National Academies Press: OpenBook

Microsurfacing (2010)

Chapter: Chapter Six - Microsurfacing Equipment Practices

« Previous: Chapter Five - Construction Practices
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Suggested Citation:"Chapter Six - Microsurfacing Equipment Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter Six - Microsurfacing Equipment Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Page 44
Suggested Citation:"Chapter Six - Microsurfacing Equipment Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
×
Page 44
Page 45
Suggested Citation:"Chapter Six - Microsurfacing Equipment Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
×
Page 45
Page 46
Suggested Citation:"Chapter Six - Microsurfacing Equipment Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
×
Page 46
Page 47
Suggested Citation:"Chapter Six - Microsurfacing Equipment Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Page 47

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43 INTRODUCTION To a great extent, the equipment used in the construction phase drives the quality and performance of microsurfacing during its service life (Bergkamp 2010). Therefore, it is critical that the construction equipment system be well defined and capa- ble of controlling the construction means and methods critical to the performance of the product. Construction practices and procedures vary from region to region and are generally asso- ciated with local equipment availability and empirical know- ledge of its use. This chapter draws information from both the survey responses and the specification content analysis to identify those microsurfacing equipment practices that are associated with successful projects. Constraints that are con- tractually articulated are identified, categorized, and reported to allow the reader to easily note the range in philosophies that naturally occur across the nation and the world. Special attention has been paid to method specifications that pre- scribe specific construction equipment or that serve to enhance equipment operation. MICROSURFACING EQUIPMENT TRAIN The microsurfacing equipment train is designed around producing the job mix in the machine that lays it down on the roadway (see Figure 1 in chapter one). This occurs in a purpose-built machine that may either be self-propelled or mounted on a truck. Therefore, all the other pieces of equip- ment support the production of the microsurfacing placement machine (Nebraska DOR 2002). Most agencies will find the following types of equipment on a typical microsurfacing project: • Microsurfacing mixing (also called a placement) machine, • Mobile support units (also called nurse or feeder trucks) to replenish the materials in the mixing machine, • Broom sweepers—rotary or suction, and • Rollers, if required—pneumatic or static. Some agencies are careful to specify the equipment char- acteristics that are of specific interest in their construction specifications. Others prefer to use a performance specifica- tion and allow the contractor the latitude to pick and choose its equipment as long as the final product conforms to speci- fied performance criteria. An example of a method specifica- tion for microsurfacing equipment specifications was found in the Georgia DOT manual and is as follows: • Blend the paving mixture using a self-propelled microsurfacing mixing machine that is: – A continuous flow mixing unit. – Able to accurately deliver and proportion the aggregate . . . emulsion, mineral filler, field control additives, and water to a revolving multi-blade, twin shafted mixer. – Able to discharge the mixed product on a continuous flow. – EXCEPTION: Blending the paving mixture may be accom- plished with a truck mounted microsurfacing mixing machine that meets the above specification, except for continuous flow, when placing the mixture on short streets or projects that are less than one-half mile (800 m) in length. • For streets or projects less than one-half mile (800 m) in length, individual truck-mounted units may be used for placement of microsurfacing. For streets or projects one-half mile (800 m) or greater, in length, place microsurfacing mixture with a machine that is equipped as follows: – Has self-loading devices that load raw materials while con- tinuing to lay micro-surfacing, thereby minimizing construc- tion joints. – Has opposite side driving stations to optimize longitudinal alignment. – Allows the operator to have full hydrostatic control of the forward and reverse speed while applying micro-surfacing material (Georgia DOT 2001). The Kansas DOT uses the following performance spec- ification: Mix and spread the microsurfacing materials with a self pro- pelled machine capable of accurately delivering and proportioning all of the required components. Operate the machine continu- ously while loading, eliminating construction joints (Kansas DOT 2008). Microsurfacing Mixing Machine Figure 22 shows pictures of the continuous self-propelled and the truck-mounted mixing machines. The major difference is that the continuous machine can have its hoppers replenished while on the move. Thus, transverse construction joints are minimized to those areas where the machine has to stop mov- ing for some reason. The truck-mounted machine has to stop to have its ingredients replenished. Table 29 shows a summary of the requirements for micro- surfacing mixing machines found in the survey and content analysis. It shows that continuous self-propelled machines are preferred, but that a large proportion of agencies accept both. CHAPTER SIX MICROSURFACING EQUIPMENT PRACTICES

Two respondents, California and Illinois, indicated that they would probably exclude the use of a truck-mounted mixing machine on projects where long stretches of road are to be microsurfaced. The above-cited Georgia DOT specification is an example of one where the truck-mounted machine can only be used on short lengths of microsurfacing. Figure 23 shows how a mobile support unit replenishes the self-propelled con- tinuous mixing machine as it moves down the road applying microsurfacing. The mixing process is tied to the application rate of the mix. Application rate is controlled by instrumentation that ties together the emulsion pump, the gate settings on the aggregate, and some form of controller for the dry additives. The survey asked if computerized controls such as the ones shown in Fig- ure 24 were specified by each agency in the population. In the United States, seven agencies answered that they do, whereas in Canada the number was four. Calibration of Microsurfacing Machinery To ensure that the mix contains the specified proportions of its ingredients the mixing machine need to be calibrated (ISSA 2010a). Additionally, calibration of individual machines is necessary because of the continuous feed nature of the mixing machine (Minnesota DOT 2005). To achieve a homogenous mix, it is important that the materials be delivered to the pug mill in the correct proportions. When calibrating, it is impor- tant to remember that the job mix formula is based on the 44 “combined weight of dry aggregate and dry mineral filler (if used). Corrections for moisture in the aggregate could be nec- essary” (ISSA 2010a). Calibration accomplishes the follow- ing tasks: • It sets the machine to the specified job mix formula. • It strives to maintain consistency with respect to the design on all mixing equipment if more than one is used for a given job. • It permits the benchmarking of data output from the calibrated machine (ISSA 2010a). Table 30 contains the ISSA-recommended procedures for calibrating the emulsion pump and the feed rates of the aggre- gate and the mineral filler. Each make of mixing machine will have its own method for feeding dry additives to the pug mill (National Highway Institute 2007). Some use a system that is mechanically connected to the head pulley. These will use a gate setting that is very similar to the one on the aggregate belt. Another makes use of a system that is hydraulically matched by means of a ratio meter. These have a hydraulic flow adjustment that must be checked. Additionally, emulsion pumps vary from manufacturer to manufacturer. Pumps are either a fixed positive displacement pump or a variable positive displacement pump that can be mechanically set to vari- ous rates of flow. Since a variable volume pump will normally not be changed during the project, a calibration is necessary only for the setting that the contractor intends to use. Variable volume pumps should be equipped with a lock to avoid accidental changes and should be locked in place once calibration is completed. Cal- ibrate emulsion to the head pulley count which is displayed on the rock/aggregate counter (ISSA 2010a). FIGURE 22 Continuous front-loaded self-propelled (left ) and truck-mounted (right) microsurfacing mixing machines (Courtesy: Bergkamp Inc. 2010). TABLE 29 SUMMARY OF MIXING MACHINE REQUIREMENTS Mixing Machine Requirement U.S. Canada Content Analysis Continuous self-propelled 14 2 18 Truck-mounted 0 0 0 Both 11 5 0 Do not know 1 1 0 Require specific model/make 0 0 0 Do not require specific model/make 23 7 3 Do not know if required or not 3 1 0

45 The survey collected information on the practice of micro- surfacing machine calibration. The results are shown in Table 31. U.S. agencies favor field calibration to certified laboratory calibration by a margin of 2 to 1, although the specification content analysis was evenly split. One-half the Canadians calibrate in the field, whereas three Canadian agen- cies do not specifically require calibration of any sort. Preparing Test Strips Even with calibration there is the possibility that the materials in use could measurably impact the design application rate (Wood 2007). ISSA (2010a) provides four factors that influ- ence actual application and need to be taken into consideration in the field: 1. Adherence to the job mix formula aggregate gradations is crit- ical. Often the designated aggregate gradations may vary in par- ticle size distribution. For example, a Type II aggregate from one supplier may be finer than a Type II aggregate from another supplier and thus could easily be applied lighter. Aggregates produced by different types of crushers from the same parent rock may produce different shaped particles. For instance an impact crusher will produce nugget-shaped particles while a cone crusher will produce flat and elongated slivers. 2. Aggregates may vary in unit weight and a thicker application of one rock may actually weigh less than a thinner application of another. It is important to recalibrate the placement machine(s) for changes in aggregate sources. 3. Surface texture [of the substrate] will affect the application rate. A smooth surface does not have as many voids to fill and thus keeps the spread rate at a minimum. A weathered, raveled, open surface will increase the spread rate as the material fills the voids at the same time it is covering the surface. 4. Surface textures will often vary on the same road between traf- fic areas and shoulders or centerline areas. Application rates will vary with surface texture and thus may vary across any given cross section of a pavement (ISSA 2010a). Many specifications account for these variations after cal- ibration by requiring the construction of a test strip before allowing the contractor to begin full production microsurfac- ing. The content analysis discovered that roughly 40% of the FIGURE 23 Continuous front-loaded self-propelled being resupplied by a mobile support unit (Courtesy: Bergkamp Inc. 2010). FIGURE 24 Typical microsurfacing machine computer control system and logic (Courtesy: Bergkamp Inc. 2010).

46 determination of break and cure time. The Louisiana DOTD had the most complete specification in the analysis regarding microsurfacing test strips: The contractor shall place a 1,000’ test strip with the microsur- facing material for each different roadway condition based on the approved job mix formula . . . Acceptance of the test strip will be based on construction technique, mixture stability, longitudinal and transverse tolerances, yield and texture. The test strip will be Asphalt Emulsion Calibration Procedure Aggregate Calibration Procedure 1. Empty machine of all aggregate. Fill the placement machine with emulsion and measure the gross weight on a platform scale. 2. Hook pump outlet to a second container capable of holding 600 to 700 gallons (2,270 to 2,650 liters), such as a distributor or mobile support unit. 3. Run a minimum of 50 counts (if 50 counts are obtainable) on the rock/aggregate counter. 4. Determine weight of emulsion pumped by reweighing the placement machine. 5. Determine the weight of emulsion pumped per count on the rock/aggregate counter. 6. Run three tests to ensure accurate results. If variable displacement pumps are used, once calibrated they must be locked to stay constant with the JMF. Consult the manufacturerís recommendation for the use of variable displacement pumps on placement machines. Calibration will have to be done for enough settings to establish a straight line graph. 7. The emulsion pump should deliver emulsion to the pug mill with such volumetric consistency that the deviation for any individual delivery rate check run shall be within 2% of the mathematical average of three runs of at least 300 gallons (1,135 liters) each. Dry Additive/Mineral Filler Calibration Procedure 1. Check that all aggregate is removed from the placement machine as the conveyor belt must turn while calibrating the fines feeder. 2. Use a small pan or other container to catch the mineral filler that falls from the feeder. Weigh this container prior to performing the next steps. 3. Using the rock/aggregate counter to count the turns of the head pulley or the fines feeder auger, run out approximately 10 counts of material into the container. 4. Weigh the container of material and subtract the weight of the container. The weight of material divided by the count of the rock/aggregate counter or the fines feeder gives weight per turn. 5. Repeat at three settings to develop a curve for the material at various gate settings. 6. Calculate the desired setting to meet JMF requirements, set the gate or hydraulic controls, and verify the delivery rate. 1. Test the moisture of the aggregate. Calculate the moisture factor. 2. Moisture factor is the percent (in decimal format) of moisture in the aggregate + 1.00. 3. Select and record three gate openings and graph. 4. Oversized aggregate should be removed by screening prior to loading into the transport vehicle or placement machine. Weighing the aggregate should be completed after the screening operation. 5. Run at least 3 tons of material per gate setting, recording the net weight conveyed and the number of counts of the rock belt for three test samples, each a minimum of 50 counts. 6. The placement machine should deliver such volumetric consistency that the deviation for any individual aggregate delivery rate check-run shall not exceed 2% of the mathematical average of three runs. 7. Determine the average dry weight per count as per the rock calibration worksheet and plot the results to the graph. If a plotted straight line is not acquired on the graph, re-run the tests. 8. Set gate to the desired setting. 9. Run a small amount of material past the gate to establish the flow and fill the gate. Remove any excess material. 10. Weigh the placement machine. (Note all weights and counts.) 11. Reset the rock/aggregate counter to zero. 12. Run material out of the machine and stop the belt just as the counter changes to a new count to avoid partial counts. 13. Remove from the belt any excess material that has passed the gate but may not have fallen into the pug mill. Re-weigh the placement machine. The net weight of run divided by the count of the rock/aggregate counter provides pounds of aggregate per revolution of the head pulley. Source: ISSA (2010a). JMF = job mix formula. TABLE 30 MICROSURFACING CALIBRATION PROCEDURES TABLE 31 SURVEY RESULTS ON CALIBRATION PRACTICES Location of Calibration U.S. Canada Content Analysis Field Calibration 15 4 8 Contractor Furnishes a Calibration Certificate 7 1 8 No Calibration 1 3 1 Do Not Know 0 0 1 microsurfacing specifications required the construction of a test strip. These ranged from 500 to 1,000 ft (152.4 to 304.8 m) in length. The purpose of the test strip is not only to validate that the calibrated machine is dispensing the precise amount of mix, but it is also to demonstrate the contractor’s ability to properly construct transverse and longitudinal joints (Wood 2007). It also allows the agency to observe and measure, if necessary, the texture of the final mix after calibration. It also allows field

47 approved by the engineer prior to continuation of construction (Louisiana DOTD 2006). This specification was selected as a good example because it called out the performance measures that will be checked for acceptance. Additionally, it recognizes that roadway condi- tions will vary from site to site and a one-size-fits-all test strip will not account for this type of variation. Finally, it supports the quality assurance program by requiring the contractor to price the test strip(s), thereby creating an opportunity to solve product quality issues before they occur on a large scale. The Minnesota DOT requires that the test strip be con- structed after dark, presumably to ensure that full production microsurfacing can be conducted at night without quality degradation. Minnesota DOT also adds: “Carry normal traffic on the test strip within one hour after application, without any damage occurring. The Engineer will inspect the completed test strip after 12 hours of traffic to determine if the mix design is acceptable” (Minnesota DOT 2009). The Minnesota test strip not only tests the calibration of the machine and the contractor’s workmanship, but also conducts a short-term field test of the job mix formula itself. These two specifications and the ISSA recommendations lead to the following effective practice: Requiring a test strip of 500 to 1,000 ft (152.4 to 304.8 m) in length be constructed and accepted allows the agency and the contractor to ensure that the equipment is properly calibrated and that any workmanship issues are resolved before full-scale microsurfacing production. If the micro- surfacing is scheduled to occur after dark, the test strip is to be constructed after dark. The final aspect of using test strips to validate the cali- bration of the microsurfacing machinery is the requirement that the machine be recalibrated every time there are changes in material sources (Minnesota DOT 2009). This notion accounts for the actuality that materials that were used to prepare the job mix formula will necessarily change as the project progresses. “To assure that the slurry system treat- ment is constructed consistent with the JMF [ job mix for- mula], the placement machine(s) must be calibrated using the actual project materials” (ISSA 2010a). Often the samples used for the laboratory tests that lead to the mix formula will come from stockpiles where the aggregate was crushed and stockpiled for some time. However, in the middle of the micro- surfacing season, the aggregate is more likely to be freshly crushed and, as such, will have marginally different properties than the test samples that necessitate a recalibration to main- tain the desired application rate and job mix formula (Wood 2007). A number of the survey respondents added a comment to their calibration frequency answer that validated the infor- mation found in the literature. This leads to the following effective practice: The microsurfacing placement machine is to be recali- brated every time there is a change in material source or composition. Brooms and Rollers Brooms and rollers are support equipment for a microsur- facing project. The brooms are used before laying the micro- surfacing to clean the road’s surface of foreign debris and materials. They may also be used after construction is com- plete to remove excess aggregate from spillage and raveling. The suction broom is generally used for post-project clean-up because it puts less shear stress on the newly laid surface than the rotary broom. Figure 25 has pictures of both machines. Rollers come in two standard types: pneumatic tired rollers and static steel rollers. It appears that the use of this piece of equipment is not standard across the U.S. and in Canada. ISSA (2010b) recommends that microsurfacing used to fill deep rut- ting be rolled using a 10- to 12-ton pneumatic roller. Never- theless, there seems to be no agreement as to whether or not rolling adds value to the microsurfacing process. The survey found that most agencies do not require rolling (see Table 32). The ISSA Recommended Guideline for Microsurfacing (2010b) contains the following clause regarding rolling: Rolling is usually not necessary for microsurfacing on roadways. Airports and parking areas should be rolled by a self-propelled, FIGURE 25 Typical rotary broom (left) and suction broom (right) (Courtesy: Broce Inc. 2010).

10-ton (maximum) pneumatic tire roller equipped with a water spray system. All tires should be inflated per manufacturer’s specifications. Rolling shall not start until the microsurfacing has cured sufficiently to avoid damage by the roller. Areas which require rolling shall receive a minimum of two (2) full coverage passes (ISSA 2010b). The Quebec MOT added an interesting comment to its answer to this question when it stated it only uses rolling to “accelerate the curing period.” A follow-up revealed that the agency believes that because rolling promotes embed- ment that the road can be opened to traffic earlier than without rolling. This appears to run parallel to the ISSA rec- ommendation that “airports and parking areas should be rolled . . .” Presumably, the purpose is to promote greater embedment or adhesion and thus at an airport rolling would reduce the amount of foreign, objects, and debris (FOD) that could damage a jet engine. Parking lots are low-volume facil- ities and hence will not experience as much traffic com- paction as a road, making rolling desirable. Going any deeper into this question is beyond the scope of this report; however, it does beg the question of whether or not rolling has any impact on microsurfacing performance, and hence makes a good area in which to conduct future research. 48 SUMMARY Good microsurfacing products are impossible without well- calibrated and well-functioning equipment. Because micro- surfacing is an equipment-intensive activity, special attention is necessary for the machines that will ultimately make micro- surfacing a profitable pavement maintenance and preserva- tion tool. The next chapter will focus on how to employ the tools and techniques of this and previous chapters to produce a high-quality microsurfacing product. The following effec- tive practices were identified in this chapter: 1. Requiring a test strip of 500 to 1,000 ft (152.4 to 304.8 m) in length be constructed and accepted allows the agency and the contractor to ensure that the equip- ment is properly calibrated and that any workmanship issues are resolved before full-scale microsurfacing production. If the microsurfacing is scheduled to occur after dark, the test strip is to be constructed after dark. 2. It is important that a microsurfacing placement machine be recalibrated every time there is a change in material source or composition. Roller Requirements U.S./State Canada/Province Specification Content Analysis/State Static Steel 1/NC 0 1/AL Pneumatic Tired 7/AL, NC, NV, NY, OK, PA, VA 1/NS 4/AL, OK, PA, VA Combination Pneumatic/Steel 1/NC 2/NS, QB 0 No Rollers Specified 17 5 13 TABLE 32 MICROSURFACING ROLLER REQUIREMENTS BY AGENCY

Next: Chapter Seven - Quality Control and Quality Assurance and Performance Measures »
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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 411: Microsurfacing explores highway microsurfacing project selection, design, contracting, equipment, construction, and performance measurement processes used by transportation agencies in the United States and Canada.

Microsurfacing is a polymer-modified cold-mix surface treatment that has the potential to address a broad range of problems on today’s highways.

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