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Six New NCAT Reports
These reports are available to download off the internet at http://www.eng.auburn.edu/center/ncat under "NCAT Publications" The files are in PDF format and can be viewed and printed using Adobe Acrobat.
Six reports have been published so far in 2002. Titles and abstracts for these reports are provided below. Several more are currently under review and should be available soon. Paper copies of the reports can be obtained for a small fee from Vinnie Hester. If you have problems downloading the PDF files, please contact Linda Kerr
Case Studies of the Tender Zone in Coarse-Graded Superpave Mixtures - NCAT 2002-01 - Buchanan & Cooley Jr.
Tender hot mix asphalt (HMA) mixes have been observed and experienced by paving contractors for many years. However, during the field compaction of coarse-graded Superpave mixes, a “tender zone”, not a true tender mix, is sometimes experienced. The tender zone is range of mix temperatures during which the mix exhibits instability during roller action. There have been many possible causes of the tender zone presented including differences in lab and production aging, mix moisture, low dust to asphalt ratio, increased asphalt binder film thickness, and a temperature differential with the lift.
A study was conducted to document and evaluate field mixes exhibiting the tender zone to determine the possible cause(s) for its occurrence. Documentation included mix, production, and construction related items. Laboratory evaluation consisted of mix gradation and volumetric testing along with Superpave asphalt binder testing on the project asphalt binder before and after steam distillation. Project results failed to clearly identify one particular reason for the tender zone occurrence. However, it is felt that the tender zone was a result of field short-term aging being less than design and increased asphalt binder film thickness acting in conjunction with an inherent temperature differential within the lift.
For more information, please contact Allen Cooley
Coarse Versus Fine-graded Superpave Mixtures: Comparative Evaluation of Resistance to Rutting - NCAT 2002-02 - Kandhal & Cooley Jr.
Both coarse and fine-graded hot mix asphalt mixtures can be designed within the gradation control points recommended within the Superpave mix design system. However, some states have begun to specify only coarse-graded mixtures (below the restricted zone) and other states are specifying only fine-graded mixtures (above the restricted zone). This study was conducted to compare coarse-graded Superpave mixtures with fine-graded Superpave mixtures in terms of resistance to rutting so as to determine whether restrictions on gradations (either coarse- or fine-graded mixtures) are justified.
Fourteen mixtures comprising two nominal maximum aggregate sizes: 9.5 and 19.0 mm; two coarse aggregates: granite and crushed gravel; and four fine aggregates: sandstone, limestone, granite, and diabase, were tested. Resistance to rutting of both coarse- and fine-graded mixtures was evaluated using three test methods: Asphalt Pavement Analyzer, Superpave shear tester, and repeated load confined creep test.
Statistical analyses of the test data obtained by the three performance tests indicate no significant difference between the rutting resistance of coarse- and fine-graded Superpave mixtures. It has been recommended that mix designs should not be limited to designing mixes on the coarse or fine side of the restricted zone.
For more information, please contact Ken Kandhal
Evaluation of Eight Longitudinal Joint Construction Techniques for Asphalt Pavements in Pennsylvania - NCAT 2002-03 - Kandhal, Ramirez, & Ingram
Premature deterioration of multilane hot mix asphalt (HMA) pavements can occur at the longitudinal joints in the form of cracking and raveling. The National Center for Asphalt Technology (NCAT) initiated a national study of evaluating various longitudinal joint construction techniques in 1992 in an effort to select technique(s) which improve the performance of longitudinal joints. Test sections were constructed in Michigan, Wisconsin, Colorado, Pennsylvania, and New Jersey. This paper gives the 6-year performance evaluation of eight different techniques utilized on a paving project in Pennsylvania in 1995.
In Pennsylvania, longitudinal joint constructed using rubberized joint material gave the best performance closely followed by the joint made with cutting wheel. Test sections using rolling from hot side 152 mm away from the joint and the New Jersey wedge joint also performed reasonably well with no significant cracking. The remaining four test sections using edge restraining device, joint maker, rolling from hot side, and rolling from cold side developed cracking at the longitudinal joint to different extents.
It has been recommended to specify minimum compaction level at the longitudinal joint to ensure its improved performance.
For more information, please contact Ken Kandhal
Development of Mix Design Criteria for 4.75 mm Mixes - NCAT 2002-04 - Cooley Jr., James, & Buchanan
The Superpave mix design system includes design criteria for a range of mixes with nominal maximum aggregate sizes (NMAS) between 9.5mm and 37.5mm. Many agencies in the United States have expressed an interest in using a 4.75mm NMAS mix. The benefits of such a mix could include a smoother riding surface, the ability to correct surface irregularities, decreased construction time, decreased permeability, provide a use for manufactured screening stockpiles and provide an economical surface mix for low volume traffic facilities.
This study was done to establish the needed standard mix design criteria for 4.75mm mixes. Based upon the findings of this study, the recommended Superpave mix design criteria includes specified gradation control of 30 to 54 percent passing on the 1.18 mm (No. 16) sieve and 6 to 12 percent passing the 0.075 mm (No. 200) sieve. During design, the following were recommended: a design air void content of 4 percent, minimum voids in mineral aggregate (VMA) of 16 percent be utilized at all traffic levels, maximum VMA of 18 percent for mixes design above 75 gyrations, voids filled with asphalt (VFA) of 75 to 78 percent for mixes designed at 75 gyrations and above, and VFA of 75 to 80 percent for mixes designed at 50 gyrations. Results of this study showed that 4.75 mm NMAS mixes can be successfully designed. These types of mixes should provide economical mixes for low volume roadways.
For more information, please contact Allen Cooley
Evaluation of Infrared Ignition Furnace for Determination of Asphalt Content - NCAT 2002-05 - Prowell
This study evaluated the Troxler Model 4730 infrared ignition furnace as compared to a standard Thermolyne ignition furnace. Comparisons were based on correction factor for aggregate loss during ignition, accuracy and variability of the measured asphalt content and aggregate degradation during ignition. Forty-eight samples, representing two nominal maximum aggregate sizes (9.5 and 19.0 mm), four aggregate types (granite, crushed gravel, limestone and dolomite) and two asphalt contents (optimum and optimum plus 0.5 percent asphalt content) were tested in each furnace.
The results indicated that the correction factors for aggregate loss during ignition were significantly different for each type of furnace, thus requiring a separate calibration for each type of furnace. Practically, the differences for all but the 9.5 mm NMAS limestone and both dolomite mixtures are less than 0.1 percent. The samples at optimum plus 0.5 percent asphalt content were tested using the calibration factors developed for a particular mix/furnace combination. The results were analyzed in terms of accuracy (bias) and variability (standard deviation). Neither the measured bias’ nor standard deviations for the two types of furnaces were significantly different. Results from four sieve sizes (NMAS, 4.75, 2.36 and 0.075 mm) were evaluated for aggregate breakdown. A comparison of the recovered aggregate gradations from both furnaces indicated no significant difference in the degree of aggregate degradation during the ignition test. Therefore, based on the comparison of a single unit each of the standard and infrared furnaces, properly calibrated, they both should produce statistically similar asphalt contents and recovered aggregate gradations.
For more information, please contact Brian Prowell
Issues Pertaining to the Permeability Characteristics of Coarse-Graded Superpave Mixes - NCAT 2002-06 - Cooley Jr., Prowell, & Brown
In order to evaluate the relationships between in-place air voids, lift thickness, and permeability, 23 on-going HMA construction projects were visited and field permeability tests conducted. Field permeability tests were conducted at 15 randomly determined locations for each project. Cores were taken at each of the 15 locations to determine pavement density using AASHTO T166. In addition, for some of the projects, cores taken from roadway were tested with the Corelok device and a laboratory permeameter.
As agencies begin to include permeability specifications, mix designers need tools they can use during the mix design process to evaluate the permeability characteristics of a given aggregate structure. Two techniques were evaluated: laboratory permeability measurements on samples compacted using the Superpave gyratory compactor and water absorption determined with AASHTO T 166 or the Corelok device.
Results of testing within this study indicated a good relationship between permeability (measured in the field and lab) and pavement density. Both the gradation’s nominal maximum aggregate size (NMAS) and the lift thickness placed in the field were shown to affect the permeability-density relationship. Increasing the NMAS requires higher densities to ensure an impermeable pavement. Also, as the lift thickness of a given pavement (and mixture) increases, permeability decreases at a given density level.
Some reasonable relationships were found between the permeability of samples compacted using the gyratory compactor and field samples. Reasonable relationships were also found between permeability and water absorption regardless of nominal maximum aggregate size.
For more information, please contact Allen Coole
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