1,721,086 research outputs found
Some key aspects in the mechanics of stress transfer between srg and masonry
Full text linkThe use of composite materials to strengthen masonry structures has become common practice within the civil engineering community. Steel-reinforced grout (SRG), which comprises high-strength steel fibers embedded in a mortar matrix, is part of the family of the fiber-reinforced cementitious matrix (FRCM) composites that represent a suitable alternative to fiber-reinforced polymer (FRP) composites for strengthening existing structures. Although studies on FRCMs have already reached a certain level of maturity, some key issues remain open, such as the role of matrix type and layout, substrate properties, and test rate. This paper focuses on some of these issues. The results of single-lap direct shear tests on masonry blocks strengthened with SRGs are presented to analyze the bond behavior between the composite material and the substrate. Four aspects are considered: (1) the change in the width of the SRG mortar matrix while keeping the width of the fiber sheet fixed; (2) the type of mortar used for the SRG; (3) the influence of the test rate, and (4) the type of substrate (i.e., concrete vs. masonry). The results obtained indicate the active role of the matrix layout and the importance of the test rate, encouraging further investigations to clarify these aspects
Advances in Knowledge of the Fracture Properties of Cohesive Materials: Fired-Clay and Tuff Bricks
No full textThis paper presents an experimental work aimed at determining the fracture properties of fired-clay and tuff brick, obtained from construction sites of existing buildings and a commercially available fired-clay brick. This paper provides a range of values of the fracture properties of natural and artificial bricks used in existing constructions. Natural and artificial stones are quasi-brittle materials and the cohesive crack model can be used to describe the propagation of cracks in these materials. However, some of the parameters of the cohesive crack model are calibrated against a large set of data of concrete specimens and might not be suitable for other quasi-brittle materials that feature a smaller fracture process zone. In this paper, digital image correlation (DIC) is used to put forward a new approach to measure the critical crack opening wc and therefore gain a new insight into the fracture parameters of bricks
The role of the fiber–matrix interfacial properties on the tensile behavior of FRCM coupons
No full textFiber-reinforced cementitious matrix (FRCM) composites are usually mechanically characterized by means of tensile and bond tests. The former, in the clevis-grip version, is referred to by the American guidelines ACI 549.4R (2013) to determine the tensile properties of the FRCM composite. The latter, in the single-lap version, is used in the Italian guidelines CNR-DT 215 (2018) to determine the effective strain. The effective strain is the strain at which debonding occurs and therefore composite action is lost. In this paper, the poliparafenilene benzobisoxazole (PBO) fiber–matrix stress transfer law, also known as cohesive material law (CML), is employed in an analytical model that describes clevis-grip tensile tests of PBO-FRCM composites. The CML was previously obtained by the authors from single-lap shear tests. The load responses provided by the model are compared with the results of tensile tests herein presented in addition to selected tests from the literature. The experimental cracking process, tensile strength, and deformation capacity can be accurately predicted by the analytical model. The comparison indicates that the knowledge of the CML of the fiber–matrix interface allows for an accurate prediction of the main tensile properties of the PBO-FRCM coupon
Tensile Testing of FRCM Coupons for Material Characterization: Discussion of Critical Aspects
No full textTensile tests on fabric-reinforced cementitious matrix (FRCM) coupons are used to evaluate the tensile mechanical properties of the composite. Bond tests, typically single-lap shear tests, are used to characterize the interfacial properties between the FRCM composite and the substrate and to identify the interface at which debonding takes place. Some FRCM composites exhibit debonding at the fiber–matrix interface, which is characterized by a cohesive material law (CML) that can be obtained from bond tests. The authors have shown that for these composites, the CML can be fed into an analytical model to predict the results of tensile tests. In this paper, the same model is used to highlight some critical aspects of the clevis-grip tensile test. In particular, it will be shown that the length of the gripping devices, the length of the specimen, and the gauge length adopted to measure the deformation of the specimen have a strong influence on the results of the tensile tests. In addition, an analogy between the clevis-grip tensile test and single-lap shear test will point out that the tensile test is a bond test and can be used to determine the bond capacity rather than the tensile properties, which will be proven to be non-uniquely defined by this test
Approximate Evaluation of Maximum Force Transferable at FRP-Masonry Interface
No full textThe debonding phenomenon between fiber-reinforced polymer (FRP) composites and masonry is influenced by the presence of mortar joints that could reduce the bond capacity of the FRP-brick interface (i.e., in the absence of the joints). The actual estimate of the maximum load transferable at the FRP-masonry interface is cumbersome. Thus, the aim of this paper is twofold. First, the paper provides a set of useful key remarks to understand how the presence of joints influence the behavior of the FRP-masonry interface with respect to the FRP-brick interface. This is an important aspect of the paper because it summarizes previous work and provides guidance for designers and researchers. Secondly, this paper proposes a step-by-step procedure to obtain an approximate estimate of the maximum transferable load at the FRP-masonry interface provided that the characteristics of the FRP-brick and FRP-mortar interfaces are known or can be derived from codes and guidelines, and the thickness of brick and joints is assigned. To the best knowledge of the authors, an approximate formulation for the FRP-masonry interface is presented in this paper for the first time together with guidance to evaluate when an approximate formulation might be necessary based on the desired acceptable error. The comparison between the maximum load transferable at the FRP-masonry interface and its approximation presented in this paper provides a relative error of roughly 0.4% for case of practical interest. Finally, the paper provides an example application of the proposed procedure
An experimental study of the bond behavior of twisted steel bars embedded in mortar cylinders and in the joints of masonry wallettes
Full text linkAmong the different strengthening techniques available to reinforce masonry buildings, the near-surface mounted technique consists in inserting reinforcing elements of high-strength material in grooves realized by removing part of the mortar in the horizontal bed joints of a masonry wall. The present study deals with two experimental campaigns on pull-out tests conducted on twisted steel bars embedded concentrically in mortar cylinders and in the mortar bed joints of masonry wallettes as in in-situ applications. In order to compare the results between cylindrical and masonry specimens, the same bonded lengths are considered
Tensile Testing of FRCM Coupons for Material Characterization: Discussion of Critical Aspects
No full textTensile tests on fabric-reinforced cementitious matrix (FRCM) coupons are used to evaluate the tensile mechanical properties of the composite. Bond tests, typically single-lap shear tests, are used to characterize the interfacial properties between the FRCM composite and the substrate and to identify the interface at which debonding takes place. Some FRCM composites exhibit debonding at the fiber-matrix interface, which is characterized by a cohesive material law (CML) that can be obtained from bond tests. The authors have shown that for these composites, the CML can be fed into an analytical model to predict the results of tensile tests. In this paper, the same model is used to highlight some critical aspects of the clevis-grip tensile test. In particular, it will be shown that the length of the gripping devices, the length of the specimen, and the gauge length adopted to measure the deformation of the specimen have a strong influence on the results of the tensile tests. In addition, an analogy between the clevis-grip tensile test and single-lap shear test will point out that the tensile test is a bond test and can be used to determine the bond capacity rather than the tensile properties, which will be proven to be non-uniquely defined by this test
Simplified Procedure to Determine the Cohesive Material Law of Fiber-Reinforced Cementitious Matrix (FRCM)–Substrate Joints
Full text linkFiber-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
FRP-masonry interfacial debonding: An energy balance approach to determine the influence of the mortar joints
No full textIn this paper a close-form relationship between the transferable load at the FRP-masonry interface and the characteristics of the FRP-brick and FRP-mortar interfaces, which are expressed in terms of cohesive material laws, is proposed. Prior to obtaining the aforementioned close-form relationship, experimental evidence is used to highlight that the fracture process at the FRP-masonry interface depends on the characteristics of the constituent materials and the geometry of the masonry. In addition, an analytical procedure previously proposed by the authors, which employs two simplified cohesive material laws for the FRP-brick and FRP-mortar interfaces, is revisited to show that the length of the stress-transfer zone of, and the transferable load at, the FRP-masonry interface vary periodically in accordance with the periodic pattern of bricks and mortar joints
A new predictive model for FRCM-confined columns: A reflection on the composite behavior at peak stress
No full textFiber-reinforced cementitious matrix (FRCM) composites have become increasingly popular in the last decade, due to the ability of this type of composites to overcome some of the drawbacks related to the use of resin-based composites. Recent studies demonstrate that the application of FRCM composites improves the compressive behavior of confined concrete and masonry columns as well as their ductility. This paper compiles the available experimental data on concrete and masonry columns confined with FRCM composites. A critical review of the experimental data on FRCM-confined columns is carried out to understand which parameters influence the response of the columns. Then, a new predictive model is proposed that takes into account the database available and the mechanics of the confinement
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