•Improvement of interfacial bonding between natural fibres and cement paste.•Crack arrest of resin coated NFR cementitious composites.•Enhanced mechanical performance of resin coated NFR cementitious ...composites.•GGBS and FA addition improved the impermeability of NFR cementitious composites.
This study investigates the interfacial bonding of natural fibre reinforced (NFR) cementitious composites by exploring the incorporation of 20–30 mm strands of uncoated and resin coated flax/wool twine into various cementitious matrices. Cementitious matrices consisting of pulverised fly-ash (FA), ground granulated blast furnace slag (GGBS) and ordinary Portland cement (OPC) in various ratios were tested with the addition of flax/wool twine (1% volume ratio). The mechanical properties of these samples were assessed at 7 and 28 days. The results showed a reduction in the flexural and compressive strength of uncoated NFR samples compared to unreinforced (UNR) counterparts due to weak interfacial bonding between the uncoated fibres and the cementitious paste, therefore, formation of voids. Epoxy (EP) and polyurethane (PU) resin were then used to coat the flax/wool twine prior to their inclusion in various cementitious pastes. The results revealed improvements in both flexural and compressive strengths exhibited at 7 and 28 days compared to UNR and uncoated NFR samples. The greatest improvement of flexural strength, 61% compared to UNR, was achieved by the mix consisting of 50% OPC and 50% FA matrix with EP resin coated flax/wool twine at 28 days. While PU coated samples exhibited an increase of 31% in flexural strength at 28 days for the same cementitious mix ratio. The morphology of the resin coated NFR samples showed an intimate interfacial bond to surrounding cementitious paste, which explains the increased mechanical performance.
The use of textile reinforced mortar (TRM) composites is considered a suitable solution to strengthen historical masonry structures. Specifically, TRM composites are characterised by a textile ...embedded in a mortar matrix. In the present work, we propose a thorough experimental analysis of basalt textile reinforced mortar. Four series of tensile tests were carried out on specimens characterised by three different basalt textile reinforcing ratio and two different kinds of mortar (usual gauged mortar and thixotropic mortar with additives). The use of digital image correlation facilitated detailed description of the displacement fields and the crack pattern during the tests. The comparison between the results of each series allowed us to evaluate the influence of the different parameters on the behaviour of the analysed TRM and to obtain useful information regarding the strengthening design.
Steel fiber reinforced geopolymer matrix (S-FRGM) composites are explored as a new tool in strengthening applications of reinforced concrete (RC) structures. The suitability of fly ash geopolymers as ...the matrix employed in S-FRGMs is investigated in this paper. Geopolymer matrix is used to embed galvanized steel fibers and bond the composite to a concrete substrate. By varying the molar concentration of the NaOH activating solution, three different geopolymer matrices are obtained and the influence of the alkaline environment on the bond behavior between fibers and matrix is examined. Physical, mechanical, and microstructural properties of the three matrices are measured and related to the interfacial bond behavior of S-FRGM-concrete joints. Results show that the decrease of the molar concentration of the NaOH activating solution leads to an increase of the interfacial bond behavior between matrix and fibers, which in turn improves the effectiveness of the composite action of the strengthening system.
AbstractUnreinforced masonry (URM) walls have been constructed for the past millennia and are still widely used today. URM walls have proven to have low shear strength and are prone to brittle ...failure when subjected to in-plane loads caused by earthquake or wind. Retrofitting URM walls is accomplished internally and externally using current techniques, such as placing steel bars in the cavities and grouting, posttensioning with steel tendons, stitching, and adhering fiber-reinforced polymers (FRP) to increase capacity and enhance pseudoductility. In this study, a fabric-reinforced cementitious matrix (FRCM) system is applied to URM walls to determine its feasibility as an alternative external strengthening technology. The experimental program consists of testing a total of nine clay brick walls under diagonal compression. Two FRCM strengthening reinforcement schemes are applied, namely, one and four reinforcement fabrics. An analytical model is used to calculate the shear capacity of strengthened URM walls and compare its results with the experimental database. The effect of limitations in design approach on shear capacity of strengthened walls is discussed.
This paper aims at developing a model that is capable to accurately predict the debonding strains in reinforced concrete (RC) members strengthened with fabric-reinforced cementitious matrix (FRCM) ...systems. A large database consisting of 393 shear bond specimens strengthened with Polyparaphenylene Benzobisoxazole (PBO), Carbon (C), Glass (G), and Steel (S) FRCM systems was firstly compiled from the published literature. A sensitivity analysis was carried out to identify the key parameters that most affected the debonding mechanism in FRCM. The notable influence of the compressive and tensile strengths of the concrete substrate, the compressive strength of FRCM mortar, and the axial stiffness of FRCM system on the debonding strains in FRCM systems was evidenced. Contrarily, the tensile strength of FRCM mortars showed slight or no impact on the FRCM debonding strains. Based on the results of the sensitivity analysis, three simple models were developed using a multivariate nonlinear regression analysis. The models were then optimized and validated against the experimental results of 41 flexural members strengthened with different types of FRCM systems. Two of the three models proved an excellent prediction performance of the debonding strains with an average predicted–to–experimental strain ratios, εpred/εexp, of 0.99 ± 0.27 and 1.02 ± 0.27 with coefficients of variation (COV) of 0.28 and 0.26, respectively. Both models could safely predict the debonding strains in FRCM-strengthened members regardless of the type of FRCM system used. Neglecting the tensile strength of the concrete substrate in the third model resulted in an average εpred/εexp ratio of 0.85 ± 0.37 with a COV of 0.44.
Despite advantages and unique features of fiber reinforced polymers (FRP) in strengthening concrete elements, problems such as being unsuitable for application on wet surfaces and low fire-resistance ...have urged researchers to seek novel alternative methods. Among them, fiber reinforced cementitious matrix (FRCM) composites have the ability to address the abovementioned problems by using inorganic matrices. In this study, 14 beam specimens with a cross-section of 250 × 300 mm and a length of 2200 mm were constructed and subjected to flexural test. The test beams were categorized into two distinct groups based on their strengthening systems. Six beams were strengthened in flexure using FRP composites, and the other eight beams were strengthened in flexure by the FRCM composites, as the main aim of this study was to compare the performance of the FRP and FRCM strengthened systems. Other parameters such as the strengthening method and the arrangement and ratio of tensile reinforcement in the beam were also investigated. Results indicate that beams strengthened by FRCM composites exhibit a significantly higher ultimate load compared to the companion beams strengthened by FRP composites.
•This study aims to compare FRP and FRCM strengthening systems.•FRCM-reinforced beams showed significantly higher ultimate loads than FRP-reinforced counterparts.•The ultimate load of EBROG-reinforced beams is unaffected by the material type (FRCM or FRP).•The FRCM-N method increased the ultimate load by 21-36% compared to the FRP-E method.•The FRCM-N method increased ultimate load by 14-20% compared to FRCM-E method.
This paper presents the results of an investigation on the bond behavior and stress-transfer mechanism of polyparaphenylene benzobisoxazole (PBO) fiber-reinforced cementitious matrix (FRCM) ...composites bonded to a concrete substrate using double-lap and single-lap shear tests. Results of double-lap shear tests with different composite bonded lengths and widths are presented and compared with those from single-lap shear tests previously reported by the authors. The idealized load response developed from single-lap shear tests is found to characterize the response of the composite in double-lap shear tests, although with a few key differences. With the double-lap shear test, load redistribution among the composite strips influences the post-peak response if debonding does not occur equally in both strips. Values of the ultimate (peak) stress determined using double-lap shear tests are generally consistent with, although slightly lower than, those determined by single-lap shear tests when the bonded length is longer than the effective bond length.
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.
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.
•The effectiveness of FRCM composite for strengthening of masonry walls was investigated.•A total of nine concrete masonry wall panels, 1.2m×1.2m in size, were tested under diagonal compression.•FRCM ...and FRP strengthened walls were compared after normalizing the data.•The experimental results were also compared with analytical model predictions.
In this paper, the in-plane behavior of un-reinforced concrete masonry walls externally strengthened with a fabric-reinforced cementitious matrix (FRCM) system is investigated. The experimental program consists of testing nine un-reinforced concrete masonry walls strengthened on both sides with two different FRCM schemes (one and four reinforcement fabrics). The analytical model as per ACI 549-13 is used to predict the shear capacity of the strengthened walls. The effects of design limitations in the approach proposed by ACI 549-13 are also discussed. Finally, experimental data from other research programs using fiber-reinforced polymer (FRP) composites are presented to demonstrate that when normalized shear capacity is related to a calibrated reinforcement ratio, the two overlay strengthening technologies match well.
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