Presentation on theme: "Hot Mix Asphalt Production and Placement"— Presentation transcript:
1 Hot Mix Asphalt Production and Placement Dr. TALEB M. AL-ROUSAN
2 Central Mixing PlantsCentral Mixing Plant: Plant or factory at which the bituminous paving mixture is produced, in a process beginning with the aggregates and bituminous materials and ending with the discharge of the mixture into hauling units for transportation to the job site.Preparation of the paving mixture at a central plant offers the advantages of:More careful proportioning of the ingredients.More uniform and thorough mixing with consequent production of more uniform mixtures.Less dependence on favorable weather conditions.Use of more viscous bituminous materials.
3 High-Type, Intermediate & Low-Type Surfaces High-type mixtures uses semi-solid binders that are mixed and laid at elevated temperatures, whereas intermediate type mixtures uses softer bituminous materials that require a moderate amount of heating (or no heating as in emulsions).Grading of aggregate components are more carefully controlled for high-type mixtures than in intermediate and low type mixtures.Density & stability requirements of the compacted mixture are much rigid for high-type pavement than intermediate pr low-type mixtures.
4 Central Mixing Plants Central Mixing Plants are described as: Portable : Small units, self contained, & wheel mounted. Or large mixing plants in which separate units are easily moved from one place to anotherSemi-portable: Plants in which separate units must be taken down, transported (trailer, trucks, or railroad cars) to new location, and then reassembled.Stationary: Plants permanently constructed in one location and are not designed to be moved from place to another.Portable & semi-portable are numerously used and have capacity range up to 400 tons/hr.
5 Types of Central Mixing Plants Batch Plants:The correct amount of aggregate & bituminous materials, determined by weight, are fed into the mixing unit of the plant.The batch is then mixed and discharged from the mixers into trucks before additional materials is introduced.Drum Mix Plants:the aggregates are proportioned prior to entry in the mixing drum by means of precision belt feeders which control the amount of each class of aggregate entering the drum.Drum feeder dries the aggregate & blends it with asphalt.The HMA discharge continuously into a surge silo, where it is temporarily stored and later loaded into trucks.
6 HMA Plant Functions Aggregate and asphalt storage. Aggregate drying. Dust collection, air pollution control.Aggregate and asphalt proportioning.Mixing.Mixture discharge/storage.
7 Aggregate Storage Separated stockpiles Good drainage Minimize segregation
8 Cold Feed System Provide uniform flow of various aggregates Flow generally controlled by:Gate openingBelt speedCoarse aggregate typically flows betterUniform feed rate is essential.
10 Aggregate Drying & Heating Dry and heat aggregates from cold feed.Large rotating metal drum.Oil or gas burner, or heater (generally located at the lower end).Drum mounted on a slope (angle to the Horizontal)Flights (steel angles or blades) in drum lift aggregatesAggregate falls in veil through hot air streamHot aggregates are then discharged from lower end, generally onto an open conveyor or enclosed hot elevator that transport it to the screens & storage bins.
11 Dust collection Dust collection Works with the drying systemEliminates dust from plant exhaustTwo basic types:Wet scrubberBaghouseCollected dust may be returned to drum if desired
12 Asphalt Storage Provides heated asphalt for mixing Steam or hot oil circulates through coils in tank.Electric heating jackets.Tanks, lines, pumps should be heatedTanks should be calibrated to allow for content determination.
16 Batch Plants-Features Aggregates dried, separated by size.Aggregates recombined by weight in weigh hopper.Aggregates introduced into pugmill, briefly mixed.Asphalt introduced by weight, mixed with aggregates.Completed HMA discharged or stored.
20 Drum PlantsAggregates are dried, mixed with asphalt in a continuous operationQuality control entirely dependent on:stockpile managementplant calibrationMixture must be stored in surge bin or silo.Plant consist of : (Aggregate Cold feed bins; Conveyor & aggregate weight system, Drum mixer, liquid asphalt storage tank & pump; hot mix conveyor; mix surge silo; control van; dust collection system).
28 Automatic Screed Controls Electronic adjustment to screed height using sensing and reference systemSensor detects elevation changes, adjusts height of tow pointSlope (transverse) controls
29 Paving Operations Maintain uniform resistance to face of screed! Keep Paver in motion 75% of the timeKeep augers turning 85% of the timeCoordinate mixture delivery, Paver speed and compaction operations
31 CompactionThe process of compressing a material into a smaller volume while maintaining the same mass.Essential to good performance!To provide shear strength or resistance to ruttingTo ensure the mixture is waterproofTo prevent excessive oxidation of the asphalt binderNeed to compact to desirable air voids levelconventional dense-grade mixtures: 4-8%gap-grade mixtures: 3-6%Compaction can only achieved if:mixture is confinedmixture is hot (workable)
32 Factors Affecting Compaction Mixture propertiesAggregateAsphaltMix TemperatureBase/subgrade support (confinement)Ambient conditionsLift thickness:Compacted lift thickness at least 3 X nominal maximum aggregate sizeparticularly important for gradations below maximum density line0.5 in nom max use 1.5 in minimum lift thickness (prefer 2 inches, especially for coarse-graded mixtures)Thicker lifts conserve heat longer, provide more time for compactionRollers
34 Compaction Equipments Screed unit on paverweight of screedtamping/vibratory unitRollersvibratory steel-wheeledstatic steel-wheeledpneumatic
35 Vibratory Rollers Commonly used for initial (breakdown) rolling tons, in wide (“heavy” rollers)lbs/linear inch (PLI)Frequency: impacts/min.Amplitude: in.For thin overlays (≤ 2 in.) use low amplitude or static modeOperate to attain at least 10 impacts/ft2-4 mph
37 Static Steel-Wheeled Rollers 10-14 ton rollers normally used for HMA compactionCommonly use vibratory rollers operated in static modeLighter rollers used for finish rollingDrums must be smooth and cleanFor initial compaction, drive wheel must face paver
39 Pneumatic-Tired Rollers Reorients particles through kneading actionLoad/tire: – 6730 Lb/tire depending on model/ballastTire pressures:~70 psi (cold) for compaction~50 psi (cold) for finish rollingTires must be hot to avoid pickupNot used for open-graded mixes or SMA
42 Paver speed must not exceed that of the compaction operation!!! Rolling PatternSpeed & lap pattern for each rollerNo. of passes for each rollerMin. temperature by which each roller must complete patternIMPORTANT:Paver speed must not exceed that of the compaction operation!!!