RELATIONSHIP OF SUPERPAVE GYRATORY COMPACTION PROPERTIES TO HMA RUTTING BEHAVIOR
This report presents the findings of a research project to examine whether there is a relationship between any property of hot mix asphalt (HMA) measurable with the Superpave gyratory compactor and the rutting behavior of HMA pavements in service. Its main finding is that the parameter N-SRmax, the number of gyrations at maximum stress ratio, may be used within certain limits to identify mix designs likely to show gross instability under the shear stresses produced by traffic in HMA pavements. The report will be of particular interest to materials engineers in state highway agencies, as well as to materials suppliers and paving contractor personnel responsible for HMA design and quality control.
Evaluation of Aggregate Structure Stability Using Superpave Gyratory Compactor
The main goal of this study was to evaluate the potential of using the gyratory compaction forces and deformation to measure the aggregate structure stability in hot mix asphalt (HMA) during the mix design process. This method is intended as a screening tool to identify aggregate structures that have low resistance to applied forces prior to conducting further performance testing. The measured compaction shear forces and deformation of a wide spectrum of hot mix asphalt (HMA) mixtures were used to calculate a stability index referred to as the Contact Energy Index (CEI). Then, the CEI values were compared to permanent deformation measured using the Asphalt Pavement Analyzer (APA), and the Hveem stability for mixes designed in accordance to the Hveem method. In addition, the aggregate texture was measured using the Aggregate Imaging System (AIMS), and the texture results were compared to CEI. The results indicated reasonable correlation between CEI and APA for mixes that have the same binder high temperature grade. For Hveem-designed mixes, CEI values correlated reasonably well with the Hveem Stability values for all tested field mixes. Reasonable correlation was found between aggregate texture and the CEI. Overall, the results indicate that the CEI has the potential to be used as a screening tool for HMA design, especially to decide upon the acceptance of the mix aggregate structure during the mix design process.
COMPRESSIVE STRESS PULSE TIMES IN FLEXIBLE PAVEMENTS FOR USE IN DYNAMIC TESTING
TRAFFIC MOVING OVER A PAVEMENT STRUCTURE RESULTS IN A LARGE NUMBER OF RAPIDLY APPLIED STRESS PULSES BEING APPLIED TO THE MATERIAL COMPRISING EACH LAYER. THIS CONDITION IS BEST CHARACTERIZED BY DYNAMIC TESTS SUCH AS THE REPEATED LOAD TRIAXIAL TEST OR FATIGUE TESTS. THE SHAPE AND DURATION OF THE COMPRESSIVE STRESS PULSE RESULTING AT DIFFERENT DEPTHS BENEATH THE SURFACE ARE STUDIED FOR SEVERAL FLEXIBLE PAVEMENT SYSTEMS AND VEHICLE SPEEDS VARYING BETWEEN 1 AND 45 MPH. A COMPARISON IS MADE OF THE COMPRESSIVE STRESS PULSES CAUSED BY A VEHICLE MOVING OVER A LAYERED PAVEMENT SYSTEM AT A SPEED OF 1 MPH USING BOTH LINEAR AND NONLINEAR ELASTIC FINITE ELEMENT THEORY. VISCOUS EFFECTS AND INERTIA FORCES ARE NEGLECTED. BECAUSE THE NORMALIZED STRESS PULSES CALCULATED USING THE LINEAR ELASTIC THEORY ARE FOUND TO AGREE CLOSELY WITH THOSE CALCULATED USING THE NONLINEAR THEORY, LINEAR ELASTIC FINITE ELEMENT THEORY IS USED THROUGHOUT THE STUDY. THE RESULTS SHOW THAT THE SHAPE OF THE COMPRESSIVE STRESS PULSE VARIES FROM APPROXIMATELY A SINUSOIDAL ONE AT THE SURFACE TO MORE NEARLY A TRIANGULAR PULSE AT DEPTHS BELOW APPROXIMATELY THE MIDDLE OF THE BASE. TYPICALLY, THE COMPRESSIVE PULSE TIME ALSO VARIES ALMOST INVERSELY WITH VEHICLE SPEED UP TO AT LEAST A SPEED OF 45 MPH, THE CALCULATED STRESS PULSE TIMES WERE EMPIRICALLY CORRECTED FOR VISCOUS EFFECTS AND INERTIA FORCES USING THE RESULTS OF FIELD MEASUREMENTS MADE AT THE AASHO ROAD TEST. CURVES ARE PRESENTED FROM WHICH APPROXIMATE COMPRESSIVE STRESS PULSE TIMES CAN BE SELECTED FOR USE IN DYNAMIC TESTING. /AUTHOR/