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.
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.
Many efforts have been made to develop a rapid and sensitive method for phytoplasma and virus detection. Taking our cue from previous works, different rapid sample preparation methods have been ...tested and applied to Candidatus Phytoplasma prunorum ('Ca. P. prunorum') detection by RT-qPCR. A duplex RT-qPCR has been optimized using the crude sap as a template to simultaneously amplify a fragment of 16S rRNA of the pathogen and 18S rRNA of the host plant. The specific plant 18S rRNA internal control allows comparison and relative quantification of samples. A comparison between DNA and RNA contribution to qPCR detection is provided, showing higher contribution of the latter. The method presented here has been validated on more than a hundred samples of apricot, plum and peach trees. Since 2013, this method has been successfully applied to monitor 'Ca. P. prunorum' infections in field and nursery. A triplex RT-qPCR assay has also been optimized to simultaneously detect 'Ca. P. prunorum' and Plum pox virus (PPV) in Prunus.
In strengthening RC elements by means of FRP wraps or strips, bonding between the FRP reinforcement and the concrete substrate is a very relevant aspect. Bonding is influenced by the mechanical and ...physical properties of concrete, composite and adhesive, as well as by the surface treatment of the concrete substrate. Several relationships are proposed in design recommendations in order to estimate the fracture energy and then the debonding load of the FRP reinforcement from the concrete substrate. These relationships generally include parameters calibrated from experimental push–pull tests. In this work an enlarged experimental database is considered combining results selected in the literature and push–pull debonding tests performed by the authors. Authors’ tests were performed with both wraps (13) and strips (17) of different lengths. The debonding load is theoretically evaluated on the basis of the fracture energy concept. The enlarged experimental database is used to achieve a refined calibration of the involved experimental parameters. As a refinement to current bond strength and fracture energy models, calibrations are performed separately for wraps and strips by two different statistical models in order to appreciate the influence of the reinforcement type and model assumption on the debonding load.
Several reinforced-concrete (RC) structural elements are subjected to cyclic load, such those employed in highway and railroad bridges and viaducts. The durability of these elements may be reduced as ...a consequence of fatigue, which mainly affects the steel reinforcement. The use of externally bonded (EB) fiber-reinforced cementitious matrix (FRCM) composites allows the moment capacity to be shared by the internal reinforcement and the EB composite, thus increasing the fatigue life of the strengthened RC member. The effectiveness of EB FRCM composites is related to the composite bond properties. However, limited research is currently available on the effect of fatigue on the bond behavior of FRCM-substrate joints. This study provides first the state of the art on the fatigue behavior of different FRCM composites bonded to a concrete substrate. Then, the fatigue bond behavior of a polyparaphenylene benzo-bisoxazole (PBO) FRCM is experimentally investigated using a modified beam test set-up. The use of this set-up provided information on the effect of fiber-matrix interface shear and normal stresses on the specimen fatigue bond behavior. The results showed that fatigue loading may induce premature debonding at the matrix-fiber interface and that stresses normal to the interface reduce the specimen fatigue life.
In this paper, the debonding strength of axially loaded double shear lap specimens between steel plates and carbon fibre reinforced plastic plates is investigated from the analytical, numerical and ...experimental point of view. Two steel plates were joined together by two carbon fibre reinforced plastic (CFRP) plates and epoxy adhesive in order to realize double shear lap specimens of different length. Failure of the steel–adhesive interface was identified as the dominant failure mode and fracture mechanics and stress based approach are presented in order to estimate the relevant failure load. A good agreement between the analytical–numerical results and experimental data is achieved.
Fabric Reinforced Cementitious Matrix (FRCM) composites are advanced cement-based materials often used for strengthening masonry or concrete structures. The system is usually composed of a dry grid ...of fibers embedded in a cementitious matrix enriched with short fibers.
An important parameter for designing the structural reinforcement is the tensile load-bearing capacity of FRCM composites. For their heterogeneity, FRCM composites show an interesting mechanical behavior in tension, that depends on the properties of the components and of the bonding strength. These values could be estimated with mechanical models but must be validated experimentally by means of proper testing campaigns.
In this work several FRCM materials made with different fiber grids were investigated. Four different types of fibers were considered: polyparaphenylene benzobisoxazole (PBO), carbon (C), glass (G) and PBO and glass (PBO-G) fibers and three different types of cementitious mortars.
The behavior of FRCM under tension and the influence of the bond properties between the dry textile and the inorganic matrix are studied developing an extensive experimental program that included the characterization both of the materials components and of the composites. A series of push–pull double lap tests and pull-off tests were performed to determine the bonding properties of FRCM composites applied to masonry structures.
The paper presents results and considerations that can provide background data for future recommendations for the use of FRCM systems in the rehabilitation of elements.
The behavior of single bricks and small masonry pillars strengthened by means of fabric reinforced cementitious matrix systems made with glass-fiber grids is discussed both from an experimental and ...numerical standpoint.
A standard Push–pull double lap test is performed on three different series of experimental set-ups for reinforced single bricks and on masonry pillars, evaluating the role played by the anchorage length on the overall behavior of the strengthened system.
Standard Italian bricks with very good mechanical properties are used, in order to evaluate the ultimate strength of the grid for delamination within the mortar. The masonry pillar is built with 3 bricks spaced out by two thick mortar joints. When dealing with the single bricks, three different anchorage lengths were tested, equal to 5, 10 and 15cm, in order to evaluate the reduction of the ultimate strength induced by an insufficient anchorage.
To suitably interpret experimental results, both a newly developed analytical–numerical approach and a recently presented 3D FEM model were utilized to have an insight into experimental results.
In the analytical–numerical approach only the glass-fiber grid was considered and modeled by means of 1D Finite Elements interacting with the surrounding mortar by means of interfaces exhibiting a non-linear stress–slip behavior deduced from experimental data.
The 3D model uses 8-noded rigid elements interconnected by inelastic interfaces exhibiting softening. The incremental non-linear problem is solved by means of a robust Sequential Quadratic Programming routine already tested on medium and large scale examples with softening materials. The grid is modeled through non-linear truss elements, interacting with surrounding mortar by means of non-linear interfacial tangential stresses. Stress–slip behavior of the interface between the mortar and the textile is deduced through ad hoc experimentation conducted on a mortar specimen reinforced with a single yarn and subjected to a standard tensile test.
Good agreement was found between experimental evidences and numerical simulations, meaning that the combined approach proposed may be considered as reference for design considerations.
Composite materials made with textile fibers both with polymeric and cementitious matrices are often adopted for the retrofitting of masonry arches and vaults. A specific project that analyzes the ...performance of ancient masonry arches and vaults strengthened with composite systems has been recently concluded at Politecnico of Milan. The project involves the experimental evaluation and the development of numerical and analytical simulations. In this paper the experimental campaign is described, whereas the numerical validation is provided in an accompanying paper (Bertolesi et al., 2017). The tests were performed in situ on ancient masonry arches and vault elements. In particular, three barrel vaults and two arches either unreinforced or reinforced with Steel Reinforced Grout (SRG), Textile Reinforced Mortar (TRM) and Fiber Reinforced Polymer (FRP) were tested.
The arches had a span equal to 3.30 m, a rise equal to 0.83 m and were built with common Italian bricks regularly spacing out two bricks laid edge on (thickness of the arch 12 cm) with two bricks (one over the other) disposed in single leaf. Barrel vaults had the same geometry of the arches but were made with a single leaf. In all cases, an eccentric vertical load was applied at 1/4 of the span and was increased up to failure. The experimental results on unreinforced structures are compared with those obtained on the strengthened ones in terms of failure mode, maximum load, stiffness and ductility.
AbstractFabric-reinforced cementitious matrix (FRCM) composites made of dry-fiber fabric embedded in an inorganic matrix are advanced cement-based materials designed for retrofitting masonry or ...concrete structures. Characterization of the tensile behavior of FRCM composites provides the parameters needed for the design of the structural reinforcement and has given rise to numerous research studies on the aspects that influence its mechanical properties. To obtain the tensile behavior characteristics of this composite under different boundary conditions, two test setups were investigated. A clevis grip (pin action) was used to reproduce field boundary conditions from typical installation and to obtain design parameters. A clamping grip was used to obtain a complete characterization of the composite by inducing a tensile failure of each constituent material. Several FRCM systems made with different fabrics were used for the investigation: polyparaphenylene benzobisoxazole (PBO), carbon (C), and glass (G), plus carbon and glass with a special protective coating. This paper offers a critical analysis of the experimental results and provides recommendations for the tensile characterization of FRCM materials.
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