John D'Angelo

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.

The Relationship of the MSCR Test to Rutting

ABSTRACT The Multi-stress Creep and Recovery (MSCR) Test is currently being considered as a replacement for the Superpave high temperature binder criteria G* sinδ. The MSCR test can distinguish between the rutting properties of both neat binders and polymer modified binders. The test is run on the same dynamic shear rheometer test equipment currently used for the current Superpave binder testing and is easy to run. The validation of the MSCR compliance value Jnr to rutting was done through extensive mix testing using laboratory rut testers, large Accelerated Loading Facilities and actual roadway sections. This paper covers the validation of the MSCR test binder compliance value Jnr to mixture rutting for modified and neat binders. Multiple binders and mix types are included in the validation and it is demonstrated that the MSCR test provides a much better correlation to mixture rutting than the existing Superpave binder criteria.

Field Evaluation of High Reclaimed Asphalt Pavement–Warm-Mix Asphalt Project in Florida: Case Study

In December 2007, a portion of State Route 11 in Deland, Florida, was milled and repaved with 45% reclaimed asphalt pavement (RAP). These high RAP mixes were produced at lower than normal hot-mix temperatures and with foamed warm-mix asphalt (WMA) technology. This project was the first large production in which the Florida Department of Transportation (DOT) allowed the use of high RAP in combination with WMA. FHWA, in cooperation with Florida DOT and the National Center for Asphalt Technology, was on site for production and placement of the high RAP-WMA. Plant-produced mix was collected by FHWA for performance testing evaluation. Two mixes were produced: a high RAP–hot-mix asphalt (HMA) control mix and a high RAP-WMA mix. Performance tests conducted by FHWA included performance grade (PG) determination of binders, dynamic modulus, and flow number. PG results of the binders indicate that the high RAP-WMA mix is softer than the high RAP-HMA control mix. This is further confirmed by flow number results, where the high RAP-WMA mix had a lower flow number than the high RAP-HMA control mix did. Dynamic modulus results indicate that the high RAP-WMA mix is slightly softer than the high RAP-HMA control mix, especially at intermediate temperatures. Comparison of measured dynamic modulus results with those predicted using the Hirsch and Witczak models confirm that complete blending occurred in the high RAP-HMA control mix. However, incomplete mixing of RAP and virgin binders may have occurred in the high RAP-WMA mix.

Proposed Refinement of Superpave High-Temperature Specification Parameter for Performance-Graded Binders

A semiempirical approach developed to predict the viscoelastic response of a binder in repeated creep recovery tests is described. This model provides an avenue to predict the rut resistance (R) as a function of loading (time and load) and temperature from data at a single frequency or frequency sweeps when needed. Thus, it can be used to develop a grading procedure for asphalt binders that not only accurately captures the delayed elasticity of modified binders but also accounts for the effect of traffic speed and traffic loading. The current Superpave binder specification attempts to capture the relative high-temperature performance (i.e., resistance to rut) of a binder via the inverse shear loss compliance, 1/J” or G*/sin δ, at 10 rad/s. This parameter represents an improvement over the absolute viscosity because it is measured at a defined rate of deformation and accounts to some degree for the viscoelasticity of the binder via the phase angle. The parameter would correctly predict the relative R for an ideally viscous (R ∞ η) material or an ideally elastic material (R = ∞). However, there is mounting evidence that at phase angles between 40° and 75°, the parameter may not fully capture the viscoelastic nature of many modified binders. Various authors have shown that the R of mixtures can be well described by models using data from dynamic creep experiments, in which the mix is subjected to a load followed by a relaxation period. More recently, Bahia proposed to capture their high-temperature performance by using a similar technique on neat binders.

Practical Use of Multiple Stress Creep and Recovery Test: Characterization of Styrene―Butadiene―Styrene Dispersion and Other Additives in Polymer-Modified Asphalt Binders

The rheological properties of asphalt binders modified by styrene–butadiene–styrene (SBS) depend on formulation variables. The most sensitive of them may be listed as polymer amount, cross-linking agent amount (percentage), and other additives such as polyphosphoric acid (PPA). The dispersion of SBS in an asphalt binder depends on the time and temperature of blending and the base asphalt binder compatibility. In this study an incompatible binder and a compatible base asphalt binder were selected and modified with various amounts of SBS. Elemental sulfur was used as a cross-linking agent in different proportions. Other additives, such as PPA at 0.5% concentration, were also used. High shear blends of SBS-modified asphalt binders were made in the laboratory by varying blending time until an optimum dispersion of polymer was obtained. The dispersion of the polymer was studied with a fluorescence microscope. A multiple stress creep and recovery (MSCR) test was used to study creep and recovery behavior of these modified binders. MSCR test results (Jnr and percentage recovery) were able to characterize the extent of dispersion of SBS in these polymer-modified asphalts (PMA). This implies that a fundamental test method is now available to discriminate between the dump-and-stir types of PMAs and those that have been optimally dispersed. This presentation discusses the effect of SBS dispersion and other additives on the MSCR test results.

Refinement of Flow Number as Determined by Asphalt Mixture Performance Tester: Use in Routine Quality Control–Quality Assurance Practice

Flow number determined with the repeated-load test is used as a criterion to characterize the rut resistance of hot-mix asphalt. The repeated-load test is performed with the asphalt mixture performance tester (AMPT). The algorithm currently used in the AMPT to determine flow number was found to be extremely sensitive to noise in the data and identifies erroneous flow number results, especially for modified mixes. A new algorithm that uses the Francken model to fit the flow number data is considered for implementation. The robustness of the Francken model is verified in this study by fitting flow number data obtained from field projects. The flow number test is time-consuming, especially for high-stiffness binders and, in many cases, can take as long as 6 h. In this study, other parameters are examined to determine whether the test can be terminated early without loss of information on rut resistance. Steady-state slope and slope at 2% strain are found to correlate well with flow number; the correlation indicates that they may be robust indicators of rut resistance. The flow number test can be terminated once steady-state slope or slope at 2% strain is reached; this substantially reduces test time. This paper discusses the findings from analysis of flow number data from the Francken model and the development of a refined, more expedient method for determining flow number.

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.

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.

Current Status of Superpave Binder Specification

Highway agencies use specifications to control the materials and construction practices used to build highways. In the development of material specifications highway agencies have been developing relationships between asphalt material properties and performance since they first started building asphalt pavements. The concept is if good performing materials are used in the construction then good performance should be achieved during the pavements life.

Refinement of Superpave High-Temperature Binder Specification Based on Pavement Performance in the Accelerated Loading Facility

Various parameters have been proposed to refine the current Superpave® high-temperature binder specification. One parameter relates the phase angle (δ) directly to accumulated strain from the creep–recovery test. Another variation suggests a formula to predict the accumulated strain using the current Superpave parameters G* and sin δ. Recently, NCHRP 9–10 project suggested a criterion based on a parameter derived from Burger’s viscoelastic model. In Europe, zero-shear viscosity is being proposed as a high-temperature specification parameter. These and other criteria proposed for high-temperature specification are evaluated by examining the correlation of each criterion to pavement performance. The experimental design consists of five asphalt binders used in the FHWA’s accelerated loading facility. Additionally, two binders used by the Nevada Department of Transportation in construction of 1–80 test sections were tested. Results show that the zero–shear viscosity correlates reasonably well with performance and is the best parameter among those evaluated. However, accurate determination of the zero-shear viscosity is time-consuming and requires expensive instrumentation, which makes it unsuitable as a specification parameter. Therefore, an easy method of obtaining zero-shear viscosity was evaluated and found to correlate to performance equally well. In this method, data from a single frequency sweep at the specification temperature are used to estimate the zero-shear viscosity. This method can be implemented immediately with little or no training or need for new instrumentation.