Yousef A. Al-Salloum

Performance of glass fiber reinforced plastic bars as a reinforcing material for concrete structures

Abstract The increasing use of fiber reinforced plastic (FRP) bars to reinforce concrete structures necessitates the need for either developing a new design code or adopt the current one to account for the engineering characteristics of FRP materials. This paper suggests some modifications to the currently used ACI model for computing flexural strength, service load deflection, and the minimum reinforcement needed to avoid rupturing of the tensile reinforcement. Two series of tests were conducted to check the validity of the suggested modifications. The first series was used to check the validity of the modifications made into the flexural and service load deflection models. The test results of the first series were also analyzed to develop two simple models for computing the service load deflection for beams reinforced with glass FRP (GFRP) bars. The second series was used to check the accuracy of the modification suggested into minimum reinforcement model. Test results of the first series indicate that the flexural capacity of the beams reinforced by GFRP bars can be accurately predicted using the ultimate design theory. They also show that the current ACI model for computing the service load deflection underestimates the actual deflection of these beams. The two suggested models for predicting service load deflection accurately estimated the measured deflection under service load, and the simpler of the two pertains better predictions than those of the models available in the literature. Test results of the second series reveal that there is an excellent agreement between the predicted and recorded behavior of the test specimens, which suggests the validity of the proposed model for calculating the required minimum reinforcement for beams reinforced by GFRP bars.

Experimental and Numerical Study for the Shear Strengthening of Reinforced Concrete Beams Using Textile-Reinforced Mortar

In this paper, the effectiveness of textile-reinforced mortars (TRMs), as a means of increasing the shear resistance of reinforced concrete beams, is experimentally and numerically investigated. Textiles comprise of fabric meshes made of long woven, knitted or even unwoven fiber rovings in at least two (typically orthogonal) directions. Mortars—serving as binders—may (or may not) contain polymeric additives usually used to have improved strength properties. These TRMs may be considered as an alternative to fiber-reinforced polymers (FRP), providing solutions to many of the problems associated with application of the latter without compromising much of the performance of strengthened members. In the present study, a new type of textile (basalt-based textile) was used as strengthening material. Two different mortar types’ viz. cementitious and polymer-modified cementitious mortars were used as binding material for the textile sheets. The studied parameters also included the number of textile layers as well as the orientation of the textile material. The experimental program comprises of testing two control beams which were intentionally designed to be deficient in shear, in addition to testing eight beams which were externally upgraded by TRM sheets for enhancing their shear capacity. On the basis of the experimental response of reinforced concrete members strengthened in shear, it is concluded that textile-mortar composite provides substantial gain in shear resistance; this gain is higher as the number of layers increases. With higher number of layers, textile with 45° orientation along with polymer-modified cementitious mortar provides the highest shear strength enhancement. Nonlinear finite-element (FE) analysis was also carried out on the tested beams using LS-DYNA, which is transient nonlinear dynamic analysis software. The numerical analysis carried out involved case studies for TRM modeled, with and without mortar. Good agreement was achieved between the experimental and numerical results especially for the ultimate load carrying capacity for the case of FE models incorporating mortar. The study was extended numerically to include additional cases of TRM-strengthened specimens with more number of TRM layers as well as a case of FRP-strengthened specimen.

Durability of GFRP Rebars in Concrete Beams under Sustained Loads at Severe Environments

Long-term behavior of composite materials is still a controversial issue among the engineering community though fiber-reinforced polymer (FRP) reinforcements are increasingly used in infrastructure applications. The main objective of this study is to investigate the effect of different local environmental conditions on the long-term behavior of glass fiber-reinforced polymer (GFRP) bars in concrete beams subjected to sustained loads. This is achieved through testing concrete beams reinforced with GFRP bars and subjected to a certain stress level. To accelerate the reaction, all beams are either completely or partially immersed in different environments (tap water and seawater) at elevated temperature. Test results are expressed in terms of tensile strength of GFRP bars and load-deflection behavior of both unstressed and stressed beams. The results show that there is significant loss in tensile strength of GFRP bars when subjected to sustained stress for the considered exposure conditions. The results of this investigation will be very useful to engineers concerned with the design of structures using composite materials.

Textile-Reinforced Mortar versus FRP as Strengthening Material for Seismically Deficient RC Beam-Column Joints

In this paper, efficiency and effectiveness of textile-reinforced mortars (TRM) on upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint has been studied. The results are then compared with that of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP)-strengthened joint specimens. Five as-built joint specimens were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing an extreme case of preseismic code design construction practice of joints and encompassing the vast majority of existing beam-column connections. Out of these five as-built specimens, two specimens were used as baseline specimens (control specimens) and the other three were strengthened with TRM, CFRP, and GFRP sheets, respectively. All five subassemblages were subjected to quasi-static cyclic lateral load histories to provide the equivalent of severe earthquake damage. The response histories of control and strengthened specimens were then compared. The test results demonstrated that TRM can effectively improve both the shear strength and deformation capacity of seismically deficient beam-column joints to an extent that is comparable to the strength and ductility achieved by well-established CFRP, and GFRP-strengthening of joints.

Ultimate strength prediction for RC beams externally strengthened by composite materials

A simple and efficient computational analysis to predict the nominal moment capacity of RC beams strengthened with external FRP laminates is presented. It considers the determination of the limits on the laminate thickness in order to assure tensile failure due to steel yielding and to avoid tensile failure due to FRP laminate rupture. The study presents the design of laminate thickness to attain a specified moment capacity in a given beam. Furthermore, the study affords the approach to determine the laminate thickness of any type of composite material available that is equivalent to FRP laminate required to achieve the desired moment capacity. This approach helps in studying comparative costs of rehabilitation using different FRP materials. The analytical and experimental results of series of beams strengthened with different number of layers of glass/epoxy or carbon/epoxy FRP laminates are presented. The results show that the design guidelines presented in this study performed well in prediction of experimental results.

Seismic Response of FRP-Upgraded Exterior RC Beam-Column Joints

Shear failure of exterior beam-column joints is identified as the principal cause of collapse of many moment-resisting frame buildings during recent earthquakes. Effective and economical strengthening techniques to upgrade joint shear resistance and ductility in existing structures are needed. In this paper, efficiency and effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in upgrading the shear strength and ductility of seismically deficient exterior beam-column joints have been studied. Four as-built joints were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing preseismic code design construction practice of joints and encompassing most of existing beam-column connections. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epo...

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.

Load Capacity of Concrete Masonry Block Walls Strengthened with Epoxy-bonded GFRP Sheets:

Epoxy bonding a thin layer of composite materials to the exterior surfaces of unreinforced masonry (URM) walls forces the individual brick or block elements to act as an integrated system. The high tensile strength of composite materials can be utilized to increase the shear and flexural capacity of URM walls significantly. This article presents the results of the experimental work carried out to investigate the capability of glass fiber reinforced polymer (GFRP) laminates in strengthening the concrete block walls subjected to out-of-plane and in-plane vertical and lateral stresses. A significant strength increase was observed for all strengthened specimens. Based on deformation compatibility and force equilibrium, simple design equations are presented to estimate the strength of masonry walls and their possible mode of failure. An excellent agreement between measured and predicted values of ultimate strength of wall specimens has been observed.

Optimality and safety of rigidly- and flexibly-jointed steel frames

Abstract A simplified procedure for the analysis and optimum design of frames with rigid or flexible connections is presented. The procedure is based on the elastic analysis and the allowable stress design of AISC specifications. The analysis procedure is simplified by using the secant stiffness as the average connection stiffness to approximate the connection behaviour in the analysis. A computer program is developed to find the optimum design of flexibly-jointed frames, as well as rigidly-jointed frames. A predictor-corrector scheme is employed to arrive at the optimum design. The predicted design vector is expressed in terms of true member forces by an implicit scheme. Dynamic scaling and a unique process of step size reduction are employed on the predicted design vector to avoid bypassing an optimum during the search procedure. A portal frame numerical example is employed to compare the optimum design of a flexibly-jointed frame to that of the same frame when assumed to be rigidly-jointed. The comparison is accomplished through the graphical presentation of the constraints and the objective function contours of each case in the design space. The safety of approximating the analysis of flexible frames as rigid is discussed. The global optimum solution, which is always safe for rigid or flexible analysis, is also presented. All these points are also addressed when employing a larger frame example.

Seismic Behavior of As-Built, ACI-Complying, and CFRP-Repaired Exterior RC Beam-Column Joints

In this paper, the efficiency and effectiveness of carbon-fiber-reinforced polymer (CFRP) sheets for upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint were studied and compared with an American Concrete Institute (ACI)-based design joint specimen. One as-built joint specimen, representing the preseismic code design and construction practice for joints and one ACI-based design joint specimen, satisfying the seismic design requirements of the current code of practice were cast. The as-built specimen was used as baseline (control) specimen. These two specimens (i.e., the as-built control and the ACI-based specimens) were subjected to cyclic lateral load histories to induce damage equivalent to damage expected from a severe earthquake. The damaged control specimen was then repaired by filling its cracks with epoxy and externally bonding CFRP sheets to the joint, the beam, and part of the column regions. This specimen was identified as the repaired specimen. The repaired specimen was subjected to a similar cyclic lateral load history, and its response history was recorded. The response histories of the as-built control, the repaired, and the ACI-based design specimen were then compared. The test results demonstrated that externally bonded CFRP sheets can effectively improve both the shear strength and the deformation capacity of seismically deficient and damaged beam-column joints to a state comparable to the ACI-based design joint.