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Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and theoretical study
Accepted Manuscript Review Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and Theoretical study Renato S. Olivito, Rosamaria Codispoti, Oscar A. Cevallos PII: DOI: Reference:
S0263-8223(16)30144-1 http://dx.doi.org/10.1016/j.compstruct.2016.03.004 COST 7301
To appear in:
Composite Structures
Please cite this article as: Olivito, R.S., Codispoti, R., Cevallos, O.A., Bond behavior of Flax-FRCM and PBOFRCM composites applied on clay bricks: Experimental and Theoretical study, Composite Structures (2016), doi: http://dx.doi.org/10.1016/j.compstruct.2016.03.004
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Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and Theoretical study
First Authors Renato S. Olivito, University of Calabria, Department of Civil Engineering, Rende (CS), Italy, E-Mail address: [email protected]
Authors Rosamaria Codispoti, University of Calabria, Department of Civil Engineering, Rende (CS), Italy, E-Mail address: [email protected]
Oscar A. Cevallos Facultad de Ingeniería Civil, Universidad Nacional de Chimborazo, Riobamba, Ecuador; E-Mails: [email protected]
Corresponding author Rosamaria Codispoti, PhD, Address: Dept. of Civil Engineering, 87036 Rende (CS), Italy. /Tel.: +39 0984 496947; fax: +39 0984 496924 / E-mail address: [email protected]
Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and Theoretical study
ABSTRACT In this paper an experimental investigation on bond adhesion between sustainable composite material and masonry support was carried out in the Civil Engineering Laboratory of the University of Calabria. This topic was addressed performing experimental tests and considering a theoretical approach. For this purpose, double-lap shear bond tests were carried out on typical masonry clay bricks, externally strengthened with composite materials (NFRCM - Natural Fibers Reinforced Cementitious Matrix). In particular, two different composite materials were studied: flax fabric-reinforced cementitious matrix (flax-FRCM) and PBO fabric-reinforced cementitious matrix (PBOFRCM) composites. In the end, a comparison between the experimental results achieved and those proposed by the Italian design code CNR 200 R1/2012 was performed, picking out some relevant aspects of the design procedures. 1. INTRODUCTION The application of composite materials to structural strengthening requires the use of special adhesives. The choice of the adhesive depends on several factors; one in particular is related to the characteristics of the support (concrete, masonry or wood) [1] [2] [3] [4] [5] [6] [7] [8]. Usually, the technical data sheet for the FRP-based structural strengthening gives specific instructions about the type of adhesive that has to be used depending on the type of work that has to be performed. There are many types of adhesives; the most popular adhesives for composite materials are based on epoxy resins. However, the current trend is directed towards the study and development of new adhesives that provide sustainability in the Industry and the use of renewable technologies in the field of civil engineering. Nowadays, minimizing energy demand, the use of renewable energy, the use of products and/or services with less environmental impact, the assessment of environmental impacts of the activities performed and the compensation of the CO2 emissions generated, represent the main topics for any application field. Consequently, researchers have turned their attention towards the development of materials obtained from renewable sources, easily recoverable or biodegradable at the end of use. In particular, in the field of civil structures, a promising area of research is the use of sustainable composite materials for strengthening or the repair of old masonry
structures that exhibit structural problems mainly due to poor tensile strength of the mortar/brick joints. The sustainable composites produced and investigated, for some years now, as a replacement of composite materials based on polyester resin (FRP) are the so called FRCM (Fibers Reinforced Cementitous Matrix), materials less harmful to the environment [9][10][11][12]. For the objective of this article the authors concentrated their study on (Natural Fibers Reinforced Cementitous Matrix). The well-known problems of debonding, especially in the structural applications, depend precisely on the relationship between the adhesive and the mechanical properties of the substrate. The bond failure of polymer matrix-based composites usually occurs with the detachment of part of the surface, so called "peeling". As a consequence of the foregoing, the mechanical properties of the support play an important role, as said: the maximum adhesion stress is, with good approximation, linearly proportional to the tensile strength of the support, and the fracture energy is, linearly proportional to the square root of the tensile strength of the same support. In this paper an experimental investigation dealing with the use of natural fibers applied to masonry substrate, was carried out in the Civil Engineering Laboratory of the University of Calabria. The experimental plan was organized in two parts: experimental tests and a theoretical approach. For this purpose, double-lap shear bond tests were carried out on typical masonry clay brick, externally strengthened with composite materials using special steel formworks. In particular two different strengthening systems were manufactured: flax fabric-reinforced cementitious matrix (flax-FRCM) and PBO fabric-reinforced cementitious matrix (PBO-FRCM) composites. The choice of using flax fibers for the production of sustainable composites is a consequence of previous studies and research carried out by the authors [13][14][15]; in fact flax fibers have shown greater compatibility with the masonry support and in particular higher mechanical properties when compared to other natural fibers such as sisal, jute or hemp. On the other hand, the choice of PBO fibers was related to the necessity to compare the mechanical performance and the bond adhesion between natural and synthetic fibers when applied to masonry substrate, highlighting the better compatibility. Considering sustainable composites materials (natural in the reinforced fibers and in the matrix), a specific standard that gives recommendation about the composition of natural fibers-based composites, indicating geometric properties or special configurations of the specimens or testing procedures still does not exist. As a consequence of this, the typical configuration of tests used for masonry substrate reinforced with GFRP or CFRP was referenced. The results obtained were compared
considering both the material used (natural and synthetic) in terms of failure modes and adhesion strength, and the mechanical properties, in terms of the maximum load capacity, fracture energy and bond stress. The interfacial bond stress–slip laws have been plotted comparing the different specimens built and the composite materials used. In the end, a comparison between the experimental results achieved and those proposed by the Italian design code CNR 200 R1/2012 was performed. It currently represents the Italian normative reference on design and construction of externally bonded FRP systems for strengthening existing structures, picking out some relevant aspects of the design procedures. More specifically, the results of the analysis allow: − the calculation of the optimal bond length of the masonry strengthened with flaxFRCM system; − the definition of the bond behavior between the composites based on natural fibers and cementitious matrix and the support masonry. − the analysis of the different failure modes; − the comparison of the performance between sustainable composite materials (flaxFRCM) and organic composite materials (PBO-FRCM); to compare the theoretical formulation and the experimental results. 2. EXPERIMENTAL INVESTIGATION The objective of the experimental program is to evaluate the validity of the theoretical approach suggested by the Italian technical standards. Therefore to analyze the behavior of the adhesion between composite materials and masonry support. In addition to this, the experimental tests are needed to underline the advantages that a sustainable reinforcing system guarantees in comparison to reinforcement based on organic material. The design of the set-up test for the NFRCM system applied on masonry support is still under experimental study, especially regarding sustainable composite materials; for this reason there is no unique set-up on the shear tests, but various procedures and equipment are used by different researchers and laboratories to investigate the delamination phenomena. Both the CNR DT200 R1 / 2013 [16] or the RILEM technical committee [17][20] propose various set-up tests, the most used are the following: − Single/Double-Lap Shear Test (Chajes et al. 1996; Taljsten 1997) [19][21]; − Double-Lap pull-pull Shear Test (Lee et al. 1999; Nakaba et al. 2001) [22][23]; − Beam-type test (De Lorenzis et al. 2001) [24];
− Double-lap push-pull Shear Test (Camli et al. 2007) [25]; The use of the different test setups, depends on several factors, related to the equipment present in laboratories but also by economic reasons and practical considerations of the laboratory, as well as the aim of the different research conducted. In this section, the results of a wide experimental program are presented. In the first part, the mechanical characterization of the constituent materials is shown: flax and PBO fiber fabrics, lime base mortars and the bricks; in the following section the preparation and the set-up procedure of the experimental tests conducted on reinforced single bricks with different bond lengths are reported. Finally, the behavior of the interface between flax-FRCM, PBOFRCM and bricks is experimentally determined carrying out double-lap shear bond tests, in order to evaluate the maximum force. 2.1. Brick properties Typical Italian clay bricks were used as a support of the experimental tests, large 200x12x50 mm3, externally strengthened using special steel formworks. The two surfaces of the bricks were previously cleaned and smoothed to eliminate all the imperfections. In addition, before the application of composite materials, each brick was soaked in water for 24 hours, in order to improve the adhesion with the mortar. Compressive and bending strength, as well as elastic modulus were characterized carrying out compressive tests according to BS EN 772-1 [26] and three point bending tests according to UNI 8942 - Part 3 [27]. The mechanical properties of the brick used, for the aim of the present paper, present a mean compressive strength of 41,77 MPa (CoV = 4%), a flexural strength of 3,16 MPa (CoV = 14%), and modulus of elasticity of 2778,46 MPa (CoV = 11,8%); all results are reported in detail in [28].
2.2. Mortar properties A natural hydraulic lime (NHL) mortar mix was used to produce fiber reinforced cementitious matrix composite materials. The mortar used contains carbonate filler and pure natural pozzolans with high reactive silica (fly-ash) content. A broad analysis of the physical and mechanical characteristics of the NLG mortar, including durability tests of natural fibers aged in this matrix have been presented by the authors [29]. This matrix is a CE-marked material and complies with the European standard BS EN 459 [30]. According to BS EN 196-1 [31], mortar NHL was mixed with 25% of water, but was not modified by the addition of any other components. The mechanical characterization of this mortar
included compression and three point bending tests performed in accordance with BS EN 1015-11 [32].
2.3. Flax fabrics and PBO fabrics properties As previously discussed, two different fabrics were used to manufacture composite materials (Figure 1): flax fabrics (natural fibers) and PBO fabrics (synthetic fibers). Both the flax and PBO fabrics were produced in Italy. Regarding the flax fibers, bidirectional balanced fabric was used, in particular 2D plain woven, in which the yarns are arranged in the two directions, warp and weft, with a ratio between the yarns equal to 2:2; on the other hand, in case of PBO fibers, unbalanced fabric was used. The mechanical properties, in terms of tensile strength, young modulus and strain to failure have been previously studied by the authors [28] and summarized in Table 1. Regarding the physical properties (Table 1), in this study the density and the mass per unit area of the PBO fabrics were evaluated in accordance to ISO 1889 [33] and ISO 3374 [34].
2.4. Flax-FRCM and PBO-FRCM strengthening system The flax and PBO fabrics here studied were used by the authors to manufacture FRCM composites, and the results of this mechanical characterization are reported in previous studies [13] [28]. All fabric strips were cut to the desired dimensions (300 mm x 55 mm), making them easy to arrange inside the molds used to manufacture the composite specimens. All the composite samples were prepared using wooden molds. The composite specimens were manufactured by the hand lay-up molding technique using untreated fabrics and the cementitious matrix. The cementitious matrix was mixed with 30% water as recommended by the supplier and according to BS EN 196-1. In order to study the mechanical behavior of the FRCM composites, only one strip of fabric was used as reinforcement of the flax- and PBO-FRCM composites. The composites 28 days old were subjected to tensile loads. The results demonstrate that an elastic response was maintained until the formation of the first matrix crack, and then a multiple cracking nature characterized the behavior of these composite materials [13][15][35][36] . 2.5. Preparation of the strengthened brick samples A total of twenty specimens were tested under double face shear tests. Specifically, two typologies of specimens were built in the laboratory, both based on the bond length and denoted as S50 and S100, using flax and PBO fabrics. To prepare the specimens, each
brick was cut in two parts and was strengthened on both sides using a single layer of flax and PBO fabric that were applied to the bricks with the manual lay-up technique; consequently, the specimens were called S50-FLAX, S50-PBO, S100-FLAX and S100PBO. The details of the geometrical size were reported in Figure 2. In addition, in order to measure the strain in the composite, one strain gauge was applied on the two opposite sides of the specimens S50 (Figure 2.a), while in the case of specimens S100 two strain gauges for each side were applied (Figure 2.b). 2.6. Test set-up All the tests were conducted using a load frame (Figure 3.a) created especially to perform tests on macro-elements. To measure the displacements, two LVDT's were applied to the opposite sides of the specimens. Moreover, the support used is a symmetric double fork (Figure 3.b), in which the base is fixed by bolting it to the machine, while the upper end of the fork is connected to the crossbar using a hinge connection (Figure 3.c), guaranteeing a uniform distribution of the load in the specimens (Figure 3.d). The tests were performed with a constant speed of the crossbar equal to 1 mm/min for the S50-PBO and S100-PBO specimens, while a constant speed equal to 0,5 mm/min was considered in the case of the S50-Flax and S100-Flax specimens, in order to avoid the sudden breakage of the single yarns that make up the fabric in the un-bonded area of the specimen. 3. EXPERIMENTAL RESULTS The results obtained, highlight a clear difference between the reinforcing system based on natural fibers (Flax-FRCM) and those based on synthetic fibers (PBO-FRCM). Various failure behaviors were observed during the tests: in the case of the first reinforcement system, using Flax-FRCM, the failure occurs slowly marked by the break in the matrix (cementitious mortar), at the beginning of the test, followed by the break/stretching of single yarns that make up the fabric (Figure 4.a). In the second reinforcement system, using PBO-FRCM, the failure is a cohesive type and takes place suddenly for most of the specimens and in some cases, uniformly (Figure 4.b), characterized by a brittle behavior, typical of FRP composites materials. According to the current standards the failure modes of the composite system are classified in four typologies: failure in the composite, cohesive failure, in the support and in the matrix, and tensile failure in the unbonded area (Figure 5). The failure mode observed, using Flax-FRCM composite materials, was different respect to the above mentioned; it is
a break that can be described as "progressive failure" (Figure 5.e), both in S50 specimens and S100 specimens. Analyzing the shear stress-displacement diagram of the first reinforcement system, plotted after the experimental tests, it is characterized by a softening final part (Figure 6.a,Figure 6.b), on the contrary for the second reinforcement system (Figure 6.c,Figure 6.d), the curve is characterized by a stiffer initial part, due to the high initial properties of the fibers. In addition, despite the high mechanical properties of PBO fibers when applied to concrete substrates, in this case the values of the maximum force achieved, but also the bond stress values, are almost similar to the values obtained using the flax fibers (Table 2). Moreover, for both of the specimens S50-Flax and S100-Flax, and for the specimens S50PBO and S100-PBO the same behavior was obtained. Some differences, although minimal, in the case of the S100-PBO specimens were observed: the value of the maximum force reached, and then the values of the bond stress, are slightly higher compared to specimens built with a bond length equal to 50mm (Table 2). This is a consequence of the bond length, due to the size equal to 100mm, of the specimens strengthened with PBO fibers, being almost the optimum bond length required to transfer the applied load to the support. In fact, in Figure 7, the break occurred for delamination. In addition, it is important to underline that the experimental tests were carried out following the standard indications on FRP composite materials. This is the reason for the results obtained in this present paper, lacking, even today, an official technical document on composite materials based on natural fibers. Another important aspect analyzed was the use of the reinforced fibers, observed during the experimental tests: flax fibers in the composite make full use of their mechanical properties compared to the PBO fibers (between 20%-30% of use), with a percentage of about 60% over the maximum load applied (Figure 8). This points out the enhanced compatibility of flax fibers with the cement matrix, unlike the PBO fibers.
3.1. Fiber strains The profiles of fiber strains were recorded and plotted for load levels between 20% and 100% of maximum force, obtained during the tests in case of only configuration (II) due to the presence of two strain-gauges, specimens S100- Flax and S100-PBO, achieving exponential curves (Figure 9). This is very similar to the typical strain distribution diagrams obtained in the case of FRP strengthening concrete/masonry elements [37] [38] [39]. As evidenced in the Figure 9.a, in case of S100-Flax specimens, the strain values measured
by strain gauges are insignificant at the ends of the bond length (x=33mm), while they reach the maximum values at the loaded ends of the specimens (x=66mm). The fiber strains became significant only for high values of the applied load; they are in the range between 0,01105-0,1197 mm/mm3. Otherwise, Figure 9.b shows the distribution of PBO strains along the bond length of the specimens. In this case the fiber strain distribution presents two different situations: in some cases, negligible values at the free end for each load level and maximum values at the loaded end were obtained as schematized in Figure 9.b1; in other cases the fiber strain values, are relevant in each point of the bonded area and for each level of load (Figure 9.b2), the average values are between 0,0126-0,3565 mm/mm3. 4. THEORETICAL APROACH 4.1. Formulation for the optimal design bond length Considering the bond between FRCM composites and masonry support, currently, a specific bond adhesion law suggested by the standards does not exist. For this reason, for the aim of this work, the typical theory of the FRP (Fiber Reinforced Polymer) composites was considered. The bond adhesion between FRP and masonry substrate is usually expressed with a relationship that takes into consideration the interfacial shear stress and the corresponding slip. The relationship typically used for the design purpose may be treated as a bilinear bond-slip law (Figure 10). In order to obtain this type of law, a typical adhesion test (Figure 11) consists in applying a barycentric parallel load to the composites; the ultimate maximum force obtained during the test, represents the maximum value that can be supported by a strengthened system before the delamination or detachment occurs. Consequently of this, the bond length, la, plays a key role. Its dimension must be such that the applied load can be transferred to the resistant material, as without that, the adhesion shear stresses to exceed the limit value. This value increases until it reaches a maximum value, above which force increments no longer occur. According to the technical Italian standard, DT200 R1 / 2013 [16] it is theoretically possible to calculate the optimal design length, led, both for flax-FRCM and PBO-FRCM composites using the following relationship: = =
∗ ∗ ∗ 1 ∗ ; 150 2 ϒ ∗
2 ∗ ,
(1)
(2)
=
∗ ∗ " # ∗ $# !
(3)
3 − ( /( % = 1 + ( /(
(4)
where Ef e tf are respectively the modulus of elasticity in the direction of force and the thickness of the FRCM; Γfd is the design value of the specific fracture energy; γRd is a partial factor for resistance models equal to 1,5 in case of brick masonry; fbd is the design bond strength between FRCM and masonry (see eq.2); su is the ultimate value of the slip between FRCM and masonry support measured during the tests by the transducers. The fracture energy was calculated following the equation 3, written as a function of the mean values of compressive, fbm, and tensile, fbtm, strength of the masonry support, k g is a corrective factor expressed in function of the typology of support, in this specific case it is equal to 0,093 mm; while kb is the geometrical corrective factor calculated in the equation 4 recently adopted in the Italian standards [16] which depends on the ratio between the width of the support, b, and the FRCM reinforced, bf; FC is a confidence factor. 4.2. Assessment of bond-slip relation The transfer of stresses between the FRCM composites and masonry support occurs in the interface of the two materials. It is necessary to define a model that is able to describe this behavior. An approach to the problem is to analyze the relationship between the shear stress at the interface (bond stress) and the relative slip of the strengthening system, making some assumptions [16]: the applied load was equally divided between the two opposite sides of the brick, the slipping occurs at the end of bonded fabric; the bond between FRCM composites and masonry support at the end of the bonded area is assumed as a perfect adhesion; the deformations of the support are neglected. Therefore, to plot the adhesion shear stress-slip law, the parameters were estimated considering the average between two subsequent strain gauges applied to the specimens during the experimental program, using the following equations [16][18][19]: +() =
E/ ∗ A/ ∗ (Ԑ234 − Ԑ2 ) ; b/ ∗ (X 234 − X 2 ) :
() = 7(Ԑ894 + Ԑ8 ) ; ;<4
(5)
(6)
where Ar and br are respectively the transversal section and the width of the reinforcement; Er is the average of the Young's modulus; ei and xi are the strain and abscissa of the i-th gauge, respectively. 5. Theoretical/experimental comparisons According to the formulation suggested by the Italian standard [16], it was possible to compare the experimental and theoretical ultimate value of the force transferred from FRCM reinforcement to the support. The equation 7 was calculated by the following assumptions: linear behavior of the adhesive; infinite length of the substrate and of the reinforcement; stiffness of the composite at its highest compared to the support. #=>
=
( ∗ ?2 ∗ ∗ ∗ !
(7)
The parameters considered are: Ef is the Young's modulus of elasticity of the FRCM reinforcement, tf and bf are the thickness and the width of the fibers (equal to 0,13 mm in case of flax fibers and 0,0455 in case of PBO fibers, while bf is equal to 50 mm for both), Γfd is the design value of the specific fracture energy (Eq. 3, equal to 1,63 N/mm) and FC is the confidence factor. The theoretical maximum force, in case of specimens reinforced with PBO fibers, is overestimated compared to the values obtained experimentally, as it is possible to note in Figure 12. This is due to the assumptions on which the equation (7) is based, which only takes account of the mechanical properties of the fibers and not the matrix or the support, and it is also a formulation studied for composites based on polymer matrix (FRP). On the other hand, in case of specimens reinforced with the flax fibers, the theoretical maximum force is comparable with the value obtained experimentally (Figure 13); obviously this is due to the higher compatibility between natural fibers and cementitious matrix, rather than synthetic fibers, because of their same order of mechanical properties. In addition, it was possible to compare the theoretical bilinear bond, as suggested by the Italian standard, with the values obtained experimentally (Figure 14); only in the case of the specimens reinforced with flax fibers and for configuration (II). Even if the theoretical curve seems to fit inside the envelope curves, bond strength - slip, obtained from maximum and minimum experimental data, further investigations are needed to take in consideration the mechanical properties of the natural fibers and the law more suitable to these materials. CONCLUSION
This work involved both the experimental and theoretical analysis of the bond adhesion between composite materials based on natural fibers and cementitious matrix (FRCM) and masonry support. The fundamental aim was to highlight the necessity to have specific standards that are concentrated on the FRCM composite materials based on natural fibers, in order to analyze the performance of the reinforcement system applied to existing masonry buildings, according to detailed requirements. From the experimental results obtained and the comparison made with the theoretical data achieved according to the Italian standards the following concluding remarks can be drawn: •
natural fabrics, especially flax fibers, have shown high mechanical properties applied to the masonry support, consequently, they appear as reinforcement fibers that are most suitable to the production of FRCM;
•
despite the high mechanical properties usually guaranteed when applied on concrete substrates, standing to the tests carried out in this case, PBO fibers are inadvisable on the masonry support in coping with its poor tensile strength, producing a visible delamination phenomena; furthermore from previous research carried out by the same authors, also the flexural performance of masonry pillars and walls, reinforced with FRCM based PBO fibers have not shown satisfactory results [28]
•
flax-FRCM composites have shown, during the experimental tests, a better bond behavior with the masonry support, than PBO-FRCM composites; furthermore, flax fibers make full use of their mechanical properties in the composite during the tests, as compared to the PBO fibers;
•
the current Italian standard needs to be revised in terms of calculation of the bond stress and the optimal bond length: the data, experimentally obtained, has revealed an overestimation of the maximum force required so that the applied load can be transferred to the substrate; moreover the design of the optimal bond length does not take into consideration the mechanical properties of the matrix but only the fibers, leading to minimum design values of the bond length;
•
finally, regarding the failure modes, the NFRCM composite materials are characterized, while increasing of the load, by a so-called "progressive” break that it is not considered in the current classification proposed by the standard; specific standards should be necessary also for this reason in order to better understand the mechanical behavior of the sustainable composites, such as Flax-FRCM composite.
According to these considerations, further researches needed on shear bond tests, especially regarding the effective geometric properties that are necessary to build the specimens, in particular the combined use of composites based on cement matrix and natural fibers (FRCM) to strengthen masonry elements.
Acknowledgements This work was supported by the DPC-ReLUIS 2014-2016 project under funding of the Italian Civil Protection - Innovative materials sector: WP2- cement-based composites, ‘‘Natural fiber reinforced cementitious matrix (NFRCM) composites applied to masonry elements’’. References [1] Bahman Ghiassi, Jose Xavier, Daniel V. Oliveira, Arkadiusz Kwiecien, Paulo B. Lourenço, Boguslaw Zajac, Evaluation of the bond performance in FRP–brick components re-bonded after initial delamination, Composite Structures, Volume 123, May 2015, Pages 271-281, ISSN 0263-8223. [2] Haixia Zhang, Luyuan He, Guochang Li, Bond failure performances between nearsurface mounted FRP bars and concrete for flexural strengthening concrete structures, Engineering Failure Analysis, Volume 56, October 2015, Pages 39-50, ISSN 13506307. [3] Mário R.F. Coelho, José M. Sena-Cruz, Luís A.C. Neves, A review on the bond behavior of FRP NSM systems in concrete, Construction and Building Materials, Volume 93, 15 September 2015, Pages 1157-1169, ISSN 0950-0618. [4] Kay-Uwe Schober, Annette M. Harte, Robert Kliger, Robert Jockwer, Qingfeng Xu, Jian-Fei Chen, FRP reinforcement of timber structures, Construction and Building Materials, Volume 97, 30 October 2015, Pages 106-118, ISSN 0950-0618. [5] Fazhou Wang, Ming Li, Shuguang Hu, Bond behavior of roughing FRP sheet bonded to concrete substrate, Construction and Building Materials, Volume 73, 30 December 2014, Pages 145-152, ISSN 0950-0618. [6] Claudio Mazzotti, Barbara Ferracuti, Alessandro Bellini, Experimental bond tests on masonry panels strengthened by FRP, Composites Part B: Engineering, Volume 80, October 2015, Pages 223-237, ISSN 1359-8368.
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[16] CNR, 2013. CNR-DT 200 R1/2013, Guide for the Design and Construction of ExternallyBonded FRP Systems for Strengthening Existing. Materials, RC and PC structures,masonry structures, CNR – Advisory Committee on Technical Recommendations for Construction. [17] Valluzzi MR, Oliveira DV, Caratelli A, Castori G, Corradi M, de Felice G, Garbin E, Garcia D, Garmendia L, Grande E, Ianniruberto U, Kwiecien´ A, Leone M, Lignola GP, Lourenço PB, Malena M, Micelli F, Panizza M, Papanicolaou CG, Prota A, Sacco E, Triantafillou TC, Viskovic A, Zaja˛c B, Zuccarino G (2012) Round Robin test for composite- to-brick shear bond characterization. RILEM Mater Struct 45(12):1761–17 [18] B. Ferracuti, M. Savoia, C. Mazzotti, Interface law for FRP–concrete delamination, Composite Structures, Volume 80, Issue 4, October 2007, Pages 523-531, ISSN 0263-8223. [19] Björn Täljsten, Defining anchor lengths of steel and CFRP plates bonded to concrete, International Journal of Adhesion and Adhesives, Volume 17, Issue 4, November 1997, Pages 319-327, ISSN 0143-7496, [20] De Felice Gianmarco, De Santis Stefano, Garmendia Leire, Ghiassi Bahman, Larrinaga Pello, Lourenço Paulo B., Oliveira Daniel V., Paolacci Fabrizio, Papanicolaou CatherineG., Mortar-based systems for externally bonded strengthening of masonry, Materials and Structures (2014) 47:2021-2037. [21] Chajes M.J., Finch W.W. jr, Januska T.F. & Thomson T.A. jr. Bond and force transfer of composite material plates bonded to concrete. ACI Structural J.(1996) Vol. 93: 208217. [22] Lee YJ, Boothby TE, Bakis CE, Nanni A (1999) Slip modulus of FRP sheets bonded to concrete. ASCE J. of Composites for Construction, 3(4): 161-167. [23] Nakaba K, Kanakubo T, Furuta T, Yoshizawa H (2001) Bond behavior between FiberReinforced Polymer laminates and concrete. ACI Structural J., 98 (3): 359-367. [24] De Lorenzis, L., B. Miller, And A. Nanni, "Bond of FRP Laminates to Concrete", ACI Materials Journal, Vol. 98, No. 3, May -June 2001, pp. 256-264. [25] Camli US, Binici B (2007) Strength of carbon fiber reinforced polymers bonded to concrete and masonry. Construction and Building Materials, 21: 1431-1446. [26] BS EN 772-1. Methods of test for masonry units. Determination of compressive strength. BSI 2011. [27] UNI 8942-3 (1986) Prodotti di laterizio per murature. Metodi di prova (in Italian).
[28] O.A. Cevallos, R.S. Olivito, R. Codispoti, L. Ombres, Flax and polyparaphenylene benzobisoxazole cementitious composites for the strengthening of masonry elements subjected to eccentric loading, Composites Part B: Engineering, Volume 71, 15 March 2015, Pages 82-95, ISSN 1359-8368. [29] Olivito R, Cevallos O, Carrozzini A. Development of durable cementitious composites using sisal and flax fabrics for reinforcement of masonry structures. Mater Des 2014;57:258–68. [30] BS EN 459. Building lime. Definitions, specifications and conformity criteria. BSI 2010. [31] BS EN 196-1. Methods of testing cement: determination of strength. BSI 2005. [32] BS EN 1015-11. Methods of test for mortar for masonry: determination of flexural and compressive strength of hardened mortar. BSI 1999. [33] ISO 1889:2009-Reinforcement yarns - determination of linear density. [34] ISO 3374-2000, Reinforcement products d mats and fabrics d determination of mass per unit area. [35] Francesca Giulia Carozzi, Carlo Poggi, Mechanical properties and debonding strength of Fabric Reinforced Cementitious Matrix (FRCM) systems for masonry strengthening, Composites Part B: Engineering, Volume 70, 1 March 2015, Pages 215-230, ISSN 1359-8368. [36] Stefano De Santis, Gianmarco de Felice, Tensile behaviour of mortar-based composites for externally bonded reinforcement systems, Composites Part B: Engineering, Volume 68, January 2015, Pages 401-413, ISSN 1359-8368 [37] Luciano Ombres, Analysis of the bond between Fabric Reinforced Cementitious Mortar (FRCM) strengthening systems and concrete, Composites Part B: Engineering, Volume 69, February 2015, Pages 418-426, ISSN 1359-8368. [38] Ernesto Grande, Maura Imbimbo, Elio Sacco, Bond behaviour of CFRP laminates glued on clay bricks: Experimental and numerical study, Composites Part B: Engineering, Volume 42, Issue 2, March 2011, Pages 330-340, ISSN 1359-8368. [39] Francesca Ceroni, Barbara Ferracuti, Marisa Pecce, Marco Savoia, Corrigendum to “Assessment of a bond strength model for FRP reinforcement externally bonded over masonry blocks” [Compos. Part B: Eng. 61 (2014) 147–161], Composites Part B: Engineering, Volume 69, February 2015, Page 612, ISSN 1359-8368. [40] Angelo D’Ambrisi, Luciano Feo, Francesco Focacci, Experimental analysis on bond between PBO-FRCM strengthening materials and concrete, Composites Part B: Engineering, Volume 44, Issue 1, January 2013, Pages 524-532, ISSN 1359-8368.
[41] Angelo D’Ambrisi, Luciano Feo, Francesco Focacci, Bond-slip relations for PBOFRCM materials externally bonded to concrete, Composites Part B: Engineering, Volume 43, Issue 8, December 2012, Pages 2938-2949, ISSN 1359-8368.
Figure captions and Figures Figure 1: PBO and Flax balance fabrics Figure 2: Specimens size of bond area (plain view): a) Configuration (I) for specimens S50; b) Configuration (II) for specimens S100. Figure 3: Set-up tests for double-lap shear bond tests: a) Set-up machine; b) Double clamps on the top and on the bottom of machine; c) Fixed plate on the crossbar; d) Schematic distribution of the load applied. Figure 4: Failure modes: Reinforcing system based on natural fibers: a) S50_ Flax-FRCM; b) S50_PBO-FRCM; reinforcing system based on synthetic fibers: a) S100_ Flax-FRCM; b) S100_PBO-FRCM. Figure 5: Failure modes according to the Italian Standards: a) Failure in the composite; b) Cohesive failure in the support; c) Cohesive failure in the matrix; d) Tensile failure nonimpregnated fabric; e) Progressive breaking typical of FRCM. Figure 6: Shear Stress - displacement diagrams obtained using configuration (I) and (II): a) Specimens S50_Flax-FRCM; b) Specimens S100_Flax-FRCM; c) Specimens S50_PBO-FRCM; d) Specimens S100_PBO-FRCM. Figure 7: Failure mode configuration (II) using PBO-FRCM reinforcing system. Figure 8: Use of fibers during the experimental tests in configurations (I) and (II) using PBO-FRCM and Flax-FRCM Figure 9: Strain distribution of some representative specimens: a) Configuration (II) S100-Flax; b) Configuration (II) S100-PBO. Figure 10: Bilinear bond-slip law Figure 11: Typical schematic adhesion tests Figure 12: Comparison between theoretical and experimental force in case of configuration (I) S100_PBO-FRCM. Figure 13: Comparison between theoretical and experimental force in case of configuration (I) S100_Flax-FRCM. Figure 14: Bilinear bond strength-slip law: Configuration (II) using Flax-FRCM.
(a) (b) Figure 1: PBO and Flax balance fabrics
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(b)
Figure 2: Specimens size of bond area (plain view): a) Configuration (I) for specimens S50; b) Configuration (II) for specimens S100.
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(b)
(c) (d) Figure 3: Set-up tests for double-lap shear bond tests: a) Set-up machine; b) Double clamps on the top and on the bottom of machine; c) Fixed plate on the crossbar; d) Schematic distribution of the load applied.
(a)
(b)
(c) (d) Figure 4: Failure modes: Reinforcing system based on natural fibers: a) S50_ Flax-FRCM; b) S50_PBO-FRCM; reinforcing system based on synthetic fibers: a) S100_ Flax-FRCM; b) S100_PBO-FRCM.
(a)
(b) (e) (c)
(d) Figure 5: Failure modes according to the Italian Standards: a) Failure in the composite; b) Cohesive failure in the support; c) Cohesive failure in the matrix; d) Tensile failure nonimpregnated fabric; e) Progressive breaking typical of FRCM.
0,75 S1 S2 S3
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Figure 6: Shear stress - displacement diagrams obtained using configuration (I) and (II): a) Specimens S50_Flax-FRCM; b) Specimens S100_Flax-FRCM; c) Specimens S50_PBOFRCM; d) Specimens S100_PBO-FRCM.
Figure 7: Failure mode configuration (II) using PBO-FRCM reinforcing system.
7000 59% 56%
Average Applied Load
6000 5000 4000 31%
Composite Load Average Fiber Load Average % Fiber
3000 21%
2000 1000 0 50_PBO
100_PBO
50_FLAX
100_FLAX
Figure 8: Use of fibers during the experimental tests in configurations (I) and (II) using PBO-FRCM and Flax-FRCM
45
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(b1) (b2) Figure 9: Strain distribution of some representative specimens: a) Configuration (II) S100Flax; b) Configuration (II) S100-PBO.
Figure 10: Bilinear bond-slip law
Figure 11: Typical schematic adhesion tests
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Figure 12: Comparison between theoretical and experimental force in case of configuration (I) S100_PBO-FRCM.
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Figure 13: Comparison between theoretical and experimental force in case of configuration (I) S100_Flax-FRCM.
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Figure 14: Bilinear bond strength-slip law: Configuration (II) using Flax-FRCM.
Table captions and Tables Table 1: Mechanical and physical properties of non-impregnated fabrics. Table 2: Average values of the experimental results of double-lap shear bond stress.
Table 1: Mechanical and physical properties of non-impregnated fabrics. Non-impregnated Fabric Properties Design thickness [mm] Design area [mm2] Fibers density [g/cm3] Mass per unit area [g/m2] Tensile strength [MPa] Young's Modulus [MPa] Strain to failure (%)
Flax 0,108 5,46 1,78 (3%) 388,4 (1%) 292,23 (3,2%) 3800 (3%) 11 (2,4%)
PBO 0,0455 1,53 1,44 (5%) 88* 3730 (11%) 155000 (15%) 2 (12%)
Table 2: Average values of the experimental results of double-lap shear bond stress. Configuration (I) FMAX [N] tMAX [N/mm2]
Configuration (II)
S50_Flax-FRCM S50_PBO-FRCM S100_Flax-FRCM S100_PBO-FRCM 1091,65 1139,74 1201,52 2290,424 32%
43%
34%
40%
0,437
0,456
0,240
0,458
32% 43% 34% Note: The coefficients of variation (CoV%) are reported in brackets.
40%
S0263-8223(16)30144-1 http://dx.doi.org/10.1016/j.compstruct.2016.03.004 COST 7301
To appear in:
Composite Structures
Please cite this article as: Olivito, R.S., Codispoti, R., Cevallos, O.A., Bond behavior of Flax-FRCM and PBOFRCM composites applied on clay bricks: Experimental and Theoretical study, Composite Structures (2016), doi: http://dx.doi.org/10.1016/j.compstruct.2016.03.004
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Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and Theoretical study
First Authors Renato S. Olivito, University of Calabria, Department of Civil Engineering, Rende (CS), Italy, E-Mail address: [email protected]
Authors Rosamaria Codispoti, University of Calabria, Department of Civil Engineering, Rende (CS), Italy, E-Mail address: [email protected]
Oscar A. Cevallos Facultad de Ingeniería Civil, Universidad Nacional de Chimborazo, Riobamba, Ecuador; E-Mails: [email protected]
Corresponding author Rosamaria Codispoti, PhD, Address: Dept. of Civil Engineering, 87036 Rende (CS), Italy. /Tel.: +39 0984 496947; fax: +39 0984 496924 / E-mail address: [email protected]
Bond behavior of Flax-FRCM and PBO-FRCM composites applied on clay bricks: Experimental and Theoretical study
ABSTRACT In this paper an experimental investigation on bond adhesion between sustainable composite material and masonry support was carried out in the Civil Engineering Laboratory of the University of Calabria. This topic was addressed performing experimental tests and considering a theoretical approach. For this purpose, double-lap shear bond tests were carried out on typical masonry clay bricks, externally strengthened with composite materials (NFRCM - Natural Fibers Reinforced Cementitious Matrix). In particular, two different composite materials were studied: flax fabric-reinforced cementitious matrix (flax-FRCM) and PBO fabric-reinforced cementitious matrix (PBOFRCM) composites. In the end, a comparison between the experimental results achieved and those proposed by the Italian design code CNR 200 R1/2012 was performed, picking out some relevant aspects of the design procedures. 1. INTRODUCTION The application of composite materials to structural strengthening requires the use of special adhesives. The choice of the adhesive depends on several factors; one in particular is related to the characteristics of the support (concrete, masonry or wood) [1] [2] [3] [4] [5] [6] [7] [8]. Usually, the technical data sheet for the FRP-based structural strengthening gives specific instructions about the type of adhesive that has to be used depending on the type of work that has to be performed. There are many types of adhesives; the most popular adhesives for composite materials are based on epoxy resins. However, the current trend is directed towards the study and development of new adhesives that provide sustainability in the Industry and the use of renewable technologies in the field of civil engineering. Nowadays, minimizing energy demand, the use of renewable energy, the use of products and/or services with less environmental impact, the assessment of environmental impacts of the activities performed and the compensation of the CO2 emissions generated, represent the main topics for any application field. Consequently, researchers have turned their attention towards the development of materials obtained from renewable sources, easily recoverable or biodegradable at the end of use. In particular, in the field of civil structures, a promising area of research is the use of sustainable composite materials for strengthening or the repair of old masonry
structures that exhibit structural problems mainly due to poor tensile strength of the mortar/brick joints. The sustainable composites produced and investigated, for some years now, as a replacement of composite materials based on polyester resin (FRP) are the so called FRCM (Fibers Reinforced Cementitous Matrix), materials less harmful to the environment [9][10][11][12]. For the objective of this article the authors concentrated their study on (Natural Fibers Reinforced Cementitous Matrix). The well-known problems of debonding, especially in the structural applications, depend precisely on the relationship between the adhesive and the mechanical properties of the substrate. The bond failure of polymer matrix-based composites usually occurs with the detachment of part of the surface, so called "peeling". As a consequence of the foregoing, the mechanical properties of the support play an important role, as said: the maximum adhesion stress is, with good approximation, linearly proportional to the tensile strength of the support, and the fracture energy is, linearly proportional to the square root of the tensile strength of the same support. In this paper an experimental investigation dealing with the use of natural fibers applied to masonry substrate, was carried out in the Civil Engineering Laboratory of the University of Calabria. The experimental plan was organized in two parts: experimental tests and a theoretical approach. For this purpose, double-lap shear bond tests were carried out on typical masonry clay brick, externally strengthened with composite materials using special steel formworks. In particular two different strengthening systems were manufactured: flax fabric-reinforced cementitious matrix (flax-FRCM) and PBO fabric-reinforced cementitious matrix (PBO-FRCM) composites. The choice of using flax fibers for the production of sustainable composites is a consequence of previous studies and research carried out by the authors [13][14][15]; in fact flax fibers have shown greater compatibility with the masonry support and in particular higher mechanical properties when compared to other natural fibers such as sisal, jute or hemp. On the other hand, the choice of PBO fibers was related to the necessity to compare the mechanical performance and the bond adhesion between natural and synthetic fibers when applied to masonry substrate, highlighting the better compatibility. Considering sustainable composites materials (natural in the reinforced fibers and in the matrix), a specific standard that gives recommendation about the composition of natural fibers-based composites, indicating geometric properties or special configurations of the specimens or testing procedures still does not exist. As a consequence of this, the typical configuration of tests used for masonry substrate reinforced with GFRP or CFRP was referenced. The results obtained were compared
considering both the material used (natural and synthetic) in terms of failure modes and adhesion strength, and the mechanical properties, in terms of the maximum load capacity, fracture energy and bond stress. The interfacial bond stress–slip laws have been plotted comparing the different specimens built and the composite materials used. In the end, a comparison between the experimental results achieved and those proposed by the Italian design code CNR 200 R1/2012 was performed. It currently represents the Italian normative reference on design and construction of externally bonded FRP systems for strengthening existing structures, picking out some relevant aspects of the design procedures. More specifically, the results of the analysis allow: − the calculation of the optimal bond length of the masonry strengthened with flaxFRCM system; − the definition of the bond behavior between the composites based on natural fibers and cementitious matrix and the support masonry. − the analysis of the different failure modes; − the comparison of the performance between sustainable composite materials (flaxFRCM) and organic composite materials (PBO-FRCM); to compare the theoretical formulation and the experimental results. 2. EXPERIMENTAL INVESTIGATION The objective of the experimental program is to evaluate the validity of the theoretical approach suggested by the Italian technical standards. Therefore to analyze the behavior of the adhesion between composite materials and masonry support. In addition to this, the experimental tests are needed to underline the advantages that a sustainable reinforcing system guarantees in comparison to reinforcement based on organic material. The design of the set-up test for the NFRCM system applied on masonry support is still under experimental study, especially regarding sustainable composite materials; for this reason there is no unique set-up on the shear tests, but various procedures and equipment are used by different researchers and laboratories to investigate the delamination phenomena. Both the CNR DT200 R1 / 2013 [16] or the RILEM technical committee [17][20] propose various set-up tests, the most used are the following: − Single/Double-Lap Shear Test (Chajes et al. 1996; Taljsten 1997) [19][21]; − Double-Lap pull-pull Shear Test (Lee et al. 1999; Nakaba et al. 2001) [22][23]; − Beam-type test (De Lorenzis et al. 2001) [24];
− Double-lap push-pull Shear Test (Camli et al. 2007) [25]; The use of the different test setups, depends on several factors, related to the equipment present in laboratories but also by economic reasons and practical considerations of the laboratory, as well as the aim of the different research conducted. In this section, the results of a wide experimental program are presented. In the first part, the mechanical characterization of the constituent materials is shown: flax and PBO fiber fabrics, lime base mortars and the bricks; in the following section the preparation and the set-up procedure of the experimental tests conducted on reinforced single bricks with different bond lengths are reported. Finally, the behavior of the interface between flax-FRCM, PBOFRCM and bricks is experimentally determined carrying out double-lap shear bond tests, in order to evaluate the maximum force. 2.1. Brick properties Typical Italian clay bricks were used as a support of the experimental tests, large 200x12x50 mm3, externally strengthened using special steel formworks. The two surfaces of the bricks were previously cleaned and smoothed to eliminate all the imperfections. In addition, before the application of composite materials, each brick was soaked in water for 24 hours, in order to improve the adhesion with the mortar. Compressive and bending strength, as well as elastic modulus were characterized carrying out compressive tests according to BS EN 772-1 [26] and three point bending tests according to UNI 8942 - Part 3 [27]. The mechanical properties of the brick used, for the aim of the present paper, present a mean compressive strength of 41,77 MPa (CoV = 4%), a flexural strength of 3,16 MPa (CoV = 14%), and modulus of elasticity of 2778,46 MPa (CoV = 11,8%); all results are reported in detail in [28].
2.2. Mortar properties A natural hydraulic lime (NHL) mortar mix was used to produce fiber reinforced cementitious matrix composite materials. The mortar used contains carbonate filler and pure natural pozzolans with high reactive silica (fly-ash) content. A broad analysis of the physical and mechanical characteristics of the NLG mortar, including durability tests of natural fibers aged in this matrix have been presented by the authors [29]. This matrix is a CE-marked material and complies with the European standard BS EN 459 [30]. According to BS EN 196-1 [31], mortar NHL was mixed with 25% of water, but was not modified by the addition of any other components. The mechanical characterization of this mortar
included compression and three point bending tests performed in accordance with BS EN 1015-11 [32].
2.3. Flax fabrics and PBO fabrics properties As previously discussed, two different fabrics were used to manufacture composite materials (Figure 1): flax fabrics (natural fibers) and PBO fabrics (synthetic fibers). Both the flax and PBO fabrics were produced in Italy. Regarding the flax fibers, bidirectional balanced fabric was used, in particular 2D plain woven, in which the yarns are arranged in the two directions, warp and weft, with a ratio between the yarns equal to 2:2; on the other hand, in case of PBO fibers, unbalanced fabric was used. The mechanical properties, in terms of tensile strength, young modulus and strain to failure have been previously studied by the authors [28] and summarized in Table 1. Regarding the physical properties (Table 1), in this study the density and the mass per unit area of the PBO fabrics were evaluated in accordance to ISO 1889 [33] and ISO 3374 [34].
2.4. Flax-FRCM and PBO-FRCM strengthening system The flax and PBO fabrics here studied were used by the authors to manufacture FRCM composites, and the results of this mechanical characterization are reported in previous studies [13] [28]. All fabric strips were cut to the desired dimensions (300 mm x 55 mm), making them easy to arrange inside the molds used to manufacture the composite specimens. All the composite samples were prepared using wooden molds. The composite specimens were manufactured by the hand lay-up molding technique using untreated fabrics and the cementitious matrix. The cementitious matrix was mixed with 30% water as recommended by the supplier and according to BS EN 196-1. In order to study the mechanical behavior of the FRCM composites, only one strip of fabric was used as reinforcement of the flax- and PBO-FRCM composites. The composites 28 days old were subjected to tensile loads. The results demonstrate that an elastic response was maintained until the formation of the first matrix crack, and then a multiple cracking nature characterized the behavior of these composite materials [13][15][35][36] . 2.5. Preparation of the strengthened brick samples A total of twenty specimens were tested under double face shear tests. Specifically, two typologies of specimens were built in the laboratory, both based on the bond length and denoted as S50 and S100, using flax and PBO fabrics. To prepare the specimens, each
brick was cut in two parts and was strengthened on both sides using a single layer of flax and PBO fabric that were applied to the bricks with the manual lay-up technique; consequently, the specimens were called S50-FLAX, S50-PBO, S100-FLAX and S100PBO. The details of the geometrical size were reported in Figure 2. In addition, in order to measure the strain in the composite, one strain gauge was applied on the two opposite sides of the specimens S50 (Figure 2.a), while in the case of specimens S100 two strain gauges for each side were applied (Figure 2.b). 2.6. Test set-up All the tests were conducted using a load frame (Figure 3.a) created especially to perform tests on macro-elements. To measure the displacements, two LVDT's were applied to the opposite sides of the specimens. Moreover, the support used is a symmetric double fork (Figure 3.b), in which the base is fixed by bolting it to the machine, while the upper end of the fork is connected to the crossbar using a hinge connection (Figure 3.c), guaranteeing a uniform distribution of the load in the specimens (Figure 3.d). The tests were performed with a constant speed of the crossbar equal to 1 mm/min for the S50-PBO and S100-PBO specimens, while a constant speed equal to 0,5 mm/min was considered in the case of the S50-Flax and S100-Flax specimens, in order to avoid the sudden breakage of the single yarns that make up the fabric in the un-bonded area of the specimen. 3. EXPERIMENTAL RESULTS The results obtained, highlight a clear difference between the reinforcing system based on natural fibers (Flax-FRCM) and those based on synthetic fibers (PBO-FRCM). Various failure behaviors were observed during the tests: in the case of the first reinforcement system, using Flax-FRCM, the failure occurs slowly marked by the break in the matrix (cementitious mortar), at the beginning of the test, followed by the break/stretching of single yarns that make up the fabric (Figure 4.a). In the second reinforcement system, using PBO-FRCM, the failure is a cohesive type and takes place suddenly for most of the specimens and in some cases, uniformly (Figure 4.b), characterized by a brittle behavior, typical of FRP composites materials. According to the current standards the failure modes of the composite system are classified in four typologies: failure in the composite, cohesive failure, in the support and in the matrix, and tensile failure in the unbonded area (Figure 5). The failure mode observed, using Flax-FRCM composite materials, was different respect to the above mentioned; it is
a break that can be described as "progressive failure" (Figure 5.e), both in S50 specimens and S100 specimens. Analyzing the shear stress-displacement diagram of the first reinforcement system, plotted after the experimental tests, it is characterized by a softening final part (Figure 6.a,Figure 6.b), on the contrary for the second reinforcement system (Figure 6.c,Figure 6.d), the curve is characterized by a stiffer initial part, due to the high initial properties of the fibers. In addition, despite the high mechanical properties of PBO fibers when applied to concrete substrates, in this case the values of the maximum force achieved, but also the bond stress values, are almost similar to the values obtained using the flax fibers (Table 2). Moreover, for both of the specimens S50-Flax and S100-Flax, and for the specimens S50PBO and S100-PBO the same behavior was obtained. Some differences, although minimal, in the case of the S100-PBO specimens were observed: the value of the maximum force reached, and then the values of the bond stress, are slightly higher compared to specimens built with a bond length equal to 50mm (Table 2). This is a consequence of the bond length, due to the size equal to 100mm, of the specimens strengthened with PBO fibers, being almost the optimum bond length required to transfer the applied load to the support. In fact, in Figure 7, the break occurred for delamination. In addition, it is important to underline that the experimental tests were carried out following the standard indications on FRP composite materials. This is the reason for the results obtained in this present paper, lacking, even today, an official technical document on composite materials based on natural fibers. Another important aspect analyzed was the use of the reinforced fibers, observed during the experimental tests: flax fibers in the composite make full use of their mechanical properties compared to the PBO fibers (between 20%-30% of use), with a percentage of about 60% over the maximum load applied (Figure 8). This points out the enhanced compatibility of flax fibers with the cement matrix, unlike the PBO fibers.
3.1. Fiber strains The profiles of fiber strains were recorded and plotted for load levels between 20% and 100% of maximum force, obtained during the tests in case of only configuration (II) due to the presence of two strain-gauges, specimens S100- Flax and S100-PBO, achieving exponential curves (Figure 9). This is very similar to the typical strain distribution diagrams obtained in the case of FRP strengthening concrete/masonry elements [37] [38] [39]. As evidenced in the Figure 9.a, in case of S100-Flax specimens, the strain values measured
by strain gauges are insignificant at the ends of the bond length (x=33mm), while they reach the maximum values at the loaded ends of the specimens (x=66mm). The fiber strains became significant only for high values of the applied load; they are in the range between 0,01105-0,1197 mm/mm3. Otherwise, Figure 9.b shows the distribution of PBO strains along the bond length of the specimens. In this case the fiber strain distribution presents two different situations: in some cases, negligible values at the free end for each load level and maximum values at the loaded end were obtained as schematized in Figure 9.b1; in other cases the fiber strain values, are relevant in each point of the bonded area and for each level of load (Figure 9.b2), the average values are between 0,0126-0,3565 mm/mm3. 4. THEORETICAL APROACH 4.1. Formulation for the optimal design bond length Considering the bond between FRCM composites and masonry support, currently, a specific bond adhesion law suggested by the standards does not exist. For this reason, for the aim of this work, the typical theory of the FRP (Fiber Reinforced Polymer) composites was considered. The bond adhesion between FRP and masonry substrate is usually expressed with a relationship that takes into consideration the interfacial shear stress and the corresponding slip. The relationship typically used for the design purpose may be treated as a bilinear bond-slip law (Figure 10). In order to obtain this type of law, a typical adhesion test (Figure 11) consists in applying a barycentric parallel load to the composites; the ultimate maximum force obtained during the test, represents the maximum value that can be supported by a strengthened system before the delamination or detachment occurs. Consequently of this, the bond length, la, plays a key role. Its dimension must be such that the applied load can be transferred to the resistant material, as without that, the adhesion shear stresses to exceed the limit value. This value increases until it reaches a maximum value, above which force increments no longer occur. According to the technical Italian standard, DT200 R1 / 2013 [16] it is theoretically possible to calculate the optimal design length, led, both for flax-FRCM and PBO-FRCM composites using the following relationship: = =
∗ ∗ ∗ 1 ∗ ; 150 2 ϒ ∗
2 ∗ ,
(1)
(2)
=
∗ ∗ " # ∗ $# !
(3)
3 − ( /( % = 1 + ( /(
(4)
where Ef e tf are respectively the modulus of elasticity in the direction of force and the thickness of the FRCM; Γfd is the design value of the specific fracture energy; γRd is a partial factor for resistance models equal to 1,5 in case of brick masonry; fbd is the design bond strength between FRCM and masonry (see eq.2); su is the ultimate value of the slip between FRCM and masonry support measured during the tests by the transducers. The fracture energy was calculated following the equation 3, written as a function of the mean values of compressive, fbm, and tensile, fbtm, strength of the masonry support, k g is a corrective factor expressed in function of the typology of support, in this specific case it is equal to 0,093 mm; while kb is the geometrical corrective factor calculated in the equation 4 recently adopted in the Italian standards [16] which depends on the ratio between the width of the support, b, and the FRCM reinforced, bf; FC is a confidence factor. 4.2. Assessment of bond-slip relation The transfer of stresses between the FRCM composites and masonry support occurs in the interface of the two materials. It is necessary to define a model that is able to describe this behavior. An approach to the problem is to analyze the relationship between the shear stress at the interface (bond stress) and the relative slip of the strengthening system, making some assumptions [16]: the applied load was equally divided between the two opposite sides of the brick, the slipping occurs at the end of bonded fabric; the bond between FRCM composites and masonry support at the end of the bonded area is assumed as a perfect adhesion; the deformations of the support are neglected. Therefore, to plot the adhesion shear stress-slip law, the parameters were estimated considering the average between two subsequent strain gauges applied to the specimens during the experimental program, using the following equations [16][18][19]: +() =
E/ ∗ A/ ∗ (Ԑ234 − Ԑ2 ) ; b/ ∗ (X 234 − X 2 ) :
() = 7(Ԑ894 + Ԑ8 ) ; ;<4
(5)
(6)
where Ar and br are respectively the transversal section and the width of the reinforcement; Er is the average of the Young's modulus; ei and xi are the strain and abscissa of the i-th gauge, respectively. 5. Theoretical/experimental comparisons According to the formulation suggested by the Italian standard [16], it was possible to compare the experimental and theoretical ultimate value of the force transferred from FRCM reinforcement to the support. The equation 7 was calculated by the following assumptions: linear behavior of the adhesive; infinite length of the substrate and of the reinforcement; stiffness of the composite at its highest compared to the support. #=>
=
( ∗ ?2 ∗ ∗ ∗ !
(7)
The parameters considered are: Ef is the Young's modulus of elasticity of the FRCM reinforcement, tf and bf are the thickness and the width of the fibers (equal to 0,13 mm in case of flax fibers and 0,0455 in case of PBO fibers, while bf is equal to 50 mm for both), Γfd is the design value of the specific fracture energy (Eq. 3, equal to 1,63 N/mm) and FC is the confidence factor. The theoretical maximum force, in case of specimens reinforced with PBO fibers, is overestimated compared to the values obtained experimentally, as it is possible to note in Figure 12. This is due to the assumptions on which the equation (7) is based, which only takes account of the mechanical properties of the fibers and not the matrix or the support, and it is also a formulation studied for composites based on polymer matrix (FRP). On the other hand, in case of specimens reinforced with the flax fibers, the theoretical maximum force is comparable with the value obtained experimentally (Figure 13); obviously this is due to the higher compatibility between natural fibers and cementitious matrix, rather than synthetic fibers, because of their same order of mechanical properties. In addition, it was possible to compare the theoretical bilinear bond, as suggested by the Italian standard, with the values obtained experimentally (Figure 14); only in the case of the specimens reinforced with flax fibers and for configuration (II). Even if the theoretical curve seems to fit inside the envelope curves, bond strength - slip, obtained from maximum and minimum experimental data, further investigations are needed to take in consideration the mechanical properties of the natural fibers and the law more suitable to these materials. CONCLUSION
This work involved both the experimental and theoretical analysis of the bond adhesion between composite materials based on natural fibers and cementitious matrix (FRCM) and masonry support. The fundamental aim was to highlight the necessity to have specific standards that are concentrated on the FRCM composite materials based on natural fibers, in order to analyze the performance of the reinforcement system applied to existing masonry buildings, according to detailed requirements. From the experimental results obtained and the comparison made with the theoretical data achieved according to the Italian standards the following concluding remarks can be drawn: •
natural fabrics, especially flax fibers, have shown high mechanical properties applied to the masonry support, consequently, they appear as reinforcement fibers that are most suitable to the production of FRCM;
•
despite the high mechanical properties usually guaranteed when applied on concrete substrates, standing to the tests carried out in this case, PBO fibers are inadvisable on the masonry support in coping with its poor tensile strength, producing a visible delamination phenomena; furthermore from previous research carried out by the same authors, also the flexural performance of masonry pillars and walls, reinforced with FRCM based PBO fibers have not shown satisfactory results [28]
•
flax-FRCM composites have shown, during the experimental tests, a better bond behavior with the masonry support, than PBO-FRCM composites; furthermore, flax fibers make full use of their mechanical properties in the composite during the tests, as compared to the PBO fibers;
•
the current Italian standard needs to be revised in terms of calculation of the bond stress and the optimal bond length: the data, experimentally obtained, has revealed an overestimation of the maximum force required so that the applied load can be transferred to the substrate; moreover the design of the optimal bond length does not take into consideration the mechanical properties of the matrix but only the fibers, leading to minimum design values of the bond length;
•
finally, regarding the failure modes, the NFRCM composite materials are characterized, while increasing of the load, by a so-called "progressive” break that it is not considered in the current classification proposed by the standard; specific standards should be necessary also for this reason in order to better understand the mechanical behavior of the sustainable composites, such as Flax-FRCM composite.
According to these considerations, further researches needed on shear bond tests, especially regarding the effective geometric properties that are necessary to build the specimens, in particular the combined use of composites based on cement matrix and natural fibers (FRCM) to strengthen masonry elements.
Acknowledgements This work was supported by the DPC-ReLUIS 2014-2016 project under funding of the Italian Civil Protection - Innovative materials sector: WP2- cement-based composites, ‘‘Natural fiber reinforced cementitious matrix (NFRCM) composites applied to masonry elements’’. References [1] Bahman Ghiassi, Jose Xavier, Daniel V. Oliveira, Arkadiusz Kwiecien, Paulo B. Lourenço, Boguslaw Zajac, Evaluation of the bond performance in FRP–brick components re-bonded after initial delamination, Composite Structures, Volume 123, May 2015, Pages 271-281, ISSN 0263-8223. [2] Haixia Zhang, Luyuan He, Guochang Li, Bond failure performances between nearsurface mounted FRP bars and concrete for flexural strengthening concrete structures, Engineering Failure Analysis, Volume 56, October 2015, Pages 39-50, ISSN 13506307. [3] Mário R.F. Coelho, José M. Sena-Cruz, Luís A.C. Neves, A review on the bond behavior of FRP NSM systems in concrete, Construction and Building Materials, Volume 93, 15 September 2015, Pages 1157-1169, ISSN 0950-0618. [4] Kay-Uwe Schober, Annette M. Harte, Robert Kliger, Robert Jockwer, Qingfeng Xu, Jian-Fei Chen, FRP reinforcement of timber structures, Construction and Building Materials, Volume 97, 30 October 2015, Pages 106-118, ISSN 0950-0618. [5] Fazhou Wang, Ming Li, Shuguang Hu, Bond behavior of roughing FRP sheet bonded to concrete substrate, Construction and Building Materials, Volume 73, 30 December 2014, Pages 145-152, ISSN 0950-0618. [6] Claudio Mazzotti, Barbara Ferracuti, Alessandro Bellini, Experimental bond tests on masonry panels strengthened by FRP, Composites Part B: Engineering, Volume 80, October 2015, Pages 223-237, ISSN 1359-8368.
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[16] CNR, 2013. CNR-DT 200 R1/2013, Guide for the Design and Construction of ExternallyBonded FRP Systems for Strengthening Existing. Materials, RC and PC structures,masonry structures, CNR – Advisory Committee on Technical Recommendations for Construction. [17] Valluzzi MR, Oliveira DV, Caratelli A, Castori G, Corradi M, de Felice G, Garbin E, Garcia D, Garmendia L, Grande E, Ianniruberto U, Kwiecien´ A, Leone M, Lignola GP, Lourenço PB, Malena M, Micelli F, Panizza M, Papanicolaou CG, Prota A, Sacco E, Triantafillou TC, Viskovic A, Zaja˛c B, Zuccarino G (2012) Round Robin test for composite- to-brick shear bond characterization. RILEM Mater Struct 45(12):1761–17 [18] B. Ferracuti, M. Savoia, C. Mazzotti, Interface law for FRP–concrete delamination, Composite Structures, Volume 80, Issue 4, October 2007, Pages 523-531, ISSN 0263-8223. [19] Björn Täljsten, Defining anchor lengths of steel and CFRP plates bonded to concrete, International Journal of Adhesion and Adhesives, Volume 17, Issue 4, November 1997, Pages 319-327, ISSN 0143-7496, [20] De Felice Gianmarco, De Santis Stefano, Garmendia Leire, Ghiassi Bahman, Larrinaga Pello, Lourenço Paulo B., Oliveira Daniel V., Paolacci Fabrizio, Papanicolaou CatherineG., Mortar-based systems for externally bonded strengthening of masonry, Materials and Structures (2014) 47:2021-2037. [21] Chajes M.J., Finch W.W. jr, Januska T.F. & Thomson T.A. jr. Bond and force transfer of composite material plates bonded to concrete. ACI Structural J.(1996) Vol. 93: 208217. [22] Lee YJ, Boothby TE, Bakis CE, Nanni A (1999) Slip modulus of FRP sheets bonded to concrete. ASCE J. of Composites for Construction, 3(4): 161-167. [23] Nakaba K, Kanakubo T, Furuta T, Yoshizawa H (2001) Bond behavior between FiberReinforced Polymer laminates and concrete. ACI Structural J., 98 (3): 359-367. [24] De Lorenzis, L., B. Miller, And A. Nanni, "Bond of FRP Laminates to Concrete", ACI Materials Journal, Vol. 98, No. 3, May -June 2001, pp. 256-264. [25] Camli US, Binici B (2007) Strength of carbon fiber reinforced polymers bonded to concrete and masonry. Construction and Building Materials, 21: 1431-1446. [26] BS EN 772-1. Methods of test for masonry units. Determination of compressive strength. BSI 2011. [27] UNI 8942-3 (1986) Prodotti di laterizio per murature. Metodi di prova (in Italian).
[28] O.A. Cevallos, R.S. Olivito, R. Codispoti, L. Ombres, Flax and polyparaphenylene benzobisoxazole cementitious composites for the strengthening of masonry elements subjected to eccentric loading, Composites Part B: Engineering, Volume 71, 15 March 2015, Pages 82-95, ISSN 1359-8368. [29] Olivito R, Cevallos O, Carrozzini A. Development of durable cementitious composites using sisal and flax fabrics for reinforcement of masonry structures. Mater Des 2014;57:258–68. [30] BS EN 459. Building lime. Definitions, specifications and conformity criteria. BSI 2010. [31] BS EN 196-1. Methods of testing cement: determination of strength. BSI 2005. [32] BS EN 1015-11. Methods of test for mortar for masonry: determination of flexural and compressive strength of hardened mortar. BSI 1999. [33] ISO 1889:2009-Reinforcement yarns - determination of linear density. [34] ISO 3374-2000, Reinforcement products d mats and fabrics d determination of mass per unit area. [35] Francesca Giulia Carozzi, Carlo Poggi, Mechanical properties and debonding strength of Fabric Reinforced Cementitious Matrix (FRCM) systems for masonry strengthening, Composites Part B: Engineering, Volume 70, 1 March 2015, Pages 215-230, ISSN 1359-8368. [36] Stefano De Santis, Gianmarco de Felice, Tensile behaviour of mortar-based composites for externally bonded reinforcement systems, Composites Part B: Engineering, Volume 68, January 2015, Pages 401-413, ISSN 1359-8368 [37] Luciano Ombres, Analysis of the bond between Fabric Reinforced Cementitious Mortar (FRCM) strengthening systems and concrete, Composites Part B: Engineering, Volume 69, February 2015, Pages 418-426, ISSN 1359-8368. [38] Ernesto Grande, Maura Imbimbo, Elio Sacco, Bond behaviour of CFRP laminates glued on clay bricks: Experimental and numerical study, Composites Part B: Engineering, Volume 42, Issue 2, March 2011, Pages 330-340, ISSN 1359-8368. [39] Francesca Ceroni, Barbara Ferracuti, Marisa Pecce, Marco Savoia, Corrigendum to “Assessment of a bond strength model for FRP reinforcement externally bonded over masonry blocks” [Compos. Part B: Eng. 61 (2014) 147–161], Composites Part B: Engineering, Volume 69, February 2015, Page 612, ISSN 1359-8368. [40] Angelo D’Ambrisi, Luciano Feo, Francesco Focacci, Experimental analysis on bond between PBO-FRCM strengthening materials and concrete, Composites Part B: Engineering, Volume 44, Issue 1, January 2013, Pages 524-532, ISSN 1359-8368.
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Figure captions and Figures Figure 1: PBO and Flax balance fabrics Figure 2: Specimens size of bond area (plain view): a) Configuration (I) for specimens S50; b) Configuration (II) for specimens S100. Figure 3: Set-up tests for double-lap shear bond tests: a) Set-up machine; b) Double clamps on the top and on the bottom of machine; c) Fixed plate on the crossbar; d) Schematic distribution of the load applied. Figure 4: Failure modes: Reinforcing system based on natural fibers: a) S50_ Flax-FRCM; b) S50_PBO-FRCM; reinforcing system based on synthetic fibers: a) S100_ Flax-FRCM; b) S100_PBO-FRCM. Figure 5: Failure modes according to the Italian Standards: a) Failure in the composite; b) Cohesive failure in the support; c) Cohesive failure in the matrix; d) Tensile failure nonimpregnated fabric; e) Progressive breaking typical of FRCM. Figure 6: Shear Stress - displacement diagrams obtained using configuration (I) and (II): a) Specimens S50_Flax-FRCM; b) Specimens S100_Flax-FRCM; c) Specimens S50_PBO-FRCM; d) Specimens S100_PBO-FRCM. Figure 7: Failure mode configuration (II) using PBO-FRCM reinforcing system. Figure 8: Use of fibers during the experimental tests in configurations (I) and (II) using PBO-FRCM and Flax-FRCM Figure 9: Strain distribution of some representative specimens: a) Configuration (II) S100-Flax; b) Configuration (II) S100-PBO. Figure 10: Bilinear bond-slip law Figure 11: Typical schematic adhesion tests Figure 12: Comparison between theoretical and experimental force in case of configuration (I) S100_PBO-FRCM. Figure 13: Comparison between theoretical and experimental force in case of configuration (I) S100_Flax-FRCM. Figure 14: Bilinear bond strength-slip law: Configuration (II) using Flax-FRCM.
(a) (b) Figure 1: PBO and Flax balance fabrics
(a)
(b)
Figure 2: Specimens size of bond area (plain view): a) Configuration (I) for specimens S50; b) Configuration (II) for specimens S100.
(a)
(b)
(c) (d) Figure 3: Set-up tests for double-lap shear bond tests: a) Set-up machine; b) Double clamps on the top and on the bottom of machine; c) Fixed plate on the crossbar; d) Schematic distribution of the load applied.
(a)
(b)
(c) (d) Figure 4: Failure modes: Reinforcing system based on natural fibers: a) S50_ Flax-FRCM; b) S50_PBO-FRCM; reinforcing system based on synthetic fibers: a) S100_ Flax-FRCM; b) S100_PBO-FRCM.
(a)
(b) (e) (c)
(d) Figure 5: Failure modes according to the Italian Standards: a) Failure in the composite; b) Cohesive failure in the support; c) Cohesive failure in the matrix; d) Tensile failure nonimpregnated fabric; e) Progressive breaking typical of FRCM.
0,75 S1 S2 S3
Shear strees [N/mm2]
0,60
S4
S5 0,45
(a)
0,30
0,15
0,00 0
2
4
6
8
10
12
14
16
Displacement [mm] 0,40 S1 S2
0,35
S3 S4 S5
Shear Stress [N/mm2]
0,30 0,25 0,20
(b) 0,15 0,10 0,05 0,00 0
2
4
6
Displacement [mm]
8
10
12
0,75 S1 S2 s3 S4
Shear stress [N/mm2]
0,60
0,45
(c)
0,30
0,15
0,00 0
0,5
1
1,5
2
Displacement [mm] 0,75 S1 S2 S3 S4
Shear Stress [N/mm2]
0,60
0,45
(d) 0,30
0,15
0,00 0
1
2
3
4
5
Displacement [mm]
6
7
8
Figure 6: Shear stress - displacement diagrams obtained using configuration (I) and (II): a) Specimens S50_Flax-FRCM; b) Specimens S100_Flax-FRCM; c) Specimens S50_PBOFRCM; d) Specimens S100_PBO-FRCM.
Figure 7: Failure mode configuration (II) using PBO-FRCM reinforcing system.
7000 59% 56%
Average Applied Load
6000 5000 4000 31%
Composite Load Average Fiber Load Average % Fiber
3000 21%
2000 1000 0 50_PBO
100_PBO
50_FLAX
100_FLAX
Figure 8: Use of fibers during the experimental tests in configurations (I) and (II) using PBO-FRCM and Flax-FRCM
45
30
20% Fu
20% Fu 25
40% Fu 60% Fu
40
80% Fu 100% Fu
35
60% Fu 80% Fu 100% Fu
20
30
e [%]
e [%]
40% Fu
15
10
25 20 15 10
5 5
0 0
33
0
66
0
Position [mm]
33
66
Position [mm]
(a) 180
25 20% Fu
160
20% Fu
40% Fu 60% Fu
140
40% Fu 60% Fu
20
80% Fu
80% Fu
100% Fu
100% Fu
120
e [%]
e [%]
15 100 80
10 60 40
5
20 0
0 0
33
Position [mm]
66
0
33
66
Position [mm]
(b1) (b2) Figure 9: Strain distribution of some representative specimens: a) Configuration (II) S100Flax; b) Configuration (II) S100-PBO.
Figure 10: Bilinear bond-slip law
Figure 11: Typical schematic adhesion tests
EXPERIMENTAL MAXIMUN FORCE [N]
6000 DT R1 200/2013 MIN VALUE MAX VALUE
5000
4000
3000
2000
1000
0 0
1000
2000
3000
4000
5000
THEORETICAL MAXIMUN FORCE [N]
Figure 12: Comparison between theoretical and experimental force in case of configuration (I) S100_PBO-FRCM.
2500
EXPERIMENTAL F Max/Min [N/mm2]
DT R1 200/2013 Experimental min value Experimental max value
2000
1500
1000
500
0 0
100
200
300
400
500
600
700
THEORETICAL MAXIMUN FORCE [N/mm2]
Figure 13: Comparison between theoretical and experimental force in case of configuration (I) S100_Flax-FRCM.
0,7 DT R1 200/2013 MAX EXPERIMENTAL VALUE
Shear strength [N/mm2]
0,6
MIN EXPERIMENTAL VALUE
0,5
0,4
0,3
0,2
0,1
0 0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Slip [mm]
Figure 14: Bilinear bond strength-slip law: Configuration (II) using Flax-FRCM.
Table captions and Tables Table 1: Mechanical and physical properties of non-impregnated fabrics. Table 2: Average values of the experimental results of double-lap shear bond stress.
Table 1: Mechanical and physical properties of non-impregnated fabrics. Non-impregnated Fabric Properties Design thickness [mm] Design area [mm2] Fibers density [g/cm3] Mass per unit area [g/m2] Tensile strength [MPa] Young's Modulus [MPa] Strain to failure (%)
Flax 0,108 5,46 1,78 (3%) 388,4 (1%) 292,23 (3,2%) 3800 (3%) 11 (2,4%)
PBO 0,0455 1,53 1,44 (5%) 88* 3730 (11%) 155000 (15%) 2 (12%)
Table 2: Average values of the experimental results of double-lap shear bond stress. Configuration (I) FMAX [N] tMAX [N/mm2]
Configuration (II)
S50_Flax-FRCM S50_PBO-FRCM S100_Flax-FRCM S100_PBO-FRCM 1091,65 1139,74 1201,52 2290,424 32%
43%
34%
40%
0,437
0,456
0,240
0,458
32% 43% 34% Note: The coefficients of variation (CoV%) are reported in brackets.
40%