Figure 8-17.-Basic drum-mix asphalt plantMixing TimeTotal mixing time begins when allthe combined mineral aggregates are in themixer and ends when the mixer dischargegate is opened. Dry mixing time, whenspecified, begins when all the combinedmineral aggregates are in the mixer andends with the introduction of the asphalt.Wet mixing time begins with the start ofthe asphalt application and ends with theopening of the mixer discharge gate.The asphalt film on aggregate ishardened by exposure to air and heat;therefore, mixing time should be theshortest time required to obtain a uniformdistribution of aggregate sizes and auniform coating of asphalt on allaggregate particles. The speed of themixer shafts and the arrangement and pitchof the paddles are factors governing theefficiency of the mixing.To aid further the operation of acontinuous mix plant, you can add orextend several automatic controls.These include the following:1. Automatic burner controls2. Automatic mix discharge3. Automatic mixer and gradationcutoffs in case of hot-bin shortage orimproper feed4. Electric interlocks that shutdown the complete plant in case of afailure anywhere in the electric systemThe operator’s manual for theparticular plant being used gives detailson the setup and adjustments of theautomatic equipment included with theplant.NOTE:Normally all automaticsystems have manual override. You shouldknow where it is located and how to useit. See the manufacturer’s manual forspecific details.DRUM-MIX ASPHALT PLANTThe drum-mix plant is shown infigure 8-3. The mixing drum for which theplant is named is very similar inappearance to the batch plant dryer drum.The difference between the two is that ina drum-mix plant the aggregate is not onlydried and heated within the drum, but itis also mixed with the asphalt cement. Ina drum-mix plant, there are no gradationscreens, hot bins, weigh-hoppers orpugmills. Aggregate gradation iscontrolled at the cold feed.The basic plant consists of a cold-feed system, a rotating drum dryer, anasphalt proportioning and dispensingsystem, and a surge silo (fig. 8- 17). Theease of setup and operation of the drum-mix plant makes it the ideal machine foroperations.Aggregate Storage and FeedAggregate gradation and uniformityare entirely dependent on the cold-feedsystem. Proper care must be exercised notonly in producing the aggregate but alsoin storage. Aggregates used for drum-mixplants must be received, handled, andstored to ensure there is no danger ofcontaminationor intermingling.Stockpiles must be properly gradedand split into different sized fractionsto control the gradation of the mixproperly. Uncorrected segregatedstockpiles will result in mix gradationdifficulties. The plant supervisor shouldestablish and maintain stockpiles in themost economical manner and correct anydeficiencies in uniformity before theaggregate is fed into the mixing plant.Since the typical drum-mix plantdoes not have a gradation unit, theaggregate must be proportioned beforeentering the mixing drum. This isaccomplished with a multiple-bin cold-feedsystem equipped with precision beltfeeders for control of each aggregate.8-16
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