Fabric Reinforced Cementitious Matrix Composites Research Articles

Strengthening and repair of RC beams using natural sisal Fabric-Reinforced Cementitious Matrix composite

The present work reports the results of an experimental program on the mechanical characterization of Reinforced Concrete (RC) beams externally strengthened/repaired by natural sisal Fabric-Reinforced Cementitious Matrix (FRCM) composite. For such, undamaged and pre-cracked RC beams were U-wrapped with a sisal FRCM composite and then tested under bending load. The results showed significant increases in shear strength, up to 25 % for beams with stirrups and 75 % for beams without them. The strengthened/repaired beams also presented stirrups yielding delay during the tests. When compared with other synthetic FRCM strengthening systems from the literature, the enhanced mechanical results of the present work were mechanically equivalent. The cracking pattern and debonding of the sisal FRCM wrap system were measured by the Digital Image Correlation (DIC) technique. The wrap application reduced the crack propagation and debonding behavior was not detected. Finally, the experimental results were confronted with an analytical approach guided by ACI 549.4R-20 and considered sufficiently accurate.

Fatigue and post-fatigue behavior of FRCM-concrete specimens under direct shear and bending conditions

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.

Shear Performance of the Interface of Sandwich Specimens with Fabric-Reinforced Cementitious Matrix Vegetal Fabric Skins

The utilization of the vegetal fabric-reinforced cementitious matrix (FRCM) represents an innovative approach to composite materials, offering distinct sustainable advantages when compared to traditional steel-reinforced concrete and conventional FRCM composites employing synthetic fibers. This article introduces a design for sandwich solutions based on a core of extruded polystyrene and composite skins combining mortar as a matrix and diverse vegetal fabrics as fabrics such as hemp and sisal. The structural behavior of the resulting sandwich panel is predominantly driven by the interaction between materials (mortar and polyurethane) and the influence of shear connectors penetrating the insulation layer. This study encompasses an experimental campaign involving double-shear tests, accompanied by heuristic bond-slip models for the potential design of sandwich solutions. The analysis extends to the examination of various connector types, including hemp, sisal, and steel, and their impact on the shear performance of the sandwich specimens. The results obtained emphasize the competitiveness of vegetal fabrics in achieving an effective composite strength comparable to other synthetic fabrics like glass fiber. Nevertheless, this study reveals that the stiffness of steel connectors outperforms vegetal connectors, contributing to an enhanced improvement in both stiffness and shear strength of the sandwich solutions.

Experimental and numerical bond behavior of PBO FRCM tested using a pull-out set-up

Fabric-reinforced cementitious matrix (FRCM) composites have become increasingly popular in the field of externally bonded (EB) reinforcement of existing concrete and masonry structures. They are used as flexural, shear, and torsional strengthening of reinforced concrete (RC) members. They are comprised of high strength textiles embedded in an inorganic-based matrix. FRCM tensile mechanical properties are characterized by clamping-grip or clevis-grip tensile tests, whereas their bond behavior by indirect or single- and double-lap direct shear tests. The eccentricity between the applied load and the restraint in single-lap direct shear tests entails for the presence of normal stresses that can affect the bond stress-transfer mechanism. Furthermore, direct shear tests require relatively large and heavy specimens. To overcome these issues, a new pull-out test set-up was proposed in the literature to investigate the bond behavior of FRCM composites that fail at the textile-matrix interface. In this paper, nine pull-out tests are performed on PBO FRCM composites. Bonded lengths shorter and longer than the textile-matrix interface effective bond length are considered. Six specimens have a short bonded length (i.e., 150 mm), whereas three have a long bonded length (i.e., 450 mm). Applied stress – global slip curves of the specimens tested highlight the presence of friction at the textile-matrix interface. The results of the pull-out tests are compared with those of corresponding single-lap direct shear tests presented in a previous work with the same PBO FRCM composite and the same bonded length and width. The applied stress – global slip curves obtained from the two sets of test show good agreement. Finally, the pull-out test set-up is validated by means of a three-dimensional finite element model.

Available measurement methods to evaluate the fiber and matrix bond performance of FRCM composites

Measuring the bond between Fabric-Reinforced Cementitious Matrix (FRCM) composites and the substrate onto which they are applied is crucial for assessing the performance of these materials when used for the strengthening of existing structures. Experimentally, this parameter has been investigated by conducting single/double lap shear tests. However, as the bond failure is usually reached at the fiber-matrix interface, it is hard to obtained reliable date of the local phenomena occurring at such location. This is caused by the impracticality of observing the behavior due to the presence of an exterior matrix layer. In this paper, a detailed review of methods used to measure the bond between the fibres and the matrix is presented. Methods studied include traditional instrumentation such as strain gauges, and more recently developed digital image correlation (DIC). In addition, due to the increase in use of new sensing technologies such as wavelength- or frequency-based fiber optic sensors (FOS), these techniques are also evaluated. The research presented in this paper emphasize the applicability of these techniques and offer a comprehensive analysis of their respective advantages and limitations, ultimately providing valuable insights into their utility for characterizing the bond behavior of FRCM composites.

Shear strengthening of RC beams with U-wrapped FRCM composites: assessment of current design guidelines

Fabric-reinforced cementitious matrix (FRCM) composites have been widely used in the past two decades as externally bonded (EB) reinforcement of existing concrete structures. They represent an effective solution for the flexural, shear, and torsional strengthening of reinforced concrete (RC) members. Shear strengthening with EB FRCM can be performed by either side bonding, U-wrapping, or full-wrapping the reinforcement around the cross-section of beams and columns. The FRCM can have a continuous or discontinuous layout, and the textile can be applied with different inclination angles with respect to the beam longitudinal axis. The contribution provided by the EB FRCM to the member shear strength is usually computed by an extension of the Mörsch truss model, which assumes the formation of a main diagonal shear crack. This approach is followed by both the Italian CNR-DT 215 (2018) and the American ACI 549.4R (2020) design guidelines, which provide similar formulations but rely on different hypotheses. In this paper, the accuracy of these formulations to determine the shear strength of U-wrapped FRCM-strengthened RC beams is assessed against a database of RC beams collected from the literature. Comparison between experimental results and analytical predictions allowed to identify the main features of the design models considered.

Bond behavior between carbon fabric reinforced cementitious matrix (FRCM) composites with added short fibers and concrete substrates

Fabric reinforced cementitious matrix (FRCM) composites show an innovative solution for strengthening existing concrete structural elements to solve the drawbacks of organic resins in fiber-reinforced polymers (FRP) system. However, the brittle fracture of the cementitious matrix significantly reduces the efficiency of the fabric. Hence, the incorporation of short fibers into the matrix is proposed to increase the fabric efficiency. For externally bonded FRCM composites to structural element surfaces, it is a crucial issue to investigate the bond behavior at the fabric-matrix and matrix-substrate interfaces, where the effect of short fibers is non-negligible and requires further investigation. In this study, a single-lap direct shear test was used to investigate the bond behavior between carbon FRCM composites with added short fibers and concrete substrates. 36 specimens were tested and discussed by considering the effects of bond length, volume content of short fibers, type of short fibers, and bond width. The experimental results were discussed in terms of failure mode, load-slip curve, and characteristic parameters. Subsequently, the contribution of weft yarns was studied and accounted for in the developed bond-slip model, which additionally takes into account the effects of the volume content of the short fibers, the type of short fibers, and the bond width.

The evaluation of the adhesion defects in FRCM reinforcements for masonry constructions by Sideband Peak Count based nonlinear acoustic technique

Fabric-reinforced cementitious matrix (FRCM) composites have emerged as reliable strengthening materials especially for historical masonry constructions. Their strengthening effectiveness strongly depends on the bond at the matrix-fibers interface and at the matrix-substrate interface; thus, the analysis of FRCM bond behavior is of crucial relevance. The strong diffusion of the FRCM composites for the reinforcement of masonry and concrete constructions requires suitable experimental techniques for assessing possible defects in the adhesion between FRCM and masonry, and between FRCM layers. To this aim, in this paper, an innovative nonlinear ultrasonic approach based on the sideband peak count technique is proposed. This approach is discussed and validated through acoustic (sonic and ultrasonic) tests performed on masonry tuff substrates reinforced with FRCM mortars embedding a basalt fibers grid and having known artificial defects at the adhesion between masonry and the reinforcement as well as in the fiber grid.

Tensile properties of carbon fabric-reinforced cementitious matrix (FRCM) at high temperatures

Despite many advantages of externally bonded FRP systems, the rapid loss of strength at high temperatures is a concern for fire scenarios. Fabric-reinforced cementitious matrix (FRCM) is deemed to perform superior at high temperatures. In this study, 84 carbon FRCM coupons were tested in tension at temperatures from 25 to 400 °C using both steady-state and transient-state test protocols. The main parameters considered in this study are the number of fabric layers and FRCM thickness, fabric orientation, target temperatures, and heating protocol. Results showed that at room temperature, the ultimate strength and cracked elastic modulus decreased by about 18% to 20% when the number of layers was increased from one to three. At 400 °C, the ultimate strength and cracked elastic modulus were reduced by up to 60 to 70% of their original values. The bidirectional FRCM specimens presented a slightly higher elastic modulus and strength than unidirectional specimens in most tests. In transient-state tests and under the same sustained load level, the failure temperature of 30 mm-thick specimens were approximately 100 °C higher than 20 mm-thick specimens. The test results showed that the carbon FRCM composites tested in this study were able to resist non-negligible tensile forces at elevated temperatures although they are significantly reduced.

Shear Strengthening of RC Beams Using Fabric-Reinforced Cementitious Matrix, Carbon Plates, and 3D-Printed Strips

Existing reinforced concrete (RC) structures suffer from degradation in their structural capacity. These structures require strengthening and retrofitting to integrate sustainability and improve their serviceability and durability. RC members strengthened with fiber-reinforced polymer (FRP) composites usually suffer from FRP debonding; therefore, researchers proposed several types of sustainable materials to overcome the shortcomings of FRP composites. Limited experimental studies have been conducted for shear strengthening of RC beams using sustainable fabric-reinforced cementitious matrix (FRCM) composites; moreover, the application of 3D-printed strips in strengthening RC beams has never been established. The current research experimentally investigates the efficiency of FRCM composites, 3D-printed sheets (CD), and CFRP plates (CP) in strengthening RC beams that are weak in shear. Various strengthening configurations were adopted, including vertical, oblique, zigzag, and several-slanted layouts. Eight simply supported beams were prepared to find the most efficient shear-strengthening configuration and material for RC beams. Test results showed that FRCM and CP are both efficient for shear strengthening in terms of maximum load capacity, initial stiffness, and ductility. However, CD showed a limited effect on enhancing the performance of shear-strengthened beams. The best shear enhancement was found in the beam strengthened with vertical CP, with improvements in load-carrying capacity, stiffness, and ductility of 43%, 23%, and 23%, respectively. The vertical and oblique strengthening configurations were more efficient than the zigzag and several-slanted layouts. The ACI 440.2R-17 model yielded accurate predictions with an average (Vc, test/Vc, ACI 440) of 1.11.

Experimental and Numerical Investigations of Punching Shear Behavior of FRCM-Strengthened Two-Way RC Slabs

Fabric-reinforced cementitious matrix (FRCM) composites have been widely studied as they constitute an alternative solution to fiber-reinforced polymer (FRP) retrofitting solutions for steel-reinforced concrete (RC) members. This study investigated the punching shear behavior of RC slabs strengthened with FRCM composites. Different tests were first performed to determine the tensile behavior of the FRCM and the interfacial bond behavior between the FRCM and concrete substrates. These tests were used as input parameters for a three-dimensional (3D) finite-element model (FEM) for RC slabs strengthened with the FRCM under shear. Subsequently, one reference slab and two strengthened slab specimens with dimensions of 1,600 mm × 1,600 mm × 100 mm were tested under punching shear. The test results demonstrate the effectiveness of FRCM strengthening under punching shear. An automation parameter calibration process was developed to identify the input parameters of Mazars’ concrete behavior law used in the 3D FEM model. The results of the numerical investigation can predict the punching shear behavior of reinforced concrete slabs with reasonable accuracy.

Numerical Modelling of the Constitutive Behaviour of FRCM Composites through the Use of Truss Elements.

The modeling of the mechanical behavior of Fabric Reinforced Cementitious Matrix (FRCM) composites is a difficult task due to the complex mechanisms established at the fibre-matrix and composite-support interface level. Recently, several modeling approaches have been proposed to simulate the mechanical response of FRCM strengthening systems, however a simple and reliable procedure is still missing. In this paper, two simplified numerical models are proposed to simulate the tensile and shear bond behavior of FRCM composites. Both models take advantage of truss and non-linear spring elements to simulate the material components and the interface. The proposed approach enables us to deduce the global mechanical response in terms of stress-strain or stress-slip relations. The accuracy of the proposed models is validated against the experimental benchmarks available in the literature.

Experimental investigation of FRCM under shear loading

Fabric Reinforced Cementitious Matrix (FRCM) can be used to improve the shear strength of masonry walls. The contribution of the FRCM is evaluated, accordingly to existing standard, taking into account the FRCM-to-substrate bond strength and the tensile strength of bare fabrics, neglecting the mortar matrix. Nevertheless, to date, there are no experimental tests that provide the actual shear capacity of the FRCM alone and not bonded to the substrate. To this scope, a four-hinge articulated steel frame was designed to investigate the shear behaviour of small-scale square specimens, as well as the mechanism of shear stress transfer between the fabric and the mortar. This paper presents the outcomes of shear tests performed on four FRCM composites, comprising of two basalt fabrics embedded in either lime- or cement-based mortars, in order to assess the influence of the single components. Digital image correlation (DIC) was also used to determine the displacement fields, strains and to detect the crack pattern of the specimens.

Matrix and fabric contribution on the tensile behaviour of fabric reinforced cementitious matrix composites

Fabric reinforced cementitious matrix (FRCM) are a type of sustainable materials for structures strengthening and envisaging to develop new construction solutions. However, the wide range of possible combinations of fabric and cementitious matrixes makes it necessary to study the particular contribution of mortar and fabric in the composite action of the FRCM. In this line, this study presents the experimental, analytical and numerical evaluation of the tensile behaviour of FRCM composites. 85 FRCM specimens of different thicknesses were manufactured, with one or two embedded fabrics of different type of fibres and geometry. Fabrics made of coated vegetal fibres were also considered to be compared with commercial fabrics of coated synthetic fibres. The results show a significant increase of the ultimate tensile strength with the increase of the FRCM thickness due to a residual contribution of the mortar after the cracking stage. The analytical model was effective for the cases that showed a cracking stabilization stage. However, the numerical model allows a closer approximation to experimental results because of considering the mortar contribution in the post-cracking stage.

Clevis-Grip Tensile Tests on Basalt, Carbon and Steel FRCM Systems Realized with Customized Cement-Based Matrices

The tensile properties of fabric-reinforced cementitious matrix (FRCM) composites are experimentally investigated through clevis-grip tensile tests (according to AC434 provisions) on FRCM coupons realized with customized (ad hoc developed in this paper) cement-based matrices. The tested FRCM coupons are reinforced with basalt, carbon, or steel fabrics, and are prepared with three different matrices: one-component mortar incorporating dispersible copolymer powders of vinyl acetate and ethylene (matrices A and B) and two-component mortar with carboxylated styrene–butadiene copolymer liquid resin (matrix C). This has made it possible to investigate the mechanical compatibility between different mortar matrices and fabrics and the resulting tensile properties of FRCM composites in the uncracked, cracking, and fully cracked phases. Experimental results are critically analyzed in terms of stress–strain curves and failure mechanisms comparatively for the analyzed FRCM systems. It has been shown that the matrix B exhibits a good compatibility with the basalt pre-impregnated fabric, while the matrix C appears to be the most suitable candidate to optimize the interfacial stress transfer at the fiber–matrix interface for all fabrics, thus exalting the mechanical performances in terms of tensile strength and ultimate strain. The results of this experimental program can be useful for designing optimized mortar mixes aimed at realizing novel FRCM composites or at improving existing FRCM systems by suitably accounting for compatibility behavior and slippage at the fabric–matrix interface.

Fabric-to-cementitious matrix bond behavior in carbon fabric reinforced cementitious matrix (FRCM) composites with added short fibers

Fabric reinforced cementitious matrix (FRCM) composites are promising composite materials with useful properties, such as non-toxicity, fire resistance, corrosion resistance, and vapor permeability. However, the brittle failure mode, fabric efficiency, and tensile strength of FRCM composites require further improvement. Accordingly, the addition of short fibers to the matrix is proposed to improve the mechanical properties of FRCM composites, which are highly influenced by the fabric-to-cementitious matrix bond behavior. The objective of this study is to investigate the fabric-to-cementitious matrix bond behavior in carbon FRCM composites with added short fibers and to propose a method to evaluate the bond parameters. In total, 75 pullout specimens were designed to investigate the effects of short fiber type, short fiber volume content, and embedded length on the fabric-to-cementitious matrix bond behavior using double-sided pullout tests. The experimental results were evaluated in terms of the peak pullout force, displacement at peak pullout force, toughness, average bond strength, and initial stiffness. A closed-form solution was presented for the pullout force-displacement relationship based on a trilinear bond-slip model that accounted for the contribution of weft yarn. Furthermore, a method for the analytical extraction of bond parameters was proposed using the main characteristics of the experimental pullout force-displacement responses.

Compressive strength of concrete cylindrical columns confined with fabric-reinforced cementitious matrix composites under monotonic loading: Application of machine learning techniques

The reinforcement of concrete columns with fabric reinforced cementitious matrix (FRCM) is one of the most challenging issues in the construction of concrete structures, as there is still an absence of a promising model to assess their performance. This is because the behavior of such elements is complex and accompanied by a high margin of uncertainty. To address this issue, this study compiles a large dataset of the performance of FRCM-reinforced concrete columns under monotonic load. The obtained dataset is then used to train an artificial neural network (ANN) as a promising method for predicting the compressive strength of concrete columns with acceptable accuracy. Afterward, using a genetic algorithm (GA)-based regression model, a simplified formula is developed as an explicit model for predicting the compressive strength of FRCM-confined concrete columns. Additionally, a reliability model is established and solved by the Monte Carlo sampling method to capture the uncertainty and provide the results probabilistically. The results indicate that with the increase of fcc, the probability of exceedance is sharply reduced, so that the failure probability of fcc greater than 68 MPa falls below 2%. Moreover, a reliability sensitivity analysis is performed to measure the effects of input parameters on the resulting exceedance probability. The results reveal that the greatest impact of column diameter and height falls within the small range of fcc between 25 and 35 MPa. The maximum effect of temperature and unconfined concrete compressive strength, however, happens in the medium range of fcc between 35 and 45 MPa. Additionally, the percentage of fiber mostly affects the large range of fcc between 45 and 49 MPa.

Long-Term Behavior of PBO FRCM and Comparison with Other Inorganic-Matrix Composites.

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.

Experimental Investigation on the Mechanical and Bond Properties of GFRP Anchors Adopted in FRCM-Masonry Strengthening

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.

Durability of a Glass Fabric-Reinforced Cementitious Matrix Composite under Different Environmental Conditions

The use of fabric-reinforced cementitious matrix (FRCM) composites for strengthening and retrofitting of existing masonry structures is nowadays a well-established practice, due to the speed and ease of application, low invasiveness, and high performance-to-cost ratio. Among different FRCMs, those comprising glass fiber textiles (GFRCM) represent an excellent choice due to the good mechanical properties and low cost of the raw material. However, limited information is available on the effect of environmental conditions on the performance of GFRCM composites and on their long-term behavior. This paper investigates the effect of different environmental conditions on the tensile performance of a GFRCM composite comprised of a cement-based matrix reinforced with an open-mesh alkali-resistant glass textile. Namely, the effect of hygrothermal conditioning, saline environment, alkali environment, freeze-thaw cycles, and dry heat conditioning were considered. After conditioning, specimens were tested using a clamping-grip tensile test configuration and the effect of the various exposure was analyzed comparing the composite tensile strength, ultimate strain, and elastic modulus of conditioned and control specimens. The experimental results show a good performance of the tested GFRCM composite with respect to the conditions considered.