Hayder A. Rasheed

Behavior of reinforced concrete beams strengthened with externally bonded hybrid fiber reinforced polymer systems

Abstract This paper presents an experimental and an analytical investigation of the behavior of Reinforced Concrete (RC) beams strengthened in flexure by means of different combinations of externally bonded hybrid Glass and Carbon Fiber Reinforced Polymer (GFRP/CFRP) sheets. In order to obtain the mechanical properties of the hybrid sheets, multiple tensile coupon tests were conducted. In addition, an experimental program consisting of a control beam and four beams strengthened in flexure with GFRP, CFRP and hybrid FRP sheets was conducted. The series of the RC beams were tested under four point bending to study the flexural effectiveness of the proposed hybrid FRP sheets. The load–deflection response, strain readings at certain locations and associated failure modes of the tested specimens had been recorded. It is observed that the increase in the load capacity of the strengthened beams ranged from 30% to 98% of the un-strengthened control RC beam depending on the combination of the Carbon/Glass sheets. It was also observed that the ductility at failure loads of the beams strengthened with glass and hybrid sheets is higher than that with a single carbon sheet. Hence, the selection of the optimum combination of hybrid sheets can lead to a strengthening material which provides an improved ductility and strength in beam behavior. The load carrying capacity of the tested specimens was then predicted by the ACI 440.2R-08 guidelines. The predicted and measured results were in good agreement, within 5% for the control beam and for beams with one layer of strengthening sheet and between 13% and 17% for beams with two or more layers of hybrid strengthening sheets. Furthermore, an analytical model was developed to predict the load–deflection response of the tested specimens and the results were compared with the measured experimental data. The results showed that the developed analytical model predicted the response of the tested beam specimens with reasonable accuracy.

RIGOROUS PROCEDURE FOR CALCULATING DEFLECTIONS OF FIBER-REINFORCED POLYMER-STRENGTHENED REINFORCED CONCRETE BEAMS

The need to upgrade the capacity of structural members is growing due to the aging infrastructure elements and the expanding volume of traffic. The use of external fiber-reinforced polymer plates bonded to the tensile face of beams has proven to be effective in increasing the ultimate strength. With this in mind, this paper develops a rigorous analytical procedure for calculating the deflection of simple beams at any load stage. A parametric study is conducted to examine the relevance of the ACI original and modified equations for a wide range of geometric and material properties as well as different loading conditions.

Delamination growth in long composite tubes under external pressure

Delamination growth is a phenomenon known to reduce the integrity of laminated composite structural elements and may lead to premature failures. In the present study, state of the art procedures of delamination growth analysis are overviewed. The energy release rate calculation is formulated for composite delaminated tubular cross sections and specialized to a finite element model for delamination buckling and growth analysis of long laminated composite tubes taking into account initial geometric imperfections, large deformations, contact between delamination faces and material degradation. It is, then, used to study the potential of delamination growth in a hybrid composite tube. Parametric studies are conducted to assess the effects of delamination length, location and geometric imperfection on growth.

Closed form equations for FRP flexural strengthening design of RC beams

FRP external strengthening has proven its success in structural rehabilitation and upgrade. Researchers and practicing engineers are working towards introducing its design procedures to standard codes of practice. The current state of the art flexural design process suggests an iterative approach, which may lead to tedious calculations. This paper provides a direct approach furnishing calculation simplicity and design efficiency. It also proposes equations to design doubly strengthened sections for the first time. The expressions derived for doubly reinforced rectangular and Tee sections are written in a compact form based on those formulated for singly reinforced rectangular sections. Verifications against experimental results are performed. The solution, using the closed form equations, is compared to that of other procedures available in the literature through design examples. Tee section design is also presented and illustrated through a comparison with an analysis example. A doubly strengthened beam design example is also solved. The prevention of premature FRP plate separation using U-wraps to develop the full flexural capacity is also discussed.

Thermal-Stress Finite Element Analysis of CFRP Strengthened Concrete Beam Exposed to Top Surface Fire Loading

A detailed 3D time domain transient thermal-stress finite element analysis is performed to study the heat transfer mechanism within a CFRP strengthened reinforced concrete beam for fire conditions initiating at the top of the beam. This loading scenario has not been investigated previously either experimentally or analytically. Accordingly, a reinforced concrete T-beam strengthened with CFRP and fire-tested by other investigators is modeled here to compare the fire rating of top and bottom exposure. The finite element results correlated very well with the experimental measured results for the bottom fire exposure. In addition, the investigation of the top fire exposure yielded important findings on the resistance of concrete beams when subjected to such fire conditions. It is concluded that heating the top surface (slab) of reinforced concrete beams seems to be beneficial in minimizing mid-span deflection.

Suppressing Delamination Failure Mode in Concrete Beams Strengthened with Short CFRP Laminates.

This study is intended to examine the suppression of delamination failure for short CFRP laminates using wide transverse CFRP U-Wrap anchors. To achieve this objective, four beams were designed, fabricated, cast, and tested. Two beams serve as control specimens with different concrete strength and primary steel yield strength. The other two beams serve as strengthened specimens where they are twins of the two control beams and they both have identical flexural and anchorage strengthening reinforcement. The beams were tested in three-point bending. The control specimens (beam 1 and 2) failed in flexure. The capacity of these beams was 33 kN and 25 kN respectively. Beams 1FRP and 2FRP failed in vertical shear cracking at the laminate tip due to the shear stress concentration at 56.5 kN and 65 kN respectively. Beam 2FRP was very close to failing in flexure. It was observed that horizontal shear and diagonal shear cracking failure modes have been suppressed by the U anchors due to the intersection of fibers to these orientations. Vertical shear was the only weak plane since it was parallel to transverse fibers. Accordingly, the strengthened beams failed in that mode. To further suppress vertical shear cracking, bidirectional fabric is proposed to be used in future experiments.

A new eccentricity-based simulation to generate ultimate confined interaction diagrams for circular concrete columns

Abstract The analysis of circular concrete columns using unconfined concrete models is a well established practice. However, there is a necessity to develop realistic analysis and design tools that predict the extreme ultimate capacity of such columns since modern codes and standards like AASHTO LRFD are introducing extreme load events. The increase in strength and ductility due to full axial confinement is not applicable to pure bending and bending plus axial load simply because the area of effective confined concrete is reduced. The higher the eccentricity the smaller the compressed portion of the confined core. Accordingly, the ultimate confined strength is gradually reduced from the fully confined value fcc′ (at zero eccentricity) to the unconfined value fc′ (at infinite eccentricity) as a function of eccentricity to diameter ratio. A numerical analysis algorithm is developed using the finite layer procedure and the secant stiffness approach within a framework of incremental-iterative moment of area computations. The resulting nonlinear section analysis requires radial loading in which the eccentricity is kept constant or the axial load is proportional to the applied moment. The results are compared with existing experimental data and the widely used Mander model to benchmark the present predictions.

Simplified nonlinear analysis to compute neutral axis depth in prestressed concrete rectangular beams

Abstract Recent research in concrete analysis and design has revealed that the shear capacity contributed by concrete correlates well with the neutral axis depth. While nonlinear analysis calculation of the neutral axis depth is trivial for reinforced concrete beams, it is iterative for prestressed concrete beams and does not lend itself to straightforward hand calculations. In this study, a program is developed to simulate the response of prestressed concrete rectangular sections subjected to monotonic bending taking into account cracking, yielding and ultimate states. This program is used to benchmark a simplified analytical procedure devised to perform the same task by hand. Accordingly, critical observations made to a large pool of experimental and analytical results reveal that the moment–curvature and moment–extreme fiber strain can be accurately modeled as trilinear relationships. The four key points that define the trilinear functions (initial, cracking, yielding and ultimate) may be computed analytically from simple equations derived based on consistent assumptions with the true behavior. Once the simplified analysis is performed, the computation of the neutral axis depth becomes a simple hand calculation. A parametric study was performed to further simplify the analytical procedure by computing the four key points that define the trilinear functions through linear relationships that were derived based on regression analysis of a large number of beam solutions. The neutral axis depth was calculated using the analytical and the simplified procedures for three beams with different ratios of prestressing steel and concrete strength. The results compared well with the iterative numerical procedure.

Analytical Solution of Interface Shear Stresses in Externally Bonded FRP-Strengthened Concrete Beams

AbstractThe fiber-reinforced polymer (FRP) plate or sheet debonding or cover delamination (concrete cover separation) failure mode in externally strengthened reinforced concrete beams has attracted a lot of attention. In this paper, a closed-form analytical solution is developed to determine the nonlinear shear stress distribution along the laminate interface and cover area for any load stage assuming a perfect bond. Trilinear moment-curvature and moment-extreme compression fiber strain is assumed to realize the analytical results. By differentiating the FRP axial tension force with respect to position along the beam, closed-form derivatives in terms of curvature and extreme compressive fiber strain are obtained. The results show three distinct regions of constant or stepwise linear shear distribution in each. These correspond to the uncracked, postcracked, and postyielded zones of the shear span. The results are shown to yield an exact match to those numerically obtained by dividing the shear spans into ...

Analytical and Finite Element Buckling Solutions of Fixed–Fixed Anisotropic Laminated Composite Columns Under Axial Compression

A generalized analytical formula is developed to predict buckling of anisotropic laminated composite fixed–fixed thin columns by using the Rayleigh–Ritz displacement field approximation. Based on the generalized constitutive relationship, the effective extensional, coupling and flexural stiffness coefficients of the anisotropic layup are determined using dimensional reduction by static condensation of the 6×6 composite stiffness matrix. The resulting explicit formula is expressed in terms of the flexural stiffness since the coupling and extensional stiffness coefficients drop out of the formulation for this boundary condition when following the standard Rayleigh–Ritz formulation steps. This formula is similar to the Euler buckling formula in which the flexural rigidity is expressed in terms of the flexural stiffness coefficient of laminated composites. Motivated by reducing some of the discrepancy with the finite element results, the pre-buckling solution was substituted into the bifurcation expression to yield an updated formula that includes the coupling and extensional stiffness coefficients. The analytical results are verified against finite element Eigen value solutions for a wide range of anisotropic laminated layups yielding high accuracy. A parametric study is then conducted to examine the effect of ply orientation and material properties including hybrid carbon/glass fiber composites. Relevance of the numerical and analytical results is discussed for all these cases. In addition, comparisons with an earlier buckling solution for cross-ply laminated columns are made.