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State of California, Department of Transportation - page 37 / 52

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metered. Baghouse dust is normally blown or augured back into the mixer, usually at the asphalt add point. Asphalt Binder Storage Tanks Asphalt binder is typically stored in heated and insulated tanks. The storage tanks must be calibrated at 100 gallon intervals (charts for converting inches of asphalt to gallons of asphalt are available at the plant), and are to be accessible at any time for measuring (stabbing) the volume of asphalt remaining in the tank.

Caltrans specifications require the discharge end of the asphalt binder circulating pipe be maintained below the surface of the asphalt binder in the storage tank. If hot asphalt binder is discharged into open air, the liquid asphalt will oxidize, thus changing the original composition and viscosity. When hot asphalt binder is discharged into open air, air bubbles are introduced into the liquid asphalt. Asphalt meters other than mass flow coriolis type can not differentiate between liquid asphalt and air bubbles causing the asphalt meter to send a false signal to the controller. One obvious problem that occurs when proportioning liquid asphalt containing air bubbles to aggregates is a dry looking mixture.

A ½” to ¾” diameter asphalt binder sample valve, located in the feed line to the plant, is used to take a one quart sample of asphalt binder. The valve is to be located 24” to 30” above the standing

platform and Housekeeping

shielded from hot piping.

in

this

area

is

very

important, spilled asphalt should cleaned up as soon as possible. Drainage receptacle must be provided

be A for

flushing

the

valve,

with

a

minimum

of

one

gallon

of liquid

binder

samples.

asphalt,

before

taking

the

The binder storage tank shall be equipped with a device that will automatically shut down the plant when the level of asphalt binder is lowered to the point that the pump suction line is exposed.

A temperature-sensing device, accurate to 10 F shall be installed in the asphalt feed

line.

A

temperature

indicating

device

(LED, digital readout, etc.) located in the control room.

shall

be

The requirements for a Caltrans QC/QA project specify that an automatic continuous recording device be provided

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