Externally bonded reinforcement

The advantages of externally bonded reinforcement over other methods of strengthening concrete structures include the ability to strengthen part of a structure whilst it is still in use, minimum effect on headroom, low cost and ease of maintenance (see Fig. 6.8, for example). The method has been in use for over 20 years, mainly to enhance flexural capacity, and has been found to produce effective and economical solutions to particular problems. 

Historical development. The technique was introduced in France in the late 1960s where the first reported application was a major 

Two bridges on the M20 in Kent were strengthened shortly after construction in 1977. Cracks were discovered in one of the side spans which were found to have insufficient longitudinal steel. Steel plates were subsequently bonded to the side span soffits and the top surface of the deck over the supports (Fig. 6.9). 

The floor beams of a North London building were strengthened in 1978 to allow for increased floor loadings following a change in use. The factor of safety provided by the existing structure, in the 

Midlands police car park soon after construction, as a result of a deficiency in conventional reinforcement, has been well publicised. Here again, additional bonded plate reinforcement was installed in order to control further cracking under live load. An 11 m span, 2 m deep reinforced concrete beam at a shop in Bootle was strengthened to provide a 10% increase in ultimate moment capacity. In so doing it was calculated that the mean horizontal shear stress resisted by the adhesive was 9.84 N/mm.  

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Fibre-reinforced polymers (FRP) rebars, usually made of an epoxy matrix reinforced with carbon or aramide fibres, have also been proposed both as prestressing wires and reinforcement. Nevertheless, they are not discussed here, because these applications are still in the experimental phase and there is a lack of experience on their durability. In fact, while they are not affected by electrochemical corrosion typical of metals, they are not immune to other types of degradation. FRP are also used in the form of laminates or sheets as externally bonded reinforcement in the rehabilitation of damaged structures this application will be addressed in Chapter 19. 

In general, for externally bonded reinforcement (EBR), fabrics and strips are available. Commonly used fibres include carbon, glass and aramid. 

PMC and in particular FRP composites have generated a lot of interest and there are future expectations for their use in construction in coming years. They are already being used to improve the performance and durability of new as well as deteriorated facilities (for repair/rehabilitation or upgrading), as either stand alone structural members, as reinforcement for concrete, (i.e., as FRP bars or as externally bonded reinforcements, EBR) [38] or in combination with other structural materials. FRP are especially suitable for difficult and complex applications both for load bearing (beams, columns, etc.), or on secondary elements (infill, partition walls and so on) [39]. 

A relatively new repair technique for concrete structures, including prestressed structures, consists of externally bonding flexible sheets of FRP composites to the concrete surface. Depending on the type of application, the function of the externally bonded reinforcement... 

During the last decade, there have been continuous improvements in the selection of constitutive materials, in the manufacturing processes and the field implementation of polymer composites for civil engineering applications. The use of FRP materials as externally bonded reinforcement is now an attractive solution to increase the lifespan of existing concrete infrastructures, while their use as internal reinforcing bars or as structural members is intended to prevent corrosion problems and increase the overall durability of new constructions exposed to severe environments. Nevertheless, key issues regarding the long-term durability of FRPs for infrastructure applications have been only partially addressed, and there remain considerable needs for research and development in this field ... 

Adhesion Epoxy Bonding reinforcements on external surface of combustion chamber... 

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

External reinforcement For externally bonded FRP, the bond of the FRP to the substrate can be affected by both elevated and low temperatures. Experience has shown that voids - regions between the concrete... 

Externally bonded flexural reinforcement (EBFR) for the strengthening of concrete members using unstressed fibre reinforced polymer (FRP) strips... 

For the cross-section analysis, different bond properties must be considered by using the bond coefficients k = 0.7 and Kt = 0.9 for the internal reinforcement steel bars and the externally bonded strips, respectively. For compatibility relationships in the cross-section analysis, the mean values of the strains, s, in the internal steel reinforcement and ft in the EBFR can be used however, for the equilibrium relationships, the maximum values of the strains for the internal reinforcement steel bars and the externally bonded strips. 

Usually, for shear design the well-known classical truss model is used. The shear strengthening design also requires knowledge and understanding of the cmcial failure modes of the externally bonded shear reinforcement (EBSR). 

Figure 4.20 shows the principle of the three states. In the figure, the numbers 1 and 2 indicate the start of the yielding of internal shear reinforcement and the failure of externally bonded shear reinforcement, respectively. 

ACI 440.7R-10 (2010). Guide for the Design and Construction of Externally Bonded Fibre-Reinforced Polymer Systems for Strengthening Unreinforced Masonry Structures. 

ISIS-Canada (2001). Strengthening Reinforced Concrete Structures with Externally-Bonded Fibre Reinforced Polymers. Design Manual No. 4. Winnipeg, Manitoba, ISIS-Canada. 

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. 

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. 

Marsh G,Seismic retrofit provides opportunities for FRP, Reinforced Plastics, 38-43, Mar 1998. Meier U, Rehabilitation and retrofitting of existing structures through external bonding of thin carbon-fiber sheets. Materials and Structures, 28(176), 105-106, 1995. 

Mitsui Y, Murakami K, Takeda K, Sakai H, Study on shear reinforcement of reinforced concrete beams externally bonded with carbon fiber sheets. Composite Interfaces, 5(4), 285-295, 1998. 

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