Key Engineering Materials Vol. 916

Abstract: Fiber-reinforced cementitious matrix (FRCM) composites have been increasingly adopted as externally bonded reinforcement (EBR) of existing concrete and masonry members. Being debonding at the matrix-fiber interface one of the most frequent failure mechanisms of externally bonded FRCM, the matrix-fiber bond behavior represents a fundamental aspect for the effectiveness of the external reinforcement. A cohesive material law (CML) that describes the interface where debonding occurs can be used to model the bond behavior observed. In this paper, a rigid-trilinear CML is used to solve the differential equation that governs the bond problem at the matrix-fiber interface of an FRCM composite. The CML adopted has peculiar characteristics that entail for a finite length of the bond stress transfer zone (BSTZ). Furthermore, it allows for a simple and accurate analytical solution of the bond problem. The analytical solution obtained is compared with the results of an experimental campaign comprising single-lap direct shear tests of a polyparaphenylene benzobisoxazole (PBO) FRCM composite specifically designed for masonry substrates. Different calibrations of the rigid-trilinear CML are proposed, also considering the matrix-fiber free end slip.

Abstract: The strengthening and retrofitting of existing masonry built heritage has become an increasingly important issue in the last decades. Among the innovative solutions developed by the construction industry, the application of externally bonded fabric-reinforced cementitious matrix (FRCM) composites attracted a great interest, proving to be an easy, effective, and cost-efficient strengthening/retrofitting technique. FRCM composites were shown to be particularly suitable for applications on masonry due to the good compatibility between the composite inorganic matrix and the masonry substrate, which also promotes their durability.A crucial point for the effectiveness of externally bonded FRCM is the bond within the composite strip and between the composite and substrate. Indeed, composite debonding is the commonly observed failure mode. In order to improve the bond with the substrate, connectors (anchors) can be used to improve the bond capacity of the FRCM composite.In this paper, the mechanical and bond properties of a glass fiber reinforced polymer (GFRP) anchor spike, designed for FRCM strengthening, are investigated. First, tensile tests are performed to determine the elastic modulus and tensile strength of the anchor. Then, the anchor-masonry bond behavior is experimentally investigated using pull-out tests. Three different masonry substrates, namely a solid clay brick masonry, a tuff block masonry, and a stone masonry were adopted in the pull-out tests. The results show the influence of the substrate type on the anchor-masonry bond capacity and failure mode observed.

Abstract: Masonry existing buildings are subjected to significant structural damages when seismic events occur. Over the last decades, innovative techniques like composite materials based on inorganic mortar (FRCM – Fiber Reinforced Cementitious Matrices) have emerged as attractive solutions for the strengthening of civil structures. FRCM shows better compatibility with masonry substrates with respect to Fiber Reinforced Polymers. The effectiveness of FRCM reinforcement systems relies on the composite-substrate bond behavior which is affected by many parameters, leading to different failure mechanisms. Although numerous studies investigate the FRCM-substrate bond, few attentions have been paid to the study of fiber grid-matrix interface behavior. In this study, the preliminary results of a wider experimental campaign aimed at investigating the interface behavior between fiber and mortar accounting for the contribution of transversal grid wires are presented. Different typologies of fiber and mortar were tested and the results are compared and discussed.

Abstract: The mechanical properties of fiber-reinforced cementitious matrix (FRCM) composites are derived from tensile tests of composite coupons and shear tests of composite strips bonded to the substrate. Different test set-ups are used for tensile coupons, which lead to different tensile responses depending on the mechanical properties of the matrix and bond properties of the fiber-matrix interface. Direct shear tests are employed to study the stress-transfer between the composite and the substrate onto which the composite is bonded. These tests can be employed to obtain the cohesive material law (CML) that describes the bond behavior at various interfaces, such as the matrix-substrate, matrix-matrix, and fiber-matrix interface. In this paper, the cohesive material law associated with the fiber-matrix interface of a polyparaphenylene benzo-bisoxazole (PBO) FRCM composite is employed in an analytical model to reproduce the tensile response of the FRCM composite, when the fibers are gripped directly. The results of the model are compared with corresponding experimental results of tensile tests of the same FRCM composite employed to calibrate the CML. The experimental work includes digital image correlation (DIC) analysis of the cracking process. A comparison between the analytical and the experimental results is performed in terms of load response focusing on the coupon deformation and opening of the cracks.

Abstract: This study addresses a numerical investigation of the bond behaviour exhibited by an FRCM system when subject to tensile and single direct shear tests. A reinforcement system, based on a polyparaphenylene benzobisoxazole (PBO) bi-directional fibre mesh and a mixed cement-pozzolanic mortar is selected. The system is characterized by the presence of coated glass-fibre yarns and dry polypropylene yarns alternated to the PBO yarns in the warp and weft directions, respectively. The mechanical characterization of composite constituent materials is carried out together with tensile and direct shear tests. Concerning mechanical interpretation of the tests, within a mode II fracture mechanics, and assuming a trilinear cohesive material law (CML), the stress transfer law between the fibre and the matrix is back calibrated from single direct shear test results. The CML obtained is employed into a finite-difference model developed for the purpose. Tensile tests are modelled providing adequate boundary conditions. Results satisfactorily agree with the tested behaviour of the FRCM system.

Abstract: The use of inorganic matrix in fiber reinforced composites has been studied in the last years for strengthening applications in masonry construction. At the moment different systems are available after a technical qualification that allows a safe and certified use in construction industry. In the field of historical masonry the benefits of such materials are well known respect to the most known Fiber Reinforced Polymers (FRPs), due to a very poor substrate. In this study the experimental results of a larger research program are presented and discussed. A Fiber Reinforced Cementitious Matrix (FRCM) system has been tested in order to measure the tensile mechanical properties and bond properties respect to different substrates: clay masonry and natural tuff masonry. Tensile properties of the FRCM composite were measured in presence of a cement mortar, and results are illustrated. In addition pull-off tests and bond shear lap tests of the FRCM are described and commented respect to the two substrates. Tensile tests on glass fiber mesh and glass FRP (GFRP) connectors were performed and results are presented in the paper.

Abstract: The present study investigates the use of Flax Textile-Reinforced Mortars (FTRM) as a strengthening and seismic retrofitting solution for unreinforced masonry. The FTRM system comprised flax textiles embedded in lime-based mortar and was externally bonded to the surface of four medium-scale masonry walls on both sides, in strengthening configurations including one and two FTRM layers. One bare wall and one wall strengthened only with lime-based mortar were additionally examined as reference samples. All specimens were tested in in-plane shear under quasi-static cyclic loading conditions, while axial load equal to 10% of the masonry compressive strength was constantly applied throughout the test. The effectiveness of the developed FTRM system is assessed in terms of strength, deformability, energy dissipation and failure modes. The obtained results highlight the promising potential of this system as an in-plane strengthening solution for masonry, with FTRM-retrofitted specimens able to promote strain redistribution and ensure the structural integrity. Two-layer configurations were evidenced to sustain up to 118% higher load capacity, improved ductility, and provided significant energy dissipation capacity.

Abstract: The increasing need of society to develop more sustainable and renewable materials has made vegetal fibers an interesting potential substitute for synthetic fibers in strengthening systems, due to their considerable strength and deformation capacity. This paper aims to increase the knowledge on how the materials interact between them in vegetal FRCM composites to strengthen concrete structures. To do it, two fibers were selected: cotton (CO), due to its good deformation capacity, and hemp (HE), due to its high strength. A low-viscosity and high adherence epoxy resin was used to coat the yarns to protect them from the alkali environment of the cementitious matrix. To study the FRCM-substrate interaction, an adaptation of the test methodology described in ISO 10080:2005 was developed and performed. Three different lengths (30, 40, and 50 cm) were used to obtain the optimal bonding length for hemp case. A single case (50 cm) for cotton was tested to compare its behaviour against hemp. In the FRCM-substrate interaction, it is noticed that hemp-FRCM shows complete bonding as all except one specimen failed by mesh failure. In the case of cotton-FRCM, its deformation capacity (at least 4 times hemp-FRCM) made all specimens deform until the geometric end of the testing set up without the mesh breakage, bearing a lower load but keeping it constant through fiber-matrix friction. In terms of load, hemp-FRCM reached the highest load, 10% higher than cotton-FRCM’s peak load. To conclude, the testing method for assessing steel reinforcement bonding in reinforced concrete was proved to be satisfactory at assessing FRCM-concrete interaction, being able to transmit the load from the substrate to the composite without the slippage of the vegetal-FRCM.

Abstract: An experimental campaign has been carried out with the aim of using some natural materials as structural reinforcement. In a first stage, jute fabrics are investigated through uniaxial tensile tests. At this level, the chemical treatment with Ca(OH)₂ tends to reduce the strength of the vegetal reinforcement. Nevertheless, this treatment seems to improve the bond between fabric and cementitious matrix. Indeed, the strength of FRCM reinforced with treated jute or flax are larger than those untreated. Similarly to the industrially produced reinforcement, as the bond strength increases, an increasing number of cracks can be observed in FRCM reinforced with vegetal fabrics. When large structural elements, such as existing brick walls, are reinforced by FRCM containing jute or flax, the strength and the fracture toughness is larger than those measured in plain walls. In particular, the increment of ductility is in direct proportion with the number of cracks developed by FRCM under uniaxial tension.

Abstract: The latest seismic events in Ecuador have allowed to verify some damage typologies on masonry panels of reinforced concrete buildings with frame resistant structures. Therefore, some theoretical and experimental research have been carried out to justify the intervention for repairing the damage to the masonry walls to give them a certain degree of shear ductility to provide for compatibility of the lateral deformations of the structure and of the masonry panels.