Experimental Qualification of PBO-FRCM Composites for Retrofitting Masonry Structures

In the construction industry, the use of natural fabrics as a reinforcement for cement-based composites has shown great potential. The use of these sustainable composites to provide strengthening or repair old masonry structures that exhibit structural problems mainly due to a poor tensile strength of the mortar/brick joints is revealed to be a promising area of research. One of the most significant load conditions affecting the mechanical response of masonry structures occurs when axial bending loads are applied on the resistant cross-section. In this study, three different types of masonry elements were built using clay bricks and a lime-based mortar. After 28 days, the samples were subjected to concentric and eccentric compressive loads. In order to produce significant bending effects, the compressive loads were applied with large eccentricity, and a sudden failure characterized the behavior of the unreinforced masonry (URM) elements. The tested masonry specimens were repaired using fabric-reinforced cementitious matrix (FRCM) composites produced using bi-directional flax and polyparaphenylene benzobisoxazole (PBO) fabrics. The mechanical behavior of the URM and repaired samples was compared in terms of load-displacement and moment-curvature responses. Furthermore, the results achieved using flax-FRCM composites were compared with those of using PBO-FRCM composites.

FRCM/internal transverse shear reinforcement interaction in shear strengthened RC beams

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.

Composite materials employing cement-based mortars, usually known as fabric-reinforced cementitious matrix (FRCM) composites, have recently emerged as an attractive solution for the repairing and strengthening of reinforced concrete or masonry members. They represent a promising alternative to the use of Fiber Reinforced Polymer (FRP) composites when there is a need to overcome some of the drawbacks related to the epoxy resin, such as moderate matrix heat and fire resistance, difficulty of application at low temperatures, impossibility of application on wet surfaces, and lack of vapor permeability. This paper presents an analytical study on the confinement of concrete columns with FRCM composites with the twofold objective to: (a) propose new models for the estimate of the compressive strength, and (b) assess existing models available in the literature and in some international guidelines.To this purpose, a wide database including results of compression tests performed on over 290 concrete cylinders externally wrapped with FRCM was assembled from the literature. The collected results were employed to perform an overall analysis of the efficiency of the FRCM confinement by varying some of the relevant parameters, such as: type of fiber (glass, carbon, steel, PBO or basalt) and geometry of the mesh, number of employed layers, mechanical properties of the inorganic matrix and compressive strength of the unconfined concrete. Relationships for estimating the compression strength of the FRCM confined concrete were then developed through best-fit analyses, and comparisons with some formulations available in the literature and in some international guidelines were performed.

The tensile properties of fabric-reinforced cementitious matrix (FRCM) composites are experimentally investigated through clevis-grip tensile tests (according to AC434 provisions) on FRCM coupons realized with customized (ad hoc developed in this paper) cement-based matrices. The tested FRCM coupons are reinforced with basalt, carbon, or steel fabrics, and are prepared with three different matrices: one-component mortar incorporating dispersible copolymer powders of vinyl acetate and ethylene (matrices A and B) and two-component mortar with carboxylated styrene–butadiene copolymer liquid resin (matrix C). This has made it possible to investigate the mechanical compatibility between different mortar matrices and fabrics and the resulting tensile properties of FRCM composites in the uncracked, cracking, and fully cracked phases. Experimental results are critically analyzed in terms of stress–strain curves and failure mechanisms comparatively for the analyzed FRCM systems. It has been shown that the matrix B exhibits a good compatibility with the basalt pre-impregnated fabric, while the matrix C appears to be the most suitable candidate to optimize the interfacial stress transfer at the fiber–matrix interface for all fabrics, thus exalting the mechanical performances in terms of tensile strength and ultimate strain. The results of this experimental program can be useful for designing optimized mortar mixes aimed at realizing novel FRCM composites or at improving existing FRCM systems by suitably accounting for compatibility behavior and slippage at the fabric–matrix interface.

Durability of glass FRCM systems: Effects of different environments on mechanical properties

Over the past two decades, composite materials, in forms of Fiber Reinforced Polymers (FRP), have been widely spread worldwide in the field of civil and monumental construction. Design guidelines and provisions were developed and provided by national and international institutions. In the last years, a new generation of materials, named Fabric Reinforced Cementitious Matrix (FRCM) were introduced as strengthening devices for concrete and masonry structures. Their application in the field of historical masonry has grown as a result of the recent Italian earthquakes. In this paper, starting from a retrospective on what has been done in recent years in the field of FRP applications, insights will be discussed for future research and applications of FRP and FRCM in heritage buildings. Some differences between FRP and FRCM materials will be highlighted, in terms of fiber-matrix interface and delamination mechanisms. The different micromechanical behavior in terms of fracture energy will be highlighted, and the macro-mechanical implications in terms of ductility will be pointed out, as a first attempt to quantify this complex problem. By considering the last innovative and pioneering applications of FRP/FRCM in heritage buildings, criteria for structural enhancement will be shown and discussed. This is done with a special focus on the ability, shown by these new technologies, to inhibit failure mechanisms in masonry artifacts.

Design of the shear strengthening of masonry walls with fabric reinforced cementitious matrix

When the strengthening material - fabric-reinforced cementitious matrix (FRCM) composite is used as the anode in the impressed current cathodic protection (ICCP) for reinforced concrete structures, its long-term performance will be compromised due to anode acidification. Incorporating graphene into the matrix offers a means of mitigating interface deterioration by delaying the localized calcium leaching. This paper presents the results of a study to investigate the mitigation effects of graphene on the mechanical and electrochemical properties of FRCM composites. Direct tensile and pull-out tests were performed on graphene-enhanced FRCM (Gr-FRCM) composites subjected to different ICCP current densities and graphene dosages. The results reveal that graphene significantly reduces the deterioration in tensile and interfacial properties induced by long-term ICCP. The degradation factor for the ultimate tensile strain of FRCM composites under ICCP follows an exponential relation of total electrons, whereas the addition of graphene reduces this factor by more than half. Two constitutive models were developed to predict the key mechanical parameter, the effective strain of fibres, in the FRCM composite subjected to cathodic protection with and without graphene. These models were validated against beam test results and shown to be accurate for calculations of the flexural resistance of concrete beams strengthened using Gr-FRCM composites.

This paper presents a data-driven approach to determine the load and flexural capacities of reinforced concrete (RC) beams strengthened with fabric reinforced cementitious matrix (FRCM) composites in flexure. A total of seven machine learning (ML) models such as kernel ridge regression, K-nearest neighbors, support vector regression, classification and regression trees, random forest, gradient boosted trees, and extreme gradient boosting (xgBoost) are evaluated to propose the best predictive model for FRCM-strengthened beams. Beam geometry, internal steel reinforcement area, FRCM reinforcement area, and mechanical characteristics of concrete, steel, and FRCM are the main input parameters included in the database. Among the studied ML models, the xgBoost model is the most accurate model with the highest coefficient of determination (R2=99.3%) and least root mean square (RMSE), mean absolute error (MAE), and mean absolute percentage error (MAPE). A comparative study of the performance of the proposed and existing analytical models revealed the superior predictive capability and robustness of the proposed model. The predicted flexural and load capacities of the beams based on the existing analytical models are highly scattered and either over-conservative or unsafe. A unified SHapley Additive exPlanations approach is employed to explain the output of the best ML model and identify the most significant input features and interactions that influence the capacity of FRCM-strengthened RC beams in flexure. Furthermore, a reliability analysis is performed to calibrate the resistance reduction factor (ϕ) to achieve a specified target reliability index (βT=3.5).

To address the limitations of organic adhesives in reinforcement structures, fabric reinforced cementitious matrix (FRCM) composites has been introduced as a substitute for fiber-reinforced polymers (FRPs). However, the high strength properties of the fabric cannot be fully exploited due to the brittle nature of the cementitious matrix. To address this issue, short fibers have been proposed to be incorporated into the matrix to enhance the mechanical properties of FRCM composites. In this study, the flexural properties of carbon FRCM composites plates with short fibers were investigated by four-point flexural tests. The effects of volume fraction and type of short fibers, aspect ratio of short polyethylene (PE) fibers, matrix strength, and fabric type were considered. The load–deflection curve, flexural performance parameters, and crack pattern were evaluated. A qualitative analysis of the average crack spacing was performed.

Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and theoretical study

Steel Reinforced Grout under uniaxial load: Experimental evidences and numerical modelling

The need for repair and rehabilitation is a crucial issue today in order to maintain a safe and efficient transportation network. One of the most recent innovative composite materials that has been proposed to the market is a fabric reinforced cementitious matrix (FRCM) strengthening system. This system consists of two components: a structural reinforcement mesh and a cementitious matrix. The structural reinforcement mesh consists of a polyparaphenylene benzobisoxazole (or PBO) fiber composite, while the cementitious matrix is a nontoxic grout system based on portland cement with a low dosage of dry polymers. The first aim of this study was to evaluate the fatigue resistance of reinforced concrete (RC) beams strengthened with FRCM under fatigue loading and postfatigue flexural strength to failure. The second aim was to investigate the durability performance of the FRCM after exposure to environmental conditioning including temperature, moisture, and sustained stress. All of the beam specimens exam...

Fabric-reinforced cementitious matrix (FRCM) are composite materials more and more used for the reinforcement of masonry structures. The combination of high tensile strength fabrics (or meshes) with cementitious matrices, having good thixotropic capabilities and vapour permeability, makes such composites suitable for reinforcing a large number of masonry structures, including the one belonging to the historic heritage. FRCMs are bonded to the outer surfaces of structural masonry elements and, thanks to their adhesive capacity, bear much of the tensile stresses that unreinforced masonry cannot withstand. The effectiveness of such reinforcements, which is highly dependent on their ability to adhere to the masonry substrate, is generally investigated throughout specific experimental investigations (shear tests). Almost all the papers in the literature devoted to bond-slip analysis refer to the case of flat bonding surfaces, although these reinforcements are also widely used on curved structural elements such as arches and vaults. Therefore, this paper reports and examines the results of an extensive experimental program concerning the behavior of FRCM systems applied on curved masonry specimens. The results point out the influence of both curvature and reinforcement position (intrados or extrados) on the response of specimens in terms of bearing capacity, failure mode and post-peak response.