Fouad Bayomy

The heterogeneity and mechanical response of hot mix asphalt laboratory specimens

Many pavement distresses, including rutting, fatigue cracking and low-temperature cracking, are influenced by the air void distribution in hot mix asphalt (HMA). This study investigated the influence of the type of Superpave gyratory compactors (SGCs) and specimen preparation/geometry on air void distribution and its manifestation on mechanical response. X-ray computed tomography along with image analysis techniques was used to capture and characterise the air void distribution within HMA microstructure. The mechanical response was characterised using the dynamic modulus test and static creep test. An index called heterogeneity index was developed to quantify the air void distribution in vertical and lateral directions. The study revealed that laboratory compacted specimens are heterogeneous and there is a need to prepare laboratory specimens with homogeneous air void distribution for accurate interpretation of results. In addition, the study revealed that the type of the SGC used influenced the dynamic modulus of compacted HMA specimens.

Performance of MEPDG Dynamic Modulus Predictive Models for Asphalt Concrete Mixtures: Local Calibration for Idaho

AbstractThe mechanistic-empirical pavement design guide (MEPDG) is the research version of the newly released DARWin-ME software by AASHTO. MEPDG includes two models for Levels 2 and 3 hot-mix asphalt (HMA) dynamic modulus (E*) predictions. The two models are NCHRP 1-37A and NCHRP 1-40D. The primary difference between the two is the binder stiffness parameter; viscosity or shear modulus. Moreover, MEPDG includes three levels for binder stiffness characterization: viscosity for Level 1 conventional binders, shear modulus for Level 1 superpave binders, and default values for Level 3. The influence of the binder characterization input level on the performance of the MEPDG E* predictive models is evaluated in this paper. To calibrate the models for Idaho, 27 HMA mixtures commonly used in Idaho were investigated. Results showed that the performance of the investigated models varied based on the temperature and the binder characterization method. The NCHRP1-37A E* model with MEPDG Level 3 binder inputs yielded ...

Evaluation of Asphalt Mix Stability Using Compaction Properties and Aggregate Structure Analysis

This paper presents a methodology to evaluate the aggregate structure stability in hot mix asphalt (HMA). The HMA response in the Superpave Gyratory Compactor (SGC) is used to derive an index, referred to as the Contact Energy Index (CEI), that reflects the energy utilized in establishing contacts among aggregate particles. Then, a three-dimensional (3-D) finite element model of the SGC is presented, and the CEI calculated from the finite element model is compared with experimental measurements. The 3-D finite element model is shown to give CEI results similar to measurements. Image analysis techniques are used to measure the evolution of the aggregate structure in terms of orientation and contacts during compaction. The image analysis results are compared with experimental measurements of CEI. The comparison shows strong correlation between the number of contacts, the resistance of the aggregate structure to re-orientation and the CEI value.

Evaluation of the MEPDG Dynamic Modulus Prediction Models for Asphalt Concrete Mixtures

HMA dynamic modulus (E*) is one of key inputs to the Mechanistic-Empirical Pavement Design Guide (MEPDG). There are two different E* models in the MEPDG; the NCHRP 1-37A viscosity-based model, and the NCHRP 1-40D, which is based on the binder shear modulus. This paper focuses on evaluating the influence of the binder characterization input level on the predicted E* in MEPDG. Laboratory E* tests were conducted on samples from 15 different HMA plant-produced mixtures. The shear modulus (G*) and phase angle (δ) for each binder were also determined in the laboratory. Results showed that MEPDG levels 1and 3 binder characterization inputs with both E* predictive models yielded E* values that are in excellent to fair agreement with laboratory measured E* . However, the 1-37A model showed better results than the 1-40D model. On the other hand, high bias in E* values was observed when level 1 binder characterization data was used.

Development and evaluation of hot mix asphalt stability index

This paper addresses the development and evaluation of a mix stability index referred to as the gyratory stability (GS). GS is calculated from the compaction data by summing the cumulative energies dissipated in the compaction of a gyratory sample. A wide range of commonly used mixes in the State of Idaho were selected for evaluation. Mixes were tested for dynamic modulus (E*) and flow number (F N); rutting was measured with the asphalt pavement analyser (APA). Furthermore, E* test results were used in the AASHTO Mechanistic–Empirical Pavement Design Guide (MEPDG) as level 1 inputs, to predict rutting for these mixes; GS ranked mixes similarly to the APA and F N tests and MEPDG results. Furthermore, results indicated that the GS has the potential to be used as a screening tool for asphalt mix design, especially to decide upon the acceptance of the mix aggregate structure, and as a quality control tool.

Mechanistically Based Flexible Overlay Design System for Idaho

A study was conducted on a mechanistically based overlay design procedure that incorporates the in situ pavement layer modulus values evaluated by deflection-based nondestructive testing using falling weight deflectometer data. The proposed overlay design procedure addresses the seasonal variation in the state of Idaho and adjusts the modulus values accordingly. The performance of the pavement is calculated in terms of critical strains based on the elastic multilayer theory. The study adopts the Asphalt Institute fatigue and rutting failure criteria to calculate the life of the pavement. Damage analysis is performed based on the past and expected future traffic to calculate the required overlay thickness. The procedure developed has been implemented in an event-driven, user-friendly computer program FLEXOLAY, which runs in the DOS environment. The program was tested and compared with other overlay design methods using pavement sections from the state of Idaho. The overlay thickness determined by FLEXOLAY was found to be close to some of the existing methods and far from others, depending on the existing pavement conditions.

USING LONG-TERM PAVEMENT PERFORMANCE DATA TO PREDICT SEASONAL VARIATION IN ASPHALT CONCRETE MODULUS

The seasonal variation of the asphalt concrete (AC) modulus with changes in pavement temperature is discussed. The main goals of the research was to develop (a) regression models that enable design engineers to assess seasonal changes in AC modulus and (b) an algorithm for calculating a seasonal adjustment factor (SAF) that allows estimating AC modulus in any season from a known reference value. The study is based on analyzing data collected at Long-Term Pavement Performance (LTPP) program sites in both freezing and nonfreezing zones. The data were obtained from the LTPP database in the DataPave 3.0 software. The approach adopted in this study was to select LTPP-seasonal monitoring program sites that represent various climatic regions and use the backcalculated modulus and pavement temperature data to develop regression models for the modulus-temperature relationships for various sites in both freezing and nonfreezing zones. Two regression models were developed to relate the variation in modulus with the variation in pavement temperatures in various seasons for both freezing and non-freezing zones. These models incorporate AC layer properties such as thickness, bulk specific gravity, air voids, and asphalt binder grade. A model for determining the SAF was also developed.

INNOVATIVE APPROACH TO FATIGUE CRACK PROPAGATION IN CONCRETE PAVEMENTS

An innovative approach to characterize the micro- and macromechanical damage phenomena resulting from fatigue cracking and relate them to the design and durability of pavements has been developed. In this approach, crack propagation and its associated damage are considered as irreversible processes, and hence the general framework of the thermodynamics of irreversible processes is used to model fatigue crack propagation behavior. A constitutive equation, the modified crack layer model, is used to extract material parameters characteristic of the pavement’s resistance to fatigue crack propagation. These parameters are the specific energy of damage, γ′, and a dissipative characteristic, β′, of the paving mixture. The general applicability of the proposed approach has been extended to examine fatigue crack propagation data of concrete reported in the literature. It is found that the proposed model describes fatigue crack propagation behavior of concrete pavements over the entire range of the crack driving force.

Prediction of the Subgrade Resilient Modulus for the Implementation of the MEPDG in Idaho

This paper focuses on developing two models for subgrade soil characterization for use in the Mechanistic-Empirical Pavement Design Guide (MEPDG). First, a multiple regression model can be used to predict R-value as a function of the soil plasticity index and percent passing No 200 sieve. Second, a Mr predictive model is based on the estimated R-value of the soil. Hence, the models can be used to estimate the Mr value as a Level 2 data input in the MEPDG. For the R-value model development, 8233 records were used from a historical database of Idaho Transportation Department (ITD) laboratory testing on subgrade soils. For the Mr model development, laboratory measured Mr values for different subgrade soils were gathered from literature nationwide. The R-values for these soils were then predicted using the developed R-value model. Hence, a relationship between the Mr and R-value was established. A limited sensitivity analysis was also conducted to address the influence of the material strength upon the predicted distresses by MEPDG.

Comparison of Idaho Pavement Design Procedure with AASHTO 1993 and MEPDG Methods

Several in service pavements located in different regions of Idaho that have been designed according to the ITD design method were redesigned using the AASHTO 1993 as well as the Mechanistic-Empirical Pavement Design Guide (MEPDG) procedures. All designs were conducted at a 50% reliability level. The nationally calibrated MEPDG (version 1.1) was used to predict the performance of the three design methods. Level 2 subgrade material characterization inputs were used in the MEPDG analysis. All other MEPDG inputs were level 3. Performance indicators predicted using MEPDG related to the three design methods were compared to each other. Results showed that, relative to AASHTO 1993 and MEPDG procedures, ITD design method significantly overestimates the thickness of the pavement structure, and particularly the thickness(s) of the unbound layer(s). On the other hand, the AASHTO 1993 and MEPDG guides show reasonable agreement on the resulting pavement structure.