Abstract: Several bridge columns, in which concrete was delaminated as a result of steel corrosion, were repaired in the mid 1990s. Different types of grout, including one based on expansive cement, were used to rebuild the damaged columns to their original shape; the columns were then wrapped with glass-fibre-reinforced polymers (GFRPs). The associated lab study indicated that the observed damage caused a reduction of about 20% in the axial-load-carrying capacity of the columns and much larger reductions in ductility and energy-dissipating capacity. The experimental results also showed that the strength and ductility of the columns could be recovered by repairing them with GFRP. Long-term monitoring of three columns repaired in the field using GFRP has indicated their excellent performance. No deterioration has been observed in the fibre-reinforced polymer or in the columns in more than 10 years. Monitoring has also shown a reduction in the rate and associated risk of corrosion over time; thus, this is a more durable retrofitting technique than traditional ones, such as steel jacketing.
Key words: concrete, columns, corrosion, cyclic loading, ductility, expansive cement, glass-fibre-reinforced polymers, monitoring, repair, strength.
Resume : De nombreux piliers de ponts dans lesquels le beton etait desagrege a la suite de la corrosion de l'acier ont ete repares dans le milieu des annees 1990. Plusieurs types de coulis, dont un a base de ciment expansif, ont ete utilizes pour rebatir les piliers endommages a leur forme originale avant de les envelopper de polymeres renforces de fibres de verre (<< GFRP >>). L'etude connexe en laboratoire a montre que les dommages observes reduisaient la capacite portante axiale des piliers de 20 % et causaient des reductions beaucoup plus importantes en ductilite et dans la capacite de dissiper l'energie. Les resultats de l'experience ont egalement montre que la resistance et la ductilite des piliers pourraient etre recuperees en les reparant avec des << GFRP >>. Le suivi a long terme de trois piliers repares sur le terrain en utilisant des << GFRP >> a montre leur excellent comportement. Aucune deterioration n'a ete observee dans les polymeres renforces de fibres ou les piliers en plus de 10 ans. Le suivi a egalement montre une reduction dans le temps du taux et du risque associe de corrosion, fournissant ainsi une methode de modification plus durable que les methodes conventionnelles telles que les enveloppes en acier.
Mots-cles : beton, piliers, corrosion, charges cycliques, ductilite, ciment expansif, polymeres renforces de fibres de verre, suivi, reparation, resistance.
[Traduit par la Redaction]
Introduction
There is a vast inventory of damaged concrete structures around the world that are either unsafe to use or are in dire need of repair to remain in use. The damage may be caused by extreme environmental conditions, overload, or routine aging. The extreme environmental conditions include exposure to sulfates, acids, alkalis, etc., variations in temperature during freezing and thawing conditions, and changes in moisture content. These factors affect the durability of cement paste and aggregate in concrete. In areas where deicing salt is used on roads, chloride ion ingress is one of the main reasons for the corrosion of steel in reinforced concrete structures. In addition, design inadequacies, unsound construction practices, and a lack of quality control, combined with minimal maintenance, have been responsible for a lot of structural damage.
During the last 10 years, several concrete structures in and around Toronto have been repaired or strengthened with surface-bonded fibre-reinforced-polymer (FRP) sheets and monitored for performance (Sheikh and Homam 2004). These structures include, among others, a concrete platform in an oil refinery, bridge columns and culverts along major highways, and high-rise apartment and condominium buildings. The repaired components include slabs, beams, walls, and columns.
This paper concentrates on research involving a bridge with its columns damaged primarily by chlorides from deicing salt. The bridge is in Toronto, on Highway 401, and was built in the 1960s, using concrete with a specified strength of 20.7 MPa. The highway has seven westbound and six eastbound lanes at this location. Figure 1 shows several damaged columns on the westbound part of the bridge. The columns are 920-1010 mm in diameter.
[FIGURE 1 OMITTED]
The damage seen in Fig. 1 is concentrated at the bents that are directly below the expansion joints. Over time, the joint sealant and other materials deteriorated as a result of extreme weather conditions and chemicals from de-icing salts, allowing the water-ice-salt solution to pass through the joints to the reinforced concrete bents. Accumulation of this solution at the bent top and its continuous flow downward resulted in deterioration of the concrete, as well as in corrosion of the steel. The columns may also have been subjected to splashes of salt solution from cars using the area under the bridge as a parking lot. In addition to the direct chemical attack, the structure was subjected to the usual environmental effects of freeze-thaw cycles, ultraviolet exposure, water, wet-dry cycles, etc. As a result, the spiral steel in the columns and the tie steel in the beams were severely corroded, which led to severe delamination of the cover concrete in both the columns and the girders. However, little corrosion was observed in the longitudinal bars.
Repair of columns
Conventional repair techniques have been successfully employed for decades, but in most cases they are cumbersome and require closing of the entire facility during repair. In addition, the need to repeat repairs at the same location within a few years is quite common. …
