To improve the confinement effectiveness of FRP composites for square andrectangular columns, shape modification is performed by using prefabricated FRP shellscombined with expansive cement concrete. Chemical post-tensioning using expansivecement concrete is used to change the FRP confinement from “passive” to “active”.Experimental results are presented demonstrating the effectiveness of this method. Ananalytical stress-strain model is developed for shape-modified FRP-confined columnswith expansive cement concrete which is based on the modified Willam-Warnkeplasticity model, the Popovics general stress-strain concrete model, and the dilatancybehavior obtained from the present study. This model is implemented by anincremental approach which accounts for the variable FRP confinement during theloading process. The analytical results show satisfactory agreement with theexperiments.

The sensitivity of the FRP-concrete bond failure load to changes ingeometric and material parameters is described, and initial comparisons to predictionsfrom existing bond models are made. To accomplish this, load and strain data from aseries of single-lap pull-off tests is analyzed, in which carbon fiber reinforced polymer(CFRP) strips of varying width, thickness, and bonded length were pulled from concreteblocks of varying concrete strength. It was found that the concrete compressivestrength had limited effects on the bond failure load, and longer bonded lengthsincreased the time up to failure load. Changes to the bonded width and FRP thicknesshad a significant impact on the bond failure load. Failure load predictions produced bythree studied bond models were found to be strongly influenced by the materialproperties used as input, and were occasionally insensitive to the parameters varied.

The use of hollow-core reinforced concrete (RC) sections for bridge piers hasbecome a popular engineering practice to obtain a reduction of the self-weight(especially in seismic zones) and a better structural efficiency in terms of the strength/mass and stiffness/mass ratios. In contrast to this popularity in practice, scientificstudies on the mechanical behavior of such structural elements are limited.The use of Fiber Reinforced Polymer (FRP) materials for external confinement of hollowcore columns and piers is an almost unknown field at the moment. The research workpresented herein aims at evaluating the influence of various experimental parameterson the effectiveness of FRP jackets applied to hollow concrete columns.Hollow-core concrete prisms and cylinders were tested under uniaxial compression tostudy the stress-strain relationship before and after FRP jacketing. A range ofexperimental parameters were investigated: different concrete strength, type of fibers,number of wrap layers, column shape and dimensions, and for square and rectangularsections, the corner radius and the cross-sectional aspect ratio. Axial strain wasmeasured by LVDTs, while strains in the fibers were recorded by electrical straingauges.Circular columns wrapped with FRP showed a significant increase in terms of bothstrength and ultimate displacements. Results obtained by laboratory tests were closeto those recorded for FRP-confined concrete, which means that the increase in ultimateload was found to be comparable to that found in full circular sections. Rectangularcolumns showed a lower increase in ultimate capacity, compared to circular sections,even if the results related to ultimate axial displacement encourage adopting thistechnique for seismic retrofit to fulfill higher ductility requirements in both prismatic andcylindrical columns.

Fiber-reinforced polymers (FRPs) are effective in strengthening concretestructures. However, little work has examined the effects of cold regions on thebehavior of the strengthened members, particularly the combined effects of sustainedloading and freeze-thaw exposure. This paper presents the results of an experimentalstudy on the durability of 70 normal weight, low strength, and non-air entrainedconcrete cylinders (150 x 300mm). The cylinders were confined with glass-FRP (GFRP)sheets or carbon-FRP (CFRP) sheets and exposed to 300 freeze-thaw cycles while undersustained axial compression loads. FRP-wrapped cylinders showed exceptionaldurability performance after their extreme exposure to freeze-thaw and sustainedloading with a maximum of 12% reduction in strength. Some CFRP wrapped cylindersthat were exposed to freeze-thaw without longitudinal restraint, by means of sustainedloads, and all the plain concrete cylinders were completely disintegrated with virtuallyzero residual strength.

ACI Committee 440 has proposed a design approach for evaluating theconcrete contribution to the shear resistance of FRP-reinforced concrete beams thataccounts for the axial stiffness of FRP longitudinal reinforcement. Recent shear testsconducted on beams longitudinally reinforced with different types and ratios of FRPbars indicate that the current ACI 440.1R-03 shear design approach significantlyunderestimates the concrete shear strength of such beams. This paper presents aproposed modification to the ACI 440.1R-03 shear design equation. The proposedequation was verified against experimental shear strengths of 98 specimens tested todate, and the calculated values are shown to compare well. In addition, the proposedequation was compared to the major design provisions using the available test results.Better and consistent predictions were obtained using the proposed equation.