as well as a statistical analysis of results, the validity of the proposed framework is established. The results show that the proposed framework has may have the potential for becoming an index test for evaluating the rutting potential of mixtures. Based on the findings of this paper, the proposed approach for evaluating mixture rut-potential should be studied further and validated for an even larger variety of asphalt mixtures.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Samhällsbyggnadsteknik -- Infrastrukturteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Civil Engineering -- Infrastructure Engineering (hsv//eng)

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