however, the permissible amount in each bin is usuallyspecified.Daily visual inspection of the screens for cleanlinessis recommended, preferably before the start ofoperation. When conditions warrant, the screens shouldbe cleaned.NOTE: Always make sure the bolts securing thescreens are tight.Hot BinsHot bins are used to store the heated and screenedaggregates temporarily in the various sizes required.Each bin is an individual compartment or a segment ofa large compartment divided by partitions. A properlysized hot-bin installation should be large enough toprevent running out of material when the mixer isoperating at full capacity. Bin partitions should be tight,free from holes, and of sufficient height to preventintermingling of the different size aggregates.Hot bins usually have indicators that tell when theaggregates fall below a certain level. These indicatorsmay be either electronic or mechanical. Each hot binshould be equipped with an overflow pipe to preventexcess amounts of aggregate from backing up into theother bins. The overfill pipes should be set up to stopoverfilling of the bins. When a bin overfills, the screenabove the bin rides on the aggregate, resulting in heavycarry-over and possible damage to the screen. Overflowvents should be checked frequently to ensure they arefree flowing.Sometimes, very fine aggregate particles build upin the bin corners. When this buildup of aggregatecollapses, it can result in an excessive amount of finesin the mix. This rush of fine materials normally occurswhen the aggregate in the bin is drawn down too low.This condition can be controlled by having fillet plateswelded in the bin corners to eliminate the 90-degreeangles and by maintaining the proper aggregate level inthe bin.Other potential obstacles to obtaining a good mixincludes a shortage of aggregate in one bin or excess inanother bin, worn gates (at the bottom of the bins) allowleakage of aggregate, and sweating of the bin walls.These obstacles must be overcome. Bin shortages orexcesses can be corrected by adjusting the cold feed.Sweating occurs when moisture vapor in the aggregateand in the air condenses on the bin walls. his usuallyhappens only at the beginning of the day’s operation orwhen the coarse aggregate is not thoroughly dried.Figure 8-11.-Segration of material in the bot bins.Sweating may accumulate dust that, when releasedsuddenly, will add unwanted fines in the mix.Hot-Bin SamplingMost modern hot-mix asphalt plants are equippedwith devices for sampling hot aggregate from the bins.These devices vary in design but usually serve to divertthe flow of aggregate from the feeders, or gates, underthe bins into sample containers. On continuous-flowplants, the best place to obtain a sample is from thefeeder gates as the material is deposited onto the elevatorleading to the pugmill. Sampling facilities must beconstructed and located so that the samples obtained willbe representative of the material in the bins.From the flow of aggregate over the screens, thefiner aggregates fall to the near side of the bins andcoarser aggregates fall to the far side (fig. 8-11). Whenthe aggregate is drawn from a bin by opening a gate atthe bottom, the flow of aggregate consistspredominantly of fine aggregate at one edge and coarseaggregate at the other; therefore, the position of thesampling device in the flow of aggregate determineswhether the sample will be composed of the fine portion,the coarse portion, or will be an accurate representationof all the aggregate in the bin. This condition is criticalin the bin that contains the fine aggregate since theasphalt required in the mix is influenced heavily by theaggregate from this bin.Stratification (vertical layering) of sizes in the finebin may be caused by variation of grading in thestockpiles or by erratic feeding of the cold aggregate.When this form of segregation exists, representative8-9
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