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OCR for page 43
44 Variable Application Spraying to the skill and workmanship of the distributor operator in regard to the longitudinal joints between adjacent sprays. Variable binder application rates are particularly useful for When the outside nozzle is parallel with the other nozzles, maintaining a consistent texture across the entire lane width. overlapping is essential to ensure that binder application rates Because aggregate in the wheelpaths will be embedded more are achieved. The distributor operator should make a longitu- deeply, the amount of binder required will be less than in the dinal overlap of 2 to 4 in. (50 to 100 mm) to ensure that the other areas of the lane. The use of variable nozzles permits texture of the finished surface is uniform (Supplemental Spec- the application of a reduced rate of binder in the wheelpaths, ification 882 . . . 2002). while still achieving the design binder rate outside of the wheelpaths. The justification for using variable nozzles is to combat bleeding in the wheelpaths. The modified Kearby AGGREGATE SPREADING design method recommends that the binder application rate outside the wheelpaths be 20% greater than the design rate As with the binder distributor, the aggregate spreader needs to calculated for the wheelpaths. (See Appendix C for a detailed have correct transverse alignment before commencing the case study on a New Zealand contractor's method for design- spread. To allow for timely aggregate coverage of the sprayed ing variable transverse application rates.) binder, it is essential to have two or three loaded trucks in queue behind the aggregate spreader and before the rollers. It is criti- cal that the trucks stagger their wheelpaths, to assist the rolling Construction Joints operations when backing into the spreader (Maintenance Chip Seal Manual 2000). A self-propelled spreader pulls the dump An area of both aesthetic and service life concern is found trucks through the aggregate spread area, known as a "rock with transverse construction joints. Special attention must be land." As each dump truck is emptied, the aggregate spreader paid to the transverse construction joints at the start and end operator releases that truck, and the next truck in queue is of each shot. Seamless transverse joints can be obtained attached to the aggregate spreader (Figure 49). through placing starting and finishing tar paper at the joint. This should ensure that the correct rate of application is Achievement of design application rates will generally achieved for the full length of the shot and avoid double appli- mean that the aggregate uniformly covers the binder without cations of binder. The binder application shall commence excess aggregate. Overspreading can increase the risk of wind- with a running start on a strip of tar paper. The spray bar needs shield damage as a result of dislodged aggregate, is not cost- to be stopped on the tar paper at the end of each shot to ensure effective, and requires additional post-construction sweeping a straight transverse construction joint (Figure 48). efforts. Underspreading, as evidenced by visible areas of uncovered binder, will result in aggregate loss, because the Longitudinal joints should not be in the center of the lane excess voids in the aggregate will result in the binder not ris- width, as this leaves an undesirable appearance and can lead ing high enough to securely hold the aggregate particles in to raveling. The number of longitudinal joints should be kept place. The material needs to be applied thick enough so that to a minimum and be located so that they will coincide with the tires of the dump trucks, aggregate spreader, and rollers painted lines between traffic lanes to the greatest degree pos- are not picking up the binder. If the aggregate is being sible (Seal Coat . . . 2003). Careful attention should be given applied at the calculated rate and tires are still picking up FIGURE 48 Paper used to ensure seamless transverse joint. FIGURE 49 Aggregate applied with self-propelled spreader.