Externally bonded shear reinforcement

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. 

Chaallal, O., Nollet, M. J. and Perraton, D. (1998), Strengthening of reinforced concrete beams with externally bonded fiber-reinforced-plastic plates Design guidelines for shear and flexure , Canadian Journal of Civil Engineering, Vol. 25, Issue 4, pp. 692-704. 

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. 

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. 

To account for the material evolutions, guidelines dedicated to the external or internal reinforcements of concrete structures using FRP composites usually introduce substantial durability parameters (reduction factors on FRP tensile properties and on shear characteristics of FRP/concrete bonded interfaces, creep stress levels and fatigue limits) see, for instance, ACI 440.2R-08 (2008) and ACI 440.1R-06 (2006). However, further research is still needed to refine the design codes and better calibrate the durability reduction factors taking into account synergistic effects between various environmental factors. 

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 filler-matrix interface The interface between filler and matrix is also crucial in terms of composite performance. The interface serves to transfer externally applied loads to the reinforcement via shear stresses over the interface. Controlling the strength of the interface is very important. Clearly, good bonding is essential if stresses are to be adequately transferred to the reinforcement and hence provide a true reinforcing function [1]. 

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... 

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. 

Shear strengthening of an r.c. beam can be carried out by applying one or more layers of sheet bonded to the external surface of the member to be strengthened. The reinforcement strips can be applied in a continuous or discontinuous mode and with a variable inclination angle (Fig. 5.6). 

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. 

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