General Information for Hot Mix Asphalt Operations
Two grades of asphalt binders have been historically available and used in Both the Monterey and San Francisco Bay areas, AR 4000 & AR 8000. AR 4000 has been widely used historically. The difference between the two binder grades is the viscosity. AR 8000 grade binder is stiffer and therefore holds up better under hot temperatures. AR 4000 is more flexible under cold temperatures.
Due to the extremely moderate climate of the region, the low temperature range of either binder is easily sufficient. And for a number of reasons, Graniterock has moved to the AR 8000 grade of Asphalt binders this year. The higher stiffness gives more favorable asphalt properties for the applications needed. It reduces Asphalt tenderness. It makes new pavements much harder to scuff and shear in high vertical stress areas such as parking lots and city streets. The most important property of all, however, may be that AR 8000 grade binders have higher tensile strengths and therefore resists the major concern in high traffic volume areas: RUTTING
Regarding type of aggregates used in hot mix paving, the California Department of Transportation (Caltrans) specifies two types of dense graded (well-graded) hot mix; Type A and Type B. Some of the Differences between Types A & B are as follows:
- Type A hot mix requires that at least 90% of the coarse aggregate and 70% of Fine aggregate used contains crushed particles while Type B hot mix only requires 25% and 20% respectively. The angular rock particles in a crushed aggregate results in a higher stability hot mix than those hot mixes containing uncrushed round gravel type aggregates. At Graniterock, all of our hot mix aggregates are 100% crushed and thus qualify as for producing Type A asphalt hot mix.
- Type A has a Los Angeles Rattles Specification of 10% material breakdown at 100 revolutions and 45% at 500 revolutions. The Type B Specification has only a Specification for 50% breakdown at 500 revolutions. This Measure of durability is essential for pavement stability. At Graniterock, our crushed granite is easily type A.
- Many other indicators of Aggregate and HMA quality are distinguished this way. Type A has a stability specification of 37 vs. 35 for Type B, as well as a moisture vapor susceptibility of 30 vs. 25. In all of these areas Graniterock’s Aggregate is a Type A Aggregate.