External steel plate bonding

Conventional techniques for dealing with such situations involve the construction of additional supports, prestressing or the enlargement of structural members. In the latter case this may involve the bonding of new concrete to old. An alternative solution is to use mild steel plates bonded by an adhesive to the external surface of the concrete member in question. These two techniques will now be discussed in turn. 

In 1985 cracking was noticed in the floor slabs of a multi-storey office building in Leeds. The cracks were adjacent to the external columns and the central lift well and design checks indicated a deficiency in both shear capacity and top flexural reinforcement. A combination of soffit supporting brackets and steel plates bonded to the top surface adjacent to supports was adopted to restore capacity and control cracking (Fig. 6.11). Subsequent load tests revealed that the steel plates were attracting tensile stresses up to 40 N/mm at 1.35 times design load. 

The purpose of the above is to specify requirements for a cold-cure adhesive to permit either the repair or strengthening of existing concrete structures by bonding on additional external steel plate reinforcement, or the construction of steel/concrete composite units in which wet concrete is poured on to steel freshly coated with a layer of adhesive. In both cases the adhesive serves to resist the interfacial shear stresses necessary to ensure structural composite action between the steel and concrete. For these purposes a cold-cure adhesive is defined as one which is capable of curing to the required strength between the temperatures of 10 °C and 30 °C. 

Steel Under Earthquake Excitations Reinforcing steel in external reinforced crmcrete jackets for the retrofit and strengthening of existing reinforced concrete or steel members comes mainly in the form of bars. Usually, the stress-strain behavior under tension-compression is cmcial to assess the longitudinal bars contribution. The stress-strain behavior under tension is of interest for the transverse reinforcement (stirmps). Steel dowels and stud shear connectors may involve shear (or tensile) behavior of steel- or bond-related issues. External steel plates may be considered under multiaxial loading (i.e., in cases of shear strengthening in order to estimate their yield stress). Bond-related issues may concern bar-concrete interfaces, bar-resin-concrete interfaces (NSM applications), or plate-resin-concrete interfaces under cyclic loading. 

If concrete removal is not required or supplementary reinforcing bars cannot be used, external reinforcement can be applied. For instance, steel bars may be encased in a shotcrete layer or steel plates may be bonded onto the concrete surface. Recently, the use of steel plates has been substituted by fibre-reinforced plastics (F. R.P.), that are composite materials with glass, aramide or carbon fibres embedded in a polymeric matrix (usually an epoxy system). F. R.P. are available in the form of laminates or sheets that are bonded to the concrete surface using an epoxy adhesive [11]. They are typically used to improve the flexural and shear strength or to provide confinement to concrete subjected to compression. The... 

The second part of this book is devoted to current and potential applications of adhesive materials in construction. Chapter 6 deals with both the repair and the strengthening of concrete structures, covering applications ranging from non-structural patch repairs and resin overlays to externally bonded steel plate reinforcement. The theme of repair and strengthening is extended to applications involving steel, timber and masonry structures. A number of case histories are reviewed and discussed with reference to the successes and failures, and the results of allied research work are presented. In Chapter 7 a number of applications of adhesives in new construction are described, and specific examples are given. The final chapter. Chapter 8, examines the potential for future developments in adhesive usage. 

Fig. 1.8. Strengthening of bridge structure with externally bonded steel plate reinforcement.

In the practice of adhesive bonding for applications in construction, surface pretreatment is likely to be the most difficult process to control. The choice of treatments must be tempered by the scale of operations, the nature of the adherends, the required durability, the adhesive to be used, and the cost. The performance of joints constructed with cold-cure epoxies is likely to be critically dependent upon surface preparation, as exemplified by the experience of the Scottish Irvine Development Corporation. In 1978 they elected to use vertical externally-bonded steel plate reinforcement to strengthen the abutment walls of three pedestrian underpasses. A year later, the plates were reported to be falling off, accompanied by extensive interfacial corrosion the steel surfaces had been abraded by hand, and the concrete surfaces chemically etched. 

Raithby, K.D. External strengthening of concrete bridges with bonded steel plates. Transport and Road Research Laboratory Supplementary Report 612, Crowthorne 1980. 

Roberts T and Haji-Kazemi H (1989), Theoretical study of the behaviour of reinforced concrete beams strengthened by externally bonded steel plates , ICE Proc Part 2, 87(1). 

Barnes, R. A. and Mays, G. C. (2006), Strengthening of reinforced concrete beams in shear by the use of externally bonded steel plates Part 1 - Experimental programme , Construction and Building Materials, Vol. 20, Issue 6, pp. 396-402. 

The bearing has no externally bonded steel plates. Therefore, hydrostatic stress is not a consideration. 

Article 14.7.5.4 of the current LRFD specifications requires a check of rotation versus axial strain for bearings without externally bonded steel plates. A restraint system is required whenever... 

Externally bonded FRP plates, sheets and wraps for the strengthening of reinforced concrete, steel, aluminium and timber structural members... 

Pre-bonding. Fresh concrete is poured directly onto a layer of uncured adhesive spread over the prepared steel surface. This method, as used in the reinforcement of slab units by externally bonded plates, has been the subject of extensive research by the Wolfson Bridge Research Unit(lO). Open sandwich slabs, based on either a flat soffit plate or a plate curved slightly upwards to form a shallow arch have been recommended. The technique is described in some detail in Chapter 8. 

---