Showing papers by "Rashid K. Abu Al-Rub published in 2012"

Mechanical Properties of Nanocomposite Cement Incorporating Surface-Treated and Untreated Carbon Nanotubes and Carbon Nanofibers

Abstract: To study the effects of functionalized carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the mechanical properties of cement composites, both untreated and treated CNFs and CNTs were added to cement paste in concentrations of 0.1% and 0.2% by weight of cement. The surface-treated nanofilaments were functionalized in a solution of sulfuric acid (H2SO4) and nitric acid (HNO3). The nano- filaments were dispersed by using an ultrasonic mixer and were then cast into molds. Each specimen was tested in a custom-made three-point flexural test fixture to record the mechanical properties (i.e., the Young's modulus, flexural strength, ductility, and modulus of toughness) at the age of 7, 14, and 28 days. The microstructure was analyzed by using a scanning electron microscope. Untreated CNTs and CNFs were found to enhance the mechanical properties of cementitious materials, whereas the acid-treated CNTs and CNFs degraded the mechanical properties. DOI: 10.1061/(ASCE)NM.2153-5477.0000041. © 2012 American Society of Civil Engineers. CE Database subject headings: Nanotechnology; Cement; Mechanical properties; Carbon. Author keywords: Carbon nanotubes; Carbon nanofibers; Cement; Surface treatment; Mechanical properties.

Thermodynamic-based model for coupling temperature-dependent viscoelastic, viscoplastic, and viscodamage constitutive behavior of asphalt mixtures

Abstract: SUMMARY Based on the continuum damage mechanics, a general and comprehensive thermodynamic-based framework for coupling the temperature-dependent viscoelastic, viscoplastic, and viscodamage behaviors of bituminous materials is presented. This general framework derives systematically Schapery-type nonlinear viscoelasticity, Perzyna-type viscoplasticity, and a viscodamage model analogous to the Perzyna-type viscoplasticity. The resulting constitutive equations are implemented in the well-known finite element code Abaqus via the user material subroutine UMAT. A systematic procedure for identifying the model parameters is discussed. Finally, the model is validated by comparing the model predictions with a comprehensive set of experimental data on hot mix asphalt that include creep-recovery, creep, uniaxial constant strain rate, and repeated creep-recovery tests in both tension and compression over a range of temperatures, stress levels, and strain rates. Comparisons between model predictions and experimental measurements show that the presented constitutive model is capable of predicting the nonlinear behavior of asphaltic mixes under different loading conditions. Copyright © 2011 John Wiley & Sons, Ltd.

Comparing finite element and constitutive modelling techniques for predicting rutting of asphalt pavements

Abstract: This paper focuses on a comprehensive evaluation of the effects of different finite element (FE) modelling techniques and material constitutive models on predicting rutting in asphalt pavements under repeated loading conditions. Different simplified 2D and more realistic 3D loading techniques are simulated and compared for predicting asphalt rutting. This study also evaluates and compares the rutting performance predictions using different material constitutive behaviours such as viscoelastic‐viscoplastic, elasto-viscoplastic and coupled viscoelastic, viscoplastic and viscodamage behaviours. The simulations show that the assumption of the equivalency between a pulse loading and an equivalent loading, which are commonly used as simplified loading assumptions for predicting rutting, is reasonable for viscoelastic‐viscoplastic and elasto-viscoplastic constitutive behaviours. However, these loading assumptions and material constitutive models overestimate rutting as damage grows. Results show that the 2D plane strain FE simulations significantly overestimate rutting as compared with the rutting performance predictions from more realistic 3D FE simulations.

Challenges and benefits of utilizing carbon nanofilaments in cementitious materials

Abstract: Carbon nanofibers/tubes (CNF/Ts) are very strong and stiff and as a result, are expected to be capable of enhancing themechanical properties of cementitious materials significantly. Yet there are practical issues concerning the utilization of CNF/Ts in cementitious materials. This study summarizes some of the past efforts made by different investigators for utilizing carbon nanofilaments in cementitious materials and also reports recent experimental research performed by the authors on the mechanical properties of CNF-reinforced hardened cement paste. The major difficulties concerning the utilization of CNF/Ts in cementitious materials are introduced and discussed. Most of these difficulties are related to the poor dispersibility of CNF/Ts. However, the findings from the research presented in this work indicate that, despite these difficulties, carbon nanofilaments can significantly improve the mechanical properties of cementitious materials. The results show that CNFs, even when poorly dispersed within the cementitious matrix, can remarkably increase the flexural strength and cracking resistance of concrete subjected to drying conditions.

Prediction of Micro and Nano Indentation Size Effects from Spherical Indenters

Abstract: In the present work, a micromechanical model based on dislocation mechanics for predicting indentation size effect from spherical indenters is developed and compared with the most widely used Swadener et al. (2002) model. The key idea proposed here while deriving the model is that a nonlinear coupling between the geometrically necessary dislocations (GNDs) and the statistically necessary dislocations (SSDs) can largely correct the deviation of the Swadener et al. model's prediction from hardness results for small diameter indents. Furthermore, the model can be used in identifying the material length scale from micro- and nano-indentation data.

Recent developments and applications of pavement analysis using nonlinear damage (PANDA) model

Abstract: This paper presents an overview of the development and applications of the PANDA (Pavement Analysis using Nonlinear Damage Approach) model that has been under development at Texas A&M University for the past few years. In addition to the basics of the constitutive relationships used in PANDA, this paper presents examples of calibration and validation of the model using experimental laboratory data. The results demonstrate clearly the ability of the model to describe the mechanical behaviour of asphalt mixtures in terms of resistance to damage and permanent deformation. Finally, the capabilities of the model to simulate the mesoscale response of asphalt mixtures are presented and their implications in the design of asphalt mixtures are discussed.