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State of California, Department of Transportation - page 9 / 52





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Totalizers Asphalt meters and aggregate belt scales shall be equipped with resettable totalizers. The totalizers are used to determine the total amount of asphalt and aggregate introduced into the asphalt concrete mixture. Some of the totalizers used to determine aggregate quantities include moisture, a quick calculation performed by the plant inspector results in dry aggregate totals. Some computerized plant controllers have asphalt and aggregate totalizers available within the computers operational software, ask the plant operator for assistance with this type of totalizer.

Drying - Aggregate is fed directly to a drier or a drier-drum mixer at a uniform rate, then heated by a burner system using various types of fuel. The aggregate feed rates may need to be decreased if the temperature of material leaving the drum is not adequately heated. Complete combustion of all burner fuel is required by Caltrans.

Moisture Correction - Continuous mixing AC plants must have aggregate moisture determined on a regular basis (every couple of hours) to correctly proportion asphalt to the aggregate. The moisture correction may apply to each cold feed bin, in this case, each bin must be sampled and percent moisture determined separately. The moisture correction may also apply to the combined aggregates, in this case, a combined aggregate sample is taken and the percent moisture is determined on the mixed sample.

Hot Elevator (Hot Stone/Bucket Elevator) Batch type plants use this device to carry the hot aggregate discharged from the drier drum up to the screen deck. Small troughs are continually passing through the flow of aggregate and transport it up to the top of the batch plant tower.

Drum Flights - Depending on the type of asphalt plant, the drier drum may have one or more of the following types of flights. Lifting flights are in all drier drums, they are designed to pass through the aggregate on the bottom of the drum and lift a portion of the aggregate. As the drier drum rotates, gravity takes over and causes the aggregate to fall through the exhausting gases. Farther down the drier drum mixer, mixing flights are installed to do just that, mix aggregate and asphalt binder together. At the rear of the drier drum, discharge flights do just that, discharge the hot aggregate or asphalt concrete mixture into a chute for further processing or storage in a silo.

Screen Deck - At the top of the batch plant tower a set of vibrating screens (screen deck) is used to separate the dried aggregates into different sizes. Typically a four-screen deck is used at most AC batch plant facilities. The top screen is usually the scalping screen, removing oversized materials and sending it to the overflow chute. The remaining screens would separate the aggregate into four different sizes. Screens openings may be slotted, rectangular or square depending on the gradation required.

Overflow Chutes Each hot bin is equipped with an overflow chute. The chute prevents overflow from one hot bin to another. The plant inspector should be aware of the size of aggregates being discharged from the overflow chute. If an excessive amount of 9.5mm x 12.5mm aggregate is being discharged from the chute, the cold feed bin containing that

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