however, the permissible amount in each bin is usually specified. Daily visual inspection of the screens for cleanliness is  recommended,  preferably  before  the  start  of operation.  When  conditions  warrant,  the  screens  should be  cleaned. NOTE: Always make sure the bolts securing the screens are tight. Hot Bins Hot bins are used to store the heated and screened aggregates  temporarily  in  the  various  sizes  required. Each bin is an individual compartment or a segment of a  large  compartment  divided  by  partitions.  A  properly sized hot-bin installation should be large enough to prevent  running  out  of  material  when  the  mixer  is operating at full capacity. Bin partitions should be tight, free  from  holes,  and  of  sufficient  height  to  prevent intermingling  of  the  different  size  aggregates. Hot bins usually have indicators that tell when the aggregates  fall  below  a  certain  level.  These  indicators may be either electronic or mechanical. Each hot bin should be equipped with an overflow pipe to prevent excess amounts of aggregate from backing up into the other bins. The overfill pipes should be set up to stop overfilling of the bins. When a bin overfills, the screen above the bin rides on the aggregate, resulting in heavy carry-over  and  possible  damage  to  the  screen.  Overflow vents should be checked frequently to ensure they are free  flowing. Sometimes,  very  fine  aggregate  particles  build  up in the bin corners. When this buildup of aggregate collapses, it can result in an excessive amount of fines in the mix. This rush of fine materials normally occurs when the aggregate in the bin is drawn down too low. This condition can be controlled by having fillet plates welded in the bin corners to eliminate the 90-degree angles and by maintaining the proper aggregate level in the bin. Other  potential  obstacles  to  obtaining  a  good  mix includes a shortage of aggregate in one bin or excess in another bin, worn gates (at the bottom of the bins) allow leakage of aggregate, and sweating of the bin walls. These obstacles must be overcome. Bin shortages or excesses can be corrected by adjusting the cold feed. Sweating occurs when moisture vapor in the aggregate and in the air condenses on the bin walls. his usually happens only at the beginning of the day’s operation or when the coarse aggregate is not thoroughly dried. Figure 8-11.-Segration of material in the bot bins. Sweating may accumulate dust that, when released suddenly,  will  add  unwanted  fines  in  the  mix. Hot-Bin Sampling Most modern hot-mix asphalt plants are equipped with devices for sampling hot aggregate from the bins. These devices vary in design but usually serve to divert the flow of aggregate from the feeders, or gates, under the bins into sample containers. On continuous-flow plants, the best place to obtain a sample is from the feeder  gates  as  the  material  is  deposited  onto  the  elevator leading to the pugmill. Sampling facilities must be constructed and located so that the samples obtained will be representative of the material in the bins. From the flow of aggregate over the screens, the finer aggregates fall to the near side of the bins and coarser aggregates fall to the far side (fig. 8-11). When the aggregate is drawn from a bin by opening a gate at the   bottom,   the   flow   of   aggregate   consists predominantly  of  fine  aggregate  at  one  edge  and  coarse aggregate at the other; therefore, the position of the sampling device in the flow of aggregate determines whether the sample will be composed of the fine portion, the coarse portion, or will be an accurate representation of all the aggregate in the bin. This condition is critical in the bin that contains the fine aggregate since the asphalt required in the mix is influenced heavily by the aggregate  from  this  bin. Stratification (vertical layering) of sizes in the fine bin  may  be  caused  by  variation  of  grading  in  the stockpiles or by erratic feeding of the cold aggregate. When this form of segregation exists, representative 8-9