Seismic Retrofitting of Corroded Reinforced Concrete Columns Using Carbon Composites

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

It is usually reported in the literature that accelerated corrosion tests on small specimens result in unrealistic even distributions of corrosion products. This was not the case in this investigation. Column 4 exhibited a marked unsymmetrical hysteretic load-deformation response, clearly indicating that corrosion damage was unevenly distributed. Because uneven distribution of corrosion products is expected in real-life situations, the results of these experiments represent the potential behavior of corroded reinforced concrete columns in the field.

Past work in retrofitting corroded reinforced concrete elements with ACMs has concentrated on corroded columns subjected to axial loading or beams subjected to transverse loading only. During pure axial loading of corroded columns, wrapping the columns with ACMs increases the axial capacity of the damaged concrete through high confining stresses, as was observed by Lee et al.7 and Pantazopolou et al.10 The results reported in this article, however, show that if corrosion-damaged columns were repaired using ACMs and then subjected to lateral loading, the response mechanisms would be distinctively different from those reported in the literature.

Figure 8 shows the crack-damage pattern for corroded specimens observed in this research. This pattern is consistent with that reported by Lee et al.7 Direct installation of ACMs on the surface of a corroded column with the crack pattern of Figure 8 would be of little help in improving the structural behavior of columns subjected to lateral loading. Volumetric expansion of the concrete is required before the confinement provided by the ACMs is activated, which translates into further opening of existing cracks and, hence, reduced interlock and transfer capacity across the crack surfaces. Therefore, damaged concrete must be removed and replaced before a column is wrapped with ACMs.

Effectiveness of retrofit system

Columns 1, 2, 3, and 5 were repaired with CFRP. The load capacities of Columns 2 and 3 matched or exceeded the capacity of the control column (Column 6). The ductility of Column 2 was similar to that of Column 1, whereas the ductility of Column 3 greatly exceeded the ductility of Column 1. Column 5 could not be tested to its full capacity due to the premature failure of the grouting material in the square base. Column 1 developed a bond-slip failure prior to yielding of the reinforcing bars.

Although Columns 2 and 3 both had yield and maximum load capacities exceeding those of Column 6 (control specimen), their ductility capacities differed significantly. Column 2 exhibited marked pinching, signifying bond-slip degradation, shortly after reaching its yield load while Column 3 exhibited little pinching throughout the test. This was an unexpected result because the retrofitting schemes used were expected to yield similar structural responses. That is, the CFRP theoretically provided the same degree of confinement for all columns. The reason for this behavior was attributed to differences in the consolidation of the repair concrete placed in Columns 2 and 3. Crosssectional cuts performed on the columns after testing showed that the repair concrete for Column 3 was much better consolidated than that for Column 2.