Hamdy M. Afefy

Strengthening of defected beam-column joints using CFRP.

This paper presents an experimental study for the structural performance of reinforced concrete (RC) exterior beam–column joints rehabilitated using carbon-fiber-reinforced polymer (CFRP). The present experimental program consists of testing 10 half-scale specimens divided into three groups covering three possible defects in addition to an adequately detailed control specimen. The considered defects include the absence of the transverse reinforcement within the joint core, insufficient bond length for the beam main reinforcement and inadequate spliced implanted column on the joint. Three different strengthening schemes were used to rehabilitate the defected beam–column joints including externally bonded CFRP strips and sheets in addition to near surface mounted (NSM) CFRP strips. The failure criteria including ultimate capacity, mode of failure, initial stiffness, ductility and the developed ultimate strain in the reinforcing steel and CFRP were considered and compared for each group for the control and the CFRP-strengthened specimens. The test results showed that the proposed CFRP strengthening configurations represented the best choice for strengthening the first two defects from the viewpoint of the studied failure criteria. On the other hand, the results of the third group showed that strengthening the joint using NSM strip technique enabled the specimen to outperform the structural performance of the control specimen while strengthening the joints using externally bonded CFRP strips and sheets failed to restore the strengthened joints capacity.

Enhancement of flexural behaviour of CFRP-strengthened reinforced concrete beams using engineered cementitious composites transition layer

This paper aimed to develop and evaluate an efficient strengthening method for reinforced concrete beams, based on engineered cementitious composites (ECC) to be applied as a transition layer prior to the application of the carbon fibre-reinforced polymer (CFRP) strengthening sheet. The role of the proposed transition layer is to control the cracking of concrete and detain or even avoid premature de-bonding of the strengthening CFRP sheets. As the ability of the transition layer to exhibit a strain hardening behaviour is mainly dependent on the used fibre volumetric ratio, three ECC mixes with three different polypropylene fibre volumetric ratios were used (fibre volumetric ratio of 0.5%, 1% and 1.5%). The experimental results showed that while the used CFRP strengthening sheet can increase the ultimate load by about 28.8% compared with the control un-strengthened beam, this increase can reach about 48.5% by applying the same CFRP sheet to the proposed ECC transition layer that contains a fibre volumetric ratio of 1.5%. Moreover, this layer integrated with the mention ratio of the fibre content enabled the CFRP sheet to be in a complete contact with the strengthened beam without any de-bonding up the rupture of the CFRP sheet at failure.

Bond Behavior of Embedded Reinforcing Steel Bars for Varying Levels of Transversal Pressure

Because of columns loads, reinforced concrete (RC) continuous beams in skeleton structures are subjected to transverse compressive stress at support locations. Such lateral pressure can enhance the bond between the main top reinforcing bars and the surrounding concrete because of the confinement effect. Thus, the development length can be reasonably decreased compared to the case of no lateral pressure. However, different codes of practice, such as the Egyptian code standard ECP 203-2007 and the American Concrete Institute (ACI) code standard ACI 318-11, stipulate increasing the reinforcement location factor for upper reinforcement implemented in the calculation of the development length of such bars. On the other hand, the Comite Euro-International du Beton (CEB-FIP) model code standard CEB-FIP 2010 considers this enhancement in bond stress calculations as being due to transverse compression stress. To assess such effects, pull-out tests were performed on reinforcing bars embedded in short RC columns subjected to different levels of axial pressure. The experimental findings showed the same trend as manifested in the CEB-FIP 2010 results, where increasing the lateral pressure on the reinforcing bar resulted in decreasing the development length for both smooth and deformed steel bars. In contrast, the results of development length calculations based on both ECP 203-2007 and ACI 408R-03 were constant regardless of the level of lateral pressure on the reinforcing bar. This highlights the need for both ECP 203-2007 and ACI 408R-03 to consider the effect of transverse lateral pressure on development length calculations due to varying lateral pressure on the reinforcing bar.

Innovative Repair Technique for RC Beams Predamaged in Shear

The one major practical difficulty associated with reinforced concrete (RC) structures strengthened with externally bonded (EB) carbon-fiber-reinforced polymer (CFRP) sheets is that they must be bonded to a hard uncracked concrete surface, which need may not be met in many cases. This paper assesses the usage of a temporary compressive force that was applied parallel to the beam depth in the cracking-shear zone in order to close all the existing shear cracks prior to the application of the CFRP-strengthening sheets. This force was kept constant till full curing of the CFRP epoxy adhesive, and after the epoxy had cured, it was completely removed. The advantages that result from the shear strengthening of RC beams using the new proposed technique are that closing of all existing cracks and removing the temporary compressive force generates tension force in the CFRP-strengthening strips, which in turn delays the formation and propagation of the shear cracks and develops a compressive force in the internal stirrups, which in turn increases their efficiency. These advantages were verified by testing of seven RC beams designed to fail in shear. Two beams were strengthened using the proposed technique, two beams were repaired by epoxy injection before being strengthened using CFRP, one beam was strengthened using CFRP applied directly over the cracked concrete surface, one beam was strengthened without preloading, and one beam was tested without strengthening and served as a control. Finally, the authors present and comment upon a comparison between the present experimental results and the predictions using the available formulations recommended by some international guidelines.

Statistically equivalent nonlinear finite element model for infilled R. C. framed systemsThe present work is an extension to the thesis of the first author for the Master of Science degree.

The most simple equivalent frame system with reduced degrees of freedom is proposed for handling multi‐story multi‐bay infilled frames. The system is composed of homogenized continuum for the reinforced concrete members braced with unilateral diagonal struts for each bay, which are only activated in compression. Identification of the equivalent system characteristics and nonlinear material properties are accomplished from the concepts of inverse analysis approach along with statistical tests of hypotheses is employed to establish the appropriate filtering scheme and the proper accuracy tolerance. The suggested system allows for nonlinear finite element static and dynamic analysis of sophisticated infilled reinforced concrete frames. Sensitivity analysis is undertaken to check the suitability of the proposed system to manipulate various structural applications.

Structural performance of reinforced concrete exterior beam–column joint subjected to combined shear and torsion

This article presents the results of an experimental study conducted in order to investigate the behavior of Type-I beam–column joints, where the beam was subjected to shear stresses due to combined effect of shear force and torsional moment (beam eccentric loading). Eight beam–column joint specimens were constructed and tested up to failure in order to better understand the complicated behavior due to combined loading transmitted from the beam to the column. The studied parameters were the eccentricity of beam load (torsional-moment-to-shear-force ratio Mt/Q), the configuration of beam side and compression reinforcing steel, the existence of the joint reinforcing stirrups, and the beam rigidity. Generally, the shear failure at the beam zone or at the joint panel was noticed to be the governing mode of failure for all tested specimens. The test results highlighted the importance of the configuration of both longitudinal side and compression reinforcing steel of the beam, where the inadequate embedded length of both of them into the joint panel including stirrups decreased the beam ultimate capacity but it kept the failure zone away from the joint panel. Finally, a three-dimensional truss model was presented and analyzed. The results of the three-dimensional model showed good agreement with the experimental findings.