International Concrete Abstracts Portal

A specialty contractor must consider many issues prior to deciding to participate in a plate bonding project. The most significant of these issues are feasibility, practicality, and the potential for teamwork between the owner designer, and specialty contractor. Assuming that plate bonding is technically feasible for a specific application, there are still many aspects to consider to insure a seccessful project. Acceptance is an expeditious manner by all parties, coupled with a good understanding of the existing and expected conditions, are critical to the decision. Plate bonding offers many unique advantages that are often realized when repair options are being reviewed. Typically, when this method has been used, it was done so because it was cost effective, minimized down time, did not increase deadload, and had little visual impact on the structure. A series of case histories, including bridges, post-tensioned slabs, and load-bearing walls, highlight the advantages and challenges a contractor may encounter when using this technology. When plate bonding as a repair option is selected early in the design process by all parties involved, then the benefits of a rapid, cost effective, and simple repair process can most readily be realized.

Repair and strengthening of concrete structures by external reinforcement with steel plates affords the manufacturer of concrete adhesives an attractive new and potentially large opportunity for his products. In this application, bond lines are generally thicker and environmental effects more pronounced than in more traditional uses of such adhesives. In plate bonding, the adhesive becomes an integral part of the reinforcement system and must be capable of transferring stresses without lasting hysteresis effects. High modulus epoxy adhesives with high heat deflection temperatures (HDT) have been shown to have the necessary creep resistance and shear strength. In plate bonding, the internal steel surface is not protected against corrosion by the alkalinity of concrete. Corrosion resistant primers were found either to lack bond strength at high temperature or to enhance a reaction between the freshly prepared metal surface and the hardener component of the epoxy adhesives, which led to the use of unprimed steel plates. Unfilled, low viscosity resins appear to have better barrier properties against corrosion than filled pastes. Treatment of freshly prepared steel surfaces with certain silanes appears to retard flash rust formation as claimed in the literature.

Extensive research has established that the provision of epoxy bonded steel plates as external reinforcement can be effective and efficient in enhancing the serviceability and ultimate strength of reinforced concrete beams. This plate bonding technique has many technical and practical advantages and provides an economic means of strengthening and rehabilitating damaged and/or deteriorated structures. There is, however, only limited data on the long term performance and durability of such strengthened beams. This paper presents the results of a comprehensive test program in which 21 steel plated reinforced concrete beams, both unloaded and loaded, were exposed to natural weathering for up to 12 years, without any maintenance, in an industrially polluted area. The main variables investigated include adhesive thickness, number of plates, and number and location of plate laps. The beams were examined in great detail after the exposure period; the extent of corrosion damage and the mechanisms of deterioration due to this exposure are critically evaluated and presented.

An experimental investigation was carried out on a total of 72 simply supported open sandwich beams in flexure under two symmetrical point loads. The beams were reinforced with 1.5 mm thick grit-blasted steel plates bonded to the concrete with a suitable epoxy resin adhesive. The major parameters investigated include the duration of exposure and the amount of sustained load. The results of these tests are presented and discussed in this paper. Test results indicate that for the type of adhesive selected, use of a double layer of adhesive in which the first layer has been allowed to harden for 24 hours before application of the second provides adequate bond strength under sustained loading up to 5 years of outdoor exposure. A steel surface which has been thoroughly degreased and metal grit-blasted appears to be a suitable preparation for the open sandwich beams.

Although some test data are available on the behavior of reinforced concrete beams with bonded plates in shear, there is still considerable lack of information on the effectiveness of such plates in resisting shear forces. This paper presents the results of the first stage of a comprehensive study to evaluate the role and mechanism of strengthening in shear when bonded steel plates are used either at the tension face or in the webs. The major variable in this study was the shear span-effective depth ratio, which is the critical parameter for shear failures; two different geometries of web plates were used. The effect of these bonded plates on deformations, cracking, failure loads, and mode of failure are presented and discussed. Bonded plates at the tension face enhanced the flexural stiffness of the original beams which were weak in shear, but their influence in enhancing shear behavior was minimal. Bonded web plates, on the other hand, were very effective in upgrading the structural behavior in shear. However, the latter were unable to reach their full load capacity due to shear cracking in the unreinforced part of the webs and the consequent debonding of the web plates.