Husain Abbas

Effect of blast loading on CFRP-Retrofitted RC columns - a numerical study

This study aims to investigate the effect of blast loads generated as a result of explosive charges on the existing exterior RC circular columns of a typical building in the city of Riyadh. A procedure has been developed for evaluating the dynamic characteristics of the circular column with and without retrofitting. A wide range of parametric studies have been performed as part of this investigation to examine the effects of stand-off distance, charge weight and the presence of CFRP retrofitting on the level of damage to the RC column. The nonlinear finite element analysis was carried out using LS-DYNA software with explicit time integration algorithms. Different charge weights of 100, 200, 500 and 1000 kg equivalent weight of TNT at stand-off distances of 1, 4 and 15 m were considered. Results described in this paper indicate that CFRP strengthening could be an effective solution to limit the damage caused by moderate explosions. The stand-off distance was found to play a very important role in mitigating the adverse effects of a blast. The results also indicate that the maximum lateral deflection experienced by the column decreased exponentially with the increase in the stand-off distance and also decreased for the columns strengthened with CFRP, compared with the unstrengthened columns.

Prediction of Intermediate Crack Debonding Strain of Externally Bonded FRP Laminates in RC Beams and One-Way Slabs

Interface crack propagation of FRP (fiber-reinforced polymer) strengthened reinforced concrete (RC) flexural member is often initiated from the toes of the intermediate cracks and propagates towards the supports. This type of FRP delamination is commonly termed intermediate crack (IC) debonding and is common for flexural members with high shear span-to-depth ratios. If the ultimate FRP strain at IC debonding failure is known, the moment capacity of the member can be obtained through a simple section analysis. This research deals with the prediction of ultimate FRP strain at IC debonding, using neural networks and regression models. Basic information on neural networks and the types of neural networks most suitable for the analysis of experimental results are given. A set of experimental data for FRP-strengthened RC beams and one-way slabs, covering a large range of parameters, for the training and testing of neural networks is used. The available test results were not only compared with current code provisions but with equations proposed by other researchers as well. The prediction models based on neural network are presented. A new design equation is also suggested.

Progressive Collapse Analysis of RC Buildings Against Internal Blast

This paper seeks to explore the vulnerability of a typical reinforced concrete (RC) building against progressive collapse as a consequence of internal blast. The emphasis has been on the local model analysis for which two approaches – one involving the use of CONWEP and another using fluid-structure interaction through Alternate Lagrangian Eulerian (ALE) element formulation - have been employed. The finite element model of the structure was created using LS-DYNA, which uses explicit time integration algorithms for solution. The results of the study are proposed to be used to control or prevent progressive collapse of the building. In order to validate the employed numerical models, blast test results of a RC column available in literature were validated using LS-DYNA modeling of the RC column. The deformation response of the column was compared which showed acceptable prediction.

Free vibration of tapered beams and plates based on unified beam theory

In this paper, accurate frequency solutions of tapered vibrating beams and plates using a simple and efficient displacement based unified beam theory in lieu of computationally expensive and rather complex two-dimensional plate theories are presented. The results are given in the form of Euler-Bernoulli/Timoshenko to quasi three-dimensional (3D) solutions. Lower frequency bending and axial modes, as well as torsional and biaxial bending modes corresponding to higher frequency values, are predicted which are in very good agreement with 3D finite element results as well as the published literature. The effects of different parameters like taper ratio, thickness and beam/plate-aspect ratios on the vibration frequencies of tapered structures are studied. It can be seen that due to taper, bending vibration modes become asymmetric along the longitudinal axis of the structure. Further, it can also be noticed that the vibration behavior of thicker beams and plates is characterized by the appearance of a significant number of axial modes at lower frequency values as compared to that of relatively thinner beams/plates.

Effect of some biotic factors on microbially-induced calcite precipitation in cement mortar

Sporosarcina pasteurii, a common soil bacterium has been tested for microbial treatment of cement mortar. The present study also seeks to investigate the effects of growth medium, bacterial concentration and different buffers concerning the preparation of bacterial suspensions on the compressive strength of cement mortar. Two growth media, six different suspensions and two bacterial concentrations were used in the study. The influence of growth medium on calcification efficiency of S. pasteurii was insignificant. Significant improvement in the compressive as well as the tensile strength of cement mortar was observed. Microbial mineral precipitation visualized by Scanning Electron Microscopy (SEM) shows fibrous material that increased the strength of cement mortar. Formation of thin strands of fillers observed through SEM micrographs improves the pore structure, impermeability and thus the compressive as well as the tensile strengths of the cement mortar. The type of substrate and its molarity have a significant influence on the strength of cement mortar.

Improving the Impact Resistance of Reinforced Concrete

The strategic concrete structures are often required to resist impact loads arising from the projectile strike, falling weight, blast generated missile etc. The existing structures found deficient in resisting these loads are required to be retrofitted whereas the upcoming structures are required to be designed for expected impact loads. This paper explores the ways of strengthening existing reinforced concrete (RC) structures using externally bonded carbon fiber reinforced polymer (CFRP) sheets and improving the impact resistance of concrete by mixing hybrid fibers in its production. The impact response of concrete structures is assessed using experiments involving the impact of projectiles of different nose shapes on slab specimens. The material behavior at high strain rate is established using split Hopkinson pressure bar (SHPB) testing at varying strain rates. Analytical models are developed for predicting penetration depth, scabbing thickness, ballistic limit velocity and ejected mass. The experimental results were also validated through numerical modeling using LS-DYNA.

Behavior and Design Aspects of FRP-Strengthened URM Walls under Out-of-Plane Loading

AbstractThe use of externally bonded fiber-reinforced polymer (FRP) composites for upgrading the out-of-plane flexural resistance of unreinforced masonry (URM) walls is experimentally and analytically investigated in this study. A total of six hollow concrete block walls were tested to failure using an airbag and a reaction frame to obtain a uniform load on the wall. The masonry walls were placed horizontally and tested as one-way slabs with span direction perpendicular to the bed joints. The first wall was left unstrengthened to be used as control specimen; the other five walls were strengthened using different schemes of externally attached glass-fiber-reinforced polymer (GFRP) sheets. The main parameters studied experimentally were FRP reinforcement ratio and stiffness. In addition to the experimental program, an analytical model was developed to predict the ultimate moment capacity of the URM walls. The procedure outlined in standard guidelines was also utilized to compute the flexural capacity of wal...

EFFECT OF CFRP AND TRM STRENGTHENING OF RC SLABS ON PUNCHING SHEAR STRENGTH

The paper presents experiments involving punching of RC slabs strengthened using externally bonded carbon fiber reinforced polymer (CFRP) sheet and textile reinforced mortar (TRM). Twelve RC slab specimens of two concrete grades (39.9 and 63.2 MPa) and employing two strengthening schemes (CFRP and TRM) were tested. Specimens were supported on two opposite edges. Experimental load-displacement variations show two peak loads in strengthened slabs and one peak followed by a plateau in control. Second peak or the plateau corresponds to the combined action of aggregate interlock and the dowel action of back face rebars and strengthening layers. The dowel action of back face rebars and strengthening layers had no role in ultimate punching load (i.e. first peak). Strengthened slabs showed 9-18% increase in ultimate punching load (i.e. first peak) whereas there was significant increase in the second peak load (190-276% for CFRP; 55-136% for TRM) and energy absorption (~66% for CFRP and 22-56% for TRM). An analytical model was also developed for predicting the punching shear strength (first and second peaks) of strengthened slabs showing good comparison with experiments.

Performance of concrete subjected to elevated temperature

This article shows experimental investigation carried out to study the effect of exposure to elevated temperature on concrete. Four concrete mixes obtained using different proportions of ground granulated blast furnace slag (GGBFS) and silica fume (SF) with 43 grade ordinary Portland cement were considered. Concrete cubes of 100 mm size were cast, cured and exposed to different peak temperatures (up to 700 °C) for different heating periods (1, 2, 3 and 7 h) and tested to failure under compression. Before subjecting the heated specimens to compression test, the heated cubes were closely examined for any signs of cracking, disintegration and loss of mass. The concrete specimens with 5% SF do not show any sign of cracks at different peak temperatures, whereas 10% SF cubes split from corners at 425 °C. However, the concrete cubes with 10% GGBFS did not show any sign of cracking even for 7-h exposure to a temperature of up to 500 °C. The mass loss of all concrete specimens increased sharply up to 200 °C, and after that, it increased marginally. The compressive strength of all concrete mixes increased up to 200 °C and decreased with further increase in temperature, with the exception of concrete containing 5% SF which increased even at higher temperatures but at a slower rate.

Dynamic Analysis of Tapered Plates Based on Higher Order Beam Theory

Modal solutions of plates with uniformly varying cross section using unified beam theory are presented. The results are given in the form of Euler-Bernoulli, Timoshenko and quasi 3D solutions. Numerical results for cantilever and CFCF supported rectangular planform plates are presented. Different types of modes, i.e. axial, bending and torsional modes are observed. The frequency values are in good agreement with 3D finite element results as well as published literature. Due to uniform taper in plate cross section, bending vibration modes become asymmetric along the longitudinal axis of the structure. Further, it can also be noticed that the vibration behavior of thick tapered plates is characterized by the appearance of significant number of axial and torsional modes at lower frequency values.