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State of California, Department of Transportation - page 28 / 52

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manufacturers rating for pugmill capacity if this type of mixing problem is suspected. For other causes of non-uniform AC mixtures, refer to Appendix “B”, titled “Possible Causes of Asphalt Concrete Mix Deficiencies”.

Note Appendix “A” depicts the correct and incorrect settings for pugmill paddles.

After mixing is completed, the gates are opened and the batch is dumped into a truck or sent to a storage silo if the plant is so equipped.

Storage Silos

The batch of AC travels from the pugmill to the top of the storage silo by belt conveyor and or a slat conveyor. Caltrans specifies, "Storage silos shall be equipped with a surge-batcher sized to hold a minimum of 4,000 lbs. of material. A surge-batcher consists of equipment placed at the top of the storage silo which catches the continuous delivery of the completed mix and changes it to individual batch delivery and prevents the segregation of product ingredients as the completed mix is placed into storage.”

Multiple silos may be used to store different types of asphalt concrete mixtures. The plant inspector must be sure the AC being sent to the project is same as the approved mix design.

The surge-batcher is to be thermally insulated and/or heated. Discharge gates on surge- batchers must be automatic and discharge only after 4,000 lbs. of AC mix has been collected. Discharge gates operated by interval timers are not allowed.

AC storage silos are typically insulated and or heated. Caltrans requires a minimum of 20 tons to be retained in the storage silo during AC production. A device (visual or audible) shall be provided at the batching location, indicating the storage level in each silo is maintained above the 20-ton cut off limit.

Automatic Batch Controllers Batch controllers are a series of electrical circuits or a computer system that

automatically accuracy and

controls the entire plant operation.

The controllers are checked

compliance

during

the

annual

California

Test

109.

Even

though

they

for are

checked on an annual basis, problems may arise AC plant operators have even been known to turn

that effect the overall mix proportions. off a portion of the automatic controls.

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