Ghassan R. Chehab

SPECIMEN GEOMETRY STUDY FOR DIRECT TENSION TEST BASED ON MECHANICAL TESTS AND AIR VOID VARIATION IN ASPHALT CONCRETE SPECIMENS COMPACTED BY SUPERPAVE GYRATORY COMPACTOR

Reliable materials characterization and performance prediction testing of asphalt concrete requires specimens that can be treated as statistically homogeneous and representative of the material being tested. The objective of this study was to select a proper specimen geometry that could be used for uniaxial tensile testing. Selection was based on the variation of air void content along the height of specimens cut and cored from specimens compacted by the Superpave gyratory compactor (SGC) and on the representative behavior under mechanical testing. From measurement and comparison of air void contents in cut and cored specimens, it was observed for several geometries that sections at the top and bottom and those adjacent to the mold walls have a higher air void content than do those in the middle. It is thus imperative that test specimens be cut and cored from larger-size SGC specimens. Complex modulus and constant crosshead-rate monotonic tests were conducted for four geometries—75 × 115, 75 × 150, 100 × 150, and 100 × 200 mm—to study the effect of geometry boundary conditions on responses. On the basis of graphical and statistical analysis, it was determined that there was an effect on the dynamic modulus at certain frequencies but no effect on the phase angle. Except for 75 × 115 mm, all geometries behaved similarly under the monotonic test. From these findings and other considerations, it is recommended that the 75- × 150-mm geometry, which is more conservative, and the 100- × 150-mm geometry be used for tensile testing.

Determination of Time-Domain Viscoelastic Functions Using Optimized Interconversion Techniques

ABSTRACT Several viscoelastic response functions are available to characterize the LVE behavior of asphalt concrete, some in time domain such as relaxation modulus E(t) and creep compliance D(t) and other such as complex modulus E* in frequency domain. The use of the complex modulus test has risen sharply after it has been incorporated in the M-E Pavement Design Guide and in the Superpave Simple Performance Test. With the availability of E* data it becomes advantageous to use mathematical interconversion techniques to obtain time-domain functions E(t) and D(t) which are typically used for constitutive modeling and other applications. This paper addresses the steps involved in conducting the interconversion between frequency-domain and time-domain functions. Issues considered include: a) presmoothing of raw data, b) refinement of phase angle data, c) Prony series representation of the fitted data including determination and sign-control of the Prony series coefficients, and d) interconversion techniques: approximate vs. exact. Finally, interconversion methods are evaluated by comparing D(t) data converted from E* to that measured in the lab.

Evaluation of geogrids for stabilising weak pavement subgrade

This study attempts to identify mechanical and physical properties of geogrids that are critical to their effectiveness in the stabilisation of pavement subgrade. Geogrid properties, including aperture size, wide-width tensile strength and junction strength, for four geogrid products are correlated with bench-scale interface test results, including direct shear and pull-out, and accelerated pavement testing (APT) results. APT is conducted through the use of a one-third scale APT device, the model mobile load simulator (MMLS3) on geogrid-reinforced pavement sections. The pavement sections are constructed on a subgrade soil with a low California bearing ratio. The performance of each pavement section is evaluated by measuring surface rutting at various trafficking stages. The analysis reveals a strong relationship between performance and junction and tensile strength of geogrids at small strains, whereas aperture size has a positive correlation with pull-out test results.

Implementation Initiatives of the Mechanistic-Empirical Pavement Design Guides in Indiana

The release of the Mechanistic-Empirical Design Guide for New and Rehabilitated Pavement Structures (M-E design guide) generated a new paradigm for designing and analyzing pavement structures. It is expected to replace the commonly used empirical design methodologies. The M-E design guide uses a comprehensive suite of input parameters deemed necessary to design pavements with high reliability and to predict pavement performance and distresses realistically. However, the considerable amount of input needed and the selection of the corresponding reliability level for each might present state highway agencies with complexities and challenges in its implementation. An overview is presented of ongoing investigative studies, sensitivity analyses, and preimplementation initiatives conducted by the Indiana Department of Transportation (INDOT) in an effort to accelerate the adoption of the new pavement design guide by efficiently using existing design parameters and determining those parameters that influence the ...

Implementing the mechanistic-empirical design guide procedure for a hot-mix asphalt-rehabilitated pavement in Indiana

One of the Indiana Department of Transportations (INDOTs) strategic goals is to improve its pavement design procedures. This goal can be accomplished by fully implementing the 2002 mechanistic-empirical (M-E) pavement design guide (M-E PDG) once it is approved by AASHTO. The release of the M-E PDG software has provided a unique opportunity for INDOT engineers to evaluate, calibrate, and validate the new M-E design process. A continuously reinforced concrete pavement on I-65 was rubblized and overlaid with a 13-in.-thick hot-mix asphalt overlay in 1994. The availability of the structural design, material properties, and climatic and traffic conditions, in addition to the availability of performance data, provided a unique opportunity for comparing the predicted performance of this section using the M-E procedure with the in situ performance; calibration efforts were conducted subsequently. The 1993 design of this pavement section was compared with the 2002 M-E design, and performance was predicted with t...

Flexural Behavior of Concrete Beams Reinforced with Different Types of Geogrids

This paper describes the behavior of geogrid-reinforced concrete members. The application of geogrids in concrete constitutes a new dimension for using geosynthetics in infrastructure. In pavement applications, geogrids have been used to provide confinement, stabilization, and reinforcement of unbound and asphalt concrete layers, as well as interlayers to mitigate reflective cracking. The ultimate objective of studying the behavior of geogrids in concrete members is to assess the feasibility and benefit of using geogrids in thin concrete overlays. The experimental program consisted of testing 21 simply supported plain and geogrid-reinforced beam specimens under four-point bending. The test parameters included three types of geogrids with different aperture shapes, physical and mechanical properties, and material composition. Additionally, two Portland cement concrete mixtures were used: normal and high strength. Results from testing confirm the reinforcing benefit of the geogrids as evidenced from the load-deflection response in terms of postpeak behavior, load capacity, crack mouth opening displacement (CMOD), and failure mode. Conclusions regarding correlations between the failure properties and the properties of the geogrid are made.

Evaluating Recycled Asphalt Pavement Mixtures with Mechanistic–Empirical Pavement Design Guide Level 3 Analysis

The main objective of this study is to assess the sensitivity of the predicted performance of recycled asphalt pavement (RAP) mixtures to the assumed binder grade. That is achieved with the Mechanistic-Empirical Pavement Design Guide (MEPDG) software to predict the performance of a specific flexible pavement structure with a RAP-modified hot-mix asphalt surface layer. Different design runs are conducted for which all the pavement properties and conditions are held constant except the properties of the surface layer. Specifically, a near-full factorial experiment is performed in which RAP content and effective binder PG grade are the main variables. Comparison of the predicted performance of the various runs reveals important findings on the extent and manner in which those two properties affect pavement distresses and performance. The influence of the assumed PG binder grade on the RAP mixtures, particularly the high-temperature grade, has a significant effect on the predicted amount of thermal cracking a...

Pilot-based assessment of the economics of recycling construction demolition waste

The significant amount of waste generated from construction demolition has become a chronic problem in many developing countries. Using data obtained from demolition contractors and various other sources, this paper proposes a framework for proper handling of construction demolition waste (CDW) to serve as a decision support tool in countries suffering from the lack of national CDW management guidelines. The framework is then demonstrated through a case study in the city of Beirut, Lebanon, and a sensitivity analysis is carried out to examine the economic feasibility of developing a recycling facility. The analysis showed that in order for a facility to be feasible, a gate fee should be charged in the presence of a market for recycled aggregates. The results confirm the significance of instigating and implementing legislation to control illegal dumping, constructing, and managing engineered landfills, and establishing markets for recycled CDW.

An FEM-predictive tool for simulating the cooling characteristics of freshly paved asphalt concrete layers

Accurate simulation of the cooling characteristics of newly paved asphalt concrete (AC) layers is of particular significance due to its potential use in efficient planning and scheduling of paving works. This study builds on principles of thermodynamics and heat transfer to develop a finite element framework that simulates the actual cooling of newly paved AC layers. For a given set of climatic conditions, such as wind, solar flux and air temperature, the presented model incorporates the effects of conduction, convection and radiation to predict the cooling rates of the pavement. The model is first validated using data acquired from laboratory and field set-ups. Then, the model is used to assess the effect of critical parameters such as layer thickness, time of paving, and material properties on asphalt cooling rates and their impact on paving operations.

Use of creep compliance interconverted from complex modulus for thermal cracking prediction using the M–E pavement design guide

The objective of this study is to evaluate the creep compliance (D(t)) of asphalt concrete (AC) mixtures for thermal cracking prediction of flexible pavements. Various AC overlay design factors influencing the thermal cracking resistance of flexible pavements, such as mixture properties and pavement structure, were included in the evaluation. Two sources of D(t) data were considered: (1) measurement from indirect tensile test and (2) numerical interconversion of complex modulus E*. Design input levels in the mechanistic–empirical design guide software do significantly impact the predicted thermal cracking distresses. For AC maintenance overlay design purposes, level 1 and 2 analyses yield very similar thermal cracking predictions, whereas level 3 analysis significantly underpredicts the extent of cracking when compared with to level 1 analysis. Some discrepancy exists in the thermal cracking predicted from measured and interconverted D(t) due to the inherent approximation nature of numerical interconversion methods. However, level 1 and 2 analyses using interconverted D(t) values provide results closer to the predictions from measured D(t) values than does level 3. Three mixtures are used in this study. The various analyses indicate that using a more ductile AC mixture for the surface layer significantly reduces the amount of thermal cracking at failure.