National Academies Press: OpenBook

Manual for Emulsion-Based Chip Seals for Pavement Preservation (2011)

Chapter: Chapter 7 - Construction

« Previous: Chapter 6 - Chip-Seal Design
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Suggested Citation:"Chapter 7 - Construction." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
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Suggested Citation:"Chapter 7 - Construction." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
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Page 23
Page 24
Suggested Citation:"Chapter 7 - Construction." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
×
Page 24
Page 25
Suggested Citation:"Chapter 7 - Construction." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
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Page 25

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22 7.1 Equipment Calibrations 7.1.1 Distributor Because a uniform application of material laterally and lon- gitudinally on the pavement is required, the machines used for this purpose, whether controlled by computer or not, must be calibrated prior to arrival on the project. First, the nozzles installed in the spray bar should be the appropriate size for the planned transverse application rate. Nozzles of equal size are required for a uniform transverse application rate. When the transverse rate is lower in the wheel paths, the nozzles should be sized accordingly. However, the number designa- tion of the nozzle should be checked for flow rate against the published flow rate of the manufacturer using a laboratory flow bench. Nozzles used in the spray bar should be checked as a group. Nozzles deviating by more than 10% of the aver- age flow rate of the group should be discarded, replaced, or corrected to allow flow that conforms to the average flow rate. Nozzles that are calibrated to provide uniform lateral flow must be re-installed in the spray bar following manufacturer’s recommendations and ensuring that each nozzle is aligned at the correct angle to provide desired spray overlap. Noz- zle angle can usually be adjusted using the wrench provided by the distributor manufacturer, an adjustable wrench, or appropriate-sized open-end wrench. Nozzle angle usually ranges between 15 and 30 degrees. After the nozzle angle is set properly, the height of the spray bar must be adjusted. If the bar is not set to the proper height, an excess or lack of emulsion will form ridges or streaks on the pavement, as depicted in Figure 5. This adjustment process is accomplished by shutting off the appropriate nozzles to determine where the spray pattern contacts the pavement, as shown in Figure 6. Every other noz- zle should be turned off for a double-lap application, and two nozzles should be turned off for every one that is left on for a triple-lap application. The distributor operator should spray emulsion onto the pavement surface for as short an interval as possible while an observer watches where the emulsion hits the pavement from each nozzle left open. Emulsion overlaps indicate that the bar is too high, and a gap indicates that the bar is too low. Note that the bar will rise as the distributor empties during spraying, but this rise does not usually cause significant streaking that requires spray bar adjustment. Uniformity of lateral flow from the spray bar should be determined by collecting a measured volume of emulsion in containers placed under each nozzle. This process can be done using standard 6-in. by 12-in. concrete cylinder molds lined with one-gallon freezer bags (Shuler 1991). One bag is positioned under each nozzle and emulsion is sprayed into the lined cans until approximately 75% full. Flow is then stopped and each lined can is weighed. The weight of each lined can should be within 10% of the average for all of the nozzles. Any nozzles that deviate from this should be replaced and the test rerun. The cylinder molds can be re-used and the bags with the contents discarded appropriately. Longitudinal calibration of the distributors is done by measuring the volume of the distributor before spraying and after spraying 70% to 90% of the distributor volume. The volume sprayed can be determined using the dipstick supplied with the distributor. The longitudinal spray rate can then be calculated by determining the area sprayed. This value should then be compared to that displayed by the dis- tributor computer, if equipped, to evaluate computer accu- racy; if the rate applied differs from design by more than 5%, corrective action should be taken and the calibration rerun. This calibration should be made each day. The following is an example of such a calibration: Assuming: 1,800-gal capacity asphalt distributor, 12-ft-wide spray width, trial spray distance = 4,630 fe, 0.32 gal/yd2 design spray rate, dipstick reading beginning of shot = 1,765 gal, dipstick reading end of shot = 185 gal 1. Check volume shot. 1,765 − 185 = 1,580 gal a. 1,580/1,765 = 89.5% > 70% and < 90%. OK. C H A P T E R 7 Construction

23 Bar Set Too High Streaks from Excess Emulsion Bar Set Too Low Streaks from Lack of Emulsion Figure 5. Streaks caused by incorrect bar adjustment (after Wood et al. 2006). Emulsion Contact for Double Lap Application Turn Off These Nozzles Emulsion Contact for Triple Lap Application Turn Off These Nozzles Figure 6. Obtaining no streaking for double and triple lap application. 2. Calculate spray rate = 1,580 gal/(12 ft × 4,630 ft/9 ft2/yd2) = 0.26 gal/yd2 a. 0.32 − 0.26 = 0.06 gal/yd2, which is greater than 5% of the difference between the actual spray rate and the design. 3. Therefore, make adjustments to distributor speed or spray bar until the rate applied is within 5% of 0.32. 7.1.2 Chip Spreader The chip spreader should be checked for uniform applica- tion both laterally and transversely. Lateral distribution is best checked using ASTM D 5624, “Standard Test Method for Determining the Transverse-Aggregate Spread Rate for Surface Treatment Applications.” Once accomplished and any spreader gates have been adjusted for variations, the lon- gitudinal spread rate can be measured by applying the entire spreader capacity to the pavement and measuring the appli- cation rate. The mass of chips applied can be determined from weigh tickets of trucks loading the chip spreader. Conduct the calibration for three trucks and average the results. An exam- ple follows. For 12-ton capacity tandem dump trucks, 12-ft-wide pave- ment, and 28 lbs/yd2 design spread rate: 1. For truck no. 1: a. Load = 23,803 lbs b. Spreader distance = 213 ft c. Rate = 23,803/213 × 12/3 = 27.9 lbs/yd2 2. For truck no. 2: a. Load = 23,921 lbs b. Spreader distance = 211 ft c. Rate = 23,921/211 × 12/3 = 28.3 lbs/yd2 3. For truck no. 3: a. Load = 23,848 lbs b. Spreader distance = 213 ft c. Rate = 23,848/213 × 12/3 = 28.0 lbs/yd2 4. Average rate = (27.9 + 28.3 + 28.0)/3 = 28.1 lbs/yd2 5. No adjustment needed (measured rate is within 1% of design). Compensation for moisture on chips must be taken into account when calibrating chip spreaders. 7.2 Operations 7.2.1 Pavement Preparation The substrate pavement should be structurally sound before chip sealing. Areas exhibiting alligator cracking should be patched the full depth of the pavement section using hot mix asphalt. The surface of these areas should be sprayed with a light application of slow-setting asphalt emulsion diluted 50:50 with water at the rate of 0.10 gallons per square yard and allowed to cure thoroughly before chip sealing so that the new chip-seal binder will not be absorbed into the surface of the new patch. Failure to do this could lead to loss of chips under traffic. The substrate pavement should be clean before commenc- ing chip-seal operations. Dust and debris on the surface should be removed by power brooms. Pickup type brooms should be used in urban areas to avoid spreading surface contaminants onto adjacent properties. Push brooms may be used in rural areas when spreading excess chips onto shoulders does not affect property owners. The surface of the substrate pavement should be damp to dry. A damp surface is acceptable as long as

moisture is present only in surface aggregate voids and is not present as free moisture between aggregates. 7.2.2 Environmental Conditions The pavement temperature for chip-seal operations should be a minimum of 70°F with little or no wind. However, chip- seal operations may commence before the pavement tem- perature reaches 70°F as long as pavement temperatures are expected to be 70°F and rising within 60 minutes after com- mencing work. Wind speeds in excess of 20 mph transverse to the pavement alignment can blow asphalt emulsion onto opposing traffic on two-lane facilities; therefore, chip-seal operations should be avoided under these conditions. Chip-seal operations should not be pursued if rain is threatening. A rain storm could wash asphalt emulsion onto concrete gutters or into roadside ditches. Ambient air temperatures in excess of 110°F with the sun shining or with moderate winds can cause emulsified as- phalts to form a skin on the surface such that the emulsion does not set adequately. This situation may require the spread of chips closer to the distributor to obtain proper embedment. However, high air temperatures may lead to lower viscosity emulsion residue, resulting in higher poten- tial for pickup on rubber tire rollers. Increasing the demul- sibility of anionic emulsions may help remedy this situation (Shuler 1991). There is anecdotal evidence to support limiting the season for chip sealing so chip seal construction does not occur when there may be periods of cool to cold weather. 7.2.3 Emulsion Application Each emulsion application should start and stop by spray- ing on top of 15 lb/yd2 roofing paper or similar dimensioned, equally heavy craft paper placed transverse to the centerline of the pavement. This creates a neat, sharp transverse joint. The distributor operator should position the spray bar at the rear of the paper on takeoff so that by the time the bar reaches the pavement the distributor speed is appropriate for the desired spray rate. Another sheet of roofing paper should be placed across the pavement before the distributor stops. Spraying should stop when the spray bar has passed over the paper. Cal- culating when approximately 90% of the distributor volume has been sprayed is a good method to determine the distance where the second strip of roofing paper should be placed. 7.2.4 Chip Application Chips should be applied to the surface of the fresh emulsion before it begins to set but not necessarily immediately after spraying. If the chips are applied too early, there is risk that the chips will roll over in the emulsion due to momentum created by the forward movement of the chip spreader. Thus, less binder will be available to hold the chips in place and the exposed binder becomes susceptible to being picked up on roller tires. Therefore, the adhesive quality of the emulsion should be checked to determine when to apply the chips. This may be determined by throwing a handful of chips onto the emulsion and observing whether they stick to the surface or tend to roll over. Some experimentation is necessary to es- timate the proper timing, which can be accomplished dur- ing the first distributor application. In many cases, changes in environmental conditions during construction will require this test be repeated during the day because humidity, chip moisture, emulsion properties, and ambient air temperature affect the adhesive quality of the emulsion. 7.2.5 Rolling Different types of rollers have been used to embed chips on chip seals. Pneumatic rollers have a tendency to pick up chips due to the affinity of asphalt residues for rubber tires. How- ever, these rollers do not crush chips in situ as do steel- wheeled rollers. Although lightweight steel rollers of three tons or less may provide a means of leveling the surface of a new chip seal after pneumatic rolling, caution must be applied to avoid breaking aggregate chips. Steel rollers with rubber- coated drums are also a good tool for embedding chips. How- ever, any rigid drum roller will bridge over areas of the pave- ment with permanent deformation, causing these areas to be inadequately rolled. Rollers must be able to keep up with the distributor and chip spreader and provide enough passes to embed the chips. If the rollers travel too fast, embedment will not be achieved. Therefore, the number of rollers used depends on roller speed, roller width, distributor and chip spreader speed, and the number of passes required to achieve embedment. The faster the rollers move, the more rollers will be required to achieve embedment. This is because rollers need to linger over an area of chip seal to obtain the desired chip embedment (Benson and Gallaway 1953, Elmore et al. 1995). The number of rollers required can be calculated based on this linger time and the assumption that the rollers should match the production of the distributor and chip spreader as follows (Gransberg et al. 2004): Where N = number of rollers; P = distributor speed, fpm; x = lane width, ft; and N Px A= 6 67. 24

A = area covered in one hour by rollers to get minimum “linger,” yd2. As an example, for a distributor traveling at 200 fpm and spraying 12-ft wide at a conservative 5000 square yards per hour coverage rate, N = 3.20 (i.e., use 4 rollers). Fewer rollers will not be able to keep up with the distributor and spreader while maintaining the 5000 yd2/hr rolling rate. If they do keep up, the rollers will not spend enough time embedding aggre- gates because they are traveling too fast. 7.2.6 Initial Sweeping After Rolling Light brooming should occur as soon as possible after rolling and before vehicular traffic is allowed on the surface to remove any excess chips. This should be possible when the moisture content of the chip seal reaches the level where 10% chip loss occurs after the laboratory sweep test. This moisture content, measured by the moisture loss test, was found to be approximately 15% to 25% of the total moisture present in the chip seal and indicated that total moisture consists of water in the emulsion plus moisture in the aggregate chips, but not moisture in the pavement (see attachment). Moisture content of the chip seal should be measured in areas of the project where moisture loss is expected to be least rapid, such as shady or cooler locations. Caution should be exercised when imple- menting this practice since it is based on limited research. Using vacuum brooms or push brooms with nylon (not steel) bris- tles should be applied with much care to avoid damage to the fresh seal. 7.2.7 Vehicular Traffic Under Traffic Control Vehicular traffic may be allowed on the fresh seal after ini- tial sweeping if speeds can be controlled to less than 20 mph using pilot vehicles. If speeds cannot be controlled to this speed, vehicles should not be allowed on the seal until final sweeping has been completed. Traffic control using pilot ve- hicles should be applied following the Manual on Uniform Traffic Control Devices for Streets and Highways (2009). 7.2.8 Removing Traffic Control Traffic control may be removed and vehicular traffic allowed on the fresh chip seal after brooming has been completed to remove excess or loose chips and the moisture content of the seal reaches the level that results in less than 10% chip loss after the laboratory sweep test. This moisture content, measured by the moisture loss test, is approximately 15% to 25% of the total moisture present in the chip seal (which consists of the water in the emulsion plus moisture in the ag- gregate chips, but not moisture in pavement). Moisture con- tent of the chip seal should be measured in areas of the proj- ect where moisture loss is expected to be slowest (e.g., shady or cooler locations). 25

Next: Chapter 8 - Quality Control »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 680: Manual for Emulsion-Based Chip Seals for Pavement Preservation examines factors affecting chip performance, highlights design and construction considerations, and explores procedures for selecting the appropriate chip seal materials. The report also contains suggested test methods for use in the design and quality control of chip seals.

Appendices A to J of NCHRP Report 680 provide further elaboration on the work performed in this project.

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