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Rehabilitation of Structures Using Steel Plates

Rehabilitation can be defined as an operation to bring a structure (or a structural component) that is deficient in design demand to the desired specific performance level. Many of the existing structures will soon approach their service life in a period shorter than the anticipated design life. For example, 40% of the highway bridges in USA are either structurally or functionally deficient and require some form of rehabilitation or replacement. The loss due to corrosion alone, amounts to about $100 billions per year. In Canada, 40% of the bridges are also deficient and require rehabilitation or replacement. In Europe, the annual cost of damage due to corrosion has been estimated at being 1000 million English pounds.


Rehabilitation Using Steel Plates:

In recent years, a new method has evolved   that   involves bonding of steel plates to the surfaces of members to be rehabilitated (Fig. Above). This method has considerable potential in the rehabilitation field, and has already been used to strengthen both buildings and bridges in many countries around the world. The major attractions of this technique are the relative simplicity of the implementation, the speed of application, and the relatively small consequential change in structural size. 
Concrete Beam Reinforced with Steel Plate
Concrete Beam Reinforced with Steel Plate.
In recent years, extensive experimental investigations on the factors influencing the structural performance of plated concrete beams have been reported by Swamy et al. [11,12]. The results of these investigations have shown that the bonding of thin steel plates to the tension faces of concrete beams can lead to a significant improvement in structural performance, under both service and ultimate loading conditions.

Shortcoming
The shortcoming of the strengthening method with steel plates is the possibility of corrosion at the epoxy-steel interface, which may adversely affect the bond strength. Calder and Lloyd and Calder conducted detailed theoretical and experimental studies on the long-term performance of RC beams externally strengthened by epoxybonded steel plates. The beams were subsequently exposed to three typical environmental areas throughout England (high rainfall, industrial, and marine), for duration of up to 2 years. After two years, the majority of the steel plates in all locations had suffered, to some extent, from some corrosion at the resin-steel interface, and this led to a slight reduction in the strength of the exposed beams as compared to the control beams. It was noted that there was a little difference between the failure load for the primed and non-primed test samples. The corrosion appeared to be caused by migration of water through micro-cracks in the concrete and resin through to the steel plate. This corrosion was nearly uniformly distributed over the entire face of the steel plate, suggesting that water seeped in through the concrete and resin, and not between the steel and resin at the ends of the beam. The use of a primer paint applied to the surface of the steel plates appeared to generally prevent corrosion of the steel plates, without affecting the structural strength of the member. However, there was some loss of strength in the bond between the steel and the concrete, which led to a 7-23% reduction of the failure loads for the loaded beams and to a 2-11% reduction for the unloaded beams.

Other drawbacks of steel plating include difficulty in handling the heavy steel plates at the site especially in limited space indoor applications, relative difficulty in shaping the plate to suit the structural element to be repaired, and the problem of forming clean butt joints between the relatively short plates. Also, steel plates are not suitable to be wrapped or jacked around tapered members.

An effective way of eliminating the corrosion problem and other previously listed drawbacks is to replace steel plates with corrosion-resistance synthetic materials such as fiber-reinforced polymer (FRP) composites. In addition to their higher corrosion resistance, many polymer composites have tensile and fatigue strengths that exceed those of steel.

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