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9 Experimental Design Chapter 2 describes field site selection, specimen prepara- tion, and laboratory tests. Field Site Selection The ultimate goal for this project was to develop a global model of aging for asphalt mixtures based on stiffness. This model can be used to predict the rate of mixture aging and the evolution of stiffness over the life of the pavement, or at least until the stiffness values plateau at some point in time. This evolution of stiffness over time could then be used as an input to mechanistic-empirical pavement design. This relationship will change depending on climate, and the nine field sites selectedâbased on their availability from NCHRP Project 09-52âspan a wide range of climates (Figure 4) and were constructed to allow continued evaluation of the follow- ing factors that include mixture components and production parameters: â¢ Aggregate type (high versus low water absorption) â¢ Asphalt source (rapid versus slow aging) â¢ Recycled material (RAP/RAS versus virgin materials) â¢ WMA technology (WMA versus HMA) â¢ Plant types (batch plant versus drum plant) â¢ Production temperatures (high versus low) In addition to climate, the evolution of stiffness with aging also depends on the time of year of construction. CDD is calculated in terms of degrees above 32Â°F as a reference point. Figure 5 summarizes CDDs for all the field sites up to March 2018. As the figure shows, the cooler climates in Wyoming, South Dakota, Iowa, and Connecticut all have similar rates of increasing CDDs, although these rates are lower than those for warmer climates in Florida, Texas I, New Mexico, Indiana, and Texas II. All the field sites included at least one of the factors identi- fied previously. Table 2 summarizes the field sites and indi- cates which factors were included for each. Eight out of the nine sites examined the effects of WMA technologies, which included foaming, zeolite, and liquid additives. Two sites (Iowa and Florida) included aggregates with high- and low-absorptive aggregates. Two sites (New Mexico and Texas I) also had RAP/RAS content as a factor. Only one site (Texas II) had asphalt source as a factor because of the expense of obtaining asphalt cement from two different suppliers. In NCHRP Project 09-52, the results showed no difference between continuous flow drum (CFD) and BMP at the Texas II and Indiana field sites. Therefore, only CFD specimens were collected and tested for the Texas II field site in NCHRP Project 09-52A to reduce the impact of coring on pavement performance. Although changes in produc- tion temperature were shown not to affect performance in NCHRP Project 09-52, the field samples in Iowa and South Dakota had been sampled, so the materials from these sites were tested. Mixtures from the Connecticut field site were tested at construction, but no further field cores were taken because of a concern that additional sampling would disrupt traffic. A detailed factor analysis is included in Chapter 3. Specimen Preparation All the field cores were cut, cleaned, and tested for air void (AV) content prior to laboratory testing. The cores were trimmed using a power saw, and after determining AV content (AASHTO T 269), they were air-dried and stored in a cold room until testing. Based on the laboratory aging protocols developed in NCHRP Project 09-49 and 09-49B that were later adopted in NCHRP Project 09-52, the plant mixes of three different mixtures at construction from the South Dakota field site were aged for 5 days at 185Â°F (85Â°C). In NCHRP Project 09-52, compacted cores were aged for 5 days at 185Â°F (85Â°C), so loose mixes were used in NCHRP Project 09-52A as a comparison. After being aged, these loose plant mixes were compacted to C H A P T E R 2
10 Figure 4. Locations of field sites. Dec-11 0 20000 40000 60000 80000 100000 120000 140000 May-13 Sep-14 Jan-16 Jun-17 Oct-18 Texas I Texas II New Mexico Wyoming South Dakota Iowa Indiana Florida Connecticut C um ul at iv e D eg re e D ay s (Â° F -d ay s) Coring Date Figure 5. Cumulative degree-days for field sites.
Site Route Climate Date Built Binder Grade Core Location Factors Mixtures TX II Odessa Dry, No- Freeze 8/13 PG 64-22 Center of Paving Plant Type: CFD and BMP Asphalt Source: Fast versus Slow Aging HMA (Binder V, CFD) @ 5.9%Asphalt Content HMA (Binder A, CFD) @ 5.9%Asphalt Content IN Indianapolis Residential Streets Wet, Freeze 6/13 PG 64-22 Center of Lane Mix Type: HMA versus WMA Plant Type: CFD and BMP WMA Tech: Foaming (CFD) Advera (BMP) HMA + 25%RAP (CFD) HMA + 25%RAP (BMP) WMA Foaming + 25%RAP (CFD) WMA Advera + 25%RAP (BMP) IA Boone County Fairgrounds Wet, Freeze 7/13 PG 58-28 Random across Lane Mix Types: HMA versus WMA Mix Temp: HMA @ 2 Temps, Foam @ 2 Temps Aggregate: High versus Low Absorption Low Absorption HMA + 20%RAP High Absorption HMA + 20%RAP Low Absorption WMA Foaming + 20%RAP High Absorption WMA Foaming + 20%RAP FL I-95 Welcome Center Wet, No- Freeze 10/12 PG 58-28 Parking Lot Mix Types: HMA versus WMA Aggregate: High versus Low Absorption Low Absorption HMA + 25%RAP High Absorption HMA + 25%RAP Low Absorption WMA Foaming + 25%RAP High Absorption WMA Foaming + 25%RAP NM IH 25 Dry, No- Freeze 10/12 PG 76-22 (PM) w/ Versabind, PG 64-28 w/ RAP, Versabind Center of Lane Mix Types: HMA versus WMA WMA Tech: Foaming versus Evotherm 3G RAP: RAP versus No RAP HMA HMA + 35%RAP WMA Foaming + 35%RAP WMA Evotherm 3G + 35%RAP CT SH 262 Dry, Freeze 8/12 PG 76-22 (PM) w/ RAP Center of Lane Mix Types: HMA versus WMA HMA + 20%RAP WMA Foaming + 20%RAP SD SH 262 Dry, Freeze 8/12 PG 58-34 w/ lime, RAP Center of Lane Mix Types: HMA versus WMA WMA Tech: Foam versus Evotherm 3G vs. Advera HMA + 20%RAP WMA Foaming + 20%RAP WMA Evotherm 3G + 20%RAP WMA Advera + 20%RAP WY SR 196 Dry, Freeze 8/12 PG 64-28 w/ lime Center of Lane Mix Types: HMA versus WMA WMA Tech: Foam versus Evotherm WMA Temp: 2 Temps Each HMA WMA Foaming WMA Evotherm 3G TX I FM 973 Wet, No- Freeze 1/12 PG 70-22 (PM) Center of Lane Mix Types: HMA versus WMA WMA Tech: Foam versus Evotherm DAT RAP: RAP/RAS versus No RAP HMA HMA + 15%RAP + 3%RAS WMA Foaming WMA Evotherm DAT WMA Evotherm + 15%RAP + 3%RAS Table 2. Summary of field sites.
12 an AV content of 7Â±1.0% with the Superpave gyratory com- pactor, with a height of 62 mm and width of 150 mm. The compaction temperature was 275Â°F (135Â°C) for both HMA and foaming, and 241Â°F (116Â°C) for Evotherm. The mixture with Advera was not tested because the previous research showed little difference between it and the Evotherm mixture. Laboratory Tests Based on previous experience from NCHRP Project 09-49 and from TTI laboratory tests, the MR stiffness (ASTM D7369) at 77Â°F (25Â°C) and IDEAL-CT at 77Â°F (25Â°C) were selected in this project to compare the stiffness and cracking resis- tance of asphalt mixtures with various factors (including mixture components and production parameters) with aging (Zhou et al. 2017). Figure 6 and Figure 7 show how the testing progressed for the available field sites. All the field sites had cores taken at construction and at least two sets of post-construction field cores were available for MR stiffness testing. Figure 7 shows the flowchart for the extended aging testing that was completed only for the South Dakota field site as a small exploratory experiment. MR Stiffness (ASTM D7369) at 77ÂºF (25ÂºC) To perform the MR stiffness test, a cylindrical asphalt con- crete specimen was tested on its vertical diametral plane with repetitive applications of compressive loads in a haversine waveform. This type of loading created a region of uniform tensile strain along the center of the vertical axis. Two linear variable differential transducers (LVDTs) aligned along the horizontal diametral plane measured the resulting horizontal deformations of the specimen. For each mixture at one field site, up to three replicates that had an AV content closest to 7.0% were tested for MR stiffness. The field cores were condi- tioned in a temperature chamber at 77Â°F (25Â°C) for 2 hours prior to testing and then were tested at room temperature. MR stiffness was measured in accordance with the current ASTM D7369 with a modification consisting of replacing the on-specimen LVDTs with external LVDTs aligned along the horizontal plane (i.e., gauge length as a fraction of diameter of the specimen = 1.00). As shown in Equation (2-1), the MR stiffness was calculated based on vertical load, horizontal deformation, and the asphalt mixtureâs Poisson ratio. 0.2732 (2-1) ( )= u + D M P t R Where: MR = resilient modulus stiffness of asphalt mixture. P = vertical load. u = Poissonâs ratio. t = specimen thickness. D = horizontal deformation measured by LVDTs. Figure 6. Flowchart for laboratory testing. Figure 7. Flowchart for new LTOA testing.
13 IDEAL-CT The IDEAL-CT test is an indirect tension test with mono- tonic loading (Zhou et al. 2017). The cylindrical asphalt con- crete sample is conditioned and tested at room temperature (25Â°C) with a loading rate of 2.0 in./min (50 mm/min) in crosshead displacement. When the core thickness is 2.50 in. (62 mm), Equation (2-2) should be used; when the core thickness is not 2.50 in. (62 mm), Equation (2-3) should be used: (2-2) 75 75= Ã ï£« ï£ï£¬ ï£¶ ï£¸ï£· CT G m l D index f 62 (2-3) 75 75= Ã Ã ï£« ï£ï£¬ ï£¶ ï£¸ï£· CT t G m l D index f Where: Gf = fracture energy calculated as the area under the whole curve, as shown in Figure 8. l = the measured vertical deformation. t = sample thickness. D = sample diameter. Figure 8 illustrates the calculations graphically. Zhou et al. (2017) proposed IDEAL-CT to determine the crack resistance of asphalt mixtures, which is a function of the area under the load-displacement curve from the initiation of loading to the end of loading, and the slope of the post-peak curve at a point that is 75% of the peak load. A high cracking test (CT) index indicates a better cracking resistance and less cracking in the field. According to Zhou et al.âs research, a laboratory specimen with a 7% AV content is expected to have a CT index of 75 for a typical dense-graded asphalt mix and 150 for an excellent crack-resistant mix. L o ad ( kN ) Displacement (mm) Figure 8. Illustration of the post-peak point at 75% load reduction (PPP75) and its slope (m75) (Zhou et al. 2017).