Gebran Karam | Lebanese American University (original) (raw)

Papers by Gebran Karam

Aci Structural Journal, 2020

MRS Proceedings, 1992

The most important property of cements and concretes after strength is the workability, which is ... more The most important property of cements and concretes after strength is the workability, which is controlled by the rheological characteristics of the mix. Theoretical modeling having proved complicated and in some cases mathematically untractable, cement specialists have concentrated on empirical based models for the last two decades. The major theoretical contributions to date on the properties of colloidal and concentrated suspensions are summarized and a general framework for theoretical modeling of cement viscosity is established. The empirical modeling and the experimental investigation of the rheological properties of fresh cements are reviewed and discussed. A semi-empirical model is proposed and its validity tested in the interpretation of some published experimental results.

A general modification to the models used for the study of the effect of fibre volume on the stre... more A general modification to the models used for the study of the effect of fibre volume on the strength properties of short fibre reinforced cements is presented. It considers the composite as a mix of cement, fibres and voids. The matrix contribution to the strength of the composite is expressed as a function of the void and fibre volumes. Based on the reduction of interfacial surfaces due to fibre-fibre and fibre-void interactions, a modification to the strength contribution of the fibres is proposed. Furthermore, the void content is related experimentally to the fibre volume. The effect of these modifications on the different measures of strength and toughness in fibre reinforced cements is briefly discussed. The experimentally observed nonlinear variation of the bending strength of wood fibre reinforced cement with fibre volume can be predicted by applying the proposed modifications to the rule of mixtures used to predict that strength.

Physics of Fluids, 2015

Applying a temperature gradient to a layer of a binary fluid establishes a diffusive transport me... more Applying a temperature gradient to a layer of a binary fluid establishes a diffusive transport mechanism called thermophoresis or Soret effect which separates the two fluid's components and is measured by the Soret coefficient. Recent investigations carried out on concentrated magnetic fluids showed that the intensity of the Soret effect depends on the concentration of the nanoparticles transported. The present article, therefore, deals with the concentration-dependence of the Soret coefficient using five equally composed magnetic fluids only varying in the concentration of the particles from 2 vol. % to 10 vol. % of magnetic material. The current investigations point out that the determination of the Soret coefficient and especially its dependence on the particles' concentration is based on the determination of the thermal and particle diffusion coefficient. The article, therefore, presents a theoretical approach for the determination of the thermal diffusion coefficient and adapts a commonly used Ansatz for the particle diffusion coefficient for the present case of concentrated magnetic fluids. It is thereby possible to determine a theoretical Soret coefficient in dependence on an empirical parameter α. The coefficient is compared with the experimental approaches which have been previously used, these will be referred to as "analytical approach" throughout the text. A second comparison is achieved with a hybrid Soret coefficient which fits the experimentally detected separation curves numerically. Within the investigations, the hydrodynamic concentration of the particles is used, assuming a surfactant layer's thickness of 2 nm per magnetic particle which leads to concentrations between approximately 11 vol. % and 47 vol. %. The diffusion coefficient ranges from 0.6 × 10 −11 m 2 /s to 2.5 × 10 −11 m 2 /s depending on the analytical model used. The theoretical Soret coefficient decreases with increasing particles' concentration; the experimental values derived from the analytical approach decrease from 0.06 K −1 to 0.01 K −1 for increasing particles' concentration. The numerically determined coefficient ranges from 0.11 K −1 to 0.022 K −1. The experimental values are smaller than former experimental results suggest (0.16 K −1), which is due to the fact that in former works, only magnetic concentrations had been considered. All three current investigations prove what could also be partly seen in former experiments that the higher the particles' concentration the weaker is thermophoresis. The particle diffusion coefficient has to be known for the determination of the Soret coefficient. It is carried out a proof of principle in the article showing that the horizontal thermophoresis cell can also be used to determine the rehomogenisation process which takes place after separating the fluid by applying a homogeneous temperature to the fluid. The diffusion coefficients that could be determined experimentally range from 1 × 10 −11 m 2 /s to 6 × 10 −11 m 2 /s.

Journal of Composites for Construction, 2000

In this technical note, fiber-reinforced plastics (FRP) strengthening efficiency for reinforced-c... more In this technical note, fiber-reinforced plastics (FRP) strengthening efficiency for reinforced-concrete columns will be examined from a micromechanical point of view by accounting for direct mechanical interaction between FRP sheet and concrete. Concrete compressive failure is initiated by wing-crack propagation. The mechanical interaction dictates the compressive behavior of FRP-wrapped concrete from the first bent-over point to the peak load in the load/displacement curve. A possible detailed interaction under compression is described.

Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences, 1995

Many natural materials have exceptionally high values of the mechanical performance indices descr... more Many natural materials have exceptionally high values of the mechanical performance indices described in the previous, companion paper. For beams and plates of a given stiffness or strength, or for a column of a given buckling resistance, woods, palms and bamboo are among the most efficient materials available. Their mechanical efficiency arises from their combination of composite and cellular microstructures. In this paper we analyse the microstructures which give rise to exceptional performance, describe the fabrication and testing of model materials with those microstructures and discuss the implications for design of mechanically efficient engineering materials.

Cerâmica, 2009

A targeted experimental program was carried out to establish basic reference physical and mechani... more A targeted experimental program was carried out to establish basic reference physical and mechanical properties of commercially available pre-cast marble chip terrazzo tiles and to investigate the effects of varying manufacturing process parameters on those properties. The transverse strength, density, water absorption, and abrasion resistance were measured as a function of casting pressure and residence time in the casting mold for a standard mix design. A simple surface abrasion index test was developed and applied comparing pre-cast terrazzo tiles with reference natural tiling stones. Recommendations for improving existing specifications and developing quality control measures are presented.

faults) and a generalized offshore thrust fault to accommodate convergence in the restraining ben... more faults) and a generalized offshore thrust fault to accommodate convergence in the restraining bend, and 2) continuum (velocity gradient) models that explore infinitesimal strain and rotation rates. The models suggest a displacement rate of 4.5-5.5 mm/yr along the Yammouneh fault and a counter clock wise rotation of 0.5 o-1.75 o /MA within the bend. This study provides an essential tool for assessing the seismic hazard in the vicinity of the Lebanese restraining bend.

Magazine of Concrete Research, 2004

An experimental investigation of the corner sharpness effect in square columns with carbon fibre-... more An experimental investigation of the corner sharpness effect in square columns with carbon fibre-reinforced polymer (CFRP) wraps is presented using traditional compressive testing of short square columns as well as an innovative set-up that allows testing directly for the CFRP strength when wrapped around the corner of a flat slab. Both types of test confirmed the observation that the confinement efficiency decreases as the corner sharpness increases, owing to stress concentrations that cause localised failure of the CFRP.

ABSTRACT Restraining bends along strike-slip faults are typically characterized by relative struc... more ABSTRACT Restraining bends along strike-slip faults are typically characterized by relative structural and kinematic complexity, owing to the obliquity of plate motion with respect to the structural trend. One example of a large restraining bend is the Lebanese Restraining Bend (LRB), a ~180 km long bend in the left-lateral Dead Sea Transform System. Compared with other large restraining bends, such as the Big Bend along the San Andreas Fault, the LRB has a relatively simple geological structure consisting of two large strike-slip faults (the through-going Yammouneh fault and the terminating Serghaya fault) and several smaller faults. Owing to the less complicated structure, the LRB presents an opportunity to study basic kinematics of deformation within restraining bends, in general. This study examines the present-day deformation associated with the LRB based on Global Positioning System (GPS) measurements from Lebanon, Syria, and adjacent areas. The GPS network in Lebanon consists of 23 survey-mode sites and 1 continuous site spanning 2002 - 2008, and GPS measurements in Syria span 2000 - 2008. 1-sigma uncertainties on most sites are less than 0.65 mm/yr. Application of elastic dislocation models suggests slip rates of 3.8 - 4.5 mm/yr for the Yammouneh fault and 1.0 - 1.5 mm/yr along the Serghaya fault, which are consistent with geological estimates. The relatively dense GPS network permits calculating velocity gradients that can be decomposed into infinitesimal rotations (vorticity), as well as infinitesimal strains. The results suggest counter clockwise rotations of 0.5 - 1.8 degrees/MA within the restraining bend. For this analysis, the vorticity can be decomposed into contributions from long-term fault slip (shear strain related to slip rate) and vertical axis rotations - the latter can be compared with long-term rotations from paleomagnetic studies. These kinematic constraints can be applied to testing proposed models for restraining bend kinematics involving continuous deformation and right block rotation.

The current status of the Lebanese vocational and technical education (VTE) system is assessed an... more The current status of the Lebanese vocational and technical education (VTE) system is assessed and the strategic issues and challenges facing it are identified. In addition to the economic and social challenges that are common to many developing countries, the Lebanese system suffers from idiosyncratic problems, which may require innovative and bold reform strategies. The results of the first open consultations in the history of VTE with public and private sector stakeholders and key decision makers are analysed. These results are used to confirm the analysis of the current situation and to chart a strategic planning process for policy making, reform, and improved effectiveness in Lebanese vocational and technical education. Vocational education, technical education, educational policy, educational reform, effectiveness improvement, Lebanon BACKGROUND TO THE STUDY Lebanon is a small country of 10,452 square kilometres, situated in the heart of the Middle East on the Eastern shores o...

Thin walled cylindrical shell structures are widespread in nature: examples include plant stems, ... more Thin walled cylindrical shell structures are widespread in nature: examples include plant stems, porcupine quills, and hedgehog spines. All have an outer shell of almost fully dense material supported by a low density, cellular core. In nature, all are loaded in combination of axial compression and bending: failure is typically by buckling. Natural structures are often optimized. Here we have analyzed the elastic buckling of a thin cylindrical shell supported by an elastic core to show that this structural configuration achieves significant weight saving over a hollow cylinder. The results of the analysis are compared with data from an extensive experimental program on uniaxial compression and four point bending tests on silicone rubber shells with and without compliant foam cores. The analysis describes the results of the mechanical tests well. Characterization of the microstructures of several natural tubular structures with foamlike cores (plant stems, quills, and spines) revealed them to be close to the optimal configurations predicted by the analytical model. Biomimicking of natural cylindrical shell structures and evolutionary design processes may offer the potential to increase the mechanical efficiency of engineering cylindrical shells.

The dynamic response of large historical block structures, as manifested by the response of the f... more The dynamic response of large historical block structures, as manifested by the response of the free-standing variable size drums colonnade of Baalbek due to strong ground shaking during earthquake events, is investigated experimentally for the purpose of predicting damage or collapse due to future earthquake events. Reduced single column multi drum models of scales 1/20 and 1/40 were subjected to harmonic excitations of varying amplitudes and periods on a 1D shaking table for up to 25 cycles noting collapse or survival. The results are compared to two dimensional simulations on UDEC as well as to the predictions of the Housner formula for single rigid block overturning by a half-cycle sinusoidal acceleration pulse. It is shown that the experimental results agree qualitatively with computer simulations for the reduced scale but cannot be directly extrapolated to full scale.

International education journal, 2006

Lecture Notes in Civil Engineering

Journal of Composites for Construction, 2001

A discussion of an article with the aforementioned title by Hwai-Chung Wu, published in this jour... more A discussion of an article with the aforementioned title by Hwai-Chung Wu, published in this journal (May 2000, Vol. 4, No. 2) is presented. The original article presents a theoretical model for the behavior of fiber reinforced plastic (FRP) wrapped concrete columns. The discusser proposes a method of estimating the confinement ratio as a function of the elastic material properties of the concrete and the FRP. Discussion is followed by closure from the author.

The role that microstructure plays in the mechanical efficiency of natural cellular materials is ... more The role that microstructure plays in the mechanical efficiency of natural cellular materials is examined here. The focus of this study is on elastic behaviour. Two natural materials with microstructures resistant to local bucking: plant stems and animal quills have also been examined.