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State of California, Department of Transportation - page 33 / 52





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included) cross the weighbridge, an electronic load cell sends a signal to the weigh belt controller. This electronic signal is meaningless by itself. A second electronic device is also used so that the weigh belt speed can be determined. A belt speed sensor is attached to the tail pulley or the gravity take-up pulley. With the combined signals, the weigh belt controller can calculate the TPH of wet aggregate crossing the weigh bridge.

If aggregates being carried on the belt are relatively dry, all the aggregate that passes over the weighbridge will enter the drum. If the moisture content of the aggregate is high, some of the fine aggregate may stick to the belt. This material will not be fed to the drum, but will remain on the belt. The fine aggregate will accumulate on the belt and be counted by the weighbridge, sending a “false” signal to the computer to increase the asphalt delivery. A properly adjusted belt wiper will prevent material accumulation on the belt.

The AC plant inspector should be aware of the potential problems that may effect this type of continuous weighing system. The following are some examples of things to watch for:

  • 1)

    Settlement of the concrete footings after the CT 109.

  • 2)

    Windy conditions without weighbelt shielding.

  • 3)

    Belt riding to one side of the weighbridge.

  • 4)

    Aggregate wedged between the weighbridge and the conveyor frame.

  • 5)

    Weighbelt slipping on tail pulley.

  • 6)

    Belt zero-check not being performed each morning.

  • 7)

    Weighbelt not making complete contact with the weighbridge idler.

  • 8)

    Load cell replaced and not calibrated (CT 109) by Caltrans.

  • 9)

    Weighbelt repaired, shortened, lengthened, or replaced and not calibrated (CT 109)

by Caltrans.

Typically, if any of the above items occur, the amount of aggregate being fed to the plant will be different than that indicated by the weigh belt controller. The completed mix will also have an asphalt content that is different than that specified by the plant controller.

Aggregate Sampling The asphalt plant inspector is responsible for taking a “representative aggregate sample.” Aggregate samplers for continuous mix plants are typically located at the beginning or at the end of

the weigh belt conveyor.

Samplers come

in many automatic

different styles, some are and are operated from ground








This sampler passes through the entire stream of aggregate at the end of the weighbelt and delivers the sample to ground level.

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