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- Publisher
- Springer Journals
- Copyright
- Copyright © 2015 by Chagas and Moita; licensee Springer.
- Subject
- Material Science; Surfaces and Interfaces, Thin Films; Mechanical Engineering; Biomaterials
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- 2196-4351
- DOI
- 10.1186/s40563-015-0035-3
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- See Article on Publisher Site
Abstract
juniasnc@civil.cefetmg.br Centro Federal de Educação In the past decade, the interest in repair and retrofitting of existing structures and Tecnológica de Minas Gerais, Av. rehabilitation of the damaged structures has led to the development of more effective Amazonas 7675, Nova Gameleira, 30510-000 Belo Horizonte, MG, and low invasive architectural and engineering strategies. In this aspect, the application Brazil of fibre reinforced polymer (FRP) strengthening techniques has become reasonably widespread as suitable solutions in addition to the traditional ones. They are promising techniques because of their key characteristics such as: high specific strength, high stiffness, small thickness compared to conventional materials, low influence on the global mass, little durability concerns, ease of handling, flexibility and fast installation that improve on-site productivity, and have a low impact on building functions. In this context, the use of carbon fibre reinforced polymers (CFRP) and glass fibre reinforced polymers (GFRP) for the rehabilitation of damaged small masonry walls (here called wallettes) was investigated experimentally. This study sought to measure the maximum loading carrying capacity of the wallettes and to assess the possible structural rehabilitation in the damaged masonry structures after their reinforcement with the composite polymers. For the adhesion between the wallettes and the reinforcement fibres, primer, putty and a saturant glue epoxy resins were used. Debonding between the FRP composites and the substrate has been recognized as the primary failure mechanism of this reinforcement system and it occurs when the system shear capacity is reached and the FRP is detached from the element. This phenomenon is also addressed in this paper. In general, the experimental results showed the recovery of the original compressive loading bearing capacity of the structures, in spite of the debonding of the FRP composites. Moreover, it couldbeobservedanincreasingofup to39% andupto49% of thecompressive strength for the damaged masonry wallettes reinforced with CFRP and GFRP systems, respectively. The recover (or even rise) in the loading capacity of the reinforced structures due to the external fibres bonding is a good indication of their effectiveness in these situations. Keywords: Rehabilitation; CFRP; GFRP; Masonry; Damage Background The structural masonry is a well-established traditional technology for the construction of affordable buildings. It is widely used throughout the world. Nowadays, simplicity and rationalisation of the construction process, aesthetic correctness, durability, low costs, good thermal and acoustic performance and fire resistance, among others, are characteristics that turn the masonry structures construction system into one of the most economical technology readily available [1]. In Brazil, structural masonry has been extensively used in the construction of the inexpensive buildings since the early © 2015 Chagas and Moita; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 2 of 15 1960’s and, up to now, represents one of the promising solutions for the housing deficit in the country. Nonetheless, problems with structural pathologies, failures and collapses have been reported. They are the result of the lack of more rigorous quality control for the mate- rials and inadequate production processes. In some cases, these problems also occur due to the application of inaccurate empirical dimensioning methods, without the wide use of computational tools, which would yield a more accurate structural analysis re- sults. In addition to these factors, others contribute to aggravate these problems, such as: the application of unpredicted loads, due to different uses and architectural modifi- cations of the structure; foundation settlement; wrong structural conception; natural deterioration of the materials and components; and, impacts, collisions or explosions. In such situations, the reinforcement or rehabilitation of the damaged existing struc- tures have been, often, more attractive or desirable than replacing it with a new con- struction due to heritage, economic and environmental reasons [2]. The adoption of low invasive and high efficient strengthening techniques is one im- portant aspect for the success and viability of the rehabilitation interventions. With this in mind, the usage of fibre reinforced polymers (FRP) to enhance the structural per- formance of masonry structures is a promising technique because of its high specific strength, high stiffness and small thickness compared to the conventional materials [3]. In the literature, numerous studies on the strengthening of reinforced concrete struc- tures with externally bonded FRP sheets have been published for many years. However, only more recently, experimental and numerical researches have been conducted about the usage of the FRP for the structural rehabilitation and strengthening of masonry walls. Very good results have been reported, what contribute to the success on this ap- proach [4-7]. Nonetheless, only few contributions refer to aspects concerning to the bonding and debonding behaviour between the masonry elements and the strengthen- ing system. The effectiveness of the reinforcement and the failure behaviour of fibre reinforced masonry structures are strongly influenced by the properties of the substrate where the reinforcement is applied. Therefore, this factor requires to be further explored. In fact, the stress concentrations occurring at the FRP/substrate interface could lead to the de- tachment of the reinforcement from the support and to the premature failure of the structure due to debonding [8]. The bonding behaviour of the FRP reinforcements on masonry surface has been investigated and theoretical formulations have been sug- gested by a specific Italian guide document, which are derived from the approach for concrete structures [9]. More recently, specific experimental tests were developed to investigate the nature of the bonding between composite reinforcements and masonry substrates. Moreover, the mechanism of debonding has been studied considering the influence of various factors, such as, bond length, geometry of the specimen, tests set-up, and type of the fibre re- inforcing system. It also can be observed that the wide variety of the masonry sub- strates, formed by clay or concrete bricks (or blocks), affects the overall performance of the reinforcement system [10-14]. In this work, a set of small masonry walls was built using concrete blocks. Three specimens, considered as the reference ones, were subjected to axial compressive load- ing up to their collapse in order to induce damage to the wallettes. Seven other Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 3 of 15 specimens were submitted to axial compressive loading of 75% of the average collapse loading of the reference wallettes. As far as the mechanical behaviour is concerned, ma- sonry structures subjected to a loading of 75% of their failure threshold is considered to be completely (structurally) damaged, which can be characterised by the appearance of randomly distributed cracks or micro-cracks throughout the specimens. The damaged specimens were then prepared and strengthened by the application of carbon fibre reinforced polymers (CFRP) or glass fibre reinforced polymers (GFRP), completely covering both their two main surfaces, as shown in Figure 1. An adequate chemical and physical bonding between the polymeric fibre and the substrate of the masonry was utilized. After the application of the reinforcement system, the wallettes were once again subjected to a vertical compressive load up to their collapse. This study measured the maximum loading bearing capacity of the wallettes and assessed the possible structural rehabilitation in the damaged masonry structures after the reinforcement with the FRP. Methods Materials characterisation and preparation of the specimens The masonry wallettes used in this research were built using concrete blocks and 1:2:6 (cement: hydrated lime: sand) mortar and had the following dimensions: height = 100 cm; length = 80 cm; thickness = 14 cm, as shown in Figure 1. Two different block sizes were utilised to build of the wallettes: (a) single-hole blocks (dimensions: 14 cm x Figure 1 Geometric configuration of the wallettes, with the applied compressive loading. (a) frontal view and (b) top view, with indication of the FRP reinforcement. Dimensions in centimetres. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 4 of 15 19 cm x 19 cm), and (b) two-hole blocks (dimensions: 14 cm x 19 cm x 39 cm), depicted in Figure 2, in order to allow the desired geometric configuration of the panels. Their average compressive strengths were, respectively, 6.30 MPa and 5.64 MPa. The mean compressive strength for the mortar specimens was 6.49 MPa. The experiments for the characterisation of the mechanical properties of these mate- rials were conducted according to the Brazilian standards NBR 12118/2013 [15] and 13279/2005 [16], respectively. Three specimens of the walletes, namely RW1, RW2 and RW3, were built as sche- matic illustrated in Figure 1. Subsequently, they were subjected to axial compressive loading up to failure, which meant a mean load of 427 kN. The load was applied per- pendicularly to the bed joints, in increments of the 2 kN, in an universal testing ma- chine under vertical displacement control. During the loading, the strains along the loading axis were calculated using the average displacement measurements obtained from four dial gauges placed in the panels, two in each of the main sides. The test setup was established in accordance with the Brazilian standard NBR 15961-2/2011 [17]. These samples were considered the reference wallettes. In order to cause damage to the wallettes, the seven remaining specimens were sub- mitted to axial compressive loading of 75% of the average collapse loading of the refer- ence wallettes, which resulted in a load of 320 kN. The loading was applied in the same direction as above. The applied loading was big enough to damage the specimens, as desired. From the visual inspection, micro-cracks and cracks could be observed in the blocks and the mortar joints of the structure, i.e., the wallettes were in fact damaged. Characteristics and mechanical properties of the resins and fibre reinforcement polymers The reinforcement system was made of polymeric fibre (FRP) and resins. The main mechanical properties of the FRP used in this work, given by the producer [18], were: for the CFRP (one-directional fabric mesh), Young’s modulus E = 227 GPa and tensile strength f = 3800 MPa; and, for the GFRP (two-directional fabric mesh), E = 68.9 GPa, and f = 1517 MPa. Epoxy resins provided the bonding for the reinforcement system. The resins used were a primer, a saturant and a leveling compound called putty. They Figure 2 Concrete blocks used, with the axial compressive loading applied during testing (according to NBR 12118/2013). Dimensions in centimetres. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 5 of 15 are all two-component materials consisting of resin and hardener. Their main charac- teristics and mechanical properties are given in Tables 1,2,3. Preparation of the masonry substrate and application of the reinforcement Before the application of the fibre reinforcement, the wallettes were prepared using high pressure water blasting in order remove the powder and any other particles from the substrate. They were dried in room temperature for 7 days. Subsequently, the dam- aged specimens were strengthened by the application of one-directional fabric of CFRP. These wallettes were denominated CW1, CW2, and CW3. The specimens GW1, GW2, GW3 and GW4 received two-directional fabric of GFRP. The FRP layers covered both the two main surfaces of all damaged specimens, according to Figure 1. An adequate chemical and physical bonding between the FRP and the substrate of the masonry was established. Firstly, the substrate of the wallettes was prepared with the application one layer of the primer. This primer is a two-component solvent-less epoxy system which when mixed yields a penetrating medium viscosity compound. This primer is used to penetrate the pore structure of the cementitious substrates and to provide a high bonding base coating for the FRP system. The drying of the primer on the substrate took around 1 hour in room temperature. Figure 3 illustrates the pri- mer application. Since the damaged wallettes did not present crushed parts, only cracks or micro-cracks, there was no need to fill the collapsed regions with mortar. Within a 48-hour period after the drying of the primer, a second layer of the adhesion system was applied, with a thickness of around 2 mm. This epoxy resin is known as putty. It was useful for the regularisation of any small surface imperfections and to pro- vide a smooth surface to which the reinforcement system would be applied. The dry- ing/hardening of the putty is an exothermic process that lasts around an hour. Figure 4 depicts the substrate regularisation when the putty was used. The system was glued with a resin denominated saturant applied in two coatings, again within a 48-hour period to ensure the proper adhesion. This saturant is epoxy based, solvent free, high strength adhesive. One layer is applied over the primer, or the putty, already dried. At around one hour, before the saturant became tacky, the FRP fabric was applied. Within 2 hours, a second layer of saturant was applied on top of the FRP (Figure 5). Finally, a roller was used to expel any bubbles (Figure 6). The whole cure process took 7 days in room temperature, ranging between 25 to 35°C. Table 1 Characteristics and mechanical properties of the primer Properties Compressive Tensile Flexural Yield strength 26.2 MPa 14.5 MPa 24.1 MPa Strain at yield 4.0% 2.0% 4.0% Elastic modulus 670 MPa 717 MPa 595 MPa Ultimate strength 28.3 MPa 17.2 MPa 24.1 MPa Rupture strain 10% 40% Large deformation with no rupture Poisson’s ratio _ 0.48 Pot life 40 min at 25°C Cure Fully cured at 20°C - 7 days Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 6 of 15 Table 2 Characteristics and mechanical properties of the putty Properties Compressive Tensile Flexural Yield strength 22.8 MPa 12.0 MPa 26.2 MPa Strain at yield 4.0% 1.5% 4.0% Elastic modulus 1076 MPa 1800 MPa 895 MPa Ultimate strength 22.8 MPa 15.2 MPa 27.6 MPa Rupture strain 10% 7% 7% Poisson’s ratio _ 0.48 _ Pot life 40 min at 25°C Cure Fully cured at 20°C - 7 days The wallettes GW1, GW2, CW2 and CW3 were treated with the putty regularisation. The remaining walls, CW1, GW3 and GW4, did not receive the putty treatment. The main direction of the fibre was positioned horizontally in the walls, that is, in the direction perpendicular to the axial loading application. This configuration was chosen so that a more effecting enveloping (or confining effect) in the damaged struc- tures could be obtained. The enveloping mentioned above can be understood as the wrapping effect on the wallettes, based upon the hypothesis that the thickness of the walls is much smaller than the FRP covered surfaces. As a result of such a configur- ation, an increase in the compressive strength and the shear capacity of the structures was expected. Axial compressive loading experiments After the application of the reinforcement system onto the damaged wallettes, they were again subjected to a vertical compressive loading, up to their collapse. In this sec- ond loading, the relative vertical displacement was measured until the total load reached approximately 250 kN, which was around 60% of the reference collapse load. This procedure prevented damage in the measurement equipment if a sudden structural fail should occur. The experiments were performed in accordance with the Brazilian standard NBR15961-2/2011 [17]. For comparison with the reference wallettes experimental results, the Young’s modulus was also determined for these reinforced wallettes. Table 3 Characteristics and mechanical properties of the saturant Properties Compressive Tensile Flexural Yield strength 86.2 MPa 54.0 MPa 138.0 MPa Strain at yield 5.0% 2.5% 3.8% Elastic modulus 2620 MPa 3034 MPa 3724 MPa Ultimate strength 86.2 MPa 55.2 MPa 138.0 MPa Rupture strain 5.0% 3.5% 5% Poisson’s ratio _ 0.40 _ Pot life 45 min at 25°C Cure Fully cured at 20°C - 7 days Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 7 of 15 Figure 3 Application of the primer on the damaged wallettes. Figure 4 Putty application. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 8 of 15 Figure 5 Application of the saturant resin: (a) first layer and (b) second layer on top of the FRP. Results and discussion Axial compression results The results of the experiments of specimens RW1, RW2 and RW3 under compression are shown in Table 4. Tables 5 and 6 present the efficiency obtained in the compressive strength for each of the applied reinforcement systems when compared to the reference wallettes. It can be noted, in general, that all the tested specimens were able to recover the original strength (and even achieving higher values). It can be seen from the tables that the specimens reinforced with CFRP that received the putty (CW2 and CW3) presented a much better performance in relation to mech- anical resistance as compared to the wallette that was not prepared with the putty (CW1). The overall compressive strength gain was up to 39% for CW2 and CW3, whereas CW1 achieved roughly the reference strength, with a small 4% increase. On the other hand, the wallettes reinforced with GFRP presented non-uniform results, which does not allow for a definitive conclusion over their mechanical behaviour: the wallettes treated with putty presented a compressive strength increasing of 5% and 21%, while those that did not received the putty presented a strength improvement of 17% and 49%, as shown in Table 6. Figure 6 Roller used to expel bubbles. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 9 of 15 Table 4 Compressive strength of the reference wallettes Wallettes Compressive strength Average compressive Standard Variation coefficient [MPa] strength [MPa] deviation [%] RW1 3.93 3.82 0.10 2.64 RW2 3.75 RW3 3.79 According to the manufacturers, the use of a proper adhesive system does not confer any extra mechanical strength to the FRP composite, but the adhesive is capable of cre- ating a link between the substrate and FRP system and is able to distribute the applied loads. The above results confirm that the bonding between the FRP external reinforcement and the substrate is one of the key issues for the recovery of load cap- acity for reinforced structures [9,14]. Young’s modulus and stress-strain behaviour The Young’s modulus was also determined for the reinforced wallettes and a compari- son with the reference specimens was made. The results indicated that the reference (before reinforcement) and the FRP reinforced (after reinforcement) wallettes presented very similar behaviour under the compressive loading, as shown in Tables 7 and 8. These results suggest that the stiffness of the wallettes was also recovered after the ap- plication of the FRP reinforcement. With regard to the stress-strain behaviour, the performance of the wallettes rein- forced with CFRP was very similar when compared with their GFRP counterpart. Be- sides, both reinforcement systems presented stress-strain curves comparable to the curve for the undamaged specimens (before receiving the reinforcement), as depicted in Figures 7 and 8, indicating the rehabilitation of the strengthened structures. Failure mode From the experiments, it could be observed that a fragile, localised and sudden collapse occurred in the reference wallettes. In the majority of the cases, the cracks started when the loading approached its failure limit, i.e., approximately 75% of the estimated maximum load. This confirms the low ductility of the walls and the well-known ex- pected fragile behaviour of the masonry structures [19]. Moreover, from the experiments in this study, it could be observed that the FRP reinforcement applied did not exhibit, during the entire loading process, faults or frac- ture of the adherent that could be visible to the naked eye. Figures 9 and 10 show that the CFRP reinforced wallettes that received the putty treatment (CW2 and CW3) Table 5 Obtained efficiency of the wallettes reinforced with one-directional fabric of CFRP Wallettes Set up Achieved maximum Reference strength Efficiency Standard Variation strength [MPa] [MPa] deviation coefficient [%] CW1 Without 3.96 3.82 1.04 0.77 15.84 putty CW2 With putty 5.27 1.38 CW3 5.31 1.39 Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 10 of 15 Table 6 Obtained efficiency of the wallettes reinforced with two-directional fabric of GFRP Wallettes Set up Achieved maximum Reference Efficiency Standard Variation strength [MPa] strength [MPa] deviation coefficient [%] GW1 With putty 4.02 3.82 1.05 0.72 15.28 GW2 4.62 1.21 GW3 Without putty 4.46 1.17 GW4 5.71 1.49 presented failure of the reinforcement system only after the total collapse of the struc- tures, without presenting fibre debonding, neither between the FRP and the adhesive system, nor between the concrete substrate and the adhesive system. The failure mode of the specimen CW2 (Figure 9) suggests that the fibre reinforcement allowed for the structural masonry wallette to reach its maximum working loading capability, even after suffering the imposed damaging. Figure 10 brings the failure mode of the CW3 structure, where the fragile rupture of the concrete blocks can be seen. Here, again, no debonding between the substrate and the adhesive or between the adhesive and the FRP can be observed. This fact, combined with the maximum loading bearing capacity shown by the CW2 and CW3 specimens (as in Table 5), implies that the application of the putty contributes to the rehabilitation, as well as to the increase of the loading bear- ing capacity, as the result of a better bonding of the reinforcement system to the sub- strate. However, the CW1 specimen (Figure 11) that did not receive the putty treatment offered a premature failure when compared with the specimens CW2 and CW3, as shown in Table 5. From this figure, it is possible to observe the debonding of the reinforcement fibres, when the wallette reached its original failure loading, i.e., the lack of bonding of the FRP limited its performance and it only displayed a small load- ing capacity improvement. The experimental results confirmed, in general, the recovery of the original compres- sive loading bearing capacity of the structures. Moreover, it could be seen an increasing of up to 39% and up to 49% of the compressive strength for the damaged masonry wallettes reinforced with CFRP and GFRP systems, respectively, as shown in Tables 5 and 6. The ultimate load attainable by FRP reinforcement depends essentially upon the compressive and tensile strengths of the substrate. Debonding between the FRP com- posite and the substrate has been recognised as the principal failure mechanism of the reinforcement system. Debonding occurs when the system shear capacity is reached and the FRP reinforcement is detached from the element. Since the substrate is usually weaker than the glue and the reinforcement, failure is normally associated with the Table 7 Initial tangential Young’s modulus for the wallettes reinforced with CFRP Before of the reinforcement After the reinforcement Wallettes Set up E (MPa) Average value Variation E Average Variation (MPa) coefficient (%) (MPa) value (MPa) coefficient (%) CW1 Without 5869 6100 8.50 5625 6170 8.38 putty CW2 With putty 6110 6653 CW3 6320 6233 Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 11 of 15 Table 8 Initial tangential Young’s modulus for the wallettes reinforced with GFRP Before of the reinforcement After the reinforcement Wallettes Set up E (MPa) Average value Variation E (MPa) Average Variation (MPa) coefficient (%) value (MPa) coefficient (%) GW1 5890 7050 12.09 6117 6837 9.81 With putty GW2 7078 6821 GW3 7927 6676 Without putty GW4 7306 7734 removal of a material layer during debonding. These behaviours have been widely stud- ied for applications to concrete columns and beams, both from the experimental and numerical points of view, but, as far as masonry is concerned, only a limited number of studies can be found in literature [20]. In the current investigation, this fact can be ob- served and confirmed (see Figure 11). In addition, it is believed that an “enveloping effect” was obtained with the FRP reinforcement. Also, the small confining action on the wallettes and, especially, the mainten- ance of the original geometry of the specimens were observed. These factors were considered responsible for the rehabilitation of the bearing capacity of the structures under the applied vertical compressive loads. The reinforcement application, and its potential of avoiding new cracks opening and the growth of the existing cracks, was also important to the final rehabili- tation of masonry walls. Figure 7 Mean values for stress-strain curves of the wallettes behaviour before and after the FRP reinforcement. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 12 of 15 Figure 8 Mean values for the load-displacement curves of the wallettes behaviour before and after the FRP reinforcement. Figure 9 Failure of the CW2 wallette reinforced with CFRP (with putty). (a) Frontal view and (b) lateral view. Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 13 of 15 Figure 10 Failure of the CW3 wallette reinforced with CFRP (with putty). Figure 11 Failure of the CW1 wallette reinforced with CFRP (without putty). Chagas and Moita Applied Adhesion Science (2015) 3:6 Page 14 of 15 Finally, it is relevant to comment that the long-term durability of the reinforced structures was not addressed in the current research. Conclusions The main objective of this work was to present the rehabilitation potential offered by the CFRP and GFRP applied over previously damaged masonry wallettes. The wallettes were tested under axial compressive loading, before and after the application of the FRP reinforcement. It could be noted that the damaged, and later rehabilitated, wal- lettes could stand the maximum reference loading, with gains of 4% to 49% on the compressive strength in comparison with the measured failure loading of the undam- aged reference wallettes. Both CFRP and GFRP reinforced wallettes showed load- displacement and stress-strain curves similar to those obtained from the reference wal- lettes. Debonding between the FRP composite and the substrate can be attributed as premature failure of the reinforcement system and, consequently, of the reinforced wal- lettes, as observed here. Moreover, the small confining action and the maintenance of the geometry contributed for rehabilitation of the damaged wallettes. The increase in the load carrying capacity of the reinforced structures due to the ex- ternal fibres bonding is a good indication of their effectiveness in these situations. Hence, the obtained results point out the potential and applicability of the FRP reinforcement system technique in full-scale problems for masonry structures. Competing interests The authors declare that they have no competing interests. Authors’ contributions JSCN and GFM prepared the samples, ran the experiments and wrote the paper. Both authors read and approved the final manuscript. Acknowledgments The authors would like to acknowledge CEFET-MG for their support during the course of this work. Received: 19 November 2014 Accepted: 16 February 2015 References 1. Hendry AW (2002) Engineered design of masonry buildings: fifty years development in Europe. Prog Structur Engineer Mater 4:291–300 2. Asteris PG, Giannopoulos IP (2012) Vulnerability and restoration assessment of masonry structural systems. 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Journal
Applied Adhesion Science – Springer Journals
Published: Mar 15, 2015