Pouya Teymourpour

Effect of particle mobility on aggregate structure formation in asphalt mixtures

During compaction of asphalt mixtures, aggregate structure starts building up by proximity and direct contact of aggregates. In the previous studies, it has been shown that the aggregate structure directly affects the service performance. However, the mechanisms of the aggregate structure formation are not clearly understood. This study is focused on the mechanisms affecting aggregate mobility during compaction and the effect of material properties on the aggregate structure formation. At the initial stages of compaction, there is a relatively thick layer of mastic (i.e. mix of binder and filler) between aggregates, which allows for a shearing mobility in the mix, if the mastic viscosity is sufficiently low. However, as compaction proceeds, the mastic layer at proximity zone of aggregates becomes thinner due to high stress intensity and the higher viscosity of thin mastic film or the aggregates dry contact effect increases the shearing resistance against compaction (i.e. mix becomes locked). In this study, mixes are compacted at different temperatures using one base binder and three different modified binders. The quality of the aggregate structure and packing throughout the compaction is characterised using two-dimensional imaging of mixture sections and the total aggregate on aggregate proximity length is measured as an indication of the aggregate-packing level. It is shown that for mixtures to obtain the maximum packing, the compaction temperature should be picked based on mastic viscosity. The viscosity of mastic should be low enough for lubrication, but high enough to provide sufficient film thickness at proximity zones and prevent locking of mixture at the early stages of compaction.

Sensitivity of the Illinois Flexibility Index Test to Mixture Design Factors

The semicircular bend test was recently modified to develop the Illinois flexibility index test (I-FIT). The I-FIT test quantifies the cracking resistance of asphalt mixtures by using the flexibility index (FI), which includes the fracture energy and postpeak behavior of a mixture. This paper presents results from testing asphalt mixtures in Wisconsin. A statistical analysis approach was chosen for this study, in which the I-FIT procedure was used to differentiate between mixtures on the basis of changes in mixture composition and aging treatments. Mixtures included in this study varied in terms of the percentage of reclaimed asphalt pavement, design traffic levels, binder grades, modification levels, and aging conditions. In addition, a laboratory experiment to evaluate the effects of variability in mixture production, including asphalt content and filler content, on the I-FIT testing results was performed to provide a basis for evaluating the tolerance limits of the current production and for setting reasonable specification criteria. The statistical analysis indicated that the FI could discriminate between types of asphalt mixtures and aging conditions. The FI parameter was found to be a better parameter for capturing some of the critical changes in mixture variables and aging than the fracture energy and other parameters from the I-FIT procedure. It was also clear that the FI parameter was sensitive to the variation in production binder and filler contents within the tolerances of the current construction specifications of the Wisconsin Department of Transportation.

Impacts of lubricating oils on rheology and chemical compatibility of asphalt binders

Modifiers have been widely used to improve the engineering value of asphalt binders and the performance characteristics of asphalt mixtures. In recent years, a wide range of oil-based modifications have been introduced to improve asphalt binder performance, with the enhancement of the low-temperature characteristics as the specific goal. Using oil modification was found to improve the Superpave low-temperature performance grade (PG), but at the expense of the high-temperature PG grade. This paper studies different combinations of lubricating oils and also hybrid systems of polymers and lubricating oils in order to improve characteristics of seven different types of neat binder and also satisfy the needs for covering the required PGs. The laboratory performance of binders was evaluated in three main categories based on the temperature range of interest (i.e. high, intermediate and low service temperatures) using Superpave testing, damage characterising tests and chemical compatibility using gel permeation chromatography. Oil modification showed to be significantly effective in improving the rheological properties of modified binders across all performance temperature ranges specifically when they are applied in hybrid systems with polymers.

Effects of Binder Modification on Aggregate Structure and Thermovolumetric Properties of Asphalt Mixtures

Modifiers have been widely used to improve the engineering value of asphalt binders and the performance characteristics of asphalt mixtures. Recent studies have confirmed that binder modification can influence mixture performance by two mechanisms: improving the mechanical properties of the continuous phase (i.e., binder or the asphalt–mineral filler mastic) of the mixture and changing the initial or inherent aggregate structure of the mixture (i.e., aggregates’ packing). A study identifies the relative importance of these two mechanisms on the low-temperature response of asphalt mixtures by using both experimental testing and finite element model analysis. This study uses three modifications: elastomeric, plastomeric, and a combination of elastomeric and plastomeric (i.e., hybrid). The low-temperature mechanical characterization of asphalt binders and mastics are measured with the bending beam rheometer and the glass transition temperature test. Mixture performance at low temperatures is evaluated with the asphalt thermal cracking analyzer. Internal structure of the aggregates is quantified by two-dimensional imaging analysis by using the recently developed Image Processing and Analysis System. Results indicate a significant relation between indexes describing the internal structure of the aggregates and the laboratory performance indicators at low temperatures. Experimental and finite element model simulation results confirm that modification enhances mixture responses by improving the initial internal structure of the aggregates, thus allowing a more favorable distribution of thermal strains within the binder. These results suggest new opportunities to optimize mixture resistance to cracking through binder formulation.

Use of the Hamburg Wheel-Tracking Test to Characterize Asphalt Mixtures in Cool Weather Regions

The Hamburg wheel-tracking test (HWTT) has shown promise to predict permanent deformation resistance and moisture damage potential of asphalt mixtures. Several state agencies have implemented the test as a mixture evaluation and design tool. One aspect of the test that remains a topic of research is the testing temperature. Many studies and specifications use 50°C for all testing, but some use a test temperature that depends on the base asphalt used in the mixture. Concern exists about the use of 50°C as the sole test temperature in cooler weather regions, such as Wisconsin, because the asphalts used in such regions tend to be relatively soft (high temperature grades of PG 58 and below). This paper presents findings in support of an effort to apply the HWTT to mixtures in cold climates with the use of three test temperatures and several mixture design variables. The paper presents the effects of the mixture design traffic level, the PG of the binder, and the binder modification level on the deformation resistance, creep slope, stripping slope, and stripping inflection point (SIP). The HWTT was found to be sensitive to the factors evaluated in this study. On the basis of statistical analysis of the test data, logical trends were observed. The testing temperature was found to affect not only the response variables but also the level of significance of controlled factors. The effectiveness of the SIP to characterize the moisture sensitivity of mixtures requires more research to validate the effect of moisture damage on HWTT results.

Development of Failure Master Curve for Asphalt Mastics Characterization

Low temperature performance grading currently relies solely on Bending Beam Rheometer (BBR) for determining low temperature creep stiffness (S) and rate of modulus relaxation (m-value) at 60 s, both determined at low stress-strain levels, in the pre-failure zones. This aspect raises questions with regard to applicability of properties derived from the linear viscoelastic range for prediction of asphalt binder thermal cracking behavior. Furthermore, many researchers have reported a discrepancy between field cracking severity and predictions based on asphalt binder properties since the asphalt binder-aggregate interaction is non-existent in asphalt binder testing. Therefore evaluation of asphalt mastics properties which could save a considerable amount of time and equipment in comparison to mixture testing should be prioritized. These challenges indicate that considering fracture properties of asphalt mastics could be a better approach for prediction of thermal cracking in asphalt pavements. It is believed that development of failure master curves for the damage characterization of asphalt mastics at different temperatures and loading rates would be beneficial for better characterization of resistance to thermal cracking. Therefore, this study presents framework and preliminary results on the development of such asphalt mastic failure master curves using the new BBR-SENB test for damage resistance characterization. The complexity of the visco-elastic behavior of asphalt mastics in terms of time and temperature dependency is also recognized by the sensitivity of the failure properties to changes in loading time and temperature.

Use of plastomeric additives to improve mechanical performance of warm mix asphalt

A number of asphalt modifiers are known to improve the engineering characteristics of asphalt mixtures. However, asphalt mixtures produced with Polymer Modified Bitumen (PMB) are sometimes difficult to handle, and in many cases need higher mixing and compaction temperatures whereas the asphalt industry strives to reduce these temperatures in order to decrease energy consumption and harmful emissions of typically used Hot Mix Asphalts (HMA). This paper investigates if plastomeric materials (or a combination of elastomeric and plastomeric materials) can lead to better mechanical properties while reducing the processing temperatures. It shows that some low molecular weight poly-olefins having low viscosities above their melting point can act as processing aids and change the initial/inherent aggregate structure of the mixture (i.e., aggregates’ packing) during compaction. This effect has been obtained with or without combination of elastomeric modifiers. The improved aggregate packing observed using image analysis tool (“iPas2”) has a significant impact on performance-related properties like rutting resistance and complex modulus. Moreover, when the oxidized poly-olefins are used in the PMB, the polymer modified mixture appeared to be less sensitive to moisture damage as measured in water boiling tests and in modified Lottman test (ITSR).