Hussain Bahia

Relating Adhesion and Cohesion of Asphalts to the Effect of Moisture on Laboratory Performance of Asphalt Mixtures

Antistripping additives and polymer modifications are two common modifiers used to improve the fundamental properties of asphalt binders as those properties relate to the performance of asphalt mixtures. Adhesion and cohesion are two important related properties of asphalt binders that can affect asphalt mixture performance before and after water conditioning. The purpose of this study was to quantify the effects of antistripping additives and polymers on the adhesion and cohesion of binders and to relate these effects to the performance of mixtures as measured in the laboratory before and after water conditioning. The performance tests of asphalt mixtures included indirect tensile strength, uniaxial compression permanent deformation, and Hamburg wheel tracking. Asphalt mixtures were produced with different modified binders and with two aggregate types. The binders were modified with antistripping additives and polymers and by chemical treatment and oxidization methods. Granite and limestone were selected...

Distribution of Strains Within Hot-Mix Asphalt Binders: Applying Imaging and Finite-Element Techniques

Because of several orders of magnitude difference between the stiffness of aggregate and binder and the randomness of the binder domain boundaries, the induced deformation under loading can result in a wide distribution of stresses and strains within each of the components. It is expected that although aggregates undergo small strains, most of the strain will accumulate within the binder. Although studies have covered the micromechanics of hot-mix asphalt (HMA), information about the actual typical distribution of asphalt binder domains in HMA and the resulting distribution of stresses and strains is scarce. In this study, advances in imaging techniques are applied to understand the distribution of binder and air voids in selected HMAs. The objective is to determine the strain distribution within the binder using digitized images analyzed with finite-element procedures. This approach captures the image of the specimen cross section and converts the image into finite-element mesh after image processing. The images are converted to finite-element mesh and the finite-element program ABAQUS provides numerical solutions to relate bulk stresses or strains applied to the asphalt mixture to stresses and strains within the binder domains. The results are presented including a summary of the distribution of directional binder film thickness and maximum strains in the mastic domain. Also included is a discussion of the effect of air voids and mineral fillers.

Modification and Validation of Linear Amplitude Sweep Test for Binder Fatigue Specification

Current asphalt binder specifications lack the ability to characterize asphalt binder damage resistance to fatigue loading. Multiple accelerated testing procedures that attempt to efficiently and accurately characterize the contribution of asphalt binders to mixture fatigue are under investigation. One of these tests, which has received significant acceptance by experts and has been submitted as a draft AASHTO standard, is the linear amplitude sweep (LAS) test. This procedure uses viscoelastic continuum damage mechanics to predict binder fatigue life as a function of strain in the pavement. The LAS test uses cyclic loading with systematically increasing load amplitudes to accelerate damage and provides sufficient data for analysis in less than 30 min. Although results of the current LAS testing protocol are promising, the time and the complex numerical procedures required for the analysis have raised concern. In addition, insufficient damage accumulation was observed when the strain amplitudes proposed in the LAS test were used for a set of polymer-modified binders. This paper presents simplifications of the current analysis procedures and evaluates the ability of extended strain levels to cause sufficient damage for better calculation of the binder fatigue law parameters. The effectiveness of the modified procedure was validated by comparison of the results with the fatigue performance recorded by the Long-Term Pavement Performance program with consideration of the pavement structure. The fair correlations showed the potential for effective use of the modified method for binder specifications.

Internal structure characterization of asphalt mixtures for rutting performance using imaging analysis

Characterization of the asphalt concrete microstructure using two-dimensional (2-D) imaging techniques is an economically efficient approach. However, the features that have been captured and quantified using 2-D imaging in most published research have been limited to simplistic analyses of aggregate structure. The present research focused on introducing a more elaborate method of characterization of internal structure, and proposing new indices to relate to and explain rutting resistance performance of asphalt mixtures. The aggregate internal structure provides the skeleton of the asphalt concrete, which plays an important role in rutting resistance. It is shown that this structure can be captured using a combination of image analysis indices developed in this research, namely: number of aggregate-on-aggregate contact points, contact length/area, and contact plane orientation. These parameters are defined for both the total aggregates and for the effective load bearing aggregate structure, referred to as the ‘skeleton’ in this study. Software developed in a previous study and significantly modified for this paper, is used to process digital images of a set of asphalt mixtures with different gradations, binder contents, types of modification, and compaction efforts. The results demonstrate a correlation between the internal structure indices and the mixture rutting performance. Additionally, the indices were successfully used to capture the effect of compaction effort, gradation quality, and binder modification on the mixture internal structure.

Measuring and Defining Fatigue Behavior of Asphalt Binders

Fatigue damage is a distress mechanism observed in asphalt, particularly at moderate to low temperatures. Preliminary studies have shown that unmodified asphalts are sensitive to fatigue and that the use of modifiers in asphalt binders can dramatically improve the binder’s response to fatigue. One of the major challenges encountered has been the lack of a definition of fatigue failure consistent with the actual performance of the material regardless of testing conditions. Superpave® asphalt binder specification has improved the evaluation of modified and neat asphalts, but the definition of failure for fatigue damage is still unclear. A selected set of unmodified and modified binders was chosen and tested under a range of loading modes, stress or strain amplitudes, temperatures, and frequencies. The fatigue data were analyzed using the dissipated energy ratio concept. Np, the number of cycles to crack propagation, was used as the fatigue criterion for the analysis. Using the initial dissipated energy per cycle (Wi) as the main independent variable for modeling fatigue of binders appears to be a promising technique to normalize some of the testing conditions. The parameter Np20, defined as the number of cycles at which the dissipated energy ratio shows 20% deviation from the no-damage ratio, appears to be a promising parameter to define failure. Using Np20 values, all modification methods used showed improvement in the fatigue behavior of unmodified asphalts. The level of improvement, however, was highly dependent on the modifier type and the testing conditions. Initial dissipated energy, testing frequency, and temperature were found to be important factors. If damage parameters are used in future specifications of binders, testing frequency and testing stress or strain levels should be carefully selected to represent pavement structural conditions and traffic speed.

Effect of Mineral Filler Characteristics on Asphalt Mastic and Mixture Rutting Potential

This study, part of the NCHRP 9–45 Project, analyzed the effect of mineral filler properties on asphalt mastic and the rutting potential of asphaltic mixture. The mineral filler properties were characterized by four tests: Rigden voids (RV), fineness modulus (FM), calcium oxide (CaO) content, and methylene blue value. The rheological properties of asphalt binder and mastic were characterized with the use of apparent viscosity and multiple stress creep recovery tests. Dynamic modulus and flow number tests were conducted to examine the asphaltic mixture rutting potential. The tested mixtures included several variables: four asphalt binder types, including virgin and polymer modified; two aggregate gradations; and a selected group of fillers. The study concluded that asphalt mastic performance was significantly affected by the fractional voids in the filler and possibly by the CaO content and FM. This effect, however, depended on binder type. On the one hand, the styrene–butadiene–styrene modified binder showed the strongest effect as a result of the mineral filler inclusion when tested as mastic. On the other hand, RV and CaO content showed relatively greater correlation with the mixture rutting potential, as compared with other filler properties. Addition of RV improved the prediction models for dynamic modulus and flow number. The effect of RV on the mixture rutting potential was more pronounced for the coarse mixture than for the fine mixture.

Measuring the Effect of Moisture on Asphalt–Aggregate Bond with the Bitumen Bond Strength Test

Understanding moisture damage mechanisms in asphalt pavements and evaluating the right combination of materials that are resistant to moisture damage are important. Moisture damage is the loss of strength or stiffness in asphalt mixtures caused by a combination of mechanical loading and moisture. Many test methods have been developed to evaluate loss of adhesion and cohesion in binders. However, a simple procedure to address moisture damage in the asphalt–aggregate interface is not available. The feasibility of the newly developed bitumen bond strength (BBS) test for moisture damage characterization was investigated. An experimental matrix that included various binders, modifications, and aggregates to account for the chemical and physical conditions in the aggregate–asphalt interface was completed. A statistical analysis was performed to verify reproducibility of the BBS test. The results indicated that the bond strength of asphalt–aggregate systems was highly dependent on modification and moisture exposure time. Polymers were found to improve the adhesion between asphalt and aggregate as well as the cohesion within the binder. Results from this study indicated that the BBS test was repeatable and reproducible. To further validate the effectiveness of the BBS test, a comparison of the BBS test results and the modified dynamic shear rheometer strain sweep test was conducted. The comparison showed that the BBS test could rank materials similarly to a more sophisticated and time-consuming test.

Aggregate structure characterisation of asphalt mixtures using two-dimensional image analysis

In current practice of mixture design, volumetric properties such as voids and binder content along with mechanical properties such as modulus or rutting resistance are used as the main quality indicators. Visualisation is an important tool that has not been widely used in asphalt mixtures. As part of the Reunion Internationale des Laboratoires et Experts des Materiaux activities, the aggregate structure has been identified as a possible important mixture characteristic in need of measuring and quantifying. This paper is a report on part of this effort. Software for processing and analysing two-dimensional images of asphalt concrete mixtures to provide information about the aggregate structure within a mix was developed. Images with accompanying volumetrics and gradation information can be processed with the software and a virtual sieve analysis of aggregates within the image is performed to verify a match with known measured gradations. Once images were successfully processed, analysis is performed to determine the number of contact points between aggregates as well as radial distribution and orientation of each aggregate. Segregation of aggregates within each specimen was also determined. Mixtures with a broad range of variables were compacted in the laboratory, using a number of compaction methods of various countries. In addition, mixtures with various nominal maximum aggregate sizes, aggregate type (limestone or gravel) and design ESALs (E-3 or E-10) were compacted in the US gyratory compactor, using two pressures (600 and 300 kPa) and two temperature levels (120°C and 60°C). Results indicate that the aggregate structure is affected by compaction methods and conditions although volumetrics are very similar. The results show that a fresh look at evaluating the aggregate structure within mixtures is required.

Advanced Characterization of Crumb Rubber-Modified Asphalts, Using Protocols Developed for Complex Binders

The use of crumb rubber modifiers (CRMs) to change asphalt binders was evaluated. Blending two base asphalt binders of performance grade 70-22 and 64-22 with three sizes of a CRM at two rubber contents resulted in varying grades of asphalt binders. In addition to these binders, two binders modified with reacted crumb rubber using a patented process were included in the study. Testing was conducted at different temperatures, testing frequencies, and strain conditions, using methods recently developed as part of the NCHRP 9-10 project (Superpave Protocols for Modified Asphalt Binders) for complex binders. The new testing includes the particulate additives test (PAT), which determines the volume of particulate material greater than 75 μm in asphalt, and the laboratory asphalt stability test (LAST), which measures the potential for separation and degradation of additives in asphalt. Strain sweeps, fatigue testing, and low-temperature failure and creep properties were also measured. The results indicate that the concentration and size of crumb rubber affect the viscosity of the resulting modified binders significantly and influence the volume of residue collected in PAT. It was observed that concentration and size of rubber are, however, not important compared with other factors in LAST and that keeping the binders agitated during storage can reduce separation significantly. On the basis of the rheological testing, it was found that both fatigue and strain dependencies are highly sensitive to asphalt binder type, rubber size, and rubber content. The failure properties of CRMs measured using the direct tension test indicate that failure stress and strain are reduced by increasing rubber size and concentration.

Adhesive and Cohesive Properties of Asphalt-Aggregate Systems Subjected to Moisture Damage

ABSTRACT The bond strength between asphalt and aggregate plays a fundamental role in evaluating the moisture sensitivity of HMA Mixtures. In this study the effect of water on adhesive and cohesive properties of asphalt-aggregate systems was investigated using a modified version of the PATTI. The device was used to measure the pull-off strength on different asphalt-aggregate combinations and to evaluate the influence of water immersion at two different temperatures. In particular, six asphalt binders were employed in combination with two aggregate types, having different asphalt affinity. The effect of the aggregate surface temperature during specimen preparation was also tested. In the first phase of the study the within-laboratory repeatability of the test procedure was investigated. The results showed the PATTI test is able to evaluate with good precision the pull-off strength and that its repeatability depends on the failure type (adhesive or cohesive). In the second phase of the study a full factorial experiment was employed to verify the reliability of the test for routine use in determining the adhesive and cohesive properties of asphalt-aggregate combinations and the effects of moisture damage. The results showed that, in the dry condition, the test was able to measure the internal cohesion of the asphalt binders. The results also showed the effects of water damage on the pull-off strength and the decisive role of asphalt-aggregate affinity was clearly highlighted. Using wet conditioning of the PATTI samples it was proven that water affects the adhesive bond between asphalt and aggregate much more than the asphalt cohesion. Moreover, the results indicate that aggregate temperature during sample preparation has only a limited effect on the adhesive strength.