Performance of RC Columns Retrofitted with UHPFRC Jackets Under Cyclic Displacement Reversals

It is the purpose of this study to evaluate, through combined experimental and analytical research, the performance of reinforced concrete columns with or without prior damage, after retrofitting with ultra-high-performance fiber-reinforced concrete (UHPFRC) jackets and subjected to cyclic loading that simulates earthquake effects. A series of conventionally designed RC column elements were constructed, based on detailing methodologies of older code standards. Preliminary damage was imposed, simulating the effects of previous seismic excitation on the columns, and the plastic hinge zone was repaired through cover replacement with a thin commercial-grade UHPFRC jacket, as well as bond breakers placed on the starter bars of the lap-spliced connection. Through experimental testing, the behavior of the columns was studied under reversed lateral cyclic displacements with the objective to obtain quantified evidence of the performance of the retrofit methodology. The UHPFRC-retrofitted columns demonstrated substantial improvement in strength and ductility, over the pre-damaged columns, where the repaired and strengthened specimens showed similar failure modes and comparable responses. Detailed FEA simulations of the experimental program were conducted, where cyclic performance of the UHPFRC-retrofitted models was defined in accordance with the pertinent experimental data, and the broader distribution of reinforcement strains was validated.

Companion small-scale material characterization experiments were prepared, to obtain the constitutive hysteretic properties of concrete confined with UHPFRC jackets, under axial monotonic and cyclic loading regimes, where the remarkable enhancement of the ductile response of the encased concrete was illustrated. Analytical models based on classical mechanics were applied, and the constitutive material laws were formulated, based on parameter calibration with the results collected from the experimental component.

An extensive database on bond experiments from the literature was assembled, including specimens fabricated with UHPFRC and engineered cementitious composites (ECC). The database reveals the existing trend of discrepancy in the experimentally measured bond, dependent on specimen geometry. The results of a combined experimental study, containing direct tension pullout (DTP) and anchored four-point bending (AFPB) using UHPFRC-fabricated specimens confirm the passive contribution of curvature in bond strength, and parametric sensitivity FEA was conducted, where the spurious lateral effects occurring on the various bond test setups were captured.