Selecting a Reference System.— Two types ofreference systems, such as stationary or traveling, to usewith an automatic screed control depend on thef o l l o w i n g f a c t o r s :. The condition of the surface on which the mat isto be placed.. The degree of precision required in the grade andslope of the finished pavement.l The thickness of the mat.. The amount of material available for theproject.When the surface on which the mat is to be placedhas a good longitudinal grade along its center line buthas an unsatisfactory transverse grade, a travelingreference, run along the center line, can be usedeffectively to provide the desired mat thickness at thecenter line and the transverse slope control, used toestablish the outside grade.When the longitudinal grade is erratic, a stringline should be placed to ensure a proper longitudinalgrade. When the existing surface has a good profileboth longitudinally and transversely, automatic screedcontrols may be unnecessary. The self-leveling abilityof the screed may be sufficient.When automaticcontrols are used, a traveling reference system would beadequate.SCREED HEATERS.— The screed is equippedwith heaters, used to heat the screed plate at the startof each new paving operation. The heaters are notused to heat the mix during the paving operation. Ifthe screed is not initially heated, the mix will tear andthe texture will look open and coarse, as it would ifthe mix were too cold. There are times when the paveroperator allows the mix to heat the screed plate. Thispractice almost always results in a section ofunsatisfactory pavement being laid while the screedis being heated.SCREED ACCESSORIES.— Three types ofcommonly used screed accessories are screed exten-sions, cutoff shoes, and bevel end plates.Screed Extensions.— Screed extensions (fig.16-46) are attachments that widen the screed, allowingthe paver to place a wider-than-normal mat. On someFigure 16-46.-Screed extension accessories.16-41
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