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