International Concrete Abstracts Portal

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.

Research on the design of adhesive bonding plates for strengthening reinforced concrete structural elements is presented in this paper. Different design aspects are covered, including increase of bending strength, calculation of anchoring length of epoxy bonded steel plates, temperature resistance, dowel action, and strengthening of two-way slabs taking into account orthotropic behavior of strengthened slabs. A brief overview of theoretical principles is given. The results of a large experimental program are evaluated according to the theoretical principles and are translated into practical engineering design procedures. The design procedure for strengthening two-way slabs is explained in a practical example.

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.

SP-165 The ACI Technical Session "Repair and Strengthening with Adhesive Bonded Plates" was held in Washington, D C on March 15-16, 1992. The papers presented at the technical session and the additional papers included in this special publication provide the first comprehensive publication of information on this subject in the United States. The papers cover research, design, construction methods, and case histories. The research described includes not only theoretical analysis and short-term experimental programs but also test programs that involve long-term outdoor exposure. research in the use of fiber reinforced plastic plates instead of the more commonly used steel plates is also described. The design papers illustrate practical engineering approaches and include details of anchoring plates, effect of plate dimensions, adhesive performance, and temperature limitations. Construction methods described include preparation of the plates, application of adhesive, and installation of the plates. The case histories provide a broad and historical view of the use of adhesive bonded plates to strengthen buildings and civil engineering structures.

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.