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In order to assess the effect of fracture energy of asphalt overlay mixtures on reflective cracking performance, five pavement cross-sections were chosen. The selection of these sections was done in consultation with the staff at MnDOT OMRR. These cross-sections represent a variety of overlay and underlying pavement combinations. Between these sections, asphalt overlay thicknesses vary from 3 to 5 inch. Four of the five cross-sections have a PCC slab in the underlying pavement structure and one is constructed on existing asphalt pavement. All of these pavement sections have been part of previous MnDOT asphalt performance research studies.

Details on various layer thicknesses and material types for the five pavement cross-sections used for simulation of reflective cracking performance in this task are shown in Table 9.2 thru Table 9.6. Please note that while all other cross-sections represent actual pavement sections, for I-94 section, existence of jointed plain concrete pavement underneath the overlay was assumed.

On the basis of previous load transfer efficiency (LTE) data from Dave (2013), researchers used a low LTE scenario in the simulations to represent a more critical condition from the perspective of reflective cracking. LTE for the simulated underlying pavement was approximately 35% for all cases presented herein. This condition represent existence of minimal to no load transfer capability from joints between PCC slabs or cracks in the underlying existing asphalt layers.

Table 9.2 Pavement Section: TH15

Trunk Highway 15

Pavement Layer Material Thickness Overlay Lift-2 9.5 mm, PG 58-28 (SPWEA440B) 1.5 inch Overlay Lift-1 12.5 mm, PG 58-28 (SPWEB440B) 1.5 inch Existing Asphalt 4 inch Aggregate Base Class 6 6 inch Subgrade Clayey Loam N.A.

Table 9.3 Pavement Section: TH14

Trunk Highway 14

Pavement Layer Material Thickness Overlay Lift-2 12.5 mm, PG 58-28 (SPWEB340B) 1.5 inch Overlay Lift-1 12.5 mm, PG 58-28 (SPWEB340B) 1.5 inch Existing Asphalt 5.75 inch

PCC Slab 7 inch

Subgrade Silty Clayey Loam N.A.

Table 9.4 Pavement Section: I 90

Interstate 90

Pavement Layer Material Thickness Overlay Lift-1 12.5 mm, PG 64-28 (SPWEB440E) 3 inch

PCC Slabs 8 inch

Bituminous Stress Relief 1 inch

PCC Slab 8 inch

Subbase Class 5 3 inch Subbase Class 4 3 inch Subgrade Silty Clayey Loam N.A.

Table 9.5 Pavement Section: TH280

Trunk Highway 280

Pavement Layer Material Thickness Overlay Lift-3 9.5 mm, PG 64-34 (SPWEA440F) 1.5 inch Overlay Lift-2 9.5 mm, PG 64-34 (SPWEA440F) 1.75 inch Overlay Lift-1 UTBWC (PMB) 0.75 inch

PCC Slab 9 inch

Base Class 5 3 inch

Subbase Class 4 9 inch Subgrade Silty Clay N.A.

Table 9.6 Pavement Section: I 94

Interstate 94

Pavement Layer Material Thickness Overlay Lift-2 12.5 mm, PG 70-28 (SPWEB540H) 2.5 inch Overlay Lift-1 12.5 mm, PG 70-28 (SPWEB540H) 2.5 inch

PCC Slab* 8 inch

Base Class 6 12 inch

Subgrade Silty Clay Loam N.A.

* Actual section for I-94 does not consist of PCC slab.

The mechanical properties of various pavement materials, other than overlay mixtures, as used in simulations are provided in Table 9.7. The source of these values are also shown in the table. As

discussed in previous chapter, the asphalt mixtures (overlays, ultra-thin bonded wear course, underlying existing asphalt layer and stress-relief interlayer) were simulated using linear viscoelastic material properties that are time, temperature and load history dependent. Since asphalt mixtures used in the present study were not available for lab testing, properties of asphalt mixtures closest the ones simulated here were used. The overlay mixtures evaluated in this study are similar to those tested and analyzed by Marasteanu et al. (2012) and Dave et al. (2017) for previous MnDOT research studies. Linear viscoelastic properties from these previous studies were utilized in the current project.

Table 9.7 Pavement Material Properties

Materials Elastic Modulus (psi)

Poisson's Ratio

Coefficient of Thermal Expansion and Contraction

(/°C)

Source

PCC Slab 4,600,000 0.20 1.00E-05 FHWA Class 4 Agg. Base 26,000 0.35 na

MnPAVE Late Spring

Value Class 5 Agg. Base 23,000 0.35 na

Class 6 Agg. Base 20,000 0.35 na Clayey Loam 7,000 0.40 na Silty Clayey Loam 6,100 0.40 na Silty Clay 5,900 0.40 na

9.3.2 Simulated Asphalt Mix Fracture Energy Levels

Since the main focus of this task was to determine the sensitivity of asphalt overlay fracture energy on the expected reflective cracking performance, parametric evaluations were conducted for each of the five simulated pavement sections. Over the course of this task more than 100 finite element runs were conducted. The results from over 35 finite element simulations were utilized for assessing the primary objective of this research effort. For each traditional asphalt mix layers, fracture energies of 300, 400, 500, 600 and 700 J/m2 _{were simulated. For the ultra-thin bonded wear course (UTBWC) interlayer lift}

fracture energies of 500 and 650 J/m2 _{simulated. These fracture energy values for UTBWC were chosen}

of asphalt mixtures (which was the case for four out of five simulated pavement sections), analyses were conducted by changing fracture energy of one lift at a time while others were held constant. Typically, 400 J/m2 _{of fracture energy was used as the constant value for an asphalt mix layer when fracture}

energy of other layer was changed. Various fracture energy combinations that were simulated for each of the five overlay sections are shown in Table 9.8.

Table 9.8 Fracture Energy Combinations for Simulated Overlays Highway Overlay

Lift Material

Fracture Energy Combinations (J/m2₎

1 2 3 4 5 6 7 8 9 TH 15 2 9.5 mm, PG 58-28 (SPWEA440B) 300 400 500 600 700 400 400 400 400 1 12.5 mm, PG 58-28 (SPWEB440B) 400 400 400 400 400 300 500 600 700 TH 14 2 12.5 mm, PG 58-28 (SPWEB340B) 300 400 500 600 700 400 400 400 400 1 12.5 mm, PG 58-28 (SPWEB340B) 400 400 400 400 400 300 500 600 700 I 90 1 12.5 mm, PG 64-28 (SPWEB440E) 300 400 500 600 700 na na na na TH 280 3 9.5 mm, PG 64-34 (SPWEA440F) 300 400 400 400 450 500 na na na 2 9.5 mm, PG 64-34 (SPWEA440F) 400 400 450 400 450 500 na na na 1 UTBWC (PMB) 650 650 650 500 500 500 na na na I 94 2 12.5 mm, PG 70-28 (SPWEB540H) 300 400 500 600 400 400 na na na 1 12.5 mm, PG 70-28 (SPWEB540H) 400 400 400 400 500 600 na na na

Apart from fracture energy, tensile strength of material is another input in the cohesive zone model. In the research presented here, cohesive strength values from previous studies on asphalt mixtures of similar origin were used (Marasteanu et al. 2012, Dave and Hoplin, 2015). Strength values for various asphalt mixtures in this study are provided in Table 9.9.

Table 9.9 Cohesive Strength for Simulated Asphalt Mixtures

Highway Overlay Lift Material Strength

(MPa) TH 15 2 9.5 mm, PG 58-28 (SPWEA440B) 2.00 1 12.5 mm, PG 58-28 (SPWEB440B) 2.25 TH 14 2 12.5 mm, PG 58-28 (SPWEB340B) 2.25 1 12.5 mm, PG 58-28 (SPWEB340B) 2.25 I 90 1 12.5 mm, PG 64-28 (SPWEB440E) 2.50 TH 280 3 9.5 mm, PG 64-34 (SPWEA440F) 2.50 2 9.5 mm, PG 64-34 (SPWEA440F) 2.50 1 UTBWC (PMB) 2.00 I 94 2 12.5 mm, PG 70-28 (SPWEB540H) 3.00 1 12.5 mm, PG 70-28 (SPWEB540H) 3.00