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1、Research Results Digest 370 February 2012 C O N T E N T S Introduction, 1 Workshop Results, 2 References, 14 Appendix A, A-1 Appendix B, B-1 INTRODUCTION Warm mix asphalt (WMA) refers to asphalt mixtures produced at temperatures at least 50F cooler than those typically used in the production of hot
2、mix asphalt (HMA). The goal of WMA is to produce mixtures with similar strength, durability, and perfor- mance characteristics as HMA using sub- stantially reduced production temperatures. Important environmental and health benefi ts are associated with reduced production tem- peratures, including l
3、ower greenhouse gas emissions, lower mix plant fuel consump- tion, and reduced exposure of workers to asphalt fumes. Lower production tempera- tures also can potentially improve pavement performance by reducing binder aging, pro- viding added time for mixture compaction, and allowing improved compac
4、tion during cold weather paving. The fi rst WMA pavements were con- structed in Europe in 1995 and in North America in 2004. Since that time, WMA production has substantially grown in the United States. For example, the FHWA estimates that 10% of the 358 million tons of asphalt mix placed nationwide
5、 in 2010 was WMA. As the use of WMA has widened, so have the number of WMA technologies available to the industry. In 2011, 30 or more WMA technologies were available, classified into three broad categories: (1) those using organic additives, including waxes; (2) those using chemical additives; and
6、(3) those using water-based foaming processes. Moreover, as these numbers suggest, WMA has rapidly moved from use in pilot and experimental projects to more routine, large-scale use. At present, at least 30 state departments of transportation (DOTs) have established specifi cations per- mitting the
7、use of WMA. This rapid growth in use of WMA use naturally raises questions about WMA pavement construction processes and the pavements long-term durability and GUIDELINES FOR PROJECT SELECTION AND MATERIALS SAMPLING, CONDITIONING, AND TESTING IN WMA RESEARCH STUDIES This digest summarizes the result
8、s of a “Workshop to Coordinate Key WMA Research Projects” sponsored by NCHRP Project Panels 9-43, 9-47, and 9-49 in conjunction with the AASHTO Highway Subcommittee on Materials, the Federal Highway Administration, and the National Asphalt Pavement Association. The workshop was held on May 13, 2011
9、at the National Academies Arnold and Mabel Beckman Center, Irvine, CA. The notes on which this digest is based were prepared by Mr. Tom Baker, Washington State Department of Transportation; Messrs. Matthew Corrigan and John Bukowski, Federal Highway Administration; Dr. Jon Epps, Texas Transportation
10、 Institute; Dr. David Newcomb, formerly of the National Asphalt Pavement Association; and Dr. Edward Harrigan, National Cooperative Highway Research Program. Responsible Senior Program Officer: E. T. Harrigan NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM performance. Newcomb (8) has summarized these
11、 questions as follows: Mix design: What modifi cations to asphalt mix technology, if any, are required for design- ing WMA? How is the selection of binder per- formance grades impacted by the lower WMA production temperatures? Long-term performance: How do the short- and long-term performance and du
12、rability of pavements constructed with WMA compare with those constructed with HMA? How are these qualities affected by WMA technologies using foaming or chemical additives? Does WMA present an increased potential for dis- tresses such as rutting and moisture damage? Cost benefi ts: What are the cos
13、t benefi ts of the reduced fuel consumption and emissions obtained with WMA? Plant operations: Is WMA compatible with the high production rates needed in the United States? Control of mixing process: Given that the various WMA mixing processes all differ to a greater or lesser degree from that of co
14、nven- tional HMA, are new guidelines needed for proper quality assurance of the mix? Workability at the paving site:Although the WMA may appear workable and easily com- pactable when produced, does it remain work- able at the paving site? Quick turnover to traffic: Can WMA pave- ments be opened to t
15、raffic as soon as possible after construction, in a time frame similar to or earlier than conventional HMA pavements? Answers to many of these questions are being pursued in WMA research studies informally coordi- nated through the FHWAs WMA Technical Work- ing Group. For example, more than 15 state
16、 DOTs are currently sponsoring WMA research studies, and the50 state DOTs are collectively sponsoring NCHRP projects investigating WMA mix design, the poten- tial moisture susceptibility of WMA pavements, and whether WMA and HMA pavements provide signifi - cantly different short- and long-term perfo
17、rmance. NCHRP projects planned for 2012 and later years will address the short-term laboratory aging of WMA for mix design and performance testing, characterization of foamed asphalt for WMA applications, and the use of recycled asphalt pavement (RAP) and recycled asphalt shingles (RAS) in WMA. A po
18、tential shortcoming of such a large, diverse, and dynamic program is the difficulty of comparing results between laboratory and fi eld studies using different experiment designs, conditioning protocols, and test methods to characterize the material proper- ties and performance of WMA. To address thi
19、s short- coming, the “Workshop to Coordinate Key WMA Research Projects” was organized with the goal of fos- tering cooperation and coordination among research studies. Workshop participants sought to (1) identify the laboratory and fi eld-test methods and sample preparation, curing, and conditioning
20、 procedures in use in FHWA-, NCHRP-, state DOT-, and industry- sponsored research projects and (2) agree on a core set of methods and procedures that would be com- monly used, insofar as possible, in all present and future WMA studies. In addition, the workshop participants reviewed new and existing
21、 WMA field pavements included in short- and long-term WMA performance studies to facilitate sharing of their fi eld materials and data, and developed selection criteria for future fi eld pavements. The invited participants included researchers and practitioners from the pub- licsector, academia, and
22、 industry, and representatives of the sponsoring organizations. WORKSHOP RESULTS The results of the workshop are presented in tables on the following pages. These tables present a pro- posed core set of criteria, methods, and protocols, including fi eld project selection criteria (Table 1); specimen
23、 preparation methods (Table 2); conditioning methods for laboratory-mixed, laboratory-compacted (LMLC) specimens, plant-mixed, laboratory-compacted (PMLC) specimens, and plant-mixed, fi eld-compacted (PMFC) specimens (Table 3); performance test methods for LMLC, PMLC, and PMFC specimens (Table 4, Ta
24、ble 5, Table 6, and Table 7); and binder and aggregate test methods (Table 8). These criteria, methods, and protocols were arrived at through a consensus-building activity involving all the workshop participants. They rep- resent the participants collective judgment of the minimum complement necessa
25、ry to enable correla- tion and comparison of results between or among WMA research studies. Use of this core set does not 2 (Text continues on p. 14.) 3 Table 1 Field project selection for short- and long-term performance studies. Project length Project and construction documentation Control section
26、 defi nition Minimum number of WMA technologies Other key features The minimum test section shall be 12lane-mile in one travel lane located between inter- sections or interchanges. The plant temperature may be increased to produce the HMA control section. Shorter sections may be allowed if they are
27、well planned and documented. Notes: The ideal production per test section is 8001000 tons or 12- to 1 day production or 1 tanker load of binder (400600 tons of non-foamed WMA); these amounts will vary depending on the nominal maximum aggregate size (NMAS) of the mix. Although 1 days production is of
28、ten possible, test projects with control sections often are difficult to fi nd. Selection of the minimum section length also must consider the type of WMA additive and where it is introduced. NCAT and the University of Minnesota (for cold-climate projects) have developed detailed checklists for docu
29、menting fi eld projects (see Appendix A). Notes: Key considerations are (1) a condition survey of the existing pavement, (2) the pavement cross-section, (3) evaluation of pavement structural support, and (4) WMA production and compaction temperatures. The HMA control section must be identical to the
30、 WMA sections (including any RAP or RAS content) in all aspects but the presence of WMA, with the exception that the binder content of the control section may differ if necessary to attain identical air void contents in all sections. Minimum two technologies, plus a control. However, this minimum nu
31、mber may be waived, depending on whether the project is a new pavement or an overlay on an existing pavement. (1) WMA and control sections must be surface mixes in the same travel lane and with the same pavement support throughout all sections. (2) The correct mix discharge temperatures for the WMA
32、must be verifi ed throughout the project. (3) New projects are favored, but existing projects may be used if the necessary require- ments are met. It is sometimes feasible to work with the state DOT and contractor to add WMA sections to an HMA project through a change order. (4) Specifi c and system
33、atic performance monitoring plans are required for new versus exist- ing WMA projects. Both plan types should include the provision for forensic analysis when pavements exhibit signifi cant distress. (5) In the event that a WMA project of interest was constructed without a control section, it may be
34、 possible to pair the WMA project with an otherwise unconnected HMA project constructed with similar materials, structure, condition, traffic, and climate (e.g., see Von Quintus, Mallela, and Buncher 1). (6) Future fi eld projects should consider (a) roadway functional classifi cation (average daily
35、 traffic ADT and trucks per day % trucks); (b) a variety of mix types (e.g., stone mas- tic asphalt and open-graded friction courses); and performance in intersections. (7) RAP and RAS are permitted as long as identical control mixes are available. (8) For overlay projects, the WMA and control secti
36、ons must have comparable levels of existing distress. 4 Table 2 Specimen preparation methods. WMA binders for mix design WMA binder extraction and recovery Laboratory blending with low shear mechanical stirrer or foaming with laboratory- foamed asphalt plant, per the proposed appendix to AASHTO R 35
37、 (2). Notes: (1) Aggregate coating is the key measure of foaming. Use coating to help guide selection of temperature. (2) Mix design may not require production of foamed binder in laboratory. Rather, it may be feasible to add water and binder to a bucket mixer containing aggregate and obtain compara
38、ble results. (3) At present, there are three commercial units for producing foamed asphalt. It is not known how these units compare. Further, it is very difficult to test the properties of foamed binder in bucket during production, and the foaming is typically lost during transfer of the binder for
39、mixing with the aggregate. In practice, however, no problems have been identifi ed with foamed WMA. (4) Future research is needed to better defi ne the requirements for laboratory production of foamed asphalt. A procedure such as that of Minnesota DOT is required. In the Minnesota DOT procedure, asp
40、halt binder extractions are performed using AASHTO T 164 Method A (Centrifuge Method). Toluene is used as the extraction solvent for the fi rst two washes with an 85:15 v/v mixture of toluene and 95% ethanol used for the third wash. ASTM D5404Standard Practice for Recovery of Asphalt from Solution U
41、sing Rotary Evaporatoris followed for the binder recovery method with the following modifi cations: Bath temperature and vacuum settings for toluene distillation (60C, 100mBar). Fines are removed from the extract by high-speed centrifuging at 2000 RPM for 35 minutes after volume of asphalt extract i
42、s reduced to 500ml. Notes: (1) The FHWA memorandum Extraction and Recovery Procedures at TFHRC Asphalt Laboratories (Appendix B) provides detailed information on a preferred method of binder extraction and recovery. (2) The use of trichloroethylene (TCE) as the extraction solvent is discouraged. TCE
43、 is known to harden recovered binders beyond the in situ level. (3) Western Research Institute has developed an infrared spectroscopy method for detecting residual solvent in the recovered binder. (4) Research indicates that a higher temperature and vacuum than specifi ed in ASTM D5404 may be requir
44、ed to effectively remove the toluene solvent from the recovered binder. 5 Table 2 (Continued) LMLC specimens for mix design PMLC specimens for quality assurance PMFC specimens for quality assurance and long-term performance testing 1. Mix design: 150-mm diameter 115-mm high at Ndesign. 2. Moisture s
45、ensitivity: 150-mm diameter 95-mm high at 7.00.5% air voids. Note: Some researchers may also use complementary lower and higher air voids, e.g., 4.0% and 9.0%. 3. Dynamic modulus and fl ow number with AMPT: 100-mm diameter 150-mm high cored and sawn from 150-mm diameter 175-mm high specimens at 7.00
46、.5% air voids (per appendix to AASHTO R 35 and 2011 change to AASHTO TP 79). 4. IDT creep and strength: 150-mm diameter 50-mm high prepared from gyratory specimen. Note: It is recommended to cut only one specimen from the center of each 115-mm high gyratory specimen. 5. Hamburg Test: 150-mm diameter
47、 62-mm high prepared from gyratory specimen. 6. Beam Fatigue Test: 380-mm long 63-mm wide 50-mm high beams cut from rolling- wheel compacted slabs. 7. Overlay Test: 150-mm diameter 115-mm high. 1. Verify mix design: 150-mm diameter 115-mm high at Ndesign. 2. Moisture sensitivity and resilient modulu
48、s: 150-mm diameter 95-mm high at 7.00.5% air voids. 3. Dynamic modulus, fl ow number, and AMPT fatigue: 100-mm diameter 150-mm high cored and sawn from 150-mm diameter 175-mm high specimens; 7.00.5% or fi eld air voids. 4. Indirect Tensile Test (IDT) creep and strength: 150-mm diameter 50-mm high. 5
49、. Hamburg Test: 150-mm diameter 62-mm high. 1. 150-mm diameter cores: generally suitable for (a) bond strength, (b) in-place density and thickness, (c) air voids analysis, (d) IDT creep compliance and strength, (e) IDT dynamic modulus, (f) Hamburg Test, (g) Overlay Test, and (h) moisture sensitivity. Notes: (1) Some low-temperature IDT testing may be done with 4-in. diameter specimens due to load requirements for 150-mm diameter specimens. (2) A core barrel with a 150-mm inside diameter should be used. (3) Due to lif
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