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This research program was based on laboratory and field studies. All work was limited to the use of a 10-mesh crumb rubber in the dry process. An evaluation of the asphalt-rubber interactions indicated that there should be a reasonable level of interaction between the crumb rubber and the asphalt cement selected for the project. A suggested criterion for defining an acceptable level of interaction would be to establish a minimum viscosity of 15 Poise (Brookfield viscosity) for a neat asphalt cement modified with 20 percent crumb rubber. When designing a crumb rubber modified mixture, the aggregate gradation should be substantially gapped. The target gradation used in the construction of the Babbit, Minnesota test sections should be considered as a guideline for an acceptable gapped gradation. Stockpile gradations should be adjusted for crumb rubber gradations volumetrically; generally, 1 gram of crumb rubber occupies the same volume as 3 grams of aggregate for a given sieve size. The optimum asphalt content for CRM mixtures should be based on air voids from 1.5 to 3 percent. During construction, the crumb rubber (supplied in 50 Ibs. bags) should be added through the recycled asphalt (RAP) hopper (drum plant) or directly into the pug mill (batch plant). Laydown and compaction procedures should proceed as usual. Use of vibration on the rollers should be used at the discretion of the field engineer.

The durability of selected asphalt concrete components and mixtures from six projects were evaluated with the newly recommended net adsorption test and the more common ASTM D4867 (a modified Lottman), respectively. The net adsorption test was used to assess the durability of the adhesion of the asphalt to the aggregate surface in the presence of water. The ASTM D4867 method evaluated the retained strengths of compacted mixture resistance after freeze/thaw conditioning. The net adsorption results indicated at least two of the six projects could be susceptible to moisture related adhesion problems. The test method was adjusted to use the full aggregate gradation rather than only the fine fraction. A comparison of these results to those originally reported by Strategic Highway Research Program (SHRP) researchers showed good agreement in both trends of results and within laboratory test method variability (about 0.14 mg/g). The mixture testing indicated that three of the six projects had tensile strength ratios of less than 70 percent and could be expected to show some evidence of moisture related damage. An evaluation of these results suggested that any moisture sensitivity could be due to mix design parameters such as lower film thicknesses, and lower initial strengths rather than a chemical loss of adhesion at the asphalt-aggregate interface. The influence of conventional mix design parameters on the moisture sensitivity of asphalt concrete mixtures should be more thoroughly investigated prior to any consideration of the use of additives.