NEAR-SURFACE-MOUNTED FRP SYSTEMS
Use of near-surface-mounted (NSM) FRP rods and strips can preclude delamination-type failures, frequently observed by using externally bonded reinforcement. FRP bars or strips can be inserted in specially constructed grooves within the concrete cover layer, and adhered to the concrete with epoxy adhesives as shown in Fig. 7. The NSM technique becomes particularly attractive for flexural strengthening in the negative moment regions of slabs and decks, where external reinforcement would be subjected to mechanical and environmental damage and would require protective cover, which could interfere with the presence of floor finishes. NSM steel bars have been used in Europe since 1947. Tests on concrete beams reinforced with steel bars and others reinforced with steel bars grouted into diamond-sawn grooves showed identical behaviour for both sets of specimens. Flexural or shear strengthening with NSM FRP strips showed a greater anchoring capacity compared with externally bonded FRP strips. The feasibility of NSM FRP bars and strips have been investigated experimentally by many researchers. Test results showed that the efficiency of NSM FRP strips, defined as the ratio of the percentage increase in capacity to construction cost, was three times that of the externally bonded strips. A general methodology to evaluate the development length of NSM FRP bars of different configurations was investigated by the authors. The model is based on equilibrium and displacement compatibility procedures using finite element analysis, and accounts for distinct characteristics of concrete, epoxy and FRP materials. Fig. 8 shows a schematic representation of the principal tensile stresses around a NSM FRP bar. The development length is highly dependent on the dimensions of the strips, concrete properties, adhesive properties, internal steel reinforcement ratio, reinforcement configuration, type of loading, and groove width.
Two different types of debonding failures can occur for NSM FRP bars. The first mode of failure is due to splitting of the epoxy cover as a result of high tensile stresses at the FRP-epoxy interface, and is termed ‘epoxy split failure’. Increasing the thickness of the epoxy cover reduces the induced tensile stresses significantly. Furthermore, using adhesives of high tensile strength delays epoxy split failure. Epoxy split failure usually forms with longitudinal cracking through the epoxy cover. The second mode of failure is due to cracking of the concrete surrounding the epoxy adhesive and is termed ‘concrete split failure’. This mode of failure takes place when the tensile stresses at the concrete-epoxy interface reach the tensile strength of the concrete. Widening the groove minimizes the induced tensile stresses at the concrete-epoxy interface and increases the debonding loads of NSM bars.
Analytical modelling for NSM FRP strips is based on the combined shear-bending model for externally bonded FRP plates. The model is modified to account for the doubly bonded area of NSM strips. The model accounts also for the continuous reduction in flexural stiffness due to cracking of the concrete. Debonding of NSM FRP strips occurs as a result of the high shear stress concentration at the cut-off point. For simply supported beams subjected to a concentrated load P, at mid-span, the shear stress at the strip cut-off point r can be expressed in terms of the effective moment of inertia, Jeff, and the thickness of the FRP strip, tf, as follows:
Ef is the elastic modulus of the FRP strip, Ec is elastic modulus of concrete, Ga is the shear modulus of the adhesive, fa is the thickness of the adhesive, 10 is the unbonded length of the strip and Y is the distance from the strip to the neutral axis of the transformed section. Premature debonding of NSM FRP strips is governed by the shear strength of the concrete. Other components of the system such as the epoxy adhesive and the FRP strips have superior strength and adhesion properties compared with concrete. Knowing the compressive and tensile strength of concrete, the Mohr-Coulomb line, which is tangential to both Mohr’s circles for pure tension and pure compression, can be represented, and the maximum critical shear stress for the pure shear circle can be expressed as:
where Ie is the compressive strength of concrete after 28 days and let is the tensile strength of concrete. Debonding loads for NSM FRP strips can be determined for simply supported beams loaded with a concentrated load at mid-span by equating the shear strength proposed in eq. (7) to the shear stress given in eq. (4), Other loading cases (e.g. simply supported beams subjected to a uniform load, simply supported beams subjected to two concentrated loads) are reported. In general, strengthening limits for concrete members retrofitted with FRP should be specified, such that a loss of FRP reinforcement should leave the concrete member with sufficient capacity to resist at least unfactored dead and live loads.