International Concrete Abstracts Portal

Externally bonded carbon fiber reinforced polymer (CFRP) is a feasible and economical alternative to traditional methods for strengthening and stiffening deficient reinforced and prestressed concrete bridge girders. The behavior of bond between FRP and concrete is the key factor controlling the behavior of these structures. Several experiments showed that debonding failure occurs frequently before FRP rupture and therefore the FRP strength can not be fully utilized. For design accuracy, the FRP strength must be reduced. This paper analyzes the effect of the bond properties on the response and failure modes of FRP-strengthened RC beams. A nonlinear RC beam element model with bond-slip between the concrete and the FRP laminates is used to analyze a test specimen subjected to monotonic and cyclic loads typical of seismic excitations, and to investigate the corresponding failure mode, and whether it is due to FRP rupture, debonding, or concrete crushing. The model is considered one of the earliest studies to numerically evaluate the behavior of FRP-strengthened girders under seismic loads. The model was also used to study the reduction factor of FRP tensile strength of simply supported strengthened RC girders due to debonding failure. This reduction factor seems to be directly affected by the bond strength between FRP and concrete interface. The study concludes with a numerical evaluation of the current ACI-440 guidelines for bond reduction factors.

The effectiveness of discrete and continuous CFRP wrapping arrangements for reinforced concrete (RC) short column subjected to monotonic and cyclic compressive loading is assessed in this work. The experimental program is composed of four series of RC columns with discrete wrapping arrangements and one series of full wrapped RC columns. Each series is composed of a monotonic and a cyclic test. Strain gauges were installed along the height of each column to measure the strain field in the CFRP during the test. The variation of the stiffness of the unloading and reloading branches of each loading cycle was determined. A constitutive model to simulate FRP-confined RC concrete elements subjected to cyclic compressive loading was developed and implemented into a computer program based on the finite element method. This model was appraised with the data obtained from the carried out experimental program.

A simple hysteretic model is proposed to define the cyclic response of reinforced concrete (RC) beam to column joints retrofitted with (carbon fiber reinforced polymer (CFRP) composites. This model includes the option to consider strength degradation resulting from damage within the joint region. The model consists of two nonlinear springs connected in series and positioned at the ends of linear elastic beam elements. The hysteretic models representing these springs were empirically derived from beam to column joint specimens retrofitted with CFRP composites and are capable of describing the hysteretic characteristics of reinforced concrete members in the column as well as in the joint hinge region. The model was subsequently used to evaluate the response of two test units that were retrofitted with CFRP composites at different damage levels. Key experimental results along with the proposed models and simulation results are presented and discussed in this paper. The analytical results were able to reproduce reasonably well the experimental data.

The paper discusses the potential use of fiber reinforced polymer composites for repair and retrofit of existing reinforced concrete (RC) column-tie beam assemblies. Results of an experimental program performed on large-scale specimens repaired and strengthened with two types of wet lay-up composite systems are presented. Each column-tie beam assembly specimen was subjected to a constant axial load simulating gravity loads, and incremental cyclic lateral loads simulating potential seismic forces. Displacements, strains and loads were continuously monitored and recorded during all tests. Evaluations of the observed strength and ductility enhancements of the strengthened specimens are made and limitations of such retrofit methods are highlighted for design purposes. Experimental results indicated that the two composite systems used in this study succeeded in enhancing the strength, stiffness and the ductility of the column-tie beam assembly. As compared to the unstrengthened specimens, the strengths of the retrofitted specimens were 152% and 154% for carbon/epoxy and E-glass/epoxy composite systems, respectively.

Development of the full inelastic lateral capacity of a reinforced concrete pile shaft is likely to require the formation of a plastic hinge below grade level. It has been shown through analytical and experimental investigation that the soil around the pile has a significant confining effect on the pile shaft, allowing the development of larger plastic strains in the compression zone than would be predicted based on the amount of transverse reinforcement provided. It was postulated that this confining effect could be built into precast prestressed piles by the addition of a GFRP jacket in the potential plastic hinge region during the construction process. Two large-scale prestressed pile specimens were thus fitted and tested in flexure to simulate a typical subgrade moment pattern. The piles exhibited higher flexural strength and significantly lower ductility capacity than a control specimen which did not have a GFRP jacket. Failure was through complete tendon rupture at a wide flexural crack which opened at the point of maximum moment. High clamping pressures from the jacket upon the tendons were caused by dilation of the compression zone. This pressure ‘anchored’ the tendons under the jacket, preventing bond slip over a wide region and forcing large inelastic strains into the short tendon length exposed at the major flexural crack. The ACI 318 equation for development length was found to give a reasonable quantitative prediction of the enhanced bond strength, expressed as reduced flexural transfer (i.e., development) length of the tendons by considering active confining pressure.