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61 CHAPTER SIX SURVEY RESULTS: CURRENT U.S. AND CANADIAN EXPERIENCE This chapter presents the results of a survey of U.S. states and Canadian provinces and territories summarizing the criteria that are used to establish relationships between binder chemistry and engineering properties of asphalt binders. See the appen- dices for a copy of the questions in the survey, the tabulated survey responses, and a list of respondents. An electronic survey was distributed in March 2016 to all 50 U.S. states and the District of Columbia and to 14 Canadian provinces and territories. Responses were received from 45 states and the District of Columbia (a response rate of 88.2%). Limited responses were received from eight Canadian provinces and territories. Because of the limited data on the Canadian forms, these data were examined separately and published in the comments section of Appendix A only. The general survey responses are summarized here; the percentages were calculated based on the U.S. responses. Since some respondents submit- ted multiple answers to a given question, all percentages are based on the total of 46 state responses. The detailed summary of the responses can be found in Appendix A. The state departments of transportation do not monitor the sources of their asphalt binders. Only 13.3% of the respondents could identify their sources. The questionnaire could also be directed to binder suppliers, since DOTs are not involved in production and formulation of the binders they use. Similarly, most DOTs are not familiar with the process used to produce their binders. Significant differences in asphalt binder properties over the past 10 years were reported by 42% of the respondents. Pos- sible issues with changes in the testing requirements mean that most of the grades used today are different from those used 10 years ago, so direct comparison is difficult. Binders appear to be stiffer and exhibit poor low-temperature properties, and some pavements aged prematurely. Although binders met specifications, experienced engineers could detect quarterly changes as crude feeds change. Changes associated with seasonal or market fluctuations were reported by 38% of the respondents. The physical properties measured to estimate changes in the crude stock or binder chemistry in the field are predominat- edly rheological (Figure 20). The dominant analytical procedures used for binder testing are rheological; that is, the PG pro- tocol (100% of respondents). FTIR is employed by 23.9% of the DOTs but this appears to be primarily used for research. The third most common procedure was XRF (19.6%), which was employed by respondents incorporating PPA or REOB additives. GPC is used routinely by only two DOTs (Figure 21). FIGURE 20 Types of binder testing used to quantify binder properties (Source: Survey results).
62 FIGURE 21 Rheological and analytical procedures employed (Source: Survey results). There was little interest in binder fractionation. Only 14 DOTs responded to this question and only three were working on new separation procedures. Eleven DOTs stated that they do not fractionate. This issue is considered the purview of binder suppliers. Binder aging characteristics are assessed routinely using the RTFO and PAV procedures defined by AASHTO M320 (100% of respondents). Occasionally multiple PAVs (13.3%) may be employed. The use of Glover-Rowe parameters, delta Tc, fracture energy, penetration index, and phase separation after oven aging is mentioned (Figure 22). FIGURE 22 Procedures used routinely to estimate asphalt binder aging characteristics (Source: Survey results). A number of modifiers/additives are employed, including PPA (41.3% of respondents), antistripping agents (26.1%), and softening agents (21.7%) (Figure 23). The use of the modifiers depends on suppliersâ reporting, so these data may not be complete. The addition of compaction aids (waxes, wetting agents, antistripping agents, and rejuvenators) is paving related and dictated by the job, aggregate, and plant. With the exemption of polymers and PPA, the additives in a binder are usually not specified or quantified, because that is considered the responsibility of the supplier. Some concern over the excessive use of PPA is reflected in the upper-limit specifications imposed by 12 DOTs. Comments on specific DOT procedures can be found Appendix A. The compatibility of modifiers in the final mixes is not measured by more than 95% of the 40 respondents replying to the questionnaire.
63 The most common polymers used to prepare a polymer-modified binder are linear styrene-butadiene-styrene copolymers (71.7%) and styrene-butadiene rubber (26.1%) (Figure 24). Crumb rubber or ground tire rubber is allowed by 31.4% of the DOTs. The presence of polymers is controlled by an elastic recovery test. Frequently, the suppliers choose the type of polymer used to meet the elastic recovery specification and the DOT imposed minimum concentration standard. FIGURE 23 Modifiers/additives used in final binder preparation to achieve specific asphalt binder grades (Source: Survey results). FIGURE 24 Polymer modifiers employed (Source: Survey results). Efforts to understand the impact of recycled materials by extracting binder from mixtures are focused on RAP (65.2%) and RAS (47.8%). A limited number of DOTs (fewer than 10) are using recycling (rejuvenating) agents; the most common materials employed are soft asphalt fluxes and abietic acid esters. The recycling agents appear to be used on an experimental basis for specific projects, and nine DOTs expressly state that they do not use them. A majority of the respondents (69%) did not consider the current PG criteria satisfactory in identifying compositional/ chemical properties of a given binder. PG criteria, which are physical measurements, currently appear to fail to capture some poorly performing binders. To better identify compositional properties of polymer-modified asphalt, Illinois DOT specifies PG+ tests (force ductility, elastic recovery, and separation of polymer). Louisiana and Virginia DOTs are employing GPC to
64 confirm the presence and concentration of polymers, but a quick, easy field identification of modifiers (polymer and rubber) needs to be developed to determine what is contained in the mixes. FIGURE 25 Recycled materials extracted from mixtures (Source: Survey results). The respondents do not believe that current performance grade criteria satisfactorily identify compositional/chemical properties. REOBs, PPA, and so forth can go undetected in the PG system because it is purely rheologically based. The jury is very much out on how much any of these modifiers affect the long-term performance of a binder. Chemical analysis is required to ensure that a binder has the specified composition. Virginia DOT considers this one potential benefit of running GPC as well as employing XRF on binders. Several respondents expressed the opinion that the PG 64-22s have declined in quality, based on the life of their asphalt pavements. The missing component of the PG criteria is long-term aging. A one-point aging test such as PAV does not tell the whole story. Procedures used to quantify the presence of modifiers/additives in final asphalt are surveyed. Polymer-modified binders are checked using MSCR (30.4%), XRF (15.2%), viscosity (8.7%), and FTIR (6.5%). Softening agents are checked using XRF. The blend compatibility with antistripping agents, polymers, and PPA is tested in a limited number of cases using mod- ern variants of the âcigar tube test.â In one case evidence for incompatibility in blends containing PPA was reported. The PPA neutralized the antistripping agent that negatively affected the field TSR. The PPA also reacted with the polymer, negatively affecting the PG binder. A specification defining an upper limit on PPA is used by 12 DOTs. Several laboratories are developing alternate techniques for estimating binder performance. Connecticut DOT currently specifies the ductility test (AASHTO T-51), toughness and tenacity test (Colorado CP-L 2210), and elastic recovery test (AAS- HTO T-301) as indicators for polymer modification on some grades of binder. Ontario Ministry of Transportation has already implemented the DENT, MSCR, and ash content tests (to limit REOB). It is phasing in the ExBBR test and looking at XRF and FTIR tests to quantify REOB. MOT has also developed a modified PAV procedure that is currently being evaluated. Quebecâs Ministry of Transportation has developed a method to evaluate the adhesion between a binder and an aggregate. For some binders, some criteria based on this method are added to the specifications to ensure a binder with superior adhesive properties. These binders help to reduce stripping problems of the pavements. The critical performance indicators for ensuring good high-temperature performance are either G*sinÎ´, RTFO (30.4%), or the ASSHTO M 320 protocol (21.7%) (Figure 26). The corresponding indicators for intermediate temperature performance are G*sinÎ´, PAV (28.3%), the ASSHTO M 320 protocol (15.2%), and MSCR (13%). Low-temperature performance is esti- mated using primarily BBR (34.8%) (Figure 27).
65 Although it is difficult to specify that field failures are the result of binder composition, at least eight DOTs cited concern. In the past decade one lab noticed premature cracking (top-down mostly), starting in the wheel paths and then propagating in a map cracking form. The lab recovered the binder in several cases and noticed a great loss of low temperature grade using an ExBBR test. This is an indication that the SHRP aging protocol is not sufficient on the low temperature side. FIGURE 26 Critical performance indicators for ensuring good high-temperature performance (Source: Survey results). FIGURE 27 Critical performance indicators for ensuring good low-temperature performance (Source: Survey results). Because there is no reliable field test, it is hard to determine if the failures can be attributed directly to the binder itself. Stripping, bleeding, segregation, and other failures have been blamed on a bad binder, but nothing definitive could be repro- duced in the laboratory. Failures can range from soft, tender mixes to prematurely cracked pavements. The problem is being pursued by analyzing cores taken from failure locations to try to identify (1) the performance grade of asphalt used, (2) the presence of any chemical contamination by gas chromatography/infrared spectroscopy, and (3) the presence of nonspecified modifiers such as PPA, bio-oils, or REOB.