Fiber Reinforced Cementitious Matrix Research Articles

Circular and square columns strengthened with FRCM under concentric load

Fiber-reinforced cementitious matrix (FRCM) systems have recently emerged as an innovative technique to strengthen and retrofit reinforced concrete structures. These non-corrosive systems involve the use of high strength reinforcing textiles sandwiched between layers of cementitious mortars. This paper aims to study the effect of retrofitting newly constructed short columns with FRCM using PBO type of textiles. The experimental program consisted of testing 4 rectangular columns and 4 circular columns under concentric loading. All columns had a reinforcement ratio of 0.02 and were cast with concrete of 30 MPa compressive strength. For each type of cross-section, columns were wrapped with 1, 2 or 4 layers of PBO FRCM. Overall, the strengthened columns exhibited higher load carrying capacity than their control unwrapped counterpart with an increase ranged between 5.1% and 36%. The confining effect of FRCM layers was more pronounced in the circular columns than in the square ones. It was also noticed that all columns exhibited similar responses in terms of load -strain relationship irrespective of the column shape and the number of FRCM layers used. Furthermore, the displacement levels increased as the number of layers increased which indicated an increase in ductility of columns wrapped with FRCM.

Evaluation of the vertical load capacity of masonry arch bridges strengthened with FRCM or SFRM by limit analysis

The large number of existing masonry bridges still in service in the world roadway and railway networks requires that transportation system managers carry out ordinary and extraordinary maintenance. In many of these networks, the need for increasing the speed and/or weight of the traffic loads also entails the design of strengthening interventions aimed at enhancing the load-carrying capacity of masonry bridges. Among the available techniques for the strengthening of these structures, the use of fiber reinforced cementitious matrix (FRCM) composites has gained popularity due to their advantages with respect to more traditional techniques. More recently, the use of steel fiber reinforced mortars (SFRM), has also been investigated for the strengthening of masonry bridges, with promising results. In this paper, a limit analysis for assessing the vertical load-carrying capacity of single-span bridges strengthened with FRCM or SFRM, applied at the intrados, is described, and calibrated using available experimental results. The document highlights the variables that should be considered in such procedure and also discusses the similarities and differences between the two strengthening techniques. A subsequent parametrical analysis shows that both techniques produce comparable increments in the load carrying capacity of the masonry bridges when compared to unstrengthened conditions. In addition, it was seen that if the finite friction between blocks is not considered when performing the limit analysis of masonry bridges, the ultimate load capacity of the arches can be overestimated as the failure mode can be attained due to shear sliding of the masonry blocks, a hinge mechanism or a combination of both.

Advanced Techniques for Pilotis RC Frames Seismic Retrofit: Performance Comparison for a Strategic Building Case Study

Pilotis buildings have widely spread out in developed countries since World War II onwards. From the structural point of view, Pilotis RC frames exhibit substantial lack in ductility capacity and shear resistance localized at the first floor, since they have been mainly realized before the seismic codes’ era. The present study shows the performance comparison of four advanced retrofit techniques when applied to typical Pilotis RC frame designed for gravity loads only according to Italian building code of ‘60s. A preliminary investigation has been performed to select non-linear numerical models suitable to describe the considered RC frame behavior, involving flexural inelastic hinges of RC beams and columns and in-plane axial inelastic hinges of masonry infill panels. Two seismic retrofit projects have been designed at a local level, by strengthening the masonry infilled panels with Fiber Reinforced Cementitious Matrix (FRCM) technique and alternatively by replacing infilled panels with prefabricated panels disconnected from the structure, so that no infill/frame interaction occurs. Two more retrofit projects have been designed at a global level, in order to improve the overall structural performance making use of energy dissipation and, alternatively, base isolation techniques. Nonlinear time history analysis and structural assessment have been carried out for the as-built case as well as for the four retrofit solutions according to Eurocode 8 and Italian Building Code, in order to highlight the structural deficiencies and relative improvements, respectively. Performances offered by the proposed retrofit techniques have been finally compared in terms of structural behavior, expected damage, and economic impact.

Fiber Reinforced Cementitious Matrix (FRCM) for strengthening historical stone masonry structures: Experiments and computations

Seismic events highlight the inherent fragility and vulnerability of stone masonry buildings, which represent a large part of the existing historical and artistic heritage. In order to preserve these structures, numerous reinforcement techniques are typically used on masonry walls, including mortar injections, reinforced drilling, and reinforced concrete plaster. Nowadays new and less invasive strengthening techniques are preferred; among them Fiber Reinforced Cementitious Matrix (FRCM) system with lime-based mortar, which is considered to be more compatible with the intrinsic properties of these ancient structures as compared to cement-based mortar. This work aims to investigate experimentally and computationally FRCM applied as reinforcement to ancient stone masonry. In particular, the paper presents results from diagonal compression tests carried out at the University of L’Aquila (Italy) on stone masonry specimens strengthened with layers of Glass-FRCM (GFRCM). In comparison with unreinforced panels, those strengthened by the GFRCM exhibited a significant increase in shear modulus and shear strength. A computational framework based on the Lattice Discrete Particle Model (LDPM) was then used to reproduce the experimental results. The fracture behavior and the damage evolution in masonry panels were investigated under different assumptions on the GFRCM system features (bond behavior, mortar thickness, fiber anchors and fiber grid). The good agreement between experimental results and the LDPM simulations show that this approach predicts well the mechanical behavior and the damage evolution in stone masonry under quasi-static loading conditions. Moreover, it can be considered a viable tool for engineers in developing effective reinforcement techniques.

The role of the fiber–matrix interfacial properties on the tensile behavior of FRCM coupons

Fiber-reinforced cementitious matrix (FRCM) composites are usually mechanically characterized by means of tensile and bond tests. The former, in the clevis-grip version, is referred to by the American guidelines ACI 549.4R (2013) to determine the tensile properties of the FRCM composite. The latter, in the single-lap version, is used in the Italian guidelines CNR-DT 215 (2018) to determine the effective strain. The effective strain is the strain at which debonding occurs and therefore composite action is lost. In this paper, the poliparafenilene benzobisoxazole (PBO) fiber–matrix stress transfer law, also known as cohesive material law (CML), is employed in an analytical model that describes clevis-grip tensile tests of PBO-FRCM composites. The CML was previously obtained by the authors from single-lap shear tests. The load responses provided by the model are compared with the results of tensile tests herein presented in addition to selected tests from the literature. The experimental cracking process, tensile strength, and deformation capacity can be accurately predicted by the analytical model. The comparison indicates that the knowledge of the CML of the fiber–matrix interface allows for an accurate prediction of the main tensile properties of the PBO-FRCM coupon.

Lime-Based Mortar Reinforced by Randomly Oriented Short Fibers for the Retrofitting of the Historical Masonry Structure.

Recent seismic events prompted research to develop innovative materials for strengthening and repair of both modern and historic masonry constructions (buildings, bridges, towers) and structural components (walls, arches and vaults, pillars, and columns). Strengthening solutions based on composite materials, such as the Fiber Reinforced Polymers (FRP) or the Fiber Reinforced Cementitious Matrix (FRCM), have been increasingly considered in the last two decades. Despite reinforcement made of short-fibers being a topic that has been studied for several years from different researchers, it is not yet fully considered for the restoration of the masonry construction. This work aims to experimentally investigate the enhancement of the mechanical properties of lime-based mortar reinforced by introducing short glass fibers in the mortar matrix with several contents and aspect ratios. Beams with dimensions of 160 mm × 40 mm × 40 mm with a central notch were tested in three-point bending configuration aiming to evaluate both the flexural strength and energy fracture of the composite material. Then, the end pieces of the broken beams were tested in Brazilian and compressive tests. All the tests were performed by a hydraulic displacement-controlled testing machine. Results highlight that the new composite material ensures excellent ductility capacity and it can be considered a promising alternative to the classic fiber-reinforcing systems.

Bond behavior of steel fiber-reinforced mortar (SFRM) applied onto masonry substrate

Masonry was the most used material during the last centuries to build constructions. Most of the existing masonry structures (buildings, bridges, etc.) were built without considering some important structural considerations that are important nowadays. Moreover, due to factors such as the increasing of service loads, materials aging, structural damage, etc., the existing masonry structures require strengthening interventions. The definition of optimal strengthening strategies using traditional and innovative materials is still an important issue of the scientific research. In fact, during the last decade, many researchers focused their attention studying innovative composites materials, such as fiber-reinforced polymers and fiber-reinforced cementitious matrix composites, for the strengthening of existing masonry structures. This research has focused on aspects such as the bond behavior between the substrate and the composite materials, the structural behavior of the strengthened masonry and concrete structures, and the compatibility and reversibility of these materials when bonded to existing substrates. In this study, the bond behavior of a composite material known as steel fiber-reinforced mortar (SFRM), recently used as for the strengthening of existing structures, applied onto masonry structures is analyzed experimentally and numerically. First, the material is characterized experimentally with the aim of getting insight on its behavior and applicability when applied as an innovative technique for the strengthening of masonry and to obtain mechanical parameters required for the numerical models. Mechanical properties of the SFRM studied included flexural and compressive strength, tensile strength, and residual flexural strength. The SFRM bond behavior on masonry substrates was evaluated by means of double shear lap tests. In addition, the experimental tensile and bond behavior of the SFRM is studied numerically through finite-element models validated using the results obtained during the experimental tests. Results show that if an adequate bonded length is provided, the SFRM can fully develop its tensile strength as detachment from the substrate is not observed.

Comparison between Design Formulations and Numerical Results for In-Plane FRCM-Strengthened Masonry Walls

Strengthening strategies and structural rehabilitation of existing buildings with innovative materials and techniques are today one of the main activities in the field of structural engineering. Externally bonded Fiber Reinforced Cementitious Matrix (FRCM), materials are spreading as an alternative strengthening technique to the more traditional Fiber Reinforced Polymer (FRP) ones, especially for masonry elements. This research focuses on the assessment of the shear strength of in-plane FRCM-strengthened masonry walls by means of code predictions and numerical results, with particular attention on the interpretation of diagonal compression tests simulated by means of Finite Element (FE) models. Firstly, the various approaches commonly adopted for the interpretation of diagonal compression tests in terms of shear strength were examined, since codes give generally provisions in terms of maximum shear force, while several experimental and numerical results of diagonal compression tests are available in literature. Then, the numerical simulations of diagonal compression tests obtained by several FE analyses in a previous work were examined in light of these different approaches; the corresponding predictions of the shear force are compared with code indications in order to individuate the most reliable approach both for the un-strengthened and the FRCM-strengthened walls. For the latter ones, a detailed analysis of the actual strain levels in the reinforcement was carried out, too, by means of FE analysis.

Fatigue Behavior of PBO FRCM Composite Applied to Concrete Substrate.

Several reinforced-concrete (RC) structural elements are subjected to cyclic load, such those employed in highway and railroad bridges and viaducts. The durability of these elements may be reduced as a consequence of fatigue, which mainly affects the steel reinforcement. The use of externally bonded (EB) fiber-reinforced cementitious matrix (FRCM) composites allows the moment capacity to be shared by the internal reinforcement and the EB composite, thus increasing the fatigue life of the strengthened RC member. The effectiveness of EB FRCM composites is related to the composite bond properties. However, limited research is currently available on the effect of fatigue on the bond behavior of FRCM-substrate joints. This study provides first the state of the art on the fatigue behavior of different FRCM composites bonded to a concrete substrate. Then, the fatigue bond behavior of a polyparaphenylene benzo-bisoxazole (PBO) FRCM is experimentally investigated using a modified beam test set-up. The use of this set-up provided information on the effect of fiber-matrix interface shear and normal stresses on the specimen fatigue bond behavior. The results showed that fatigue loading may induce premature debonding at the matrix-fiber interface and that stresses normal to the interface reduce the specimen fatigue life.

Relationship between the effective strain of PBO FRCM-strengthened RC beams and the debonding strain of direct shear tests

Debonding of fiber-reinforced composites can limit the increase of strength that a reinforced concrete (RC) beam can achieve due to the addition of the external layer of reinforcement provided by the composite. Small-scale direct shear tests can help understand the debonding phenomenon. However, researchers have debated for decades whether the results of direct shear tests can be used to predict the strain level at which the composite would fail in strengthened beams. This paper is the first systematic attempt to address this issue. A database of RC beam flexural tests and of direct shear tests that feature externally bonded polypara-phenylene-benzo-bisoxazole (PBO) fiber-reinforced cementitious matrix (FRCM) composite is compiled. For the beam tests, two methods are employed to determine the strain in the composite at failure due to debonding, referred to herein as the cross-sectional analysis method and the ΔM method, and the results are compared with those of direct shear tests. The relationship between the effective strain in the FRCM when composite action is lost and the debonding strain in direct shear tests is critically discussed taking into account what is prescribed by current design guidelines. The limitations of the cross-sectional analysis method to determine the effective strain are also highlighted.

Constitutive Numerical Model of FRCM Strips Under Traction

In this paper the tensile behavior of Fiber Reinforced Cementitious Matrix (FRCM) strips is investigated through Finite Element (FE) models. The most adopted numerical modeling approaches for the simulation of the fiber-matrix interface law are described. Among them, the cohesive model is then used for the generation of FE models which are able to simulate the response under traction of FRCM strips tested in laboratory whose results are available in the technical literature. Tests on basalt, PBO and carbon coated FRCM specimens are taken into account also considering different mechanical ratios of the textile reinforcement. The comparison between FE results and experimental data allows validating the adopted numerical modelling approach. Finally, some considerations are provided on the effects of the fiber fabric mechanical ratio and the strength and stiffness of the interface on the tensile capacity of the FRCM strip.

Modeling of the Tensile Behavior FRCM Systems for Repair and Strengthening Interventions of Masonry Structures

Fiber Reinforced Cementitious Matrix (FRCM) systems have been recently introduced as an alternative solution for strengthening interventions of existing masonry and concrete constructions. The current literature provides interesting experimental and numerical studies which underline the potentialities of FRCMs together and their specific features. Direct tensile tests are generally used for characterizing the behavior of FRCM. Indeed, these tests underline the occurrence of important phenomena influencing the behavior of FRCM, such as the cracking of the matrix and the debonding of the reinforcement from the matrix. The present paper aims at numerically analyzing the response of FRCM systems during tensile tests carried out throughout different setup configurations. To this end, a simple model carried out from literature and opportunely modified is presented in the paper and validated with reference to experimental case studies. Moreover, the obtained results are critically analyzed in order to emphasize the role of the proposed model for the interpretation of the results carried out from standard tensile tests.

Horizontal capacity of single-span masonry bridges with intrados FRCM strengthening

The strengthening of masonry arch bridges, which account for a significant portion of the railway and road bridges in the world’s infrastructure network, is a pressing need for the operators of such networks, and even more so in the seismic areas.As any interruptions to road and especially rail traffic are particularly costly, and consequently the most suitable solution involves the design of a strengthening system applied to the intrados of the masonry arch bridge. However, in these circumstances a more compatible and reversible solution compared to traditional strengthening techniques or FRP (Fiber Reinforced Polymer) systems involves the use of FRCM (Fiber Reinforced Cementitious Matrix).This work thus examines the effectiveness of FRCM strengthening on the intrados of masonry arch bridges, taking into account the actual geometric relationships that characterise the arches of bridges and considering the effect of Coulomb friction. The results obtained using lower bound limit analysis highlight the optimal conditions for the use of this strengthening, and the situations in which, due to the actual geometry of the bridges and the friction coefficient value, the use of this strengthening system would not be recommended.

Parametric 3D finite element analysis of FRCM-confined RC columns under eccentric loading

Fiber reinforced cementitious matrix (FRCM) is emerging as a viable retrofit and confinement technique, in lieu of fiber reinforced polymer (FRP) system which suffers from a number of issues related to the use of synthetic binders. While many studies have been conducted on the use of FRCM in shear and flexural applications, few were dedicated to confinement of slender columns, particularly those related to finite element (FE) analysis. In this study, a nonlinear three-dimensional FE model has been developed to study the behavior of reinforced concrete (RC) columns confined by (FRCM) jackets, and loaded concentrically and eccentrically. Drucker-Prager (DP) concrete model, which has several improvements over traditional DP models, was used to model the concrete core. Composite failure in the fibers comprising FRCM system and column buckling were also considered in the developed FE model. The model was validated by comparing its predictions with those of three control and 8 FRCM-confined RC columns from literature. Consequently, a parametric study utilizing 96 additional models, was performed on five parameters, namely: cross-sectional shape (square, circle, hexagon, and octagon), and for rectangular columns; aspect (h/b) ranging from 0.5 to 3, at 0.5 increment; slenderness (KL/r) ratio, considering four values, 10, 25, 50, and 75; load eccentricity (e) as a ratio (e/h) to side length (h), varying from 0 to 2.5; and concrete compressive strength (f́c), studying three values: 20, 35, and 50 MPa. Effects of these parameters on the column’s maximum load (Pmax) and general behavior, are discussed in details in Section 6 and summarised in the conclusions part. In general, Pmax increased by 0–32% as a result of applying one layer of FRCM jacket, and showed great dependence on the examined parameters.

Numerical simulation of the tensile behavior of FRCM strengthening systems

The direct tensile test is one of the experimental procedures used for the characterization of Fiber Reinforced Cementitious Matrix (FRCM) materials. From this test generally emerge the main phenomena characterizing the tensile response of FRCM: cracking of the matrix and de-bonding at the reinforcement-matrix interface. These phenomena, that in some cases are quite difficult to experimentally monitor and understand, particularly influence the performance of FRCM as strengthening systems. In this paper the authors present a simple numerical modeling approach able to simulate the tensile response of FRCM by reproducing the occurrence of both the cracking of the matrix and the de-bonding at the reinforcement-matrix interface. The numerical analyses reported in the paper show the reliability of the proposed approach and, in particular, its role as support tool for understanding the influence of local failure phenomena on the global tensile response of FRCM systems.

Determination of the interfacial cohesive material law for SRG composites bonded to a masonry substrate

Fiber reinforced cementitious matrix (FRCM) composites, also known as textile reinforced matrix (TRM) composites, are a suitable alternative to fiber reinforced polymer (FRP) composites to strengthen reinforced concrete and masonry structures. In the toolbox of FRCMs, a recently-developed composite that employs high-strength steel fibers embedded in a hydraulic mortar is particular appealing for applications on historical masonry constructions. This type of composite is known as steel reinforced grout (SRG). In this paper, an extensive experimental work is presented. Single-lap shear tests are performed to study the debonding of SRG strips from a masonry substrate, which is the critical failure mode for strengthening applications. For SRGs, debonding typically occurs at the fiber-matrix interface. A large scatter of the experimental results is observed, which is related to the variability of hydraulic mortars and their ability to impregnate the fibers. Although strain gauges can be applied directly to the fibers to obtain the experimental strain profile along the fibers, because of the presence of the matrix these measurements are complex and in some cases not reliable. Thus, indirect method based on the global response of the test is proposed to obtain the interfacial properties.

Experimental behavior of reinforced concrete columns confined with carbon-FRCM composites

Confinement of concrete elements by means of fiber reinforced polymer (FRP) or fiber reinforced cementitious matrix (FRCM) has been largely investigated in literature, however such studies dealt mainly with small scale specimens rather than with fully reinforced concrete (RC) ones. Here, the results of an experimental campaign where twelve RC columns were tested under axial compressive loading are shown. Among the test specimens, eight were confined with carbon-FRCM (CFRCM) and four were used as control. The following variables were considered as influencing parameters: section geometry, i.e. columns with circular or square cross-sections; amount of transverse steel reinforcement, i.e. two stirrups spacings; and lastly the number of fiber layers, namely one or two layers. Stress vs. axial strain curves and fiber hoop strains were analyzed to derive the strength gain and specimens’ ductility. Additionally, strains of transverse steel reinforcement were monitored, to assess how the internal reinforcement interacts with the confinement performance of the external CFRCM jacket. Results obtained here represent better the in-situ condition of axially-loaded members, that include inner longitudinal and transverse reinforcement.

Full-range behavior of fiber reinforced cementitious matrix (FRCM)-concrete joints using a trilinear bond-slip relationship

Interfacial debonding of fiber reinforced cementitious matrix (FRCM)-concrete joints can be considered as a mainly mode-II fracture process, a problem that can be solved by accounting for one-dimensional interfacial shear stress-slip relationships. This paper presents an analytical approach to predict the load response of FRCM-concrete joints by adopting a trilinear bond-slip relationship consisting of a linear-elastic branch, a softening branch, and a friction branch. The applied load-global slip response of FRCM-concrete joints with (relatively) long bonded length includes five stages: elastic, elastic-softening, elastic-softening-debonding, softening-debonding, and debonding stages. Closed-form solutions of the interfacial slip, shear stress, and axial stress (or strain) distribution along the bonded length are provided. The response of FRCM-concrete joints with (relatively) short bonded length is examined. The effective bond length and a critical length for the existence of the snap-back phenomenon are derived. Experimental results reported in the literature are used to calibrate the parameters needed for the analytical approach. The analytical results are then compared with experimental results and with numerical results determined using a finite difference method (FDM). Finally, the capability of determining the parameters in the trilinear bond-slip relationship using a neural network (NN) with the experimental load response as the input is investigated.

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.

Out-of-plane testing of masonry infills strengthened using fibre reinforced matrix with prior in-plane damage

Performance of fibre reinforced cementitious matrix (FRCM) strengthened masonry infills under sequential bi-directional earthquake induced lateral loading, which is far from well-understood, was experimentally investigated herein. A total of eight single bay reinforced concrete frames were constructed with unreinforced hollow concrete masonry infills, of these one was tested non-retrofitted that served as control test assembly and the remainder were strengthened using varying FRCM types and configurations. The test assemblies were subjected to cyclic in-plane loading gradually increasing to 1% story drift, replicating lateral loading due to a moderate earthquake. These already in-plane damaged test assemblies were then subjected to gradually increasing monotonic OOP loading until failure. Resulting damage patterns, failure mechanisms, measured force-displacement response and stiffness characteristics were discussed. FRCM strengthening delayed the onset of out-of-plane cracking, instigated a more controlled failure mode, with strengthened infills exhibiting out-of-plane strength of 1.72.0 times that of the control specimen.