Seismic Retrofitting of Corroded Reinforced Concrete Columns Using Carbon Composites

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

Current experimental studies on the behavior and seismic retrofitting of corroded reinforced concrete elements have been limited to small-scale specimens. To the best knowledge of the authors, the large-scale accelerated corrosion tests presented in this paper are the first of their kind.

EXPERIMENTAL PROGRAM

Specimen details

Six identical reinforced concrete columns 500 mm (20 in.) in diameter and 2400 mm (96 in.) in height were built in the laboratory. Laboratory specimen details are shown in Fig. 1. The columns were reinforced longitudinally with 12 No. 8 steel bars that were spliced to eight No. 8 steel bars protruding as dowel bars from a square base. The 750 mm (30 in.) extension of the dowel bars above the foundation beam duplicated the 30-bar diameter extension that is the standard extension for Illinois bridge columns constructed between 1950 and 1990. It is important to recognize that in regions of high seismic risk, current codes require 40-bar diameter extensions. A 30-bar diameter extension was used in this research to be consistent with the as-built condition of the prototype field columns. In a previous investigation,12 it was shown that the structural response of columns with inadequate lap-splice lengths can be successfully upgraded with the retrofitting schemes used in this research.

The compressive strength of the concrete used in the columns was 32 MPa (4600 psi) at 28 days. All reinforcing bars were Grade 60 steel. The concrete mixture had an air content equal to 5% to provide for resisting severe freezing-and-thawing conditions. The CFRP system used for retrofitting the columns consisted of unidirectional carbon fiber sheets and an epoxy resin. The dry carbon fiber sheets had a thickness of 0.63 mm (0.025 in.) and contained a fiber volume fraction of approximately 26%. Carbon fiber sheets with the same properties were used to jacket a selected group of field columns located in a high seismic risk area in Southern Illinois.

Five columns were subjected to accelerated corrosion by using external currents and one was kept in the as-built condition as a control column. Four of the five deteriorated specimens were repaired using CFRP and one column was kept in the deteriorated state for comparison purposes. The research program also included a study of the effects of freezing-and-thawing cycles, applied prior to lateral loading, on the CFRP repaired columns. The structural responses of the repaired columns were not affected by freezing and thawing. Further details of the freezing-and-thawing experiments are presented in Reference 1. The results of the freezing-andthawing experiments are not reported in this paper.

Accelerated corrosion process

The external current method1 was used to induce corrosion in the columns. A parallel circuit arrangement was used, as shown in Fig. 2. Although a series circuit arrangement is preferable for achieving the same level of corrosion in all specimens in a given amount of time, a parallel arrangement was used to reduce the voltage needed to induce the current demand required for these large-scale specimens. The power supply was connected directly to the column bars only. The dowel bars and column bars, however, were in electrical contact at local areas through metal spacers placed between bars. The bar layout and contact simulated construction details found in the field.