Journal of Composites for Construction

Repair, strengthening, and retrofit of reinforced and prestressed concrete members have become increasingly important issues as the World’s infrastructure deteriorates with time. Buildings and bridges are often in need of repair or strengthening to accommodate larger live loads as traffic and building occupancies change. In addition, inadequate design and detailing for seismic and other severe natural events has resulted in considerable structural damage and loss of life, particularly in reinforced concrete buildings. Numerous buildings and bridges suffer damage during such events and need to be repaired. The use of carbon fiber reinforced polymer (CFRP) composite fabric bonded to the surface of concrete members is comparatively simple, quick and virtually unnoticeable after installation. The use of composites has become routine for increasing both the flexural and shear capacities of reinforced and prestressed concrete beams. Earthquake retrofit of bridge and building structures has relied increasingly on composite wrapping of columns, beams and joints to provide confinement and increase ductility. This paper presents the results of cyclic testing of three large-scale reinforced concrete slab–column connections. Each of the specimens was a half-scale model of an interior slab–column connection common to flat-slab buildings. The specimens were reinforced according to ACI-318 code requirements and included slab shear reinforcement. While supporting a slab gravity load equivalent to dead load plus 30% of the live load, the specimens were subjected to an increasing cyclic lateral loading protocol up to 5% lateral drift. The specimens were subjected to the same loading protocol after they were repaired with epoxy crack sealers and CFRP sheet on the surfaces of the slab. Repair with epoxy and CFRP on the top surface of the slab was able to restore both initial stiffness and ultimate strength of the original specimen.

The present work reports the test results of seven full-scale reinforced concrete slab-column edge connections strengthened against punching shear using different methods. In this study, three slabs contained openings in the vicinity of the column, and the other four were without openings. The dimensions of the slabs were 1,540×1,020×120 mm with square columns (250×250 mm). The openings in the specimens were square (150×150 mm) with the sides parallel to the sides of the column. The slabs were reinforced with an average reinforcement ratio of 0.75%. Except for the two reference slabs, two different strengthening techniques were considered. Technique I applies externally bonded fiber reinforced polymer (FRP) flexible sheets on the slab around the column in two schemes with one or two layers of FRP sheets glued to the tension face or both tension and compression faces of the slab. Both glass and carbon FRP sheets were considered. Technique II applies externally bonded FRP sheets using either the first or second scheme combined with installing steel bolts through holes across the slab thickness around the column. Based on the test results, it is concluded that the presence of FRP sheets and steel bolts substantially increased the punching capacity of the connections. Code design expressions were conservative in predicting the experimental results.

Six prototype one-way RC slabs with openings were strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) systems and subjected to concentrated line loads. The results were compared to those of a solid slab without opening and a slab with an unstrengthened opening. The CFRP system proved to be effective in enhancing the load-carrying capacity and stiffness of RC slabs with an opening, provided that premature failure due to CFRP debonding is excluded. An analytical model based on the modified yield line method is presented, in which four failure modes comprising one orthogonal and three nonorthogonal yield line patterns were considered. The analytical model predicts the load carrying capacity of the strengthened slabs very well. The opening width has a more prominent effect on the load-carrying capacity than does the opening length.

For reinforced concrete beams with the same shear and flexural reinforcements, shear failure is most likely to occur in deep beams rather than in regular beams. Thus, retrofitting of deep beams with shear deficiencies is of great importance. Externally bonded reinforcement such as carbon fiber reinforced polymer (CFRP) provides an excellent solution in these situations. In order to investigate the shear behavior of deep beams with externally bonded CFRP shear reinforcement, 16 deep beams without steel shear reinforcement were cast at the concrete laboratory of New Jersey Institute of Technology. After the beams were kept in the curing room for 28 days, carbon fiber strips and fabrics were applied outside of the beams at various orientations with respect to the axis of the beam. All beams were tested on a $979\mathrm{kN}$ $\left(220\mathrm{kip}\right)$ MTS testing machine. Results of test demonstrate the feasibility of using externally applied, epoxy-bonded CFRP system to restore or increase the shear capacity of deep beams. The CFRP system can significantly increase the serviceability, ductility, and ultimate shear strength of a concrete beam, thus restoring deep beam shear strength using CFRP is a highly effective technique. An analysis and design method for shear strengthening of deep beams using externally bonded CFRP has also been proposed as well.

The paper is principally aimed at analyzing the role of externally applied fiber reinforced plastic (FRP) sheets in the shear ultimate behavior of reinforced concrete elements. A theoretical model for predicting the shear resisting contribution of FRP sheets is illustrated. The proposal is based on a complete equilibrium/compatibility approach for reinforced concrete beams failing in shear and considers the possible interactions between the composite contribution and the resisting mechanisms of an ordinary reinforced concrete beam. The proposal is discussed and tested by means of an experimental investigation carried out on beams reinforced by glass FRP composite sheets with a shear span to depth ratio equal to 3. Further comparisons are then performed that consider the predictions of other existing approaches reported in the literature.

This paper presents a simplified shear design method for reinforced concrete beams strengthened externally with fiber reinforced polymer (FRP) sheets. This design method combines both the strip method and the shear friction approach. The background of the strip method is presented in detail, including the interface shear strength curve, which is compared with some available bond test data found in the literature. A parametric study is performed to propose two simplified equations, which describe the FRP sheet contribution. This contribution is added to the discrete shear friction formulation and, by derivation, a continuous and simplified design equation is proposed. This method well describes the interaction between the concrete, the stirrups, and the FRP sheets. A variable concrete crack angle is used, which enhances the accuracy of the model. No iteration is required. The proposed design formulation gives conservative predictions with 35 experimental test results found in the literature.

This paper investigates strengthening masonry walls using glass-fiber reinforced polymer (GFRP) sheets. An experimental research program was undertaken. Both clay and concrete brick specimens were tested, with and without GFRP strengthening. Single-sided strengthening was considered, as it is often not practicable to apply the reinforcement to both sides of a wall. Static tests were carried out on six masonry panels, under a combination of vertical preload, and in-plane horizontal shear loading. The mechanisms by which load was carried were observed, varying from the initial, uncracked state, to the final, fully cracked state. The results demonstrate that a significant increase of the in-plane shear capacity of masonry can be achieved by bonding GFRP sheets to the surface of masonry walls. The experimental data were used to assess the effectiveness of the GFRP strengthening, and suggestions are made to allow the test results to be used in the design of sheet GFRP strengthening for masonry structures.

At present, fiber reinforced polymer (FRP) composite materials are extensively used to strengthen concrete structures and a main application is wrapping compression members such as building and bridge columns for improved strength and ductility. In this case, FRP laminates are intended to provide confinement to the concrete and the cross section shape plays an important role on the effectiveness of the method. The primary purpose of this paper is to introduce a test device and a test method designed to determine the effect of corner radius on the strength of the FRP laminate and on the distribution of the resulting radial stress on the substrate material. Various curvatures were investigated. In the proposed device, they can be realized by using interchangeable inserts. Strain distribution around the corner, failure load, and failure mode of the FRP laminate were monitored and analyzed. The stress concentration in the laminate is studied numerically using the finite element method and compared with experimental results. The relationship between radial stress distribution and corner radius is determined to provide guidance in practical cases.

Seven beams were tested in bending to determine the concrete contribution to their shear resistance. The beams had similar dimensions and concrete strength and were reinforced with carbon fiber reinforced polymer bars for flexure without transverse reinforcement. They were designed to fail in shear rather than flexure. The test variables were the shear span to depth ratio, varying from 1.82 to 4.5, and the flexural reinforcement ratio, varying from 1.1 to 3.88 times the balanced strain ratio. The test results are analyzed and compared with the corresponding predicted values using the American Concrete Institute, the Canadian Standard, and the Japan Society of Civil Engineers (JSCF) fiber reinforced polymer design recommendations. Based on these results and previous experimental data, it is shown that the ACI recommendations are extremely conservative whereas the Canadian and JSCE recommendations, albeit still conservative, are in closer agreement with the experimental data. Overall the Canadian Standard’s predictions are in better agreement with experimental data than the JSCE predictions.

An experimental program has been carried out to investigate the structural behavior of adhesively bonded glass fiber reinforced polymer (GFRP) sandwich girders. The girders, conceivable for main spans up to 20 m, are composed of translucent double sandwich element webs and adhesively bonded pultruded shape flanges. The continuous adhesive connections between web and flanges are loaded in a highly favorable manner without peeling stresses. Despite the complex load-carrying and failure behavior of the girders, simple calculation models can be applied. The successful girder experiments allowed for the development of a material-adapted construction method for new GFRP bridges and buildings. The integration of architectural aspects such as transparency, translucency, lighting, and color, as well as building physical aspects for buildings such as thermal insulation, substantially increases the overall value of the proposed constructions. Together with the lower life-cycle costs, this further justifies these new materials’ higher initial costs as compared with traditional materials.

The time-dependent creep behavior of pultruded composite sheet piling was investigated. Two panels were tested under equally spaced third point bending at a span to depth ratio of 48; one was subject to a constant load of 50% of $P_{\max }$ and the other 25% of $P_{\max }$. Tensile creep, shear creep, and deflection creep were recorded over 1 year. The time-dependent tensile and shear moduli were obtained using the simplified Findley’s model, and the deflection creeps were predicted based on both Findley’s model and Timoshenko’s equation. It was found that the time exponents in Findley’s model for tensile, shear and deflection creep were of close value and could therefore be averaged to provide a viscoelastic material constant for the composite sheet piling. With the averaged viscoelastic parameters, Timoshenko’s equation resembled the Findley’s power law model for the prediction of deflection creep and agreed well with experimental results up to 1 year. Over 30 years, it is estimated that the viscoelastic tensile and shear moduli will be reduced to 68 and 36% of their respective initial values and the creep deflection will reach 50% of its static deflection.