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State of California, Department of Transportation - page 27 / 52





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shall be provided to document the asphalt binder temperatures during production. Caltrans a recording type temperature indicator is used, it shall be maintained in a working condition.

Asphalt Binder Weigh Pots The liquid asphalt is proportioned by weight,

as a percent

of total

dry aggregate.


example, the


content is to

be 5

percent, require 5

100 lbs. of lbs. of liquid

dry aggregate would asphalt, and so forth.

The weigh pot is filled with liquid asphalt while the aggregate hot bins are discharging into the weigh bin. The asphalt binder shall be introduced uniformly into the mixer along

the center of the mixer parallel to the shafts, or by pressure spraying.

mixer The

beginning weight

of the

weigh pot

should be

checked regularly to verify are draining completely.

all the contents Asphalt binder

temperature is to be maintained between 250°F and 375°F when added to the aggregate.

Pugmill (Mixing Chamber) This portion of the plant uses mixing paddles attached to two horizontal shafts that rotate in opposite directions to blend aggregates and asphalt. The Aggregate is first discharged from the weigh hopper into the pugmill and is mixed for about 2 seconds (dry mix time). While the aggregate is still being discharged from the weigh hopper, liquid asphalt is either gravity fed or pumped from the weigh pot to the center of the mixing chamber. After all aggregates and the asphalt binder have been discharged into the pugmill, the controller automatically starts a new weighing cycle with aggregate and asphalt.

At the end of the dry mix time, the aggregate and asphalt are mixed together for a minimum of 30 seconds, to "form a homogeneous mixture of uniformly distributed and properly coated aggregates of unchanging appearance" (wet mix time). Caltrans requires an interval timer for the control of the total mix time. Also, the interval timer must be interlocked so that the mixer can not be discharged until the required mix time has elapsed.

If pug mill paddle tips are worn, wet mix time may need to be increased slightly. Typically, an increase of 3 seconds will produce a well-coated mixture. If more than 3 seconds is required, examine paddle tips for excessive wear or even missing paddles. Clearance between paddle tips and the pugmill liner, exceeding one-half the maximum aggregate size, are considered excessive wear. Non-uniform AC mixtures may also occur when the pugmill is over or under filled. The AC plant inspector should refer to the

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