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

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

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45 (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 FIGURE 23 Continuous front-loaded self-propelled being rock may produce different shaped particles. For instance an resupplied by a mobile support unit (Courtesy: Bergkamp Inc. impact crusher will produce nugget-shaped particles while a cone crusher will produce flat and elongated slivers. 2010). 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) The survey collected information on the practice of micro- for changes in aggregate sources. 3. Surface texture [of the substrate] will affect the application surfacing machine calibration. The results are shown in rate. A smooth surface does not have as many voids to fill and Table 31. U.S. agencies favor field calibration to certified thus keeps the spread rate at a minimum. A weathered, raveled, laboratory calibration by a margin of 2 to 1, although the open surface will increase the spread rate as the material fills specification content analysis was evenly split. One-half the the voids at the same time it is covering the surface. 4. Surface textures will often vary on the same road between traf- Canadians calibrate in the field, whereas three Canadian agen- fic areas and shoulders or centerline areas. Application rates cies do not specifically require calibration of any sort. will vary with surface texture and thus may vary across any given cross section of a pavement (ISSA 2010a). Preparing Test Strips Many specifications account for these variations after cal- ibration by requiring the construction of a test strip before Even with calibration there is the possibility that the materials allowing the contractor to begin full production microsurfac- in use could measurably impact the design application rate ing. The content analysis discovered that roughly 40% of the FIGURE 24 Typical microsurfacing machine computer control system and logic (Courtesy: Bergkamp Inc. 2010).

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46 TABLE 30 MICROSURFACING CALIBRATION PROCEDURES Asphalt Emulsion Calibration Procedure Aggregate Calibration Procedure 1. Empty machine of all aggregate. Fill the 1. Test the moisture of the aggregate. Calculate the placement machine with emulsion and measure the moisture factor. gross weight on a platform scale. 2. Moisture factor is the percent (in decimal format) 2. Hook pump outlet to a second container capable of moisture in the aggregate + 1.00. of holding 600 to 700 gallons (2,270 to 2,650 liters), 3. Select and record three gate openings and graph. such as a distributor or mobile support unit. 4. Oversized aggregate should be removed by 3. Run a minimum of 50 counts (if 50 counts are screening prior to loading into the transport vehicle obtainable) on the rock/aggregate counter. or placement machine. Weighing the aggregate 4. Determine weight of emulsion pumped by should be completed after the screening operation. reweighing the placement machine. 5. Run at least 3 tons of material per gate setting, 5. Determine the weight of emulsion pumped per recording the net weight conveyed and the nu mber count on the rock/aggregate counter. of counts of the rock belt for three test samples, 6. Run three tests to ensure accurate results. If each a minimum of 50 counts. variable displacement pumps are used, once 6. The placement machine should deliver such calibrated they must be locked to stay constant with volumetric consistency that the deviation for any the JMF. Consult the manufacturers individual aggregate delivery rate check-run shall recommendation for the use of variable not exceed 2% of the mathematical average of three displacement pumps on placement machines. runs. Calibration will have to be done for enough settings 7. Determine the average dry weight per count as to establish a straight line graph. per the rock calibration worksheet and plot the 7. The emulsion pump should deliver emulsion to results to the graph. If a plotted straight line is not the pug mill with such volumetric consistency that acquired on the graph, re-run the tests. the deviation for any individual delivery rate check 8. Set gate to the desired setting. run shall be within 2% of the mathematical average 9. Run a small amount of material past the gate to of three runs of at least 300 gallons (1,135 liters) establish the flow and fill the gate. Remove any each. excess material. Dry Additive/Mineral Filler Calibration Procedure 10. Weigh the placement machine. (Note all 1. Check that all aggregate is removed from the weights and counts.) placement machine as the conveyor belt must turn 11. Reset the rock/aggregate counter to zero. while calibrating the fines feeder. 12. Run material out of the machine and stop the 2. Use a small pan or other container to catch the belt just as the counter changes to a new count to mineral filler that falls from the feeder. Weigh this avoid partial counts. container prior to performing the next steps. 13. Remove from the belt any excess material that 3. Using the rock/aggregate counter to count the has passed the gate but may not have fallen into the turns of the head pulley or the fines feeder auger, pug mill. Re-weigh the placement machine. The net run out approximately 10 counts of material into the weight of run divided by the count of the container. rock/aggregate counter provides pounds of 4. Weigh the container of material and subtract the aggregate per revolution of the head pulley. 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. Source: ISSA (2010a). JMF = job mix formula. microsurfacing specifications required the construction of a test determination of break and cure time. The Louisiana DOTD strip. These ranged from 500 to 1,000 ft (152.4 to 304.8 m) in had the most complete specification in the analysis regarding length. The purpose of the test strip is not only to validate that microsurfacing test strips: the calibrated machine is dispensing the precise amount of mix, but it is also to demonstrate the contractor's ability to properly The contractor shall place a 1,000' test strip with the microsur- facing material for each different roadway condition based on the construct transverse and longitudinal joints (Wood 2007). It approved job mix formula . . . Acceptance of the test strip will be also allows the agency to observe and measure, if necessary, the based on construction technique, mixture stability, longitudinal texture of the final mix after calibration. It also allows field and transverse tolerances, yield and texture. The test strip will be TABLE 31 SURVEY RESULTS ON CALIBRATION PRACTICES Content Location of Calibration U.S. Canada 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

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