Fabric Reinforced Cementitious Matrix (FRCM) Materials For Structural Rehabilitation.

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1 Fabric Reinforced Cementitious Matrix (FRCM) Materials For Structural Rehabilitation. Giuseppe BIANCHI [a], Diana ARBOLEDA [b], Francesca Giulia CAROZZI [c], Carlo POGGI [d], Antonio NANNI [e] [a]master Student, Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, Italy; [b]phd Candidate, Department of Civil Architectural and Environmental Engineering, University of Miami, U.S.A.; [c]phd Student, Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, Italy; [d]full Professor, Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, Italy; [e]full Professor and Chair, Department of Civil Architectural and Environmental Engineering, University of Miami, U.S.A. and University of Naples-Federico II, Italy; Keywords: FRCM, tensile strength, constitutive law, structural rehabilitation, material characterization Field of Interest: Refurbishment and Rehabilitation. Main paper focus: technological innovation for existing buildings; preservation and maintenance of the built heritage. Abstract: Existing reinforced concrete (RC) structures often need to be repaired, strengthened, and upgraded to satisfy current code requirements. Fabric Reinforced Cementitious Matrix (FRCM) composite systems represent one novel possibility for external reinforcement. The aim of this research was to study the main characteristics of an FRCM system realized with PBO (polyparaphenylene benzobisoxazole) fabric. First, characteristics of the matrix (mortar) and fabric were investigated. For the mortar, the dry specific weight, the fresh specific weight and the void content were determined. For the fabric, uniaxial tensile tests were performed on both a single yarn and a thin band made of five yarns. Second, characterization tests were performed in order to establish the constitutive relationship of the FRCM composite system. The constitutive relationship was studied by performing tensile tests, using two different gripping mechanisms, rigid and clevis type grips. It was shown that the tensile performance of the FRCM specimens anchored with rigid grips is equivalent to the performance of the yarns-only tests, while the specimens anchored with the clevis grips are representative of the field application boundary condition. The results of this study form a basis for the tensile characterization of the FRCM system so that design parameters can be established. 1.: Introduction and Background 1.1: Historical Background Cement-based materials like mortar and concrete have high compressive strength, low tensile strength and toughness, and are brittle in nature. Therefore, the use of these materials in practice involves the use of reinforcement; typically, steel bars in conventional reinforced concrete (RC). In addition, during the last two decades, the use of pre-fabricated cement-bonded fiberboard around the world has increased. Such elements

2 are used for many applications like wall panels, exterior siding, pressure pipes, and roofing and flooring tiles. The reinforcement can be either short fibers (Fiber Reinforced Concrete, FRC) or continuous fibers in a fabric form (Textile Reinforced Concrete, TRC). Fabric Reinforced Cementitious Matrix (FRCM) composites represent a particular type of TRC where a dry-fiber fabric is applied to a structure through a cementitious mortar enriched with short fibers. FRCM as a technology should not be confused with the use of FRP reinforcing grids embedded in concrete or mortar. The difference between FRP and the structural fabric used for FRCM consists in the following: the continuous reinforcement used for FRP is fully impregnated in an organic resin, whereas the dry-fiber fabric used for FRCM is applied with an inorganic matrix (mortar) which does not fully penetrate and impregnate the fiber strands. 1.2: Importance of Rehabilitation In addition to the use of TRC for new construction such as cladding applications or industrially manufactured products [1], TRC composite systems have also been widely used as external reinforcement applied to existing structures in need of rehabilitation. The need for further research in developing new methods of repairing, strengthening, and retrofitting of structures may be illustrated by the fact that in the United States alone, there has been an estimated $20 trillion investment in currently operating civil infrastructure systems (National Science Foundation, NSF, 1995). Because of aging, overuse, exposure, misuse, and neglect, many of these systems are deteriorating and becoming inadequate for the more stringent design requirements recently introduced. Europe has a similar situation. For example, in seismic areas, where design was performed according to old seismic codes, structures have to meet performance levels required by current standards. Italy represents, perhaps, the clearest example of this, because only recently the entire national territory was declared subject to seismic risk. Since it would be prohibitively costly and disruptive to replace the built stock, the clear solution is rehabilitating existing structures. FRP technology has been utilized, even with some drawbacks attributed to the organic resins used to bind and impregnate the fiber reinforcement [2]. FRCM materials were designed to overcome these drawbacks, and represent an alternative to FRP [3]. In order to start using the new technology, knowledge of the mechanical characteristics of the material is essential. Such a necessity promoted this and other studies aimed to define the material properties according to AC434 [4]. 1.3: Materials Presentation and Application Procedure This work intends to characterize the FRCM material called Gold-750, designed for concrete structures [5]. Gold-750 consists of a fabric made up by PBO fibers, applied to the structures using its own specific inorganic matrix (i.e. grout system based on Portland cement) enriched with short polymeric fibers [5]. Before the application, the surface has to be clean and moist, ensuring a saturated surface dry condition so as not to change the water/cementitious ratio of the mortar being applied and thus its characteristic strength and adhesion properties. Then, the first layer of matrix can be applied with a trowel on the support surface with a thickness of 3-5 mm, as smooth as possible. The pre-cut fabric, with the appropriate fiber orientation, has to be spread out on top of the first matrix layer and pressed lightly with a trowel to embed it into the matrix. Finally, the second layer of mortar can be applied (same thickness). This operation can be performed up to four times in order to create a stronger rehabilitation system. The mortar needs to cure for at least 28 days before completely developing its characteristics in terms of strength and adhesion. Even though an embedding procedure is followed, FRCM cannot be considered a system where the fibers are completely embedded. Indeed, only the external filaments in the yarns are in contact with the matrix, while the inner filaments remain dry. 2.: Mortar and Fabric Characterization 2.1: Mortar Specific Weight and Void Content

3 The void content of a mortar affects the durability characteristic of the material itself. Both air and water can propagate into a material that is rich of voids and in relationship to the dimensions and the interconnections between the voids. In addition, mortar has the fundamental role of developing the adhesion to the support [5]. If the mortar has high void content and big voids, a good adhesion will not be provided, since the contact surface area will be reduced by the voids. Dry unit weight of the mortar of 2.84 g/cm 3 was obtained by performing tests in accordance to ASTM C 188 [6]. Fresh unit weight and void content tests were performed following ASTM C138 [7]. Five test repetitions were performed on different bags of mortar. The average result for fresh unit weight is 1.90 g/cm 3 with a void content of 8.37%. These results were obtained by mixing the mortar using a high velocity drill. A higher degree of variability was noted with other mixing methods such as concrete mixer, low velocity drill, and by hand. 2.2: Fiber Tensile Tests on PBO Two types of tensile test were performed: on a single yarn of material and on a segment of fabric made by five yarns. The yarns were dry except for test set C. The material used for gripping was different, as described in Table 2.1. Table 2.1: Details of experimental set up for fiber tensile tests. TYPE OF SPECIMEN GRIPPING MATERIAL REPETITIONS TEST SET Fiberglass - Araldite 1 A Single yarn Cardboard Loktite 5 B Epoxy Glue 3 C Fabric (five yarns) Fiberglass - Araldite 3 D Cardboard Loktite 3 E The PBO fibers have a modulus of elasticity of 270 GPa and an ultimate strength of 5.8 GPa [5], as provided by the manufacturer. In the following graph (Figure 2.1) the average results for each test are shown and compared with the individual filament behavior. Figure 2.1: Average results of tensile tests. The modulus of elastic behavior (slope) of the yarns tends to fall below that of the individual fiber or filament, and the maximum strength of the filament is not reached. This difference is expected, as it accounts for the inherent variability in single filament that is a known behavior of materials composed of multiple filament

These results are important as they provide further understanding of the characteristics of the composite material behavior investigated in the next section. 3.: FRCM Constitutive Law 3.

4 strands. The single yarn average modulus is 240 GPa, while the multiple yarn modulus is 210 GPa Compared with the maximum 270 GPa. The ultimate stresses were calculated as the average of the ultimate stresses reached for every single test. The single yarn average strength is 3.7 GPa, while the multiple yarn strength is 3.2 GPa. These results are important as they provide further understanding of the characteristics of the composite material behavior investigated in the next section. 3.: FRCM Constitutive Law 3.1: Two Approaches In order to apply the tensile load on the rectangular specimens, two gripping mechanism approaches were used in order to study their influence in the measured response (figure 3.1). One approach was to use a wedge type grip where lateral pressure is applied at the specimen ends. With this approach the specimen is fully constrained providing a fixed end support. A second approach was to bond metal tabs at the specimen ends and use a clevis type grip [8]. In this case the load is transferred only through the matrix to the fabric. The specimen has multiple degrees of freedom providing a pinned end support. Figure 3.1: Wedge type grip (left) and clevis type grip (right). Researchers from Politecnico di Milano (PoliMi) used the wedge type grip anchor, while University of Miami (UM) researchers used the clevis type anchor to perform the tensile tests. The specimens were cut from larger panels made in a flat mold and cured for 28 days. Individual coupons were cut with a wet saw in a fixture to ensure consistent width dimension. Each specimen had an average thickness of 10 mm and was 400 mm long [9]. 3.2: Experimental Campaign Analysis Different specimen widths were tested to ensure full coverage of the specimen ends by the grips. Table 3.1 summarizes the tests performed. Table 3.1: Tensile tests on FRCM-PBO specimens. GRIP METHOD SPECIMEN WIDTH REPETITIONS TEST SET SPEED STRAIN RECORDING METHOD Wedge 35 mm 3 A 40 mm 4 B 0.5 mm/min 100 mm gage length Extensometer

5 Clevis 50 mm 5 C 0.25 mm/min 100 mm gage length Extensometer Figure 3.2 shows the results for the three sets of tests (A, B and C) with representative curves. The dashed line represents the single fiber theoretical behavior. Figure 3.2: Representative results of tensile tests on FRCM-PBO specimens. Strains were recorded as indicated in table 3.1 while stresses were calculated dividing the load by the fabric area. The fabric area was calculated using the equivalent thickness given by the manufacturer [5]. Ideally, two or three linear segments can be identified from each curve. The initial linear segment corresponds to the uncracked linear elastic behavior of the material and it is characterized by the tensile modulus of elasticity, E1 of the matrix. A second linear segment corresponds to the behavior when the mortar is cracking. This phase is characterized by the "in cracking" tensile modulus of elasticity, E2. The last phase corresponds to the behavior when there is no more cracking of the mortar and the fabric is the only material engaged, E3 [9]. The tri-linear curve results only when the wedge gripping method is used, while for the clevis gripping method a bi-linear curve is obtained consistently, indicating the significant influence of the gripping approach. Since the fabric and the matrix do not have a perfect bond because not all the fibers in the yarn can be evenly wetted, slippage of the fibers can occur. With the wedge grip the fibers are constrained from slipping and the behavior of the dry fabric tests can be closely approximated, whereas with the clevis grip the main failure mode is by slippage of the fabric. From each test, the following values were recorded: E1, un-cracked modulus of elasticity; E2, in cracking modulus of elasticity; E3, completely cracked modulus of elasticity; ffu, ultimate stress; εfu, ultimate strain; fft, stress at the transition points; εft strain at the transition points. Table 3.2 shows the average results.

6 Table 3.2: Results of tensile tests performed on FRCM-PBO specimens following the two grip approaches. E1 [GPa] E2 [GPa] E3 [GPa] fft1 [GPa] εft1 [%] fft2 [GPa] εft2 [%] ffu [GPa] εfu [%] A Av B C 7.03% 39.11% 6.68% 16.37% 19.92% 33.93% 19.30% 8.77% 2.09% A CoV 19.97% 14.09% 1.30% 6.73% 20.70% 14.65% 12.76% 4.21% 3.03% B 26.5% 22.44% % 28.77% % 17.58% C For sets A and B, E3 and the ultimate strength reached are the same as with the dry fiber yarn tests in the previous section. Set C shows the behavior of the material when the fibers are allowed to slip and is more indicative of behavior in field application where the fibers cannot be constrained. These values represent the mechanical properties of the FRCM-PBO composite system and can be used for development of design parameters. 4.: Conclusions For the mortar, the dry and fresh unit weights were 2.84 g/cm 3 and 1.90 g/cm 3 respectively with a void content of 8.37%. The tensile tests on a single yarn of PBO and on a fabric made up by five single yarns confirm the value of its modulus of elasticity. The ultimate stress reached during the tests was lower than the theoretical value (5.8 GPa). The single yarn has shown a greater strength than the fabric. These results are expected since a filament is always stronger than yarn. In addition, the width of the fabric mesh does not allow uniform loading of the five yarns so that the failure occurs when the most loaded yarn fails. The gripping system plays an important role in the tensile characterization as the mode of failure is either slippage (clevis type grip) or fabric failure (constrained wedge type grip). Field application and expected system boundary conditions should determine the methodology. For FRCM application fibers are not constrained and clevis type grip tensile testing can be used for characterization. Wedge grip tensile testing can be used for theoretical analysis. Further studies on FRCM materials are on going with regards to the compression and tensile characterization of the mortar, adhesion strength to the substrate, inter-laminar shear, and durability characteristics. 5.: Acknowledgments

7 The authors gratefully acknowledge NSF for the support provided to the Industry/University Center for Integration of Composites into Infrastructure (CICI) under grant IIP and its industrial member Ruredil S.p.A., San Donato Milanese, Italy. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors also thank Politecnico di Milano for the precious support provided. List of References: [1] Nanni, A. FRCM strengthening a new tool in the concrete and masonry repair toolbox, Concrete International, Design and Construction, 34(4), (2012) pp [2] Triantafillou, T.C.; Papanicolaou, C.G. Shear strengthening of reinforced concrete members with textile reinforced mortar (TRM), Materials and Structures, 39(1), (2006) pp [3] Weiland, S.; Ortlepp, R.; Brückner, A.; Curbach, M., Strengthening of RC Structures with Textile Reinforced Concrete, ACI SP (2007), pp [4] AC434: 2013 ICC-Evaluation Service, Whittier, CA Acceptance Criteria for Masonry and Concrete strengthening using Fabric-Reinforced-Cementitious-Matrix (FRCM) Composite Systems. [5] Mantegazza, G.; Valentino, S. Di.Te.R, Instructions for the Planning of Static Consolidation Interventions through the use of Fibre Reinforced Cementitious Matrix FRCM, San Donato Milanese, [6] ASTM C Standard Test Method for Density of Hydraulic Cement. [7] ASTM C138/C138M 10b Standard Test Method for Density (Unit Weight), Yield and Air Content (Gravimetric) of Concrete. [8] Arboleda, D.; Loreto, G.; De Luca, A.; and Nanni, A. Material characterization of fiber reinforced cementitious matrix (FRCM) composite laminates, Proceedings for 10 th International Symposium on Ferrocement and Thin Reinforced Cement Composite, Havana, October [9] Ombres, L. Failure modes in reinforced concrete beams strengthened with PBO fiber reinforced cementitious mortars (FRCM), Proceedings, 9 th International Symposium on Fibre Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-9), Sydney, July 2009.

RESILIENT INFRASTRUCTURE June 1 4, 2016

RESILIENT INFRASTRUCTURE June 1 4, 2016 FLEXURAL STRENGTHENING OF RC BEAMS USING GLASS-FRCM Abdulla Jabr University of Windsor, Canada Amr El Ragaby University of Windsor, Canada Faouzi Ghrib University