Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
7 CHAPTER 1. BACKGROUND PROBLEM: FATIGUE IN ASPHALT PAVEMENTS Traffic-associated fatigue damage is one of the major distresses in which flexible pavements fail. This type of distress is the result of many thousandsâor even millions of wheel loads passing over a pavement. Each load repetition induces stresses and strains in the pavement that gradually damage the pavement and lead to failure. Figure 1 is a photograph of severe fatigue cracking in an asphalt concrete pavement at the NCAT Test Track. This type of cracking is often called âalligatorâ cracking because the pavement surface resembles an alligatorâs hide. Figure 1. Alligator Cracking at the NCAT Test Track. The alligator cracking shown in Figure 1 is typical of pavement failure resulting from bottom-up fatigue cracking, meaning fatigue cracking that originates at the bottom of the asphalt concrete layers and gradually works up towards the surface. Bottom-up fatigue cracking is easy to explain, since tensile strains in a flexible pavement will almost always be largest at the bottom of the asphalt structure. Engineers focused on this type of fatigue cracking for many years in their testing procedures, materials specifications, and pavements design and analysis methods. However, it has become clear over the past 10 to 15 years that fatigue cracking can also originate at the surface and work down towards the center of the pavement. The source of the stresses and strains are more complex in this type of cracking, originating from tire-pavement interactions, shear stresses in the pavement surface near the outside edge of the tire, and tensile stresses in the pavement surface some distance from the tire. Thermal gradients within the pavement may also contribute to this type of fatigue cracking. Top-down fatigue cracking usually has a different appearance from bottom-up cracking, typically occurring as longitudinal cracks near or in between the wheel paths. There is no reason to suspect that these two mechanisms for fatigue damage (three including thermally induced damage) are mutually
8 exclusive. In fact, it is likely that bottom-up fatigue, top-down fatigue and thermally induced fatigue damage generally occur simultaneously in asphalt pavements. A third type of fatigue cracking, reflective cracking occurs in asphalt concrete overlays when a crack or joint in the underlying pavement gradually works its way up through the overlay due to repeated load applications or movement of the underlying pavement. The underlying pavement can be portland cement concrete or asphalt concrete. Figure 2 is a photograph of reflective cracks in an asphalt concrete overlay (Miller and Bellinger, 2003). Although this is a complex mode of distress that is difficult to analyze mechanistically, it is likely that measures to improve traffic associated fatigue and thermal fatigue performance will also improve resistance to reflective cracking. Figure 2. Reflective Cracking in Asphalt Concrete Overlay (Miller and Bellinger, 2003). Factors Affecting Fatigue Cracking in Asphalt Concrete Pavements Traffic associated fatigue cracking of any type in flexible pavements is a complex phenomenon that depends on many factors. Perhaps most important is the severity of traffic loadingâthe heavier the axle loads on a pavement and the higher number of heavy vehicles, the more quickly fatigue cracking will occur, all else being equal. The pavement structure is also extremely important. For full-depth asphalt pavements, thicker pavements will exhibit lower stresses and strains under a given traffic load; thus, they will resist fatigue cracking significantly longer than thinner pavements. The subgrade underneath the pavement will also affect the occurrence of fatigue cracking; weak subgrades will result in higher stresses and strains in the overlying structure, which in turn will reduce the fatigue life of the pavement system. Pavement drainage and moisture damage are also related to the subgrade. When moisture is present in the subgrade and the pavement drainage system is not properly designed or constructed, water can infiltrate the pavement and accelerate any existing fatigue damage, both by weakening the subgrade and increasing the strains in the bound layers and through moisture damage. This can be a severe problem resulting in very rapid failure. Moisture damage and fatigue damage often occur together, and it can be difficult to sort out the relative contributions of the two failure mechanisms.
9 Another important factor affecting the fatigue behavior of asphalt concrete is the modulus of the material. This relationship is complex and is the result of several related factors. In a flexible pavement system, as the modulus of the asphalt concrete increases, the stiffness of the pavement system will increase and strains under traffic loading will decrease, which will tend to reduce fatigue damage. At the same time, the amount of strain that an asphalt mixture can withstand without failure tends to decrease with increasing modulus, which will increase the fatigue damage caused under a given traffic loadâalthough it should be noted that using polymer- modified binders and/or special mix designs such as SMA may tend to offset this trend. To further complicate matters, the relative importance of these two offsetting factors depends largely upon the pavement structure. In stiff pavement structuresâwith thick asphalt concrete layers, a substantial base layer and a good quality subgradeâincreasing the modulus of a mix will generally improve fatigue performance because this will lower than strains in the bound material. In thinner pavements, especially with poor quality subgrades, strains are largely independent of the surface layer stiffness; increasing the modulus of the asphalt mix in this situation will decrease fatigue performance due to decreased strain tolerance in the bound material. The modulus of asphalt concrete depends on several factors: temperature, loading rate, mixture composition, and the asphalt binder used in the mix. Temperature and loading rate have very large effects on asphalt concrete modulus but are the result of climate and traffic conditions, which cannot be controlled through specifications or construction practice. Mix composition is measured and controlled through parameters like air void content and voids in the mineral aggregate (VMA). Although the composition of asphalt concrete is controlled through a variety of tests and specifications during mix design, mix production, and pavement construction, the effect of composition on modulus is relatively small. The most important factor affecting the modulus of asphalt mixture that can be controlled by engineers is the modulus of the asphalt binder used in the mix. Another factor affecting the fatigue performance of asphalt pavements, and one that is critical in addressing the objectives of NCHRP 9-59, is the inherent strain tolerance of the binder. The term inherent strain tolerance as used here means the overall strain tolerance of the binder, compared to what is average or typical for a wide range of binders. As mentioned above, as binder modulus increases, binder failure strain decreases, and the relationship between modulus and failure strain tends to be similar but not identical for all asphalt binders. Some binders however, because of their rheologic type and/or the presence of polymer modifier exhibit unusually high failure strain over a wide range of modulus. Similarly, some binders exhibit low failure strains at any given modulus value. This is what is meant by inherent strain toleranceâ not the absolute failure strain of a binder under specific testing conditions, but the failure strain relative to other asphalt binders over a wide range of conditions. Binders with good inherent strain tolerance will exhibit better fatigue performance than those exhibiting poor strain tolerance. As discussed later in this Chapter, the NCHRP 9-59 research team focused much of
10 their efforts during the project on characterizing the inherent strain tolerance of a wide range of binders, using both mixture and binder tests. PREVIOUS RESEARCH RELATED TO ASPHALT BINDER FATIGUE PERFORMANCE How Asphalt Binders Affect the Fatigue Performance of Asphalt Mixtures The recently developed AASHTO mechanistic-empirical design guide (ARA Inc., 2004) for flexible pavements uses the following equation for estimating cycles to failure for bottom-up traffic-associated fatigue cracking, calibrated for general use in the United States: 281.19492.3 1 1100432.0 ï·ï· ï¸ ï¶ ï§ï§ ï¨ ï¦ ï·ï· ï¸ ï¶ ï§ï§ ï¨ ï¦ = HMAt ff E CN Îµ Î² (1) Where Î²f1 and C are variables that depend on the asphalt concrete thickness and composition, respectively, Îµt is the maximum tensile strain at the bottom of the asphalt concrete layer, and EHMA is the modulus of the asphalt concrete. The only place where this equation includes binder properties is indirectlyâthe mixture modulus EHMA is strongly dependent on the binder modulus. The strong relationship between binder modulus and mixture modulus is demonstrated by several equations for estimating mixture modulus, such as the Hirsch model and various versions of Witczakâs equation, which all include binder modulus in some form as one of the primary predictors of mix modulus (Christensen and Anderson, 1992; Andrei et al., 1999). Equation 1 suggests that all else being equal, mixture fatigue life will increase with decreasing mixture and binder modulus. The current binder fatigue specification limits the binder loss modulus, |G*| Ã sin Î´, to a maximum of 5.0 kPa. This limit was based in part on the observation that for several test roads constructed in the 1950s and 1960s, fatigue cracking increased dramatically when the binder modulus at typical pavement temperatures exceeded this value (University of California, 1994). Thus, the current specification is largely based on the relationship between binder modulus and mixture fatigue. Equation 1 was based on a fatigue equation originally proposed by the Asphalt Institute (Shook et al., 1982; Monismith et al., 1972). An alternative approach to addressing traffic- associated fatigue in flexible pavements based on the Shell fatigue equations was considered but later abandoned. There are two such equations, one for stress-controlled fatigue and one for strain-controlled fatigue, as shown here (Bonnaure et al., 1980): ( )[ ] 8.155112.00454.00085.017.0 âââ+â= HMAtbbff EVVPIPIAN Îµ (2)
11 Where Nf is the number of cycles to failure, Af is a correction factor to account for the typical difference between fatigue life in the laboratory and in an actual pavement, PI is the penetration index of the asphalt binder, Vb is the binder content by volume for the mix, Îµt is the maximum tensile strain at the bottom of the asphalt concrete layer, and EHMA is the modulus of the asphalt concrete. Equation 2 indicates that asphalt binders affect mix fatigue in two ways: through their effect on mixture modulus, and through the penetration index PI. The penetration index is no longer widely used in the U.S. to characterize asphalt binders but was commonly used until the 1990s. The PI is considered to be an indicator of the rheologic type of an asphalt binder, which is related to both the shape of the modulus versus time function for the binder as well as its chemistry. Although PI is no longer commonly used to characterize asphalt binders, a more rational and related parameterâthe R-value, also called the rheological indexâcan be easily calculated from data produced in the bending beam rheometer (BBR) test. The R-value in this case refers to one of the variables in the Christensen-Anderson (CA) model for asphalt binder modulus (Christensen and Anderson, 1992). Figure 3 shows the relationship between PI and R- value for the SHRP core asphalts (University of California, 1994). Although R has not been widely used in developing fatigue models for asphalt concrete mixes, there should be a relationship between the binder R-value and asphalt mixture fatigue performance because of its close relationship to PI and the use of PI in the Shell fatigue equations. Figure 3. Relationship between Penetration Index and Rheological Index (R-Value) for SHRP Core Asphalts (University of California, 1994). In addition to modulus and R-value, various binder fatigue and fracture properties have been related to mixture fatigue performance. A recently completed study at the Federal Highway Administrationâs Accelerated Loading Facility (FHWA ALF) did show some relationships between binder fracture properties and the fatigue performance of mixtures made with a wide variety of asphalt binders (Gibson et al., 2012). The experiment was designed so that most of the
12 evaluated mixes used identical (or nearly identical) aggregates and volumetric designs, although this was not possible with one of the crumb-rubber mixes. Table 1 summarizes the correlations between mix fatigue performanceâas measured both in laboratory testing and in the observed fatigue performance for the ALF lanesâand a variety of binder tests. Quite a few of the tests showed at least moderate correlations, including several tensile fracture tests and shear fatigue tests. This supports the intuitive idea that binder fracture and fatigue properties should correlate to mixture fatigue performance and might also serve as the basis for a binder fatigue specification. Many of the test methods listed in Table 1 are discussed in more detail later in the work plan for this project. Table 1. Correlation of Binder Tests with Mix Fatigue Performance from FHWA ALF Study (Gibson et al., 2012)* Binder Test Method Type of Test Coefficient of Multiple Correlation (r), with: Uniaxial Fatigue Tests Fatigue Performance in ALF Test Lanes Double-edge notched tension (DENT), crack tip opening displacement (CTOD) Tension/fracture 0.95 0.98 Binder yield energy Shear fracture 0.87 0.80 Time sweep test Shear fatigue 0.79 0.88 Direct tension failure strain Tension/fracture 0.83 0.85 |G*| Ã sin Î´ (loss modulus) Modulus -0.73 -0.66 Large strain time sweep surrogate Shear fatigue -0.74 -0.67 DENT elastic work of fracture (EWF) Tension/fracture 0.43 0.50 Bending beam rheometer m-value Flexural creep 0.52 0.38 Stress sweep Shear fatigue -0.79 -0.73 *For lanes with 100-mm asphalt concrete pavement thickness Low Temperature Cracking and Thermal Fatigue Damage Over approximately the past 15 years, an increase in premature failures of asphalt concrete pavements has been observed in the U.S. and Canada, especially in colder regions (Ahearn, 2015; Reinke, 2015; Marks 2015). The rate and severity of these failures seems to be increasing and has been the topic of significant research as suggested by their specific mention in the recently released Request for Proposals (RFP) for NCHRP 9-60. Although not specifically mentioned in the RFP, this increase in premature failures is certainly an issue that should also be considered in conducting NCHRP 9-59 research for several reasons. Low temperature crackingâalso called transverse crackingâoccurs when an extreme cold weather event causes the temperature of a pavement to drop quickly to very low temperaturesâin Minnesota, for example, pavement temperatures can drop to â30Â°C or lower. This rapid decrease in temperature causes thermal stresses to build up in the pavement, which can eventually exceed its tensile
13 strength, leading to failure through cracks that typically occur every 6 to 12 meters and run transversely across the pavement. There is also evidence that such cracking can occur from thermal fatigue, that is from sub-critical cooling events that are not severe enough to cause failure during a single event, but that can cause enough damage so that over time failure can occur. Thermal fatigue is not as well understood as single-event thermal cracking, but the potential for a thermal fatigue component to low temperature cracking suggests that this type of distress should also be considered within NCHRP 9-59 if feasible. Because traffic-induced stresses should be expected to superimpose on thermally-induced stresses, interaction between these failure modes should be expected, again suggesting that some consideration must also be given to low temperature cracking to maximize fatigue performance of asphalt pavements. The recent increase in premature failures largely linked to non-load associated cracking suggests that the current binder specification is not adequately addressing binder resistance to low-temperature cracking. One potential problem is the physical hardening that occurs in the bending beam rheometer (BBR) test. Canadian research has clearly documented substantial errors in BBR grading due to physical hardening (Kanabar, 2010; Marks, 2015). This often results in grading a binder to a lower temperature than is should be, significantly increasing the chances for premature failure. As a result, Ontario has implemented a BBR specification incorporating extended conditioning to address the errors associated with physical hardening during the BBR test (Kanabar, 2010). Other researchers have linked these premature failures to large differences in BBR grading for stiffness and m-value. The parameter ÎTc = Tc(S) â Tc(m), which is the critical temperature based on stiffness minus the critical temperature based on m- value, has been corelated to failure in numerous studies. Specifically, when ÎTc becomes too negative a failure through non-load associated cracking is more likely to occur. Because ÎTc is directly related to the Christensen-Anderson R-value, this suggests that the rheologic type of the binderâthat is the overall flow characteristics or shape of the modulus master curveâhave a significant effect on thermal cracking independent of low temperature stiffness. As discussed later in this report, these phenomena both appear to be significant problems potentially increasing the likelihood of low temperature cracking in asphalt pavements, but fortunately both can be addressed with simple changes to our current binder specification. Another parameter recently correlated to non-load associated cracking is the Glover-Rowe parameter (Glover et al., 2005; Anderson et al., 2011; Rowe, 2011). This parameter was originally developed as a surrogate for ductility, in order to evaluate the durability of asphalt binders (Glover et al., 2005). It was subsequently slightly modified by Rowe in order to make its application more straightforward. It has been correlated to surface cracking potential of pavements in several studies (Glover et al., 2005; Anderson et al., 2011). Its correlation to ductility also ties it in with many old studies linking pavement durability to ductility and the relationship between ductility and penetration. In fact, it will be shown later in this report that the GRP relates well to the results of the DENT test, and perhaps more importantly to binder fatigue strain capacity (FSC). The GRP apparently does a good job of addressing the effects of both
14 modulus and rheologic type on binder strain capacity. It therefore is not surprising that it should correlate to non-load associated cracking in pavements. Recent research on non-load associated cracking suggests that both rheologic type, as indicated most commonly by ÎTc, and binder FSC, as indicated by the GRP, both have a significant effect on non-load associated cracking. Because these parameters (or other closely related parameters) can be measured relatively easily using existing equipment and test methods, these are promising approaches to an improved binder specification to reduce non-load associated cracking. Binder Rheology, Adhesion and Healing Some of the recently observed premature pavement failures have been characterized by severe raveling and associated surface distress (Ahearn, 2015). This would suggest a significant loss of adhesion between the asphalt binder and aggregate. Standard laboratory moisture resistance tests did not indicate that these mixes were unusually susceptible to moisture damage, which in turn suggests that the cause is mechanical. One possible explanation for this distress is that the asphalt binder, because of physical changes due to aging, undergoes a significant loss of adhesiveness at intermediate temperature. Rheologically, tackiness and adhesion are related to the phase angleâas the phase angle decreases, the tackiness and adhesion of an asphalt binder (or any similar material) will in general decrease. For instance, an elastic solid with a phase angle near zero will have essential no tackiness or adhesion. An example of this relationship, for a polymer gel, is shown in Figure 4 (Grillet et al., 2012). Similar findings were made by another group of researchers studying hot melt adhesives (Tse et al., 1995). It is also well established in polymer science that adhesive tack is directly related to the storage modulus Gâ(Ï). In fact, there is rule, called the Dahlquist criterion, which states that adhesive tack for pressure-sensitive polymers will be essentially completely lost when the storage modulus is above 100 kPa (Dahlquist, 1959). Pressure-sensitive adhesives are simply materials which bond to other materials primarily through the application of pressure and as a result of the tackiness of the material. It seems possible that the Dahlquist criterion could apply to asphalt concrete mixtures. As discussed later in this report, the hypothesis that mixture healing can be related to binder rheology was difficult to evaluate, but appears to be true, although the relationship is perhaps not as strong as hoped and is somewhat complicated. Mixture healing appears to be related more strongly to the binder phase angle than to the storage modulus, with healing increasing with increasing phase angles above about 35 degrees. At lower phase angles, there appears to be little or no healing. Because the phase angle at any given modulus value will be lower for binders with higher R values, this leads to the important finding that overall healing potential will decrease with increasing R-values.
15 Figure 4. Relationship Between Adhesion and Loss Tangent for a Polymer Gel (Grillet et al., 2012). Laboratory Aging of Asphalt Binders and Mixtures Many researchers have concluded that in characterizing asphalt binder and mixture properties in studying the recent premature failure, significantly more age hardening is needed than is provided in standard laboratory aging procedures. Much of the recent research presented at FHWA ETG meetings over the past several years has made use of extended PAV aging of binders, using 40 hours or more instead of the standard 20 hours (Reinke et al., 2015; Bennert, 2015). The 40-hour PAV protocol is probably the most widely used method of extended binder aging and can be completed in a reasonable amount of time. Therefore, the research team decided to use 40-hour PAV aging (after RTFOT aging) for most of the binder tests performed as part of NCHRP 9-59. An important question for NCHRP 9-59 was what sort of mixture conditioning would provide similar age hardening to the RTFOT/40-hour PAV. After reviewing several studies, including preliminary data from NCHRP 9-54, the NCHRP 9-59 research team selected loose mix aging, using a temperature of 95Â°C for five days (120 hours). As presented later in this report, this mixture aging protocol seems to closely match the RTFOT/40-hour PAV binder aging. As results from NCHRP 9-59 and related research are implemented, it is essential to consider the ramifications of field aging and laboratory aging on any binder test specification. Any specification limits established directly using data from RTFOT/40-hour PAV aged binders can only be applied to binders aged using this same protocol. Using different laboratory aging protocols will mean that any suggested specification limits will have to be adjusted. In the same
16 way, it is important that other engineers and researchers evaluating the results of NCHRP 9-59 carefully consider any differences in binder and/or mixture aging protocols while comparing data from different sources. What are the Problems with the Existing Binder Fatigue Specification Test? Before discussing the results of NCHRP 9-59, it is important to understand the perceived shortcomings to the current approach to ensuring adequate fatigue performance. The existing specification was developed during SHRP and was largely based on the observation that a dramatic increase in fatigue cracking was observed when the estimated loss modulus of the binder exceeded a value of about 3 MPa for the Zaca-Wigmore test road, as shown in Figure 5 (University of California, 1994). This value was later raised to 5.0 MPa during implementation of the binder specification. Figure 5. Plot of Gâ = |G*| Ã sin Î´ at 25Â°C and 10 rad/s for Zaca-Wigmore Test Road, Estimated from Penetration Data (University of California, 1994). Several researchers have pointed out shortcomings in the Superpave binder specification (Deacon et al., 1997; Bahia et al., 2001; Gibson et al., 2012). The asphalt binder specification parameter, |G*| sin Î´, does not seem to correlate well to fatigue performance in the field (Deacon et al., 1997). Part of the underlying problem is likely because the test is largely empirical in nature. It was based on a correlation between observed fatigue cracking and modulus values estimated from penetration values for a single test road. This problem is especially relevant with respect to polymer-modified binders, which can show significantly enhanced fatigue and fracture properties compared to a non-modified binder. There does appear to be several other tests that probably relate more strongly to mixture fatigue performance (refer to Table 1 above, from the 2012 FHWA ALF study). There is another potential source of problems for the current binder fatigue specification. The specification might be valid for relatively thin pavements where the strains are affected more by
17 the stiffness of the underlying structure than the modulus of the pavement. However, the specification potentially has a problem when applied to thicker pavements, where the strains can be significantly reduced by an increase in the pavement modulus. In evaluating the current binder fatigue specification it is essential to understand two important aspects of this specification that often seem to be misunderstood, or simply ignored: (1) as discussed above, the specification is meant to address the field performance of actual pavement systems and not the performance of mixes as tested in the laboratory; and (2) the specification was originally based on the loss modulus (Gâ = |G*| Ã sin Î´) at some intermediate temperature characteristic of a particular climate, not on an intermediate temperature based on binder grades. In other words, the temperature at which Gâ should evaluated should be based on the local climateâor the base binder grade used in a given locale, and not on binder grades âbumpedâ for extreme traffic levels. The latter approach can lead to maximum Gâ values that are excessively high for a given climate and violate the original intent of the specification. Because of these factors, it is difficult to evaluate the current binder fatigue specification in laboratory tests. An additional potential problem in the current binder specification was discussed earlier in connection with low-temperature crackingâphysical hardening can cause significant errors in BBR test data. This will in general result in a binder grade lower than it should be, increasing the potential for transverse cracking. Ontario has already implemented a BBR specification using extended aging to address this problem (Marks, 2015). Other simpler approaches to addressing physical hardening may be possible, as discussed later in this report. Review and Selection of Binder Fatigue Tests for In-Depth Evaluation in NCHRP 9-59 Appendix A presents a review and evaluation of binder fatigue tests identified by the research team at the start of NCHRP 9-59. This includes a literature review of potential tests, and a detailed numerical ranking of the most promising. The section below presents a summary of the information given in this appendix. Through the literature review seven primary binder tests/parameters were identified as candidates for further evaluation: 1. The linear amplitude sweep (LAS) 2. The double-edge notched tension (DENT) test 3. The Glover-Rowe parameter (GRP) and other similar rheological parameters that can be calculated from DSR data and have been related to performance, such as the R-value 4. The direct tension (DT) test 5. The single edge notched bending (SENB) test 6. And the ductility test The ductility test was included in the ratings because of its historical significance, because it is still widely used in other countries and because there is substantial data in the literature linking ductility to asphalt concrete pavement durability and fatigue performance. It was not expected
18 that this test would be selected for further evaluation. Five criteria were selected for evaluating the tests: 1. Additional cost to run the test, primarily purchase of equipment that a commercial testing lab is unlikely to currently possess. 2. Active time requirementâthe actual working time required to run the test 3. Correlation with performance based upon published research. 4. Engineering soundnessâa subjective evaluation of how strongly the test is based on engineering principles, as opposed to being empirical. 5. Technical difficulty and ease of implementationâalso a subjective evaluation of how difficult the test is to perform and the potential hurdles that would be encountered in implementing the test. These criteria are explained in detail in Appendix A. The final criteria and weights used in this ranking were developed in consultation with the NCHRP 9-59 panel. Table 2 is a summary of the resulting ranking of the candidate tests based on these criteria and their weights. It should be emphasized that the rating in Table 2 was not meant to be the only means for selecting the final tests including for evaluation in the laboratory testing phase of NCHRP 9-59. Instead, this table was meant to provide guidance to the researchers and panel in making the final selection. The highest rated tests were the LAS, GRP and related rheological parameters, and the DENT test. Although the ductility test scored relatively highly in this rating, the research team feels that there would be little support and significant opposition to reconsideration of this procedure. As mentioned above, it has been included partly for historical reasons and partly because it is still widely used in other parts of the world. The two remaining testsâdirect tension and SENBâare similar in that they provide information on the strain capacity of asphalt binders at low temperature but use different geometries. Table 2. Ratings of Candidate Binder Fatigue Tests Criteria Weight LAS DENT GRP DT SENB Ductility Equipment cost (additional/new) 10 5 3 5 2 4 2 Active time requirement 20 3 1 3 2 3 2 Correlation with performance 50 5 5 5 2 1 4 Engineering soundness 5 3 3 5 5 5 1 Technical difficulty/ease of implementation 15 5 1 5 1 3 1 Total 100 4.5 3.3 4.6 2.0 2.2 2.8
19 Selection of Final Asphalt Binder Tests The NCHRP 9-59 research team has emphasized from the beginning of the projectâeven during writing the initial proposalâthat the funding and time allotted for the project was not adequate for the development, refinement and validation of a completely new test procedure for evaluating the fatigue performance of asphalt binders. Therefore, the approach used in selecting tests for laboratory evaluation emphasized identifying and evaluating tests that had already been developed and could be implemented quickly and easily. It was also essential that potential tests had shown promise by exhibiting good correlation to mixture fatigue performanceâeither in the laboratory or the field, preferably both. Based upon these considerations and the ratings summarized in Table 2, the following tests were selected for detailed evaluation in Phase II of NCHRP 9-59: â The LAS test â The simplified DENT test â Various rheological parameters, including the GRP, loss modulus, storage modulus and phase angle These tests for the most part meet the criteria developed by the research team for candidate binder tests to be included in Phase II of NCHRP 9-59 as they have gone through initial development; they have been correlated to field performance; and they can be realistically implemented as specification tests. Details concerning these test procedures are presented later in this report. Characterizing Mixture Fatigue Performance in the Laboratory Probably the most important aspect of NCHRP 9-59 was relating selected binder test properties to mixture fatigue performance as measured in the laboratory. For this reason, selection of laboratory tests for evaluating mixture fatigue performance was a critical activity. Three test methods were considered for use in NCHRP 9-59: bending beam fatigue, the overlay test, and uniaxial fatigue testing. Appendix B provides a review of these methods. The most important criteria for laboratory fatigue tests in NCHRP 9-59 was having a good record of correlation to field fatigue performance. Also, very important was evidence of correlation to binder properties. Unfortunately, none of currently used laboratory fatigue tests have exhibited consistently good correlation with either field performance of binder properties. As discussed later in this report, this is perhaps not because of shortcomings in the test methods, but because of problems in traditional methods of analyzing asphalt mixture fatigue data. After evaluating the three candidate mixture tests, and in consultation with the project panel, the NCHRP research team initially decided to rely primarily on uniaxial testing, but to also use bending beam flexural testing and overlay testing in characterizing the fatigue resistance of mixtures during NCHRP 9-59. However, testing plans were modified early in the project and additional overly testing was abandoned. All 16 of the NCHRP 9-59 mixtures were tested using uniaxial fatigue at two or three temperatures, while 9 of 16 of these mixtures were tested using
20 bending beam flexural tests at 10 and 20Â°C. Details of the mixture tests are discussed later in this report. OBJECTIVE OF NCHRP 9-59 The objectives of NCHRP 9-59 are given in the Request for Proposal for the project: 1. Determine asphalt binder properties that are significant indicators of the fatigue performance of asphalt mixtures. 2. Identify or develop a practical, implementable binder test (or tests) to measure properties that are significant indicators of mixture fatigue performance for use in a performance-related binder purchase specification such as AASHTO M 320 and M 332. 3. Propose necessary changes to existing AASHTO specifications to incorporate the identified binder properties and their specification limits. 4. Validate the binder fatigue properties, test(s), and changes to existing and/or proposed AASHTO test methods and specifications with data from field projects, accelerated loading facilities, or both, supplemented, as necessary, with data from additional laboratory-prepared specimens. This research shall emphasize the potential use of binder test equipment used currently in AASHTO binder test methods and consider a range of asphalt binder types, unmodified and modified, appropriately conditioned (i.e., aged), and with fatigue performance expected to vary widely. Field performance data for validation shall be drawn from existing sources such as accelerated loading facilities, LTPP Specific Pavement Studies (including SPS-10), Asphalt Research Consortium (ARC) field sections, and NCHRP Projects 9-47A, 9-49, 9-49A, 9-52, 9-53, 9-54, 9-55, and 9-58. SCOPE OF NCHRP 9-59 NCHRP 9-59, like other projects within the NCHRP, had a limited timeframe and budget. Therefore, the testing of binders and mixtures had to be limited to what could be reasonably accomplished given these constraints. Not all aspects of this problem could be addressedânot every potential binder test evaluated, not every specific binder type tested. The research team assumed that because of the limited resources, the best chance for success was to focus on binder fatigue and fracture tests that had already gone through most of the development process and had shown correlation to field performance. Ideally candidate binder tests would already be widely used and adopted by one or more highway agencies in a specification. Three binder tests were selected for in-depth evaluation during NCHRP 9-59: the LAS test, a simplified version of the DENT test; and an array of rheological parameters such as the GRP. The research team similarly felt that the laboratory mixture fatigue tests used in NCHRP 9-59 should be ones that have been widely used by pavement engineers, with a reasonable history of correlation to field performance, although realistically, as mentioned above, it has historically been difficult to link
21 mixture fatigue tests to field performance. As discussed in more detail later in this report, the main mixture tests used in NCHRP 9-59 were uniaxial fatigue and bending beam (flexural) fatigue. A total of 16 binders were selected for inclusion in the NCHRP 9-59 primary test program. These were selected in close consultation with the panel to represent a wide range of binder types and potential fatigue performance. In order to expand the NCHRP 9-59 data set beyond these 16 binders, substantial use was made of data from previous research projects and asphalt mixture fatigue. This includes the fatigue data collected during the Strategic Highway Research Program (University of California, Berkeley, 1994) and the first two FHWA ALF fatigue experiments (Stuart et al., 2002: Gibson et al., 2012. Binders from these projects were collected and tested as part of NCHRP 9-59, and the results compared to the observed fatigue performance of the mixtures. The primary experiment in NCHRP 9-59 involved performing mixture fatigue testsâboth uniaxial and flexuralâand relating these results to the results of the selected binder tests to determine which showed the best correlation. The method used to make this comparison was to calculate the fatigue/fracture performance ratio (FFPR) value for each binder from the two mixture tests, and then compare these to binder test data, which were in some cases also FFPR values and in some cases other binder test parameters. Some comparisons were also made between binder test data and field performance as a means of validating the findings of the project. The research team believes that the chosen approach was very successful and has led to a better understanding of asphalt mixture fatigue and will also provide a good basis for a few simple modifications in current binder specifications that will significantly improve mixture fatigue performance. Two important products of NCHRP 9-59 are proposed modifications to existing specification, and a plan for implementing these specification changes. A mixture healing experiment was also performed as part of NCHRP 9-59. Because of budget and time constraints, this experiment was less extensive than those involving mixture fatigue, but did provide useful information on the relationship between binder properties and mixture healing. Early discussions between the NCHRP 9-59 research team and the panel resulted in the issue of how to address the use of RAP and RAS being eliminated from the scope of the project. However, Dr. Walaa Mogawer and Dr. Ahmed Soliman of the University of Massachusetts Dartmouth (UMass) offered to perform research on the relationship between the binder parameters being looked at as part of NCHRP 9-59 and mixture semi-circular bend (SCB) flexibility index. This work was conducted without support from NCHRP 9-59, other than the limited time required from Dr. Christensen to coordinate their efforts with NCHRP 9-59, and to incorporate an appendix into the NCHRP 9-59 Final Report. This information should be useful in helping engineers use the SCB and other mixture tests to evaluate the fatigue performance of mixtures containing RAP and RAS in a way consistent with the findings, conclusions and recommendations of NCHRP 9-59.
22 Organization of this Report This report is organized following the standard NCHRP format. Following this Chapter on background is Chapter 2: Research Approach. This chapter explains the concepts outlined above in more detail, and provides more information on the binders, aggregates and mixtures used in NCHRP 9-59. Methods of analysis are also described in Chapter 2. Chapter 3: Findings and Applications presents the results of the testing and analysis performed during the project. It also discusses the resulting findings and how these can be applied to the problem of developing an improved binder fatigue specification. Chapter 4: Conclusions and Suggested Research is a listing of the most important conclusions from NCHRP 9-59 and suggestions for further research. This report includes numerous Appendices, where various details on information collected during NCHRP 9-59 can be found. In most cases, the body of this report only summarizes information that is presented more thoroughly in one of the appendices. The ancillary work performed by the UMass team on the relationships among binder rheological parameters and mixture SCB flexibility index in described in Appendix G.