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30 predicted at this time on the basis of aggregate and/or binder · Several 12.5-mm Superpave mixtures placed in Florida in type. Air void content also has a significant effect on age 1996, as reported by Choubane et al. in their permeability hardening--as air void content increases, the amount of age study (3); hardening increases. This is most likely because increasing air · A large number of Superpave mixtures placed at the NCAT voids will cause an increase in permeability, in turn causing an Test Track (6); and increase in age hardening. Although not observed in the test- · Typical SMA mixtures, according to information reported ing and analysis performed as part of NCHRP Projects 9-25 during NCHRP Project 9-8 (49). and 9-31, it is likely that increasing aggregate specific surface will also reduce age hardening because this would decrease Two different SMA mixtures are given--one compacted mixture permeability. During this research, a model was using 50-blow Marshall and one compacted using a gyratory developed for estimating mix permeability from air void compactor with Ndesign = 100. In order to compare compaction content and aggregate specific surface (Equations 1012). levels for Marshall compaction and gyratory compaction, the This model was combined with a modification of the number of blows for Marshall compaction must be converted MirzaWitczak global aging system to provide a means for to equivalent gyrations. As discussed in Chapter 2, it appears evaluating the relationships among mixture characteristics that for modeling rutting, Marshall blows are roughly equiva- and age hardening. However, the results of this analysis lent to number of gyrations. When modeling fatigue, it was should be considered approximate, since there are many found that 50 Marshall blows is approximately equivalent to questions concerning the accuracy of the global aging system. 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- Recent Evolution of HMA tionship relating Marshall blows to design gyrations: Ndesign. Composition and Effects Examining Table 5, several observations can be made. Com- on Performance paction levels are relatively high for the Superpave mixes Some insight into the practical aspects of the relationships although it should be noted that the most common compaction among HMA composition and performance can be gained by level for Superpave mix designs is probably 75 gyrations, which examining recent changes in typical mix designs. Table 5 is a is close to the equivalent Ndesign value for the Marshal mixes.VBE summary of average characteristics for five different projects is significantly lower for the Superpave mixes than for the other and/or mix types: HMA types. The SMA mixes have the highest VBE values. The MnRoad Marshall mixes and the Superpave mixes included in · A large number of Marshall mix designs as reported by the Florida permeability study have low values for aggregate Brown and Cross in their National Rutting Study (8); specific surface; the Superpave mixes placed on the NCAT Test · Ten Marshall mix designs placed on MnRoad in 1992 and Track have much higher aggregate specific surface values than 1993 (5); do the Superpave mixes placed on MnRoad. Table 5. Typical composition of various HMA mixtures. SMA/ Compositional Marshall/ Marshall/ Superpave Superpave 50-blow SMA/ Characteristic c. 1970/80 c. 1992/93 c. 1996 c. 2000 Marshall Ndesign = 100 Project (reference) National MnRoad Florida NCAT NCHRP NCHRP Rutting (5) Perm. Test Track Project Project 9-8 Study (8) Study (3) (6) 9-8 (49) (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 6.42 4.83 4.20 6.46 7.90 7.90 (m2/kg) VTM, as designed 4.1 3.7 4.3 4.0 4.0 4.0 (Vol. %) VTM, in-place 5.1 6.4 8.1 6.2 6.0 6.0 (Vol. %) Relative Density 0.994 0.971 0.960 0.977 0.979 0.979
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31 The effect of these trends on performance--rut resistance, would no doubt exhibit substantially improved performance. fatigue resistance, and permeability--can be calculated using It should also be noted that many other state highway agen- the models presented in Chapter 2. Figure 22 illustrates these cies probably placed similar mixes in the mid-1990s. Another estimated trends expressed as relative performance--higher surprising observation is that the Superpave mixes placed at values indicating better performance. Relative performance the NCAT Test Track exhibit excellent values for estimated values for rut resistance were calculated as the average rutting relative performance. The significant difference in the relative rate for the six mixes divided by the rutting rate for the given performance of the two sets of Superpave mixes is due to mix, multiplied by 100. Thus a relative performance of 50 for three factors--the NCAT mixes had substantially higher val- rut resistance indicates a rutting rate twice the average value. ues for aggregate specific surface, were compacted much bet- Relative performance for fatigue resistance was calculated as ter during construction, and had higher effective binder the number of cycles to failure for the given mix divided by contents. As should be expected, the relative performance of the average number of cycles to failure. Relative performance the SMA mixes is very good to excellent. The high perform- for permeability was determined as follows. For mixes with a ance of the SMA mixes is attributable to (1) high VBE values, permeability value near zero, performance was set at 100%. (2) high aggregate specific surface, and (3) good field com- For mixes with non-zero permeability, relative performance paction (if constructed as specified). was calculated by dividing the estimated permeability by 100 This analysis suggests that under the current Superpave sys- and multiplying by 75%. Thus, a mix meeting the minimum tem, requirements for volumetric composition could poten- requirement suggested by Choubane et al. (a maximum per- tially be improved. The wide difference in potential performance meability of 100 cm/s) would have a relative performance of between the Superpave mixes included in the Florida perme- 75%. Because materials specifications for Marshall mix ability study and those placed at the NCAT Test Track are of designs in the 1970s and 1980s were so much different than particular concern and suggest that research is needed to those for Superpave mixes, it was felt that the rutting/ address the workability and ease of compaction of HMA. There resistivity model could not be accurately applied to the may also be a need to refine requirements for aggregate fine- National Rutting Study data; therefore, no relative perform- ness in order to avoid mixes deficient in fines, leading to poor ance data for rutting is given for this case. rut resistance and high permeability. Agencies concerned with In interpreting Figure 22, it must be remembered that dif- the fatigue resistance of their HMA mixes should consider ferences in binder properties were not considered in con- modest increases in minimum VBE. The excellent performance structing this plot--the relative performance values reflect predicted for SMA mixes is consistent with experience and only differences in composition and compaction level. It is lends credence to the findings of this analysis. It also suggests surprising that over all, the worst mixes appear to be the that optimal performance for Superpave mixes and other Superpave mixes included in the Florida permeability study. HMA mix types can be ensured through three steps: The poor relative performance of these mixes is primarily due to (1) poor compaction, (2) low aggregate specific surface val- 1. Including enough asphalt binder to ensure good fatigue ues, and (3) low VBE. As noted previously, these represent resistance, early Superpave mix designs and specifications and construc- 2. Including adequate mineral filler and fine aggregate to tion practice in Florida have evolved since these mixes were keep permeability low and rut resistance high, and placed and Superpave mixtures currently placed in Florida 3. Obtaining proper compaction in the field. 200 Rut Resistance Relative Performance, % Fatigue Resistance 150 Impermeability 100 50 0 Marshall Marshall Superpave Superpave SMA SMA NRS MN/Road FL Perm. NCAT 50-Blow 100-Gyr. Figure 22. Relative Performance of Various HMA Mixes Based on Volumetric Composition and Compaction; Differences in Binder Properties Ignored.