The Superpave Mix Design System: Anatomy of a Research Program (2012) / Chapter Skim
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51 What's In a Name? The NCHRP 9-6 research project developed recommendations for a mix design system called the Asphalt-Aggregate Mixture Analysis System (AAMAS)
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52 4.5.1 Binder Specification and Supporting Tests The binder specification (AASHTO MP 1) , as originally configured at the end of SHRP, is shown in Table 5.
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53 PERFORMANCE GRADE PG 46- PG 52- PG 58- PG 64- 34 40 46 10 16 22 28 34 40 46 16 22 28 34 40 10 16 22 28 34 40 Average 7-day Maximum Pavement Design Temp, °C <46 <52 <58 <64 Minimum Pavement Design Temperature, °C >-34 >-40 >-46 >-10 >-16 >-22 >-28 >-34 >-40 >-46 >-16 >-22 >-28 >-34 >-40 >-10 >-16 >-22 >-28 >-34 >-40 ORIGINAL BINDER Flash Point Temp, T48, Min °C 230 Viscosity, ASTM D4402, Max, 3 Pa·s, Test Temp, °C 135 Dynamic Shear, TP 5, G*
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54 PERFORMANCE GRADE PG 64- PG 76- PG 82- 10 16 22 28 34 40 10 16 22 28 34 10 16 22 28 34 Average 7-day Maximum Pavement Design Temp, °C <70 <76 <82 Minimum Pavement Design Temperature, °C >-10 >-16 >-22 >-28 >-34 >-40 >-10 >-16 >-22 >-28 >-34 >-10 >-16 >-22 >-28 >-34 ORIGINAL BINDER Flash Point Temp, T48, Min °C 230 Viscosity, ASTM D4402, Max, 3 Pa·s, Test Temp, °C 135 Dynamic Shear, TP 5, G*
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55 4.5.2 Other Binder-Related Products The A-003B team, studying asphalt-aggregate interactions, produced two major products from their work: models for adhesion and stripping, and the net adsorption test. The net adsorption test provided a method for determining the affinity of an asphalt-aggregate pair and its sensitivity to water.
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56 Level 1 Level 2 Level 3 Volumetrics • Volumetrics • Volumetrics • Performance Tests • Performance Tests • Performance Predictions • Performance Predictions 106 107 ESALs Level 3 also included an optional proof testing scheme that would allow the mix to be subjected to tests simulating the actual traffic and environmental conditions to confirm that the mix actually would perform at the desired level. In level 2 mix design, fewer tests were to be performed at fewer temperatures than for level 3 mix design.
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57 Aggregate Selection Asphalt Selection Volumetric Mixture Design include Moisture Susceptibility Measurement of Performance Based Material Properties Level 1 Permanent Deformation Fatigue Low Temperature Cracking Estimate of Pavement Performance Final Mix Design for Production Level 3* Level 2 Permanent Deformation Fatigue Low Temperature Cracking Estimate of Pavement Performance Field Mixture Control Tests Proof Testing Optional *
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58 Table 6 Comparison of Level 2 and Level 3 Mix Design Methods Permanent Deformation/ Fatigue Cracking Low-Temperature Cracking Test Types Level 3 considers more states of stress and requires two additional test methods No difference between level 2 and level 3 Test Temperatures Level 3 considers range of temperatures from 4 to 40°C Level 3 considers three temperatures Level 2 uses one effective temperature for fatigue cracking and one for permanent deformation Level 2 considers tensile strength at one temperature only Performance Prediction Level 3 breaks the year into seasons No difference between level 2 and level 3 Level 2 considers the entire year as a single season As originally configured, equipment and test protocols supporting the Superpave mix design system included the following: Gyratory or Rolling Wheel Compaction. Short and Long-Term Aging (Forced Draft Oven)
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59 Research addressing modified binders was conducted under NCHRP 9-10, Superpave Protocols for Modified Binders, and other research. 4.5.5 The Gyratory Story One of the most visible differences in the current Superpave method of mix design is the gyratory compactor.
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60 The mold was a piece of pipe with an inside diameter of 4 inches and a wall thickness of ½ inch. It was placed between two horizontal plates with an opening ½ inch greater than the height of the mold.
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61 the mold was twisted so opposite corners of the mold contacted the top and the bottom plates. By chance, the angle happened to be approximately 5 degrees and 40 minutes.
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62 In 1959 a delegation from France visited the United States and observed both the Texas gyratory compactor and the Corps of Engineers compactor. Curious about parameters of gyratory compaction, the Laboratoire Central des Ponts et Chausees (LCPC)
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63 4.5.5.2 Selection of SHRP Gyratory Compactor In May 1991, Gerry Huber was part of a panel that travelled to France to review French highway technology. The LCPC laboratories in Nantes, where Francis Moutier was located, was one of the stops.
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64 On this gyratory the angle was induced by a cam on a lever. A new cam was made to change the angle from 5 degrees to one degree.
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65 Speeds greater than 30 gyrations per minute were not investigated because the maximum speed of the Texas gyratory was 30 gyrations per minute. Changing the mechanism of the Texas 6-inch gyratory to increase the speed was too difficult.
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66 The mix design for the project had been done on the modified Texas gyratory at the Asphalt Institute but the prototype was to be used for quality control in the FHWA field laboratory. Immediately there were problems.
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67 The next field trial occurred three weeks later in Waukesha, Wisconsin, on I-43. Again, the mix designs were done at the Asphalt Institute on the modified Texas 6-inch gyratory.
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68 So, How Did the Angle Change? It was based on a mistake.
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69 0.25 0.5 0.75 1.0 2.0 Height to Diameter Ratio 4.5.5.4 Density Gradient in Gyratory Specimens The LCPC compactor could be used with 80, 120 and 160 mm diameter molds. LCPC had done some experiments looking at these three mold sizes using 10 mm maximum size and 20 mm maximum size mixtures.
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70 AAMAS study evaluated several different laboratory compactors.
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71 Table 8 Density Gradient in Specimen Compacted on Prototype Gyratory Compactor (14) 4.5.5.5 Field Compaction of Superpave Mixes Early in Superpave implementation there were some issues with achieving density, especially in areas where the Marshall mixes had been easy to compact.
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72 In the gyratory mold, a change in specimen height means a change in the zone of disturbance and a different compaction efficiency. As a result, the decision was made to keep the compaction efficiency in the mold the same for all mixtures (i.e., to keep the height to diameter ratio the same)
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73 "You mean it can predict rutting and cracking? " "Well, you are trying to design pavements that don't rut.
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74 Discussion of the GTM waned and implementation of the Superpave gyratory compactor continued. 4.5.5.7 Rolling Wheel Compaction A significant debate occurred within the SHRP Asphalt Research team about using a rolling wheel compactor, such as that shown in Figure 35, as the laboratory compaction method.
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75 Before continuing discussion about the type of compaction, it is important to understand the thought process in place in 1991 when the compaction experiment was completed. At the beginning of SHRP, the vision for mix design was a new method where properties of candidate mixtures would be measured and performance would be predicted.
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76 The view of the A-001 team was that rolling wheel compaction was impractical for use in mix design. Although the gyratory-compacted specimens might not have exactly the same properties as rolling wheel compacted specimens, evaluation of compacted mixture gave the same trends for both.
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77 laboratory compaction still had several unanswered questions and that a study was required with a larger scope than the A-003A experiment. In the end, it probably doesn't matter who was right, A-001 or A-003A (or both, or neither)
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78 Modulus would be the fundamental engineering property. Unfortunately, the behavior of HMA is more complicated, and, depending on the temperature and the time of load, its behavior may be linear elastic, non-linear elastic, visco-plastic, or plastic.
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79 The old properties were needed. The question was "How to get them?
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80 precipitation levels, July temperature, coldest winter temperature, traffic level and depth from the pavement surface. The results from the second questionnaire were tabulated and discussed.
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81 is more variable. Therefore, the group consensus was to use Gsb for all aggregates, absorptive and non-absorptive alike.
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82 I hope the above explanation gives you some insight into the logic of SHRP gradation bands. I know that the SHRP gradation controls are significantly different than some existing specifications as you point out and states will be faced with change during the implementation process.
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83 and absorption properties. Key products of these studies, the net adsorption test (NAT)
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84 be) developed in an attempt to identify asphalt concrete mixes susceptible to water damage.
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85 4.5.8 Analysis of a Meeting The SHRP Asphalt Research Program was a high pressure experience for the participants. A meeting held on July 25, 1991 at the Green Building (National Academy of Sciences at 2001 Wisconsin Ave NW)
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86 Page 1 1. Gyratory • 1º • 6 rpms Hughes (Chuck Hughes A-006 team)
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87 Below are some comments on the points in the Kennedy note. The first page of notes is an outline of what the mix design system would be.
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88 cooperative experiment by Dr. Ted Vinson and Dr.
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89 4.6 PEOPLE AND ORGANIZATIONS In addition to the program staff, committee members and researchers, there are a couple of other groups or people that deserve special mention. This section describes the spin-off Canadian Strategic Highway Research Program, C-SHRP, and those involved with that parallel effort.
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90 Technology transfer. As research results from the SHRP Program became available for implementation, the Canadian participants provided guidance for suitability and implementation in Canada.
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91 The Canadian Technical Asphalt Association (CTAA) , which is a technical association holding annual meetings at various locations throughout Canada, became a key tool for dissemination of research results and coordination of implementation activities.
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92 While many SHRP loaned staff members worked in a variety of areas, a few focused on the asphalt research area or later became predominantly asphalt researchers. The experience of working on an unprecedented research effort like SHRP could be very heady stuff.
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93 far short of the initial vision of a mixture analysis system. In part this is because of the initial obsession with binder chemistry and only superficial focus on asphalt-aggregate mixes.
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94 regular reinforcement of the evolutionary nature of the research: only those ideas/concepts/products that furthered the goal would be pursued; all others would be jettisoned ‒ another concept with which most academic researchers were unfamiliar. The quarterly, pro bono reviews of voluminous technical reports by the ETGs, were, not surprisingly, uneven at best – superficial at worst.
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