across the cross section of the drum. The aggregatesmust not only rotate with the revolving motion of thedrum but must also spread out sufficiently to makeheating and drying of all particles quick and efficient.The spiral flights are located at the charging(burner) end of the drum-mixer and direct wetaggregates into the drum in such a manner as to attainuniform drum loading. Tapered lifting flights pick up theaggregates and drop them in an even veil through theburner flame.Burner OperationThe burner inside the drum-mixer provides the heatnecessary to heat and dry the aggregates used in the finalmixture. The burner provides this heat by burning fueloil, gas, or both.When oil is burned, a low-pressure air draft is usedto atomize the fuel oil for burning. Depending on thetype of fuel used for the burners, the fuel feed and airblower must be balanced to ensure that the properproportions of fuel and air are being introduced into theburner to ensure efficient combustion. Lack of balancecan lead to incomplete burning of the fuel. Especially,when fuel oil or diesel fuel is used, this can leave an oilycoating on the aggregate particles. An imbalancebetween the fuel feed and air flow can be adjusted byeither decreasing the fuel rate or increasing the bloweror draft air.Surge SiloThe drum-mix plant produces a continuous flow offresh asphalt hot mix and has a surge silo for temporarystorage of the hot-mix material. The surge silo is alsoused for controlled loading of trucks. A weight systemis normally connected to the holding bin of the silo tomonitor the amount of hot mix loaded into each truck.Weight measurements are normally recorded by theweight system control panel.BITUMINOUS SURFACINGMATERIALSBituminous materials are tremendously importantin the construction of roads and airfields for bothmilitary and civilian use. A basic knowledge of thesebituminous materials, their origin, composition, types,and grades are essential for an understanding of their usein construction.Bituminous surfaces are composed of compactedaggregate and bitumen (binder). The aggregatetransmits the load from the surface to the subgrade, takesthe abrasive wear of the traffic, and provides a nonskidsurface. The binder binds the aggregate together, thuspreventing the displacement and loss of the aggregate.The binder also provides a waterproof cover for the basethat keeps surface water from seeping into andweakening the material.Bituminous surfaces are particularly adaptable tostage construction. Additional courses can be added toexisting pavements to provide further reinforcementwhen loads or traffic density increases. The flexibilityof bituminous surfaces permits slight adjustment causedby settlement of the subgrade without detrimental effect.Properly designed bituminous wearing surfaces, whencompared with concrete, are less affected bytemperature strains. The surfaces resist wear,weathering, and deterioration from aging with onlyminimal maintenance.Bituminous materials are highly versatile and serveadmirably in temporary, expedient, and light trafficsituations where concrete is not justifiable. It is equallytrue that thicker bituminous pavement designed forheavy and continuing duty is fully comparable toconcrete designed for heavy volumes of traffic or heavywheel loads; however, bituminous wearing surfaceslack appreciable bearing action to carry wheel loadsover weak spots in the subbase. For this reason, thesubgrade must have an adequate, uniform bearingstrength and the base course must have adequatethickness, bearing capacity, and cohesion.TYPES AND GRADES OF ASPHALTAsphalt is a natural or man-made by-product ofpetroleum distillation. Natural asphalt is found in natureas either lake (or pit) asphalt or rock asphalt. Thecommon bituminous surfacing materials are asphaltcements, asphalt cutbacks, asphalt emulsions, road tars,and road tar cutbacks. For identification purposes, thesematerials are divided into three classes:asphaltbitumens, emulsions, and tars. The classification ofthese materials is based on the extent to which theydissolve in a distillate of petroleum or coal. Asphaltcements and asphalt cutbacks are asphalt bitumens (orasphalts). Road tars and road tar cutbacks are tars.Asphalt CementsAsphalt cements are solid products of petroleumrefining (fig. 8-19). An asphalt cement is designated bythe letter symbol AC, followed by the penetration gradethat represents its relative hardness. The numbers range8-18
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