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88 thickness, the 20-year MEPDG design fails the reliability 20% of the lane area to be cracked at the end of the design criteria for terminal international roughness index (IRI), life whereas the perpetual pavements would not be expected total pavement rutting, and HMA rutting. The HMA thick- to have any cracking. This significantly changes the required ness must be increased to 14 in. to produce an acceptable maintenance and rehabilitation requirements in a life-cycle reliability for terminal IRI. The HMA thickness must be cost analysis. increased to 30 in. to produce an acceptable reliability against total pavement rutting. The reliability for HMA rutting can not Considerations for Incorporating be achieved for this level of traffic in the NCAT Test Track's the Endurance Limit into climate using a PG 67-22 binder. The perpetual thickness M-E Design Procedures determined using the MEPDG is significantly thicker than that determined using PerRoad when considering a typical In the preceding section, sensitivity analyses were presented axle distribution. demonstrating the affect of incorporating the endurance limit Although the pavement thicknesses appear similar between into two M-E programs, the MEPDG and PerRoad. Certainly conventional empirical or mechanistic designs and the per- the predicted performance from the MEPDG in terms of petual pavement thickness determined using PerRoad, again bottom-up cracking was improved compared to a conven- the implications are very different. Based on the conven- tional 20-year design. Using the experimentally determined tional MEPDG results, the 11-in. HMA pavement would be endurance limits from this study, there was no increase in the expected to have maximum cracking of 4.8% of the lane design thickness determined using the MEPDG for a 20-year area after 20 years. The 90% reliability for bottom-up crack- or perpetual design. The thicknesses determined, 11.0- and ing is 22.72% of the lane area. Similarly, for the 12-in. pavement 10.0-in., respectively, for the PG 67-22 and PG 76-22 mixes after 40 years, maximum bottom-up cracking was predicted at optimum asphalt content are consistent with Nunn's (10) at 5.9% with a 90% reliability of 23.99% of the lane area. Based recommendations that long-life pavements should range on the conventional analysis, the pavement will have failed between 7.9 and 15.4 in. Further, Section N3 and N4 of the and be in need of reconstruction, whereas the perpetual 2003 NCAT Test Track have now gone through two test track analysis suggests that at a similar thickness there should be loading cycles without any observed fatigue cracking (77). This no bottom-up fatigue cracking after 40 years. This difference indicates that pavement thicknesses close to those designed would have a significant effect on life-cycle cost analysis. as perpetual pavements (N3 and N4 are 9 in. thick) with the MEPDG are performing well after a fairly high number (20 million ESALs) of load applications. However, the sen- Summary of Sensitivity Analyses sitivity of the required pavement thickness to the measured The design thickness for a perpetual pavement is very endurance limit also has been demonstrated, as well as sensitive to the measured endurance limit using both the the apparent sensitivity to temperature as evidenced by the MEPDG and PerRoad. Considering a typical traffic stream, increased pavement thicknesses determined using PerRoad. a 50-ms change in the endurance limit resulted in a 7- to 8-in. Therefore, consideration should be given as to whether the or 4-in. change in HMA thickness, respectively, with the endurance limit is really best represented by a single value, MEPDG and PerRoad. This sensitivity highlights the need for determined at a single temperature. accurate determination of the endurance limit. To improve One hypothesis is that the fatigue endurance limit is driven, accuracy, the number of strain levels used to predict the in part, by the ability of asphalt mixtures to heal. Healing endurance limit in Appendix A was increased from two to occurs more readily at higher temperatures. Therefore, a three with three replicates at each level from that used in mixture's fatigue capacity or endurance limit may be higher at Phase II of the study. Additional samples should reduce the higher temperatures. Testing was only conducted at a single standard error of the log-log regression and result in a smaller temperature, 20°C, as part of this study. Tsai et al. (87) tested t-value when calculating the lower, one-sided prediction limit mixes at three temperatures, 10°C, 20°C, and 30°C, as part of (endurance limit). a reflective-cracking study. A total of six samples was tested Again considering a normal traffic stream, the PerRoad at each temperature, three each at two strain levels. The sam- perpetual design thickness was slightly less than that deter- ples were compacted to 6% air voids (target). Five binders mined using the 1993 AASHTO Pavement Design Guide, and were tested: AR-4000, type G asphalt rubber (RAC-G), and approximately the same as that required to satisfy the fatigue three modified binders termed MB4, MB15, and MAC16. requirements of a 20- or 40-year MEPDG design (not consid- The endurance limit was predicted from published data ering the endurance limit). However, the predicted conditions included in Tsai et al. (87) using the procedure described in of the pavement at the end of the design life are significantly Appendix A. The results are shown in Table 7.16. Varia- different. At 90% reliability, the MEPDG would predict over tions in the predicted endurance limit were observed both
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89 Table 7.16. Predicted endurance limit as a function of test temperature. Predicted Lower 95% Test Temperature, Binder Endurance Limit, Prediction Limit, °C ms ms 10.2 101 39 AR-4000 19.9 52 12 30.4 105 80 10.3 130 91 RAC-G 20.2 190 106 30.0 183 124 10.0 176 127 MAC15 20.0 255 178 30.3 461 100 9.9 377 284 MB4 20.5 394 230 29.8 555 247 10.1 348 261 MB15 20.0 215 135 30.4 171 82 with changes in test temperature and binder. Three of the Based on measured strains from the NCAT Test Track from binders generally followed the expected trend of increasing sections that have not experienced fatigue cracking, Willis (77), endurance limit with increasing test temperature. In two of proposed designing perpetual pavements based on a cumula- the cases, the predicted endurance followed the expected tive frequency distribution of allowable strains. A similar con- trend, while the 95% lower prediction limit was more vari- cept was initially proposed by Priest (76). The proposed upper able, due to variability in the beam fatigue test results. For bound for a cumulative frequency distribution of endurance the RAC-G, the 95% lower prediction limit showed a trend limit strain is shown in Table 7.17 for Sections N3 and N4 of of increasing endurance limit with increasing temperature. the NCAT Test Track. A cumulative frequency distribution The MB15 binder indicated decreasing endurance limit with defines the percentage of observed data below a given value. increasing temperature. The AR-4000 binder indicated its Based on Table 7.17, 50% of the in-service strain values should lowest endurance limit at 20°C. be less than 181 ms to prevent fatigue cracking. It should be Table 7.17. Cumulative distribution of strain criteria for long-life pavements (77). Percentile Upper Bound Fatigue Maximum Fatigue Ratio Limit 99 394 2.83 95 346 2.45 90 310 2.18 85 282 1.98 80 263 1.85 75 247 1.74 70 232 1.63 65 218 1.53 60 205 1.44 55 193 1.35 50 181 1.27 45 168 40 155 35 143 30 132 25 122 20 112 15 101 10 90 5 72 1 49
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90 noted that the mean annual air temperature measured during the 55th percentile. It should be reiterated that these distri- the 2003 NCAT Test Track cycle was 65.5°F, which corre- butions are based on two sections for which bottom-up fatigue sponds to a pavement temperature (based on Equation 44) of cracking has not been observed after the application of approx- 73.8°F. This is greater than the 20°C (68.8°F) test temperature imately 20 million ESALs. It is possible that fatigue cracking used for the beam fatigue tests. Hence, the fact that the 50% could occur with additional loading. strain values are greater than the endurance limits measured The last concept that needs to be considered in long-life for this study is not unexpected. Table 7.17 also presents strain pavement design is how different rates of loading may be ratios, which are ratios of the upper bound for the allowable accommodated. In addition to designing against damage strain at a given percentile to the measured endurance limits from expected axle loads, frequency of application needs to (151 ms and 146 ms for the PG 67-22 and PG 76-22 mixes, be considered. Low volume roads may, in some cases, experi- respectively) determined as part of this study. This offers an ence the same distribution of axle loads over time, but differ- opportunity to adjust the distribution based on measured ent frequencies of application. Infrequent load applications material properties. may offer more time for healing to occur and hence less accu- Both concepts were developed based on observations mulated damage. If load frequency is not considered, all per- that the cumulative frequency distribution of measured petual pavements designed for the same distribution of axles strains of sections that did and did not crack differed above on the same subgrade will have the same thickness.