Chip Seal Best Practices (2005)

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33 INTRODUCTION The quality of the equipment and the appropriate use of its capabilities undoubtedly play roles in successful chip seal proj- ects. This chapter explores the state of the practice in chip seal equipment selection and use. For this synthesis study, particu- lar attention was paid to the types and sizes of equipment typ- ically specified. The survey responses and literature review identified a number of equipment technologies being widely used abroad, with which North Americans have little familiar- ity or expertise. They are discussed at the end of the chapter. The following major types of equipment are typically used on all chip seal projects: • Asphalt binder distributors, • Aggregate (chip) spreaders, • Rollers, • Dump trucks, and • Sweeping equipment. BINDER DISTRIBUTOR The binder distributor is essentially an asphalt tank with spray- ing equipment mounted on a truck chassis. Analysis of binder distributors has paid particular attention to binder distributor components, production characteristics, controls and calibra- tion, and spraying operations. The binder distributor has gone through some significant technological advancements, with most manufacturers now offering binder distributors with par- allel spray bars (also called wheelpath bars) that enable vari- able spray rates across the lane. In this discussion, particular attention is being paid to the binder distributor, with a special focus on the use of variable nozzles and multiple spray bars. The use of computerized distributors is becoming more com- mon in North America, with 63% of agencies in Canada and the United States requiring computerized rate-controlled dis- tributors in their specifications, as shown in Figure 29. Inter- national specifications requiring computerized distributors appear to be more stringent, with 88% of international respon- dents indicating that they mandate this technology. Distributor Components A straightforward way of understanding a distributor is to break it down into its four essential components: 1. Insulated asphalt tank, 2. Heating system and circulation pump, 3. Spray bar and nozzles, and 4. Distributor controls and gauges. Insulated Asphalt Tank The distributor’s tank must be capable of efficiently storing the binder at temperatures that allow the heated binder to remain consistent with the appropriate viscosity for spraying opera- tions and within the design specifications. Most of the asphalt distributor tanks used for chip seal work hold from 1,000 to 4,000 gal of liquefied asphalt. They should be equipped with baffles to prevent pressure surges resulting from the asphalt sloshing in the tank when starting and stopping. Heating System and Circulation Pump Depending on the make and size of the distributor, either one or two burners are used. These burners are supported at the rear of the tank and positioned with a configuration that directs the flames into the insulated tank’s flues. A constant volume circulation pump maintains a pressurized system so that the binder can be uniformly heated. The circulation pump must also spray a constant volume for the entire length of the spray bar for each application. In addition, the pump enables the distributor operator to load the tank with binder from a storage tank. Spray Bar and Nozzles Figure 30 shows a typical distributor spray bar. There are many different bar widths available, with typical spray bars on North American distributors being 12 ft wide, whereas agencies that prespray as a method of surface preparation use spray bars as wide as 24 ft (Sprayed Sealing Guide 2004). Spray bars connect a series of evenly spaced nozzles along its length. Nozzles are manufactured with different sizes of openings to permit different volumes to be pumped from the same pump pressure. The nozzles control the spray pattern of bituminous binder shot from the distributor. Appropriate selection of nozzles is critical to achieving a consistent and accurate spray pattern. Nozzles with larger openings need to be considered for viscous asphalts such as CHAPTER SIX EQUIPMENT PRACTICES

34 it ensures a uniform distribution of binder across the shot width and that no areas are missed. However, to do so, the spray bar must be adjusted to the correct height or the spray pattern will become distorted. A spray bar with a positive shutoff called a cut-off valve will avert problems with noz- zle dribbling. This is particularly important on the end noz- zles, which might also be equipped with a deflector to develop a sharp edge on each side of the shot or by changing the angle of the end nozzles. Distributor Controls and Gauges Typical controls and gauges include tachometers, volume measuring devices, pressure gauges, and a thermometer. In addition, most distributors manufactured today have comput- erized systems that not only regulate the pressure of the mate- rial to compensate for the speed of the vehicle, but also allow the operator to quickly make accurate rate adjustments, adjust the spray bar height and width, and even shut off individual spray bar sections from the cab. Before the development of computerized rate control systems, a distributor would require more than one operator. Figure 33 shows a contem- porary computerized control panel for a binder distributor. Such a panel is capable of allowing the operator to control all distributor operations from the cab of the distributor. crumb-rubber binders (Sprayed Sealing Guide 2004). One may be able to modify the spray bar on the asphalt distribu- tor so that it has smaller nozzles in the wheelpaths, a practice that results in more binder in the nontraffic areas than in the traffic areas (Gransberg et al. 1998). The nozzles are installed in the spray bar so that the fan-shaped spray is at an angle to the axis of the spray bar. The angle varies from manufacturer to manufacturer. Figure 31 shows that this angle is usually between 15° and 30°, depending on the manufacturer. All nozzles must be set at the same angle to avoid distortion of the spray pattern. The spray bar and nozzles are designed to provide an appropriate fan width to ensure uniform transverse distribu- tion, without any corrugation or streaking. Chip seal projects require either double- or triple-lap coverage, as shown in Fig- ure 32. The advantage of using double or triple lapping is that 63% 37% 63% 37% 88% 12% 0% 20% 40% 60% 80% 100% United States Canada AU, NZ, UK, SA Yes No FIGURE 29 Respondents requiring computerized distributors. FIGURE 30 Distributor spray bar. FIGURE 31 Spray bar nozzle alignment.

35 AGGREGATE (CHIP) SPREADER The aggregate (chip) spreader must apply a uniform, even layer of aggregate across the full width of the binder. Tail- gate box spreaders are commonly used for spot (strip) sealing, whereas self-propelled chip spreaders are used on larger-scale projects. Truck-mounted box spreaders or self- propelled spreaders are equally capable of aggregate appli- cation, although the self-propelled spreaders are more con- trollable, providing more accurate and uniform rates of spread. The most obvious drawback of using a tailgate box spreader is that there are considerable interruptions between loads being spread; it is not a smooth and contin- uous process like that of the self-propelled spreader. Figure 34 shows a typical box spreader attached at the rear of a dump truck bed. A self-propelled spreader, equipped with a receiving hopper in the rear, belt conveyors to carry the aggregate to the spreading hopper, and a spreading hopper with adjustable discharge gates, is generally specified for most chip seal projects in North America. A discharge roller that assists in ensuring uniform transverse application rates is located at the bottom of the discharge gate. These spread- ers can be equipped with variable-width spreading hop- pers that will hydraulically extend to adjust to changing spread widths, such as a shoulder widening. Most manu- facturers offer chip spreaders equipped with computerized controls that allow the gates to open and close hydrauli- cally, to compensate for the speed of the spreader. This ensures a constant application rate, regardless of travel speed. Some models also come equipped with a vibratory hopper that further improves the uniformity of the dis- charge. Figure 35 shows a typical self-propelled aggregate spreader. A sufficient number of dump trucks should be available to circumvent any interruption in the supply of chips to the aggregate spreader. The dump trucks used on nearly all chip seal projects are tandem axles, because single-axle trucks require additional hookups and therefore increase the chance of spillage and damage to the constructed seal. The dump trucks used for transporting the aggregate need to be com- patible with the aggregate spreader, meaning that their hitches must match and that the dump truck bed will not dam- age the aggregate spreader’s receiving hopper. Compatibil- ity of the dump truck’s bed and spreader is essential to ensure that aggregate is not spilled onto the roadway. Dump trucks or aggregate spreaders are sometimes equipped with aprons to ensure that the aggregate is effectively dumped into the aggregate spreader’s hopper. FIGURE 32 Spraying lap coverage. FIGURE 33 Computer rate control panel for binder distributor. FIGURE 34 Dump truck bed aggregate spreader. FIGURE 35 Self-propelled aggregate spreader.

ROLLERS The covering aggregate is rolled for the following reasons (Maintenance Technical Advisory Guide 2003): • To orient the aggregate to their least dimension, • To embed the aggregate into the binder, and • To achieve mechanical interlock between the individ- ual pieces of aggregate. It must be pointed out that the roller’s purpose is to achieve the desired aggregate embedment depth. It achieves this by redistributing the aggregate and seating it in the binder (Ben- son and Gallaway 1953). To realize proper embedment and orientation, particular attention must be paid to the time between the aggregate spread and initial rolling, selection of the most appropriate roller type, and determination of rolling requirements such as rolling patterns and number of rollers (Gransberg et al. 2004). Achievement of the full design life of a chip seal is not possible without the bonding that results from proper embedment and orientation of the chips. Figure 36 shows the types of rollers being typically used by the respondents. Respondents were permitted to identify more than one type of roller typically used on their projects. Pneumatic Rollers For all practical purposes, pneumatic (rubber-tired) rollers are being universally used. There are two primary functions for rolling chip seal: embed the aggregate into the binder and orient the chips so that maximum bonding can occur. Pneu- matic rollers exploit the machine’s weight per unit area of 36 surface contact to provide the forces needed to embed the aggregate firmly in the binder. Pneumatic rollers are capable of ballast loading, with either water or sand, which allows the weight of the machine to be varied “from four to six tons” (Maintenance Technical Advisory Guide 2003) or “not less than eight tons” (2003 Standard Specifications . . . 2003) to be able to achieve the specified contact pressure, which typ- ically is around 80 lb/in.2 (Minnesota Seal Coat Handbook 1998; Maintenance Technical Advisory Guide 2003). In addition to the machine’s weight, the number of tires, tire size, and inflation pressure determine the machine’s contact pressure (Minnesota Seal Coat Handbook 1998). The orien- tation of the aggregate is facilitated by the seating action of the rubber tires. Most pneumatic rollers are 60 to 80 in. wide and have two axles, with four tires on the front axle and five tires on the rear axle. The alignment of the axles is such that the rear axle tires, when inflated to proper pressure, can com- pact the voids untouched by the front-axle tire, as illustrated in Figure 37. It should be noted that if the tires are inflated to their maximum pressures on some models of rollers, there may not be 100% overlapping coverage between the front and back tires. Static Steel-Wheeled Rollers Static steel-wheeled rollers use a smooth-surfaced cylin- drical steel drum to exert rolling forces. Use of steel- wheeled rollers should be carefully observed, because these types of rollers can crush and degrade the aggregate. Steel-wheeled rollers used for surface treatments typically weigh between 3 and 6 tons, and thus are comparatively lighter than those used in asphalt paving (Seal Coat . . . 2003). Steel-wheeled rollers may be used on the surfacing 6% 22% 50% 20% 20% 20% 13% 13% 13% 38% 100% 0% 20% 40% 60% 80% 100% Static Steel Vibratory Steel Pneumatic Combination Steel/Pneumatic Rubber-Coated Steel Proportion of Respondents United States Canada AU, NZ, UK, SA FIGURE 36 Typical types of rollers used.

37 to “tighten it up” and create a uniformly prepared surface (McLeod 1969). Steel-wheeled rollers will have difficul- ties when the underlying pavement is rutted, because they will bridge over the ruts and fail to properly seat the aggre- gate in the wheelpaths. Other Roller Types The other types of rollers are variations on either the pneu- matic or steel-wheeled roller. The rubber-coated, steel-wheeled roller has a layer of rubber attached to the drum and causes less damage or degradation to the aggregate while retaining the high contact pressure inherent to the static steel-wheeled roller. With the vibratory steel-wheeled roller, the machine vibrates as it rolls and is thought to better seat the aggregate owing to the vibration’s effect. Finally, the combination pneumatic and steel-wheeled roller has a set of pneumatic tires on one end and a steel drum on the other. This piece of equipment attempts to combine the advantages of both roller types in a single machine. SWEEPING EQUIPMENT There are two main tasks for sweeping on a chip seal project: cleaning the existing road surface of dust and foreign materi- als before placing the chip seal and removing excess aggregate from constructed chip seals. There are three different types of sweeping equipment typically used in chip seal construction: rotary brooms, pickup sweepers, and vacuum sweepers. Rotary Brooms Rotary brooms, such as the one shown in Figure 38, are employed to remove the excess aggregate from the surface of the chip seal without dislodging the embedded particles. The downward pressure must be kept to a minimum as the broom’s bristles will remove the aggregate with a flicking action. The main concern with rotary brooms pertains to bris- tle selection. Steel bristles are unquestionably more success- ful than other types in removing foreign materials from the surface of the existing pavement surface before placing the chip seal, but they are more likely to dislodge embedded aggregate after construction. Plastic bristles require earlier replacement; however, they are not as likely to damage the new chip seal. Rotary brooms generate dust, which can affect visibility for traffic. Additionally, they move the excess aggregate to the side of the road, where it can eventually be swept back onto the traveled way by either rain or vehicles using the shoulder for parking. Pickup and Vacuum Sweepers Pickup sweepers are generally used wherever dust must be minimized and it is desirable to remove all excess aggregate from the project limits. A pickup sweeper features a broom that sweeps the aggregate to a suction head that deposits the material in a storage tank. Pickup sweepers are particularly useful in urban areas where aggregate accumulation in gut- ters or along the edge of the roadway is undesirable. Vacuum sweepers represent purpose-built equipment that removes the excess aggregate through suction only. The lack of contact with the chip seal’s surface minimizes damage and is the pre- ferred method of loose aggregate removal in Australia (Sprayed Sealing Guide 2004), as well as in some parts of the United States. UNIQUE EQUIPMENT An effort has been made to associate construction methods with unique pieces of construction equipment. Thus, a num- ber of pieces of equipment that are not found in North Amer- ica have been identified and are described here to furnish information to agencies that may be looking for new solu- tions to their chip seal equipment concerns. FIGURE 37 Pneumatic roller tire configuration. FIGURE 38 Typical rotary broom.

Aggregate Precoating Loader Precoating of aggregate with a purpose-built aggregate loader is a common practice in Australia and New Zealand (Sprayed Sealing Guide 2004). The loader is a unique piece of equip- ment that takes windrowed aggregate in one end and screens out the dust. It then precoats the aggregate through a trom- mel screen and loads the precoated aggregate directly into the dump trucks. VicRoads, an Australian road agency equiva- lent to an American state DOT, contends that using front-end loaders is not acceptable for loading chip seal aggregate owing to the increased fines associated with aggregate degra- dation during handling (Bituminous Sprayed Surfacing Man- ual 2003). Figure 39 shows this special piece of equipment in operation. Low-Drop Aggregate Spreader Low-drop aggregate spreaders are used in Australia. This machine was developed to minimize the bouncing and turning of aggregate as it hits the freshly shot surface. It is believed that this machine increases the uniformity of the aggregate’s spread by placing it as close to the surface as possible. From Figure 40, one can see that the operator is facing forward, which also con- tributes to better control of the aggregate spreading operation. Rubber-Coated Drum Rollers Rubber-coated drum rollers are purpose built for chip seal operations. The rubber coating supposedly reduces the crush- ing of the aggregate inherent with steel-wheeled rollers, while achieving the embedment and mosaic characteristics not capable with a pneumatic roller. The survey responses indicated that rubber-coated drums are commonly used on chip seals (known as surface dressings in British terminol- ogy) in the United Kingdom, and they are also used when required on sprayed seals in Australia. British Columbia was the only North American agency that prescribes the use of rubber-coated drum rollers. 38 Water-Retexturizing Machine A water-retexturizing machine can eliminate the effects of bleeding by using carefully directed high-pressure water to remove any binder that is submerging the aggregate on the existing surface. These machines have been used in Aus- tralia and the United Kingdom to treat bleeding surfaces. Such equipment is especially valuable for preparing the surface of the road before receiving a chip seal. Figure 41 illustrates a water-retexturizing machine in use in the United Kingdom. Combination Vibratory Pneumatic Rollers Finally, none of the respondents indicated that they used combination vibratory pneumatic rollers, probably owing to the recent development of the rollers. Trials of vibratory pneumatic rollers are suggested, because they are specif- ically designed for achieving both the benefits of aggregate particle orientation and embedment compaction. They appear to offer some significant advantages to standard pneumatic FIGURE 39 Aggregate precoating loader. FIGURE 40 Low-drop aggregate spreader. FIGURE 41 Water-retexturizing machine.

39 rollers in seating the aggregate as quickly as possible in the roller operation. EQUIPMENT CONCLUSIONS AND BEST PRACTICES Superior equipment does not prevent all failures. However, the findings of this portion of the study do result in several best practices with respect to chip seal equipment. 1. Use computerized distributors. 2. Preproject analysis of the ability of the chip seal equip- ment fleet to keep up with the production rate of the dis- tributor is important. 3. The use of variable nozzles reduces the amount of binder that is sprayed in the wheelpaths. 4. Plastic bristles for rotary brooms will minimize aggre- gate dislodgment during brooming. Although not a best practice pertaining to this synthesis, the equipment and procedures used overseas show great promise for application in North America. The use of water- retexturizing machines to prepare the road’s surface texture and therefore allow a constant rate of binder to be shot should be investigated for North American projects. Doing so would be particularly useful on any roads whose surface is bleed- ing. Equipment best practices cannot be isolated from the presence of experienced equipment operators and the use of regimented construction practices. The following chapter describes how the construction phase ultimately determines which projects are successes.