Search Results (23)

This study presents an analytical and numerical framework to describe the debonding behavior of fiber-reinforced cementitious matrix (FRCM)-reinforced flat masonry elements under direct shear tests. A sawtooth shear stress–slip law, initially proposed for Steel Reinforced Grout (SRG) systems by two of the authors, is calibrated for a PBO-FRCM system based on the experimental results available in the literature. These recent experimental outcomes on flat masonry pillars serve to validate the model by capturing essential interface behaviors, including residual strength and pseudo-linear hardening. Furthermore, a finite element (FE) model of the specimens has been developed to simulate the interface response, allowing for a comparison between numerical predictions and experimental results. The sawtooth law is implemented directly in commercial FE software without the need for custom coding. Additionally, a mesh sensitivity analysis was performed to verify numerical stability and identify the optimal discretization parameters for consistent model response. Results show good agreement among experimental observations, the sawtooth analytical model, and FE simulations. The analytical model slightly underestimates the experimental peak load by about 4–6%, while the FE predictions differ from the experimental results by less than 10%, confirming the reliability of the proposed modeling framework. Full article

In the case of strengthening building structures, the process usually involves elements that have a certain loading history and are typically subjected to loading during the strengthening process. In scientific research, on the other hand, strengthening is usually applied to elements that are not representative of real structures. This article presents a study of the effect of pre-damage on the behavior of eccentrically compressed concrete cylinders confined with PBO-FRCM (fabric-reinforced cementitious matrix with PBO fibers) composite. Concrete confinement introduces a favorable triaxial stress state, which leads to an increase in the compressive strength of concrete. FRCM systems are an alternative to FRP (fiber-reinforced polymer) composites. Replacing the polymer matrix with a mineral matrix primarily improves the fire resistance of the strengthening system. The elements were made of concrete with a compressive strength of about 40 MPa, which is typical for current reinforced concrete columns. Pre-damage was induced by loading the test elements to 80% of the average compressive strength and then fully unloading. The elements were then strengthened with three layers of PBO-FRCM composite and subjected to axial or eccentric compression with force applied at two different eccentricities. In addition to electric strain gauges, a digital image correlation system was used for measurements, to identify the initiation of PBO mesh overlap delamination. This study analyzed the elements in terms of load-bearing capacity, deformability, ductility, and failure mechanisms. In general, there was no negative effect of pre-damage on the behavior of the tested elements. Full article

FRCM (Fabric-Reinforced Cementitious Matrix) composites, while providing an effective alternative to FRP (Fiber-Reinforced Polymer) strengthening systems when epoxy resins cannot be used, typically fail to achieve their full strengthening potential. Research indicates that appropriate mesh anchorage systems can minimize some of the undesirable effects that limit FRCM composite performance. This study investigates the effectiveness of different anchorage systems for PBO (p-Phenylene Benzobis Oxazole) fibers in FRCM composites used for strengthening reinforced concrete slabs. A series of unidirectionally bent RC slabs were tested under four-point bending: an unstrengthened control element, slabs strengthened with PBO-FRCM without anchorage, with bar anchorage (GFRP bar in a groove), and with cord anchorage (PBO cord through the slab). The research focused on analyzing the load–deflection behavior and key strain mechanisms that influence structural performance. The findings indicate that a single layer of PBO-FRCM increases bending capacity, raises yield load, and delays initial cracking. Most significantly, the research reveals substantial differences in composite mesh utilization efficiency. This study confirms that mechanical anchorage, particularly bar anchorage, significantly enhances the effectiveness of PBO-FRCM strengthening systems by delaying composite detachment and allowing for greater utilization of the high-strength fiber material. These results contribute valuable insights for RC slabs using FRCM composite systems and the anchorage of their mesh. Full article

This article explores the impact of strengthening reinforced concrete beams under different load levels, focusing on the use of polyphenylene benzobisoxazole (P.B.O.) fibers in a stabilized inorganic matrix to enhance the shear capacity. This research examines the interaction between modern composite materials and existing reinforced concrete structures, highlighting the practical challenges when the full unloading of structures is impossible. The experiments demonstrate that strengthening significantly increases the shear strength, with a maximum enhancement of 25%. However, the effect decreases as the load applied during strengthening increases, dropping to 16% at 70% of the ultimate load. This research also highlights the importance of refining current calculation methods due to the complex stress–strain state of beams and the unpredictable nature of shear failures. It concludes that composite materials, especially fiber-reinforced cementitious matrix (FRCM) systems, provide a practical solution for enhancing structural performance while maintaining the integrity and safety of concrete elements. This article emphasizes that the strengthening efficiency should be adjusted based on the applied load, suggesting a 5% reduction in effectiveness for every 10% increase in the initial load level. The findings support the empirical hypothesis that the shear strength improvement diminishes linearly with higher load levels during strengthening. Full article

Fiber-reinforced cementitious matrix (FRCM) composites have been largely used to strengthen existing concrete and masonry structures in the last decade. To design FRCM-strengthened members, the provisions of the Italian CNR-DT 215 (2018) or the American ACI 549.4R and 6R (2020) guidelines can be adopted. According to the former, the FRCM effective strain, i.e., the composite strain associated with the loss of composite action, can be obtained by combining the results of direct shear tests on FRCM–substrate joints and of tensile tests on the bare reinforcing textile. According to the latter, the effective strain can be obtained by testing FRCM coupons in tension, using the so-called clevis-grip test set-up. However, the complex bond behavior of the FRCM cannot be fully captured by considering only the effective strain. Thus, a cohesive approach has been used to describe the stress transfer between the composite and the substrate and cohesive material laws (CMLs) with different shapes have been proposed. The determination of the CML associated with a specific FRCM–substrate joint is fundamental to capture the behavior of the FRCM-strengthened member and should be determined based on the results of experimental bond tests. In this paper, a procedure previously proposed by the authors to calibrate the CML from the load response obtained by direct shear tests of FRCM–substrate joints is applied to different FRCM composites. Namely, carbon, AR glass, and PBO FRCMs are considered. The results obtained prove that the procedure allows to estimate the CML and to associate the idealized load response of a specific type of FRCM to the corresponding CML. The estimated CML can be used to determine the onset of debonding in FRCM–substrate joints, the crack number and spacing in FRCM coupons, and the locations where debonding occurs in FRCM-strengthened members. Full article

Externally bonded fiber-reinforced polymers (FRPs) have been widely used for strengthening and retrofitting applications. However, their efficacy is hindered by the poor resistance of their epoxy resins to elevated temperatures and their limited compatibility with concrete substrates. To address these limitations, fabric-reinforced cementitious matrix (FRCM), also known as textile reinforced mortar (TRM), systems have emerged as an alternative solution. In this study, experimental tests were performed on concrete cylinders confined with FRCM systems that consisted of mineral mortar and poliparafenilenbenzobisoxazole fabric (PBO). The cylinders with concrete strengths of 30, 45, and 70 MPa, were confined with one or two FRCM layers, and were subjected to different target temperatures (100, 400, and 800 °C). The experimental results highlighted the confinement effect of FRCMs on the compressive strength of the tested cylinders. Cylinders exposed to 100 °C exhibited a slight increase in their compressive strength, while no specific trend was observed in the compressive strength of cylinders heated to 400 °C. Specimens heated up to 800 °C experienced a significant reduction in strength, reaching up to 82%. Full article

An experimental investigation on partially PBO (short of Polyparaphenylenebenzobisthiazole) FRCM (Fiber Reinforced Cementitious Mortar) confined clay brick masonry columns has been conducted. Ten small-scale specimens measuring 445 mm high with a square cross-section of the 250 mm side have been tested under monotonic axial loading until collapse. Two columns were unconfined, while the remaining ones were confined with single-layer PBO FRCM jackets varying the geometric configuration along their height. The vertical spacing ratio sf’/sf, being sf’ and sf the center-to-center and the net spacings between two consecutive jackets, respectively, was considered as the key parameter of the confinement configuration. The failure modes, stress–strain curves and peak axial stress and strain values are reported. The experimental results have been compared to the predictions of models found in the Italian guidelines CNR DT 215/2018 and the American ACI 549-R20 standards. The main aspects analyzed involved (i) the evaluation of the effectiveness of partial confinement on the mechanical response of columns, (ii) the definition of the mechanical and geometrical parameters that influence the structural response of partially confined columns, and (iii) the development of appropriate analytical models for the prediction of the resisting capacity of masonry columns partially confined with PBO FRCM. Full article

The article presents an analysis of the static work of bent solid-wood beams reinforced with FRCM–PBO (fiber-reinforced cementitious matrix–p-phenylene benzobis oxazole) composite. In order to ensure better adhesion of the FRCM–PBO composite to the wooden beam, a layer of mineral resin and quartz sand was applied between the composite and the wooden beam. Ten wooden pine beams with dimensions of 80 × 80 × 1600 mm were used for the tests. Five wooden beams, unreinforced, were used as referenced elements and another five were reinforced with FRCM–PBO composite. The tested samples were subjected to a four-point bending test in which the static scheme of a simply supported beam subjected to two symmetrical concentrated forces was used. The main purpose of the experiment was to estimate the load capacity, the flexural modulus and the maximum bending stress. The time needed to destroy the element and the deflection were also measured. The tests were carried out based on the PN-EN 408: 2010 + A1 standard. The material used for the study was also characterized. The methodology and assumptions adopted in the study were presented. The tests confirmed a significant increase in destructive force by 141.46%, maximum bending stress by 118.9%, modulus of elasticity by 18.32%, time needed to destroy the sample by 106.56% and deflection by 115.58% compared to the reference beams. The unusual method of wood reinforcement presented in the article can be considered as innovative, characterized not only by a significant load capacity margin exceeding 141%, but also by simplicity of application. Full article

The article describes the results of pull-off adhesion strength of the FRCM-PBO (Fiber Reinforced Cementitious Matrix-p-Phenylene benzobis oxazole) composite adhered to the epoxy resin layer which is the connector with the timber beam. In addition, this paper shows the results of the tests of resistance to pull-off the epoxy resin layer from the pine beam. The tests were carried out based on the Polish Standard PN-EN 1542. The Pearson linear correlation analysis was also carried out in order to determine the correlation between the obtained results and the destructive forces. The factors that occurred during the test that may affect its results, such as the method of applying the bursting force, surface preparation of the tested elements and the types of substrate destruction, were also characterized. The experimental data show that in all the tested samples, non-initial adhesive destruction between the adhesive layer and the disc was observed. Full article

Fabric-reinforced cementitious matrix (FRCM) composites, comprising high-strength fiber textiles embedded within inorganic matrices, represent an effective, cost-efficient, and low-invasive solution for strengthening and retrofitting existing masonry and reinforced concrete structures. Among different textiles employed in FRCM composites, polyparaphenylene benzo-bisoxazole (PBO) textiles are adopted due to their high tensile strength and good adhesion with the matrix. Although several experimental, numerical, and analytical works were performed to investigate the mechanical properties of PBO FRCM composites, limited information is available on their long-term behavior, as well as in the case of exposure to aggressive environments. This paper presents and discusses the results of a wide experimental campaign aimed at investigating the effect of different environmental conditions on the long-term tensile behavior of a PBO FRCM composite. Tests are performed using a clamping-grip tensile test set-up. The effect of various aggressive environments on the composite matrix cracking stress, composite tensile strength, ultimate strain, and fully cracked stage slope is investigated by comparing the results of nominally equal conditioned and unconditioned (control) specimens. These results are also compared with those of other FRCM composites comprising glass and carbon textiles subjected to the same conditionings, collected from the literature. The results show only limited reductions in the tensile properties, even after long exposure to aggressive environments. Full article

This paper aims to examine the nonlinear flexural behavior of continuous RC beam specimens strengthened with fabric-reinforced cementitious matrix (FRCM) composites through experimental testing and numerical modeling. A total of nine two-span RC beam specimens were constructed and tested. Test parameters included the type of FRCM (carbon (C-FRCM) and polyparaphenylene benzobisoxazole (PBO-FRCM)), location of strengthening (sagging and hogging regions) and number of FRCM layers (two and four layers). Test results indicated that sagging strengthening resulted in a strength gain in the range of 17 to 29%, whereas hogging strengthening increased the load capacity by 9 to 17%. The use of C-FRCM resulted in a higher strength gain than that provided by PBO-FRCM composites. Specimens strengthened with PBO-FRCM exhibited, however, higher ductility and deformational capacity than those of their counterparts strengthened with C-FRCM. Doubling the number of FRCM layers resulted in no or insignificant increase in the load capacity but reduced the beam ductility. Specimens strengthened in the sagging regions exhibited moment redistribution ratios of 13 to 26% between the hogging and sagging regions. Insignificant moment redistribution was recorded for the specimens strengthened in the hogging region. Three-dimensional (3D) numerical simulation models, with and without an interfacial bond-slip law at the fabric–matrix interface, were developed. The inclusion of the bond-slip law in the modeling had an insignificant effect on predicted response. Although the models tended to underestimate the deflection, the predicted load capacities were within a 12% error band. Numerical findings were in agreement with those obtained from laboratory testing. Full article

This paper presents the results of an experimental study conducted to understand the bond capacity through single-lap, direct-shear tests of fiber-reinforced cementitious matrix (FRCM)-concrete joints under an alkaline and hot water environment. The experimental campaign was focused on a FRCM system equipped with two different types of fibers, (PBO) and Carbon. After the conditioning, the specimens conditioned were subjected to visual inspection, and the experimental results were compared with the unconditioned specimens. Moreover, in this present work, the number of layers and the conditioning time were varied. Full article

In recent years, inorganic-matrix reinforcement systems, such as fiber-reinforced cementitious matrix (FRCM), composite-reinforced mortars (CRM), and steel-reinforced grout (SRG), have been increasingly used to retrofit and strengthen existing masonry and concrete structures. Despite their good short-term properties, limited information is available on their long-term behavior. In this paper, the long-term bond behavior of some FRCM, CRM, and SRG systems bonded to masonry substrates is investigated. Namely, the results of single-lap direct shear tests of FRCM-, CRM-, and SRG-masonry joints subjected to wet-dry cycles are provided and discussed. First, FRCM composites comprising carbon, polyparaphenylene benzobisoxazole (PBO), and alkali-resistant (AR) glass textiles embedded within cement-based matrices, are considered. Then, CRM and SRG systems made of an AR glass composite grid embedded with natural hydraulic lime (NHL) and of unidirectional steel cords embedded within the same lime matrix, respectively, are studied. For each type of composite, six specimens are exposed to 50 wet–dry cycles prior to testing. The results are compared with those of nominally equal unconditioned specimens previously tested by the authors. This comparison shows a shifting of the failure mode for some composites from debonding at the matrix–fiber interface to debonding at the matrix-substrate interface. Furthermore, the average peak stress of all systems decreases except for the carbon FRCM and the CRM, for which it remains unaltered or increases. Full article

This paper describes methods, procedures, and results of cyclic loading tensile tests of a PBO FRCM composite. The main objective of the research is the evaluation of the effect of low- and high-cycle fatigue on the composite tensile properties, namely the tensile strength, ultimate tensile strain, and slope of the stress–strain curve. To this end, low- and high-cycle fatigue tests and post-fatigue tests were performed to study the composite behavior when subjected to cyclic loading and after being subjected to a different number of cycles. The results showed that the mean stress and amplitude of fatigue cycles affect the specimen behavior and mode of failure. In high-cycle fatigue tests, failure occurred due to progressive fiber filaments rupture. In low-cycle fatigue, the stress–strain response and failure mode were similar to those observed in quasi-static tensile tests. The results obtained provide important information on the fatigue behavior of PBO FRCM coupons, showing the need for further studies to better understand the behavior of existing concrete and masonry members strengthened with FRCM composites and subjected to cyclic loading. Full article

Fabric-reinforced cementitious matrices (FRCMs) are a novel composite material for strengthening structures. Fabric contributes to tying cross-sections under tensile stress. The complexity of the interfaces between the fabric and the matrix does not allow having a simple and accurate model that enables practitioners to perform feasible calculations. This work developed an analytical approach and a numerical simulation based on the reduction of FRCMs’ strength capabilities under tensile stress states. The concept of effective strength was estimated for different types of fabrics (basalt, carbon, glass, poly p-phenylene benzobisoxazole (PBO), and steel) from experimental evidence. The proposed models calculate the ultimate bending moment for reinforced concrete (RC) structures strengthened with FRCMs. The numerical models performed simulations that reproduced the moment–deflection curves of the different tested beams. Steel fabric showed the highest contribution to strength (78%), while PBO performed the worst (6%). Basalt and carbon showed irregular contributions. Full article