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41 sensitivity of a wide range of typical paving-quality aggre- formance of aggregate prisms pressed into a pot of bitumen gates. This relatively fast and simple test was developed to (109). This test uses expensive test equipment and involves provide a rapid, simple, quantitative measure of the amount elaborate test sample preparation. The testing procedure con- of asphalt adhered to aggregate after exposure to water. It sists of fixing a rectangular aggregate prism into the upper was used to evaluate the affinity of asphalt for aggregate and jaw of an Instron apparatus, pressing the prism into a layer of to determine the water sensitivity of a given asphalt-aggregate bitumen held in a container or pot, and extracting the prism pair. The test is composed of three parts. First, asphalt is from the bitumen under controlled conditions. The INAPOT flowed over and adsorbed onto aggregate from a toluene solu- requires that aggregate prisms be cut from lump rock sam- tion using a recirculating column. The adsorption step is ples representative of the aggregate being assessed because allowed to run for 7 h. Second, a small amount of water is of the inability of the Instron jaws to grasp individual aggre- introduced into the toluene solution, and the adsorbed asphalt gate particles. Instead, prisms 18 mm 10 mm 30 mm were that is sensitive to the presence of water is desorbed from the cut; the face to be assessed was left as a natural uncut surface. aggregate. Third, the amount of asphalt remaining on the Woodward's study (109) showed that the INAPOT was able aggregate after the introduction of water is determined. This to quantify the variation in tensile adhesion characteristics amount is termed "net adsorption"; it gives a measure of the for three individual constituents from the same greywacke "affinity" of the asphalt for the aggregate by water and serves quarry. The results agreed with in-service experience that the as an indicator of the water sensitivity of the pair. The aggre- poor initial coating, adhesion, and premature stripping aggre- gate properties predominated in the test, showing a stronger gate gave the worst INAPOT results. His study also indicated influence than the asphalt on the initial amount of asphalt that the influence on adhesion of different rock types was adsorbed, on the amount of asphalt desorbed by water, and much less than that of temperature. on the amount of asphalt remaining--the net adsorption. The French Vialit Plate test was originally developed in the After the NAT was proposed, it was validated using both early 1960s to simulate conditions experienced on-site with laboratory and field data by Hicks et al. (107) and Terrel et al. the use of chippings applied as surface dressing. Through his (108). Both research teams used two accelerating rutting study, Woodward (109) concluded that this method offers tests: Oregon State University (OSU) wheel tracker and SWK/ the engineer a quick and simple means of predicting the in- UN (SWK Pavement Engineering in Nottingham, UK) wheel service performance of aggregate used in surface courses tracker to evaluate water sensitivity in the validation of the under a wide range of simulated in-service conditions. NAT. The prediction of water sensitivity of the binder as pro- posed by the SHRP A-003B NAT shows little or no correla- tion to either these two wheel tracking tests or to the SHRP 2.6.8 Summary of Aggregate Tests Related to Moisture Damage A-002 predications for permanent deformation. As a conclu- sion, the authors suggested that the NAT is a poor indicator This section gives the state of practice for test procedures of the moisture sensitivity of the binder. used to evaluate aggregate moisture damage potential. Many Woodward's research in his Ph.D. dissertation (109) showed factors influence moisture damage: HMA characteristics the ability of NAT to rank aggregate-asphalt pairings. He (aggregate, asphalt binder, and type of mixture); weather dur- stated that the NAT test was able to highlight how optimum ing construction; environmental effects after construction; levels could be achieved in terms of predicating performance; and pavement surface drainage. Aggregate tests related to however, this result has not been validated by a wide range moisture damage generally fall into two categories: tests to of aggregate resources. identify clay-like fines and tests that evaluate the surface prop- erties of the aggregate related to the adhesion of the binder to 2.6.7 Other Aggregate Tests Related the aggregate. to Moisture Damage The Superpave method currently specifies the Sand Equiv- alent Test (AASHTO T176) to identify clay-like fines. Con- In addition to the NAT, the SHRP A-003B researchers troversial results and findings exist for the sand equivalent developed two "specialty" tests (106). The limestone reactiv- test, PI, NAT, and other tests. In some cases, the sand equiv- ity test is a quick and reliable method for determining the alent test identifies crusher fines as harmful clay-like parti- amount of active sites present on the aggregate surface. It can cles. It appears that the MB test may be the best method to be used to differentiate among limestone sources. Another quantify the amount of harmful clays in fine aggregate. "specialty" test assesses the reactivity of the asphalt-aggregate The NAT was developed during SHRP to evaluate the systems to the addition of anti-stripping agents. interaction between the asphalt binder and aggregate in the Woodward (109) used the Vialit Plate Test and the Instron presence of water; however, validation work conducted as Adhesion Pull-Off Test (INAPOT) along with the NET to part of SHRP indicated a poor predictive ability for the test, predict the adhesion property in the laboratory. The INAPOT and it has not been widely used since. At the present time, the was developed as a method to quantify the effect of steady surface energy techniques appear to be promising. The pro- load and temperature conditions on the adhesive bond per- cedures are relatively new. Results and efforts from NCHRP