* Corresponding author: Department of Civil Engineering, Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh E-mail addresses: zahurul90@gmail.com (S. M. Zahurul Islam)
33
FRP-Adhesive Materials for Stregthening of RC Beams at Flexure and Shear Region
S. M. Zahurul Islam1,*, B. Ahmed1, Md. Sahinur Islam1, Md. Ryhanul Ferdous1, Md. Washim Ali21Department of Civil Engineering, Rajshahi University of Engineering and Technology, Rajshahi-6204, Bangladesh
2Executive Engineer, Bangabandhu Multipurpose Bridge Authority, Bangladesh
ARTICLE INFORMATION ABSTRACT
Received date: 22 Jan 2019 Revised date: 25 April 2019 Accepted date: 08 May 2019
It is promising technique of the strengthening for new construction and retrofitting of Reinforced Concrete (RC) Beams by an advanced composite material Fibre Reinforced Polymers (FRP). Fibre reinforced polymer (FRP) and CFRP are used in strengthening and retrofitting in flexure and shear failure regions during earthquake and application of point load. The roll and effect of those applications are still subject to investigate with various number of influencing factors, significantly. The aim of this study is to evaluate the flexure and shear region and strengthening and retrofitting of those regions of RC beams. The beams are strengthened and retrofitted at different position of shear and flexure region experimentally. A series of test was conducted under three points and four-point loading in this research for FRP strengthen beam. The collapse loads, collapse modes and the load-deformation behaviour of the FRP strengthen and retrofitting of RC beam are also presented. Based on test results, it was found that the FRP strengthen and retrofitting of RC beam provided better performance significantly. Therefore, it is concluded that FRP materials can be apply for strengthening of RC beam efficiently.
Keywords
CFRP
Concrete structure Flexural member Retrofitting Strengthening
1. Introduction
Recently, strengthening of RC beams has get extra importance for reduction of strength and load carrying capacity of structures due to unpredictable climatic conditions, use of sub-standard materials, impact loads, corrosion or deterioration of concrete, settlement and poor workmanship. The use of unconventional composite materials in strengthening and retrofitting of concrete beams and columns has also gained wide reception in the construction industry [1]. The shear and flexural performance of the strengthened members depends on the flexural stiffness of FRP for different loading cases. The technique of strengthening is an auspicious skill for rising the flexural and shear capacity of reinforced concrete members (RC beams). Some cases, extra additional loads or upgraded seismic load had been taken in consideration which was not included in the original design. Fiber reinforced polymers for structural retrofitting is increasingly used in different concrete structures [1-6]. Byrne and Tikka [6] conducted a research on repair and strengthening of severely damaged concrete beams with externally bonded CFRP. Failure in flexure region occurs when some additional loads has been introduced in the existing structure or deterioration of the structure. In literature reviews, last two decades, different researcher had been carried out research on this field. The shear and
Journal of Engineering and Applied Science
Contents are available at www.jeas-ruet.ac.bd
34 flexural strength of structural members were enhanced using Fiber reinforced polymer (FRP) sheets by Alagusundaramoorthy [7] and Esfahani [8]. Many investigations have been performed on flexural repair and strengthening of damaged RC beams using CFRP materials [9-13].
The shear regions are vulnerable to failure caused by the quite common occurrence during earthquake as well as in case of significant point load application. Shear strengthening on deep beam and T-beam conducted by Zhang [14]
and Bousselham and Chaallal [15].Use of FRP composites for retrofitting of RC beams after shear collapse is investigated by Donchev[16].Using CFRP, seismic retrofit of shear-critical RC Beams was conducted by Colalillo and Sheikh [17] and Lombard [18]. A study about the behaviour of concrete beams reinforced with carbon FRP stirrups was carried out by Ahmed [19] and Lorenzis and Nanni [20]. Soliman [21] studied experimental and numerically on RC beams strengthened in bending with near surface mounted CFRP.
Flexural behaviour of reinforced concrete beams strengthened by CFRP were conducted by Esfahani et al. [22] and Kotynia [23]. Using FRP laminates at the tension face of the beam, flexural and shear strengthening of reinforced concrete beams was first introduced by Meier’s group [24]. Since then, wide-ranging experimental and analytical investigations (Colalillo and Sheikh [17]; Saxena [3]; Choi [11]; Nitereka and Neal [12]; Brena [13]; Bonacci, and Maalej [2]) Nordin et al. [25] have been carried out all over the world on flexural strengthening of concrete beams.
The efficiency of different FRP and different adhesives on RC beams, aluminium and stainless steel was investigated by Islam and Young [26-28]. Lorenzis and Nanni [29] investigated bond between near-surface mounted fiber- reinforced polymer rods and concrete.
However, a little research has been conducted on application of CFRP materials for strengthening and retrofitting of RC beams for flexural and shear failure. Therefore, it is an innovative approach to study on strengthening and retrofitting of RC beams by FRP for flexural and shear failure. A series of test was conducted in this research for CFRP strengthen beam under three points and four points loading by universal testing machine. The shear and flexure regions of RC beam has been strengthened and retrofitted by CFRP materials to observe the failure mode, deflection, extra load carrying capacity for its different sizes. Based on test results, it was found that the CFRP strengthen and retrofitting of RC beam provided better performance significantly. The research concludes that the CFRP materials can be apply for strengthening and retrofitting of RC beam effectively for better performance against shear and flexure.
2. Field applications of CFRP in civil engineering structures
Bridges, buildings, tunnels, chimneys, electric poles, and box culverts are the main fields for application of CFRP.
But applications of CFRP in bridges and buildings have been getting more importance [4]. Now a day’s tunnel lining also can be the important field of application of CFRP. Potential benefits of using CFRP in construction industry can be attain by strengthening concrete, masonry, steel, cast iron, and timber structures. The existing structures are strengthened by retrofitting or alternative reinforcing (or pre-stressing material) instead of steel from the outset of a project in the industry [25]. The common use of CFRP strengthening in bridge piers, girder, plates and building beams, columns and plates.
Approximately 25% of the bridges in the world are functionally obsolete or structurally deficient. The functionally obsolete or structurally deficient bridges can still provide services but it requires some form of maintenance or major rehabilitation to make them functional or restore them to their original condition to have their original load carrying capacity [1, 2, 3, 16]. Retro-fitting is popular technique in many instances. The strengthening cost using CFRP is comparatively lower than the cost of replacing the deficient structure. The CFRP composite deck systems help to extend service life of existing structures. Using CFRP in deck make it lightweight, high strength and high performance.
It also resists chemical corrosion and makes the construction easy to handle and high-quality shop fabrication [2].
Resistance to collapse under seismic actions is increased by wrapping bridge sections. Such ‘seismic retrofit’ is the major application in earthquake-prone areas and it is much more economic than alternative methods of retrofitting [17, 18]. Concrete piers, pier caps, concrete arch and cracked beams are retrofitted by fiber wrapping systems.
CFRP laminates, rods and wet lay-up fabrics is also a very popular repair technique for bonded concrete repair. Many concrete and composite bridges are being strengthening by surface mounted strengthening and repair applications [9, 23, 25, 26, 29]. This method is cost effective, design simple, install and inspect.
The existing aged chimneys require some repair and strengthening to keep it operation. The chimney is needed protect from high temperatures, chemical actions, and structural reliability. Generally, it should have low life cycle cost and needs low maintenance cost. CFRP chimney liners can meet the above requirements normally. Application of Carbon
35 Fiber composites for retrofitting Beam, Column and Slab is very important. Bending strength of flexural reinforced concrete members improves for addition of carbon [21]. Beams are strengthened by pasting CFRP plates at the bottom and U shaped around the sides and bottom. Both techniques increase the strength of beam, deflection capacity of beam, shear resistance and stiffness [19,20]. The lateral deflection of column can be reduced by wrapping with CFRP and higher strength attained. The tensile strength resistance of slab increases by applying CFRP strips at tension face.
CFRP strips helps to perform in better way.
FRP is used for mitigating the brittle failure mechanisms, shear failure of beams and/or columns, and lap splice failure.
Field application of flexural and shear strengthen using CFRP is shown in Fig. 1. The buckling of column and the longitudinal bars used in the column is minimized by applying the FRP and it helps to enhance the overall deformation and energy dissipation capacities of the concrete structure and develop its global behavior. Due to the resistance to corrosion, FRP composites can be employed on interior and exterior structural members in different of environments [4].
Figure 1: Shear strengthening using CFRP [4].
Table 1: Tensile properties of FRP [30]
Sample Reference FRP materials
Specimen Reference Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
Width (mm) 14.39 13.85 14.67 14.82 14.42
Thickness (mm) 0.164 0.167 0.167 0.167 0.167
Maximum load (N) 9217.16 10025.62 9711.69 9650.89 10807.13
Tensile strength (Mpa) 3800 4330 4730 3900 4490
Elongation at break (%) 1.74 1.86 1.70 1.73 1.88
Elastic modulus (Mpa) 257000 246000 267000 253000 242000
Mode of failure MMV MMV XVV XVV XVV
* Mode of failure: MMV-Multi-mode, Multi areas, Various; XVV- Explosive, Various, Various
During the construction Bongobondhu multipurpose bridge over Jamuna river some cracks are developed at the approach viaduct, top of pier head, surface of deck, bottom face of soffit, pier and girder connection, 6E segment adjacent to the hinge segment, pile cap shell and the pile stem. Jamuna Multipurpose Bridge Authority (JMBA) had taken actions to repair it immediate by the Department of Civil Engineering, Bangladesh University of Engineering and Technology (BUET) [5]. FRP and adhesive test were conducted in Singapore for repair of cracks inside the box girders and strengthening of the superstructures of the of Bongobondhu Jamuna Multipurpose Bridge [30]. Lab test of FRP and adhesive was conducted in Singapore for crack repairing of Bongobondhu Jamuna Multipurpose Bridge.
The mechanical properties of FRP and adhesive for using crack repairing of Bongobondhu Jamuna Multipurpose Bridge are shown in Table 1 & 2 respectively.
36 Table 2: Tensile Strength of Adhesive [30]
Sample Reference Adhesive material
Specimen Reference Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6
Width (mm) 13.40 13.03 13.19 12.98 12.91 13.29
Thickness (mm) 3.39 3.66 3.22 3.22 3.35 3.22
Cross sectional area (mm2) 45.43 47.80 41.99 41.80 44.52 41.57 Maximum load (N) 1648.26 2023.71 1708.25 1747.57 1975.12 1727.95 Tensile strength (Mpa) 36.30 42.30 40.70 41.80 44.40 41.60 3. Methodology
3.1. Material properties
FRP has three main components of Carbon (c), glass (g) or are mid (a) fibers. All components are bonded together in a matrix of epoxy, vinyl ester or polyester. The load carrying capacity component is fibers in FRP whereas the plastic, the matrix material, transfers shear. Structural strengthening and rehabilitation are done by using the strips, sheets and laminates of FRP products [10].
3.1.1. Material properties of FRP
Retrofitting and strengthening of concrete structures are mainly performed by three are different types of FRP.
Different types of FRP components are symbolized by ‘a’ to ‘f’ shown in Table 3. The detailed material properties of each FRP provided in the specifications are also listed in Table 3.
Table 3: Mechanical properties of FRP given by manufacturer specifications [27]
Types of FRP Symbol t u Eo f
(MPa) (MPa) (GPa) (%)
Sika Wrap-300C/60 (CFRP) a 0.166 3900 230 1.50
Sika Wrap-430G/25 (GFRP) b 0.172 2300 76 2.80
Tyfo UC laminate (Laminate Plate) c 1.400 2790 155 1.80
Sika CarboDur S1214 (Laminate Plate) d 1.400 3100 165 1.70
Sika CarboDur M614 (Laminate Plate) e 1.400 3200 210 1.35
Sika CarboDur H514 (Laminate Plate) f 1.400 1500 300 0.45
3.1.2. Material properties of adhesive
External bonded FRP strengthening depends on the properties of adhesive. The key mechanical properties of adhesive for strengthening structures are effective bond strength, elastic modulus, and elongation. Sika 330, Sika 30, Tyfo TC, Araldite 2011, Araldite 2015, and Araldite 420 are the six different types of adhesive, which are symbolized as ‘A’ to
‘F’ in alphabetical order as shown in Table 4. The outer surface of web of the aluminum sections are bonded by two component adhesives of FRP. The different adhesives had a pot life of 30-45 minutes at room temperature and were fully cured after 7 days at room temperature. The main material properties of these six different types of adhesive are measured by Tensile coupon tests. The measured dimensions of the adhesive tensile coupons are given in Table 4.
The mould of the adhesive coupon specimens, test setup and the stress-strain curves are shown in Fig. 2. Six different adhesives tensile coupon test specimens are shown in Fig. 3. The failure pattern of the adhesive is shown in Fig. 4.
The materials properties determined from the coupon tests and typical stress-strain curves for each adhesive are presented in Table 2 and Fig. 5 respectively. The rages of Elastic modulus from 1.6 to 11.6 GPa and tensile strain at fracture from 0.004 to 0.045 based on 50 mm gauge length from Table 2.
37 Sika CarboDur H514 laminate plate and adhesive Araldite 2015 was used for the strengthening of the aluminum tubular sections in the second and third phases of the investigation. The high modulus CFRP Sika CarboDur H514 laminate plate and adhesive Araldite 2015 was symbolized as ‘F’ and ‘E’, respectively.
Figure 2: Dimension of mould of adhesive tensile coupon test specimen
Figure 3: a) Six different adhesive tensile coupon test specimen; and b) Adhesive tensile coupon test
Figure 4: Failed specimen of adhesive tensile coupon test
(a) (b)
38 Figure 5: Stress-strain curves of different adhesives obtained from tensile coupon tests
Table 4: Measured dimensions and material properties of adhesives obtained from tensile coupon tests.
Types of adhesive
Symbol
bc tc Ac u Eo f
(mm) (mm) (mm2) (MPa) (GPa) (%)
Sika 330 A 9.83 4.609 45.31 31.8 4.6 0.8
Sika 30 B 9.82 4.722 46.37 22.0 11.6 0.4
Tyfo TC C 9.69 4.481 43.42 19.6 2.3 1.3
Araldite 2011 D 9.96 4.606 45.86 23.1 1.6 4.5
Araldite 2015 E 9.52 4.461 42.47 19.7 1.8 3.3
Araldite 420 F 9.58 4.594 44.01 24.3 1.6 3.2
3.2. Properties of FRP and adhesive for this research
External bonded strengthening highly depends on the properties of adhesive and FRP. The main mechanical properties of adhesive for strengthening structures are effective bond strength, elastic modulus, and elongation. CFRP materials are composite materials that typically consist of fibres embedded in a resin matrix. Epoxy resin is the most widely used resin. CFRP could be more than 10 times higher strength than aluminium and stainless-steel material [25]. Fig.
6 represents the CFRP sheet, epoxy adhesive ( primer & saturant). Typical tensile properties of CFRP materials and epoxy adhesives are presented in Table 5 and Table 6 respectively.
3.3. Test specimen
A series of concrete beam made for testing to meet the main aim of this research. A total of ten beams were tested for this research. Five beams were tested for shear region failure and the other five beams were tested for flexure region failure as shown in Fig. 7. Among five beams which were subjected for shear failure, one beam was reference beam and the other four beams acted as test beam. Same procedure was followed for the other five beams tested to measure the flexure failure. The cross section of all the beams tested for shear and flexure failure were identical 6ʺx 6ʺ but the
0 5 10 15 20 25 30 35
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Strain (%)
Stress (MPa)
Sika 330 (A) Sika 30 (B) Tyfo TC (C) Araldite 2011 (D) Araldite 2015 (E) Araldite 420 (F)
B A
C
F D
E
39 length of the beam was different. The beams tested for shear failure were 30ʺ in length but the beams tested for flexure failure were 72ʺ in length.
Figure 6: CFRP sheet and epoxy adhesive (primer & saturant).
Table 5: Properties of CFRP
Table 6: Properties of epoxy adhesive.
Aspect Free flowing fluid
Mixing Ratio by Weight 100 (Base): 50 (Hardener)
Bond Strength to Concrete >1 Mpa or concrete failure
Pot Life 40 minutes @ 25 Celsius
Tack Free Time 6 hours @ 25 Celsius
(a) (b) (c)
Figure 7: (a) Specimen for shear test; (b) Specimen for flexure test; and (c) Cross section view of specimen
Fiber -Reinforcement Carbon-High modulus
Ultimate Tensile Force @ 0.2% strain/m width 200 KN
Fiber Modulus 640 Gpa
Ultimate Tensile Strength 2650 Mpa
Ultimate Tensile Elongation (Strain) 0.4 %
Fibre Density 2.1 gm per cubic cm
Thickness 0.19 mm
30" 72"
6"x 6"
40 All four faces of the beams were cleaned properly and painted with white chalk powder to identify the cracks clearly.
The casing area was cleaned and made rough manually before the application of epoxy adhesive. Primer (MBRACE PRIMER) was applied first in the case area and then saturate (MBRACE SATURANT) was applied. After that CFRP was glued to the casing area and kept for 7 days to dry the adhesive and have enough bonding of CFRP with the beam casing area. The FRP surface cleaning, smoothing and application of epoxy adhesive to CFRP to the selected area is shown in Fig. 8.
Figure 8: (a) Surface cleaning & roughing, (b) Application of epoxy adhesive; and (c) Application of CFRP 3.4. Test procedure
Ten reinforced concrete beams were constructed, one control beam and eight beams that were strengthened using externally bonded CFRP. Shear and flexure failure testing were conducted according to ASTM standard. The schematic view of three and four-point loading is shown in Fig. 9.
(a) (b)
Figure 9: (a) Schematic view of four points loading, and (b) Schematic view of three points loading The lab test set up of three and four-point loading conditions are shown by photographs in Fig. 9. A servo-controlled hydraulic universal testing machine was used to apply a concentrated compressive force to the test specimens. Dial gauge was attached/fixed to measure the beam deflection. The deflection with corresponding load was recorded for each case and load-deflection curve using the obtained data was plotted.
(a) (b)
Figure 10: Test setup for (a) Actual four-point loading test setup; and (b) Actual three-point loading test setup
(a) (b) (c)
41 4. Results and Discussions
The test results of RC beam strengthening by FRP at flexure and shear region is depicted in this section. The failure mode of shear test was observed both reference beam and CFRP strengthened beam as shown in Fig. 11. Based on the observation, the cracks appeared on both end of the support in case of shear test. The cracks propagated inward with the increasing load and at a certain point there was no significant increase in load carrying capacity and thereby the beam failed finally. For the test sample one end of the beam near the support was CFRP treated and the other end was as before. It was noticed that the CFRP treated end was crackles even though the untreated end of the beam gone through failure.
(a) (b) (c)
Figure 11: (a) Failure mode without FRP, (b) Failure mode of beam with FRP and (c) Crack of beam with FRP The failure mode for flexure test was observed that failure crack was appeared on mid span as shown in Fig.12. With the increasing of load the cracks propagated inward and at a certain point there was no significant increase in load carrying capacity and thereby the beam failed afterwards. For the test sample the crack moved from centre to the end of CFRP treated area. FRP treated area was crackles and crack initiated from where CFRP treated zone ends. It can be concluded that the increase the CFRP treated zone, the crack can be shifted to end of CFRP end.
(a ) (b)
Figure12: (a) Failure mode of beam without FRP; and (b) Failure mode of beam with FRP
Load-deflection comparison curve for shear failure test is shown in Fig. 13. In the comparison graph, blue line refers to the reference beam and the red line refers to the test beam with FRP. The highest peak point of red line indicates that the load carrying capacity of FRP treated beam is higher than the reference beam corresponding to respective deflection till failure. The load carrying capacity FRP strengthened beam was higher that reference beam without FRP beam.
Load-deflection comparison curve for flexure failure test is shown in Fig.14. In flexure, the load carrying capacity of FRP strengthened beam was higher that reference beam without FRP beam. From the graphical presentation, it was found that load carrying capacity was increase and deflection was reduced due to FRP strengthening. From the test results, it was found that the flexural strength and stiffness of the strengthened beams increased compared to the without strengthen specimens.
Crack initiating from mid-span
Crack has been shifted from mid- span because of CFRP
42 Figure 13: Load-deflection comparison curve for shear failure test.
Figure 14: Load-deflection comparison curve for flexure failure test.
Table 7: Maximum load deflection due to flexural and shear test.
Specimen
Flexure test results Shear test results Max Load
(kN)
Max Deflection (Inch)
Max Load (kN) Max Deflection (Inch)
Beam without CFRP 28.22 0.17 57.51 0.12
Beam 1 with CFRP 38.50 0.13 76.90 0.15
Beam 2 with CFRP 39.80 0.16 75.25 0.15
Beam 3 with CFRP 40.02 0.14 77.09 0.15
Beam 4 with CFRP 42.56 0.16 72.10 0.15
0 10 20 30 40 50 60 70 80 90
0 0.05 0.1 0.15
Load ( kN )
Deflection (inch) Reference Beam
Beam with frp Sample 1 Beam with frp Sample 2 Beam with frp Sample 3 Beam with frp Sample 4
0 10 20 30 40 50
0 0.05 0.1 0.15 0.2 0.25
Load ( kN )
Deflection (inch) Beam without FRP
Beam with frp Sample 1 Beam with frp Sample 2 Beam with frp Sample 3 Beam with frp Sample 4
43 Maximum load -deflection relationship due to flexural and shear test is shown in Table 7. Data obtained from shear and flexural test which gives a comparison between reference beam and CFRP treated test beam. Comparison shows that the load carrying capacity of FRP treated beam has increased though the deflection also reduced most of the cases.
The load carrying capacity was increased about 33.63% while the beam was strengthened with FRP for shear. Data obtained from flexure failure test shows that for both case of four sample of test beam the recorded deflection was smaller than the reference beam and the load carrying capacity for flexure also increased by 36.43% for test beam sample 1, 41.03 % for test beam sample 2, 41.81 % for test beam sample 3 and 50.82 % for test beam sample 4. The percentage of increase in load carrying capacity for shear varied from 25-34% for all four specimens. FRP strengthening enhancement trends are almost similar to previous results at flexural and shear region that is indicate that test results were satisfactory. The effectiveness of this FRP and adhesive is proven as a power full technique for strengthening and retrofitting of RC beam.
5. Conclusions
Experimental investigation on FRP strengthening of reinforced concrete beam at flexure and shear region has been presented in this study. A series of concrete beam specimens were tested under three point and four-point conditions the flexural and shear strengthen behaviour of reinforced concrete beams were investigated. The failure loads, failure modes and the load-web deformation behaviour of CFRP strengthened concrete beam are also presented. The FRP wrapping is the method with the minimum deflection recorded which indicates that the shear stiffness at this case is highest. The FRP wrapping were the easiest method to apply; it is less expensive than the FRP laminated plate and with relatively small deformability of the beam. In both cases, the shear and flexure failure test the load carrying capacity was increased. The percentage of increase in load carrying capacity varied from 25-34% for shear and 36- 50% for flexure. The FRP was not subjected to full of its load carrying capacity because before the FRP was subjected to full of its capacity debonding of FRP or crushing of concrete occurred. This percentage of increase in load carrying capacity can be increased and effectively used for concrete structure. The test result was promising enough to apply FRP not only for strengthening but also for retrofitting. The effectiveness of the applied methods is estimated qualitatively and proved that analysed types of FRP shear and flexural strengthening are effective for repairing of damaged (pre-cracked) reinforced concrete beams.
Acknowledgements
The authors are grateful to the Rajshahi University of Engineering and Technology (RUET), Rajshahi-6204, Bangladesh, The University of Hong Kong and Bangladesh Bridge Authority for supporting the test program. Also sincere thanks to the technicians and laboratory attendants for the assistance and co-operation while performing the test in laboratory.
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