Figure 8-9.-Gradation control unit.transferred to a storage silo. This material is often usedin the hot mix.When the material removed from the dust collectorcan be recombined satisfactorily with the aggregates inthe mix, some or all of it may be returned to the plant.The amount returned depends upon the combinedgrading of the finished mix. When the collected dust isunsatisfactory or is prohibited by the mix specifications,it is removed from the bottom of the collector andwasted.Hot ScreensAfter the aggregates have been heated and dried,they are carried by a hot elevator to the gradation unit,In the gradation unit, the hot aggregate passes over aFigure 8-10.-Screen wear.series of screens. The function of the screens is toseparate the hot aggregate into the specified sizesaccurately and deposit those sizes in hot bins.The gradation control unit (fig. 8-9) or screeningunit includes a set of several different-sized vibratingscreens. The top screen is a scalping screen that rejectsand carries off oversized aggregates. This is followed byone or two intermediate-sized screens, decreasing insize from the top to bottom. The very bottom screen isnormally a sand screen. The effective screening areamust be large enough to handle the maximum amountof feed delivered to separate the hot aggregates properly;therefore, the capacity of the screens should be checkedagainst the capacity of the dryer and the capacity of thepugmill.When too much material is fed to the screens or thescreen openings are plugged, many particles that shouldpass through ride over the screen and drop into a bindesignated for larger sized aggregate. When screens areworn (fig. 8-10) or torn, resulting in enlarged openingsand holes, oversized material will go into bins intendedfor smaller sized aggregate. Fine aggregate misdirectedinto bins intended for larger aggregate is know as“carry-over.”Carry-over can cause a lack of uniformity in theaggregate gradation and in the mixture. Additionally,excessive carry-over adds to the amount of fineaggregate in the total mix, thus increasing the surfacearea to be coated with asphalt. Excessive carry-over, orits fluctuations, can be detected by a sieve analyses madefrom the contents of the individual hot bins and must becorrected immediately. Corrective measures include thecleaning of screens, the regulation of the quantity ofmaterial coming from the cold feed, or a combination ofboth. Some carry-over is permitted in normal screening;8-8
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