Seismic Retrofitting of Corroded Reinforced Concrete Columns Using Carbon Composites

ACI Structural Journal, May/Jun 2007 by Aquino, Wilkins, Hawkins, Neil M

Column 1: Retrofitted with continuous CFRP jacket-The load-deformation plot for Column 1 is shown in Fig. 5(f). The hysteretic response for this column was similar for both directions and the column showed stiffness degradation early in its loading history. The response showed no distinctive yield point. Successive load peaks increased following a nonlinear trend until a maximum load of 120 kN (27 kip) and a corresponding displacement of 43 mm (1.7 in.) were reached. For greater displacements, the response showed increasing softening and the test was stopped when the load dropped to 70 kN (15.7 kip). Marked pinching characterized the load-deformation response from early stages of loading. The cable transducers attached to the column indicated negligible rigid body translation or rotation during the test. Hence, it can be inferred that the pinching was caused mainly by bond-slip failure. The strain gauges installed on the steel bars indicated that the reinforcement did not reach its yield point, confirming bond-slip problems.

Load-circumferential strain plots for the CFRP jacket are shown in Fig. 6(f). Strains started to increase rapidly at the relatively low load of 50 kN (11 kip). A maximum strain of 0.2% was registered by the lower gauge and of 0.13% by the upper gauge, despite the relatively low load applied to the column. Again, this result indicates that a bond-slip type of failure occurred.

DISCUSSION

Bond degradation dependent on the extent of corrosion was the primary factor controlling the structural response of the specimens. Figure 7 conceptually illustrates the extent of the damage in the concrete surrounding the column and dowel bars as a result of the corrosion process. Only column bars were directly connected to the external power source so that dowel bars experienced little damage from direct corrosion. The lateral load capacity of Column 4 (corroded and not repaired), however, was 80% of the capacity of the control column, whereas the maximum displacement was only 25% of that of the non-corroded control column. These facts, along with observations of the interior of the specimen after cyclic testing, document that damage in the concrete extended to the exterior surface of the dowel bars, and that there was degradation of the concrete between the dowel and column bars.

The degradation of Column 4 can be explained as follows. During loading, tensile stresses are transferred from dowel bars to column bars through shear stresses in the concrete between bars. Limited capacity exists in corroded columns to carry these shear stresses because of cracking and accumulation of rust products in the concrete between dowel and column bars. Axial stresses in column and dowel bars cause radial stresses in the surrounding concrete due to the interlocking of bar lugs and concrete. These radial stresses in turn widen the existing cracks produced by the build up of corrosion products, resulting in further loss of bond and causing loss of column flexural capacity.