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epoxy composites
Materials and Design 89 (2016) 225–234
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Effect of carbon nanotubes on strengthening of RC beams retrofitted with carbon fiber/epoxy composites Mohammad R. Irshidat a,⁎, Mohammed H. Al-Saleh b, Hashem Almashagbeh a a b
Department of Civil Engineering, Jordan University of Science and Technology, Irbid, Jordan Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 9 August 2015 Accepted 30 September 2015 Available online 3 October 2015 Keywords: CNTs RC beams Carbon fiber Strengthening CFRP Retrofitting
a b s t r a c t The effect of carbon nanotubes (CNTs) in improving the strengthening efficiency of carbon fiber/epoxy composites retrofitted reinforced concrete (RC) beams was investigated. A total of sixteen simply supported RC beams were prepared and tested under four-point loading. The incorporation of CNTs within the systems was done by modifying the epoxy resin using CNTs and/or coating the carbon fiber sheets with CNT enriched sizing agent. The effects of epoxy modification with CNTs, incorporation of CNT enriched sizing agent, anchorage length, and number of retrofitting layers were investigated through crack patterns, failure modes, load-deflection curves, and scanning electron microscopy (SEM) micrographs of fractured surfaces. Experimental results showed that using CNT modified epoxy resin enhanced the ultimate load and stiffness of retrofitted beams. The enhancement efficiency highly depends on the level of dispersion of CNT, anchorage length, and number of retrofitting layers. SEM characterization showed that CNTs could improve the adhesion at the concrete/epoxy interface and carbon fiber/epoxy interface leading to improvement in the load transfer and ultimate load of the strengthened beams. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Strengthening and upgrading of structural elements such as reinforced concrete (RC) beams are necessary to extend its service period, overcome the original design limits and to minimize the effect of construction or design defects. One of the most common strengthening techniques for flexural RC members is the use of externally bonded fiber reinforced polymer (FRP) composites. This method of strengthening is becoming more popular due to the outstanding properties of FRP materials, such as high strength to weight ratio, high stiffness, excellent corrosion resistance, ease of application, and minimal change in the geometry. Over the last three decades, extensive research has been conducted on the strengthening of RC beams with carbon FRP (CFRP) composites. It was reported that retrofitting RC beams with CFRP sheets significantly enhances its flexural properties in terms of strength and ductility. The level of enhancement depends on several factors, namely: type, length, width, and thickness of FRP sheet, number of FRP layers, beam size, concrete cover, and the resin type [1–30]. Despite the mentioned advantages, strengthening RC beams using FRP composites has some drawbacks regarding the performance of the transition layer between the FRP sheets and concrete [5,15,18,19,21,31,32]. These disadvantages may include the debonding of FRP sheets and incompatibility of epoxy resins and concrete. These weaknesses make the debonding ⁎ Corresponding author at: Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan. E-mail address: [email protected] (M.R. Irshidat).
http://dx.doi.org/10.1016/j.matdes.2015.09.166 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
of FRP sheets the most common failure mode of strengthened beams, which makes the total utilization of the tensile strength of the FRP materials impossible [19,21]. One possible solution to the above problems would be modifying the resin properties for better impregnation of the fibers and binding between the FRP and concrete. Carbon nanotubes (CNTs) may be considered as one of the most promising nanofiller that can act as matrix modifiers owning to their unique properties. A host of studies have investigated the influence of CNT addition on the properties of epoxy resin. It was reported that modifying epoxy with CNTs improves its tensile strength [33,34], flexural strength [35], toughness [34, 35], fracture strain [33], and young modulus [33,35,36]. However, the enhancement efficiency depends on many factors such as CNT dispersion, alignment, and interfacial adhesion between CNTs and the polymer matrix [37–39]. Recently, using CNT modified epoxy resin as polymer matrix to produce hybrid FRP composites has attracted significant attention. Most of the studies reported on the influence of using CNT modified epoxy resin on the mechanical properties of carbon fiber/epoxy nanocomposites. The results of these studies indicated that incorporation of CNTs into epoxy resin and use it along with carbon fiber reinforcements caused an enhancement in the nanocomposite flexural strength and modulus [40], fracture toughness [41–43], interfacial shear strength [44–47], and interfacial adhesion between epoxy matrix and carbon fiber [45,46,48]. It is evident that good understanding of the behavior of carbon fiber/ epoxy with CNTs has been established. However, very limited studies are focused on using these nanocomposites in strengthening RC structures. For example, Rousakis et al. [49] studied the effect of using CNT
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total of sixteen simply supported RC beams with cross section of (100 mm × 150 mm) and 1550 mm length were casted, retrofitted, and tested under four-point loading. Scanning electron microscopy (SEM) imaging was conducted to investigate the microstructure of the fractured surfaces. The studied parameters in this research are: epoxy modification with CNTs, using CNT enriched sizing agent, anchorage length, and number of retrofitting layers.
2. Experimental program Fig. 1. Dimensions and reinforcement details of the test specimens.
2.1. Test specimens A total of sixteen simply supported RC beams with dimensions of 100 mm × 150 mm × 1550 mm were constructed. All beams have the same flexural and shear reinforcements. Two 12-mm diameter and two 10-mm diameter steel bars were used in the longitudinal direction at the bottom and top, respectively. The shear reinforcement included 6 mm diameter steel stirrups at small spacing of 50 mm, to ensure that failure would be controlled by flexural yielding. General layout and reinforcement details of the test specimens are shown in Fig. 1. The following test series were conducted: o Two unstrengthened beams, designated as control o Eight beams were strengthened with a single layer of carbon fiber sheet bonded on the tension side of the beam (Fig. 2a) under the following conditions: • Two beams were retrofitted with CF/neat epoxy, designated as B-NE • Two beams were retrofitted with CF/CNT modified epoxy, designated as B-CNT • Two beams were retrofitted with CF coated with CNT enriched sizing agent/neat epoxy, designated as B-S-NE • Two beams were retrofitted with CF coated with CNT enriched sizing agent/CNT modified epoxy, designated as B-S-CNT o Four beams were partially strengthened with a single layer of carbon fiber sheet embedded in CNT modified epoxy resin bonded on the tension side of the beam: • Two with one 1.0 m-strip at the middle of the beam, designated as B-M-CNT (Fig.2b) • Two with one 0.5 m-strip at the middle of the beam, designated as B-MT-CNT, (Fig.2c)
Fig. 2. Anchorage length a) full retrofitting b) B-M-CNT specimen c) B-MT-CNT specimen.
enriched epoxy resins for external confinement of concrete columns using glass fiber sheet. Their results indicated that using CNT modified epoxy enhanced the bearing load of concrete specimen confined with glass fibers compared with non-reinforced polymer. This study aims at investigating the effectiveness of CNTs on strengthening RC beams retrofitted with carbon fiber/epoxy composites. The incorporation of CNTs was achieved by either modifying the epoxy resin with CNTs or coating the carbon fiber with CNT enriched sizing agent. A
o Two beams were strengthened with two layers of carbon fiber sheet embedded in CNT modified epoxy resin bonded on the tension side of the beam, designated as B-2 L-CNT.
A summary of test program and specimens designation is summarized in Table 1.
Table 1 Test program and specimen designation. Designation
No. of specimen
Epoxy type
Using CNT modified epoxy
Using CNT enriched sizing agent
Strengthening technique
Number of layers
Control B-NE B-CNT B-S-NE B-S-CNT B-M-CNT B-MT-CNT B-2 L-CNT
2 2 2 2 2 2 2 2
No Type I Type I Type I Type I Type I Type I Type I
No No Yes No Yes Yes Yes Yes
No No No Yes Yes No No No
No Full retrofitting Full retrofitting Full retrofitting Full retrofitting Partially retrofitting Partially retrofitting Full retrofitting
0 1 1 1 1 1 1 2
M.R. Irshidat et al. / Materials and Design 89 (2016) 225–234 Table 2 Concrete mix design.
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Table 4 Properties of epoxy resin.
Ingredient
Quantity
Property
Cement water Coarse aggregate Fine aggregate Silica sands
490 kg/m3 230 kg/m3 817 kg/m3 468 kg/m3 117 kg/m3
Density (kg/lt) Viscosity @ 25 °C Tensile strength
1.13 4000 ± 500 17 MPa
In this study, concrete with 28-day average compressive strength of 42.7 MPa and a slump of 40 mm was prepared. Ordinary Portland cement (type I), crushed coarse limestone aggregates having a maximum size of 12.5 mm, and a 20/80 mixture of silica sand and fine limestone were used to prepare the concrete mix following ACI 211 mix design procedure. The water to cement ration (w/c) was kept at a ratio of 0.47. Concrete mix design is shown in Table 2. Steel rebars (diameter: 12 mm and 10 mm, average yield stress of 418 MPa) were used as longitudinal reinforcement. The transverse reinforcement was provided with rectangular ties made of bars (diameter: 6 mm, yield stress: 290 MPa). Commercially available unidirectional carbon fiber sheet (MBrace CF 230/4900, BASF, Germany) and epoxy resin (MBrace Saturant, BASF, Germany) were used in this study. The main properties of CF and epoxy systems are summarized in Tables 3 and 4, respectively. A master-batch (MB) of CNT reinforced epoxy (EpoCyl™ NC R128-02, Nanocyl, Belgium) was used as a source of CNT. This MB is based on liquid Bisphenol-A epoxy and contains 20 wt.% of NC7000 multi-walled CNT (Nanocyl, Belgium). According to the manufacturer, these nanotubes are 1.5 μm in length and 9.5 nm in diameter. Liquid sizing agent (SIZICYL™ XC R2G, Nanocyl, Belgium) containing high concentration of CNTs was also used in this study.
repaired using cementitious material. After that, the retrofitting process was applied according to the following procedure: The test specimens were cleaned before the resin was applied. A layer of epoxy or CNT modified epoxy was directly applied on the beam surface. The CF sheet was then carefully applied on the resin-layer and rolled by special plastic roller until the resin was reflected on the external surface of the CF sheet to ensure that the sheet is saturated with the resin. For systems in which CNT enriched sizing agent was used, the CF sheet was coated with a thin layer of sizing agent before applying it on the resin-layer. Finally, a second layer of epoxy or CNT modified epoxy resin was applied. The retrofitted specimens were left at room temperature for one week before testing. The specimens were tested under four-point loading using a testing machine with a capacity of 2000 kN. The load was gradually increased using displacement control and three linear variable displacement transducers (LVDT) with a gage length of 150 mm were used to record the displacement of the beams. Two of the LVDT were placed at the bottom side of the beam in a direction parallel to the fiber orientation in order to measure the elongation of CFRP sheet. The third LVDT was used to measure the vertical displacement at the mid-span of the beam, as shown in Fig. 3. After testing, scanning electron microscopy (SEM) analysis was conducted to investigate the microstructure of the nanocomposite (CNT/epoxy), the epoxy/CF interface, and the concrete/epoxy interface using QUANTA FEG 450 SEM machine. The specimens were fractured in liquid nitrogen and coated with a thin layer of gold prior to imaging.
2.3. Preparation of CNT modified epoxy
3. Experimental results and discussion
The CNT/epoxy mixtures were prepared according to the manufacturer recommendations; where 221 g of the MB were diluted with 779 g of neat epoxy (Part A). To ensure good distribution and dispersion of the CNT particles within the mixture, the blend's components were mechanically stirred for 5 min followed by sonication for 30 min using an ultra Sonicator (Qsonica Q700, Qsonica, LLC., USA). Afterwards, the hardener was added at a ratio of 100 g resin/30 g hardener and the whole mixture was compounded at 600 rpm for 4 min. Based on this formulation, the concentration of CNT in the epoxy nanocomposite was 3.4 wt.%.
During the test, crack patterns and failure modes of all tested beams were carefully observed and reported. In addition, loads and mid-span deflections were collected, plotted, and characterized in terms of ultimate load, initial stiffness (slope of the initial linear part of the curve), and toughness (area under the load-deflection curve). SEM
2.2. Material properties
2.4. Specimen preparation and test setup Test beams were cast into horizontally-positioned wooden molds. Twenty four hours after casting, the beams were de-molded and wetcured for 28 days. All observed surface defects and irregularities were
Table 3 Properties of carbon fiber sheet. Property Width Fabric thickness Sheet thickness Areal weight Tensile strength Tensile E-modulus Strain at break
500 mm 0.34 mm 1 mm (per layer) 600 g/m2 ± g/m2 4900 MPa 28 GPa 2.1%
Fig. 3. Instrumentation details.
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M.R. Irshidat et al. / Materials and Design 89 (2016) 225–234 Table 5 Response parameters for tested beams. Specimen
Ultimate load (kN)
Initial stiffness kN/m
Toughness (kN·mm)
Control B-NE B-CNT B-S-NE B-S-CNT B-M-CNT B-MT-CNT B-2 L-CNT
44.8 66.2 69.7 60.6 62.7 51.8 44.2 79.5
3300 3500 4720 4120 3740 3910 3500 4800
1848 1064 1364 746 1093 395 585 935
are similar to the trends reported previously by many researchers [3,8, 20,30]. The beams strengthened with CF sheet embedded in neat epoxy (B-NE) were failed by sudden debonding of CF sheet starting from the center to one end of the beam as shown in Fig. 4. Although the crack formation pattern was similar to that of the control specimens, the presence of the CF sheets delayed the initial cracks formation and helped in distributing the flexural cracks regularly along the length of the beams resulting in smaller crack width. This behavior can be attributed to the ability of the retrofitting layer in bridging the cracks and consequently restricting their growth and propagation. The first crack loads for B-NE specimen was 11.7 kN. Similar behavior was reported in literature [3,8,30].
Fig. 4. (a) Failure mode of control specimen (b) failure mode of B-NE specimen (c) failure mode of B-CNT specimen (d) concrete splitting region of B-CNT specimen.
micrographs were used to explore the microstructure of the fractured surfaces and to explain the obtained results.
3.1. Control and neat epoxy specimens 3.1.1. Crack patterns and failure modes The control specimens were failed in a conventional flexural manner. The first flexural crack appeared in the mid-span of the beam, at a load of 5.8 kN, and was followed by the formation and propagation of many other cracks in the high moment zone. Shear cracks were then initiated and propagated in an inclined direction at higher load levels. Upon yielding of steel reinforcement, the cracks continued to propagate toward the compression zone leading to concrete crushing and failure of the beam at a load of 44.8 kN. The crack patterns and failure mode
3.1.2. Flexural behavior and load-deflection response The load versus midspan deflection curves for control, B-NE, and BCNT specimens are depicted in Fig. 5. The characteristics of the curves are summarized in Table 5. The control specimens exhibited the typical bilinear response of RC beams [8,32]. The initial stiffness, ultimate load, and toughness of control specimen were equal to 3300 kN/m, 44.8 kN, and 1848 kN · mm, respectively. On the hand, all retrofitted beams had almost same flexural behavior. Fig. 5 shows that strengthening of RC beams with carbon fiber sheet embedded in neat epoxy (B-NE) increased the ultimate load by 48% compared to the unstrengthened beams. The significant improvement in the flexural strength is referred to the contribution of carbon fiber sheet in increasing the tensile strength at the tension zone of the RC beams [8,50]. The flexural toughness of retrofitted beams B-NE showed a considerable reduction by about 42% compared to control specimens. The loss of ductility for retrofitted beams is due to the low ductility and high strength of carbon fiber sheet attached to the tension side of the RC beams. Moreover, the strengthening process did not affect the initial stiffness of B-NE specimen compared to control specimen.
Fig. 5. Load-deflection response of control, B-NE, and B-CNT specimens.
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Fig. 6. SEM images show (a) the fractured surface of B-CNT specimen (b) the cracks formulation and propagation within neat epoxy (c) the cracks formulation and propagation within CNT modified epoxy.
Fig. 7. SEM images show epoxy matrix debris attached to the carbon fiber in the case of (a) B-CNT specimen (b) B-NE specimen.
Fig. 8. SEM images show carbon fiber/epoxy resin and concrete/epoxy resin interfaces of (a) B-NE specimen (b) B-CNT specimen.
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the interphase between carbon fiber and epoxy matrix, as reported in [47,51], thus enhances the carbon fiber/epoxy resin and concrete/ epoxy resin adhesion [40] leading to proper load transfer between the retrofitting material and the strengthened beams. The improvement in the toughness can be assigned to the ability of CNTs to restrict the microcracks of the matrix which leads to increase the energy absorption of the whole system before debonding or sheet rupturing.
Fig. 9. (a) Failure mode of B-S-NE (b) failure mode of B-S-CNT specimen (c) region of fiber sheet rupture of B-S-CNT specimen.
3.2. Effect of epoxy matrix modification using CNTs 3.2.1. Crack patterns and failure modes The beams retrofitted with CF sheet embedded in CNT modified epoxy (B-CNT) were failed by sudden debonding of CF sheet starting from the center to one end of the beam with concrete splitting as shown in Fig. 4. However, the presence of CNTs delayed the propagation and debonding of the CF sheets. This may be attributed to the improvement in concrete/epoxy adhesion [44,45]. The crack pattern was similar to that of B-NE specimens. The first crack formation was insignificantly delayed in the case of B-CNT compared to B-NE specimen. The first crack loads for B-CNT was 12.7 kN.
3.2.2. Flexural behavior and load-deflection response Fig. 5 and Table 5 clearly show that modifying epoxy resin by adding CNTs (B-CNT specimens) slightly improved the beams ultimate load by 5%, but significantly enhanced its stiffness and toughness by 35% and 28%, respectively, compared to B-NE specimens. The enhancement in these properties can be ascribed to the ability of CNT in effectively suppressing the formation and propagation of micro-cracks at
3.2.3. Scanning electron microscopy (SEM) imaging Fig. 6a shows SEM micrograph of the surface of B-CNT specimen. Considerable number of well dispersed CNTs with uniform distribution and random orientation are embedded within the epoxy resin as shown in the micrograph. The well dispersed CNT effectively delays the formation and propagation of microcracks within epoxy matrix as shown in Fig. 6b and c and reported in the literatures [47,51]. Fig. 7 shows that more matrix is attached to the fiber in the case of BCNT specimen (Fig. 7a) compared to B-NE specimen (Fig. 7b). This observation indicates better adhesion between the carbon fiber sheet and CNT-modified epoxy compared to that with neat epoxy [47, 52,53]. This better interfacial bonding will improve the load transfer process at the polymer matrix/carbon fiber interface. Moreover, SEM micrograph (Fig. 8) and visual examination of the fractured surfaces of B-NE and B-CNT specimens show that more concrete fragments are attached to the epoxy resin in the later. This observation indicates that CNT modification also enhances the concrete/epoxy adhesion hence it improve the load transfer between composite materials and retrofitted beams [34,54]. These observations may also explain the delay in the debonding propagation of the carbon fiber sheet for the case of B-CNT specimen compared to B-NE specimen.
3.3. Effect of using CNT enriched sizing agent 3.3.1. Crack patterns and failure modes The beams retrofitted with sized CF sheet embedded in neat epoxy (B-S-NE) were failed by sudden debonding of carbon fiber sheet starting from the center to one end of the beam as shown in Fig. 9a. The crack formation pattern and the initiation of first crack are similar to these of B-NE specimens. Unlike other specimens, beams strengthened with sized CF sheet embedded in CNT modified epoxy (B-S-CNT) were failed by sheet rupture at the mid-span of the beams combined with concrete splitting. The CF sheet was still intact with concrete as shown in Fig. 9. This behavior could be attributed to the improvement in the adhesion at carbon fiber/epoxy and concrete/epoxy interfaces due to the presence of CNTs as shown later in the SEM images.
Fig. 10. Load-deflection response of control, B-NE, B-S-NE, and B-S-CNT specimens.
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Fig. 11. SEM images show (a) the fractured surface of B-S-CNT specimen (b) epoxy matrix debris attached to the carbon fiber in the case of B-S-NE specimen (c) epoxy matrix debris attached to the carbon fiber in the case of B-S-CNT specimen.
3.3.2. Flexural behavior and load-deflection response The load versus midspan deflection of control, B-NE, B-S-NE, and BS-CNT specimens are plotted in Fig. 10. It is clear that coating the carbon fiber with CNT enriched sizing agent and use it with epoxy (B-S-NE and B-S-CNT) reduced the beams ultimate load but enhanced its stiffness compared to B-NE specimens. Visual examination and SEM micrographs of fractured surfaces show that even though that coating the carbon fiber with sizing agent enhance the adhesion between the fibers and epoxy resin, it prevent the resin to immerse inside the fiber sheet leading to reduction in the amount of fibers that participate in carrying the load.
3.3.3. Scanning electron microscopy (SEM) imaging Fig. 11a shows SEM micrograph of the fractured surface of B-S-CNT specimen, on which the high concentrations of well dispersed CNTs are clearly seen. In the case of using sized fiber (B-S-NE and B-S-CNT specimens), more matrix debris were found to stick on the fiber surface as shown in Fig. 11b and c. This observation could be attributed to the high concentration of CNTs inside the sizing agent which improves the carbon fiber sheet/epoxy adhesion [55,56]. On the other hand, in the case of using sized fibers, little amount of epoxy resin were observed inside the fibers since coating the CF with a thin layer of sizing agent might influence their impregnation with the resin matrix. This observation is consistent with the results reported in previous works [57,58]. 3.4. Effect of anchorage length 3.4.1. Crack patterns and failure modes Unlike B-CNT specimens, B-M-CNT and B-MT-CNT beams were failed by concrete cover delamination starting from one end of the CF sheet extended toward its other end as shown in Fig. 12. This failure mechanism may be attributed to the fact that B-M-CNT and B-MT-CNT specimens do not have a full anchorage length outside the high moment zone, hence higher shear stress concentration will occur at the end of the CF sheets compared to the case of B-CNT specimen [8]. 3.4.2. Flexural behavior and load-deflection response Fig. 13 shows the load-deflection curves of control, B-CNT, B-M-CNT, and B-MT-CNT specimens. The figure indicates that strengthening RC beams using CNT modified epoxy/carbon fiber sheets with different anchorage lengths enhanced its ultimate loads and stiffness. The strengthening efficiency clearly depends on the length of retrofitted region which consists with the literature [8]. 3.5. Effect of CFRP layer number 3.5.1. Crack patterns and failure modes Beams retrofitted with either one or two layers of CF sheets embedded in CNT modified epoxy had same mode of failure. Both B-CNT and B-2 L-CNT specimens failed by sudden debonding of CF sheet starting from the center of the beam and extended to its end.
Fig. 12. Failure mode of (a) B-M-CNT specimen (b) B-MT-CNT specimen.
3.5.2. Flexural behavior and load-deflection response The load-deflection behavior shown in Fig. 14 indicates that strengthening RC beams with one layer of nano modified CF sheets enhanced their ultimate load, stiffness, and toughness by 5%, 35%, and 28%, respectively, compared to B-NE specimens. Adding another layer of retrofitted material increase the enhancement in ultimate load and stiffness to be 20% and 37%, respectively, compared to B-NE specimen. It is
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Fig. 13. Load-deflection response of control, B-CNT, B-M-CNT, and B-MT-CNT specimens.
clear that the enhancement in the ultimate load and stiffness depends on the number of retrofitting layers. These results are consistent with the results reported in [8,22]. 4. Conclusions This paper investigated the effect of CNTs on improving the strengthening efficiency of RC beams retrofitted with carbon fiber/ epoxy composites. A total of sixteen simply supported RC beams were casted, retrofitted, and tested under four-point loading. The incorporation of CNTs was achieved by either modifying the epoxy resin using CNTs and/or coating the carbon fiber with CNT enriched sizing agent. The effect of epoxy modification with CNTs, using CNT enriched sizing agent, anchorage length, and number of retrofitting layers were studied through crack patterns, failure modes, load-deflection curves, and SEM micrographs of fractured surfaces. The following conclusions were drawn: 1. Modification of epoxy resin with CNTs did not change the crack patterns and failure mode of retrofitted beams but delayed the propagation of carbon fiber sheet debonding. 2. Modification of epoxy resin with CNTs (B-CNT specimens) slightly improved the beams ultimate load by 5%, but significantly enhanced its stiffness and toughness by 35% and 28%, respectively, compared to B-NE specimens. 3. Coating the carbon fiber sheet with CNT enriched sizing agent enhances the adhesion between the fibers and epoxy resin, but prevents the resin to immerse inside the fiber sheet, hence reduce the amount of fibers that participate in carrying the load.
4. SEM micrographs showed that more matrix debris and concrete fragments are attached to the carbon fiber in the case of using CNT modified epoxy (B-CNT specimens) compared to B-NE specimen. This observation reflects the enhancement in the load transfer process of retrofitted beams. 5. Strengthening RC beams using CNT modified epoxy/carbon fiber sheets, with different lengths and number of layers, enhanced its ultimate loads and stiffness. The strengthening efficiency clearly depends on the anchorage length and number of retrofitting layers.
Acknowledgments The authors would like to thank the Scientific Research Support Fund, Amman, Jordan, for the financial support of this research (EIT/1/ 12/2012).
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Fig. 14. Load-deflection response of control, B-CNT, and B-2 L-CNT specimens.
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Effect of carbon nanotubes on strengthening of RC beams retrofitted with carbon fiber/epoxy composites Mohammad R. Irshidat a,⁎, Mohammed H. Al-Saleh b, Hashem Almashagbeh a a b
Department of Civil Engineering, Jordan University of Science and Technology, Irbid, Jordan Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 9 August 2015 Accepted 30 September 2015 Available online 3 October 2015 Keywords: CNTs RC beams Carbon fiber Strengthening CFRP Retrofitting
a b s t r a c t The effect of carbon nanotubes (CNTs) in improving the strengthening efficiency of carbon fiber/epoxy composites retrofitted reinforced concrete (RC) beams was investigated. A total of sixteen simply supported RC beams were prepared and tested under four-point loading. The incorporation of CNTs within the systems was done by modifying the epoxy resin using CNTs and/or coating the carbon fiber sheets with CNT enriched sizing agent. The effects of epoxy modification with CNTs, incorporation of CNT enriched sizing agent, anchorage length, and number of retrofitting layers were investigated through crack patterns, failure modes, load-deflection curves, and scanning electron microscopy (SEM) micrographs of fractured surfaces. Experimental results showed that using CNT modified epoxy resin enhanced the ultimate load and stiffness of retrofitted beams. The enhancement efficiency highly depends on the level of dispersion of CNT, anchorage length, and number of retrofitting layers. SEM characterization showed that CNTs could improve the adhesion at the concrete/epoxy interface and carbon fiber/epoxy interface leading to improvement in the load transfer and ultimate load of the strengthened beams. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Strengthening and upgrading of structural elements such as reinforced concrete (RC) beams are necessary to extend its service period, overcome the original design limits and to minimize the effect of construction or design defects. One of the most common strengthening techniques for flexural RC members is the use of externally bonded fiber reinforced polymer (FRP) composites. This method of strengthening is becoming more popular due to the outstanding properties of FRP materials, such as high strength to weight ratio, high stiffness, excellent corrosion resistance, ease of application, and minimal change in the geometry. Over the last three decades, extensive research has been conducted on the strengthening of RC beams with carbon FRP (CFRP) composites. It was reported that retrofitting RC beams with CFRP sheets significantly enhances its flexural properties in terms of strength and ductility. The level of enhancement depends on several factors, namely: type, length, width, and thickness of FRP sheet, number of FRP layers, beam size, concrete cover, and the resin type [1–30]. Despite the mentioned advantages, strengthening RC beams using FRP composites has some drawbacks regarding the performance of the transition layer between the FRP sheets and concrete [5,15,18,19,21,31,32]. These disadvantages may include the debonding of FRP sheets and incompatibility of epoxy resins and concrete. These weaknesses make the debonding ⁎ Corresponding author at: Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan. E-mail address: [email protected] (M.R. Irshidat).
http://dx.doi.org/10.1016/j.matdes.2015.09.166 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
of FRP sheets the most common failure mode of strengthened beams, which makes the total utilization of the tensile strength of the FRP materials impossible [19,21]. One possible solution to the above problems would be modifying the resin properties for better impregnation of the fibers and binding between the FRP and concrete. Carbon nanotubes (CNTs) may be considered as one of the most promising nanofiller that can act as matrix modifiers owning to their unique properties. A host of studies have investigated the influence of CNT addition on the properties of epoxy resin. It was reported that modifying epoxy with CNTs improves its tensile strength [33,34], flexural strength [35], toughness [34, 35], fracture strain [33], and young modulus [33,35,36]. However, the enhancement efficiency depends on many factors such as CNT dispersion, alignment, and interfacial adhesion between CNTs and the polymer matrix [37–39]. Recently, using CNT modified epoxy resin as polymer matrix to produce hybrid FRP composites has attracted significant attention. Most of the studies reported on the influence of using CNT modified epoxy resin on the mechanical properties of carbon fiber/epoxy nanocomposites. The results of these studies indicated that incorporation of CNTs into epoxy resin and use it along with carbon fiber reinforcements caused an enhancement in the nanocomposite flexural strength and modulus [40], fracture toughness [41–43], interfacial shear strength [44–47], and interfacial adhesion between epoxy matrix and carbon fiber [45,46,48]. It is evident that good understanding of the behavior of carbon fiber/ epoxy with CNTs has been established. However, very limited studies are focused on using these nanocomposites in strengthening RC structures. For example, Rousakis et al. [49] studied the effect of using CNT
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total of sixteen simply supported RC beams with cross section of (100 mm × 150 mm) and 1550 mm length were casted, retrofitted, and tested under four-point loading. Scanning electron microscopy (SEM) imaging was conducted to investigate the microstructure of the fractured surfaces. The studied parameters in this research are: epoxy modification with CNTs, using CNT enriched sizing agent, anchorage length, and number of retrofitting layers.
2. Experimental program Fig. 1. Dimensions and reinforcement details of the test specimens.
2.1. Test specimens A total of sixteen simply supported RC beams with dimensions of 100 mm × 150 mm × 1550 mm were constructed. All beams have the same flexural and shear reinforcements. Two 12-mm diameter and two 10-mm diameter steel bars were used in the longitudinal direction at the bottom and top, respectively. The shear reinforcement included 6 mm diameter steel stirrups at small spacing of 50 mm, to ensure that failure would be controlled by flexural yielding. General layout and reinforcement details of the test specimens are shown in Fig. 1. The following test series were conducted: o Two unstrengthened beams, designated as control o Eight beams were strengthened with a single layer of carbon fiber sheet bonded on the tension side of the beam (Fig. 2a) under the following conditions: • Two beams were retrofitted with CF/neat epoxy, designated as B-NE • Two beams were retrofitted with CF/CNT modified epoxy, designated as B-CNT • Two beams were retrofitted with CF coated with CNT enriched sizing agent/neat epoxy, designated as B-S-NE • Two beams were retrofitted with CF coated with CNT enriched sizing agent/CNT modified epoxy, designated as B-S-CNT o Four beams were partially strengthened with a single layer of carbon fiber sheet embedded in CNT modified epoxy resin bonded on the tension side of the beam: • Two with one 1.0 m-strip at the middle of the beam, designated as B-M-CNT (Fig.2b) • Two with one 0.5 m-strip at the middle of the beam, designated as B-MT-CNT, (Fig.2c)
Fig. 2. Anchorage length a) full retrofitting b) B-M-CNT specimen c) B-MT-CNT specimen.
enriched epoxy resins for external confinement of concrete columns using glass fiber sheet. Their results indicated that using CNT modified epoxy enhanced the bearing load of concrete specimen confined with glass fibers compared with non-reinforced polymer. This study aims at investigating the effectiveness of CNTs on strengthening RC beams retrofitted with carbon fiber/epoxy composites. The incorporation of CNTs was achieved by either modifying the epoxy resin with CNTs or coating the carbon fiber with CNT enriched sizing agent. A
o Two beams were strengthened with two layers of carbon fiber sheet embedded in CNT modified epoxy resin bonded on the tension side of the beam, designated as B-2 L-CNT.
A summary of test program and specimens designation is summarized in Table 1.
Table 1 Test program and specimen designation. Designation
No. of specimen
Epoxy type
Using CNT modified epoxy
Using CNT enriched sizing agent
Strengthening technique
Number of layers
Control B-NE B-CNT B-S-NE B-S-CNT B-M-CNT B-MT-CNT B-2 L-CNT
2 2 2 2 2 2 2 2
No Type I Type I Type I Type I Type I Type I Type I
No No Yes No Yes Yes Yes Yes
No No No Yes Yes No No No
No Full retrofitting Full retrofitting Full retrofitting Full retrofitting Partially retrofitting Partially retrofitting Full retrofitting
0 1 1 1 1 1 1 2
M.R. Irshidat et al. / Materials and Design 89 (2016) 225–234 Table 2 Concrete mix design.
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Table 4 Properties of epoxy resin.
Ingredient
Quantity
Property
Cement water Coarse aggregate Fine aggregate Silica sands
490 kg/m3 230 kg/m3 817 kg/m3 468 kg/m3 117 kg/m3
Density (kg/lt) Viscosity @ 25 °C Tensile strength
1.13 4000 ± 500 17 MPa
In this study, concrete with 28-day average compressive strength of 42.7 MPa and a slump of 40 mm was prepared. Ordinary Portland cement (type I), crushed coarse limestone aggregates having a maximum size of 12.5 mm, and a 20/80 mixture of silica sand and fine limestone were used to prepare the concrete mix following ACI 211 mix design procedure. The water to cement ration (w/c) was kept at a ratio of 0.47. Concrete mix design is shown in Table 2. Steel rebars (diameter: 12 mm and 10 mm, average yield stress of 418 MPa) were used as longitudinal reinforcement. The transverse reinforcement was provided with rectangular ties made of bars (diameter: 6 mm, yield stress: 290 MPa). Commercially available unidirectional carbon fiber sheet (MBrace CF 230/4900, BASF, Germany) and epoxy resin (MBrace Saturant, BASF, Germany) were used in this study. The main properties of CF and epoxy systems are summarized in Tables 3 and 4, respectively. A master-batch (MB) of CNT reinforced epoxy (EpoCyl™ NC R128-02, Nanocyl, Belgium) was used as a source of CNT. This MB is based on liquid Bisphenol-A epoxy and contains 20 wt.% of NC7000 multi-walled CNT (Nanocyl, Belgium). According to the manufacturer, these nanotubes are 1.5 μm in length and 9.5 nm in diameter. Liquid sizing agent (SIZICYL™ XC R2G, Nanocyl, Belgium) containing high concentration of CNTs was also used in this study.
repaired using cementitious material. After that, the retrofitting process was applied according to the following procedure: The test specimens were cleaned before the resin was applied. A layer of epoxy or CNT modified epoxy was directly applied on the beam surface. The CF sheet was then carefully applied on the resin-layer and rolled by special plastic roller until the resin was reflected on the external surface of the CF sheet to ensure that the sheet is saturated with the resin. For systems in which CNT enriched sizing agent was used, the CF sheet was coated with a thin layer of sizing agent before applying it on the resin-layer. Finally, a second layer of epoxy or CNT modified epoxy resin was applied. The retrofitted specimens were left at room temperature for one week before testing. The specimens were tested under four-point loading using a testing machine with a capacity of 2000 kN. The load was gradually increased using displacement control and three linear variable displacement transducers (LVDT) with a gage length of 150 mm were used to record the displacement of the beams. Two of the LVDT were placed at the bottom side of the beam in a direction parallel to the fiber orientation in order to measure the elongation of CFRP sheet. The third LVDT was used to measure the vertical displacement at the mid-span of the beam, as shown in Fig. 3. After testing, scanning electron microscopy (SEM) analysis was conducted to investigate the microstructure of the nanocomposite (CNT/epoxy), the epoxy/CF interface, and the concrete/epoxy interface using QUANTA FEG 450 SEM machine. The specimens were fractured in liquid nitrogen and coated with a thin layer of gold prior to imaging.
2.3. Preparation of CNT modified epoxy
3. Experimental results and discussion
The CNT/epoxy mixtures were prepared according to the manufacturer recommendations; where 221 g of the MB were diluted with 779 g of neat epoxy (Part A). To ensure good distribution and dispersion of the CNT particles within the mixture, the blend's components were mechanically stirred for 5 min followed by sonication for 30 min using an ultra Sonicator (Qsonica Q700, Qsonica, LLC., USA). Afterwards, the hardener was added at a ratio of 100 g resin/30 g hardener and the whole mixture was compounded at 600 rpm for 4 min. Based on this formulation, the concentration of CNT in the epoxy nanocomposite was 3.4 wt.%.
During the test, crack patterns and failure modes of all tested beams were carefully observed and reported. In addition, loads and mid-span deflections were collected, plotted, and characterized in terms of ultimate load, initial stiffness (slope of the initial linear part of the curve), and toughness (area under the load-deflection curve). SEM
2.2. Material properties
2.4. Specimen preparation and test setup Test beams were cast into horizontally-positioned wooden molds. Twenty four hours after casting, the beams were de-molded and wetcured for 28 days. All observed surface defects and irregularities were
Table 3 Properties of carbon fiber sheet. Property Width Fabric thickness Sheet thickness Areal weight Tensile strength Tensile E-modulus Strain at break
500 mm 0.34 mm 1 mm (per layer) 600 g/m2 ± g/m2 4900 MPa 28 GPa 2.1%
Fig. 3. Instrumentation details.
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M.R. Irshidat et al. / Materials and Design 89 (2016) 225–234 Table 5 Response parameters for tested beams. Specimen
Ultimate load (kN)
Initial stiffness kN/m
Toughness (kN·mm)
Control B-NE B-CNT B-S-NE B-S-CNT B-M-CNT B-MT-CNT B-2 L-CNT
44.8 66.2 69.7 60.6 62.7 51.8 44.2 79.5
3300 3500 4720 4120 3740 3910 3500 4800
1848 1064 1364 746 1093 395 585 935
are similar to the trends reported previously by many researchers [3,8, 20,30]. The beams strengthened with CF sheet embedded in neat epoxy (B-NE) were failed by sudden debonding of CF sheet starting from the center to one end of the beam as shown in Fig. 4. Although the crack formation pattern was similar to that of the control specimens, the presence of the CF sheets delayed the initial cracks formation and helped in distributing the flexural cracks regularly along the length of the beams resulting in smaller crack width. This behavior can be attributed to the ability of the retrofitting layer in bridging the cracks and consequently restricting their growth and propagation. The first crack loads for B-NE specimen was 11.7 kN. Similar behavior was reported in literature [3,8,30].
Fig. 4. (a) Failure mode of control specimen (b) failure mode of B-NE specimen (c) failure mode of B-CNT specimen (d) concrete splitting region of B-CNT specimen.
micrographs were used to explore the microstructure of the fractured surfaces and to explain the obtained results.
3.1. Control and neat epoxy specimens 3.1.1. Crack patterns and failure modes The control specimens were failed in a conventional flexural manner. The first flexural crack appeared in the mid-span of the beam, at a load of 5.8 kN, and was followed by the formation and propagation of many other cracks in the high moment zone. Shear cracks were then initiated and propagated in an inclined direction at higher load levels. Upon yielding of steel reinforcement, the cracks continued to propagate toward the compression zone leading to concrete crushing and failure of the beam at a load of 44.8 kN. The crack patterns and failure mode
3.1.2. Flexural behavior and load-deflection response The load versus midspan deflection curves for control, B-NE, and BCNT specimens are depicted in Fig. 5. The characteristics of the curves are summarized in Table 5. The control specimens exhibited the typical bilinear response of RC beams [8,32]. The initial stiffness, ultimate load, and toughness of control specimen were equal to 3300 kN/m, 44.8 kN, and 1848 kN · mm, respectively. On the hand, all retrofitted beams had almost same flexural behavior. Fig. 5 shows that strengthening of RC beams with carbon fiber sheet embedded in neat epoxy (B-NE) increased the ultimate load by 48% compared to the unstrengthened beams. The significant improvement in the flexural strength is referred to the contribution of carbon fiber sheet in increasing the tensile strength at the tension zone of the RC beams [8,50]. The flexural toughness of retrofitted beams B-NE showed a considerable reduction by about 42% compared to control specimens. The loss of ductility for retrofitted beams is due to the low ductility and high strength of carbon fiber sheet attached to the tension side of the RC beams. Moreover, the strengthening process did not affect the initial stiffness of B-NE specimen compared to control specimen.
Fig. 5. Load-deflection response of control, B-NE, and B-CNT specimens.
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Fig. 6. SEM images show (a) the fractured surface of B-CNT specimen (b) the cracks formulation and propagation within neat epoxy (c) the cracks formulation and propagation within CNT modified epoxy.
Fig. 7. SEM images show epoxy matrix debris attached to the carbon fiber in the case of (a) B-CNT specimen (b) B-NE specimen.
Fig. 8. SEM images show carbon fiber/epoxy resin and concrete/epoxy resin interfaces of (a) B-NE specimen (b) B-CNT specimen.
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the interphase between carbon fiber and epoxy matrix, as reported in [47,51], thus enhances the carbon fiber/epoxy resin and concrete/ epoxy resin adhesion [40] leading to proper load transfer between the retrofitting material and the strengthened beams. The improvement in the toughness can be assigned to the ability of CNTs to restrict the microcracks of the matrix which leads to increase the energy absorption of the whole system before debonding or sheet rupturing.
Fig. 9. (a) Failure mode of B-S-NE (b) failure mode of B-S-CNT specimen (c) region of fiber sheet rupture of B-S-CNT specimen.
3.2. Effect of epoxy matrix modification using CNTs 3.2.1. Crack patterns and failure modes The beams retrofitted with CF sheet embedded in CNT modified epoxy (B-CNT) were failed by sudden debonding of CF sheet starting from the center to one end of the beam with concrete splitting as shown in Fig. 4. However, the presence of CNTs delayed the propagation and debonding of the CF sheets. This may be attributed to the improvement in concrete/epoxy adhesion [44,45]. The crack pattern was similar to that of B-NE specimens. The first crack formation was insignificantly delayed in the case of B-CNT compared to B-NE specimen. The first crack loads for B-CNT was 12.7 kN.
3.2.2. Flexural behavior and load-deflection response Fig. 5 and Table 5 clearly show that modifying epoxy resin by adding CNTs (B-CNT specimens) slightly improved the beams ultimate load by 5%, but significantly enhanced its stiffness and toughness by 35% and 28%, respectively, compared to B-NE specimens. The enhancement in these properties can be ascribed to the ability of CNT in effectively suppressing the formation and propagation of micro-cracks at
3.2.3. Scanning electron microscopy (SEM) imaging Fig. 6a shows SEM micrograph of the surface of B-CNT specimen. Considerable number of well dispersed CNTs with uniform distribution and random orientation are embedded within the epoxy resin as shown in the micrograph. The well dispersed CNT effectively delays the formation and propagation of microcracks within epoxy matrix as shown in Fig. 6b and c and reported in the literatures [47,51]. Fig. 7 shows that more matrix is attached to the fiber in the case of BCNT specimen (Fig. 7a) compared to B-NE specimen (Fig. 7b). This observation indicates better adhesion between the carbon fiber sheet and CNT-modified epoxy compared to that with neat epoxy [47, 52,53]. This better interfacial bonding will improve the load transfer process at the polymer matrix/carbon fiber interface. Moreover, SEM micrograph (Fig. 8) and visual examination of the fractured surfaces of B-NE and B-CNT specimens show that more concrete fragments are attached to the epoxy resin in the later. This observation indicates that CNT modification also enhances the concrete/epoxy adhesion hence it improve the load transfer between composite materials and retrofitted beams [34,54]. These observations may also explain the delay in the debonding propagation of the carbon fiber sheet for the case of B-CNT specimen compared to B-NE specimen.
3.3. Effect of using CNT enriched sizing agent 3.3.1. Crack patterns and failure modes The beams retrofitted with sized CF sheet embedded in neat epoxy (B-S-NE) were failed by sudden debonding of carbon fiber sheet starting from the center to one end of the beam as shown in Fig. 9a. The crack formation pattern and the initiation of first crack are similar to these of B-NE specimens. Unlike other specimens, beams strengthened with sized CF sheet embedded in CNT modified epoxy (B-S-CNT) were failed by sheet rupture at the mid-span of the beams combined with concrete splitting. The CF sheet was still intact with concrete as shown in Fig. 9. This behavior could be attributed to the improvement in the adhesion at carbon fiber/epoxy and concrete/epoxy interfaces due to the presence of CNTs as shown later in the SEM images.
Fig. 10. Load-deflection response of control, B-NE, B-S-NE, and B-S-CNT specimens.
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Fig. 11. SEM images show (a) the fractured surface of B-S-CNT specimen (b) epoxy matrix debris attached to the carbon fiber in the case of B-S-NE specimen (c) epoxy matrix debris attached to the carbon fiber in the case of B-S-CNT specimen.
3.3.2. Flexural behavior and load-deflection response The load versus midspan deflection of control, B-NE, B-S-NE, and BS-CNT specimens are plotted in Fig. 10. It is clear that coating the carbon fiber with CNT enriched sizing agent and use it with epoxy (B-S-NE and B-S-CNT) reduced the beams ultimate load but enhanced its stiffness compared to B-NE specimens. Visual examination and SEM micrographs of fractured surfaces show that even though that coating the carbon fiber with sizing agent enhance the adhesion between the fibers and epoxy resin, it prevent the resin to immerse inside the fiber sheet leading to reduction in the amount of fibers that participate in carrying the load.
3.3.3. Scanning electron microscopy (SEM) imaging Fig. 11a shows SEM micrograph of the fractured surface of B-S-CNT specimen, on which the high concentrations of well dispersed CNTs are clearly seen. In the case of using sized fiber (B-S-NE and B-S-CNT specimens), more matrix debris were found to stick on the fiber surface as shown in Fig. 11b and c. This observation could be attributed to the high concentration of CNTs inside the sizing agent which improves the carbon fiber sheet/epoxy adhesion [55,56]. On the other hand, in the case of using sized fibers, little amount of epoxy resin were observed inside the fibers since coating the CF with a thin layer of sizing agent might influence their impregnation with the resin matrix. This observation is consistent with the results reported in previous works [57,58]. 3.4. Effect of anchorage length 3.4.1. Crack patterns and failure modes Unlike B-CNT specimens, B-M-CNT and B-MT-CNT beams were failed by concrete cover delamination starting from one end of the CF sheet extended toward its other end as shown in Fig. 12. This failure mechanism may be attributed to the fact that B-M-CNT and B-MT-CNT specimens do not have a full anchorage length outside the high moment zone, hence higher shear stress concentration will occur at the end of the CF sheets compared to the case of B-CNT specimen [8]. 3.4.2. Flexural behavior and load-deflection response Fig. 13 shows the load-deflection curves of control, B-CNT, B-M-CNT, and B-MT-CNT specimens. The figure indicates that strengthening RC beams using CNT modified epoxy/carbon fiber sheets with different anchorage lengths enhanced its ultimate loads and stiffness. The strengthening efficiency clearly depends on the length of retrofitted region which consists with the literature [8]. 3.5. Effect of CFRP layer number 3.5.1. Crack patterns and failure modes Beams retrofitted with either one or two layers of CF sheets embedded in CNT modified epoxy had same mode of failure. Both B-CNT and B-2 L-CNT specimens failed by sudden debonding of CF sheet starting from the center of the beam and extended to its end.
Fig. 12. Failure mode of (a) B-M-CNT specimen (b) B-MT-CNT specimen.
3.5.2. Flexural behavior and load-deflection response The load-deflection behavior shown in Fig. 14 indicates that strengthening RC beams with one layer of nano modified CF sheets enhanced their ultimate load, stiffness, and toughness by 5%, 35%, and 28%, respectively, compared to B-NE specimens. Adding another layer of retrofitted material increase the enhancement in ultimate load and stiffness to be 20% and 37%, respectively, compared to B-NE specimen. It is
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Fig. 13. Load-deflection response of control, B-CNT, B-M-CNT, and B-MT-CNT specimens.
clear that the enhancement in the ultimate load and stiffness depends on the number of retrofitting layers. These results are consistent with the results reported in [8,22]. 4. Conclusions This paper investigated the effect of CNTs on improving the strengthening efficiency of RC beams retrofitted with carbon fiber/ epoxy composites. A total of sixteen simply supported RC beams were casted, retrofitted, and tested under four-point loading. The incorporation of CNTs was achieved by either modifying the epoxy resin using CNTs and/or coating the carbon fiber with CNT enriched sizing agent. The effect of epoxy modification with CNTs, using CNT enriched sizing agent, anchorage length, and number of retrofitting layers were studied through crack patterns, failure modes, load-deflection curves, and SEM micrographs of fractured surfaces. The following conclusions were drawn: 1. Modification of epoxy resin with CNTs did not change the crack patterns and failure mode of retrofitted beams but delayed the propagation of carbon fiber sheet debonding. 2. Modification of epoxy resin with CNTs (B-CNT specimens) slightly improved the beams ultimate load by 5%, but significantly enhanced its stiffness and toughness by 35% and 28%, respectively, compared to B-NE specimens. 3. Coating the carbon fiber sheet with CNT enriched sizing agent enhances the adhesion between the fibers and epoxy resin, but prevents the resin to immerse inside the fiber sheet, hence reduce the amount of fibers that participate in carrying the load.
4. SEM micrographs showed that more matrix debris and concrete fragments are attached to the carbon fiber in the case of using CNT modified epoxy (B-CNT specimens) compared to B-NE specimen. This observation reflects the enhancement in the load transfer process of retrofitted beams. 5. Strengthening RC beams using CNT modified epoxy/carbon fiber sheets, with different lengths and number of layers, enhanced its ultimate loads and stiffness. The strengthening efficiency clearly depends on the anchorage length and number of retrofitting layers.
Acknowledgments The authors would like to thank the Scientific Research Support Fund, Amman, Jordan, for the financial support of this research (EIT/1/ 12/2012).
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