Force V will decrease at this point and be balanced byforce W.3. To reduce mat thickness, tilt the screed plate sothat less material crowds under the screed plate.4. The amount and condition of the material,leaving the auger, can change the equilibrium of the fourforces. Excessive flow of material increases force H. Acold, stiff mix will increase H and to some extent V. Anexcessively hot, fluid mix decreases H and V. Stoppingand starting the paver also cause changes in equilibriumamong the forces. The key to controlling the action ofthe screed is to maintain in a uniform manner thoseforces acting on the screed.5. The secret of good paver operations is a balanceof the forces and uniformity to maintain that balance.When balance and uniformity are attained, the screedpath follows the paver in a plane parallel to that of thepivot point. As the paver goes up over an irregularity,the pivot point rises and the screed begins to rise also.However, because the screed reacts to changes inelevation more slowly than the pivot point does, thescreed rises very little and thereby maintains the planeof the surface of the mat over the irregularity, and theimpact of the irregularity is reduced. The same is nottrue of long irregularities (longer than several lengths ofthe paver). Grade line irregularities of this type shouldbe corrected before placing surface courses with thepaver.6. Screeds, equipped with tamping bars andvibratory mechanisms, are designed to strike off andthen compact the mixture slightly, as it is placed. Thetwo purposes for this screed action are that it achievesmaximum leveling of the mat surface and ensures thatminimum distortion of the mat surface occurs withsubsequent rolling.TAMPING BAR TYPES OF SCREEDS—Tamping bar types of screed compactors compact themix, strike off the excess thickness, and tuck the materialunder the screed plate for leveling. As figure 16-41shows, the tamper bar has two faces:. A beveled face on the front that compacts thematerial, as the screed is pulled forward.l A horizontal face that imparts some compaction,but primarily strikes off excess material, so the screedcan ride smoothly over the mat being laid.The adjustment that limits the range of downwardtravel of the tamping bar is the single most importantadjustment affecting the appearance of the finished mat.At the bottom of its stroke, the horizontal face should16-38extend one sixty-fourth of an inch (about the thicknessof a fingernail) below the level of the screed plate.When the bar extends down too far, mix builds upon thescreed face. This buildup scuffs the surface of the mixbeing placed and also causes the tamping bar to lift thescreed slightly on each downward stroke, and this oftencauses rippling of the mat surface.When the horizontal face of the tamping bar isadjusted too high (either by poor adjustment or due towear of the bottom of the horizontal face), the bar doesnot strike off excess mix from the mat. Consequently,the screed plate begins to strike off the material, and thisresults in surface pitting of the mix being placedbecause the leading edge of the screed plate drags thelarger aggregates forward. For this reason, the tamperbar should always be checked before operating thepaver, and it should be adjusted if necessary. Before thetamper bar approaches knife-edge thinness, it should bereplaced.VIBRATING TYPES OF SCREEDS.— Theoperation of vibratory screeds is similar to that oftamping screeds, except that the compactive force isgenerated either by electric vibrators, rotating shaftswith eccentric weights, or hydraulic motors (fig. 16-43).On some pavers both the frequency (number ofvibrations per minute) and the amplitude (range ofmotion) of the vibrators can be adjusted. In others, thefrequency remains constant and only the amplitude canbe adjusted.Frequency and amplitude must be setaccording to the type of paver, the thickness of the mat,the speed of the paver, and the characteristics of themixture being placed.Once set, the frequency andamplitude do not normally need adjustment until the matthickness or mix characteristics change.Figure 16-43.-Vibratory type of screed.
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