Atypical rubber scrap from tire tread buflings has the following composition:
Screen size: Passing, percent by weight 4 100 A typical mat scrap ground to A? on a Cumberland grinder has the following screen analysis:
Screen size Passing, percent by weight 4 100 6 90.6 14 6.0 30 0.4
It should be appreciated that the particle size of this ground rubber scrap or other rubber particles used in this invention should have a range such as that indicated in the screenings above. Thus by using a range of particle size, for instance a 50/50 mixture of ground mat scrap and tire buflings, the percent voids in the rubberized bitumen compositions is decreased and the amount of rubberized bitumen required is reduced. Not only is the amount of rubberized bitumen required reduced but it appears the load carrying ability and the stability of the finished composition is materially increased.
Rubberized bitumen suitable for use in this invention will normally be sufficiently fluid at about 300 to 400 F. to permit the particulate rubber to be incorporated therein. The particulate rubber may be incorporated in the fluid rubberized bitumen in any suitable mixer, such as a Banbury or a pug mill. Normally the ratio of rubberized bitumen to particulate rubber is controlled to give the desired load bearing qualities to the bituminous aggregate compositions. In general about to 50 parts of rubberized bitumen is used for each 100 parts of particulate rubber with the preferred range of rubberized bitumen being from about to par-ts by weight. Also, where the particulated rubber is very active or lively, it is preferred to use a higher ratio of rubberized bitumen containing a high percentage of microscopically dispersed vulcanized rubber which exhibits a network structure.
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