Mohsen Shahawy

Tests and modeling of carbon-wrapped concrete columns

Abstract There is an urgent need for models that can accurately predict performance of fiber-wrapped concrete columns. Axial compression tests on a total of 45 carbon-wrapped concrete stubs of two batches of normal and high-strength concrete and five different number of wraps were used to verify a confinement model, which was originally developed for concrete-filled glass FRP tubes. Also, a nonlinear finite element model with a non-associative Drucker–Prager plasticity was developed. Both models compared favorably with test results. It was concluded that the adhesive bond between concrete and the wrap would not significantly affect the confinement behavior. Moreover, the same confinement model can be applied to carbon and glass fibers, as long as the model has incorporated the dilation tendency of concrete as a function of the stiffness of the jacket. However, it is of utmost importance to establish the effective hoop rupture strain of the wrap through a reliability analysis by setting proper confidence level for design purposes.

A new concrete-filled hollow FRP composite column

An effective use of fiber reinforced plastic (FRP) shapes in infrastructure is in the form of composite construction with reinforced concrete. A novel composite column is proposed that is similar to the classic concrete-filled steel tubes, except that steel has been replaced with a hollow FRP shell. The FRP shell, while an integral part of the structure, is also the pour form for concrete. The shell may be a filament-wound or a multi-layer FRP pipe with a layer of longitudinal fibers sandwiched between two piles of circumferential fibers. The proposed column offers high strength and ductility in addition to excellent durability. Behavior of the proposed column is studied by developing two analytical tools; a new passive confinement model for externally reinforced concrete columns, and a composite action model that evaluates the lateral stiffening effect of the jacket. Results are compared with recent studies of fiber-wrapped columns.

PERFORMANCE OF REINFORCED CONCRETE T-GIRDERS STRENGTHENED IN SHEAR WITH CARBON FIBER-REINFORCED POLYMER FABRIC

Results are presented from an experimental investigation into the performance of 20-ft-long reinforced concrete (RC) T-girders strengthened in shear using epoxy-bonded bidirectional carbon fiber reinforced polymer (CFRP) fabric. The aim was to evaluate and gain insight into the effectiveness of shear strengthening of large-scale girders with externally bonded CFRP under a low shear span condition. Four series of tests, corresponding to stirrup spacings of 5.5, 8, 16, and 24 in, were considered. Each series of girders included control specimens with no CFRP wrap and specimens retrofitted in shear with 1, 2, and 3 layers of CFRP wrap. Results indicate that for unwrapped specimens, values for nominal shear predicted by ACI underestimated, by 40-80%, the shear resistance of beams developing arch action, such as those considered herein. For wrapped specimens, the maximum shear force as well as the midspan deflection generally increased with the number of CFRP layers. The optimum number of layers to achieve the maximum gain in shear resistance was found to depend on the internal shear steel reinforcement provided. The effective CFRP strain used to calculate the contribution of the CFRP to the shear capacity was correlated to the total shear reinforcement ratio consisting of steel stirrups and CFRP wrap. Retrofitting RC girders in shear with CFRP wrap also increased the ductility. Experimental evidence shows an optimum combination of CFRP layers and steel stirrups exists for a maximum increase in ductility.

PERFORMANCE OF FIBER-REINFORCED POLYMER-WRAPPED REINFORCED CONCRETE COLUMN UNDER COMBINED AXIAL-FLEXURAL LOADING

Many countries around the world have the tremendous need to repair and strengthen their existing infrastructure. This paper presents results of an experimental investigation into the performance of reinforced concrete beam-columns strengthened with externally applied bidirectional carbon fiber-reinforced polymer (CFRP) material. The external moment was applied to the specimens through corbels that were part of the columns. The overall length of the column specimens, including the corbels, was 11.8 ft (3.6 m). Six series of tests were performed on the specimens. The first five series, corresponding respectively to eccentricities of 0, 3, 6, 12, and 16 in. (0, 75, 150, 300, and 400 mm), were performed under a combined axial-flexural loading condition. The sixth series was tested in four-point pure flexure with no axial load. Results indicate that the strength capacity of beam-columns improved significantly as a result of the combined action of the longitudinal and the transverse weaves of the bidirectional composite fabric. The longitudinal CFRP elements contributed mostly to flexural capacity, whereas the transverse elements enhanced the compressive capacity of the compression zone through confinement action. The maximum capacity gain achieved was slightly below 30% in pure compression, and over 54% in pure flexure. Under combined axial force-bending moment conditions, the gain in moment capacity attained 70%. The increase in the compressive strain attributed to the confinement effect varied from 49% to 166%. The transverse confinement was engaged in the compression zone from the early stage of loading. Finally, within the conditions and the limits of this study, the proposed design procedure, based on the stress of confined concrete in the compression zone in conjunction with an effective confinement ratio that takes into account the rectangular shape of the beam-columns, compared reasonably well with experimental results.

Reinforced concrete rectangular beams strengthened with CFRP laminates

Abstract Flexural behavior of reinforced concrete rectangular beams with epoxy bonded carbon fiber reinforced plastic (CFRP) laminate is experimentally investigated. Comprehensive test data are presented on the effect of CFRP laminates, bonded to the soffit of a beam, on the first crack load, cracking behavior, deflections, serviceability loads, ultimate strength and failure modes. The increase in strength and stiffness provided by the bonded laminates is assessed by varying the number of laminates. The results generally indicate that the flexural strength of strengthened beams is significantly increased. Theoretical analysis using a specially developed computer software is presented to predict the ultimate strength and moment deflection behavior of the beams. The comparison of the experimental results with theoretical values is presented, along with an investigation of the failure nodes.

CONFINEMENT MODEL FOR AXIALLY LOADED SHORT RECTANGULAR COLUMNS STRENGTHENED WITH FIBER-REINFORCED POLYMER WRAPPING

A confinement model, describing the behavior of rectangular concrete columns retrofitted with externally bonded fiber-reinforced polymer (FRP) material and subjected to axial stress, is presented. The derivation of the proposed model is based on the findings of an extensive experimental investigation involving the testing of 90 rectangular specimens representing 3 cross-sectional aspect ratios, 2 concrete strengths, and 5 different numbers of FRP layers. The proposed model is trilinear, both in the axial and lateral directions, and captures the key parameter observed; namely, the ratio of the stiffness of the FRP jacket in the lateral direction to the axial stiffness of the column. The proposed model was found to fairly accurately describe results reported in other similar research studies.

LARGE BEAM-COLUMN TESTS ON CONCRETE-FILLED COMPOSITE TUBES

The concept of concrete-filled composite tubes was developed to address the corrosion problems associated with reinforced and prestressed concrete piles. Beam-column tests on a total of 16 2.75-m specimens demonstrated the feasibility of off-the-shelf fiber-reinforced polymer products. Two types of tubes were used: spin-cast (I) and filament-wound (II). Based on their respective brittle-brittle reinforcement ratios, Type I and II specimens were considered as over-reinforced and under-reinforced concrete sections, respectively. The tests showed that over-reinforced specimens performed superior as beam-columns. They deflected to a lesser extent and failed at much higher axial and lateral loads, while their failure was still gradual and ductile. They were also more efficient, as a smaller portion of their sectional capacity was consumed by secondary moment effects. Bond failure was not an issue in beam-columns. Therefore, off-the-shelf tubes can be used as long as end conditions and connections are properly designed. It is necessary, however, to provide a shear transfer mechanism for beams. In comparison with prestressed concrete columns, the 348 mm diameter Type I specimens were found comparable to 584 mm diameter circular sections prestressed with 20 strands of Grade 1862 MPa, whereas the 369 mm Type II specimens were found comparable to 460 mm square sections prestressed with eight strands.

Experimental investigation on structural repair and strengthening of damaged prestressed concrete slabs utilizing externally bonded carbon laminates

This paper presents the results of a feasibility study to investigate the flexural behavior of structurally damaged full-scale pretensioned concrete slabs retrofitted with bonded carbon fiber reinforced plastic (CFRP) laminates. The effect of CFRP laminates bonded to the soffit of precracked solid and voided slabs is investigated in terms of flexural strength, deflections, cracking behavior and failure modes. The results, generally, indicate that strengthening of significantly cracked slabs by bonding CFRP laminates is structurally efficient and that the retrofitted slabs are restored to stiffness and strength values nearly equal to or greater than those of the original undamaged slabs. The results indicate that the retrofitted slabs maintained adequate structural integrity and composite action at all stages of testing up to failure.

Dynamic behavior of horizontally curved I-girder bridges

Abstract The purpose of this paper is to investigate the dynamic behavior of horizontally curved I-girder bridges due to one or two trucks (side by side) moving across rough bridge decks. The bridge is modeled as a planar grillage beam system composed of horizontally curved beam elements and straight beam elements. Warping torsion is taken into consideration in the analysis. The analytical vehicle is simulated as a nonlinear vehicle model with 11 independent degrees of freedom according to the HS20–44 truck design loading contained in the American Association of State Highway and Transportation Officials (AASHTO) specifications. Four different classes of road surface roughness generated from power spectral density function for very good, good, average, and poor roads are used in the study. The analytical results are very significant and show that the dynamic behavior of curved I-girder bridges is quite different from that of straight girder bridges. The impact factors of bending and shear for inside girders of curved I-girder bridges are significantly smaller than those for outside girders.

Analysis and field tests on the performance of composite tubes under pile driving impact

Abstract Composite tubes provide a feasible alternative to concrete piles by eliminating formwork, reinforcing cage, and additional corrosion-deterrent cover. Field driving of concrete-filled composite tubes showed no damage to the tube or concrete. Driving stresses in filled tubes were found comparable to those for prestressed concrete piles. Empty tubes may buckle or rupture under driving impact, unless driven at shallow depths and in soft soils, or with a steel mandrel. A detailed parametric study using the wave equation further confirmed that there is no difference in the drivability of filled FRP tubes and prestressed concrete piles of the same cross-sectional area and concrete strength. The typical refusal rate for conventional concrete piles can be safely adopted for filled tubes. However, empty tubes are susceptible to compression failure, and can only endure diving stresses up to 40–50% of the refusal rate of concrete piles.