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.
Newcomb, Dave; Stroup-Gardiner, Mary; Kim, Jong Ryeol; Allen, Bruce; Wattenhoffer-Spry, Jeff.
Polymerized crumb rubber modified mixtures in Minnesota.
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