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13 To focus on the desired structural properties of the recycled materials, the dynamic modulus and RLPD tests were performed to investigate the stiffness and rutting susceptibility, respectively, of each test specimen. Before testing, the project team had to decide if the testing and expected properties of the recycled materials best matched those of an asphalt mixture or of a loosely bound granular material. In reality, the field performance probably lies somewhere between the two extremes. For this study, the project team decided that testing the materials assuming asphalt mixture-like behavior was the proper path. This decision was based on previous experience with these materials and the following observations: (1) the stiffness values of the recycled materials often were very near those of asphalt mixtures (and were thus greatly underrepresented in previ- ous versions of the MEPDG), and (2) the stiffness values were found to vary with respect to changes in temperature like an asphalt mixture and unlike a granular material. 3.1 Test Specimen Fabrication After the cores were unboxed, allowed to dry in ambient laboratory conditions, and photo- graphed, test specimens were fabricated. Following the procedures outlined in Bowers, Diefenderfer, and Diefenderfer (2015), small-scale cylindrical specimens were fabricated for testing. Small-scale cylindrical specimens were used based on the research teamâs desire to have the same boundary conditions and use the same specimen geometry for all laboratory tests. Cylindrical 50 mm diameter test specimens were extracted by sub-coring perpendicular to the long axis of a field core using a sample holder as shown in Figure 6. The field core was fastened horizon- tally beneath a hollow drill bit with a nominal 50 mm interior diameter. If the recycled layer was of sufficient thickness, multiple 50 mm diameter sub-cores were obtained. Figure 7 shows a series of specimens from which sub-cores were extracted. Before adopting the small-scale cylindrical test specimen geometry, a series of comparisons with full-size specimens was performed. Following the sub-coring procedure, the ends of the 50 mm diameter sub-cores were trimmed with a diamond wet saw to create a 110 mm tall specimen with flat ends. The trimming process was performed so that an equal portion of each end was removed. The trimmed specimens were next placed in a forced-draft oven at 40Â°C for approximately 24 hours to remove any surface water added during the sub-coring and trimming steps. The specimens were then further dried in accor- dance with AASHTO PP 75, Standard Practice for Vacuum Drying Compacted Asphalt Speci- mens, to ensure that any internal water was removed without further aging the test specimens. After the test specimens were dried, the diameter and length of each specimen was measured at four locations around its perimeter and at three locations along its length. The bulk density was then determined from these measurements and the mass of the test specimen. Table 2 shows the average bulk density of the test specimens from each project location. C H A P T E R 3 Specimen Preparation and Testing Methods
14 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete Next, the bulk specific gravity of each test specimen was determined in accordance with AASHTO T 331, Standard Method of Test for Bulk Specific Gravity (Gmb) and Density of Com- pacted Hot Mix Asphalt (HMA) Using Automated Vacuum Sealing Method. The average bulk specific gravity from each project location also is shown in Table 2. Once the bulk specific gravity was determined, the test specimens were subdivided for dynamic modulus or RLPD testing. The test specimens were divided assuming that their parent cores were taken sequentially along the length of a project. Thus, the project team distributed the test specimens such that the tests would represent conditions from the entire length of the project. Figure 6. Core drill sample holder used to extract small-scale cylindrical specimens from a field core. Figure 7. Field cores from which test specimens were extracted.
Specimen Preparation and Testing Methods 15 Location Project ID Average Bulk Density (lb./ft.3) Coefï¬cient of Variation (%) Average Bulk Speciï¬c Gravity (g/cm3) Coefï¬cient of Variation (%) Kansas 13-1093 127.0 1.5 2.045 0.04 Ontario 13-1111 130.4 3.2 2.097 0.06 Ontario 13-1112 134.2 2.4 2.151 0.06 Ontario 13-1113 134.1 1.8 2.144 0.02 Ontario 13-1114 136.1 3.4 2.183 0.09 Alberta 13-1115 119.4 0.8 1.971 0.02 Alberta 13-1116 124.8 1.8 2.030 0.04 Alberta 13-1117 126.7 2.2 2.062 0.04 California (San Jose) 13-1124 130.7 4.8 2.155 0.06 Colorado 13-1127 127.6 1.9 2.053 0.04 California (Los Angeles) 14-1001 136.8 4.8 2.155 0.13 California (Los Angeles) 14-1002 118.6 0.9 1.927 0.03 California (Los Angeles) 14-1003 134.7 2.6 2.174 0.05 West Virginia 14-1011 123.3 4.0 1.967 0.09 Delaware 14-1025 138.6 2.7 2.242 0.06 Delaware 14-1026 138.4 2.7 2.256 0.07 Delaware 14-1027 136.5 2.7 2.220 0.05 Delaware 14-1028 127.3 3.4 2.063 0.07 Utah 14-1055 129.1 3.1 2.057 0.08 Georgia 14-1057 124.2 2.8 2.003 0.06 Washington State 14-1058 131.9 0.7 2.122 0.07 Colorado 14-1062 126.9 2.5 2.019 0.09 Maine (Lyman) 15-1001 128.4 4.8 2.085 0.06 Maine (Corinna, Exeter) 15-1002 130.5 1.6 2.095 0.04 Table 2. Average bulk density and bulk specific gravity of test specimens.
16 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete 3.2 Small-scale Test Specimen Geometry The |E*| and RLPD tests frequently are performed using the AMPT. In accordance with AASHTO PP 60, the |E*| and RLPD test specimen geometry must be nominally 100 mm in diameter and 150 mm in height, and the specimens are cored from mixtures compacted using the Superpave Gyratory Compactor. Test specimens having this geometry can be produced using gyratory compacted samples, but it is much more difficult to attain the specified size from field cores because most pavement layers are placed at a lift thickness less than or equal to 150 mm. Given this limitation, many efforts have been made to eval- uate alternate specimen geometries for calculating the dynamic modulus values of road cores such as prisms (Pellinen et al. 2006) or by testing in indirect tensile mode (Kim et al. 2004). During the course of this project, a study by Li and Gibson (2013) investigated the use of small-scale cylindrical specimens for dynamic modulus testing and found a good correlation to full-size specimens. These specimens were 38 mm in diameter and 110 mm or 140 mm in height. The benefit of using cylindrical specimens is that the boundary conditions are gen- erally the same as those in the full-size specimen; thus, the RLPD testing should be feasible using this geometry (whereas prismatic specimens or indirect tensile geometries would not allow the same specimen type to be used for dynamic modulus and RLPD testing). Bowers et al. (2015) and Diefenderfer and Bowers (2015) furthered the work of Li and Gibson (2013) and found strong agreement between full-size and small-scale (50 mm diameter, 110 mm length) specimens for dynamic modulus testing. Dynamic modulus and RLPD testing for this study were performed in a commonly used AMPT. Because the research team was using small-scale specimens, some test fixture com- ponents had to be custom-machined to accommodate the reduced specimen size before testing. Specifically, custom arms for the linear variable displacement transducer (LVDT) stud gluing jig and reduced diameter testing platens for the AMPT were manufactured from aluminum (see Figure 8 and Figure 9, respectively). Figure 8. LVDT stud gluing jig showing custom arms.
Specimen Preparation and Testing Methods 17 A unique set of gluing jig arms was needed for the small-scale specimens because of their decreased diameter. The upper portion of each gluing jig arm had to be extended so that the arm could apply pressure to the LVDT stud as it was being glued to the specimen. For the 50 mm diameter specimens, the upper portions of the arms were extended by approximately 25 mm. All other dimensions for the custom arms were the same as for the stock gluing arms. The custom testing platens for the AMPT were also machined to facilitate centering the specimen during testing. A unique set of platens was fabricated to match the diameter of the small-scale specimens. Removable spacer blocks, used as the lower support for the small-scale specimens, also were manufactured for the different specimen heights. Testing was conducted using standard LVDT gauges and studs; the LVDT stud spacing was 70 mm. Figure 10 shows an example of a CCPR specimen ready for testing. Figure 9. AMPT testing platens with removable pacer blocks for 38 mm and 50 mm diameter small-scale specimens. Figure 10. Test specimen from Maine CCPR project ready for dynamic modulus testing.
18 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete 3.3 Dynamic Modulus Testing The relationship between stress and strain under continuously applied sinusoidal loading for a linear viscoelastic material is defined as the complex modulus (E*) with the absolute value of this term defined as the dynamic modulus (|E*|). The dynamic modulus also may be defined as the maximum dynamic stress (s0) divided by the peak recoverable axial strain (e0). Given that asphalt mixtures are viscoelastic materials, the peak strain will lag behind the peak stress by an amount that depends on the properties of the materials, the test temperature, and the loading frequency. This relationship can be expressed as follows: * * cos * sin (1)E E i E( ) ( )= Ï + Ï For a purely elastic material, Ï will equal zero and the complex modulus E* will equal the dynamic modulus |E*|; for a purely viscous material, Ï will equal 90Â°. Additional information can be found in Witczak et al. (2002). Unconfined dynamic modulus testing was conducted on the small-scale cylindrical speci- mens extracted from the field cores. The test was performed generally in accordance with AASHTO TP 79. Modifications to the specification included using a reduced set of tempera- tures (4.4Â°C, 21.1Â°C, and 37.8Â°C), using small-scale cylindrical specimens, and adjustments to the accepted test result variability. The specimen preparation and test procedures for the small-scale specimens are detailed in Bowers et al. (2015); this procedure incorporates a slight deviation from AASHTO TP 79-15, Appendix X3, by using 50 mm diameter specimens in lieu of smaller (38 mm diameter) specimens. An evaluation of the small-scale specimen geometry found that a larger diameter test specimen better correlated with full-size speci- mens when using larger particle sizes (Bowers, Diefenderfer, and Diefenderfer 2015). Testing was conducted at loading frequencies of 25 Hz, 10 Hz, 5 Hz, 1 Hz, 0.5 Hz, and 0.1 Hz at each of the three temperatures. Approximately 80% of the dynamic modulus tests at 10 Hz had a within-batch COV of less than 20% for the tests at 4.4Â°C and 21.1Â°C. For approximately 5% of the tests at 10 Hz at 4.4Â°C and 21.1Â°C, the within-batch COV was higher than 50%. At the 37.8Â°C test temperature, approximately 45% of the tests had a within-batch COV less than 20%; however, the percentage of tests with a COV greater than 50% was similar to that at other test temperatures. Generally, larger COVs were found at the higher test temperatures and lower test frequencies. AASHTO TP 79 defines an acceptable range of COV from 9% to 24% for a single operator testing asphalt mixtures having a 25 mm nominal maximum aggregate size (NMAS) over the range of stiffness values experienced in this study for recycled materials; however, this allow- ance in AASHTO TP 79 is based on asphalt mixtures and not on cold-recycled materials. The higher-than-allowable COV was expected, based on the authorsâ experiences and the literature (Cross and Jakatimath 2007; Schwartz and Khosravifar 2013; Diefenderfer and Link 2014), and it suggests that revisions to AASHTO TP 79 tolerances are needed when recycled materials are evaluated. The measured dynamic moduli for replicate specimens were averaged with respect to tem- perature and frequency and were then shifted to construct a master curve using standard time- temperature superposition techniques as follows: E e tR = Î´ + Î± + Î²+Î³ log * 1 (2) log in which |E*| is the dynamic modulus, tr is the reduced time at the reference temperature, Î´ is the minimum value of |E*| (lower shelf), Î´ + Î± is the maximum value of |E*| (upper shelf), and
Specimen Preparation and Testing Methods 19 Î², Î³ are shape parameters. The temperature dependency of the modulus is incorporated in the reduced time parameter tR: t t a TR ( )= (3) in which t is the actual loading time, a(T) is the shift factor as a function of temperature, and T is temperature. A simple quadratic polynomial is used to fit the temperature shift factors: a T aT b cT( ) = + +log (4)2 in which a, b, and c are the polynomial constants. Figure 11 shows an example of a master curve. 3.4 RLPD Testing The rutting susceptibility of the recycled mixtures was assessed in accordance with AASHTO TP 79. Modifications to the test included using a lower test temperature (45Â°C) and the same small-scale cylindrical specimen geometry as used for the dynamic modulus testing. A repeated deviator stress of 482.6 kPa was applied at a constant confining stress of 68.9 kPa. The test temperature was determined following the procedure developed in NCHRP Project 9-30A (Von Quintus et al. 2012). As a check, LTPPBind software was used to predict the average high pavement temperatures for selected stations in the eastern United States (ranging from Tampa, Florida, to Caribou, Maine). These predicted temperatures were found to range from approxi- mately 34Â°C to 56Â°C at depths of 100 mm and 150 mm. The depths of 100 mm and 150 mm were chosen as typical depths of the recycled layer for high volume pavement structures. Given the limited number of cores from each project and because the RLPD test specimens were not reusable, multiple test temperatures were not attempted. The RLPD test results were analyzed following the procedures developed in NCHRP Proj- ect 9-30A (Von Quintus et al. 2012). A power-law function was used to characterize the second- ary stage of the permanent deformation behavior as follows: ANp BÎµ = (5) Figure 11. Sigmoidal master curve function (Pellinen et al. 2004).
20 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete in which ep is the permanent strain, N is the number of cycles applied, A is the regression con- stant representing the intercept in log-log space, and B is the regression constant representing the slope of the line in log-log space. This analysis methodology is particularly appropriate when testing mixtures in a confined state that tend not to reach tertiary flow. The power-law equation was regressed starting at 2,000 cycles, as recommended by Khosravifar et al. (2015), and continued through 10,000 cycles with the exception of five specimens for which there was a plastic failure before 10,000 cycles. In the case of a plastic failure before 10,000 cycles, the range of regressed equation was limited to between 2,000 cycles and the point at which the plastic failure occurred. In all cases, the coefficient of determination (R2) was above 0.99.