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40 smectite (montmorillinite) group, organic matter, and iron The surface energy theory will be discussed in the following hydroxides present in fine aggregate (96). The principle of section. the test is to add quantities of a standard aqueous solution Results from ongoing project NCHRP Project 4-19(2), of the dye (methylene blue) to a sample until adsorption of "Validation of Performance-Related Tests of Aggregates for the dye ceases. Use in Hot-Mix Asphalt Pavements," showed a very signifi- A representative sample of dry fine aggregate is screened cant relationship between the rutting performance of the wet through the No. 200 sieve. The portion of the sample passing pavements and the MBV (23). A high MBV may be associ- the No. 200 sieve is tested for methylene blue adsorption ated with a high amount of harmful material or a more active value (MBV). Ten grams of the sample are dispersed in 30 g clay mineral type. of distilled water in a beaker. One gram of methylene blue (MB) is dissolved in enough distilled water to produce 200 2.6.5 Surface Free Energy Theory ml of solution so that 1 ml of solution contains 5 mg of MB. This MB solution is titrated stepwise in 0.5 ml aliquotes from The mechanism of moisture damage can be explained by the burrette into the continually stirred fine aggregate sus- the theories of adhesion. Four broad theories have been pre- pension. After each addition of MB solution and stirring for sented to explain adhesion of asphalt binder to aggregate: the 1 min, a small drop of the aggregate suspension is removed mechanical theory, the chemical reaction theory, the surface with a glass rod and placed on a filter paper. Successive addi- free energy theory, and the molecular orientation theory. The tions of MB solution are repeated until the end point is surface energy theory is being used by several researchers in reached. Initially, a well-defined circle of MB-stained dust is evaluating the moisture potential of asphalt mixtures (100 formed and is surrounded with an outer ring, or corona, of clear 104). The surface energy theory primarily involves calculat- water. The end point is reached when a permanent light blue ing the surface energies of the asphalt binder and the aggre- coloration, or "halo," is observed in this ring of clear water. gate. The bonding energy between asphalt and aggregate can The MB value of a specific fine aggregate fraction is then be calculated either between the two components alone reported as milligrams of MB per gram of specific fine aggre- or in the presence of a third liquid such as water. Cheng et al. gate fraction, such as MBV = 5.3 mg/g, 0/No. 200. The MBV (101) stated that the bonding strengths helped to select the expresses the quantity of MB required to cover the total sur- most compatible mixtures, to improve the adhesive bond, face of the clay fraction of the sample with a mono-molecular and to reduce debonding potential in the presence of mois- layer of the MB. Therefore, the MBV is proportional to the ture. Several methods have been developed to measure the product of the clay content times the specific surface of the surface energy of an asphalt-aggregate system. Elphingstone clay (97). (102) measured the surface energies of various kinds of The MB test is simple and practical, and its cost is reason- asphalts using the Wilhelmy Plate technique and measured able. Cross and Voth (98) used this test as a reference test for the contact angle of many asphalt samples. Unfortunately, he evaluating the APA's suitability for predicting moisture sus- could not obtain the surface energies for a number of sam- ceptible mixtures. Kandhal et al. (95) found that the MB test ples with his technique because of errors in the contact angle is the fine aggregate test that is best related to stripping of measurements. Li (103) measured the surface energies of a HMA (compared with sand equivalent test and PI) and then variety of European aggregates. Cheng et al. (104) measured recommended the MB test to be used to indicate the presence surface energies of some widely used aggregates and asphalt of detrimental plastic fines, which may induce stripping in binders in the southern United States using the Universal HMA mixtures. Sorption Device (USD) and Wilhelmy Plate method, respec- Aschenbrener and Zamora (99) evaluated several special- tively. Later, Cheng et al. (105) developed the adhesion failure ized aggregate tests and their relation to HMA performance. model. Comparison between mechanical test results (repeated The tests include MB, Rigden voids index, stiffening power, load permanent deformation tests) and the adhesion failure and dust coating on aggregates. The evaluation was con- model showed the same trends of moisture damage potential ducted using aggregate sources from 20 projects with known for the aggregates and asphalts evaluated. Although the surface energy theory is not new, methods to evaluate moisture dam- field performance. It has been found that the MB, dust coat- age potential based on this theory and testing protocols need ing on aggregates, and Rigden voids index, or stiffening additional study. power, when used with one another, accurately identified aggregate problems in the stripping pavements. The study indicated that the MB test, the other aggregate tests, or both 2.6.6 Net Adsorption Test can be used to isolate the potential problematic components of the HMA if an HMA fails a performance-related test, such The net adsorption test (NAT) was applied to the HMA as the Hamburg wheel-tracking device. industry by the SHRP A-003B contractor to predict moisture The MB test was also used by Harders and Noesler (100) damage (stripping) in asphalt-aggregate mixes (106). It was to address different surface activities (i.e., surface energies). developed to determine the adsorptive nature and the water