Evaluation of The Laboratory Asphalt Stability Test

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2. EXPERIMENTAL DETAIL

Equipment Used as the LAST Device

The LAST device used for evaluation was a commercial version manufactured using stock materials. This is detailed in table 1 and shown in figure 2. The equipment design equipment was based on available blueprints that maintained the scaled-down dimensions recommended in the original study, but included certain modifications to correct for some perceived deficiencies. The various modifications to the research prototype that were incorporated in the design are noted below:

• Though the device was equipped to provide a nitrogen flow, no nitrogen purge was used. It was believed that the stripping of volatile components due to the purging of nitrogen would be more deleterious than the limited exposure to air during conditioning and sampling.
• No baffle was used, because the shortcomings of this device overshadowed the merits.
• A considerably lower agitation rate of 365 rpm was used. Researchers believed that the excessively high agitation rate of 2000 rpm used in the original study would generate shear rates that could degrade some modifiers.
• Though the propeller shaft was capable of accommodating two impellers a single propeller with a 40° pitch was used, because it was adequate for keeping the modified asphalts stabilized.
• The internal heating that was used in the original study was replaced by an external heat source using side and bottom heating mantles.

Equipment Used for Rheological Characterization

The Rheometrics DSR was used to generate the dynamic data at an intermediate temperature of 16 °C with a set of parallel plates of 8 millimeters (mm) diameter, and a high temperature of 75 °C with a set of parallel plates of 25 mm diameter following the procedure given in the American Association of State Highway and Transportation Officials' (AASHTO) Provisional Standard TP5 (Edition 1A). The samples for the test were prefabricated using a silicone rubber mold. The rheometer and the temperature-controlled unit were operated through a personal computer, and the data acquisition/analysis was done by using specialized software running under Windows® 95. Frequency sweep data from 1 radian/second (s) to 100 radians/s were generated on the binders using a strain low enough to be within the linear viscoelastic response range of the material.

Figure 2. The commerical LAST device

Materials Used

The modified asphalts used in NCHRP 90-07 were investigated in this study. These included elastomers, plastomers, elasto-plastomers, and crumb rubber. The eight binders chosen all met the Superpave performance grading of 70-28 (PG70-28) and included the following polymer-modified systems: Elvaloy® (B6228), Styrene-Butadiene-Styrene Linear-Grafted (B6229), Styrene-Butadiene-Styrene Linear (B6230), Styrene-Butadiene-Styrene Radial-Grafted (B6231), Ethylene-Vinyl Acetate (B6232), Ethylene-Vinyl-Acetate Grafted (B6233), Ethylene Styrene Interpolymer (B6243), and Chemically Modified Crumb Rubber (B6251).

The PG number shown is based on the Superpave system description. The polymer-modified grades were obtained by adding various amounts of different polymers to the asphalt from the same source, namely, Venezuelan crude blend of Boscan and Bachaquero-PG64-28 (base) or the PG52-28 (flux), or mixture of the PG64-28 (base) and the PG52-28 (flux)) in different proportions to achieve the same performance grading. All these asphalts are part of the extensive ongoing polymer research program at the Pavement Testing Facility located at TFHRC. A list of the various binders used in this study and some of their select properties are presented in table 2.

Testing Protocol

Measured quantity of modified binder was heated to 165 °C in a 113.4-gram tin. The tin was removed from the oven and stirred for about 1 minute with an electric mixer to ensure that the asphalt and the modifier were adequately blended. The modified binder then was poured into the vessel, which was preheated to 165 °C. The filled vessel was placed in the heating mantel and connected to the manifold. With the stirrer set to 365 rpm and the temperature of the heating mantles set to 165 °C, the experiment was initiated.

Aliquots of asphalt were withdrawn from the top and bottom of the vessel. These samples were obtained following 0, 6, 24, 31, and 48-hour (hr) conditioning periods. This procedure was repeated twice; once with agitation (365 rpm), and once without agitation (static). Agitation was halted briefly while the bottom aliquot was withdrawn. The withdrawn samples were poured directly into silicone molds of 8 mm and 25 mm in preparation for analyses on the DSR. Frequency sweep data was generated from 1 radian/s to 100 radians/s at two temperatures (16 °C and 75 °C), and the G* and d determined at 10 radians/s at these two temperatures was used to calculate the separation ratios, R_s and degradation ratios, R_d.

The protocol discussed above is radically different from the conventional CTS test. In the CTS test, a homogeneous sample is placed in the tube and conditioned in the oven at 165 °C for 2 days. Following this, the material is quench-cooled in a freezer at -20 °C. The frozen specimen is retrieved and cut into three sections. The rheological properties taken from the top and bottom sections are compared, and deviations larger than 10 percent are considered to be representative of phase separation. The CTS test was also used in this study for comparison purposes.

Table 2. List of modified binders and some select Superpave values