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45 binder, then the binder application rate is either too high and The number of rollers should be determined by the width the aggregate is rolling over on contact with the binder or the of the area to be covered, as well as the nominal aggregate aggregate is too wet. When the aggregate spreader is pro- size and traffic volume. As nominal aggregate size in- ceeding too fast or alternatively when the binder is too vis- creases, the area that can be effectively covered by each cous, the aggregate may roll over. roller decreases (Sprayed Sealing Guide 2004). Develop- ment of rolling guidelines such as patterns and minimum Minor aggregate spread deficiencies such as corrugation or rolling time should be directed toward achieving full lane missed areas can be corrected with the use of a drag broom or coverage and a similar number of passes for all areas of the hand rake. Drag brooms are typically fitted on the roller doing lane. The survey responses indicated that most highway the initial roller pass and will assist in redistributing minor agencies use an average of only two rollers, with some spread inequalities. If the aggregate is uneven, nonuniform, or agencies "requiring" only one. Such a phenomenon is in irregular for any reason, it should be drag-broomed or hand- agreement with a Texas study, which found that rolling raked immediately after spreading and before initial rolling. requirements are often ignored in the field because QC test- ing associated with chip seal rolling operations is not com- mon (Gransberg et al. 1998). Excess Aggregate Achievement of the service life of a chip seal is not pos- Loose aggregate, the application of excess chips, is a serious sible without the bonding that results from proper embed- concern for all chip seal projects. There is also a tendency to ment and orientation of the chips. A recent paper offered apply more aggregate than is required when it is paid for by a straightforward mathematical equation to compute the the ton, which ultimately just gets swept off the road and required number of rollers based on maximizing distributor wasted (Asphalt Surface Treatments--Specifications undated). production rate (Gransberg et al. 2004). Distributor speed for Application of chip quantities in excess of 10% of the designed the desired asphalt rate can be calculated from Eq. 1 (Epps rate makes sweeping challenging, and as the likelihood of et al. 1981). Spray bar output is dependent on the type of the unswept excess aggregate increases, so does the likelihood binder sprayer used. of vehicle damage from flying stones. Excess aggregate can also dislodge embedded chips under traffic, leading to 9Gt the failure of the chip seal (Shuler 1990). A leading cause Sf = (1) WR of excess aggregate is an improperly calibrated aggregate spreader. In an attempt to curtail excess aggregate spread, where Montana performs a sweeping test in the field (Maintenance Sf = distributor speed (ft/min), Chip Seal Manual 2000). Excess aggregate spread will be dif- Gt = spray bar output (gal/min), ficult for the sweeping equipment to remove and therefore W = sprayed width (ft), weighing the quantity of unswept chips is an indication of R = rate of binder application (gal/yd2), and whether or not the design rate was exceeded. Montana 9 = conversion factor (from yd2 to ft2). requires that the amount of excess chips be less than 10% of the design rate. The only possible exception to regulations Distributor speed, Sf, can be modeled as the distribu- about minimizing excess aggregate are when dealing with tor production rate (P) by converting speed from feet per areas where extensive stopping and turning movements take minute to lineal miles per hour. Next, assuming that the place (Janisch and Gaillard 1998). In these locations, appli- production rate of the rollers must be greater than or equal cation of a controlled amount of excess aggregate may reduce to the planned production of the distributor to ensure that the dislodging of the aggregate in the same fashion as with the the maximum system production is achieved (Peurifoy et al. racked-in seal method. 2001), the required number of rollers can be calculated by using Eq. 2 if a specified rolling time (also called linger time) is known. ROLLING OPERATIONS 1760 PX The roller is the tool used to seat the aggregate and create N = (2) embedment in the hot asphalt or emulsion binder. Rolling A operations need to be preplanned and carefully spelled out where for field personnel to follow. Verification of the rolling pat- tern should be done with a visual analysis of lane coverage, N = required number of rollers, aggregate orientation, and embedment (Minnesota Seal P = distributor production (lineal mph) obtained by con- Coat Handbook 1998; Seal Coat . . . 2003). Careful roller verting distributor speed (from ft/min to lineal mph), operation will ensure that the roller itself does not cause X = shot width (yd), damage to the freshly constructed chip seal, especially by A = roller linger time (yd2/h), and displacing aggregate. 1760 = conversion factor (from yd to mi).
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46 Similar computations can be made for each of the different minimum rolling time or number of passes. Figure 51 shows shot widths that will be encountered during a chip seal project. the typical rolling requirements used by survey respondents. The inspector can then be given a list of how many rollers are required with which to enforce the rolling time provisions in It is interesting to note that none of the North American the contract. Figure 50 is taken from the aforementioned paper respondents specify a rolling time. This does not mean that (Peurifoy et al. 2001) and illustrates the need to plan rolling there are none followed in North America. Some agencies patterns and roller coverage. It can be seen that the use of three have specified roller linger times in the range of 1,000 to rollers in this example results in an uneven roller coverage-- 5,000 yd2/h (Gransberg et al. 1998). Times in Australia range and the least amount of rolling in the areas outside of the from 3,000 to 7,000 yd2/h depending on traffic volume, as wheelpaths where possible loss of aggregate is the greatest shown in Table 9 (Bituminous . . . 2003). potential. It should be noted that this roller has an effective rolling width of only 69.3 in. (176 cm), although it is widely New Zealand uses the following equation (Eq. 3) to cal- accepted as being a 72-in. (183-cm) roller. culate the required rolling time (T ) based on volume of binder to be shot (Vt ), the rolling speed (S ), and the number Additional rollers will be required when the viscosity of the of rollers (N ) (Notes . . . 2002): binder is increased, "such as with the use of polymer-modified binders or during cool weather construction" (Notes . . . 2002). Vt T = (3) It may seem counterintuitive, but the Minnesota study found 450( S )( N ) that the greatest rolling attention needs to be paid to roads that have light traffic volumes. The reason being that traffic assists where in the orientation and embedment of aggregate, actually pro- T = rolling time (h), viding a level of rolling not obtainable with pneumatic rollers Vt = volume of binder (L), (Janisch and Gaillard 1998). S = roller speed (km/h), N = number of rollers, and The Mn/DOT constructability study (Janisch and Gaillard 450 = conversion factor. 1998) also found that aggregate loss typically occurs outside and between the wheelpaths where the roller coverage is min- A consensus from the survey responses is that a maximum imum with use of three rollers on a 12-ft (3.7-m) shot width (see number of passes in roller coverage graphs in Figure 50). speed of approximately 5 mph (8 km/h) for pneumatic rollers The use of four rollers provides a uniform coverage and should be mandated, to prevent the roller's tires from dis- twice as much rolling between the wheelpaths as three rollers placing the aggregate. Special attention should be given to for the case study discussed earlier. ensure that the tire pressures of the rollers are set to obtain optimum embedment of the material without undue crushing Prompt rolling is critical to achieve adequate aggregate of the aggregate. Pneumatic-tired rollers should have a total embedment. It is important that the aggregate is rolled before coverage width of not less than 60 in. (1.82 m), while also pro- the binder becomes cold or too viscous to achieve proper viding a minimum contact pressure of 40 lb/in.2 (2.81 kg/cm2) embedment. The justification behind prompt rolling is that to the surface (Asphalt Surface Treatments--Specifications as the binder cools, its viscosity may increase, which in turn undated). Texas specifications require that all tires on pneu- increases the amount of rolling energy required to achieve the matic rollers be inflated so that there is no more than 5 lb/in.2 same embedment. Therefore, rolling must follow as closely variation within all tires (Seal Coat . . . 2003). as practical behind the spreader. Typically, additional rolling is beneficial to the success of Allowing emulsions to break before applying aggregate the chip seal, unless aggregate degradation is occurring. Aus- contributes to aggregate loss (Jackson et al. 1990). This tralian agencies have recognized that the failure to achieve emulsion-specific issue indicates that by waiting too long, the design embedment depth is primarily a function of achieving ability of the rollers to properly seat the aggregate is QC requirements for rolling time. Thus, minimum rolling greatly reduced. The binder will be brown in color at applica- times are specified, and the importance of adequate rolling tion and will turn black as it breaks. On a hot, low-humidity has led to practices in Australia whereby extra equipment day, the binder will break in 3 to 5 min (Janisch and Gaillard operators relieve the roller operators during breaks and even 1998). A rule of thumb is that the first pass should roll the extend rolling operations into the evening after the construc- aggregate just before the binder breaking (Asphalt Surface tion has ceased (Bituminous . . . 2003). Table 9 highlights the Treatments--Specifications undated). necessity to pay particular attention to monitoring rolling times on roads with low traffic volume. Rolling Requirements Steel-Wheeled Rollers Effective rolling specifications should detail the roller types permitted, number of rollers required, time between aggre- Steel-wheeled rollers are primarily used on multiple-course gate spread and initial rolling, maximum speed limit, and chip seals to rapidly achieve levels of embedment not possible
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47 FIGURE 50 Roller patterns and coverage (Source: Peurifoy et al. 2001).