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Areas of the country which are not fortunate enough to have ample accessible supplies of good quarry stone rely on diminishing gravel deposits for the bulk of their construction aggregates. This paper intends to show how producers in those areas which have strict minimum fracture requirements for asphalt material can utilize a larger portion of the natural supplies.
Wear course asphalt specifications often require 80 to 100 percent of the particles in the asphalt mix which are retained on the #4 mesh to have at least one fractured face.
Field experience has shown consistently that centrifugal impact crushing of natural aggregates that are already within this size range achieves fracture counts of 80 to 95 percent, often in a single pass. The operator can then utilize this material and reduce or eliminate the need to separate, handle, and stockpile a waste material.
A centrifugal impactor can accomplish this because it does not depend on a “closed side setting” to achieve crushing as do compression units, so virtually every particle entering the machine has an equal chance of fracturing, even if it is already product size.
On the other hand, the vertical shaft impactor has the ability to minimize the loss to fines (minus #10 mesh) often a problem with conventional horizontal shaft impactors. This is a direct result of the crusher's geometrical design and its control of the trajectory of the rock. The design minimizes scuffing on the rock and incorporates no grinding path, consequently allowing the option of reducing production of fines.
The requirement for fractured faces on asphalt aggregate is an important parameter to assure proper interlocking of particles and stability of the resulting asphalt mix. But with the proper equipment the operator can meet this requirement, waste less material, and conserve more of the often scarce high-quality aggregates.
aggregates, concrete, cement, fracture (materials), gravel
Application engineer, Spokane Crusher Mfg. Co., Spokane, Wash.