Some vibratory screeds require a pre-strike-off unit.This unit is a rounded moldboard that controls theamount of mix passing under the screed.SCREED CONTROLS (ADJUSTMENTS).—In operating the screed, two types of controls areessential:l Control of the thickness of the matl Control of the crown, formed in the mat forproper drainageBoth functions are regulated by controls built intothe paver (fig. 16-44).It is important to understand that, when the paveris operating, control adjustments, made to the screed,take time to go into effect. For example, when athickness control screw is adjusted to change thethickness of the mat, the paver is likely to move adistance of several feet before the change iscompleted, and the mat is produced in the newthickness. For this reason, it is necessity that a screedoperator know the effective delay involved in makingadjustments to a particular screed unit and be able toanticipate adjustments accordingly. Furthermore, it isimportant that after such adjustment of the thicknesscontrols, the paver be allowed to travel far enough forthe correction to be completed before anotheradjustment is made.Excessive adjustment orovercontrol of the thickness controls is one of theprincipal contributors to poor pavement smoothness.The condition of the screed unit is important whena high-quality mat is to be placed. To ensure thescreed control linkage is snug, the operator shouldcheck the wear points. Also, the screed plates shouldbe checked regularly for signs of wear, such as pittingand warping. The plates should always be properlyadjusted before paving begins. Both the leading andtrailing edges of the screed have a crown adjustment.The leading edge should always have slightly morecrown than the trailing edge. This provides a smoothflow of material under the screed. Too much leadcrown results in an open texture along the edges ofFigure 16-44.-Mat thickness and crown controls.16-39
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