Emad Kassem

Measurements of Moisture Suction and Diffusion Coefficient in Hot-Mix Asphalt and Their Relationships to Moisture Damage

A procedure for measuring the total suction in hot-mix asphalt (HMA) mixes was developed in this study. Suction measurements were related to physical and chemical properties of HMA that influence moisture damage. This procedure was achieved in two phases. In the first phase, the total suction was measured in HMA specimens with different types of aggregates and with different air void distributions and aggregate gradations. The results showed that the drying test using a 60° C temperature-controlled room was the most appropriate setup for measuring the total suction in HMA with thermocouple psychrometers. The characteristics of suction-moisture content curves were found to be related to the air void distribution in HMA and to the resistance of the mixtures to moisture damage. In the second phase, total suction was measured in sand asphalt specimens. These specimens had different combinations of aggregates and binders that exhibited different field resistances to moisture damage. The suction measurements we...

Measurements of the moisture diffusion coefficient of asphalt mixtures and its relationship to mixture composition

The presence of moisture in asphalt pavements detrimentally affects the bond between the aggregate and binder and the bond within the binder. The loss of these bonds leads to the deterioration of asphalt pavements. In regions with low rainfall, moisture diffusion is an important source of moisture transport in asphalt mixtures. The diffusion coefficient is a necessary input for models simulating moisture transport and, consequently, moisture damage in asphalt pavements. In this study, an experimental protocol was developed for determining the moisture diffusion coefficient of asphalt mixtures. The experimental set-up relies on measuring suction over time in test specimens using thermocouple psychrometers. The proposed protocol successfully determined a wide range of diffusion coefficients (i.e. from 5.67 × 10− 5 to 2.92 × 10− 6 cm2/s) of different asphalt mixtures. X-ray computed tomography was used to characterise the air void structure of test specimens. The results revealed good correlation between the diffusion coefficient value and the per cent of air voids and the average radius of air voids in the test specimens. The diffusion coefficient values determined in this study can be used as an input in models for determining the rate of moisture diffusion in asphalt mixtures with different per cents of air voids.

Influence of Air Voids on Mechanical Properties of Asphalt Mixtures

ABSTRACT Air voids play an important role in influencing the performance of asphalt mixtures. This study utilized several mechanical tests to characterize the influence of air void distributions on mechanical properties and response of asphalt mixtures. The overlay tester, Hamburg Wheel-Tracking Test (HWTT), and a repeated tensile test were used to characterize the resistance to fatigue cracking, permanent deformation, and moisture damage, respectively. The overlay test results showed that specimens with more uniform air void distribution had less variability in terms of resistance to fatigue cracking compared with specimens with less uniform air void distribution. Consequently, recommendations were made to improve air void distribution and reduce the test variability. The HWTT results were found to be insensitive to the air void distribution. The repeated tensile test was used to measure material properties that were required inputs to a fracture-based model for the analysis of moisture damage. The repeated tensile test and model results showed that the detrimental effect of moisture was the highest for specimens with the highest percent of air voids. In addition to the characterization of mechanical properties, this study evaluated the effect of temperature distribution in Superpave Gyratory Compacted (SGC) specimens on air void distribution. It was possible to make some minor changes to the compaction procedure in order to improve the uniformity of temperature distribution and, consequently, enhance the uniformity of air void distribution.

Viscoelastic Poisson's Ratio of Asphalt Mixtures

AbstractThe Poisson’s ratio (PR) of asphalt mixtures is a fundamental material property that is an important input parameter to viscoelastic pavement models. It is typically assumed that PR of asphalt is time independent or, in cases where the time dependency of viscoelastic PR is considered, calculated as the negative ratio of the time-dependent transverse and axial strains measured in a uniaxial creep test. This paper presents analytical and experimental results demonstrating the errors involved in these assumptions. The first part of this study derives an expression for viscoelastic PR. The second part of this study reports the results of various experimental tests to measure axial and transverse strains of asphalt concrete specimens under different loading conditions. The viscoelastic PR increased with time in the tensile relaxation test and ramped tensile test, whereas it slightly increased at the beginning of the compressive relaxation test and was subsequently virtually constant. The error introduc...

Characterization of Asphalt Pavement Materials in the State of Qatar

ABSTRACT The state of Qatar is experiencing tremendous economic growth, and traffic loading has increased rapidly as a consequence of the need to increase road transport efficiency. This study was initiated to evaluate the properties of asphalt pavement materials and make recommendations for materials and test methods that can be used to build long-lasting roads that are necessary to support economic growth. Many tests were conducted to evaluate the physical, chemical, and mechanical properties of asphalt pavement materials. Some of these results will be valuable as the state of Qatar moves toward mechanistic-empirical design. The imported aggregates referred to as gabbro were found to have better resistance to degradation under mechanical forces and better skid resistance in comparison to the local limestone aggregates. However, the surface energy results reveal that gabbro has less resistance to moisture damage than the limestone aggregate. The dynamic mechanical analysis of the mixtures demonstrates the benefits of using modified asphalt binders instead of the current unmodified binders used in the country. The results of the mechanical tests that were conducted on extracted field cores show that good resistance to permanent deformation can be expected under traffic; however, these mixtures could be prone to fatigue cracking, given the low asphalt binder used in these mixtures.

Comprehensive Evaluation of Compaction of Asphalt Pavements and a Practical Approach for Density Predictions

The compaction of asphalt mixtures has a significant influence on their performance. Insufficient compaction leads to several distresses, such as premature permanent deformation, excessive aging, and moisture damage, even if all desirable mixture design characteristics are met. Currently, no acceptable method relates asphalt pavement density to the compaction pattern. The compaction pattern required to achieve the desired density is established on the basis of experience and trial and error, a process that is time-consuming and expensive. This study involved comprehensive experimental evaluation of several test sections that were constructed with different asphalt mixtures by various compaction methods. Results of these experiments were used to determine the effects of compaction temperature, compaction method, mixture design, and base type on the compactability of asphalt mixtures. In addition, the paper presents a systematic method to determine the compaction pattern required to achieve the desired density of asphalt pavements. This method lends itself to integration in an automated system that can be used by roller operators to estimate density during construction.

Relationship between bond energy and total work of fracture for asphalt binder-aggregate systems

Surface free energy is a thermodynamic material property representing the work required to create new surfaces of unit area in a vacuum. Surface free energy has been used to quantify and screen both the cohesive bond energy of asphalt binders and the adhesive bond energy of asphalt binder–aggregate interfaces in wet and dry conditions. The bond energy is computed based on the surface free energies of the constituent materials. The total work of fracture is the cumulative effect of energies applied to the sample to create two new surfaces of unit area. These energies include the bond energy, calculated from surface free energy, dissipated plastic energy, and dissipated viscoelastic energy. This paper presents experimental results from a series of pull-off tests using asphalt binder-aggregate samples that demonstrate the relationship between bond energy and total work of fracture. In order to fully explore this relationship, temperature, loading rate, specimen geometry, and moisture content were varied.

Quantification of Damage in the Dynamic Complex Modulus and Flow Number Tests Using X-Ray Computed Tomography

The dynamic modulus test is conducted based on the assumption that no damage is induced during the testing process and that measurements are conducted within the linear viscoelastic region. In addition, the level of damage that occurs in the flow number test has not been quantified. The objectives of this study are to quantify the levels of damage in the dynamic complex modulus and flow number tests by using X-ray computed tomography (CT) and to characterize the microstructural properties of asphalt mixtures under loading. Four Superpave mixtures, including one conventional hot-mix asphalt (HMA) and three warm-mix asphalt (WMA) mixes, were evaluated. Two WMA processes (i.e., water foaming and Rediset additive) were used in the preparation of the WMA mixes. In addition, reclaimed asphalt pavement (RAP) was used in the preparation of the mixes at a content ranging from 15–30%. Results of the experimental program indicated that the damage incurred in the dynamic modulus test is minimal and homogeneous, but the damage incurred in the flow number test is significant and heterogeneous. In addition, specimen preparation may significantly influence the three-dimensional air voids distribution in HMA.

Finite element modelling of field compaction of hot mix asphalt. Part II: Applications

A constitutive model is developed and implemented in the finite element system three-dimensional computer-aided pavement analysis for the simulation of hot mix asphalt field compaction. The details of this model are presented in a companion paper (Masad et al., Finite element modelling of field compaction of hot mix asphalt. Part I: Theory, International Journal of Pavement Engineering, Accepted, 2014). This model is based on nonlinear viscoelasticity theory and can accommodate large deformations that occur during the compaction process. The model was used to study the influence of frequency and amplitude of vibratory compaction rollers on the level of compaction. In addition, it was used to analyse the influence of various methods for compacting longitudinal joints on the percent air voids near these joints. The model was used to simulate the compaction of asphalt pavements with different structures and compacted using various equipment and patterns. The finite element results of the level of compaction and percent air voids were in reasonable agreement with the measurements. The model offers the opportunity to simulate and predict the compaction of asphalt mixtures under various rolling patterns and for different pavement structures.

Stochastic Identification of Linear-Viscoelastic Models of Aged and Unaged Asphalt Mixtures

The behavior of asphalt mixtures is typically described using linear viscoelastic models at low-strain applications. The time and temperature dependency of asphalt mixture properties is described by forming a master curve that includes three components: time-temperature shift factors, and storage and loss moduli (or compliances). Mathematical models are needed to describe the master curve, which are used in calculating asphalt pavement responses to load and also to compare the overall properties of various mixtures at a wide range of temperatures. This paper proposes a rigorous approach to mathematically describe the master curves using stochastic identification techniques. These techniques have the advantage over current deterministic methods in their ability to account for the uncertainty associated with the constructed models, which could be contributed to variation in the material properties of the asphalt mixture phases as well as their spatial distribution, measurement errors, modeling errors, and inadequate available information. Consequently, uncertainty can be accounted for in the analysis and design of asphalt pavements. The stochastic approach is used successfully in this paper to identify the linear viscoelastic master curve for asphalt mixtures that have been aged to different time durations.