Fiber Reinforced Cementitious Matrix Composites Research Articles

Simplified Procedure to Determine the Cohesive Material Law of Fiber-Reinforced Cementitious Matrix (FRCM)-Substrate Joints.

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.

Strengthening non-engineered building using vegetal FRCM retrofitting: A numerical modeling approach for seismic assessment

The concept of developing alternative rehabilitation techniques to enhance the performance of structures with low seismic resistance has come to light in recent years. Substantial research and methods have been devoted to evolving strengthening techniques worldwide for structures with economic value, with the ultimate goal of enhancing seismic resilience in them. Among these, Fiber Reinforced Cementitious Matrix (FRCM) stands out as a reliable alternative to conventional retrofitting approaches due to its high strength-to-density ratio and negligible mass increment. However, the inadequacy of full-scale modeling and the application of FRCM in building components persist in the research domain. Therefore, this paper aims to develop a numerical model for the full-scale application of vegetal FRCM composites in retrofitting a representative non-engineered building. The full-scale application of the four vegetal FRCM materials (hemp, cotton, flax, and sisal) in 1-ply and 2-ply configurations was undertaken to strengthen a non-engineered building. The results of pushover analysis show that buildings retrofitted with 2-ply hemp and 2-ply flax FRCM exhibited the most significant improvement in base shear strength, with increments of about 30% and 28.5% respectively, compared to the peak base shear of the bare frame. Additionally, the retrofitted models demonstrated an increase in their ductility. A subsequent fragility analysis highlighted a reduction in the seismic hazard vulnerability of the building for the relevant earthquake spectrums. Finally, the results from the numerical analysis confirm that the use of vegetal fibers as cementitious matrix can have significant potential to be an effective and sustainable strengthening technique for seismically deficient buildings.

Experimental investigation on a FRCM bio-composite for sustainable retrofitting of masonry buildings

Strengthening masonry structures by using green and natural materials has received increasing attention. Natural fibers can be obtained from vegetables, animals or mineral sources and can be used to meet modern buildings safety standards and to reduce carbon dioxide emissions. This paper presents a novel method to strengthen masonry buildings through the application of a mortar coating reinforced with natural fibers made of a jute or a basalt grid. This class of materials is known as Fiber Reinforced Cementitious Matrix (FRCM). FRCM composites can produce an increment in the mechanical properties of masonry members and contribute to the thermal-acoustic insultation of the building envelope. The mortar coating is made of a hydraulic lime: to increase thermal properties this was admixed with short spelt fibers. Both matrix (lime mortar) and fibrous material were initially mechanically and thermally characterized. Laboratory tests have been carried out to study the bond characteristics between the FRCM composite and brickwork masonry. It was found the low mechanical properties of jute fibers are aggravated by a low FRCM-to-masonry bonding, making difficult to use this material for structural retrofit. On opposite, basalt fibers exhibit high mechanical properties and their use in conservation is surely promising and interesting.

Shear capacity prediction for FRCM-strengthened RC beams using Hybrid ReLU-Activated BPNN model

This study presents a robust Hybrid ReLU-Activated Backpropagation Neural Network (BPNN) model for predicting shear strength (VFRCM) in RC beams reinforced with Fiber-Reinforced Cementitious Matrix (FRCM) composites. The model demonstrates exceptional accuracy, boasting a coefficient of determination (R2) of 0.924 and a mean root mean squared error (RMSE) of 10.611 kN on the test set. Leveraging k-fold cross-validation, the model's performance is rigorously assessed across ten data splits, emphasizing its reliable predictive capabilities. Hyperparameter optimization, guided by Particle Swarm Optimization (PSO), reveals an optimal value of α = 0.5757, minimizing the Mean Squared Error (MSE) to 74.53 for the dataset. Feature importance analysis underscores the significant role of critical parameters in shear strength prediction, with the longitudinal reinforcement ratio (ρl) emerging as the most influential feature, contributing 37.55 % to the model's predictive power. Additionally, the effective beam depth (d) is identified as the second most influential feature, providing approximately 16 % to the accurate prediction of VFRCM in RC beams strengthened with FRCM composites.

Prediction of Flexural Strength of RC Circular Columns Considering Lateral Confinement Effects of FRCM and Transverse Steel Reinforcement

There has recently been growing interest in making a sustainable and durable fiber reinforced cementitious matrix (FRCM) to improve the seismic performance of RC column members. However, most studies evaluating the lateral confinement effect on FRCM jacketing concrete have excluded the confinement effect of transverse steel reinforcement and focused solely on mechanical properties. This paper, based on existing studies, proposes a peak axial stress formula that considers the lateral confinement effects of FRCM composite and transverse steel reinforcement. Additionally, the study assesses the structural performance of FRCM composites with flexural strengthening in actual size RC circular columns and predicts the maximum flexural strength by applying the proposed formula. The proposed peak axial stress formula, considering the influence of transverse reinforcement, was compared with the experimental value (databases) and the predicted value, and the suitability was confirmed with a standard deviation of 27.6% and a coefficient of variation of 0.856. In addition, the maximum flexural strength of actual size RC circular columns predicted by applying this formula ranged from 0.97 to 1.02, which effectively predicted the experimental values.

EVALUATION OF RETROFIT EFFECTIVENESS AND COST IMPLICATIONS OF MRF STRUCTURES USING DISTINCT FRCM COMPOSITE SYSTEMS

This paper summarizes the results of a numerical investigation to assess the effectiveness of retrofitting pre-seismic code concrete framing structural systems using two distinct fiber-reinforced cementitious matrix (FRCM) composites. While FRCM composites were focused in the literature at the member level, this study focuses on implementing this retrofit technique effectively at the structure level. Several static pushover analyses (SPAs) and inelastic dynamic simulations (IDSs) are carried out for FRCM-retrofitted RC frames representing the lateral force-resisting system of a moment-resisting frame (MRF) structure. Correlating the numerical simulation results and experimental results obtained from another study verified the selected fiber discretized section modeling technique for FRCM retrofitted MRFs and reflected the effectiveness of the adopted approach in prolonging local and global damage states. The verified modeling technique of the risk-mitigation measure is then employed to evaluate the performance of an upgraded RC building using three-dimensional fiber-based numerical models. Finally, a parametric study using SPAs and IDSs using multiple seismic scenarios is conducted to select the most effective FRCM retrofit material for the seismic performance enhancement of the benchmark structure. The study concludes that using the selected FRCM retrofit technique effectively mitigated the structural collapse damage state of the substandard building at the design and maximum considered seismic intensity levels by 28% and 32%, respectively.

A UNIFIED MACHINE LEARNING MODEL TO PREDICT ULTIMATE TENSILE AND BOND CAPACITY OF FRCM

Fiber-reinforced cementitious matrix (FRCM) is a new-era material for strengthening concrete and masonry structures. Unlike Fiber reinforced polymer (FRP), it has excellent substrate bond properties. However, FRCM shows intra-fiber slip and Fiber-matrix debonding as its major failure modes, effectively reducing its ultimate load-carrying capacity. It has led to broad research scope, and researchers have used permutations and combinations of various types of fibers, matrices, coatings, and substrates to improve the tensile and bond properties of the FRCM composites. Additionally, other experimental parameters, such as rate of loading and experimental setup (clevis, hydraulic or mechanical), affect the tensile and bond properties of FRCM composites. Complex failure modes and parameters make tensile and bond capacity prediction very difficult. Previous attempts by researchers showed the effectiveness of machine learning in predicting the bond capacity of the FRCM composites. Artificial neural network (ANN) based models have also been used successfully to predict the flexural capacity of FRCM-strengthened masonry panels. This study develops a unified machine learning-based model to predict bond and tensile capacity using complex tree-based algorithms such as extra tree regressor, categorically boosted trees, extreme Gradient boosted trees, etc. The authors extracted 603 data points from the literature to develop and validate the model; this includes 345 data points from tensile testing and 258 data points from FRCM-concrete bond testing. It is found that the developed tree-based model predicts the ultimate tensile and bond capacity with about 93% and 95% overall accuracy, respectively.

Shake-table testing and numerical simulation to select the FRCM retrofit solution for flexure/shear deficient RC frames

This paper presents shake-table and numerical investigations to select a retrofit strategy for reinforced concrete (RC) frame structures using fiber-reinforced cementitious matrix (FRCM) composites. Two FRCM alternatives are investigated, namely polyparaphenylene benzobisoxazole (PBO) and carbon FRCM composites, denoted as ‘P-FRCM’ and ‘C-FRCM’, respectively. The study focuses on implementing the selected retrofit material with spike anchorage systems to delay the debonding failure and addresses different failure modes of RC framing systems, including shear or flexure failure. The first phase of this study involves pre shake-table test (STT) simulation to investigate the effectiveness of the selected retrofit technique in delaying local and global damage states and failure modes. STT campaigns are undertaken in the second phase to assess the seismic performance of framing systems retrofitted with different FRCM materials and verify their fiber-based modeling approach. The verified numerical modeling approach of the retrofit techniques is utilized in the third phase to upgrade a benchmark RC moment-resisting frame (MRF) building. A parametric study involving inelastic pushover analyses (IPAs) and multi-record incremental dynamic analyses (IDAs) is finally conducted to assess the vulnerability of the retrofitted building with different FRCM alternatives. It is concluded that the adopted two retrofit techniques significantly reduced the collapse damage state at the design and maximum considered earthquake intensities by up to 59% and 46%, respectively. Although the P-FRCM composite has 38% higher tensile strength and 76% higher ultimate tensile strain than C-FRCM, it slightly improves the overall seismic performance by up to 9% based on the adopted seismic performance index. Owing to the higher cost of P-FRCM compared with its counterpart, a performance-to-cost index confirmed that the C-FRCM composite is the preferred retrofit technique for MRF structures.

Experimental and numerical investigation on the tensile behavior of PBO FRCM composites with textile lap splice

Fiber-reinforced cementitious matrix (FRCM) composites are comprised of high strength textiles embedded within inorganic matrices. Due to their peculiar mechanical and physical properties, they represent a valid alternative to fiber-reinforced polymer (FRP) composites for strengthening existing reinforced concrete and masonry members. Depending on the textile and matrix adopted, FRCM composites provide a peculiar mechanical behavior, which shall be assessed to obtain the mechanical properties to be used in the design of FRCM-strengthened members. Since FRCM can be applied to large surfaces, reinforcing textile lap splice is needed to guarantee the continuity of the composite mechanical performance. Thus, the behavior of FRCM with textile lap splice should be properly investigated. In this paper, the tensile behavior of a polyparaphenilene benzobisoxazole (PBO) FRCM with continuous and overlapped textiles is studied using clevis-grip tensile tests. The experimental results are modeled using two finite element approaches, which accounted for the non-linear behavior of materials and for the interface between matrix and textile fiber. The results obtained showed that clevis-grip tests can be used to determine the minimum lap splice length needed to obtain the full composite load-carrying capacity and confirmed the validity of the method previously proposed by the authors to estimate such lap splice length.

Beam tests for the determination of the interfacial properties of FRCM composites

Fiber Reinforced Cementitious Matrix (FRCM) materials are widely used for strengthening reinforced concrete (RC) structures. Experimental studies show that the collapse of RC elements reinforced with FRCM composites occurs due to the loss of bond of the reinforcing material. For this reason the issue of the shear stress transfer between reinforcement and concrete substrate has been extensively studied in the last years. The single-lap shear test is the most frequently adopted test configuration. In this paper the results of bond tests between a FRCM composite and concrete performed in the beam test configuration are presented. The results are used to determine the cohesive material law (CML) of the tested composite and the obtained CML is compared with that obtained on the basis of the results of single-lap shear tests. The comparison shows that the two test configurations provide similar CML.

Open issues on the investigation of PBO FRCM-Concrete debonding

This paper addresses some open issues of the bond behavior of fiber-reinforced cementitious matrix (FRCM) composites applied to a concrete substrate. The study is focused on the effect of the number of fiber layers on the peak load, the effect of the loading rate on the load response, the relationship between peak load and bonded length (considering also bonded lengths substantially longer than the effective bond length), and the information that can be gained by measuring the slip at the free end of the bonded composite. The FRCM composite investigated is comprised of polyparaphenylene benzobisoxazole (PBO) fibers and a polymer-modified cementitious mortar. The experimental results are compared with results from the literature. The results indicate that the peak load must be carefully interpreted considering the effect of friction (interlocking), which is observed to occur between fibers and matrix and the fibers themselves. In addition, the effect of the loading rate requires additional investigation. Finally, the peak load of specimens with two layers of PBO fiber mesh is approximately double the peak load of tests with one layer of fiber mesh if the bonded length is longer than the effective bond length determined from tests with one layer of fiber mesh.

Effect of temperature variations on the bond behavior of FRCM applied to masonry

In the last decades, Fiber Reinforced Cementitious Matrix (FRCM) composites were successfully introduced to repair and strengthen existing masonry structures. The good mechanical performances of these materials determined their efficiency as a strengthening technique; however, their durability is still an open issue. As a matter of fact, FRCM composites may be exposed to a combination of different environmental conditions and, additionally, to temperature variations due to solar radiation. The objective of this research was to study the effects of temperature variations on the bond behavior of a FRCM composite, constituted by a basalt grid and a lime-based mortar matrix, applied to masonry. For this purpose, an experimental investigation on thermally conditioned FRCM-strengthened masonry wallets is presented, in which 14 single-lap shear tests were performed. Before testing, samples were exposed to different target temperatures inside a climatic chamber: 32, 40, 50, 60 and 80 °C. Thermocouples were embedded within the FRCM reinforcing layers at two different depths to detect the inner temperature profiles and to control the conditioning process. The single-lap shear tests were then carried out inside the same climatic chamber, while maintaining the target temperature constant. A decrease in terms of peak-axial stress was observed by increasing temperature, along with a progressive change in the failure mode, from fiber rupture outside the bonded area to fiber slippage within the mortar matrix layers.

Seismic behavior of damaged RC columns repaired with FRCM composites

In literature, a limited number of studies were carried out on the use of externally bonded composites to repair reinforced concrete (RC) columns subject to cyclic increasing lateral loads. Among them, very few deal with the use of fiber reinforced cementitious matrix (FRCM) composites. Thus, this work experimentally analyzes the behavior of two real scale RC columns repaired through FRCM with carbon fibers, subject both to axial and lateral loads, the latter applied through fully-reversed increasing cycle steps. The goal of the repair interventions was to restore the initial strength and ductility of the specimens after severe damage was applied to the original columns. Two repair methods were adopted: in one case, concrete confinement at the column base was realized; in the second case, other than confining jacket, flexural reinforcement was also used. Cracking pattern, load-displacement curves, ductility, stiffness and strength deterioration, energy dissipation and curvature development along the plastic hinge zone were investigated, comparing the performance of the control specimens with the repaired ones. Results were also compared with ones obtained applying a simple analytical model. Fibers exploitation was experimentally monitored in the repaired columns. Results demonstrate that FRCM confinement jackets can adequately repair seismically-damaged RC columns, restoring the initial lateral load capacity. Repaired columns showed also a similar ductility and energy dissipation compared to the undamaged control ones, while damping ratio resulted slightly lower for the repaired columns.

A new predictive model for FRCM-confined columns: A reflection on the composite behavior at peak stress

Fiber-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.

Relationship between Results of Tensile Test of FRCM Composites and the Fiber-Matrix Bond Properties

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.

Calibration of a Rigid-Trilinear Cohesive Material Law to Describe the Matrix-Fiber Bond Behavior in FRCM Composites

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.

Shear Transfer Mechanisms of FRCM-Masonry Systems: A Critical Analysis of an Extended Database

In the context of the mechanical behaviour of FRCM (Fiber Reinforced Cementitious Matrix) composites as strengthening materials for masonry structures, an important issue is the evaluation of the shear transfer mechanisms between the FRCM material and the masonry. In this work a wide database of more than 500 experimental results of shear bond tests on FRCM-masonry specimens is collected. The data concern specimens made with different masonry supports and various FRCM materials. The data are subdivided into homogeneous classes. The occurrence of the different failure modes is studied and some considerations are assessed. Furthermore, the average peak shear stress and its variability is evaluated and a critical analysis is performed. Finally, a comparison among the failure tensile stress in the yarns obtained by i) the tensile tests on the dry textile meshes, by ii) the tensile tests on the FRCM specimens, and by iii) the bond tests on FRCM-masonry panel is provided.

Experimental Study on the Tensile and Bond Properties of a FRCM System for Strengthening of Masonry Construction

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.

A tension stiffening model for FRCM reinforcements calibrated by means of an extended database

In the context of the mechanical behavior of the FRCM (Fiber Reinforced Cementitious Matrix) composites as structural retrofitting technique, an accurate analysis of its tensile performance by collecting an extended database of experimental results is presented. The experimental data concern the results of more than 750 tensile tests carried out on FRCM coupons, made with different types of constituent materials (mortar and fiber grid), different geometrical properties and tested with different test set-ups. The data are subdivided into uniform classes and some exclusion criteria are adopted. The influence of the mechanical and geometrical parameters of the constituents on the tensile response of the FRCM reinforcement is assessed. Thus, the relations between the peculiar points of the experimentally-obtained stress–strain law (cracking stress, failure stress, initial elastic modulus, stiffness of the specimens at failure) and the mechanical properties of the constituents are defined. Those relations are the basis to derive an analytical provisional formulation able to reproduce the stress–strain law of the FRCMs, starting from geometrical and mechanical features of the constituents. The proposed tension stiffening model is validated through comparisons with experimental results of FRCM made with different grid types and different mortar nature.

Characterization and Design of Multilayer PBO FRCM Composite Reinforcements for Concrete Structures

Fiber-reinforced cementitious matrix (FRCM) composites can be preferred to other techniques to strengthen reinforced concrete (RC) members. Due to the limited cross-sectional area of textiles in FRCM, multilayer composites are often needed to provide adequate strength increase. Although multilayer FRCM composites have been already employed, quite limited research is available regarding the bond behavior of multilayer FRCM–concrete joints. Direct shear tests of FRCM–concrete joints and bending tests of FRCM-strengthened RC beams showed that when one or two textile layers are employed, debonding at the matrix–fiber interface generally occurs. When more than two layers are employed, detachment of the FRCM strip was observed in FRCM-strengthened RC beams, while limited studies of corresponding FRCM bond behavior were performed. The bond behavior of a multilayer bidirectional polyparaphenylene benzobisoxazole (PBO) FRCM composite applied onto a concrete substrate is investigated in this paper. Single-lap direct shear tests of PBO FRCM–concrete joints comprising two or four textile layers are performed, along with tensile tests of bare textile strips and single-layer FRCM coupons. Tests were conducted in both textile directions. The results obtained provide fundamental information on the effect of the textile direction and number of layers to be considered when designing the reinforcing system.