International Concrete Abstracts Portal

Externally bonded FRP has been established as the technology of choice tostrengthen RC beams. Researchers and practicing engineers have recently developeddesign guidelines for FRP strengthening. However, the current state of the art flexuraldesign procedure suggests an iterative process. No earlier efforts have been devotedto develop direct strength design equations on the failure mode of FRP rupture that canfacilitate structural calculations. This study develops exact and approximate sets ofclosed form equations to design singly and doubly reinforced strengthened rectangularsections that fail by FRP rupture. Comparisons with reported experimental strength dataindicate excellent agreement. A comprehensive parametric study has yielded a simplelinear regression equation that has an almost perfect statistical correlation and isequally applicable in cases of analysis and design. Comparison between the exactsolution and the regression equation confirms the accuracy of the latter. The latter isused in a design example.

Five square columns were constructed to model shear-deficient columnsand tested under constant axial compression and reversed cyclic lateral load,simultaneously. The retrofitting scheme consisted of wrapping the column along its endparts, i.e. around the plastic hinge area, by use of FRP in the form of three-centimeterwide belts. Both carbon and aramid/epoxy belts were used in this study. Moreover, fortwo of the specimens, the FRP belts were prestressed before applying the lateral loadto the columns, and thereby, the effects of active confinement in addition to passiveconfinement were investigated. The proposed prestressing technique is an innovativemethod and can be applied manually. According to test results, while the originalcolumn exhibited brittle shear failure, all retrofitted columns developed ductile flexuralresponse. Despite the different initial confining pressures, yet the same lateralstiffness of the confining device, the deformation ductility of all retrofitted columns wassimilar.

Limited information is known about the effects of environmental conditionsduring installation on quality and performance of the bond between carbon fiberreinforced polymer (CFRP) reinforcement and substrate material. This research studyinvestigates the effect of surface moisture, relative humidity and temperature on thebond strength between concrete and CFRP reinforcement. Three test methods includinga surface pull-off bond test, a surface shear-torsion bond test, and a flexural test wereused to evaluate the bond performance of the FRP fabric under various installationconditions. Test results revealed that the high surface moisture content, extremehumidity and extreme low temperature can be detrimental to bond strength. Althoughthe high temperature improved the bond strength, it is not recommended because ofdecreased set-time and saturant workability. Based on the results presented in thispaper, a maximum allowable limit on surface moisture content, relative humidity, andtemperature of 4.3%, 82%, and 90oF, respectively, at installation is recommended.

Fiber reinforced polymer (FRP) casings, in the form of stay-in-placeformwork, provide an attractive alternative to conventional confinement reinforcementfor concrete columns. These casings can fulfill multiple functions of; i) formwork, ii)confinement reinforcement, and iii) protective shell against corrosion, weathering andchemical attacks. This paper investigates the use of stay-in-place FRP formwork asconcrete confinement reinforcement for HSC columns with circular and square cross-sections. Large-scale specimens with 270 mm cross-sectional dimension and up to 90MPa concrete strength, were tested under combined axial compression andincrementally increasing lateral deformation reversals. FRP casings were manufacturedfrom carbon fiber sheets and epoxy resin. One of the square columns was providedwith internal FRP crossties, a new technique introduced by the authors, to provide well-distributed lateral restraints along the column face, thereby improving the mechanismof confinement. The results indicate that the deformation capacity of HSC columns canbe improved significantly by using FRP casings. The results further indicate that theconfinement effectiveness of casings of square columns is significantly affected by thecorner radius of casings. Confinement efficiency of these casings improves with the useof FRP crossties.

For under-reinforced concrete sections reinforced with FRP, failure of amember is initiated by rupture of the FRP bar and the typical ACI compression stress-block might not be applicable. This is because of the fact that the corresponding strainat the extreme fiber of the concrete will not reach the ultimate strain in concrete.Therefore, accurate computation of flexural capacity requires developing equivalentstress-block parameters that represent the stress distribution in the concrete at aparticular strain level. While the ACI 440 permits the use of a simplified approach tocalculate moment capacities that do not require equivalent stress-block calculations,the significance of this simplification needs to be examined. This paper suggests afamily of curves based on the extreme fiber strain in concrete using three existingstress-strain models. The paper highlights the significance of these curves for differentvalues of compressive strengths of concrete.