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State of California, Department of Transportation - page 34 / 52





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be pushed through the aggregate flow by the plant inspector. During the CT 109, a sample of aggregate taken with the sampler is compared with a sample taken from a belt cross section. The two gradings are compared, if the amount passing each sieve size is within 2 percent when compared directly, the sampler will be approved for use. If an aggregate sampler is modified at a future time, this comparison grading must be done again before it may be used.

The following items will help the plant inspector to obtain representative samples:

  • 1)

    The sampling device must be wider than the flow of aggregate being sampled.

  • 2)

    The sampling device must be sized large enough to accommodate a full cross section of the aggregate stream and not over flow.

  • 3)

    Aggregate samplers must pass through the aggregate stream at a “constant speed”.

  • 4)

    If the AC plant inspector is not familiar with a sampling device, the plant operator should be asked for assistance when sampling aggregates.

Aggregate Drying And Heating Aggregates are delivered by a series of belts (collection, weigh and slinger) to the drum for heating and drying. The continuous mix dryer drum is a revolving cylinder up to 10 feet in diameter and up to 60 feet in length. The following drum configurations are used for a continuous mixing AC plant:

  • 1)

    Parallel Flow Drum: Heated air, aggregate, completed mix and exhaust gases travel in the same direction, all in a downhill direction.

  • 2)

    Counter Flow Drum: Heated air and exhaust gases travel in one direction, while the heated aggregate and completed mix travel in the other direction.

  • 3)

    Double Drum: this design utilizes a drum within a drum, where heated air, aggregate and exhaust gases pass through the inside drum. Heated aggregate passes into the outer drum where liquid asphalt is injected and mixed together in the outside drum.

  • 4)

    Parallel/Counter Flow Drum with Coater: Heated air, aggregate and exhaust gases pass through the drum. The heated aggregate is discharged into a pugmill with the metered asphalt binder and mixed together.

Fuel used to heat the aggregate may include LPG, propane, natural gas, diesel, heavy fuel oil or reclaimed oil. A blower fan is used to supply air for combustion of the fuel, and an exhaust

fan is

used to



create a draft through the inside of the drum is

equipped channels,

with longitudinal troughs or called flights, that lift the

aggregate and the hot gases. drum, rotation

drop it in veils through The slope of the dryer speed, diameter, length

Burner/Blower assembly at discharge end of drum.

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