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9 Nishizawa et al. (11) reported an endurance limit of 200 ms void levels were targeted, resulting in relationships between based on the analysis of in-service pavements in Japan. Simi- fatigue life and both air void content and asphalt content. larly, strain levels at the bottom of the asphalt layer of between Voids filled with binder (VFB) have been a typical parame- 96 and 158 ms were calculated based on back-calculated stiff- ter used in fatigue life prediction equations. Harvey and Tsai ness data from the falling-weight deflectometer for a long-life (15) caution against the use of VFB since various combina- pavement in Kansas (12). Others (13, 14) report similar find- tions of air voids and asphalt content will produce the same ings, particularly, the absence of bottom-up fatigue cracking VFB. Maupin and Freeman (9) note that there was little in thick pavements and the common occurrence of top-down increase in fatigue life resulting from an increase of 0.5% cracking. binder, but significant increases were seen with an increase of 1.0% binder. Factors Affecting Fatigue Life Strategies to Produce A significant amount of fatigue research was conducted in Long-Life Pavements the 1960s and 1970s. Epps and Monismith (1) provide a summary of the effects resulting from binder stiffness, A number of strategies have been put forth to promote the asphalt content, aggregate type, aggregate gradation, and air likelihood of constructing a long-life pavement, including: void content. Table 2.1 indicates the relative affects of these polymer modification, rich bottom layers, and high-modulus components. The authors conclude that binder stiffness and asphalt bases. air void content have a larger influence on fatigue life than aggregate type and gradation. The SHRP A-404 research (3) Polymer Modification noted that angular aggregates tended to produce both stiffer mixes and longer fatigue lives. Harvey and Tsai (15) Fatigue testing and analyses of asphalt mixtures made evaluated the effect of air voids and asphalt content on with modified asphalt binders has been performed in sev- fatigue life. In most previous evaluations, a constant com- eral studies. In 1988, Goodrich presented an early study on the paction effort was used to produce samples. This results in fatigue performance of polymer modified mixes (17). In this air voids being highly correlated with the asphalt content of study, three unmodified asphalt binders with different tem- a given mix in which case there is little relationship between perature susceptibilities and two modified asphalt binders-- asphalt content and fatigue life. In this instance, specific air produced using one base asphalt and two levels of modification Table 2.1. Factors affecting the stiffness and fatigue behavior of hot mix asphalt (16 in 1). Factor Change in Effect of Change in Factor Factor On Stiffness On Fatigue On Fatigue Life in Life in Controlled Controlled Stress Mode of Strain Mode Test of Test Asphalt Penetration decrease increase increase decrease Asphalt Content increase increasea increasea increaseb Aggregate Type increase in increase increase decrease rough texture and angularity Aggregate Gradation open to increase increase decreased dense Air Void Content decrease increase increase increased c Temperature decrease increase increase decrease Notes: a Reaches optimum level above that required by stability considerations. b No significant amount of data; conflicting considerations of increase in stiffness and reduction of strain in asphalt make this speculative. c Approaches upper limit at temperature below freezing. d No significant amount of data.
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10 (not identified)--were evaluated using mixture fatigue tests. Testing was conducted on mixtures using 10 Hz sinusoidal The intent was to correlate binder properties with mixture loading with a peak-to-peak strain of 800 ms. The test tem- fatigue performance. Laboratory testing was conducted using perature was selected for each mixture as the temperature flexural beam fatigue at 25°C and 1.67 Hz loading frequency where the intermediate temperature requirement was met in controlled stress mode. Tests were conducted at an initial (G*sin = 5000 kPa). So, unlike other studies in which the strain level of 400 ms, and findings indicated that the fatigue temperature was fixed (usually at 20°C), the viscous compo- lives of the two modified asphalt binders were an order of nent of the shear modulus was fixed. magnitude greater than the fatigue life of one of the unmod- A brief examination of the data in the report indicates that ified asphalt binders (produced from the same source as the the fatigue lives of the modified mixtures could be signifi- base asphalt used to create the modified asphalt binders). The cantly different. Mixtures made with a PG 82-22 asphalt base asphalt properties appear to be important in the perform- binder produced using a radial styrene-butadiene-styrene ance of the modified asphalt binders. Mixtures made with one (SBS) modifier had fatigue lives at 24°C that were two to five unmodified asphalt binder with low temperature susceptibil- times the fatigue lives (tested at 32°C) of mixtures made with ity, had approximately two to three times the fatigue life of the an unmodified (oxidized) PG 82-22 asphalt binder. Since the polymer modified mixtures. G*sin value was the same for each of these mixtures at their During the Strategic Highway Research Program (SHRP), respective test temperatures, the researchers considered the the A-003A contractor evaluated the use of the flexural beam intermediate temperature criterion in the PG binder specifi- fatigue test as a mixture performance test for fatigue. The cation to be inadequate for assessing the fatigue characteris- modified asphalt mixtures experiment (MAME), described tics of asphalt binders. in SHRP Report A-404, was performed to determine if the A temperature equivalency experiment conducted during fatigue characteristics of modified mixtures could be evalu- SHRP (18) indicated a strong relationship between temper- ated using the flexural beam fatigue test (18). Asphalt mixtures ature and fatigue life at a given strain level, with fatigue life were made using one aggregate source, three asphalt binders decreasing as the temperature decreased. Thus, it could be (AAF-1, AAG-1, and AAK-1), and three modifiers (identified hypothesized that the difference in fatigue life between the as M-405, M-415, and M-416). Test results indicated that SBS-radial modified PG 82-22 mixtures and the oxidized the addition of modifier M-405 to each of the three asphalt PG 82-22 mixtures would be greater if the test temperatures binders decreased the fatigue life compared to the unmod- had been equal. ified asphalt mixes. The addition of modifiers M-415 and Monismith et al. (20) reported on the development of the M-416 had a negative effect on the fatigue life of mixes made design and specifications for the California I-710 rehabilita- with AAG-1, but substantially increased the fatigue life (by tion. In the study, AR-8000 (roughly equivalent to a PG 64-10 approximately three to five times) of mixtures made with or PG 64-16) and PBA-6a (PG 64-40) asphalt binders were AAK-1 compared to the unmodified mixtures. A validation used to prepare mixtures for testing. When tested at 20°C study was performed using slab wheel track tests on mixtures using the procedure described in AASHTO T321, the mea- made with AAG-1 and the three modifiers. Although the sured fatigue life of the PBA-6a mixtures was approximately results were similar for the M-405 modifier (decrease in an order of magnitude (10 times) greater than the fatigue fatigue life), the M-415 and M-416 modifiers resulted in an life of the AR-8000 mixtures. This relationship seemed to be increase in fatigue life from the slab wheel track test. This was affected by the applied strain resulting in an increased dif- contrary to the findings of the flexural beam fatigue tests. ference between the two sets of fatigue lives at higher strain Shortly after the implementation of the Superpave levels. performance-graded asphalt binder tests and specification, Lee et al. (21) reported on a laboratory evaluation of the users recognized that the properties of modified asphalt effects of aggregate gradation and binder type on mixture binders may not be characterized properly using the Super- fatigue life. Using uniaxial tension fatigue test results (con- pave binder tests. This led to the funding of NCHRP Project ducted at 25°C) and a viscoelastic fatigue model, the authors 9-10, "Superpave Protocols for Modified Asphalt Binders," calculated that mixtures made with an SBS-modified PG 76-22 in 1996. The research (19) was conducted by the University asphalt binder had 10 times greater fatigue life than mixtures of Wisconsin-Madison, Asphalt Institute, and NCAT, under made with an unmodified asphalt binder, regardless of aggre- the direction of Hussain Bahia. gate gradation. The NCHRP 9-10 research evaluated the effectiveness of the Research performed in 2002 by the Asphalt Institute for intermediate temperature stiffness requirement (G*sin the Asphalt Pavement Alliance evaluated the possibility of a 5000 kPa) by performing flexural beam fatigue tests on four fatigue endurance limit by testing mixtures made with two aggregate structures (gravel and limestone, coarse and fine asphalt binders (PG 64-22 and PG 76-22) and two asphalt gradation) using nine different modified asphalt binders. binder contents at various strain levels. The results, shown in
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11 1.E+09 1.E+08 Cycles to Failure 1.E+07 64-22 Opt 64-22 Opt+ 1.E+06 76-22 Opt 1.E+05 76-22 Opt+ 1.E+04 1.E+03 10 100 1,000 Strain (x10-6) Figure 2.3. Fatigue life comparison for modified and unmodified mixes (22). Figure 2.3, indicate that the mixtures made with the PG 76-22 reported in SHRP Report A-404, one asphalt-aggregate asphalt binder have approximately an order of magnitude (i.e., mixture (RB aggregate and AAG-1 asphalt binder) was used 10 times) greater fatigue life than the mixtures made with the to prepare specimens at three different levels of air voids PG 64-22 asphalt binder. and two different levels of asphalt binder content (4.5 and Von Quintus (23) conducted a study to quantify the effects 6.0%). The effect of asphalt binder content on fatigue life was of polymer modified asphalts. Based on a literature review, Von a primary focus of this study since the 8 × 2 expanded test Quintus reported that PMA mixtures generally last about 25% program experiment did not evaluate asphalt binder content longer than conventional mixtures. Some premature failures as a variable. A separate 2 × 2 pilot test program used two caused concern. However, most of the failures were " . . . found asphalt binder contents defined as optimum and high--with to occur prior to the adoption of the Performance Graded (PG) the high asphalt binder content established as 0.6% higher binder specification and can be traced back to inferior con- than the optimum asphalt binder content. For the 2 × 2 struction (for example, high air voids), inferior materials, pilot test program, a statistical analysis of results from flex- and/or inadequate design thickness." The study also notes ural beam fatigue (controlled stress and controlled strain) and "One of the more important findings from the recent field other tests indicated that asphalt content did not signifi- experiments is that many of the PMA pavements are not cantly affect fatigue life (18). exhibiting fatigue cracking or have less load-related crack- The results of the mix design fatigue experiment indicate ing than the control sections (unmodified mixtures)." that asphalt binder content significantly affected flexural stiff- In summary, there appears to be significant historical data ness (decreasing by 8% as asphalt binder content increased) indicating that the laboratory fatigue performance of modi- and fatigue life--increasing the fatigue life by 67% as the fied asphalt mixtures is greater than mixtures made with asphalt binder content increased from 4.5% to 6%. As in other unmodified asphalt binders. In some reported cases, modi- experiments, increasing the air void content resulted in a fied asphalt mixtures have exhibited an order of magnitude decrease in flexural stiffness of 33% and a decrease in fatigue greater fatigue life compared to unmodified asphalt mix- life by 45% as air voids increased from 4% to 8% (18). tures. The fatigue characteristics appear to be dependent on Harvey and Tsai (25) conducted a study on the effects of the base asphalt binder used for modification. asphalt content and air void content on mixture fatigue and stiffness. Samples were produced at five asphalt contents: 4.0%, 4.5%, 5.0%, 5.5%, and 6.0% by weight of aggregate and Rich Bottom Layers three ranges of air voids: 1% to 3%, 4% to 6%, and 7% to 9%. The concept of a rich bottom layer originated from the In this experiment, a constant compaction effort was not Australian experience (24) and was explored during SHRP used, so air voids were independent of asphalt content. Con- experiments. Two potential benefits are created through stant strain tests were performed at two strain levels: 300 and the use of a rich bottom layer: increased asphalt binder con- 150 ms. Analysis of the data indicated that higher asphalt con- tent and decreased air voids in the bottom layer (as a result of tent and lower air voids resulted in longer fatigue lives. Lower easier compaction created by the additional asphalt binder). asphalt content and lower air voids resulted in higher initial Several known literature sources confirm the rationale stiffness. Instead of using stiffness in mixture fatigue life pre- for these concepts. In the mix design fatigue experiment diction models, the authors recommended evaluating the