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OCR for page 67
67 CHAPTER 4 REVIEW OF PERFORMANCE DATA FROM FIELD TEST SECTIONS AND FULL-SCALE ACCELERATED TESTING Currently, a large percentage of the HMA produced in the Unfortunately, traffic data were only available in DataPave United States is designed using the Superpave mix design 3.0 for five adjacent sections in one state. There is a weak system. The earliest Superpave projects were placed in 1992 relationship (R2 = 0.46) between uncompacted voids content (171). A number of experimental field sections have been built and rut depth divided by the square root of ESALs for the five and documented by agencies. In 1999, NCAT evaluated the sections. As shown in Figure 29, the relationship from the early performance of 44 Superpave sections placed between limited LTPP data does not match the relationship from the 1992 and 1998 (172). Some of these sections were revisited NCAT National Rutting Study (10). in 2001. Unfortunately, the consensus and source aggregate properties are not documented in these reports. There are a number of accelerated loading facilities in the 4.2 MNROAD United States. However, aggregate properties have not been experimental factors in the majority of testing completed MnRoad's mainline test road contains 16 HMA sections. to date. One exception is the Indiana DOT/Purdue Univer- Fourteen of these sections, constructed in 1992 and 1993, use sity APT Facility in West Lafayette, Indiana. Numerous the same three stockpiles: a coarse crushed gravel, fine gravel, research results from this facility were discussed previously and crushed granite (174). All fourteen original mainline HMA (23, 61, 6467). In addition, there are three test tracks that sections were constructed using the same gradation. The have been active since the completion of the Superpave mix MnRoad mainline experimental variables were design com- design system: MnRoad, WesTrack, and the NCAT Test paction effort, binder grade, and thickness. Two additional Track. Superpave sections were constructed in 1997, one coarse and one through the restricted zone. Originally, 11 HMA sections were constructed on the low-volume test road using the same 4.1 LTPP aggregate stockpiles and gradations as the mainline. Since aggregate type and gradation were not experimental factors There are 773 SPS 1, 5, and 9 HMA test sections (there is in the MnRoad experiment, it is not possible to analyze the often more than one test section per site) listed in LTPP relationships between aggregate properties and performance. DataPave 3.0 (173). A limited number of the SPS 1 and 5 sec- tions may have been designed using the Superpave method. The majority of the SPS 9 sections were designed using the 4.3 WESTRACK Superpave method. Unfortunately, the only Superpave con- sensus aggregate property stored in DataPave is uncompacted Aggregate type was not an experimental factor for the 26 voids in fine aggregate. A survey of DataPave 3.0 indicates original WesTrack sections; a crushed gravel was used for all there are records for 91 HMA layers (multiple layers per sec- of the sections (175). Two gradations were used, coarse- tion) representing a range of uncompacted void results from graded and fine-graded, both 19.0-mm NMAS. The fines con- 37.6% to 47.9%. Some sand equivalent test results are stored tent of the fine-graded mix was also varied. Due to premature in DataPave 3.0, but only for seal coat treatments. failure, a number of the original sections were replaced: eight A data extraction was performed to obtain uncompacted sections with a crushed andesite aggregate matching the orig- void content, rut depth, date of construction, date of last rut inal coarse gradation and two sections with two slightly dif- depth measurement, and annual ESALs. In total, uncom- ferent dense-graded gradations, one with the original crushed pacted voids contents were available for 55 surface mixes. gravel and one with the crushed andesite. However, design Rut-depth measurements were available for all of these sec- compaction effort (Hveem instead of gyratory) and binder tions. There was no trend between total rut depth and uncom- grade were also modified for the dense-graded sections. One pacted voids content. This was expected because of the range comparison that can be made is the effect of fractured face in age of the pavement sections and varying levels of traffic. count between the two coarse aggregate sources used in the

OCR for page 67
68 0.0007 NCAT Rutting Study 0.0006 y=-0.0000791x + 0.0036647 R2 = 0.67 Rut Depth, in/Sqrt. ESALs 0.0005 0.0004 0.0003 LTPP Data 0.0002 y = -8E-06x + 0.0004 R2 = 0.4569 0.0001 0 35 37 39 41 43 45 47 Uncompacted Voids, % Figure 29. Uncompacted voids versus rut depth normalized by traffic (10). coarse-graded sections. Hand et al. (176) plotted coarse aggre- They were placed in service during hot weather. Data from gate angularity (percent two crushed faces) versus rut depth the NCAT Test Track (177) show that rutting did not occur divided by the square root of ESALs along with data from the when the 7-day average air temperature dropped below 28C. NCAT National Rutting Study (17) as shown in Figure 30. The average age of the sections evaluated as part of the The results from the WesTrack "fine" and "fine plus" mixes National Rutting Study was 5.7 years (10). Therefore, on aver- plot close to the regression line. The error on the "coarse" age the sections in the National Rutting Study that were used mixes is higher, but still comparable with the other sections. to develop the relationship had been through five winters. It is Prior to the implementation of the Superpave method, the use unlikely that rutting would occur during these cooler periods, of such coarse mixes was uncommon. Therefore, it is unlikely and yet ESALs would continue to accumulate. Accelerated that similar mixes were included in the National Rutting loading was applied to the replacement sections during warm Study. The "replacement" sections appear to be outliers in the weather. Based on these considerations, the WesTrack results relationship. The replacement sections failed very quickly (at do not invalidate the results for coarse aggregate angularity approximately 500,000 ESALs) after being placed in service. from the National Rutting Study. 0.0012 Replacement Rut Depth/Sq. Root ESAL 0.001 Y = 0.001235 - 0.0000098(X) Coarse 0.0008 R2 = 0.42 0.0006 Fine Plus 0.0004 0.0002 Fine 0 50 55 60 65 70 75 80 85 90 95 100 Coarse Aggregate Crushed Faces (%) Figure 30. National Rutting Study and WesTrack coarse aggregate angularity relationship to performance (176). = National Rutting Study. = WesTrack.