National Academies Press: OpenBook

Volumetric Requirements for Superpave Mix Design (2006)

Chapter: Chapter 3 - Interpretation, Appraisal, and Applications

« Previous: Chapter 2 - Findings
Page 29
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 29
Page 30
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 30
Page 31
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 31
Page 32
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 32
Page 33
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 33
Page 34
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 34
Page 35
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 35
Page 36
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 36
Page 37
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 37
Page 38
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 38
Page 39
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 39
Page 40
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 40
Page 41
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Volumetric Requirements for Superpave Mix Design. Washington, DC: The National Academies Press. doi: 10.17226/13999.
×
Page 41

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.

29 The purpose of this chapter is to discuss and interpret the findings presented in Chapter 2, with special emphasis on the practical application of these findings. This chapter is pre- sented in four sections: 1. A summary of the relationships among HMA mixture characteristics and performance; 2. A discussion of how HMA mix design specifications have evolved over the past 30 years, and how the resulting changes have affected potential pavement performance; 3. A discussion of potential revisions in Superpave require- ments for HMA composition and compaction, and how these revisions might affect various aspects of perform- ance; and 4. A discussion of the implementation of the results of this research, which includes an Extended Work and Validation Plan. Summary of Relationships Among HMA Mixture Characteristics and Performance The findings presented in Chapter 2 dealt primarily with relationships among mixture characteristics and various aspects of performance. Before discussing the practical impli- cations of these findings, a summary of these relationships is useful. The following factors tend to improve the rut resist- ance of Superpave and other HMA mix types: • Increasing binder viscosity; • Decreasing VMA; • Increasing aggregate specific surface; • Increasing design compaction (Ndesign); and • Increasing field compaction (decreasing in-place air voids). The relationship among these factors and observed rut resistance for a wide range of field data has been quantified in a rutting/resistivity model (Equations 1 and 2). This model allows some quantitative (but approximate) estimates to be made regarding how changes in the composition require- ments of HMA might affect rut resistance. The following factors tend to improve the fatigue resistance of Superpave and other HMA mix types: • Increasing effective asphalt binder content, at given levels of Ndesign, design air voids, and in-place air voids; • Increasing Ndesign, at given levels of VBE, design air voids, and in-place air voids; and • Decreasing in-place air voids, at given levels of design voids, VBE, and Ndesign. Asphalt binder rheologic type, as reflected in the rheo- logical index R, also affects fatigue resistance; in the labora- tory, increasing values for R tend to improve fatigue resistance. However, significant experience with actual pave- ments suggests that HMA made using binders with high R values often exhibit extensive premature surface cracking, contradicting the results of most laboratory fatigue tests. Therefore, it is not recommended that mix designers attempt to improve the fatigue resistance of HMA mixes by selecting binders with high R values. These findings on fatigue resistance are largely based on continuum damage theory, which was used to analyze a large amount of labora- tory fatigue data, including data gathered during NCHRP Projects 9-25 and 9-31 and flexural fatigue data collected during SHRP. This analysis resulted in a fatigue model (Equation 9) for predicting the number of cycles required to reach a given damage level for an HMA with specified char- acteristics (VBE, Ndesign, etc.). The relationships between mixture composition and age hardening are not as easily quantified as other aspects of performance. A large amount of the age hardening observed in a given HMA in laboratory tests appears to be a function of the specific asphalt-aggregate composition and cannot be C H A P T E R 3 Interpretation, Appraisal, and Applications

predicted at this time on the basis of aggregate and/or binder type. Air void content also has a significant effect on age hardening—as air void content increases, the amount of age hardening increases. This is most likely because increasing air voids will cause an increase in permeability, in turn causing an increase in age hardening. Although not observed in the test- ing and analysis performed as part of NCHRP Projects 9-25 and 9-31, it is likely that increasing aggregate specific surface will also reduce age hardening because this would decrease mixture permeability. During this research, a model was developed for estimating mix permeability from air void content and aggregate specific surface (Equations 10–12). This model was combined with a modification of the Mirza–Witczak global aging system to provide a means for evaluating the relationships among mixture characteristics and age hardening. However, the results of this analysis should be considered approximate, since there are many questions concerning the accuracy of the global aging system. Recent Evolution of HMA Composition and Effects on Performance Some insight into the practical aspects of the relationships among HMA composition and performance can be gained by examining recent changes in typical mix designs. Table 5 is a summary of average characteristics for five different projects and/or mix types: • A large number of Marshall mix designs as reported by Brown and Cross in their National Rutting Study (8); • Ten Marshall mix designs placed on MnRoad in 1992 and 1993 (5); • Several 12.5-mm Superpave mixtures placed in Florida in 1996, as reported by Choubane et al. in their permeability study (3); • A large number of Superpave mixtures placed at the NCAT Test Track (6); and • Typical SMA mixtures, according to information reported during NCHRP Project 9-8 (49). Two different SMA mixtures are given—one compacted using 50-blow Marshall and one compacted using a gyratory compactor with Ndesign = 100. In order to compare compaction levels for Marshall compaction and gyratory compaction, the number of blows for Marshall compaction must be converted to equivalent gyrations. As discussed in Chapter 2, it appears that for modeling rutting, Marshall blows are roughly equiva- lent to number of gyrations. When modeling fatigue, it was found that 50 Marshall blows is approximately equivalent to 73 gyrations, while 75 Marshall blows is approximately equiv- alent to 92 gyrations. In calculating equivalent values of Ndesign in Table 5, these data were used to develop a power law rela- tionship relating Marshall blows to design gyrations: Ndesign. Examining Table 5, several observations can be made. Com- paction levels are relatively high for the Superpave mixes although it should be noted that the most common compaction level for Superpave mix designs is probably 75 gyrations, which is close to the equivalent Ndesign value for the Marshal mixes.VBE is significantly lower for the Superpave mixes than for the other HMA types. The SMA mixes have the highest VBE values. The MnRoad Marshall mixes and the Superpave mixes included in the Florida permeability study have low values for aggregate specific surface; the Superpave mixes placed on the NCAT Test Track have much higher aggregate specific surface values than do the Superpave mixes placed on MnRoad. 30 Compositional Characteristic Marshall/ c. 1970/80 Marshall/ c. 1992/93 Superpave c. 1996 Superpave c. 2000 SMA/ 50-blow Marshall SMA/ Ndesign = 100 Project (reference) National Rutting Study (8) MnRoad (5) Florida Perm. Study (3) NCAT Test Track (6) NCHRP Project 9-8 (49) NCHRP Project 9-8 (49) Comp. Method Marshall Marshall Gyratory Gyratory Marshall Gyratory Blows/Gyrations 52 56 109 100 50 100 Ndesign /equivalent Rutting 52 56 109 100 50 100 Fatigue 75 78 109 100 73 100 VBE (Vol. %) 12.4 11.6 9.8 10.7 13.5 13.5 Specific Surface (m2/kg) 6.42 4.83 4.20 6.46 7.90 7.90 VTM, as designed (Vol. %) 4.1 3.7 4.3 4.0 4.0 4.0 VTM, in-place (Vol. %) 5.1 6.4 8.1 6.2 6.0 6.0 Relative Density 0.994 0.971 0.960 0.977 0.979 0.979 Table 5. Typical composition of various HMA mixtures.

The effect of these trends on performance—rut resistance, fatigue resistance, and permeability—can be calculated using the models presented in Chapter 2. Figure 22 illustrates these estimated trends expressed as relative performance—higher values indicating better performance. Relative performance values for rut resistance were calculated as the average rutting rate for the six mixes divided by the rutting rate for the given mix, multiplied by 100. Thus a relative performance of 50 for rut resistance indicates a rutting rate twice the average value. Relative performance for fatigue resistance was calculated as the number of cycles to failure for the given mix divided by the average number of cycles to failure. Relative performance for permeability was determined as follows. For mixes with a permeability value near zero, performance was set at 100%. For mixes with non-zero permeability, relative performance was calculated by dividing the estimated permeability by 100 and multiplying by 75%. Thus, a mix meeting the minimum requirement suggested by Choubane et al. (a maximum per- meability of 100 cm/s) would have a relative performance of 75%. Because materials specifications for Marshall mix designs in the 1970s and 1980s were so much different than those for Superpave mixes, it was felt that the rutting/ resistivity model could not be accurately applied to the National Rutting Study data; therefore, no relative perform- ance data for rutting is given for this case. In interpreting Figure 22, it must be remembered that dif- ferences in binder properties were not considered in con- structing this plot—the relative performance values reflect only differences in composition and compaction level. It is surprising that over all, the worst mixes appear to be the Superpave mixes included in the Florida permeability study. The poor relative performance of these mixes is primarily due to (1) poor compaction, (2) low aggregate specific surface val- ues, and (3) low VBE. As noted previously, these represent early Superpave mix designs and specifications and construc- tion practice in Florida have evolved since these mixes were placed and Superpave mixtures currently placed in Florida would no doubt exhibit substantially improved performance. It should also be noted that many other state highway agen- cies probably placed similar mixes in the mid-1990s. Another surprising observation is that the Superpave mixes placed at the NCAT Test Track exhibit excellent values for estimated relative performance. The significant difference in the relative performance of the two sets of Superpave mixes is due to three factors—the NCAT mixes had substantially higher val- ues for aggregate specific surface, were compacted much bet- ter during construction, and had higher effective binder contents. As should be expected, the relative performance of the SMA mixes is very good to excellent. The high perform- ance of the SMA mixes is attributable to (1) high VBE values, (2) high aggregate specific surface, and (3) good field com- paction (if constructed as specified). This analysis suggests that under the current Superpave sys- tem, requirements for volumetric composition could poten- tially be improved.The wide difference in potential performance between the Superpave mixes included in the Florida perme- ability study and those placed at the NCAT Test Track are of particular concern and suggest that research is needed to address the workability and ease of compaction of HMA. There may also be a need to refine requirements for aggregate fine- ness in order to avoid mixes deficient in fines, leading to poor rut resistance and high permeability. Agencies concerned with the fatigue resistance of their HMA mixes should consider modest increases in minimum VBE. The excellent performance predicted for SMA mixes is consistent with experience and lends credence to the findings of this analysis. It also suggests that optimal performance for Superpave mixes and other HMA mix types can be ensured through three steps: 1. Including enough asphalt binder to ensure good fatigue resistance, 2. Including adequate mineral filler and fine aggregate to keep permeability low and rut resistance high, and 3. Obtaining proper compaction in the field. 31 0 50 100 150 200 Marshall NRS Marshall MN/Road Superpave FL Perm. Superpave NCAT SMA 50-Blow SMA 100-Gyr. R el at iv e Pe rf o rm an ce , % Rut Resistance Fatigue Resistance Impermeability Figure 22. Relative Performance of Various HMA Mixes Based on Volumetric Composition and Compaction; Differences in Binder Properties Ignored.

Further insight into potential improvements in the volu- metric requirements for Superpave can be gained by examin- ing the performance history of Superpave mixes. In general, the rut resistance of Superpave mixes has been very good to excel- lent. The notable exception was the extreme rutting observed in most of the mixes placed at WesTrack. The resistivity/rutting model accurately predicts that these mixes should be prone to rutting because of high VMA, low specific surface, marginal compaction, and a binder that would perhaps be adequate in a normal pavement but was marginal in an accelerated loading environment because there was little opportunity for long- term age hardening to occur within the pavement surface. The most important lesson of WesTrack is that the various factors that affect pavement performance are additive; individual fac- tors that may not result in serious degradation of performance can cause premature failure if a number of them acts simulta- neously. This should be kept in mind in any attempts to mod- ify current requirements for volumetric composition. Many state highway agencies have become concerned over the durability of Superpave mixes because of high levels of permeability and an apparent increase in the incidence of top-down cracking; this is evidenced by the recent large num- ber of research projects dealing with these topics (3, 50–54). The analysis above offers a clear explanation for the recent increase in HMA permeability—the decreased aggregate spe- cific surface and the increased difficulty of field compaction can both act together to substantially increase the permeabil- ity of mixes designed using the Superpave system. Explaining the increase in top-down cracking is not as easy. Certainly sig- nificant increases in the permeability of surface-course mix- tures would contribute to top-down cracking by increasing age hardening and by decreasing resistance to moisture dam- age at the pavement surface. Poor compaction would also tend to produce a relatively weak pavement surface, prone to cracking under repeated loading. The lower asphalt binder contents that have been used under the Superpave system would also contribute to top-down cracking by reducing the fatigue resistance of the HMA. However, it is not clear if these are the primary factors that are increasing the extent of sur- face cracking in HMA pavements. Other factors that could contribute to top-down cracking include increased tire-pave- ment stresses, changes in asphalt binder chemistry and flow properties, and a general increase in pavement modulus brought about by the use of stiffer asphalt binders (50). Possible Revisions in Volumetric Requirements for Superpave Mixtures and Their Effect on Performance In the Survey of State Practice, it was found that a number of highway agencies have modified volumetric requirements for Superpave mixes. The most common such modifications are allowing a design air voids range of from 3.0% to 5.0%, rather than using a single target value of 4.0%, and establish- ing a maximum VMA 1.5% or 2.0% above minimum require- ments. A number of states have increased minimum VMA requirements by 0.5% to 1.0%. An optional modification to original Superpave requirements that already is included in AASHTO specifications is an increase in the dust-to-binder ratio from 0.6–1.2 to 0.8–1.6. The methods used above can be applied to analyze the potential effects on HMA performance of these modifications to Superpave requirements. Changing Design Air Voids There are two different ways in which design air void con- tent can be changed: (1) at constant VMA and (2) at constant VBE. If design air void content is changed at constant VMA, then VBE must change an amount equal in magnitude but opposite in sense from the change in design air voids. For example, if the design air void content for a 9.5-mm NMAS mixture is changed to 3% from 4% and no changes are made in VMA, then the minimum VBE content will be increased from 11% to 12%. If design air void content is changed at constant VBE, then VMA will change along with changes in air void content—if design air voids are decreased 1%, then VMA will decrease 1%, and vice versa Changing design air voids can affect HMA performance through two mechanisms: (1) the change in design air voids relative to in-place air voids will effect relative compaction; and (2) changes in design air voids will change either VBE or VMA, depending on the manner in which the change is accomplished. Reducing design air voids will decrease as con- structed relative density if in-place air voids are assumed constant—this will tend to reduce both rut resistance and fatigue resistance. Similarly, increasing design air voids will increase as constructed relative density, improving rut resist- ance and fatigue resistance. However, if design air voids are reduced at constant VMA, VBE will increase, which will tend to improve fatigue resistance, somewhat offsetting the effect of the reduced relative density. Increasing design air voids at constant VMA will decrease VBE, decreasing fatigue resist- ance and, again, somewhat offsetting the effect of increased as-constructed relative density. The net effects on perfor- mance for changes in design air voids are summarized in Table 6. These estimates are based on an HMA mix with a design air void content of 4%, a VMA of 14%, a VBE of 10%, and an Ndesign of 75 gyrations. The in-place air void content is assumed to be 7%. At constant VMA, reducing design air voids from 4% to 3% decreases rut resistance about 18% and fatigue resistance about 9%. Increasing design air voids from 4% to 5% improves rut resistance about 22% and fatigue resistance about 8%. When air voids are changed at constant VBE, a reduction in design air void content from 4% to 3% increase rut resistance 32

a small amount (5%) while fatigue resistance is decreased by about 22%. An increase in design air void content from 4% to 5% has almost no effect on rut resistance, but increases fatigue resistance by about 29%. When contemplating increases in design air void content, engineers should consider that this will increase the difficulty of field compaction, which in some cases might increase in- place air voids. This would reduce or eliminate all of the potential benefits of a higher design air void level and would also increase the permeability of the pavement surface, ren- dering it more susceptible to age hardening and moisture damage. On the other hand, even though decreasing design air void content to 3% reduces performance when in-place air void contents are held constant as discussed in Chapter 2, the effect on performance would be negligible if the reduction in design air voids were to be accompanied with a similar decrease in allowable in-place air void contents. This might be an effective approach to reducing permeability of surface- course mixtures for some agencies. Establishing Maximum Limits for VMA A number of state highway agencies have established max- imum limits for VMA, generally either 1.5% or 2.0% above the minimum VMA for a given aggregate size.AASHTO M323-04 includes a note warning that mixtures made with VMA values more than 2.0% above the specified minimums might be prone to rutting and flushing. For surface-course mixtures designed for higher traffic levels (3 million ESALs and above), capping VMA at 2.0% above current minimum values is largely a matter of practicality and does not have a significant effect on performance. The reason for this is that current Superpave requirements include indirect limits on VMA that result from the interaction of design air void requirements and maximum values for VFA. Table 7 lists the current, indi- rect limits on VMA as given in AASHTO M323-04. The max- imum VMA values range from 1.7% to 3.0% above the minimum values. For mixtures made using 12.5-mm NMAS and smaller, the maximum values are 1.7% to 2.2% above the minimum. For most surface-course mixtures designed at high traffic levels, establishing explicit maximum VMA values has little effect on allowable ranges for VMA and HMA perform- ance. However, it is extremely important to realize that if design air void levels are allowed to change, and no other changes are made in volumetric requirements, the indirectly specified VMA values inherent in the Superpave system can change dramatically. For example, for a 12.5-mm mixture for heavy traffic, the maximum allowable VFA is currently 75%. At 4% design air voids, this translates to 4/[1−(75/100)] = 16% maximum VMA. However, if design air voids are increased to 5%, the maximum allowable VMA becomes 20%; if design air voids decrease to 3%, the maximum VMA becomes 12%—lower than the minimum VMA of 14% for this aggregate size. Agen- cies that alter design air void levels in the Superpave system must either adjust VFA requirements to establish reasonable VMA limits, or they should eliminate VFA requirements and rely on explicit maximum VMA limits. Indirect maximum VMA values are higher at design traffic levels below 3 million ESALs because maximum VFA limits increase to 78% and 80%, depending on the traffic level. At design traffic levels below 0.3 million ESALs, the maximum VFA of 80% results in a maximum allowable VMA of 20% for all aggregate sizes. At design traffic levels of from 0.3 million ESALs, the maximum VFA is 78%, resulting in maximum VMA of 18.2%. Some of the complexity and confusion inher- ent in this system could be avoided by eliminating VFA requirements and relying solely on VMA and air voids to con- trol this aspect of mixture composition. As discussed previ- ously, this has an advantage in that it is effective even when design air void levels vary. A reasonable scheme would be to allow maximum VMA to vary according to design traffic level: •  0.3 million ESALs: maximum VMA of 4.0% above min- imum value; • ≥ 0.3 to  3 million ESALs: maximum VMA of 3.0% above minimum value; and • ≥ 3 million ESALs: maximum VMA of 2.0% above mini- mum value. Other similar schemes are possible. Establishing explicit limits on VMA is simpler than the current system and involves less possibility of misinterpretation. It is also more 33 Approach Design Air Void Content Change in Rut Resistance Change in Fatigue Resistance 3% – 18% – 9% 4% — —Constant VMA 5% +22% +8% 3% +5% – 22% 4% — —Constant VBE 5% – 3% +29% Aggregate NMAS Mm Minimum VMA % Maximum VMA % VMA Range % 19.0 13.0 16.0 3.0 12.5 14.0 16.0 2.0 9.5 15.0 16.7 1.7 4.75 16.0 18.2 2.2 Table 6. Effect of change in design air void content on HMA performance. Table 7. Minimum and maximum VMA values for mixes designed using the Superpave system at 4% design air voids and for traffic levels of 3 million ESALs and higher (AASHTO M323-04).

flexible, but maintains a similar degree of control over VMA, VFA, and related factors. Increasing Minimum VMA Limits A number of agencies have increased minimum VMA lim- its by 0.5% to 1.0%. Given the recent concern over top-down cracking and high permeability in mixtures designed using the Superpave system, this modification would appear to be an attempt to improve durability of HMA pavements. Increasing VMA will improve fatigue resistance, but applying the models developed during NCHRP Projects 9-25 and 9-31, this improvement is probably not as large as what many engi- neers might suppose—only about 17% improvement for a 1% increase in VMA (assuming constant design air voids). Furthermore, unless aggregate specific surface is increased along with minimum VMA, there is a risk that rut resistance will be decreased. This decrease could be as much as 19% or roughly equal to the gain in fatigue resistance. To avoid a reduction in rut resistance, agencies should consider increas- ing the minimum dust-to-binder ratio when increasing min- imum VMA requirements. Although only a 17% improvement in fatigue resistance is predicted for an increase of 1% in VMA, other factors may be at work that will improve the durability of HMA made with higher VMA and binder contents. An important issue is the ease of compaction of the mixture. It is possible that higher VMA and binder contents might improve field compaction for HMA, which would substantially improve durability above and beyond the improvement resulting from increased binder content alone. It is also possible that alternate cycles of fatigue damage and healing could mean that relatively small improvements in fatigue resistance might significantly improve field performance. The fact that top-down cracking in HMA pavements has recently been observed to increase at the same time VMA requirements have been reduced sup- ports the use of increased VMA to improve HMA durability. Further research is needed to better understand the relation- ship between HMA mixture characteristics, binder rheology, laboratory fatigue test data, and fatigue performance in situ. Increasing Dust-to-Binder Ratio and Related Modifications AASHTO M323-04 already includes an option for an increase in dust-to-binder ratio from 0.6–12 to 0.8–1.6 for mixtures made using coarse aggregate gradations. It is not clear why this requirement should only be applied to such mixtures although in most cases, fine and dense gradations would probably meet this stricter requirement anyway, so enforcing this increased dust-to-binder ratio for such mix- tures might be redundant. In any case, the findings of NCHRP Projects 9-25 and 9-31 suggest that including an ade- quate amount of fines—mineral filler and material in the 75- to 300-μm-size range—is one of the most important fac- tors affecting HMA performance. It should be emphasized that a general increase in the aggregate specific surface for all mixtures does not appear to be needed—what is needed is an increase in the minimum requirements for aggregate fineness in order to avoid designing and producing HMA mixtures deficient in fines. Such mixtures will potentially exhibit poor rut resistance and high permeability, which will in turn increase their susceptibility to age hardening and moisture damage. There are three possible specification modifications related to this aspect of HMA composition. First, the dust-to-binder ratio could be increased either to the currently optional range of 0.8 to 1.6 or to some other value. Second, the requirements for dust-to-binder ratio could be replaced with requirements for minimum FM300 as a function of VMA or aggregate NMAS. Although this approach may at first seem somewhat more complicated than the current system, dust-to-binder ratios are generally calculated on the basis of effective asphalt content, the determination of which is not simple. Further- more, controlling FM300 as a function of VMA or aggregate NMAS would be a somewhat more effective approach since FM300 is a better indicator of aggregate specific surface com- pared with the percent passing 75 μm alone. A third approach would be to eliminate the requirements for dust-to-binder ratio and increase the requirements for percent finer than 75 μm in the aggregate gradation specifications. This approach is the simplest of all and probably is just as effective as controlling dust-to-binder ratio. Before these alternate approaches are evaluated, it is useful to try to determine some rational limits for aggregate specific surface. With the exception of the WesTrack mixtures, there is little evidence that mixtures designed according to the Super- pave system are prone to rutting. On the other hand, recent research suggests that there is significant concern about the relatively high permeability of HMA mixtures being placed in North America. Furthermore, increasing minimum require- ments for aggregate specific surface can only improve rut resistance because of its beneficial effect on resistivity. There- fore, permeability requirements will control any modification to the dust-to-binder ratio requirements. Control of in-place permeability is relatively straightforward since permeability is mostly a function of in-place air voids and aggregate spe- cific surface (Equations 10–12). If in-place air voids are assumed constant, at an average value of 7.0%, permeability then becomes only a function of aggregate specific surface, which in turn can be estimated from either percent finer than 75 μm or FM300 (see Figures 1 and 2). A complicating factor in this analysis is the variability in the relationships between the percent finer than 75 μm and 34

FM300 and specific surface (referring again to Figures 1 and 2). The relationship between FM300 and specific surface is signif- icantly better than that between the percent finer than 75 μm and specific surface, and neither is close to perfect. To account for variability in these relationships, the following approach was used in analyzing the effectiveness of different means of controlling aggregate specific surface and permeability. The data set used was the same as that plotted in Figures 1 and 2 and included data from eight projects: NCHRP Project 9-9, the NCAT Test Track, Pooled Fund Study 176, the Florida per- meability study, MN/Road, FHWA’s ALF rutting study, Wes- Track, and data from NCHRP Projects 9-25 and 9-31 (2, 3, 5, 6, 29–31). Four approaches to controlling specific surface were initially considered: (1) direct control of specific surface (minimum value for calculated Sa), (2) minimum value for percent finer than 75 μm, (3) minimum value for FM300, and (4) minimum dust-to-binder ratio (by weight, calculated using effective binder content). It was decided (somewhat arbitrarily) that three levels of control would be examined, corresponding to rejection rates of about 30%, 20%, and 10%. “Rejection rate” in this case means the percentage of mixtures that fail to meet the given criteria. The minimum values for the various control parameters were varied until the rejection rate matched the target value as close as possi- ble, then, the average permeability and maximum permeabil- ity for the mixes meeting this criteria were estimated using Equations 10–12. Because Choubane et al. recommended lowering target in-place air voids from 7.0% to 6.0%, both of these values were used in the analysis. Using this analysis, the more effective a given approach, the lower will be the average and maximum values for permeability. The results are sum- marized in Table 8. With no control, the average permeability for the data set is 100 cm/s and the maximum value is 510 cm/s. The high degree of control would most likely be con- sidered too restrictive to be practical: the lowest level of con- trol appears to yield only modest decreases in permeability at 7% in-place air voids, although the reduction at 6% in-place air voids is significant. It would appear that to gain significant reductions in in-place permeability, either the moderate level of control is needed while maintaining current levels of in- place air voids, or the low level of control can be applied along with a 1% reduction in target in-place air voids. Rather than control permeability at a constant level, an alternative approach is to try to control permeability as a function of aggregate NMAS. For example, FM300 limits can be established as a function of aggregate NMAS; an example of this type of control is given in Table 9. In this case, maxi- mum FM300 values range from 19 for 19.0-mm NMAS to 25 for 9.5-mm NMAS. The rejection rate for this scheme using the same data set used in the previous analysis was 15%. This approach allows for a very low permeability level for mixtures with small NMAS, with gradually increasing permeability 35 Control Method Control Level Average In- Place Air Voids Vol. % Property Minimum Aggregate Specific Surface m 2/kg Minimum Percent Passing 75 μm Min. FM300 Min. Dust/ Binder Ratio Avg. k × 105, cm/s 100 7.0 Max. k × 105, cm/s 510 Avg. k × 105, cm/s 50 None 6.0 Max. k × 105, cm/s 400 Min. Value 4.5 4.5 24 1.1 N/A % Failing Spec. 28 28 28 27 Avg. k × 105, cm/s 30 40 30 50 7.0 Max. k × 105, cm/s 170 250 230 320 Avg. k × 105, cm/s 4 10 10 20 High 6.0 Max. k × 105, cm/s 60 140 120 210 Min. Value 4.4 4.3 22 1.0 N/A % Failing Spec. 19 21 20 17 Avg. k × 105, cm/s 50 50 50 70 7.0 Max. k × 105, cm/s 230 320 270 350 Avg. k × 105, cm/s 20 20 20 30 Moderate 6.0 Max. k × 105, cm/s 120 210 160 240 Min. Value 4.0 4.0 20 0.9 N/A % Failing Spec. 9 5 12 8 Avg. k × 105, cm/s 70 80 60 80 7.0 Max. k × 105, cm/s 290 370 280 370 Avg. k × 105, cm/s 30 40 30 40 Low 6.0 Max. k × 105, cm/s 180 260 170 260 Table 8. Effectiveness of various approaches to controlling HMA permeability and aggregate specific surface, 7% in-place air voids.

levels with increasing NMAS. This is consistent with the vari- ation of VMA and VBE with NMAS—this approach is con- sistent with the general trend of increasing fatigue resistance and durability with decreasing NMAS. An advantage of this approach is that it provides for mixtures with very low per- meability while maintaining an overall moderate level of con- trol. Another advantage is that this method of controlling aggregate specific surface tends to provide similar levels of resistivity regardless of aggregate NMAS. Since specific sur- face increases with increasing NMAS, it will tend to increase with increasing VMA. A related approach would be to control FM300 directly as a function of VMA. An example of this type of control is given in Table 10. In this case, FM300 limits were calculated to give the same values for resistivity regardless of VMA; the FM300 values were calculated using the formula: (14) The resulting limits are listed at the top of Table 10. This particular example is slightly more restrictive than that given in Table 9, with a rejection rate of 19%. It provides slightly less control over permeability compared with the previous exam- ple, but has the advantage of very good control over resistivity since specific surface is linked directly to VMA. This approach would however be slightly more difficult to implement. Although controlling dust-to-binder ratio was listed in Table 8 with several other approaches that tend to provide similar levels of specific surface and permeability regardless of aggregate NMAS, this approach does in fact tend to result in some variation in aggregate specific surface and perme- FM VMA300 3 0 50 15= ×( ). . ability with NMAS since as NMAS decreases, VMA and VBE will increase, increasing the amount of mineral filler that must be used to obtain the required minimum dust-to- binder ratio. Table 11 summarizes the control of permeabil- ity as a function of aggregate NMAS that results from setting a minimum dust-to-binder ratio of 1.0. This approach is moderately restrictive and is, in fact, identical to the dust/binder/moderate control level summarized in Table 8— the permeability values have now simply been broken down by aggregate NMAS. This approach appears to be similar to that given by linking FM300 to VMA. The main advantage of this approach is simplicity and that it is consistent with the current Superpave system. As with linking FM300 to aggregate NMAS, this method provides some control over resistivity, but not as good as does linking FM300 to VMA. A few comments are needed concerning the analysis pre- sented above. Although the data set used is relatively large and robust, the results should be considered as only guidelines. Further analysis with additional data is needed to provide more confidence in the specific degree of control exerted by the various approaches. A large number of approaches were presented here because it is not clear at this time which approach will be most effective and efficient for the largest number of users over the widest range of situations—this is a decision that will be made during implementation. Further- more, in some areas the aggregates locally available may be deficient in fines, and the cost of obtaining additional fines may be prohibitive. Such situations require flexibility and judgment when developing approaches for controlling aggre- gate specific surface and mixture permeability. General Approaches to Improving the Durability of Mixtures Designed According to the Superpave System and Other HMA Mix Types As discussed previously, there is substantial evidence that mixtures designed according to the Superpave system are more permeable and somewhat more prone to top-down cracking compared with HMA that is designed and placed using the Marshall system. There is therefore a desire within 36 Permeability×105 (cm/s) at Average In-Place Air Voids 7.0 % 6.0 % NMAS Min. FM300 Avg. Max. Avg. Max. 9.5 mm 25 3 20 0 0 12.5 mm 22 40 250 20 150 19.0 mm 19 110 290 50 180 Overall: 60 290 20 180 VMA, Vol. % 11 12 13 14 15 16 17 Min. FM300 14 16 18 20 23 25 27 Permeability×105 (cm/s) at Average In-Place Air Voids 7.0 % 6.0 % NMAS Avg. Max. Avg. Max. 9.5 mm 10 160 4 50 12.5 mm 50 320 20 210 19.0 mm 110 290 50 180 Overall: 60 320 30 210 Permeability×105 (cm/s) at Average In-Place Air Voids 7.0 % 6.0 % NMAS Avg. Max. Avg. Max. 9.5 mm 30 230 10 120 12.5 mm 40 320 20 210 19.0 mm 130 350 60 240 Overall: 70 350 30 240 Table 9. Control of permeability by limiting FM300 as a function of NMAS. Table 11. Control of permeability as a function of NMAS by limiting dust-to- binder ratio to 1.0. Table 10. Control of permeability as a function of NMAS by limiting FM300 as a function of VMA.

some highway agencies to improve the durability of HMA designed using Superpave methods, while still maintaining the excellent rut resistance that these materials have exhibited. Before discussing how this goal might be achieved, it must be noted that much evidence suggests it is not advisable to make large changes in the current requirements for HMA designed using the Superpave system—implementing drastic changes may have the effect of significantly decreasing rut resistance of HMA mixtures or causing other unforeseen problems. Furthermore, the differences in HMA composition between materials designed under the Superpave system and materi- als designed using the traditional Marshall system, although significant, are not so large to suggest that complete revision of the Superpave system is needed. Changes to the Superpave system implemented by various highway agencies support the advisability of measured changes in current specifications. Therefore, any modifications to current requirements in the Superpave system should be kept relatively minor. Based upon the findings given in Chapter 2 and the dis- cussion presented above, there are four critical aspects to improving HMA durability while maintaining good rut resistance: 1. Effective binder content should be increased to provide better fatigue resistance. 2. Aggregate fineness should be increased to decrease mix- ture permeability. 3. Design air voids can be decreased to improve compaction— lowering in-place air voids and decreasing permeability— but unless in-place air voids are in fact significantly decreased, both rut resistance and fatigue resistance will decrease if design air void content is reduced. 4. Requirements for in-place air voids can be decreased, improving both rut resistance and fatigue resistance. These aspects to improving HMA durability can be com- bined in a number of reasonable ways. Listed below are two promising approaches, some aspects of which have already been implemented by highway agencies in attempts to improve the performance of HMA designed using the Super- pave system: • Approach 1: 1. Increase minimum VMA from 0.5% to 1.0% while main- taining design air voids at 4.0%; this produces an increase in minimum VBE of 0.5% to 1.0%. 2. Apply optional increased dust-to-binder ratio of 0.8 to 1.6 or even further to 1.0 to 2.0. Alternately, one of the other methods presented earlier for controlling aggregate spe- cific surface can be used. 3. Review field compaction requirements to ensure that in- place air voids are sufficiently low to provide for low per- meability and overall good performance. • Approach 2: 1. Maintain current minimum VMA values while decreasing design air voids to 3.0% to 3.5%; this produces an increase in minimum VBE of 0.5% to 1.0%. 2. Reduce maximum allowable in-place air voids by an amount equal to the decrease in design air voids; also, review field compaction requirements to ensure that desired level of in-place air voids will in fact be achieved. 3. Consider applying optional increased dust-to-binder ratio of 0.8 to 1.6. Alternately, one of the other methods pre- sented earlier for controlling aggregate specific surface can be used. However, reducing in-place air void requirements should reduce the need to increase minimum require- ments for aggregate specific surface since mixture perme- ability will be significantly lower because of the improved field compaction. The resulting improvements in performance for these two approaches, as estimated using the various models presented in this report, are summarized in Table 12. This example is for a 12.5-mm NMAS design, with Ndesign = 75. The “current HMA” design assumes a VMA of 14.0% and a design air void 37 Characteristic Current 12.5-mm HMA Approach 1 Approach 2 Composition Ndesign 75 75 75 VMA, Vol. % 14.0 15.0 14.0 VTMdesign, Vol. % 4.0 4.0 3.0 VTMin-place, Vol. % 8.0 7.0 6.0 Dust/binder 0.6 0.8 0.6 Sa, m2/kg 3.8 4.6 3.9 Estimated Performance Relative Rut Resistance, % 100 150 140 Relative Fatigue Resistance, % 100 150 150 Permeability × 105, cm/s 440 200 200 Table 12. Relative performance of 12.5-mm NMAS mix designs modified using different approaches.

content of 4.0, resulting in a VBE of 10.0%. The dust-to- binder ratio is 0.6, corresponding to the current minimum value for the standard range for this characteristic in the Superpave system. Field air voids are assumed to be 8.0%, slightly above the standard assumed field air void content of 7.0%. The amount of aggregate finer than the 75-μm sieve was estimated from the mix composition and the dust-to- binder ratio and the aggregate specific surface then estimated using the relationship shown earlier in Figure 2. For Approach 1, the VMA was increased 1.0% to 15.0% while maintaining the design air voids at 4.0%, resulting in an increase in VBE to 11.0%. The dust-to-binder ratio was increased to 0.8, corresponding to the minimum value for the optional, higher range for this characteristic. It is assumed that a review of field compaction data has shown that field air voids are not as low as desired, and that modification in acceptance plans and enforcement result in achieving the desired level of 7.0% air voids in-place. Approach 2 keeps the VMA at 14.0% while decreasing design air voids to 3.0%, increasing VBE to 11.0%. The dust-to-binder ratio is kept at 0.6. It is assumed that field compaction is significantly improved by revising acceptance plans, resulting in an aver- age in-place air void content of 6.0%. The resulting improve- ments in estimated performance are significant. Both rut resistance and fatigue resistance improve by 40% to 50% for both modifications, while permeability is roughly cut in half. These approaches are only meant as general examples as to the type and magnitude of modifications that might prove successful in improving the durability of HMA designed according to the Superpave system while maintaining good rut resistance. Other approaches are possible and will be effective if proper consideration is given to how each specifi- cation change will affect various aspects of performance. Highway agencies must consider their local climate, traffic levels, and materials characteristics when attempting to mod- ify requirements for HMA. Furthermore, although evaluating specification changes with performance models is a useful tool, engineers should note that existing performance mod- els (including the ones developed as part of this research) pro- vide only approximate results and should be used with discretion. When making adjustments in the requirements for aggre- gate fineness—be it through dust-to-binder ratio, FM300, per- cent finer than 75 μm, or some other means—it should be kept in mind that the analyses presented here were based on aggregate gradations from QC data—that is, these were aggregates that had gone through most or all of the batching, mixing, and transport processes. The amount of fine mate- rial, and the specific surface, and related parameters will therefore be somewhat higher than for aggregates taken from stockpiles without going through an HMA plant. This increase in fines during production compared with labora- tory mix designs should be carefully considered when modi- fying requirements for aggregate fineness. For example, the data set used in the analyses of aggregate specific surface pre- sented earlier in Chapter 3 included a total of 94 points. Of these, only 1 point had a dust-to-binder ratio below 1.60, and only 3 had a dust-to-binder ratio below 0.80. On the other hand, 42 mixtures had dust-to-binder ratios above 1.20, and 19 had values above 1.60. Although in some cases the mixes may have been purposely designed with gradations outside Superpave limits, much of this discrepancy between the observed dust-to-binder ratios and current specification lim- its is probably due to an increase in fines during batching, mixing, transport, and placement. The best approach to deal- ing with this problem would be to try to obtain information concerning the changes that occur in aggregate gradation within a specific plant during production and to adjust stock- pile aggregates to try to mimic these changes in laboratory mix designs. Alternately, requirements for dust-to-binder ratio (or FM300 or percent finer than 75 μm) could be relaxed somewhat during the mix design process: dust-to-binder ratio could be set at 1.0 for production purposes, but allowed to go to 0.80 during the mix design process. It should how- ever be understood that the increase in fine material that occurs in actual plant production will cause other changes in HMA characteristics—typically, air void content and VMA will decrease. This is why it is best to try to mimic aggregate gradations as they come out of the plant, rather than to make adjustments in going from a laboratory mix design to a pro- duction job mix formula. Lowering Ndesign to Improve HMA Durability Some engineers may suggest that simply lowering Ndesign will provide significant improvement in durability, believing that this will increase design binder content and improve field compaction, resulting in improved fatigue resistance and lowered permeability. However, lowering Ndesign will not nec- essarily increase design binder content—in this situation, many producers will adjust their aggregate gradation so that the design binder content remains as low as possible since this will minimize the cost of the HMA and maximize profits. Paying for asphalt binder as a separate item removes the incentive to minimize binder content, but in no way guaran- tees that binder contents will be sufficient for good fatigue resistance. If an agency believes that current minimum binder contents are too low for adequate fatigue resistance and/or durability, the most effective and efficient remedy is simply to increase these minimum values. A similar situation exists for field compaction. Lowering Ndesign values will tend to make HMA mixtures easier to compact, but will not guarantee that in-place air voids will decrease. Assuming most successful contractors are motivated not by maximizing losses but by 38

maximizing profits (and therefore staying in business), the competitive marketplace demands that they adjust their com- paction methods to optimize their profits, based on the cost of performing compaction and the penalties and/or bonuses that result from different levels of compaction. Lowering Ndesign will help improve field compaction, but unless this is combined with a payment schedule adjusted to provide addi- tional incentive for thorough field compaction, in the long run it will not likely result in significant lowering of in-place air voids. Linking Aggregate NMAS and Minimum VMA Traditionally, HMA design has linked aggregate NMAS with requirements for minimum VMA—as NMAS decreases, minimum VMA increases. There are two reasons for this link- age: (1) smaller NMAS is usually associated with higher aggre- gate specific surface so that too maintain a more or less constant apparent film thickness, more binder is needed, and VMA should therefore increase; (2) smaller NMAS is in gen- eral associated with higher VMA—that is, all else being equal, aggregate gradations with smaller NMAS will tend to yield higher VMA. However, neither of these trends is extremely strong; aggregates with large NMAS may have a large amount of fines, leading to a high specific surface and requiring higher VMA to maintain a reasonable apparent film thickness. Simi- larly, aggregates with small NMAS may have inherently low VMA, making it difficult in the mix design process to achieve the higher VMA values required for aggregates with small NMAS. It can be questioned whether aggregate NMAS and minimum VMA should be linked. A theoretically more sound approach might be to establish aggregate NMAS on the basis of lift thickness and to set minimum VMA on the basis of desired fatigue resistance and durability. However, this would be a monumental change in the way people think about HMA and design mixes. It would be very difficult to implement and would probably lead to much confusion among engineers and technicians. Within the current approach, it is still possible to provide HMA mixtures with a reasonable range in fatigue resistance and durability. Maintaining the current system will help ease implementation of the more critical findings of this research, while still providing engineers with an adequate slate of mixtures to address most paving applications. Effect of Multiple Changes in HMA Specifications A number of the analyses presented previously involve mul- tiple, simultaneous changes in specifications and illustrates how these changes work together to affect performance. Any engineer or agency contemplating changes in Superpave spec- ifications, or in specifications for other HMA types, should consider in some way the way in which all changes in a speci- fication act together to affect performance. This includes not only changes in volumetric composition and compaction, but also changes in materials specifications; especially important are how changes in aggregate specifications might affect per- formance. The models and analyses presented in this report were largely developed on HMA mixtures made with aggre- gates that either meet or come close to meeting current Super- pave specifications. The models may or may not be accurate for aggregates that do not meet these requirements. An example of how specification changes can negatively affect performance is useful to illustrate the importance of considering how these changes can work together. Consider again the mixtures listed in Table 12. Imagine a third alterna- tive modification, Approach 3, in which design air voids are lowered to 3% to improve fatigue resistance. At the same time, Ndesign is reduced to 50 in order to try to improve compaction, but no effort is made to make specifications for field com- paction more stringent so that in-place air voids remain at 8.0%. It might at first seem that these changes would be ben- eficial to fatigue resistance and durability, but the analysis suggests otherwise—rut resistance in this case is reduced by 30%, fatigue resistance is reduced by 40%, and permeability remains nearly constant at 440 × 10−5 cm/s. The proposed changes have significantly decreased fatigue resistance, done nothing beneficial for permeability, and decreased rut resist- ance. If these changes were to be simultaneously implemented with reduced standards for aggregate angularity, the results could go from simply being bad to being disastrous. It is sug- gested that agencies considering both changes in materials specification and changes in specifications for volumetric composition and compaction should implement and evalu- ate such changes separately, to avoid unanticipated negative impacts on pavement performance. Implementation Because of the execution of NCHRP Project 9-33, a com- plete and formal implementation of the results of NCHRP Projects 9-25 and 9-31 would be redundant. Most of the findings and recommendations of this research are being evaluated and refined as appropriate for possible incorpora- tion into the Mix Design Manual being developed under NCHRP Project 9-33. It is therefore suggested that imple- mentation of the results of this project be kept simple and informal. The initial phases of implementation have already taken place through publication of several papers dealing with the various models developed during this research. It is expected that one or two additional publications will be sub- mitted summarizing the final results of NCHRP Projects 9-25 and 9-31. 39

Agencies that experience severe performance problems in a significant proportion of their HMA pavements may find it necessary to implement some of the recommendations of this research prior to development of the Mix Design Manual being developed as part of NCHRP Project 9-33—some agencies have already made significant modifications to the Superpave system. This report has intentionally been structured to pro- vide flexibility in helping engineers to evaluate a wide range of possible changes in volumetric requirements for HMA mix- tures designed using the Superpave system. Any such evalua- tion of the effects of changes in HMA specifications should be done not only using reasonably accurate performance models, but also using the experience of the engineer with local condi- tions and materials. Such changes should be done gradually and with caution. Demonstration projects using the proposed changes should be constructed and observed for several years prior to full-scale adoption of the proposed specification. Extended Work and Validation Plan The most significant general findings of NCHRP Projects 9-25 and 9-31 can be summarized as follows: • The impact on performance of various changes in HMA composition and compaction can be estimated using models. Several such models were developed as part of NCHRP Projects 9-25 and 9-31 using a combination of laboratory data and a number of large data sets taken from the literature. These and other such models are useful tools for evaluating the effects of changes in HMA specifications for mixture composition and compaction. • Fatigue resistance of HMA tends to increase with increas- ing VBE and with increasing Ndesign. • Rut resistance of HMA tends to increase with decreasing VMA, increasing aggregate specific surface, increasing binder stiffness at high temperature,and increasing Ndesign. • HMA permeability decreases with decreasing in-place air voids and increasing aggregate specific surface. • Fatigue resistance and rut resistance increase, and per- meability decreases, with decreasing field air voids. Of particular significance to fatigue and rut resistance is the in-place air void content relative to the design air void con- tent: the lower in-place air voids relative to design air voids, the higher the fatigue and rut resistance of the pavement. • There is significant evidence that the implementation of the Superpave system has resulted in an increase in the permeability and a decrease in the fatigue resistance of HMA pavements. A number of approaches to correcting these problems are possible, involving various combina- tions of increased VBE, increased aggregate specific sur- face, and/or improved field compaction. • The effect of changes in mixture composition are addi- tive and must be considered together when evaluating potential changes in requirements for HMA designed using the Superpave system and other mix types. The research performed during NCHRP Projects 9-25 and 9-31 involved 9.5-, 12.5-, and 19-mm aggregate blends. The purpose of the Extended Work and Validation Plan is to extend the results of NCHRP Projects 9-25 and 9-31 to larger aggregate sizes and also to validate the findings of the research through accelerated pavement testing, evaluation in test roads, or full-scale field evaluation. Specifically, there is a need to extend the laboratory testing to 25- and 37.5-mm aggre- gate sizes. Because such mixtures should not be used for surface-course mixtures, there is no need for testing related to rut resistance or resistance to age hardening. However, there is a need to evaluate the permeability of such mixtures and their fatigue resistance. Because the permeability of most of the mixtures tested during NCHRP Projects 9-25 and 9-31 was very low, the permeability model developed during this research relied on permeability data gathered from other research projects—notably, research performed in Florida on the permeability of Superpave surface-course mixtures. In addition to evaluating the permeability of mixtures made using larger aggregate sizes, there is also a need to confirm the findings on permeability by testing Superpave surface-course mixtures prepared at air void contents of 6% to 10%. Objective The objective of this research is to extend the results of NCHRP Projects 9-25 and 9-31 to mixtures made using 25- and 37.5-mm NMAS aggregates and to validate the find- ings of these research projects through accelerated pave- ment testing, pavement test tracks, or evaluation of full-scale pavements. Tasks It is anticipated that the research will include the following nine tasks. • Phase I, Task 1—Review the findings of NCHRP Projects 9-25 and 9-31 and related research, including NCHRP Projects 9-19, 9-29, 9-33, and 1-37A. Also, results of performance tests conducted using accelerated pavement testing facili- ties and test tracks or from monitoring of full-scale pave- ments should be identified and summarized. Performance data should include information on rutting, fatigue crack- ing, and age hardening. Some initial analyses of these data may be conducted, but the primary purpose of this effort is to identify data for analysis during Phase II. Special 40

emphasis should be made in considering field data ana- lyzed during Phase II of NCHRP Project 9-33. • Phase I, Task 2—Survey current practice among state high- way agencies in their implementation of volumetric specifi- cations for Superpave HMA. A Survey of Current Practice was performed during the initial phases of NCHRP Project 9-31 and was updated near the completion of NCHRP Projects 9-25 and 9-31; the objective of this task is to review and update this survey. • Phase I, Task 3—Develop a revised Phase II Work Plan. The task descriptions below represent an initial summary plan for Phase II of the Extended Work and Validation Plan. After the review of NCHRP Projects 9-25 and 9-31 and related research and updating the survey of current prac- tice, a revised, more detailed plan for Phase II should be developed. This will be included in the Interim Report to be submitted as Task 4. • Phase I, Task 4—Submit an Interim Report to NCHRP within 4 months of the start of work. This Interim Report will include as a minimum the review of findings of NCHRP Projects 9-25 and 9-31 and related research, an updated Survey of Current Practice, and the Revised Phase II Work Plan. Approximately 1 month will be allotted for review of the Interim Report by NCHRP. • Phase II, Task 5—Analyze the field performance data that was identified and summarized during Task 1 using the methods recommended in NCHRP Projects 9-25 and 9-31. This analysis will include statistical comparisons of pre- dicted and measured performance, including estimates of overall error compared with the error estimates reported in NCHRP Projects 9-25 and 9-31. Recommendations con- cerning the accuracy of the models will be made based upon the results of this analysis and the review of the find- ings of NCHRP Projects 9-25 and 9-31. • Phase II, Task 6—Execute accelerated pavement tests and/or field tests according to the Phase II Work Plan. This should include 8 to 12 test sections of pavements at an accelerated pavement testing facility such as FHWA’s ALF at the Turner–Fairbank Highway Research Center or at a test track such as exists at NCAT. Alternately, test sections could be constructed in actual pavements, but these must be care- fully designed and constructed so that valid comparisons among the mixtures tested can be made. Approximately one-half of the test sections should represent a variety of mixtures prepared according to the procedures given in the NCHRP Projects’ 9-25 and 9-31 final report; the balance should represent mixtures made according to current Superpave specifications, but in such a way that significant contrasts are made with the NCHRP Projects’ 9-25 and 9-31 mixture designs. Of particular interest are contrasts in effective binder content, VMA, aggregate fineness relative to VMA, and high temperature binder grade. The per- formance of the sections should be evaluated and com- pared with the performance as predicted using the models developed during NCHRP Projects 9-25 and 9-31. This analysis should emphasize the apparent effects of specific recommendations of NCHRP Projects 9-25 and 9-31. • Phase II, Task 7—Perform laboratory testing. These tests should include permeability tests and uniaxial fatigue tests. The permeability tests should be performed on a wide range of mixtures, including mixtures made using 9.5-, 12.5-, 19-, 25- and 37.5-mm NMAS aggregates. The speci- mens should be fabricated using rolling wheel compaction and/or should be field cores since specimens prepared using the gyratory compactor often exhibit significantly lower permeability values than do cores taken from pave- ments. The procedure used should be the Florida perme- ability test or similar procedure. The fatigue tests should be done in uniaxial mode, following the same procedures used in NCHRP Projects 9-25 and 9-31. The results should be analyzed using continuum damage methods and com- pared with the models developed during NCHRP Projects 9-25 and 9-31. • Phase II, Task 8—Evaluate and/or recalibrate performance models. The specific findings of NCHRP Projects 9-25 and 9-31 were based on several semi-empirical models relating mixtures volumetrics to different aspects of pavement per- formance. These models are being refined and recalibrated as part of NCHRP Project 9-33. These models should be further refined using the results of the field tests and labo- ratory tests performed during Tasks 6 and 7 and also using the field data from other projects analyzed in Task 5. If appropriate, the proposed models should be refined or recalibrated using an expanded data set including both the data generated during NCHRP Projects 9-25, 9-31, and 9-33 and the data collected during this research. If the mod- els appear to be inappropriate, alternate models should be proposed and evaluated. The final result of this task should be a final, refined set of recommendations concerning the volumetric composition of Superpave mixtures. • Phase II, Task 9—Prepare the Final Report. This will consti- tute a clear and concise summary of all of the significant research performed during the extend work/validation effort. The report will be prepared according to NCHRP guidelines. Detailed information concerning laboratory test- ing, analyses, or derivations should be included in appen- dixes. Three months will be allowed for NCHRP review of the Draft Final Report, after which the contractor will pre- pare the Revised Final Report based upon the comments received from NCHRP after review of the draft report. 41

Next: Chapter 4 - Conclusions and Recommendations »
Volumetric Requirements for Superpave Mix Design Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB's National Cooperative Highway Research Program (NCHRP) Report 567: Volumetric Requirements for Superpave Mix Design examines whether changes to the recommended Superpave mix design criteria for voids in mineral aggregate, voids filled with asphalt, and air voids content might further enhance the performance and durability of hot-mix asphalt.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!