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Refining the Simple Performance Tester for Use in Routine Practice (2008)

Chapter: Chapter 4 - Simple Performance Test Specimen Fabrication System

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Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
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Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
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Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
×
Page 29
Page 30
Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
×
Page 30
Page 31
Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
×
Page 31
Page 32
Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
×
Page 32
Page 33
Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
×
Page 33
Page 34
Suggested Citation:"Chapter 4 - Simple Performance Test Specimen Fabrication System." National Academies of Sciences, Engineering, and Medicine. 2008. Refining the Simple Performance Tester for Use in Routine Practice. Washington, DC: The National Academies Press. doi: 10.17226/14158.
×
Page 34

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27 4.1 Recommended Standard Practice for Performance Test Specimen Fabrication A recommendation made by several reviewers of AASHTO TP62 was that the test specimen fabrication procedures should be removed from AASHTO TP62 and moved to a separate standard practice so that additional guidance on specimen fabrication could be provided. Since NCHRP Project 9-29 was developing equipment for specimen fabrication, it was logical that a recommended standard practice of performance test specimen fabrication be developed by the project team. The resulting practice is contained in Appendix A. Major items addressed in this practice include: • HMA mixture preparation; • Over-sized gyratory specimen preparation; • SPT test specimen preparation; • SPT test specimen air void content; and • SPT test specimen storage. The recommended practice also includes two important appendices that provide additional guidance for preparing SPT specimens. The first is a procedure for obtaining the target air void content for specimens from mixtures that the technician is not familiar with. This procedure was developed at Arizona State University during NCHRP Project 9-19. The second appendix provides a method for evaluating the uniformity of air void contents within SPT test specimens. The appendix is intended help identify the gyratory specimen height that yields the most uniform air voids for a given laboratory. 4.2 Automated Specimen Fabrication Equipment The remainder of this chapter documents the development of the automated coring and sawing device for the SPT. This device was developed to simplify and automate test specimen fabrication for the SPT. Based on a thorough specimen size and geometry study conducted during NCHRP Project 9-19, the required test specimen for the SPT is a 100 mm (4 in.) di- ameter by 150 mm (6 in.) tall cylindrical specimen that is cut and cored from a larger 150 mm (6 in.) diameter by 175 mm (6.9 in.) gyratory specimen prepared in a Superpave Gyratory Compactor (4). The specimen size, 100 mm (4 in.) in diam- eter with a height to diameter ratio of 1.5, is needed to ensure that fundamental material properties are measured in the SPT. The test specimen is sawed and cored from a larger gyratory compacted specimen to minimize air void gradients in the specimen, and to provide smooth sides for attaching instrumentation and flat, parallel ends to minimize end effects during testing. Specimens prepared in the Superpave gyratory compactor have higher air void contents near the ends and circumference of the specimen. Test specimen preparation for the SPT is a multi-step process. Appendix A presents a draft standard practice for preparing SPT test specimens. First, tall gyratory specimens must be prepared to an air void content that is 1 to 2 percent higher than the desired air void content of the test specimen. During this step, it is critical that the mold be loaded in a manner that minimizes segregation in the specimen. Next, the 100 mm (4 in.) diameter test specimen must be cored from the larger gyratory specimen. Finally, the test specimen is cut to the appropriate length by sawing approximately 12.5 mm (1/2 in.) from each end of the specimen. During the cor- ing and sawing operations, it is critical the test specimen be properly clamped and the cutting be performed at the proper rate to ensure a smooth specimen with flat parallel ends is prepared. In evaluating the specimen preparation process, it was determined that an automated system for coring and sawing the specimens would be beneficial to the future implementa- tion of the SPT. Such a system would reduce the amount of skilled labor needed to prepare test specimens. It also would C H A P T E R 4 Simple Performance Test Specimen Fabrication System

28 minimize the potential for errors in the coring and sawing operations that result in specimen rejection due to noncom- pliance with the SPT specimen dimensional tolerances. 4.2.1 Equipment Selection Process General requirements for an automated coring and sawing device were set forth in the First Article Equipment Specifi- cation for the Simple Performance Test Specimen Fabrication System developed by the research team based on experience with the fabrication of many SPT specimens for other re- search projects. The major requirements of this specification are given in Table 12. Proposals were solicited from several manufacturers that expressed interest in building the equipment during a work- shop held in Phase I of the project. Only two manufacturers responded to the RFP issued on January 2, 2002 for the sys- tem: Shedworks, Inc. and Pine Instrument Company. Shed- works proposed an innovative approach where the gyratory specimen is gripped by a chuck similar to that used in a lathe. Automated diamond-tipped cutoff blades saw the gyratory specimen to length, and an automated diamond-core barrel then cores the test specimen from the gyratory specimen. Pine’s system included a portable laboratory core drill, a spe- cially designed clamp to hold the gyratory specimen during the coring operation, and a milling machine to cut the cored test specimen to the appropriate length. Based on a compre- hensive evaluation of the two proposals, the equipment proposed by Shedworks was selected for purchase in NCHRP Project 9-29. Although the Shedworks approach was consid- ered to be more risky, it had the potential to automate and simplify the specimen fabrication operations and thereby accelerate the implementation of the SPT. The system pro- posed by Pine represented only a marginal improvement over available equipment, and did not address the primary objec- tive of the Simple Performance Test Specimen Fabrication System, which was to automate and simplify the specimen fabrication operations. 4.2.2 Equipment Development A purchase order for the equipment was issued to Shed- works, Inc. on March 9, 2002. A five month schedule was provided for final design, fabrication, and delivery of the equipment. After completing the design, Shedworks, Inc. elected to subcontract the fabrication of the automated chuck components to another company. The automated chuck was designed to tighten against the specimen when rotated. This aspect of the design was not only important for automating the specimen fabrication process, but it also Requirement Specification Assembled Size No larger than 60 in. by 96 in. by 72 in. high. Maximum Component Size No wider than 30 in. Electrical Power Single phase 115 or 230 VAC Cutting Fluid Air or Water Air Supply 125 psi max pressure, 10.6 cfm max volume Specimen Preparation Time Less than 15 min. Item Specification Note Average Diameter 100 mm to 104 mm 1 Standard Deviation of Diameter 0.5 mm 1 Height 147.5 mm to 152.5 mm 2 End Flatness 0.5 mm 3 Specimen Dimensions End Perpendicularity 1.0 mm 4 Notes: 1. Using calipers, measure the diameter at the center and third points of the test specimen along axes that are 90 apart. Record each of the six measurements to the nearest 0.1 mm. Calculate the average and the standard deviation of the six measurements. 2. Measure the height of the test specimen in accordance with Section 6.1.2 of ASTM D 3549. 3. Using a straightedge and feeler gauges, measure the flatness of each end. Place a straightedge across the diameter at three locations approximately 120 apart and measure the maximum departure of the specimen end from the straightedge using tapered end feeler gauges. For each end record the maximum departure along the three locations as the end flatness. 4. Using a combination square and feeler gauges, measure the perpendicularity of each end. At two locations approximately 90 apart, place the blade of the combination square in contact with the specimen along the axis of the cylinder, and the head in contact with the highest point on the end of the cylinder. Measure the distance between the head of the square and the lowest point on the end of the cylinder using tapered end feeler gauges. For each end, record the maximum measurement from the two locations as the end perpendicularity. Table 12. First Article Specimen Requirements.

29 allowed the chuck to adjust for creep that occurs in as- phalt concrete under sustained loads. With this design, the gyratory specimen will not loosen during the sawing and coring operations as a result of the self-tightening action. Unfortunately, the chuck components were complex, and the subcontractor was not able to satisfactorily fabricate the components. In an attempt to keep the project on schedule and within budget, Shedworks fabricated a manual chuck that held the gyratory specimen in place using screws that were manually tightened. This allowed Shedworks to as- semble the device and shop test it in early November 2002. This version of the device was powered by a small single phase electric motor, used air to cool the cutting blades and the core barrel, and pneumatic actuators to automate the sawing and coring. The shop testing revealed several serious problems with this design as summarized in Table 13. Shed- works requested and was granted additional time to resolve the problems. Over the next 15 months, Shedworks designed and fabri- cated modifications to address each of the problems identi- fied during the November 2002 shop test. The following major modifications were made: 1. Cooling fluid: The cooling fluid was change from air to water. 2. Motor size: The motor was increased to a 3 hp 208/230 V three phase motor. The phase conversion is done internal to the machine so that only a single phase supply is needed. 3. Actuator fluid: The actuator fluid was changed from air to a hybrid air/hydraulic system to provide better control over the cutting and coring forces and speeds. In some cases these modification resulted in changes to other components in the device. For example, the decision to change to water as the cooling fluid required that the chuck bearing seals be redesigned to be watertight and that corro- sion resistant materials or finishes be used on all parts that would be exposed to water. In July, 2004, Shedworks delivered the first version of the Shedworks FlexPrep™ system. This unit, shown in Figure 25, incorporated the improvements listed above, but still used the manual screw chuck. During testing by the research team, it was determined that this chuck was not acceptable. Gyra- tory specimens loosened in the chuck approximately 50 per- cent of the time, and when this occurred, a test specimen could not be obtained. Additionally, the core barrel tended to break through the specimen leaving a ragged edge at the top. The chuck failure rate was greatest for samples made with softer binders and harder aggregates. When the specimen did not loosen in the chuck, a specimen meeting the tolerances in Table 12 was obtained except at the top edge where the break- through was occurring. Because the FlexPrep™ system showed promise, Shedworks was granted additional time and funding to develop an improved chuck and a back-up plate to eliminate the breakthrough. The breakthrough problem was resolved by adding a back- up plate. The back-up plate is held tight against the top of the specimen by a pneumatic actuator. Shedworks considered many alternatives for the chuck, ultimately deciding that the original concept was the only acceptable alternative. Several design changes were made to simplify the chuck mechanism, and in July, 2005 Shedworks produced a prototype version of the chuck that functioned as designed. The major issue that remained was to develop seals to keep water and grit from entering the bearings and operating mechanism of the chuck. This required a number of iterations. Finally a slinger-type seal was developed and the self-tightening chuck and seals were installed on the FlexPrep™ System. This final version of Problem Possible Cause Possible Solution Saw Blade Flexure. The saw blades flexed after approximately ½ in deep cut. Cutting was stopped to avoid blade damage. Actuator force or speed too high. Add blade bearing strips to support the saw blade against flexure Core barrel able to stall motor when cutting at appropriate coring force. Motor size too small 1. Increase motor size. Will require 208/230 V single phase power. 2. Switch to a hydraulic motor. Heat build-up when cutting at reduced coring force melted binder and caused specimen to slip in the chuck. Inefficient cutting due to reduced coring force. Air cooling may not be adequate. 1. Increase motor size to allow higher coring force. 2. Use coarser diamond blades and core barrels to provide greater heat dissipation. 3. Use water for cooling. Actuator control. Current pneumatic actuators functioned smoothly under no load conditions. There is concern that control under load when completing cuts may not be acceptable. Compressibility of air. Switch to hydraulic actuators. Table 13. Operational problems identified during November, 2002 shop testing.

30 Figure 25. Shedworks, Inc. FlexPrep™ System, serial number 001. the device was delivered in September 2006 and subjected to the specification compliance testing as described below. 4.2.3. Specification Compliance Testing Table 12 summarized the requirements contained in the first article equipment specification. The size, electrical power, air supply, and specimen preparation time were checked through measurements or information contained on compo- nent label plates. A small experimental plan was developed to check the dimension of specimens prepared with the device. This plan was based on 20 gyratory specimens that included the following variables: • Aggregate type: limestone and granite. • Nominal maximum aggregate size: 9.5 mm and 19.0 mm. • Binder grade: PG 58-28 and PG 64-22. • Air void content: 4 and 7 percent. • Height: 165 and 175 mm. The sections that follow present the findings of the specifi- cation compliance testing. 4.2.3.1 Physical and Operational Requirements The FlexPrep™ is very compact measuring 37 in. wide by 30 in. deep by 44 in. high and weighing approximately 400 lb. The system operates on single phase 208/230 V AC power and according to the manufacturer’s specifications requires only a modest air-flow of 3 cfm at 60 psi pressure. For the specifi- cation compliance testing, the equipment was operated with 208 V power with air pressure regulated at 75 psi. The ma- chine is capable of completing the sawing and coring opera- tions within the specified time of 15 min. The first step in preparing a test specimen with the Flex- Prep™ is to secure the gyratory specimen in the chuck of the machine. This is done by opening the top door and moving the specimen back-up plate to the open position as shown in Figure 26. The chuck is opened (See Figure 27) by turning the motor in the reverse direction using a socket wrench while the chuck is held stationary by an air actuated pin. The reverse force on the chuck opens the chuck mechanism, which has springs to ensure a minimum contact pressure when closed. Once the chuck is opened, the gyratory specimen is dropped into the chuck as shown in Figure 28. The core barrel is used to center the specimen vertically in the chuck as shown in Fig- ure 29. When the gyratory specimen is centered, the chuck is closed by turning the motor in the forward direction with the socket wrench. The back-up plate is secured as shown in Figure 26. Top view of FlexPrep™ chuck with upper door and specimen back-up plate open.

31 Figure 29. Centering the gyratory specimen vertically using the core barrel. Figure 27. Opening chuck using a socket wrench. Figure 30, then the top and front doors are closed, and the machine is ready to prepare the SPT test specimen. The FlexPrep™ system automatically performs the sawing and coring operations. First, the cutoff blades are advanced to trim the specimen ends. Once the ends are trimmed, the cutoff blades retract, and the core barrel advances from the bottom to core the test specimen. The finished specimen is removed from the core barrel by removing a cap on the bot- tom of the core barrel as shown in Figure 31. The waste ring is removed by opening the chuck mechanism as described above. Figure 32 shows the finished specimen after removal from the core barrel. Figure 28. Inserting gyratory specimen in the FlexPrep™ chuck. Figure 30. Securing the back-up plate.

32 Figure 32. Finished test specimen. Figure 31. Removing finished test specimen from the bottom of the core barrel. Figure 33. Water circulation system. The operator can adjust the speed of the cutoff blades and the core barrel using controls on the machine. The feed rates should be adjusted to obtain smooth cuts, generally slower for harder aggregates. Additionally the feed rates must be such that the current draw for the motor remains below 10 amps, otherwise the motor circuit breaker will trip. An amp meter is provided to aid in setting the feed rates. The FlexPrep™ circulates the cooling water. The system includes a pump and settling tank under the machine as shown in Figure 33 to capture then circulate the cooling water. 4.2.3.2 Specimen Dimensions One of the objectives of the specification compliance test- ing was to investigate the effect of several specimen variables on the finished dimensions of specimens fabricated with the FlexPrep™ system. The planned experiment included differ- ent binder grades, different nominal maximum aggregate sizes and aggregate hardness, high and low air void content specimens, and gyratory specimens compacted to two heights. Twenty test specimens were fabricated, and the dimensions of the test specimens were measured and com- pared to the tolerances listed in Table 12. Table 14 summa- rizes the measurements. As shown, all of the specimens meet the SPT specification requirements for diameter, height, and end perpendicularity. The top of several specimens fail the flatness requirement. The failing specimens are highlighted in bold. All of these specimens had aggregate torn from the top end near the middle of the specimen as shown in Figure 34. As the cutoff blade moves through the specimen, it tends to lift the waste material from the specimen as it cuts. If the cut- ting speed it too fast or there is an air void near the middle of the specimen, the waste material breaks from the specimen. If it breaks below the plane of the cutting blade, a divot is

33 Specim en Characteristics Diam eter Height Flatness Perpendicularity No. Mix Size, mm Aggregate Type Binder Grade Gyratory Height, mm Air Voids, % Average, mm Standard Deviation, mm Average, mm Top, mm Bottom, mm Top, mm Bottom, mm 1 9.5 Li me stone 58 165 7 101.3 0.1 149.5 0.25 0.05 0.05 0.05 2 9.5 Li me stone 58 165 7 100.7 0.2 148.3 0.30 0.05 0.10 0.05 3 19 Granite 58 175 7 100.8 0.1 149.5 0.30 0.10 0.05 0.30 4 9.5 Li me stone 64 165 7 101.3 0.1 149.8 0.45 0.05 0.15 0.05 5 9.5 Li me stone 64 165 7 101.1 0.1 149.9 0.45 0.05 0.15 0.05 6 9.5 Li me stone 64 165 4 101.3 0.0 149.8 0.20 0.05 0.15 0.05 7 9.5 Li me stone 64 165 4 101.1 0.1 149.7 0.25 0.10 0.10 0.05 8 9.5 Li me stone 64 165 7 101.0 0.1 149.9 0.15 0.05 0.05 0.15 9 9.5 Li me stone 58 165 7 101.0 0.1 148.0 0.10 0.05 0.05 0.30 10 19 Granite 64 165 7 101.4 0.1 149.3 0.30 0.25 0.10 0.10 11 19 Granite 64 165 7 101.3 0.1 150.0 0.30 0.30 0.15 0.10 12 19 Granite 64 175 4 101.1 0.1 150.3 1.00 0.15 0.15 0.15 13 19 Granite 64 165 4 101.0 0.1 150.5 0.85 0.05 0.50 0.10 14 9.5 Li me stone 64 165 4 101.2 0.1 150.6 0.90 0.10 0.50 0.10 15 9.5 Li me stone 58 165 4 101.0 0.0 149.8 0.90 0.05 0.40 0.30 16 9.5 Li me stone 64 175 4 101.2 0.1 150.5 0.45 0.05 0.30 0.10 17 9.5 Li me stone 64 175 7 101.3 0.1 150.5 1.45 0.45 0.40 0.10 18 19 Granite 64 175 7 101.2 0.1 150.6 1.25 0.15 0.30 0.05 19 19 Granite 58 165 7 100.8 0.1 149.9 0.80 0.15 0.25 0.20 20 19 Granite 58 165 4 101.1 0.0 150.7 1.25 0.20 0.30 0.20 Table 14. Dimensions of specimens prepared using the FlexPrep™ system. created as shown in Figure 34. None of the variables included in the experiment affected the top end flatness failure rate. Although both the top and bottom cutoff blades have the same shape, the failures only occurred on the top of the specimen. This is likely the result of the air void gradient produced by the Interlaken compactor used to fabricate the gyratory specimens for this study. The Interlaken compactor produces high air voids at the top of the specimens and low air voids at the bottom. It is likely that the higher air voids at the top are the reason the failures always occurred at that end during the specification compliance testing. The effect of not meeting the specimen end tolerance on the measured properties of the specimens was not evaluated in this study. The defects may not significantly affect the measured properties because the instrumentation is located far from the specimen end and aggregate interlock, and the resulting redistribution of stress and strain likely produces more uniform conditions at the center of the specimen. Additionally, it may be possible to fill the defects with plaster or some other material without significantly affecting the measured material properties. 4.2.4 Needed Improvements The FlexPrep™ System was found to be in substantial com- pliance with the first article equipment specification and was accepted by the research team. The machine complies with the physical size and power requirements. It can produce specimens meeting the dimensional tolerances for specimens for the SPT. The cycle time for cutting and coring test speci- mens is less than the specified 15 minutes. Although the FlexPrep™ System was accepted under NCHRP Project 9-29, the specification compliance testing Figure 34. Divot in top of specimens created by the FlexPrep™ system.

34 identified several improvements that should be made in future production units. These are summarized below: • Cutoff blade. Shedworks should perform additional devel- opment work to improve the cutoff blades and their con- trol to minimize the potential for aggregate being torn from the specimen. • Controls. The control system requires further improve- ment. In some cases, the limit switches that detect the completion of the cutting or the coring failed to trip. When this occurs, there is no manual override that allows the program to continue from its current point. The only alternative is to reset the machine and restart the cutting and coring operation from the beginning. Restarting from the beginning wastes time and increases the possi- bility that the test specimen will not meet the dimensional tolerances. • Water circulation system. The settling tank is undersized and requires frequent cleaning. Additionally the cooling water heats-up after several specimens are cut in succession. The water heats sufficiently that specimens made with soft binders and high air void contents may creep beyond the range of the self-tightening chuck or break while being cored. • Test specimen removal. It is difficult to remove the cap on the core barrel to remove the test specimen. A different type of core barrel cap is needed. • Front doors. The machine has a wide front door that pro- vides access for removing the test specimen and cleaning the system. This door is made from a polycarbonate mate- rial. It is relatively wide and split horizontally in the middle. A spring loaded pin-type latch is included in the top half of the door to close it. This door is difficult to use particularly when coring grit builds up on the door. Additionally, the door leaks at the bottom as shown in Figure 33.

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TRB's National Cooperative Highway Research Program (NCHRP) Report 614: Refining the Simple Performance Tester for Use in Routine Practice explores the develop of a practical, economical simple performance tester (SPT) for use in routine hot-mix asphalt (HMA) mix design and in the characterization of HMA materials for pavement structural design with the Mechanistic-Empirical Pavement Design Guide.

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