Fiber Reinforced Cementitious Matrix Composites Research Articles

Numerical Study of Bond – Slip Mechanism in Advanced Externally Bonded Strengthening Composites

Detrimental effects of various environmental phenomena such as earthquakes on Reinforced Concrete (RC) structures make repairing, strengthening and rehabilitation necessary to satisfy current code requirements. For achieving compliance and safer structures, many efforts involve attaching composites to damaged or identified damage-prone zones of RC members. In recent years, a type of novel composites, named Fiber Reinforced Cementitious Matrix (FRCM) has been developed which comprises of two internal and external layers of mineral cement based matrix cover, reinforced with one or more layers of fiber. In any newly developed composite, among the first steps for performance evaluation is the investigation into bond characteristics of the various structural elements. In this respect, direct shear tests, owing to their straightforward outputs, are the most aptly utilized kind of tests in order to study behavior of composite bonds. This paper investigates various aspects of FRCM bond performance on concrete blocks in shear tests. A numerical bond - slip relation was developed based on experimental data on FRCM strengthened concrete blocks. The results obtained from the numerical model show good agreement with the experimental results and provide in depth information about stress - slip relation, load responses, width effect as well as slip and strain profiles in FRCM composites.

Out-of-plane maximum resisting bending moment of masonry walls strengthened with FRCM composites

Fiber reinforced composites represent an alternative to more traditional techniques for strengthening and retrofitting existing masonry structures. Numerous studies have been carried out to characterize the mechanical and bond behavior of Fiber Reinforced Cementitious Matrix (FRCM) composites applied to different substrates and to provide reliable parameters for the estimation of the FRCM composite contribution to the strengthened member. However, limited work was done to provide reliable design procedures for FRCM strengthened structures and the only available guideline is the American ACI 549.4R-13.In this paper, a new analytical procedure to estimate the out-of-plane maximum resisting bending moment of FRCM strengthened masonry walls is provided. The accuracy of the proposed procedure is assessed by comparison of the analytical provisions with corresponding experimental results from a database comprising 63 FRCM strengthened walls collected from the literature. The results obtained are compared with those computed according to the ACI 549.4R-13 approach and to a method based on the procedure by Ascione et al. (2015). The new procedure proposed provides accurate results, whereas the ACI 549.4R-13 approach and the method based on the procedure by Ascione et al. (2015) generally tend to underestimate the experimental maximum resisting bending moment providing a low accuracy.

Estimation of a matrix-fiber interface cohesive material law in FRCM-concrete joints

The application of composite materials to strengthen existing structural elements is a valid alternative to traditional strengthening techniques. Fiber reinforced cementitious matrix (FRCM) composites have been successfully employed to strengthen existing reinforced concrete (RC) and masonry structures in bending, shear, torsion, and to confine axially loaded elements. Although failure of FRCM strengthened elements depends on different parameters, such as the composite and substrate geometrical and mechanical properties, debonding at the matrix-fiber interface is generally the failure mechanism. Therefore, the study of the bond behavior of FRCM composites is a key topic to develop reliable design procedures. Numerous experimental campaigns were carried out recently to study the bond behavior of different FRCM composites. An analytical model is employed in this paper to describe the bond behavior of FRCM-concrete joints and different trilinear cohesive material laws are defined based on the experimental results. The experimental and corresponding analytical load response, strain profile, slip profile, and shear stress profile along the bonded length are compared. An analytical formulation of the bonded length needed to fully develop the stress-transfer mechanism at the matrix-fiber interface, i.e. the effective bond length, is provided for the trilinear cohesive material law employed.

Three-dimensional finite element modeling of inorganic-matrix composite materials using a mesoscale approach

Fiber reinforced cementitious matrix (FRCM) composites represent a promising alternative to the use of fiber reinforced polymer (FRP) composites for strengthening existing structures. FRCM composites are comprised of a high-strength fiber net (textile) embedded within an inorganic matrix, which is responsible of the stress-transfer between the composite and the substrate. FRCM composites comprising one layer of fiber are generally reported to fail due to debonding of the fibers from the embedding matrix. However, depending on the FRCM material employed, failure at the matrix-substrate interface, interlaminar (delamination) of the matrix, and fiber rupture may also occur. When stitch-bonded textiles are employed, i.e. textiles where longitudinal and transversal bundles are firmly connected, the interlocking between fibers and matrix plays a key role in the matrix-fiber stress-transfer mechanism and failure generally occurs due to cracking of the matrix. In this paper, the bond behavior of FRCM-masonry joints comprising one layer of a carbon stitch-bonded textile embedded within a lime-based matrix is studied by means of single-lap direct-shear tests of FRCM-masonry joints. A mesoscale three-dimensional finite element approach, which accounts for the matrix-fiber bond behavior and matrix-textile interlocking, is adopted to study the formation and propagation of the matrix cracks in the FRCM strip that eventually led to failure of the FRCM-masonry joints. The finite element analysis was carried out by using a dynamic explicit approach, which allowed for overcoming convergence difficulties associated with severe nonlinearities of the model.

Behavior of RC beams strengthened in shear with FRP and FRCM composites

This paper presents the results of an experimental campaign carried out to investigate the behavior of reinforced concrete (RC) beams strengthened in shear with externally bonded composites. Two different types of composites were studied: Fiber Reinforced Polymer (FRP) and Fiber Reinforced Cementitious Matrix (FRCM) composites. In addition, different types of fiber (carbon and steel) were employed, and the influence of internal transverse steel reinforcement ratio and the presence of composite anchors were investigated. Internal-external shear reinforcement interaction, i.e. reduction of the stirrup strain due to the presence of the composite, was observed for both FRP and FRCM strengthened beams, but the interaction was less pronounced for those with FRCM composites. The anchors employed in this study did not affect the shear strength of the beams, but changes in the concrete crack pattern, mid-span displacement, and failure mode were observed. For FRCM strengthened beams, strains measured in the fibers showed higher exploitation ratios, i.e. the ratio between the maximum measured fiber strain and the rupture strain, for beams with carbon FRCM than those with steel FRCM. Effective strains computed using available models were considerably lower than the maximum measured fiber strains.

A New Pull-Out Test to Study the Bond Behavior of Fiber Reinforced Cementitious Composites

Fiber reinforced cementitious matrix (FRCM) composites are gaining increasing popularity in the civil engineering community. FRCM composites are comprised of high-strength fiber textiles embedded within inorganic matrices that are responsible for the stress-transfer mechanism between the composite and the substrate. Failure of FRCM composites including one layer of textile is generally reported to be debonding of the fibers from the embedding matrix. Therefore, the bond behavior of the matrix-fiber interface is of critical importance for these types of composites.This paper presents the results of an experimental campaign carried out to investigate the bond behavior of an FRCM composite comprising PBO fibers. Specimens were tested using a newly-developed pull-out test set-up. The results obtained are compared with those obtained by different authors on single-lap direct-shear tests with the same FRCM composite.

An Investigation of the Debonding Mechanism between FRCM Composites and a Masonry Substrate

Fiber reinforced cementitious matrix (FRCM) composites have recently become a hot topic in Europe as an alternative to traditional fiber reinforced polymer (FRP) composites for several strengthening applications of existing masonry buildings. The terrific success of this new retrofitting system is mainly due to some advantages that it offers when compared to FRP, such as the possibility of application of the composite to wet surfaces and the vapor permeability featured by the inorganic matrix. In this work, the stress transfer between FRCM composites and a masonry substrate is investigated. FRCM strips comprised of ultra-high-strength steel fibers embedded in a cementitious grout are externally bonded to masonry blocks. Single-lap direct shear tests are performed. Parameters studied are bonded length and density of the steel fibers. Load responses are presented and failure modes are discussed. Change in the bond behavior and load carrying capacity with increasing bonded length is analyzed to determine the effective bond length.

Destructive <i>In Situ</i> Tests on Masonry Arches Strengthened with FRCM Composite Materials

In the last few decades, fiber reinforced polymer (FRP) composites have been widely employed in several strengthening and rehabilitation applications of existing masonry buildings. Fiber reinforced cementitious matrix (FRCM) composites are a newly-developed strengthening technique comprised of high strength fibers embedded in a cementitious matrix. FRCMs usually offer several advantages such as the high resistance to fire and high temperatures or vapor permeability with masonry substrate, therefore they appear to be a promising alternative to traditional FRP strengthening systems. In this experimental work, the results of destructive in situ tests performed on existing masonry arches strengthened with FRCM composites are reported. FRCM strips consist of a balanced bi-axial mesh made of basalt fibers embedded in a cementitious grout. Three different configurations of the strengthening system have been considered. Load responses and failure modes are presented.

Confinement of Masonry Columns with Steel and Basalt FRCM Composites

The rehabilitation of existing masonry elements by means of jacketing of columns using composite materials is becoming a remarkable technique in several applications that aim to increase the strength of existing masonry buildings. Fiber reinforced cementitious matrix (FRCM) composites are a newly developed strengthening system that consist of high-strength fibers embedded in a cementitious grout and externally bonded to the substrate. High resistance to fire and high temperatures, ease of handling during application, and vapor permeability with the substrate are some of the characteristics that make FRCMs a promising alternative to traditional organic composites such as fiber reinforced polymer (FRP) composites. This work presents the results of an experimental study carried out to understand the behavior of masonry columns with a square cross-section confined by steel and basalt fiber sheets embedded in a mortar matrix subjected to monotonic concentric compressive load. The effectiveness of the confinement is studied in terms of load-bearing capacity with respect to unconfined columns. The effect of corner radius for columns confined with basalt fibers is investigated.

Experimental Behavior of Glass-FRCM Composites Applied onto Masonry and Concrete Substrates

The use of Fiber Reinforced Polymer (FRP) composites has become a popular solution for retrofitting and strengthening of existing concrete and masonry structures. However, some drawbacks of this technique, mainly associated with the use of organic resins, have been reported. To overcome such drawbacks, the development of composite materials in which the organic resins are replaced with inorganic matrices has recently caught the attention of the civil engineering industry. Among these newly developed systems, Fiber Reinforced Cementitious Matrix (FRCM) composites, which are comprised of high strength fibers embedded within an inorganic matrix, have shown promising results. However, research on this topic is still limited and important aspects, such as the bond behavior between the composite and the substrate, are not fully understood and require further study. This paper presents the results of an experimental campaign aimed at investigating the influence of the type of matrix and substrate on the bond behavior of FRCM composites. Glass-FRCM composite strips were applied onto concrete and masonry substrates and then tested by means of a classical push-pull single-lap direct-shear test set-up. A cementitious and a lime-based matrix were employed to apply the same type of fiber on concrete and masonry substrates, respectively. FRCM-concrete and FRCM-masonry joints reported the same failure mode. However, higher values of the peak load were obtained for the lime-based glass-FRCM composite applied onto masonry substrates than with the cementitious glass-FRCM composite applied onto concrete substrates.

An indirect method to calibrate the interfacial cohesive material law for FRCM-concrete joints

The cohesive material law (CML), i.e., relationship between the interfacial shear stress and slip between a fiber reinforced composite and the substrate, is a fundamental tool to model the structural behavior of composite-strengthened elements. A crucial problem that researchers working in the field of strengthening applications with fiber reinforced cementitious matrix (FRCM) composites face is how to obtain the CML. A direct method to determine the CML could be applied if the longitudinal strain is measured along the bonded length. However, since the critical interface for some FRCM composites appears to be the interface between the fibers and matrix, measuring the fiber strain in FRCM composites is a difficult task due to the presence of the matrix that embeds the fiber textile. To overcome this difficulty, an indirect method is proposed in this paper. The parameters of the CML are determined by fitting experimental data in terms of peak load measured for different composite bonded lengths. The procedure is applied to single-lap shear test results previously published by the authors. The CML obtained shows good correlation with the CML obtained from direct calibration of strain profiles measured along the bonded length and is able to predict the experimental load responses.

Experimental Investigation of Glass and Carbon FRCM Composite Materials Applied onto Concrete Supports

In recent decades the growing need for strengthening and retrofitting existing structures has led to the development of innovative strengthening materials. Fibre reinforced composites have been shown to be an effective strengthening solution for flexural and shear strengthening and for confinement of axially/eccentrically loaded elements. Fibre Reinforced Cementitious Matrix (FRCM) composites, comprised of high-strength fibres and an inorganic matrix, are a newly-developed type of composite that has better resistance to high temperature and compatibility with the substrate than traditional fibre reinforced polymer (FRP) composites. This paper investigates the behaviour of FRCM composites comprised of a glass or carbon fibre net tested using single-lap direct-shear tests. Observations regarding the load response and failure mode of FRCM-concrete joints with different geometrical and mechanical characteristics are provided.

Influence of the substrate characteristics on the bond behavior of PBO FRCM-concrete joints

Fiber Reinforced Cementitious Matrix (FRCM) composites can be employed as a valid alternative to Fiber Reinforced Polymers (FRP) composites for strengthening existing reinforced concrete (RC) structures. The weakness of FRCM-concrete joints has been reported to be at the matrix–fiber interface rather than within the concrete substrate, as is usually reported in the case of FRP-concrete joints. The experimental investigations regarding FRCM strengthening available in the literature have been performed by applying the FRCM composite onto a surface that was treated before the application, usually by grinding or sandblasting. However, the role of the substrate characteristics on the failure mechanism of the FRCM composite has not been properly investigated. This paper presents the experimental results obtained from FRCM-concrete joints tested using single-lap direct-shear tests. FRCM composites with different bonded lengths and bonded widths were applied onto concrete specimens with different surface preparations and different concrete strengths. The influence of shrinkage cracks and load distribution along the composite width on the failure modes observed is discussed. In order to avoid the possibility of sudden composite detachment failure, the authors recommend that the substrate surface should be treated, and the matrix shrinkage should be controlled.

Fatigue and post-fatigue behavior of PBO FRCM-concrete joints

Fiber-reinforced cementitious matrix (FRCM) composites may represent a valid alternative to the use of traditional techniques for strengthening and retrofitting existing reinforced concrete (RC) and masonry structures. Although FRCM composites appear to be an effective strengthening technique for RC structures, limited work is available in the literature. In this paper the behavior of FRCM-concrete joints subjected to fatigue and post-fatigue quasi-static monotonic loading is presented and discussed. The single-lap direct-shear test set-up was adopted in this study. The fatigue loading protocol was designed to investigate the effects of different frequencies and different load ranges on: (a) the interfacial slip; (b) the dissipated energy during cycles; (c) the stiffness degradation of the interface; and (d) the post-fatigue quasi-static monotonic behavior. In general, it was observed that different combination of amplitude and mean value of the load range implies different damage measured in terms of slip, energy dissipation, and stiffness degradation. A fracture mechanics approach was proposed to describe the intermediate range of the fatigue crack growth for different frequencies.

Investigation of Bond Behavior of Polyparaphenylene Benzobisoxazole Fiber-Reinforced Cementitious Matrix-Concrete Interface

This paper presents the results of an experimental study conducted to understand the behavior and stress-transfer mechanism of fiber-reinforced cementitious matrix (FRCM) composites externally bonded to a concrete substrate for strengthening applications. The FRCM composite was comprised of a polyparaphenylene benzobisoxazole (PBO) fiber net embedded within two layers of polymer-modified cement-based mortar. Single-lap shear tests were conducted on specimens with composite strips bonded to concrete prisms. Parameters that varied were bonded length and width of composite. Additionally, the external coating layer of matrix was omitted on a limited number of specimens to examine the interfacial behavior between fibers and matrix and the role of the matrix in the stress transfer. Strain measurements along the composite bonded length were used to investigate the stress-transfer mechanism. Results suggest that the effective bond length of this composite is within the range of 250 to 330 mm (10 to 13 in.). Unlike with fiber-reinforced polymer (FRP) composites, no width effect was observed in terms of the maximum load. Finally, the stress-transfer mechanism at the matrix-fiber interfaces on either side of the fiber net was found to be unequal.

Experimental Analysis of the Bond Behavior of Glass, Carbon, and Steel FRCM Composites

In recent decades, the construction industry has witnessed a rapid growth of interest in strengthening and retrofitting of existing reinforced concrete (RC) and masonry structures. Fiber reinforced polymer (FRP) composites have gained great popularity, and several studies are now available in the literature on their use in strengthening and retrofit applications. Promising newly-developed composite materials are represented by the so-called fiber reinforced cementitious matrix (FRCM) composites. FRCM composites are comprised of high strength fibers embedded within a cementitious matrix that is responsible for the stress transfer between the existing structure and the strengthening material. FRCM composites are still in their infancy, and very limited results are available in the literature on RC and masonry strengthening applications. This study presents an experimental campaign conducted on different FRCM composites comprised of glass, carbon, or steel fibers embedded within two different cementitious matrices and applied to concrete prisms. The single-lap direct-shear test was used to study the stress-transfer mechanism between the FRCM composite and the concrete substrate. Two different composite bonded lengths were investigated. Debonding occurred at the matrix-fiber interface for some of the composites tested and at the concrete-matrix interface for others. This work contributes to the study of the bond behavior of FRCM composites, which represents a key issue for the effectiveness of FRCM composite strengthening.

Confinement of Masonry Columns with PBO FRCM Composites

The overarching goal of this work is to provide a fundamental understanding of the behavior of solid brick masonry columns confined with fiber reinforced cementitious matrix (FRCM) composites. FRCM is a newly-developed type of composite material comprised of a cementitious inorganic matrix (binder) and embedded fibers that are usually bundled to improve the bond between the matrix and the fibers. Compression tests were carried out to investigate the influence of the FRCM confinement and the brick patterns on the load-carrying capacity of the confined columns. Compression tests were conducted on brick masonry columns with different brick configurations. Digital image correlation measurements on the surface of the composite and on the surface of the brick for the control specimens were attempted in order to understand the role of the mortar joints and the arch effect across the section of the columns due to the confinement. The experimental results indicate that FRCM composites can effectively increase the load-carrying capacity of brick masonry columns and the failure mode could be different from the one observed for masonry columns confined with fiber-reinforced polymer (FRP) composites.

Effect of eccentric compression loading on the strains of FRCM confined concrete columns

The paper describes the effect of eccentric compression on the strains of RC columns, reinforced with FRCM (Fibre Reinforced Cementitious Matrix) composites. Over the span of a dozen or so years, many research studies have been dedicated to the learning of the behaviour of FRP confined concrete elements. The comprehensive test results of axially and eccentrically compressed elements as well as numerous theoretical analyses provide an opinion on the effectiveness of FRPs (Fibre Reinforced Polymers). Under the FRP system, the bonding of a composite material with concrete is made using epoxy resin, on the other hand, under the FRCM system, a composite material is bonded with concrete using a mineral mortar. The FRCM strengthening is relatively poorly recognised, and the research studies presented aim at the widening of the knowledge acquired to date. The testing was performed on 15 slender reinforced concrete columns that were strengthened longitudinally and/or transversally with a composite mesh under the FRCM system. The elements were subjected to axial compression and on the eccentrics: h/6 and h/12. The paper presents the testing preparation and performance method. The description of the strengthening effectiveness was made basing on the values of the energy of destruction. On the grounds of the analysis of the Poisson’s ratio value, the impact of the strengthening method and of the eccentric on the state of stress of the material being tested was reviewed.

Matrix–fiber bond behavior in PBO FRCM composites: A fracture mechanics approach

The results of single-lap shear tests, conducted on specimens with fiber reinforced cementitious matrix (FRCM) composite strips bonded to concrete blocks, are presented in this paper. The FRCM composite was comprised of polyparaphenylene benzobisoxazole (PBO) fibers and polymer-modified cement-based mortar. This study indicates that in PBO FRCM–concrete joints debonding mainly occurs at the matrix–fiber interface. Friction between fiber filaments and between fibers and matrix is observed after the debonding process initiates. The experimental data suggest that a width effect does not exist among the fiber bundles, and an effective bond length can be defined and is approximately 260mm. Axial strain profiles along the bonded length are analyzed to investigate the stress-transfer mechanism at the matrix–fiber interface.

Fibre Reinforced Cementitious Matrix confined concrete elements

This paper discusses outcomes of experimental research into FRCM (Fibre Reinforced Cementitious Matrix) and CFRPs (Carbon Fibre Reinforced Polymers) confined concrete elements. FRPs (Fibre Reinforced Polymers) composite materials use epoxy resin to bond concrete with external composite reinforcement, whereas FCRM uses composite jacket embedded in mineral mortar. Since mineral adhesive is used for FRCM composites, they are an alternative to FRP composites for applications in elevated temperatures. The two-stage comparative research was carried out to evaluate the confinement effectiveness and behaviour of FRCM and CFRP confined concrete elements. At the first stage, the effect of different number of transverse reinforcement layers of FRCM and CFRP sheets not exposed to elevated temperature was investigated. At the second stage, elements with single layer transverse reinforcement of FRCM and CFRP sheets was exposed to elevated temperature and then tested to failure. This paper reviews results of limit load capacity research, damage patterns were described and stress–strain curves were plotted and compared for both confinement types.