width of the road being paved. It is then struck off tothe desired shape and thickness and compacted. Threegeneral methods of spreading and shaping the materialare in use today: hand spreading, blade spreading, andmechanical spreading.HAND SPREADING.— Hand spreading is theoldest method used to spread and shape the mixedmaterial. For this method, the mix is dumped from thetrucks onto dump boards from which the material isshoveled onto the road or runway. After placement, itis raked smooth to grade and contour and compactedwith a roller.WARNINGAsphalt and bituminous materials containcoal tars, benzene, and other components whichare suspected or known carcinogens. Workersshould avoid inhalation of the vapors andprolonged skin contact with these materials.Review the Materials Safety Data Sheet(MSDS) for specific hazards and precautions.Because of the high cost of labor and the inabilityto obtain a smooth and even-textured surface, handspreading is not used to any great extent. It is usedprimarily to supplement the other spreading methods.For example, hand spreading is used effectively foradjacent curbing and around manholes.When placing the material by hand, you should beextremely careful to prevent segregation of the mix. DoNOT throw the material a long distance and do NOTdump it from too great a height. Dump the material insmall piles and level the material with shovels, rakes,and lutes. Use the shovel to move the excess materialand the lute and rakes to level it. The material shouldbe as level as possible before compacting it.BLADE SPREADING.— Blade spreading is donewith a grader by a skilled operator. The grader blade canobtain reasonably good surface smoothness. Eachsuccessive pass of the grader blade reduces theirregularities in the surface. Often, blade spreading isused in areas too large for hand spreading andinaccessible to mechanical spreading.MECHANICAL SPREADING.— Specializedmachines have been developed to spread bituminouspaving materials. Self-propelled, these machines havecrawler, wheels, or rollers which run on the base coursefoundation or surface.The mix from the plant isdumped into a hopper on the front of the paver. Thepaver places the mix evenly on the road itself. Figure16-6 shows a bituminous paver that can handle any typeof asphaltic mix.Compacting Plant-Mix MaterialsThe most important phase of flexible pavementconstruction is compaction. When the specified densityof asphalt pavement mix is not obtained duringconstruction, subsequent traffic will further consolidatethe pavement. This consolidation occurs principally inthe wheel paths and appears as channels in the pavementsurface.Most mixtures compact quite readily when they arespread and rolled at temperatures that assure properasphalt viscosity.Rolling should start as soon aspossible after the material has been spread by the paverbut should be done with care to prevent undulyroughening of the surface.Mix temperature is a principal factor affectingcompaction.Compaction can only occur while theasphalt binder is fluid enough to act as a lubricant.When it cools enough to act as an adhesive, furthercompaction is extremely difficult to achieve. The besttime to roll an asphalt mixture is when its resistance tocompaction is the least, while at the same time, it iscapable of supporting the roller without excessiveshoving of the asphalt material. The best rollingtemperature is influenced by the interparticle friction ofthe aggregates, the gradation of the mix, and theviscosity of the asphalt; therefore, it can change if anyof these factors change. The critical mix temperature inFigure 16-6.-Bituminous paver.16-11
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