Carbon Fiber Reinforced Cementitious Matrix Research Articles

A pull-out equivalent telescopic layered failure model of carbon fiber bundles in seawater sea-sand cementitious matrix

Based on the excellent mechanical properties and piezoresistive effect of carbon fiber, carbon fiber reinforced cementitious matrix (CFRCM) has been used in the strengthening and self-monitoring research of reinforced concrete (RC) structures. In this study, the assumptions of the telescopic layered model theory of CFRCM were refined. The factors influencing the piezoresistive effect of carbon fibers were subsequently analyzed. Simultaneously, the resistance calculation theory for the pull-out process of CFRCM bundles was established by combining the parallel circuit theory. Based on the above theories, a pull-out equivalent telescopic layered failure model of carbon fiber bundles in the cementitious matrix was developed, considering the trilinear cohesive material law (CML) and the piezoresistive effect. Pull-out tests on carbon fiber bundles in both freshwater standard sand and seawater sea-sand matrices were conducted. This model was successfully used to simulate the piezoresistive effect during the pull-out process of CFRCM bundles with two matrices and different embedded lengths. The number of fiber filaments in each layer, the rupture damage of the fiber filaments during the loading process, and the degree of matrix infiltration into the carbon fibers were calculated. This work lays the foundation for advancing the development of structural strengthening and structural health monitoring.

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.

Analytical study of the bond behavior of fiber reinforced cementitious matrix (FRCM)-substrate joints based on a two-stage nonlinear cohesive material law

Externally bonding fiber-reinforced cementitious matrix (FRCM) composite to the surface of concrete or masonry members is an effective technique to improve the performance of existing structures. In many cases, interfacial debonding between the FRCM fiber textile and embedding matrix governs the capacity of FRCM-strengthened structures. The interfacial debonding process can be studied analytically by assuming a cohesive material law (CML), which represents the relationship between interfacial shear stress and slip. In this study, a two-stage function with an exponential stage and a constant stage is proposed to describe the CML associated with the matrix-fiber interface. The latter stage is characterized by a constant shear stress to account for the friction/interlocking between the matrix and fiber observed in experimental tests. With the assumed CML, the full-range loading response was obtained. Additionally, the interfacial slip, fiber axial strain, and interfacial shear stress were analytically derived. The parameters of the two-stage CMLs for PBO, glass, and carbon FRCM were inversely determined by matching the analytical relationships of peak applied axial stresses associated with different bonded lengths with the experimental ones. Considering the inversely determined CMLs, the predicted load responses and strain profiles showed good agreement with the measurements of direct shear test specimens.

Strengthening the hogging and sagging regions in continuous beams with fiber-reinforced cementitious matrix (FRCM): Experimental and analytical investigations

This paper reports on the flexural performance of ten reinforced concrete (RC) continuous beams strengthened with fabric-reinforced cementitious matrix (FRCM) systems. The test parameters included the type of the strengthening system used, the location of the deficient section that required strengthening, and the number of the fabric layers used. Apart from two unstrengthened control beams, two beams were strengthened with two layers of Polyparaphenylene benzobisoxazole (PBO) FRCM. The other six beams were strengthened with four layers of PBO-FRCM, two carbon FRCM (C-FRCM), and one layer of carbon fiber-reinforced polymer sheet (CFRP), all having the same axial stiffness. The test results confirmed the excellent rotational capacity of the FRCM-strengthened beams and their ability to efficiently redistribute moments between their critical sections. The moment redistribution ratio ranged between 20 and 31% for beams strengthened with FRCM in their hogging regions (51 and 77% of that of the control beams) compared to 8 and 13% for those strengthened in their sagging regions (42 and 65% of that of the control beams). The FRP-strengthened hogging and sagging sections showed moment redistribution ratios that ranged between 15 and 2%, respectively (37 and 9% of that of the control beams). The FRCM-strengthened beams also showed a remarkable ductile response with an average ductility index of 90% of that of their control beams compared to 42% only for the FRP-strengthened ones. Analytically, the capacity of the FRCM-strengthened beams predicted using a strain model that was previously developed by the authors agreed well with the experimental capacity obtained during the tests with an experimental-to-predicted ratio of 0.99 ± 0.02.

Effect of Wet-Dry Cycles on the Bond Behavior of Fiber-Reinforced Inorganic-Matrix Systems Bonded to Masonry Substrates.

In recent years, inorganic-matrix reinforcement systems, such as fiber-reinforced cementitious matrix (FRCM), composite-reinforced mortars (CRM), and steel-reinforced grout (SRG), have been increasingly used to retrofit and strengthen existing masonry and concrete structures. Despite their good short-term properties, limited information is available on their long-term behavior. In this paper, the long-term bond behavior of some FRCM, CRM, and SRG systems bonded to masonry substrates is investigated. Namely, the results of single-lap direct shear tests of FRCM-, CRM-, and SRG-masonry joints subjected to wet-dry cycles are provided and discussed. First, FRCM composites comprising carbon, polyparaphenylene benzobisoxazole (PBO), and alkali-resistant (AR) glass textiles embedded within cement-based matrices, are considered. Then, CRM and SRG systems made of an AR glass composite grid embedded with natural hydraulic lime (NHL) and of unidirectional steel cords embedded within the same lime matrix, respectively, are studied. For each type of composite, six specimens are exposed to 50 wet–dry cycles prior to testing. The results are compared with those of nominally equal unconditioned specimens previously tested by the authors. This comparison shows a shifting of the failure mode for some composites from debonding at the matrix–fiber interface to debonding at the matrix-substrate interface. Furthermore, the average peak stress of all systems decreases except for the carbon FRCM and the CRM, for which it remains unaltered or increases.

Compression tests of C-FRCM jacket confined hollow section RC columns

This paper describes an experimental investigation on column tests of carbon fiber-reinforced cementitious matrix (C-FRCM) jacket confined hollow section reinforced concrete (RC) columns. The experimental program consists of twenty columns on two series of confined specimens and two series of unconfined specimens. Test specimens comprise of columns having solid section and hollow section with different hollow ratios. The material properties of C-FRCM were measured through tensile coupon tests. The load carrying capacities, load-end shortening responses, load–strain curves and failure modes of RC column specimens are presented and discussed in this paper. It is shown that the hoop strain of columns under compression decreases with the increase of the hollow ratio, which means the reduction of confinement effect. The effect of hollow ratio on the compressive strength of confined concrete was discussed and an equation was proposed. It is shown that the predictions from the proposed equation agree with the test results generally well.

Study of Materials Behavior in a Monumental Vault Strengthened by a Carbon Net in a Mineral Matrix Subjected to Seismic Influence

The application of the elasto-plastic material model known as the Barcelona Model (BM) for numerical assessment of a historical vault subjected to earthquake sequence is presented in this work. As a case study, part of a masonry vault erected in Southern Poland in the 12th century was chosen. For the study purposes, a 3D finite element model (FEM) of the vault was prepared using the ABAQUS/Standard software program. The essential details of the structure geometry were taken from the 3D scan of the vault. The first variant of the masonry vault was the structure without any strengthening, whereas the second variant was with strengthening system realized by application on composite materials, i.e., the carbon fiber reinforced cementitious matrix (C-FRCM). The results of the dynamic analysis revealed that an evident nonlinear performance of the masonry materials of the vault in both cases was detected for both FE models of the structure. The analysis proved that the foreshock–mainshock–aftershock sequence caused substantial damages in structural parts of the masonry vault. The distribution of plastic strains and damages allowed assessment of the impact of the full seismic sequence on the masonry vault. In the case of the unstrengthen vault the level of cracking and stiffness loss reached 90%. In the case of the vault strengthened with the FRCM system the tensile damage level was significantly lower. It did not exceed 30%. In addition, the first plastic zone of the unstrengthened masonry structural elements of the vault became visible after the foreshock.

Repair of severely damaged RC columns through FRCM composites

This study presents the results of an experimental campaign aimed at testing the effectiveness of carbon fiber reinforced cementitious matrix (CFRCM) confinement as a repair technique for severely damaged reinforced concrete (RC) columns. To evaluate shape effect and transverse steel reinforcement (TSR) – fiber interaction, two different cross-section shapes, and two TSR rates were investigated. Nine full-scale specimens were analyzed considering three control ones, three undamaged specimens confined with two CFRCM layers, and three severely damaged specimens repaired through confinement by two CFRCM layers. Axial stress-strain curves were analyzed to derive strength and ductility enhancement. Finally, TSR and fiber strains were monitored to better comprehend the contributions of internal and external reinforcement to the overall confinement and possible TSR-FRCM interaction.

Hybrid UHPC/NSM CFRP strips vs. traditional systems for flexural upgrading of RC beams – Experimental and FE study

In this study, the efficacy of traditional versus innovative systems for enhancing the flexural strength of RC (reinforced concrete) beams is investigated experimentally as well as numerically. Four-point bending tests were conducted on seven RC beams. Test matrix comprised of two control and five strengthened beams. Strengthening techniques included: bonded steel plate, externally attached CFRP (carbon fiber reinforced polymer) composite, NSM (near-surface mounted) steel rebars, externally attached CFRCM (carbon fiber reinforced cementitious matrix) composite, and innovative hybrid system comprising of ultra-high performance concrete (UHPC) layer combined with NSM CFRP strips. Different strengthening systems were designed to provide approximately the same flexural strength enhancement. The performance of strengthened specimens was compared in terms of load–deflection characteristics. The peak load of the tested specimens was analytically predicted using the equations of ACI 318-19 code and ACI 440.2R-17 guidelines. Nonlinear FE (finite element) modeling was also carried out, and a comparison was conducted between the experimental and FE results showing good agreement. The validated FE models were extended for some useful parametric studies of interest.

C-FRCM Jacket Confinement for RC Columns under Impressed Current Cathodic Protection

In coastal regions, chloride penetration causes steel reinforcing bar (rebar) corrosion in reinforced concrete structures, leading to durability problems in existing structures. A new intervention method, impressed current cathodic protection and structural strengthening (ICCP-SS), was adopted to rehabilitate sea–sand concrete columns. A carbon fiber-reinforced cementitious matrix (C-FRCM) was used as a dual-functional material in the ICCP-SS system, wherein the C-FRCM served as both an anode and a strengthening material. This study aimed to consider the effects of the total charge density on the confinement effect of C-FRCM jackets and the compressive strength of columns under ICCP-SS intervention to demonstrate the long-term effectiveness of the ICCP-SS intervention method for sea–sand RC columns and to investigate the appropriateness of existing strength models for RC columns strengthened by C-FRCM jackets under impressed current cathodic protection (ICCP). The experimental program included a total of nine reinforced concrete stub columns. Prior to the compression tests, the columns were subjected to 270 days of accelerated corrosion and 250 days of cathodic protection under protective cathodic current densities of 20 and 60 mA/m2. This paper presented an experimental program, a comparison between short-term and long-term test results of ICCP-SS, a comparison of existing strength models, and a discussion on the appropriateness of the existing models for C-FRCM jackets subjected to an applied current.

Evaluation of carbon fiber reinforced cementitious matrix as a recyclable strengthening material

The use of fiber-reinforced polymer (FRP)-epoxy resin is a common retrofitting technique usually employed by engineers. In the research presented in this paper, a new cement-based material was developed introducing chopped carbon fiber. The carbon-fiber-reinforced cementitious matrix (C-FRCM) containing a cement-based matrix and embedded carbon-fiber mesh is an alternative solution to FRP-epoxy resin, addressing cost, durability, and reversibility issues. The flexural performance of carbon-fiber-reinforced cementitious matrix plates with chopped carbon fibers was explored in this study. A series of three-point bending tests were conducted to investigate the effects of chopped carbon fiber content. The measured mechanical properties of the proposed C-FRCM mix were compared with those made of commercial formulations. The flexural strengths and failure modes of the carbon-fiber-reinforced cementitious matrix plates were obtained and recorded during tests, and then the interlaminar shear strengths were calculated. Upon the completion of three-point bending tests, scanning electron microscopy was used to observe the microscopic appearance of tested specimens. The optimum content of chopped carbon fibers in the cement-based materials was determined based on the structural behavior and microstructural analysis. The results also show that the carbon fiber mesh is effective in strengthening the carbon-fiber-reinforced cementitious matrix.

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.

Carbon-FRCM materials for structural upgrade of masonry arch road bridges

Background/PurposeNumerous bridges of the Italian arterial road network were designed for live loads significantly lower than those produced by the current vehicular traffic. Many of them are masonry arch bridges whose load carrying capacity should be assessed for defining the necessary strengthening interventions. In the present paper the design criteria for strengthening masonry bridges with carbon fiber reinforced cementitious matrix (C-FRCM) materials are presented with reference to a masonry arch road bridge built right after the second world war. MethodsThe structure is analyzed both in its original and in its strengthened configuration following the approach of the collapse mechanisms. The considered approach allows to capture the strengthening effect of the C-FRCM material in terms of modification of the collapse mechanism and increase of the load collapse multiplier. Two different configurations of C-FRCM strengthening material applied at the extrados have been considered. In the first configuration the ends of the C-FRCM material are anchored at the vaults imposts, while in the second configuration they are not anchored at the imposts. ResultsTo obtain load collapse multipliers greater than one three layers of C-FRCM strengthening material have to be applied at the extrados of all the three vaults in the case of end anchored strengthened material, while in the case of unanchored strengthening material four layers of C-FRCM strengthening material have to be applied at the extrados of all the three vaults. ConclusionThe performed analyses show a lack of load carrying capacity of about 60% with respect to the load carrying capacity required by current codes. This lack can be filled up adopting the considered strengthening technique.

Confinement Effectiveness in Eccentrically Loaded Masonry Columns Strengthened by Fiber Reinforced Cementitious Matrix (FRCM) Jackets

Abstract.The paper is devoted to the analysis of the effectiveness of the Carbon Fiber Reinforced Cementitious Matrix (C-FRCM) as confinement system of the masonry and the characterization of the structural response of masonry column confined by C-FRCM jackets, accounting for all geometrical and mechanical involved parameters. At this aim, tests on confined masonry column under eccentric axial load were carried out. Five masonry column having an overall length of650 mm, with rectangular cross-section,250 x250 mm, were eccentrically loaded until collapse; two specimens were un-confined and used as control specimens, two specimens were confined with one layer of C-FRCM and one specimen was confined with two layers of C-FRCM. The eccentricity values considered weree/H=0ande/H=0.20, being H the height of the section. Failure modes and load-strain diagrams were considered to analyze test results and to evaluate the effectiveness of the confinement both in terms of strength and ductility.

Effects of confinement level, cross-section shape and corner radius on the cyclic behavior of CFRCM confined concrete columns

The main results of an experimental research aiming to investigate the behavior of medium-size low-strength concrete columns wrapped with Carbon Fiber Reinforced Cementitious Matrix (CFRCM) under monotonic and cyclic compressive axial loads are presented. Thirty columns with circular, square and rectangular cross-sections were tested under monotonic and cyclic axial loads to investigate the effect of the confinement level, the cross-section shape and the corner radius on the stiffness, strength, and ductility of CFRCM confined concrete columns under cyclic loads. The results prove that CFRCM confining jackets provide substantial gain in compressive strength, deformability and absorbed energy.