Inorganic Matrix Composites Research Articles

Mechanical properties of fabric-reinforced geocomposites in accordance with novel size-independent flexure-test technique

In this paper, two silica-based geopolymer matrix systems reinforced composites were synthesized and fabricated at optimal conditions. The composites contained approximately 40 wt.% of Carbon HTA twill (TohoTenax) and 48 wt.% of advanced S-glass twill (Saint-Gobain, Vetrotex), respectively. The resulting composites exhibited quite high toughness, with the shear stress playing a greater role compared to other ceramic matrix composites. To determine the flexural properties of the composites, a universal testing machine was used under the three-point bending mode. This evaluation was conducted using a novel size-independent technique based on testing specimens at different scales of the sample height (H) to span length (L) ratio. Microstructure analysis of the composites was performed using a scanning electron microscope (SEM). The analysis aimed to assess the effective impregnation and adhesion between the fiber and geopolymer matrix, as well as to identify micro-cracks, which are inborn defects in inorganic matrix composites. The findings presented in this study highlight the potential of the synthesized silica-based geopolymer matrix composites and their applicability in various engineering fields. Keywords: Geopolymer composite, fabric fiber, Size-independent flexure-test technique, flexural properties and microstructure.

Upcycling plastic waste into fully recyclable composites through cold sintering.

Plastics have substantial societal benefits, but their widespread use has led to a critical waste management challenge. While mechanical recycling dominates the reuse of post-consumer plastics, it is limited in efficacy, especially for composites. To address this, we propose a direct reprocessing approach that enables the creation of hybrid, long-lasting, and durable composites from difficult-to-recycle plastics. This approach utilizes cold sintering, a process that consolidates inorganic powders through fractional dissolution and precipitation at temperatures far below conventional sintering; these temperatures are compatible with plastic processing. We show that this process can create inorganic-matrix composites with significant enhancements in tensile strength and toughness over pure gypsum, which is commonly found in construction waste. These composites can be recycled multiple times through direct reprocessing with the addition of only water as a processing promoter. This approach to recycling leads to composites with orders of magnitude lower energy demand, global warming potential, and water demand, when compared against common construction products. Altogether, we demonstrate the potential for cold sintering to integrate waste into high-performance recyclable composites.

Diagonal Compression Tests on Unfired and Fired Masonry Wallettes Retrofitted with Textile-Reinforced Alkali-Activated Mortar

This paper discusses the integration of an alkali-activated mortar (AAM), based on industrial waste, into a novel composite material fit for structural upgrading purposes and rendered with high temperature endurance and a low CO2 footprint. The AAM combined with carbon fiber textiles form a new generation of sustainable inorganic matrix composites—that of textile-reinforced alkali-activated mortars (TRAAM). A test program was designed to assess the effectiveness of carbon TRAAM overlays in increasing the shear capacity of masonry wall specimens comprising solid clay bricks bonded with lime-based mortar and furnished with TRAAM jackets on both sides. The initial and the residual capacity of the reinforced walls were evaluated, the latter by performing diagonal compression tests after exposure to 300 °C and 550 °C. It was shown that TRAAM jacketing can increase the shear capacity of unfired masonry walls by 260% and 335% when a single or a double layer of textile is used, respectively. Rapid heating to temperatures up to 550 °C, one-hour-long steady-state heating, and natural cooling bore no visible thermal cracks on the specimens and had little effect on their residual capacity. Based on these results, the prospect of using TRAAM for retrofitting applications for fire-resilient structures seems very auspicious.

Effects of Defects on Masonry Confinement with Inorganic Matrix Composites

Fabric-reinforced cementitious matrix (FRCM) composites are currently considered a suitable solution for strengthening existing structures. Confinement applications are still being investigated, since experimental programs showed significant scatter in the results and theoretical models are struggling to become established as a consequence. The main aim of this study is the identification of potential sources of scatter in the confinement efficiency of FRCM wrappings, in defects such as fiber slip within the matrix or imperfect straightening of fibers, or premature failure of fibers once exposed after complete matrix cracking. A theoretical incremental approach is proposed to simulate such effects. The approach is incremental, but not iterative, so that no convergence is required and the incremental step size has an impact only on the smoothness of the nonlinear theoretical stress vs. strain curves of the FRCM confined material, among other simulation results. Theoretical results are compared to experimental outcomes of previous tests. The main source of variability can be identified in the cited defects, and the approach can be considered satisfactory to simulate the effects of defects and the high scatter found in experimental results; however, further uncertainties in the behavior of materials can be included in future refinements of this study.

Design of Strain-Hardening Natural TRM Composites: Current Challenges and Future Research Paths

This paper discusses the challenges in using natural fibers for the development of textile-reinforced mortar (TRM) composites with pseudo-strain-hardening and multiple cracking behavior. The particular characteristics of natural vegetal fibers are analyzed with reference to data from the literature. It is concluded that the efficient use of these fibers as composite reinforcement requires the development of treatment or impregnation protocols for overcoming durability issues, eliminating crimping effects in tensile response and imparting dimensional stability. Relevant experimental research on the synthesis and performance of natural TRMs is reviewed, showing that the fabrication of such systems is, at present, largely based on empirical rather than engineering design. In order to set a framework regarding the properties that the constituents of natural TRM must meet, a comparative analysis is performed against inorganic matrix composites comprising synthetic, mineral and metallic reinforcement. This highlights the need for selecting matrix materials compatible with natural fibers in terms of stiffness and strength. Furthermore, a rational methodology for the theoretical design of natural TRM composites is proposed. First-order analysis tools based on rule-of-mixtures and fracture mechanics concepts are considered. Based on the findings of this study, paths for future research are discussed.

Production of Graphene/Inorganic Matrix Composites through the Sintering of Graphene Oxide Flakes Decorated with CuWO4·2H2O Nanoparticles.

There is growing interest in graphene-reinforced inorganic matrix composites, but progress in this field is far behind that of polymer matrices due to difficulties in the processing of carbon materials in aggressive sintering environments, including oxidation and solubility in the host matrix. Copper-tungsten matrices are of particular interest in the power switching field but are difficult to produce due to the mutual insolubility of metals and poor wetting. Herein, composites were produced by decorating graphene oxide flakes with 8 nm diameter CuWO4·2H2O nanoparticles and then sintering them to form the final shape. The oxide nanoparticles were found to self-assemble into platelets on the surfaces of graphene flakes. Upon sintering, the presence of graphene was found to change the grain morphology from elongated needles to a polyhedral shape. It was found that, despite the nanosize of the CuWO4·2H2O particles used, the sintering conditions did not reduce the matrix to a pure metal; the sintered composites were found to be of mixed phase with copper tungstate and copper oxide present. Raman spectroscopy indicated that the graphene oxide became hydrogenated during the sintering process as a result of the reducing hydrogen atmosphere used.

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

Fabric-reinforced cementitious matrix (FRCM) composites, comprising high-strength fiber textiles embedded within inorganic matrices, represent an effective, cost-efficient, and low-invasive solution for strengthening and retrofitting existing masonry and reinforced concrete structures. Among different textiles employed in FRCM composites, polyparaphenylene benzo-bisoxazole (PBO) textiles are adopted due to their high tensile strength and good adhesion with the matrix. Although several experimental, numerical, and analytical works were performed to investigate the mechanical properties of PBO FRCM composites, limited information is available on their long-term behavior, as well as in the case of exposure to aggressive environments. This paper presents and discusses the results of a wide experimental campaign aimed at investigating the effect of different environmental conditions on the long-term tensile behavior of a PBO FRCM composite. Tests are performed using a clamping-grip tensile test set-up. The effect of various aggressive environments on the composite matrix cracking stress, composite tensile strength, ultimate strain, and fully cracked stage slope is investigated by comparing the results of nominally equal conditioned and unconditioned (control) specimens. These results are also compared with those of other FRCM composites comprising glass and carbon textiles subjected to the same conditionings, collected from the literature. The results show only limited reductions in the tensile properties, even after long exposure to aggressive environments.

Long-Term Durability of Inorganic Matrix Composite (IMC) Systems under Sustained Tensile Loading

Inorganic Matrix Composite (IMC) systems have been extensively studied in recent years as an effective alternative to the most known organic Fiber Reinforced Polymer (FRP) systems, especially for strengthening applications on masonry. Despite intensive research has been conducted to study mechanical properties and bond mechanisms of such systems, also under different environmental conditions, little information is available on their long-term behaviour under sustained load. In the framework of a large assessment plan for the analysis of fabric reinforced inorganic matrix composites, long-term tensile tests have been performed on different systems in a newly developed test setup. The paper will aim at describing the proposed setup and instrumentation, as well as the results in terms of measured displacements for different combination of fabrics and matrices. Results in terms of retained tensile strength will also be presented, showing the little influence of the long-term loading on the performance of the studied composite systems.

A macro-modelling approach for arches strengthened with externally bonded inorganic matrix composites

The use of externally bonded inorganic matrix composites is becoming a common technique for strengthening arches and vaults to improve the load-carrying capacity with respect to both static and seismic loads. Experimental evidence has shown a complex behaviour where failure may occur within the reinforcement matrix, with a strong coupling between pure opening mode (mode I) and shear mode (mode II), as a consequence of the curvature of the substrate.Aiming at providing a tool for simulating the structural behavior of masonry arches reinforced with inorganic matrix composites, a discrete macro-modeling approach is proposed in which the reinforcement is described by means of piecewise rigid plates, which interact with the masonry through zero-thickness nonlinear interfaces. A refined bond constitutive law, specifically conceived to account for both the cohesive and the frictional contributions, as well as the coupling between mode I and mode II, in terms of strength and softening behavior, is proposed and implemented in a macro-elements model. The proposed model is then validated through comparison with analytical predictions and experimental direct shear tests performed with straight and curved substrates. Eventually, the model is used to reproduce the behaviour of a barrel vault strengthened with steel reinforced grout and tested up to failure.

Experimental analysis on tensile strengthening properties of steel and glass fiber reinforced inorganic matrix composites

The fiber-reinforced inorganic matrix (FRIM) composite is a new type of composite, which has many economical and performance advantages. Beside the direct shear and bending tests, the tensile tests form an integral part in determining the mechanical properties of these composites. In this paper, to understand the tensile behavior of the FRIM composites, some strip specimens of the composites were tested which were clamped at the both ends, and the strains were measured using the extensometers installed at the middle of each strip. The inorganic matrix composites studied in this paper were constructed using two types of steel and glass fibers together with two different types of inorganic lime and geopolymer mortars. The results of direct tensile tests showed that the inorganic geopolymer mortars had the higher potential to increase the tensile load bearing of the specimens compared with the lime mortars. In addition, in most cases, the maximum values of stress, strain and the stiffness at the final stage of response in the tensile tests of composites were consistent with the results reported from the tensile test of textile fibers without the mortar. Moreover, clamping specimens by applying sufficient compressive force prevents slipping of fibers within the surrounding mortar.

Targeting functionalised carbon nanotubes at the interphase of Textile Reinforced Mortar (TRM) composites

Tensile performance of textile reinforced inorganic matrix composites strongly depends on the matrix-to-fabric bond strength, that is the weak chain in the system. In this work, we investigate the role of multi-walled carbon nanotubes (MWCNT) dispersion in an amorphous silica nano-coating for AR-glass and carbon fabric Textile Reinforced Mortar (TRM) composites. Two lime mortars are considered at 56-day curing. Comparative mechanical testing in uni-axial tension show remarkable enhancements in terms of mean ductility, strength and energy dissipation capabilities. Besides, coating successfully hinders telescopic failure and delamination, which significantly narrows data scattering and benefits design limits. Crack pattern analysis reveals that coating promotes diffuse cracking in the specimen, with gradual and progressive damage buildup. Indeed, mean crack width and mean crack spacing are consistently reduced. BET, optical and E-SEM microscopy supports the action mechanism of the coating, that promotes wettability, surface roughening and imparts a remarkable increase in the specific surface area of the reinforcement.

Effect of fiber-to-matrix bond on the performance of inorganic matrix composites

The strengthening of masonry structures is nowadays performed by means of high-strength fibers embedded in inorganic matrix (FRCM) where lime or cement-based matrix is used instead of epoxy adhesive to reduce debonding issues between substrate and matrix. However, some sliding phenomena and cohesive failures between fibers and the matrix mortar can occur. The paper examines the effect on the FRCM efficiency of the mechanical properties of fiber and matrix and potential geometrical defects, which are possible in real field applications or in qualification tests. The model application to simulate bond tests on typical PBO-FRCM and Glass-FRCM allowed to analyse slips as well as normal and shear stresses both in the bundle and in the matrix constituting the FRCM, for different defects due to application issues. The result of numerical simulations seems to interpret well the results of the qualification tests with a multi-bundle effect that justifies their scatter.The approach can be applied by varying main mechanical properties of the materials (e.g. elastic modulus, fiber cross section, bond properties) to consider their intrinsic variability in the assessment of the performance of the FRCM system or by changing the type of materials (i.e. mortar and fibre) to optimize the FRCM system.

A New Fabric Reinforced Geopolymer Mortar (FRGM) with Mechanical and Energy Benefits

A large part of the European building Heritage is dated back over centuries. Consequently, its structural and thermal performances are often inadequate. Commonly, different interventions are proposed for solving these issues separately. However, reasonable drawbacks arise when the structural retrofitting requires a direct contact with the target-member while the insulation layer is potentially interposed in between. In this scenario, the present research proposes a novel and unique system able to guarantee both the energetic and the structural retrofitting. Inorganic Matrix Composites (IMCs) are a promising solution in this sense. Among them, the Fabric Reinforced Cementitous Matrix (FRCM) is one of the most used; or rather a composite made of a fabric (open grid or mesh) within an inorganic matrix (lime or cement based). Even if the inorganic matrix has a relevant thickness (if compared with the one of the fabric), its thermal resistance is insufficient. The novelty of this work consists in assessing a new geo-polymeric FRCM-system by combining fly-ash binder (reused material) and expanded glass aggregate (recycled material). Direct tensile tests, for measuring the tensile strength, ultimate strain and elastic modulus, were performed in addition to thermal conductivity tests. The results were compared with those of traditional FRCM (commercially available). The potentiality of the proposal for structural and energy retrofitting is discussed and examples of its possible application are also reported.

Effect of corner over-reinforcing strips on the compressive behaviour of TRM confined masonry columns

Several recent works studied the efficiency of inorganic matrix composites, namely Textile Reinforced Mortar (TRM) or Fabric Reinforced Cementitious Matrix, for enhancing the strength and shortening capacity of masonry columns subjected to axial load. Literature studies were addressed to study the great number of variables involved in the problem, such as the nature and the grade of mortar, the strength of the fabric, the number of reinforcing layers, the type, the arrangement and the strength of the masonry, and helped to draw the first technical guidelines for practitioners and designers. All the experimental works highlighted that the actual performance of TRM confinement in masonry members was limited by the premature failure of the external layer in correspondence of the corners of sharp-edged sections, due to the stress concentration, limiting the confinement pressure achievable from the external wrap. This paper presents the result of an experimental investigation which aims to study the effect of a corner strengthening detail on the compressive behaviour of calcarenite masonry columns wrapped with glass TRM. The study was performed by varying the number of textile layers and the grade of the mortar adopted for the composite. The results show that the introduction of a fabric strip in correspondence of the corners can limit the stress concentration at the edges, allowing to exploit the confinement pressure of the TRM and achieving greater strength increases.

Experimental procedures for the mechanical characterization of composite reinforced mortar (CRM) systems for retrofitting of masonry structures

Fiber-reinforced composites can be arranged in the form of bi-dimensional grids and employed as internal reinforcement of mortar plasters to realize composite reinforced mortar (CRM) systems. Recently, CRM were applied as externally bonded reinforcement of existing masonry members showing promising improvements of load-carrying and deformation capacities. However, since CRM systems are still in their infancy, limited research is available regarding their mechanical properties and their bond behavior with respect to masonry substrates. In this paper, a series of experimental tests are performed on a CRM system comprising a glass fiber-reinforced composite grid and a lime-based matrix. Namely, tensile tests of bare grid yarns and of CRM coupons, shear tests of grid joints, and single-lap direct shear tests of CRM-masonry joints were performed. These tests are aimed at providing a comprehensive mechanical characterization of the CRM, which results can be used to design strengthening applications with this system. Namely, the tensile properties of bare grid yarns in warp and weft direction are obtained and compared with those of CRM coupons tested following the indications of the Italian and U.S. acceptance criteria for inorganic-matrix composites. Furthermore, the grid joints are subjected to shear tests to determine if the yarns orthogonal to the applied load direction provide a contribution to the system load-carrying capacity. Finally, CRM-masonry joints are subjected to single-lap direct shear tests to study the CRM bond behavior. This work provides an insight on the behavior of CRM that can be useful to formulate reliable design procedures for these systems.

Study of the influence of interface normal stresses on the bond behavior of FRCM composites using direct shear and modified beam tests

In the last decades, the increasing need for strengthening and retrofitting of existing masonry members has led the scientific community toward the development of innovative strengthening systems. Among these systems, inorganic-matrix composite materials have been gaining increasing popularity due to their effectiveness, relatively low cost, ease of the application, and reversibility of the intervention. Inorganic-matrix composites, usually referred to as fiber- (or fabric-) reinforced cementitious matrix (FRCM) composites, were successfully used as externally bonded reinforcement (EBR) of masonry members subjected to various load combinations. However, their effectiveness is strictly related to the FRCM-substrate bond behavior. Therefore, numerous set-ups were employed to investigate the bond properties of FRCM composites. Among them, single-lap direct shear test set-ups were the most commonly employed and are now recommended by available design guidelines for FRCM composites.In this paper, a modified beam (MB) test set-up is used to study the bond behavior of two different FRCM composites: a polyparaphenylene benzobisoxazole (PBO) FRCM and a carbon FRCM. The parameters studied are the dimensions of the blocks used in the MB tests and the presence of bare or embedded fibers at the gap between the blocks. The bond behavior observed is compared with that of single-lap direct shear tests of the same FRCM composites. The results obtained provide an insight on the influence of normal stress components at the matrix-fiber interface on the FRCM bond behavior, which may play an important role in the capacity of FRCM strengthened curved members.

Prepregs for Temperature Resistant Composites.

In this paper, carbon fabric reinforced inorganic matrix composites were prepared. The inorganic matrix based on alkali activated aluminosilicate was used because of its resistance to fire and the temperatures up to 1000 °C. Influence of heat treatment of fabric, high temperature treatment of composite and preparation method on the mechanical properties and morphology of the composites were studied. The preparation of composites with the subsequent steps of impregnation, layering and curing of the composites was compared with the prepreg preparation method, which separates the impregnation of the reinforcement from the production of the composite. The SEM photographs show no differences in morphology between composites prepared from heat treated fabric and composites prepared from original fabrics. All four series of samples were comparatively saturated with matrix. Despite this, tensile properties of heat-treated fabric composites were negatively affected. While composites with heat-treated fabric reached the tensile strength up to 274 MPa, composites prepared without heat-treated fabric exhibited strengths higher than 336 MPa. Samples exposed to temperatures reaching 600 °C retained up to 40% of their original strength. The effect of composite preparation method on the tensile properties of the composites has not been proved.

Designing epoxy viscosity for optimal mechanical performance of coated Glass Textile Reinforced Mortar (GTRM) composites

Preliminary epoxy coating of the reinforcing fabric provides an effective approach for improving matrix-to-fabric strength in inorganic matrix composites. We investigate the effect of epoxy resin dilution in acetone on uni-axial tensile performance of coated alkali-resistant (AR) glass fabric embedded in a lime-based matrix. Remarkably, it is found that dilution has a mixed effect on performance and this trend is consistently retrieved for strength, ductility and energy dissipation. Indeed, performance initially decays and then it suddenly raises to a level close to or even exceeding that of the undiluted specimens. It is postulated that this behaviour is caused by resin viscosity, that falls off exponentially with the dilution degree. Once a viscosity threshold is breached, epoxy is capable of penetrating inside the yarn and thereby prevents telescopic failure, that is the sliding of the outer over the inner glass filaments. Furthermore, the interphase surface area increases dramatically and this enhances performance and narrows scattering. Besides, optimal viscosity is reached at an unexpectedly high dilution degree, whence material cost is significantly reduced. A cost-to-performance comparison of common strengthening technologies is presented, which shows that diluted epoxy composites score comparably to FRPs. It is concluded that epoxy coating optimization plays an important role in designing inorganic matrix composites.

Diagonal Compression of Masonry Walls Strengthened with Composite Reinforced Mortar

The use of inorganic matrix composites to strengthen and retrofit existing masonry and concrete structures has been gaining increasing interest in the last years. Among them, composite reinforced mortar (CRM) systems are a promising solution to increase the shear and flexural capacity of masonry panels. CRMs are comprised of a relatively thin inorganic matrix layer reinforced with a bi-dimensional grid made with high-strength fibers impregnated with an organic matrix. They are compatible with the substrate due to the use of inorganic matrix, have good durability and high-strength-to weight ratio due to the use of reinforcing composite materials. CRM systems are still in their infancy and limited research is available in the literature. In this paper, masonry walls constructed with historical bricks typical of the north of Italy were strengthened with a CRM system including a glass composite grid and a lime-based mortar and were subjected to diagonal compression. Three walls were strengthened with the CRM and one was used as a control specimen. The results obtained showed that the CRM system significantly increased the shear strength of the masonry panels subjected to diagonal compression.

Comparison between different tensile test set-ups for the mechanical characterization of inorganic-matrix composites

Inorganic-matrix composite materials can be effectively employed to strengthen and retrofit existing reinforced concrete and masonry structures. These composite materials comprise high-strength fiber textiles embedded within inorganic matrices. Different textile layouts and types of matrix and fibers can be employed resulting in a multitude of different composite materials and mechanical behavior thereof. As in most composite materials, the mechanical characterization of the single constituents (matrix and fiber) does not provide indications on the behavior of the composite material. Therefore, the mechanical characterization of the entire composite material is of fundamental importance to understand the interaction (i.e. composite action) between the fiber textile and the embedding matrix. Although different tensile test set-ups were proposed in the literature, a shared standard testing method for the mechanical characterization of inorganic-matrix composites is not yet available.In this paper, the mechanical properties of four different inorganic-matrix composites comprising carbon, glass, basalt, and steel fibers embedded within cement- and lime-based matrices are investigated using two different tensile test set-ups. Analog LVDTs and digital image correlation (DIC) measurements are employed to study the specimen longitudinal strain behavior. Two criteria to assess the reliability of results of tensile tests are proposed. Comparison between results described in this paper shows the strong influence of the test set-up on the tensile behavior of inorganic-matrix composites and helps to shed light on the reliability of the mechanical parameters obtained.