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State of California, Department of Transportation - page 24 / 52

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Excessive carry-over will add to the amount of fine aggregate in the mix. The No. 2 bin is the critical bin for carry-over. This bin will receive the finest aggregate in a carry-over situation and will affect the demand for asphalt the most. Typically, the carry-over into the No. 2 bin should not exceed 10 %. To quickly check the amount of carry-over, run a sample of the No. 2 bin material over a No. 8 sieve to determine the percentage passing.

Aggregate hot bins temporarily store the screened aggregates in various sizes required for the type of mix being made. The partitions dividing hot bins must be free of holes and high enough to prevent intermingling of aggregates. Each bin shall be equipped with an overflow pipe to prevent excess aggregates from backing up and overflowing into adjacent

bins. large

When bin overflow does occur, amounts of aggregate will be

discharged from

plant

inspector

the reject chute.

should

be

able

The to

This is a cross section of a typical batch plant screen deck as it separates the aggregate sizes.

determine the aggregate size discharged from the reject chute. information should be passed on plant operator who then can

being This to the make

adjustments to cold feed bin

reduce the feed rate of the containing that aggregate

fraction. Overflow pipes should be checked frequently for obstructions. Gates at the bottom of hot bins should be checked regularly for proper operation and leaks. Aggregate leaking from a bin will adversely affect gradation of the final mix.

Segregation of coarse and fine aggregate occurs when a bin is allowed to run low, hot bins should never be allowed to run empty. Bin shortages or excesses are corrected by adjusting the cold feed rates. For example, if the fine bin is overflowing and the coarse bin is running empty, the speed of the cold feed supplying the fine aggregate should be reduced while the speed of the cold feed supplying the coarse aggregate should be increased.

AC plant inspectors must know the aggregate gradings for each of the individual hot bins. Then, when a bin percentage change is required because the gradings are out of tolerance, calculations may be made with the proposed bin percentages to determine if the new combined grading is going to bring the grading back into specifications.

Aggregate Sampling At Hot Bins The asphalt plant inspector is responsible for taking a “representative aggregate sample” from each hot bin. From the flow of material over the screen deck, fine aggregates will

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