Thomas Harman

Quantifying Laboratory Compaction Effects on the Internal Structure of Asphalt Concrete

The performance of asphalt concrete (AC) mixtures is influenced by its internal structure, which refers to the arrangement of aggregates and their associated air voids. Currently, most of the discussion on the effects of internal structure on AC performance is qualitative. This study proposes computer-automated image analysis procedures to quantify the internal structure of AC. Internal structure is quantified in terms of aggregate orientation, aggregate contacts, and air void distribution. The new procedures are useful tools to describe and compare AC materials produced by different compaction methods and mix designs. The new procedures are used to study the difference in internal structure of AC specimens compacted with the Superpave gyratory compactor (SGC) and the linear kneading compactor (LKC). Specimens compacted with the SGC were found to have aggregates with more preferred orientation and fewer contacts than specimens compacted with the LKC. In addition, SGC specimens were found to have more air voids at the top and bottom, whereas air voids in LKC specimens were found to increase from the top to the bottom.

X-RAY TOMOGRAPHY TO CHARACTERIZE AIR VOID DISTRIBUTION IN SUPERPAVE GYRATORY COMPACTED SPECIMENS

Air void distribution has considerable influence on the mechanical properties of asphalt mixtures. Several factors such as the compaction effort, method of compaction, aggregate gradation, and aggregate shape control the air void distribution. An X-ray computed tomography (CT) system along with image analysis techniques are used in this study for non-destructive characterization of air void distribution in gyratory specimens prepared using different gradations and compaction efforts. The air void distributions in gyratory specimens are quantified using parameters that describe the change in percent and volume of air voids along the horizontal and vertical directions. Air voids are shown to be more concentrated in the top and bottom regions that are in contact with the base plates, as well as in the outer region that is in contact with the mold. The non-uniformity of the distribution increases with an increase in compaction effort. The difference in aggregate gradations used in this study is shown to have just a slight influence on the air void distribution.

Accuracy of Current Complex Modulus Selection Procedure from Vehicular Load Pulse: NCHRP Project 1-37A Mechanistic-Empirical Pavement Design Guide

The Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the complex modulus to simulate the time and temperature dependency of hot-mix asphalt (HMA). To account for the time dependency of HMA, MEPDG recommends calculation of the frequency of the applied load as a function of the vehicle speed and the pavement structure. By this approach, the Odemark method of thickness equivalency is first used to transform the pavement structure into a single-layer system, and it is then assumed that the stress distribution occurs at a constant slope of 45° in the equivalent pavement structure. Concerns were raised that the current MEPDG methodology may be overestimating the frequency, which would result in underconservative distress predictions. Therefore, to evaluate the MEPDG methodology for calculation of the loading time, the results of the MEPDG procedure were compared with those of an advanced three-dimensional (3-D) finite element (FE) approach that simulates the approaching-leaving rolling wheel at a specific speed. The model developed accurately simulated actual tire rib sizes and the applicable contact pressure for each rib. In addition, laboratory-measured viscoelastic properties were incorporated into the FE model to describe the constitutive behavior of HMA. Comparison of these two methods shows that the frequencies calculated on the basis of the MEPDG procedure are greater than the ones determined by the 3-D FE method, which indicates that the loading time determined from MEPDG is not conservative. Ultimately, this would result in underestimation of the pavement response to a load and, therefore, greater errors in calibrations of the pavement response to field distress. Correction factors are thus presented to ensure the correctness of the loading time calculation in MEPDG. Adoption of the proposed factors within the MEPDG software does necessitate a recalibration of the performance models.

History and future challenges of gyratory compaction: 1939 to 2001

History is a dynamic tapestry of facts and perceptions, dates and personalities. This work attempts to capture the events and rationale of those who contributed significantly to the use of gyratory compaction in the design and field management of hot-mix asphalt and discusses the challenges ahead. Throughout the evolution of asphalt mix design several different types of laboratory compaction devices have been developed. In general, the goal of these laboratory devices is to fabricate a specimen for volumetric or physical characterization, or both. Depending on the system, specimens can be cylindrical, trapezoidal, or rectangular and compaction can be achieved through impact, kneading, or vibration. Gyratory compaction applies a kneading effort to fabricate cylindrical specimens. The evolution of gyratory compaction has resulted in several unique devices and a variety of methods.

Development of Temperature-Effect Model for Predicting Rutting of Asphalt Mixtures Using Georgia Loaded Wheel Tester

Use of the Georgia loaded wheel tester (LWT) to evaluate rutting susceptibility of asphalt mixtures has gained acceptance by the asphalt paving industry. The test is typically conducted at 40°C for 8,000 cycles and the rut-depth value measured at the end of the test is compared with a maximum criteria of 5.0 mm or 7.5 mm to assess rutting susceptibility of the mixture. A temperature effect model (TEM) was developed using the LWT test data from seven asphalt mixtures. The TEM developed can be used to predict the rut-depth values of an asphalt mixture at different temperatures and number of loading cycles from the LWT performed on the asphalt mixture at one testing condition. The predicted rut-depth values from the TEM compared very closely with the measured values. For the five dense-graded hot-mix asphalt mixtures (HMA), only 7 out of 170 predicted rut-depth values deviated from the measured values by more than 0.8 mm. For the two stone-matrix asphalt mixtures (SMA), only 2 out of 64 predicted values deviated from the measured values by more than 0.8 mm. The TEM can be used to determine the equivalent rut-depth acceptance values at a lower number of rut-testing cycles, and thus can shorten the testing time for performing the LWT rutting susceptibility acceptance test. This can be useful for the field quality control of HMA. Using this predictive model, the LWT rutdepth acceptance criteria can be developed for asphalt mixtures at the temperatures more closely related to the actual pavement temperatures in the field.

Noninvasive Measurement of Three-Dimensional Permanent Strains in Asphalt Concrete with X-Ray Tomography Imaging

Because of the stiffness differences between aggregates and mastics, permanent deformation of asphalt concrete is localized mainly in the soft mastics. Therefore, studies on the microresponse of the aggregates and mastics could provide a better understanding of the macrobehavior of asphalt concrete. A method is presented to measure the deformations in the mastics; this was correlated with the permanent deformation resulting from the asphalt pavement analyzer test. An automated procedure with tomography images to reconstruct three-dimensional particles was developed. The translations of the particles can be obtained from the coordinate differences of the mass centers before and after testing. The microstrains in the mastics and macrostrains in the mixture can be calculated on the basis of particle translations. The procedures described have significance for future study on how the aggregated shape, size, and arrangement affect the macroresponse of the mixture in three dimensions and how the relative stiffness of aggregates and mastics affects rutting potential.

Target and Tolerance Study for Angle of Gyration Used in Superpave Gyratory Compactor

Five companies offer eight models of the Superpave® gyratory compactor (SGC) in the United States. Each model uses a unique method of setting and inducing the specified angle of gyration. However, all angles are set externally relative to the mold and none of the manufacturer’s calibration systems can be universally applied to all models. The specified external angle of gyration (α) is 1.25° FHWA, in partnership with Test Quip, Inc., developed a dynamic angle validation kit that measures the dynamic internal angle (DIA) of gyration during loading. The DIA accounts for equipment compliance issues, such as bending of the platens during compaction, which is not apparent when the angle is measured externally. Differences in specimen density produced by different compactors have been attributed to differences in compliance. Thus, the SGC test method needs to be revised to obtain uniformity. However, it would be inappropriate to assign the external angle of 1.25° to the DIA because the internal angle is always lower than the external angle. Use of a 1.25° internal angle would result in an increase in the compactive effort applied by all SGCs, which would invalidate the Ndesign table. FHWA investigated the DIA by using the original two SGC models. These models were used to refine the Ndesign table that is in use today. The study found that the DIAs for these original SGCs were 1.176° and 1.140°, which resulted in a proposed DIA of 1.16° The proposed tolerance is ±0.03°.