SCREED CONTROLS (ADJUSTMENTS).-In operating the screed, two types of controls are essential:

l Control of the thickness of the mat

l Control of the crown, formed in the mat for proper drainage

Both functions are regulated by controls built into the paver (fig. 16-44).

It is important to understand that, when the paver is operating, control adjustments, made to the screed, take time to go into effect. For example, when a thickness control screw is adjusted to change the thickness of the mat, the paver is likely to move a distance of several feet before the change is completed, and the mat is produced in the new thickness. For this reason, it is necessity that a screed operator know the effective delay involved in making adjustments to a particular screed unit and be able to anticipate adjustments accordingly. Furthermore, it is important that after such adjustment of the thickness controls, the paver be allowed to travel far enough for the correction to be completed before another adjustment is made. Excessive adjustment or overcontrol of the thickness controls is one of the principal contributors to poor pavement smoothness. The condition of the screed unit is important when a high-quality mat is to be placed. To ensure the screed control linkage is snug, the operator should check the wear points. Also, the screed plates should be checked regularly for signs of wear, such as pitting and warping. The plates should always be properly adjusted before paving begins. Both the leading and trailing edges of the screed have a crown adjustment. The leading edge should always have slightly more crown than the trailing edge. This provides a smooth flow of material under the screed. Too much lead crown results in an open texture along the edges of

Figure 16-44.-Mat thickness and crown controls.

the mat and too little results in an open texture in the center. The trailing edge is what actually sets the crown. Crown adjustments may be made independently or simultaneously during the paving operation.

AUTOMATIC SCREED CONTROLS.- The screed controls must be adjusted by the screed operator as paving progresses. Automatic screed controls are designed to adjust automatically to place a uniform mat of the desired thickness, grade, and shape (fig. 16-45).

Types and Operating Principles.- Automatic screed controls can be used in several different ways, but all automatic screed control operations require a reference system for the automatic system to follow. This reference system can be the base on which the asphalt hot mix is being placed, the lane next to the material being placed, or a string line. When a string line is used as a reference, the automatic control will follow the height of the string line exactly, so the mat conforms to it; therefore, placement of the string line (or other reference system) must be precise.

Automatic screed controls can also follow traveling reference systems. A traveling reference system, such as a ski attached to a control arm, notes changes in base contours and adjusts the screed automatically to compensate.

A string line or traveling reference system allows the automatic control to adjust screed height as necessary to maintain proper longitudinal (lengthwise) grade of the pavement. Automatic screed controls use a system, attached to a beam, running between the two screed pull-arms to maintain proper transverse (widthwise) grade.

A pendulum in the slope control housing moves side to side with changes in the transverse grade of the roadway, triggering necessary adjustments in the slope control mechanism.

Automatic control systems have several advantages over manually controlled screed systems. Some of the advantages are as follows:

l Automatic controls compensate for changes in grade and slope more quickly than a screed operator could.

l Automatic controls help disassociate the screed from the erratic vertical movement of the tractor unit.

. Automatic controls adjust the screed tow points to enable the screed to follow a path parallel to the grade and slope of the reference system, which may be different from the path plane of the tractor unit.

Figure 16-45.-Automatic screed reference system.

Selecting a Reference System.- Two types of reference systems, such as stationary or traveling, to use with an automatic screed control depend on the following factors:

. The condition of the surface on which the mat is to be placed.

. The degree of precision required in the grade and slope of the finished pavement.

l The thickness of the mat.

. The amount of material available for the project.

When the surface on which the mat is to be placed has a good longitudinal grade along its center line but has an unsatisfactory transverse grade, a traveling reference, run along the center line, can be used effectively to provide the desired mat thickness at the center line and the transverse slope control, used to establish the outside grade.

When the longitudinal grade is erratic, a string line should be placed to ensure a proper longitudinal grade. When the existing surface has a good profile both longitudinally and transversely, automatic screed controls may be unnecessary. The self-leveling ability of the screed may be sufficient. When automatic controls are used, a traveling reference system would be adequate.