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32 Table 11. Texture test results for selected mixtures. a Flexible Wall Permeameter. All texture and permeability test results are presented in Table 11 and Table 12. Mixture Type Sand Mix SMA OGFC Texture Category Low High High Texture (in) 0.019 0.039 0.071 3.9 Theoretical Investigation COV (%) 1.1 0.4 2.5 The effects of tack coat interface shear bond characteristics, as measured by the LISST, on pavement responses at the interface were investigated using a 2-D FE approach. Six pavement struc- tures typically used in Louisiana were simulated using the com- Table 12. Permeability test results for selected mercial FE software, ABAQUS Version 6.9-1 (see Figure 23). mixtures. Structure A consisted of a 1.5-in HMA overlay on top of a 2.0-in Mixture Type Sand Mix SMA OGFC old HMA layer and a 4.0-in crushed stone base layer. Struc- ture B consisted of a 2.0-in HMA overlay on top of a 3.0-in old Permeability Category Low Low High HMA layer and an 8.0-in crushed stone base course. Struc- 2.2 1.4 408 ture C consisted of a 2.0-in HMA overlay on top of a 6.0-in old Permeability (ft/day) 2.8 1.3 441 HMA layer and a 12-in crushed stone base layer. Structure D 2.8 1.9 401 consisted of a 2.0-in HMA overlay on top of a 4.0-in old HMA Average (ft/day) 2.6 1.8 417 layer and a 12.0-in crushed stone base course. Structure E con- Standard Deviation (ft/day) 0.3 1.5 21.1 sisted of a 2.0-in HMA overlay on top of a 2.0-in old HMA layer COV (%) 13.1 21.0 5.1 and a 12.0-in crushed stone base course. Structure F consisted of a 2.0-in HMA overlay on top of an 8.0-in old HMA layer and a 12.0-in crushed stone base course. The six structures are constructed on the same subgrade material, A-7-6 clayey soil. These mixtures were used to fabricate the bottom layer of the For the FE analyses, the tacked interface is located between specimens in the laboratory for interface shear strength test- the HMA overlay and the old HMA layer. Table 13 presents ing. The top layer of the test specimens used the mix design the assumed mechanical properties for the pavement materi- adopted for preparation of the HMA overlay at the PRF als. As shown in the table, the base and subgrade materials site. A complete specimen consisted of two layers, top and were assumed to respond elastically to the load. On the other bottom, with a tack coat placed at the interface of the two hand, the HMA overlay and old HMA layer were simulated layers. Each layer was compacted to achieve a 6 1 percent as a viscoelastic material using a Generalized Kelvin model. air void. As part of the viscoelastic definition of asphaltic materials, The diameter of each specimen was 6.0 in. The bottom half the initial instantaneous moduli, presented in Table 13, were of each specimen was prepared by compacting the mixture to used to define the elastic component of HMA. a height of 2.2 in at 165C using the SGC. Each compacted Elastic element foundations were used to simulate the bottom layer was allowed to cool to room temperature, then support provided to the pavement structure by the subgrade. its air voids content was measured. The calculated amount of These elements, which act as nonlinear springs to the ground, preheated SS-1 tack coat was then applied on the bottom half provide a simple way of including the stiffness effects of the of the sample. The tack coat was allowed to cure. Once the subgrade without fixation of nodes at the bottom of the application and curing of the tack coat was completed, the model. A dual-tire assembly applying a load of 9,000 lbf on top half of the specimen was applied by placing the bottom the pavement structure over an equivalent rectangular area half in the SGC mold and compacting the prescribed mixture was simulated with a uniform pressure of 105 psi and for a on top of the tack coated bottom half. Four-inch-diameter total loading time of 0.1 sec. The surface interactions between specimens were then cored from the SGC-compacted samples, the old HMA and the base layer and between the base and and the interface shear strength was measured at 25C. subgrade layers were assumed to be a friction-type contact Texture and permeability of the selected three mixtures (MohrCoulomb theory). Limited sliding was also allowed (see Table 9) were quantitatively measured. Mixture sur- between the aggregate layers. This formulation assumes that face texture measurements were performed according to a slave node will interact with the same local area of the mas- ASTM E 965, Standard Test Method for Measuring Pavement ter surface throughout the analysis. Macrotexture Depth Using a Volumetric Technique, which The interface conditions between the HMA overlay and is known as the sand patch test method. Permeability tests the old HMA layer was simulated according to the con- were conducted according to ASTM PS-129-01, Measure- stitutive model adopted by Romanoschi and Metcalf (35) ment of Permeability of Bituminous Paving Mixtures using for asphalt pavements. In this model, the stiffness penalty

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33 Structure A Structure B Structure C Structure D Structure E Structure F Figure 23. Pavement structures simulated in the FE analysis. Table 13. Mechanical properties of pavement materials in the FE analysis. Material Constitutive Elastic Modulus Poisson's Description Behavior (psi) ratio HMA Overlay Viscoelastic 650,000 0.25 Old HMA Viscoelastic 500,000 0.25 Base Elastic 40,000 0.30 Subgrade Elastic 6,000 0.35

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34 HMA Overlay Existing HMA Base Layer Subgrade (a) (b) Figure 24. General layout of the FE model (a) and shear stress distribution in the HMA overlay and the old HMA layer (b). method is used to describe the interface conditions. The pen- Figure 24a presents the general layout of the FE model for alty method allows relative motion between the surfaces as Structure A; in total, 7,168 elements were used to simulate long as the behavior is in the elastic region, as defined by dmax the pavement structure. The shear response of the top two (limiting displacement in the elastic region). While the sur- layers is presented in Figure 24b. As shown in this figure, the faces are sticking (i.e., t < tmax), the motion between the sur- axisymmetric shear response of the pavement structure to faces is elastic and recoverable. However, if the applied shear the applied tire load is demonstrated. In addition, while the stress exceeds the interface shear strength, the interface fails maximum shear stress is located in the middle of the layer, and the interface condition is converted to a simple friction the critical shear stress for the interface is the one calculated model, defined by a friction coefficient ( = 0.7). at the bottom of the HMA overlay.