31. Mix Design Flashcards
(19 cards)
Hot mix design
To develop an economical blend of aggregates and asphalt
Should be designed to withstand heavy traffic loads
Components
Asphalt binder, mineral aggregate, additives, air
Common requirements
Sufficient asphalt for durable pavement
Sufficient stability under traffic loads
Sufficient air voids (3-5%)
Sufficient workability
Why doesn’t all asphalt cement (AC) end up coating the aggregate particles?
Some is absorbed into the water permeable voids within the particles
Air voids, upper limit
Prevents excessive environmental damage
Air voids, lower limit
Allows room for initial densification due to traffic
Marshall mix design
Developed in the late 30s, most widely used, methods are empirical
Consists of 6 steps
Aggregate Selection
Strong, tough, angular, low porosity, clean, rough, hydrophobic
Asphalt binder evaluation
No specific method, superpave PG is most common
Sample preperation
Use any suitable method to find optimum asphalt content
Then two above, two below, and one at that content are sampled and compacted
Density and voids calculations
Determine bulk S.G. of compacted HMA and max S.G. of loose mix in lab
Calculate air voids, effective S.G., VMA, and VFA
Bulk S.G. (Gmb)
Gmb = W(air)/(WSSD(air) - W(water))
Max S.G. Loose mix (Gmm)
Gmm = W(air)/(W(air) - W(water))
VMA
Voids in mineral aggregates
Space occupied by asphalt cement and air voids
VMA = vol. of air voids + vol. effective asphalt
Selecting optimum asphalt binder content
Plot density, Marshall stability, air voids, VMA, and flow vs asphalt content
Average A from 4% air voids, max stability, and density then check using two other graphs
What compactor simulates compaction the best?
Gyratory compactor (GC)
VFA
Voids filled with asphalt
What is the compaction temperature based on?
Viscosity-temperature relationship
Asphalt binder content vs VMA
Percent VMA should decrease with increasing A, reach a minimum, and then increase
