•NSE/EB pioneer FRCM systems were examined for shear strengthening of RC beams.•Thirteen beams under three-point loading have been tested.•Carbon-FRCM showed the highest increase in the load carrying ...capacity of 83%.•PBO-FRCM showed the least increase in the load carrying capacity of 62%.•Near surface embedding of FRCM mitigate deboning.
The externally bonded (EB) fabric reinforced cementitious matrix (FRCM) has successfully been used as a structural strengthening for various applications including flexural and shear strengthening of reinforced concrete (RC) beams, flexural strengthening of RC slabs and column confinement. However, the EB-FRCM system is characterized by poor FRCM/concrete bond leading to premature debonding of FRCM off the concrete substrate, particularly for thicker FRCM. The present paper reports on an experimental study on the efficacy of a pioneer form of hybrid near surface embedded and externally bonded technique using FRCM composites (NSEEB-FRCM) for shear strengthening of RC beams. With such a technique, higher thickness of FRCM composites can be applied with less likelihood of debonding that is normally experienced when using the EB-FRCM system. Thirteen shear-deficient medium-scale RC beams were constructed, strengthened in shear and tested under three-point bending test. The test parameters were: (a) FRCM type (polyparaphenylene benzobisoxazole, carbon, and glass), (b) strengthening configuration (full versus intermittent strips), and (c) number of fabric layers.
The percentage enhancement in the shear capacity of the beams ranged from 43% to 114% indicating the successful implementation of the strengthening methods provided. An average enhancement in shear capacity of 83%, 72% and 62% were observed in carbon FRCM, glass FRCM and PBO-FRCM, respectively. The failure mode of the strengthened specimens was sensitive to the type and configuration of FRCM in addition to the number of FRCM layers. The strengthening systems also resulted in higher deflection at failure and energy absorption value of the strengthened beams with an average of 94% and 204% relative to the reference specimen, respectively.
The combined effect of cyclic loading and steel rebar corrosion can determine the premature deterioration of reinforced concrete (RC) structures subjected to dynamic and environmental actions. The ...use of fabric-reinforced cementitious matrix composites (FRCM) is a promising solution to extend the fatigue life of RC structures. Externally bonded FRCMs are applied to the tension side of structural members to reduce the rebar stress level, thus delaying fatigue crack nucleation and/or propagation. A key aspect for the design of FRCM strengthening of these structures is the matrix-fiber bond behavior under fatigue loading. In this paper, a critical review of currently available studies on the fatigue behavior of FRCM-strengthened RC beams is presented. Then, the results of 25 bond tests on PBO FRCM-concrete specimens are provided and discussed. These tests include both single-lap direct shear and modified beam tests performed in quasi-static and cyclic mode. The results obtained show that the cyclic load may induce progressive debonding at the matrix-fiber interface with rupture of fiber filaments. Modified beam tests are more affected by these phenomena than direct shear tests. The rupture of fiber filaments is confirmed by the results of post-fatigue tests that show capacities lower than those of corresponding quasi-static tests.
A fast bisection procedure embedded in a standard ODE45 solver is proposed for three different kinds of FRCM experimental tests, namely (1) coupon tensile test, (2) pull-out test and (3) single-lap ...shear test. In order to predict global and local behavior of FRCM in such kind of experiments, a mono-axial differential equations model can be derived, requiring in the most general case the numerical solution of Boundary Value Problems BVPs, involving as independent variables mortar and fiber displacement fields and their first derivatives. Assuming a nonlinear behavior of both matrix and reinforcement-matrix interface, implicit and general purpose BVPs exhibit poor numerical stability and slow convergence. Furthermore, the initial guess of the solution is an integral part of solving a BVP, making the approach even less appealing, especially to reproduce FRCM behavior in the whole possible range of deformations, where severe softening and snap back phenomena are encountered. In the paper, the BVP is transformed in an Ordinary Differential Equation ODE system with initial values, i.e. into an Initial Value Problem IVP, by means of a classic shooting method where guess values of the matrix displacement on the opposite side of the loaded edge are assumed and varied until the desired boundary condition on the loaded edge are obtained. A classic bisection procedure allows to converge quickly to the exact value of matrix displacement to assign in order to recover quickly the boundary condition of the original BVP problem imposed on the opposite side. The advantage -common to all shooting methods available- is that a BVP is transformed in a sequence of ODE problems with initial conditions, which can be solved by means of explicit Runge Kutta kernels. Any kind of non-linearity can be assumed for matrix –for which a particular saw tooth approximation of the real behavior in tension is proposed to strengthen further convergence- and matrix-reinforcement interface, for which both a newly proposed exponential law and a stepped softening relationship are adopted. The numerical procedure is tested on the aforementioned three different experimental setups, showing excellent fitting capability of both the experimental behavior and previously presented theoretical models, where available.
This paper presents the results of the mechanical characterization of composite materials comprising high strength textiles embedded in inorganic matrices. These materials are commonly termed Textile ...Reinforced Mortars (TRM) or, when comprising cementitious matrices, Fabric-Reinforced Cementitious Matrix (FRCM – despite the fact that this term is often extended to composites with cement-free matrices). Different types of fibers were employed, namely carbon, glass, and basalt, as well as steel cords, which were embedded in lime- or cement-based matrices. Results of tensile tests on single fiber yarns and composite prismatic specimens with a rectangular cross-section are shown and discussed. The effect of fiber coating and stitch-bonded joints between warp and weft yarns on the tensile behavior observed is studied. The results obtained help to shed light on the different parameters that affect tensile testing of inorganic-matrix composites contributing to the appropriate mechanical characterization of these materials.
The paper presents a semi-analytical approach for the study of the debonding phenomenon of Fiber Reinforced Cementitious Matrix (FRCM) systems externally applied to curved masonry pillars. One of the ...main features of the approach consists of considering the strengthening system composed by three separated components: external mortar layer, central fiber net and internal mortar layer. These components, assumed subjected to a longitudinal state of stress, interact one each other through tangential stresses developing at the level of zero-thickness interfaces. Regarding the latter, a tri-linear shear stress-slip relationship is assumed to account for a first elastic phase, a second phase exhibiting linear softening and a third phase with a possible non-null residual strength. Since in case of curved substrates, by equilibrium, normal stresses at the interfaces between fiber and matrix arise, which modify the peak tangential resistance and the ductility, a classic Mohr-Coulomb criterion is introduced in the approach. Additionally, the central fiber net progressively transfers along the bond length the force applied at its loaded end by means of an elastic interface interposed between the internal mortar layer and the substrate, the latter assumed rigid and infinitely resistant. The longitudinal equilibrium equations written for the two mortar layers, suitably re-arranged considering the constitutive behavior of the layers, allow to deduce a field problem governed by six first order differential equations into six unknowns. The non-linearity is tackled by means of a recursive elastic numerical algorithm where the elastic modulus of the damaged materials is progressively dropped down, subdividing the bonded length into small portions where the material properties are assumed constant. For each element the solution of the field problem is known in closed form and the only variables to determine are the integration constants coming from the solution of the differential equation system. After a standard assemblage, all constants are derived imposing the boundary conditions at the extremes of the elements, which depend on the state of cracking of the matrix layer. The validation of the proposed approach is carried out with reference to recent experimental tests carried out by the Authors. The obtained results show the reliability of the approach to account for the influence of the curvature of the substrate on the debonding process of FRCM systems.
•Experimental tests show performance differences between C-FRCM and G-FRCM materials.•Analytical model and simulation accurately estimate FRCM-confined concrete strength.•Strength of FRCM-confined ...concrete: size, radius, scale, fibre mesh & layer count.•C-FRCM and G-FRCM confined column enhanced energy, deformability & reinforcement.
In this research, the strength of confined concrete was tested and evaluated numerically by examining the influence of three variables: fibre material, corner radius, and FRCM (Fiber Reinforced Cementitious Matrix) layer number. The study used thirty C-FRCM and thirty G-FRCM specimens and developed a new empirical formula to estimate confined concrete compressive strength based on experimental results. The advanced formula was then used to determine the input parameters of the nonlinear concrete Mazar model for simulating confined concrete areas. A parametric study investigated the effects of cross-section size, corner radius, cross-section scale, fibre mesh type, and the number of FRCM layers on confined concrete compressive strength. The proposed finite element method was found to be effective in accurately predicting confined element compressive strength and nonlinear behaviour, including core concrete, FRCM layers, and stress concentrations at corners.
This paper examines the effect of PBO (P-phenylene benzobisoxazole)-FRCM (Fabric Reinforced Cementitious Matrix) reinforcement on the stiffness of eccentrically compressed reinforced concrete ...columns. Reinforcement with FRCM consists of bonding composite meshes to the concrete substrate by means of mineral mortar. Longitudinal and/or transverse reinforcements made of PBO (P-phenylene benzobisoxazole) mesh were applied to the analyzed column specimens. When assessing the stiffness of the columns, the focus was on the effect of the composite reinforcement itself, the value and eccentricity of the longitudinal force and the decrease in the modulus of elasticity of the concrete with increasing stress intensity in the latter. Dependences between the change in the elasticity modulus of the concrete and the change in the stiffness of the tested specimens were examined. The relevant standards, providing methods of calculating the stiffness of composite columns, were used in the analysis. For columns, which were strengthened only transversely with PBO mesh, reinforcement increases their load capacity, and at the same time, the stiffness of the columns increases due to the confinement of the cross-section. The stiffness depends on the destruction of the concrete core inside its composite jacket. In the case of columns with transverse and longitudinal reinforcement, the presence of longitudinal reinforcement reduces longitudinal deformations. The columns failed at higher stiffness values in the whole range of the eccentricities.
Mortar-based reinforcements are an innovative solution for retrofitting existing structures that combine effectiveness, compatibility, and sustainability. Despite the recent spreading of field ...applications, there is still insufficient knowledge on their fundamental mechanical properties, and a regulatory gap for experimental procedures and design criteria. A Round Robin Test initiative was organized by the Rilem TC 250-CSM (Composites for the Sustainable strengthening of Masonry) to investigate the tensile and bond behaviour of mortar-based composites with basalt, carbon, glass, polyparaphenylene benzobisoxazole (PBO), aramid and steel textiles. This paper presents the tests carried out on Steel Reinforced Grout (SRG) systems, comprising three textile and four mortar types, supplied by three producers. Ten laboratories from Italy, Poland and Portugal were involved for a total of 150 tests, including direct tensile tests on textiles and composites, and single-lap bond tests on masonry substrate. The influence of the layout of the textile, the mechanical properties of the mortar matrix, the manufacturing and curing conditions, as well as the testing setup and instrumentation, is discussed to contribute to the optimization of the reinforcement systems and to the development of recommendations for laboratory testing. Finally, results are combined to derive engineering parameters for qualification and design purposes.
The debonding process of an FRCM reinforcing system from the substrate is studied in a semi-analytical fashion. FRCM is modeled considering independently the central elastic fiber grid and the two ...thick upper and lower matrix layers, assumed elasto-fragile; matrix and fiber are considered in a monoaxial state of stress; they mutually exchange shear stresses at the interface, this latter characterized by a softening stress-slip relationship; the reinforcement system is then bonded with a rigid substrate by means of a further elastic interface. Under such hypotheses, a simple system of first order non-linear and coupled differential equations is derived and solved by means of a semi-analytical approach. Independent variables are the axial displacements of the three layers (upper and lower matrix, central fiber) and the corresponding axial stresses. The approach is successfully validated against two experimental datasets available in the literature, relying into different FRCM strengthening systems bonded to rigid substrates and subjected to single lap shear tests. The model is able to capture not only the global debonding behavior but also the local one, with a precise prediction along the bond length of the shape of the axial stresses into the different layers, of the interface shear stresses and of the location of the cracks inside the matrix.
•Four-point bending tests were conducted on beams with steel-FRCM composite strips.•Parameters varied were number of matrix layers, load rate, and anchorage condition.•Results were compared with ...direct-shear tests of the same composite.•Values of maximum fiber strain at debonding were determined using three methods.•U-wrap anchorages did not restrain the fiber-matrix interfacial slip.
This paper presents the results of an experimental investigation of the flexural response of reinforced concrete (RC) beams strengthened using externally bonded steel fiber reinforced cementitious matrix (steel-FRCM) composite. Steel-FRCM composite strips were bonded to the tension face of four RC beams, which were tested in four-point bending. Parameters varied were the presence/absence of the external (coating) layer of the matrix, presence/absence of U-wrap anchorages, and loading rate. Results are compared with those from single-lap direct-shear tests conducted on the same composite. The direct-shear tests showed that debonding of steel-FRCM joints is characterized by fiber slippage and fracture of the matrix layer at the internal matrix layer-fiber interface. In the beam tests, the strengthening system increased the yield load by 15–21% relative to the unstrengthened beam. The ratio of the load at which debonding occurred to the load at yielding ranged from 1.11 to 1.19 for each strengthened beam. The load rates employed and the presence of the external matrix layer did not appear to significantly affect the failure mode or the load and midspan displacement at debonding. The presence of the U-wraps helped restrain the peel-off of the composite observed in strengthened beams without the U-wrap, however, they did not restrain the fiber slippage at the ends of the composite, which inhibited composite action. Average values of the maximum fiber strain at composite debonding determined using strain profiles from strain gages, an approximate method, and moment-curvature analysis were 0.54%, 0.73%, and 0.83%, respectively.
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