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Experimental study of the behaviour of RC corbels strengthened with CFRP sheets
Case Studies in Construction Materials 9 (2018) e00181
Contents lists available at ScienceDirect
Case Studies in Construction Materials journal homepage: www.elsevier.com/locate/cscm
Case study
Experimental study of the behaviour of RC corbels strengthened with CFRP sheets Yaman Sami Shareef Al-Kamaki*, Gulan Bapeer Hassan, Gehan Alsofi University of Duhok, College of Engineering, Civil Department, Duhok, KRG, Iraq
A R T I C L E I N F O
Article history: Received 7 May 2018 Received in revised form 18 June 2018 Accepted 20 June 2018 Keywords: Short corbel Strengthening Fibre reinforced polymers (FRP) composites Reinforced concrete Experimental results
A B S T R A C T
This paper presents an experimental study of behaviour of externally strengthened short reinforced concrete (RC) corbels by carbon fibre reinforced polymer (CFRP) fabrics. For this purpose, twelve specimens were prepared and tested. The study inspected the effect of some parameters on the structural behaviour of corbels. The parameters included: the amount of internal secondary steel bars and external composite sheets configurations. The ultimate load obtained from static load resulted in up to 27% increase in the load bearing capacity through external CFRP composite reinforcement compared to control samples. The diagonal 45 CFRP reinforcement constrained widening and growth of the shear cracks, and hereafter, improved the gain in the load capacity and axial toughness. The addition of secondary reinforcement at the mid-height of the corbels produced additional strength increase. Finally, the cracking and failure pattern modes of corbels are presented. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
1. Introduction Short reinforced concrete (RC) cantilever members with shear span/depth ratio (a/d) less than 1.0 are called corbels (brackets). Such members are used to transfer vertical and horizontal forces to principle members. Mainly corbels are used to transfer eccentric loads over the wall or column elements. However, horizontal force must be considered in the design to restrained shrinkage, creep or temperature change, unless special precaution is taken in their consideration. Corbels cannot be adequately designed using beam theory concept due to discontinuity in load and geometry [1–7]. The failure mechanisms (crack pattern failure) of corbels can be classified in 6 basic modes: concrete crushing on the strut, bending, shear at the interface between the column and the corbel, loss of anchorage of the main tie (stirrups) reinforcement, concrete crushing under the steel loading plate (bearing pad) and horizontal load [6,8–13]. Kris and Rath [8] carried out the first pioneering experimental and analytical work on the behaviour of RC corbels using normal strength concrete (NSC). The following parameters have been taken into consideration: reinforcement ratio, shearspan to depth ratio, concrete strength and applied load [vertical load only or combined vertical and horizontal loads]. The authors concluded that the tension reinforcement and horizontal stirrups have the same effect on corbels strength increase when subjected to vertical load only, while the horizontal load reduce their strength significantly. Depending on the same parameters taken by Kris and Raths [8], Mattock et al. [14] reported that minimum amount of horizontal stirrups should be provided to avoid diagonal tension failure and to permit the main reinforcement to reach to the yield strength.
* Corresponding author at: University of Duhok (UoD), College of Engineering, Civil Department, Duhok, KRG, Iraq. E-mail address: [email protected] (Y.S.S. Al-Kamaki). https://doi.org/10.1016/j.cscm.2018.e00181 2214-5095/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).
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Series of researches reported to investigate the effect of adding different types of steel fibres reinforced (SFR) to concrete corbels [15–20]. Earlier, Fattuhi [16,17] indicated that flexural, shear strength and ductility of the corbels improved by the addition of fibres or stirrups. It was also concluded that the use of stirrups needed the complex reinforcement detailing while the fibres eliminated this. Hughes and Fattuhi [18] investigated that in addition to that SFR improved the capacity of tested corbels and the mode of failure changed from diagonal splitting to flexure when fibres with efficient bond characteristics were used. Generally, there are many causes may be responsible for the need to strengthen of existing structures by FRP materials, deterioration and aging, inadequate maintenance, design / construction errors, revisions of code requirements, change in the facility use and natural or human caused fires [21]. In the last two decades, different types of fibre reinforced polymers (FRPs) composites were utilized in upgrading structural members to increase their capacity. That is because of their high tensile strength, light weight, resistance to corrosion, high durability, and ease of installation. For example, the retrofitting of the concrete approach viaducts of the West Gate Bridge in Melbourne, is the largest FRP strengthening program in the world to date [22]. Moreover, numerous strengthening steel bridges projects have been undertaken around the world, including the Ashland bridge in the USA and the Acton bridge in England were rehabilited by applying CFRP composites to the bottom of the girders [23]. However, available research on the behaviour of FRP strengthened corbels are limited [24,25]. Campione et al. [26] studied experimentally and analytically the flexural behaviour of RC corbels by comparing the effect of traditional steel reinforcement with externally wrapped carbon fibre reinforced polymers (CFRP) and SFR. They stated that by adding SFR or CFRP sheets, the flexural capacity of corbels was increased. Elgwady et al [2] reported experimentally, that the load carrying capacity of the corbels was increased by using different strengthening configuration of CFRP fabrics. The failures were brittle and sudden without adequate warning due to increase the stiffness of the corbels. It was also concluded that the all corbels were failed due to debonding of CFRP strips and spalling of concrete cover. Ozden and Atalay [27] investigated the strength and post-peak performance of using glass fibre reinforced polymer (GFRP) in strengthening corbels subjected to vertical load only. The main parameters were shear span / effective depth ratio, ratio of main reinforcement, layers number and changing the orientation of GFRP. Findings concluded that GFRP wrapping with 45 fibre orientation was more active than lateral wrapping. The strength gain increased due to reinforcement ratio and number of GFRP layers increase. Anis and Muhammad [28] studied the effect of CFRP strips on behaviour of rehabilitated RC corbels. The variables included in this study were location, direction, amount of CFRP strips and the shear span / effective depth ratio (a/d). While the dimension, flexural reinforcement ratio and without shear reinforcement were fixed for all specimens. It is concluded that the capacity of the corbels was significantly influenced by the direction of CFRP strips, the inclined strips increase the capacity more than the horizontal strips. Ivanova et al. [24] present a study on performance of externally strengthened short RC corbel by CFRP fabrics a horizontal form (front and rear face) and in a wrapping form. The test results stated that the attached fabrics on the tensile face of the corbel have a greater effect. The aim of the experimental program in this paper is to investigate the influence of changing the orientation of CFRP on the behaviour and bearing capacity of RC corbels with or without the presence of stirrups.
Fig. 1. Geometry of corbels.
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2. Experimental program 2.1. Materials and detail of reinforcement The RC corbels were made using ready-mixed normal strength concrete (NSC) arranged by a local provider. Casting process was carried out using local available materials. Maximum size of coarse aggregate was 12.5 mm and specific gravity of 2.6 and 2.7 for sand and gravel respectively. The column segment of the corbels was reinforced longitudinally with 4 Ø 12 mm deformed bars, and Ø 6 mm stirrup placed 100 mm c/c. Two deformed bars with Ø 10 mm were used as primary reinforcement and the end portion were bend to ensure an adequate anchorage length. Only six corbels were reinforcing with Ø 8 mm deformed bars as a secondary reinforcement (stirrup), see reinforcement no. 2 shown in Fig. 1. Details of reinforcement mechanical properties are shown in Table 1. Unidirectional woven CFRP fabric from Sikawrap-230 C were used as strengthening materials. CFRP roll was of 600 mm width and 0.131 mm fabric design thickness (based on fibre content). The epoxy resin (epoxy adhesive) used to bond the CFRP to the concrete surface was from Sikadur-330 type consisting of two components of resin and hardener (A:B) ratio of 4:1. Table 2 shows the mechanical properties of CFRP and epoxy resin provided by the manufacturer. 2.2. Specimen dimensions A total of twelve identical RC corbel specimens were cast, six of which contained horizontal stirrups reinforcement and six without. Two short cantilever beam (corbel) connected to column measuring (200 150 150) mm, column crosssection dimensions were (150 150) mm with total height of 450 mm. The geometry of all corbels was same as shown in Fig. 1. As there were identical corbels in each group, only one representative corbel from each pair was taken for comparison. Table 3 contains the details of the RC corbels which divided into three groups.
Table 1 Details of mechanical properties.
f (mm)
fy (MPa)
fu (MPa)
eu (%)
8 10 12
742 518.5 537
810 617.3 660
11.7 21.2 17
Table 2 Material manufacturer’s characteristics of CFRP sheet & adhesive epoxy. Characteristics
CFRP sheet 0.131 mm thickness
Adhesive epoxy
Ultimate tensile strength (MPa) Elastic modulus (GPa) Tensile strain capacity %
4300 238 1.8
30 3.8 0.9
Table 3 Details of RC corbels. No.
Group
Corbel Symbol
CFRP Layers
Description
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12
1
CN-WO CN-WO CN-W CN-W INC-WO INC-WO INC-W INC-W HOR-WO HOR-WO HOR-W HOR-W
0 0 0 0 1 1 1 1 1 1 1 1
Control corbels without horizontal stirrups
2
3
C = corbel, CN = control, WO = without, W = with, INC = inclined, HOR = horizontal.
Control corbels with horizontal stirrups Corbels with 1 inclined CFRP layer & without horizontal stirrups Corbels with 1 inclined CFRP layer & with horizontal stirrups Corbels with 1 horizontal CFRP layer & without horizontal stirrups Corbels with 1 horizontal CFRP layer & with horizontal stirrups
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Fig. 2. Shows various aspects of the pouring operations.
Fig. 3. Corbels CFRP configurations.
2.3. Casting operation The molds were made of wood and they were placed horizontally, oiled then the concrete was poured, all corbels were cast at the same time along with six standard cylinders, three cylinders to measure the compressive strength of concrete and three for splitting test. Each corbel was cast by three concrete layers and each layer was vibrated about 20 s and the final layer was vibrated until no air bubble appear on the concrete surface and the surface troweled to a smooth finish. After 24 h, the corbels were demolded and covered by wet cloth for about 28 days while the cylinders were kept in curing tank for the same period. Fig. 2 shows the molds, the steel reinforcement cages and specimens after pouring operations. 2.4. Bonding of CFRP to corbels After curing time, the specimens were strengthened with one layer and with two different CFRP orientation (i.e. two configurations), horizontal and diagonal 45 configurations as shown in Fig. 3. Prior to bonding CFRP to the specimens, the corners of all specimens were rounded then the locations were wrapped by CFRP cleaned from dirt. CFRP was cut to the required width and length and the surface of CFRP strips were kept clean. A thin coat of a prime was applied to the clean corbel surface and allowed to cure for at least of one hour. Resin and hardener were weighed as ratio (4:1) due to manufacturer recommendations and mixed until the color was gray then applied to both concrete surface and CFRP strips. Finally, CFRP strips were fixed to their place by brush. 2.5. Testing technique The corbels were tested to their ultimate load (Vu) by Universal computerized testing machine Walter + Bai AG/ Switzerland type, 3000 kN capacity as shown in Figs. 4 and 5 respectively.
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Fig. 4. Detail of corbel supports and measurement instrumentations.
Fig. 5. View of specimen showing the measurement units.
The test samples were inverted column with two corbels and the corbels were seated on steel roller supports. The position of the support from the faces of the column were fixed (a = 110 mm) yielding a/d = 0.846. The displacement in each corbel was measured by using linear variable differential transducer (LVDTs reading accuracy of 0.001 mm) supported as shown in Figs. 4 and 5 respectively. Mechanical foil strain gauges were used to measure strain, one strain gauge was placed on one of the two main bars near to the column-corbel junction. As for the stirrups, the strain gauges were mounted at the corbel column interface. It should be noted that strain gauges were also used on strengthened samples and placed on the diagonal line from centre of support to the column-corbel junction to the direction of CFRP. All the specimens were loaded monotonically in small increment (1 kN/s) up to ultimate load. At each increment, the readings were recorded and the development of cracks were observed and marked later on the specimens.
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Table 4 Test results of ultimate load, ultimate deflection and ultimate strains. No.
Group
Corbel Symbol
Pu kN
Aver. du (mm)
eu in steel micron
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12
1
CN-WO CN-WO CN-W CN-W INC-WO INC-WO INC-W INC-W HOR-WO HOR-WO HOR-W HOR-W
90.65
1.33
3557
95.45
1.56
93.45
1.82
6041
115.35
1.25
8435
104.25
1.22
115.1
1.31
2
3
eu in CFRP micron 5 cm
3470 4451
C = corbel, CN = control, WO = without, W = with, INC = inclined, HOR = horizontal.
3. Results and discussion In the following sections, the observed behaviour and the failure mode results of RC corbels will be discussed. The specimen test results are listed in Table 4 in the terms of ultimate load, the corresponding deflections and foil gauges strain measurements. By comparing Table 4 with Table 2, it is obvious that the strain in the CFRP at failure is typically less than the strain data provided by the manufacturer. This could be due to the difference in testing a single fibre (manufacturer’s data) and testing a practical specimen. The variation in such data could also be due to the fact that the strain were gained using conventional strain gauges. In some cases, the foil gauges de-bonded prior to failure, and these data points are therefore being lost. 3.1. Load versus deflection response Load-deflection curves for tested representative pairs of corbels described in Table 4 are shown in Fig. 6. The curves are for both control and CFRP bonded specimens with 1-layer. Form each group of corbels labeled in Table 4, two corbels were reinforced with longitudinal steel bars in addition to stirrups. The average experimental deflections of the samples were taken from LVDTs movements under load relative to supports. From the trend of the relationships it can be detected that the overall response of the corbel was quite comparable in terms of both maximum load, initial stiffness and the failure. The first segment is very steep, tracked by a kink. A reasonable increase in the fracture toughness, ductility and ultimate strength resulted when strengthen corbels with CFRP sheet. 3.2. Ultimate load capacity and ductility enhancement due to CFRP reinforcement Table 4 shows the results for the ultimate capacity of the tested corbels. It can be seen from the table that the maximum enhancement in the ultimate load capacity of RC corbels is achieved when external CFRP overlays are applied. CFRP can increase the ultimate load, ductility and enhances the stiffness of RC corbels. The level of improvement depends on the orientation of CFRP fabrics (configuration) and presence of main bars plus stirrups. In general, the ultimate load was larger in corbels strengthened with CFRP with the presence of stirrups than those of unstrengthen corbel specimen (control corbel). The tougher corbel failed at an ultimate load of 115.3 kN with an increase in strength of about 27% compared to control specimen, see Fig. 7. It can be noticed that the percentage of increase for inclined configuration is almost similar (i.e. 27%) to horizontal scheme compared to control corbel. This is almost resulted from insufficient CFRP overlap, but if sufficient overlap was provided, the enhancement rate will be higher. The axial toughness exhibited by a RC corbel can be obtained to be reflected by the area under the respective loaddeflection curves. These are shown in Fig. 8 for all tested specimens and were calculated using Grapher [29] software. The Figure shows that there is a steady increase in axial toughness with increasing CFRP layers with the presence of stirrups. It also illustrates that the presence of CFRP improves the axial toughness significantly with the presence of stirrups. In general, the axial toughness for inclined CFRP configuration with only one layer of CFRP is higher than that of the horizontal outline. 3.3. Effect of secondary reinforcement In the presence of transverse reinforcement (stirrups), finer cracks formed and the ultimate load capacity measured was higher than that of corbels with main steel reinforcement only, see Fig. 7. Because of the bridging action of transverse reinforcement against the principal cracks, more ductile behaviour was observed. Test results revealed that providing stirrups increase the level of load enhancement of corbels in ranges between 6% and 24%, for both control CFRP overlay attached corbels.
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Fig. 6. Load-deflection curves.
Fig. 7. Ultimate load enhancement.
3.4. Effect of CFRP orientation Test results revealed that the level of strengthening of corbels with CFRP sheets ranges between 13% and 17% (see Fig. 7), depending on the orientation of the CFRP material compared with control specimens without and with secondary reinforcement respectively. The resistant mechanism involved in the CFRP bonded corbels was also categorized by the contact phenomena at the interface between the CFRP overlay and concrete surface. The external strengthening of a corbel using CFRP can enhance the corbel capacity when the CFRP is adequately arranged. However, in this study the inadequate overlap was observed and thus the diagonal CFRP configuration increased the ultimate load carrying capacity of the corbel by 27% of the control specimens similar to that of horizontal arrangement.
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Fig. 8. Axial toughness enhancement.
Fig. 9. Crack pattern and failure mode for specimens.
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3.5. Failure modes The modes of failure were identified as follows: For corbels without CFRP strengthening (i.e. control samples), the first predicted bending crack patterns were vertical cracks appearing at the corbel faces close to the column side. The other, major cracks were two diagonal cracks at an angle of almost 45 when the load increased, this crack started to widen and propagate, leading to collapse. For samples strengthened with an externally bonded horizontal strip of CFRP fabric, only one major diagonal crack started at the bearing strip and propagated toward the junction of column and face of corbel. The carbon fibre fabric is broken (fabrics peeling off) in this case, as in Fig. 9. The external CFRP composite limited widening and growth of the flexural crack developed at the junction of the tension face of the corbel and the face of the column. The inclined shear cracks developed in the corbels with external longitudinal composite strengthening were more curved toward the outer surface of the corbels, relative to the cracks developed in the corresponding specimens with internal steel reinforcement only. It was noted that the mode of failure of most tested strengthened corbels was shear failure represented by diagonal tension cracks. In the other word strengthening against diagonal tension failure cannot be fully controled, to prevent shear type of failure. However, such type of failure in strengthened corbels is not differ widely from the flexural failure because the corbel can withstand moderately high deformation before final collapse as discussed before and well observed from the load-deflection relationships. It was also noticed that the provision of secondary reinforcement reduces crack widths, improves ductility, and for beams failing in compression may change the failure mode from diagonal splitting to compression strut crushing. Therefore, they advised the provision of a minimum quantity of horizontal stirrups, similar to that for normal-strength concrete, 4. Conclusions The following conclusions can be drawn based on the results of the present study: 1 2 3 4
There is an enhancement in both load bearing capacity and ductility due to CFRP enhancement. The first segment of load-deflection is very steep, followed by a kink. Test results showed that providing secondary reinforcement increase the level of load enhancement of corbels up to 24%. Due to inadequate CFRP overlap, the diagonal CFRP configuration increased the ultimate load carrying capacity of the corbels by 27% of control specimen similar to that of horizontal arrangement. 5 All wrapped RC corbels failed by tensile rupture of the CFRP wrap.
Conflict of interest None Acknowledgements The authors would like to express their gratitude to the staff of Bekhal Concrete for their assistance. References [1] O.Q. Aziz, Shear strength behavior of crushed stone reinforced concrete corbels, 26th Conference on Our World in Concrete & Structures, Singapore, 2001, pp. 767–775. [2] M.A. Elgwady, M. Rabie, M.T. Mostafa, Strengthening of Corbels Using CFRP an Experimental Program, Cairo University, Giza, Egypt, 2005, pp. 1–9. [3] ACI-318, Building Code Requirements for Structural Concrete American Concrete Institute, (2011) . [4] A.M.A. Hafez, M.M. Ahmed, H. Diab, A.A.M. Drar, Shear behaviour of High strength fiber reinforced concrete corbels, J. Eng. Sci. Assiut Univ. 40 (2012) 669–987. [5] R.M.F. Canha, D.A. Kuchma, M.K. El Debs, R.A.D. Souza, Numerical analysis of reinforced high strength concrete corbels, Eng. Struct. 74 (2014) 130–144. [6] W. Kassem, Strength prediction of corbels using strut-and-tie model analysis, Int. J. Concr. Struct. Mater. 9 (2) (2015) 255–266. [7] L.D.M.A. Attiya, A.A. Mohamad-Ali, Experimental behaviour of reinforced concrete corbels strengthened with carbon fibre reinforced polymer strips, Basrah J. For. Eng. Sci. (2012) 31–45. [8] L.B. Kriz, C.H. Raths, Connections in precast concrete structures: strength of corbels, PCI J. (1965) 16–61. [9] F. Torres, Theoretic-Experimental Analysis of Reinforced Concrete Corbels, Dissertation (Master Science in Structural Engineering)-Engineering School of São Carlos, São Paulo University, São Carlos, (In Portuguese), 1998. [10] G. Russo, R. Venir, M. Pauletta, G. Somma, Reinforced concrete corbels - shear strength model and design formula, ACI Struct. J. 103 (1) (2006) 3–10. [11] A. Yousif, Prediction of ultimate load capacity of high-strength reinforced concrete corbels, Al-Rafidain Eng. J. 17 (4) (2009) 12–27. [12] L.A.G. Yassin, Q.A.M. Hasan, Reinforced concrete Corbels–State of the art, J. Mater. Eng. Struct. 2 (4) (2016) 180–205. [13] D.L. Araújo, S. Azevedo, E. Muniz, E.O. Silva, L. Oliveira Júnior, Strength evaluation of concrete corbels cast in a different stage from the column, Revista IBRACON de estruturas e Materiais 10 (2) (2017) 509–546. [14] A.H. Mattock, K.C. Chen, K. Soongswang, Behavior of reinforced concrete corbels, J. Prestressed Concr. Inst. 21 (2) (1976) 52–77. [15] N.I. Fattuhi, Strength of FRC corbels in flexure, J. Struct. Eng. (United States) 120 (2) (1994) 360–377. [16] N.I. Fattuhi, B.P. Hughes, Ductility of reinforced concrete corbels containing either steel fibers or stirrups, ACI Struct. J. 86 (6) (1989) 644–651. [17] N. Fattuhi, Corbels with shear reinforcement in the form of stirrups or fibres, Proceedings of the 3rd RILEM International Symposium on Developments in Fibre Reinforced Cement and Concrete, University of Sheffield, (1986) paper No. 8.
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Y.S.S. Al-Kamaki et al. / Case Studies in Construction Materials 9 (2018) e00181 B.P. Hughes, N.I. Fattuhi, Reinforced steel and polypropylene fibre concrete corbel tests, Struct. Eng. Lond. 67 (4) (1989) 68–72. N.I. Fattuhi, SFRC corbel tests, ACI Struct. J. 84 (1) (1987) 119–123. N.I. Fattuhi, Reinforced corbels made with plain and fibrous concretes, ACI Mater. J. 91 (5) (1994) 530–536. Y.S.S. Al-Kamaki, Strengthening of Fire-Damaged RC Columns Using CFRP Fabrics, Swinburne University of Technology, Australia, 2015. R. Kalfat, R. Al-Mahaidi, Investigation into bond behaviour of a new CFRP anchorage system for concrete utilising a mechanically strengthened substrate, Compos. Struct. 92 (11) (2010) 2738–2746. M. Kamruzzaman, M.Z. Jumaat, N.H. Ramli Sulong, A.B.M.S. Islam, A review on strengthening steel beams using FRP under fatigue, Sci. World J. 2014 (2014) 702537. I. Ivanova, J. Assih, A. Li, D. Dontchev, Y. Delmas, Experimental investigation into strengthened short reinforced concrete corbels by bonding carbon fiber fabrics, J. Adhes. Sci. Technol. 29 (20) (2015) 2176–2189. J. Assih, I. Ivanova, D. Dontchev, A. Li, Concrete damaged analysis in strengthened corbel by external bonded carbon fibre fabrics, Appl. Adhes. Sci. 3 (1) (2015). G. Campione, L. La Mendola, M. Papia, Flexural behaviour of concrete corbels containing steel fibers or wrapped with FRP sheets, Materials and Structures/Materiaux et Constructions 38 (280) (2005) 617–625. S. Ozden, H.M. Atalay, Strengthening of reinforced concrete corbels with GFRP overlays, Sci. Eng. Compos. Mater. 18 (1-2) (2011) 69–77. A.A. Mohamad-Ali, M.A. Attiya, Experimental behavior of reinforced concrete corbels strengthened with carbon fibre reinforced polymer strips, Basrah J. Eng. Sci. 12 (2012) 31–45. Grapher, Grapher, Golden Software, Golden, Colorado, US, 2009.
Contents lists available at ScienceDirect
Case Studies in Construction Materials journal homepage: www.elsevier.com/locate/cscm
Case study
Experimental study of the behaviour of RC corbels strengthened with CFRP sheets Yaman Sami Shareef Al-Kamaki*, Gulan Bapeer Hassan, Gehan Alsofi University of Duhok, College of Engineering, Civil Department, Duhok, KRG, Iraq
A R T I C L E I N F O
Article history: Received 7 May 2018 Received in revised form 18 June 2018 Accepted 20 June 2018 Keywords: Short corbel Strengthening Fibre reinforced polymers (FRP) composites Reinforced concrete Experimental results
A B S T R A C T
This paper presents an experimental study of behaviour of externally strengthened short reinforced concrete (RC) corbels by carbon fibre reinforced polymer (CFRP) fabrics. For this purpose, twelve specimens were prepared and tested. The study inspected the effect of some parameters on the structural behaviour of corbels. The parameters included: the amount of internal secondary steel bars and external composite sheets configurations. The ultimate load obtained from static load resulted in up to 27% increase in the load bearing capacity through external CFRP composite reinforcement compared to control samples. The diagonal 45 CFRP reinforcement constrained widening and growth of the shear cracks, and hereafter, improved the gain in the load capacity and axial toughness. The addition of secondary reinforcement at the mid-height of the corbels produced additional strength increase. Finally, the cracking and failure pattern modes of corbels are presented. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
1. Introduction Short reinforced concrete (RC) cantilever members with shear span/depth ratio (a/d) less than 1.0 are called corbels (brackets). Such members are used to transfer vertical and horizontal forces to principle members. Mainly corbels are used to transfer eccentric loads over the wall or column elements. However, horizontal force must be considered in the design to restrained shrinkage, creep or temperature change, unless special precaution is taken in their consideration. Corbels cannot be adequately designed using beam theory concept due to discontinuity in load and geometry [1–7]. The failure mechanisms (crack pattern failure) of corbels can be classified in 6 basic modes: concrete crushing on the strut, bending, shear at the interface between the column and the corbel, loss of anchorage of the main tie (stirrups) reinforcement, concrete crushing under the steel loading plate (bearing pad) and horizontal load [6,8–13]. Kris and Rath [8] carried out the first pioneering experimental and analytical work on the behaviour of RC corbels using normal strength concrete (NSC). The following parameters have been taken into consideration: reinforcement ratio, shearspan to depth ratio, concrete strength and applied load [vertical load only or combined vertical and horizontal loads]. The authors concluded that the tension reinforcement and horizontal stirrups have the same effect on corbels strength increase when subjected to vertical load only, while the horizontal load reduce their strength significantly. Depending on the same parameters taken by Kris and Raths [8], Mattock et al. [14] reported that minimum amount of horizontal stirrups should be provided to avoid diagonal tension failure and to permit the main reinforcement to reach to the yield strength.
* Corresponding author at: University of Duhok (UoD), College of Engineering, Civil Department, Duhok, KRG, Iraq. E-mail address: [email protected] (Y.S.S. Al-Kamaki). https://doi.org/10.1016/j.cscm.2018.e00181 2214-5095/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).
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Series of researches reported to investigate the effect of adding different types of steel fibres reinforced (SFR) to concrete corbels [15–20]. Earlier, Fattuhi [16,17] indicated that flexural, shear strength and ductility of the corbels improved by the addition of fibres or stirrups. It was also concluded that the use of stirrups needed the complex reinforcement detailing while the fibres eliminated this. Hughes and Fattuhi [18] investigated that in addition to that SFR improved the capacity of tested corbels and the mode of failure changed from diagonal splitting to flexure when fibres with efficient bond characteristics were used. Generally, there are many causes may be responsible for the need to strengthen of existing structures by FRP materials, deterioration and aging, inadequate maintenance, design / construction errors, revisions of code requirements, change in the facility use and natural or human caused fires [21]. In the last two decades, different types of fibre reinforced polymers (FRPs) composites were utilized in upgrading structural members to increase their capacity. That is because of their high tensile strength, light weight, resistance to corrosion, high durability, and ease of installation. For example, the retrofitting of the concrete approach viaducts of the West Gate Bridge in Melbourne, is the largest FRP strengthening program in the world to date [22]. Moreover, numerous strengthening steel bridges projects have been undertaken around the world, including the Ashland bridge in the USA and the Acton bridge in England were rehabilited by applying CFRP composites to the bottom of the girders [23]. However, available research on the behaviour of FRP strengthened corbels are limited [24,25]. Campione et al. [26] studied experimentally and analytically the flexural behaviour of RC corbels by comparing the effect of traditional steel reinforcement with externally wrapped carbon fibre reinforced polymers (CFRP) and SFR. They stated that by adding SFR or CFRP sheets, the flexural capacity of corbels was increased. Elgwady et al [2] reported experimentally, that the load carrying capacity of the corbels was increased by using different strengthening configuration of CFRP fabrics. The failures were brittle and sudden without adequate warning due to increase the stiffness of the corbels. It was also concluded that the all corbels were failed due to debonding of CFRP strips and spalling of concrete cover. Ozden and Atalay [27] investigated the strength and post-peak performance of using glass fibre reinforced polymer (GFRP) in strengthening corbels subjected to vertical load only. The main parameters were shear span / effective depth ratio, ratio of main reinforcement, layers number and changing the orientation of GFRP. Findings concluded that GFRP wrapping with 45 fibre orientation was more active than lateral wrapping. The strength gain increased due to reinforcement ratio and number of GFRP layers increase. Anis and Muhammad [28] studied the effect of CFRP strips on behaviour of rehabilitated RC corbels. The variables included in this study were location, direction, amount of CFRP strips and the shear span / effective depth ratio (a/d). While the dimension, flexural reinforcement ratio and without shear reinforcement were fixed for all specimens. It is concluded that the capacity of the corbels was significantly influenced by the direction of CFRP strips, the inclined strips increase the capacity more than the horizontal strips. Ivanova et al. [24] present a study on performance of externally strengthened short RC corbel by CFRP fabrics a horizontal form (front and rear face) and in a wrapping form. The test results stated that the attached fabrics on the tensile face of the corbel have a greater effect. The aim of the experimental program in this paper is to investigate the influence of changing the orientation of CFRP on the behaviour and bearing capacity of RC corbels with or without the presence of stirrups.
Fig. 1. Geometry of corbels.
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2. Experimental program 2.1. Materials and detail of reinforcement The RC corbels were made using ready-mixed normal strength concrete (NSC) arranged by a local provider. Casting process was carried out using local available materials. Maximum size of coarse aggregate was 12.5 mm and specific gravity of 2.6 and 2.7 for sand and gravel respectively. The column segment of the corbels was reinforced longitudinally with 4 Ø 12 mm deformed bars, and Ø 6 mm stirrup placed 100 mm c/c. Two deformed bars with Ø 10 mm were used as primary reinforcement and the end portion were bend to ensure an adequate anchorage length. Only six corbels were reinforcing with Ø 8 mm deformed bars as a secondary reinforcement (stirrup), see reinforcement no. 2 shown in Fig. 1. Details of reinforcement mechanical properties are shown in Table 1. Unidirectional woven CFRP fabric from Sikawrap-230 C were used as strengthening materials. CFRP roll was of 600 mm width and 0.131 mm fabric design thickness (based on fibre content). The epoxy resin (epoxy adhesive) used to bond the CFRP to the concrete surface was from Sikadur-330 type consisting of two components of resin and hardener (A:B) ratio of 4:1. Table 2 shows the mechanical properties of CFRP and epoxy resin provided by the manufacturer. 2.2. Specimen dimensions A total of twelve identical RC corbel specimens were cast, six of which contained horizontal stirrups reinforcement and six without. Two short cantilever beam (corbel) connected to column measuring (200 150 150) mm, column crosssection dimensions were (150 150) mm with total height of 450 mm. The geometry of all corbels was same as shown in Fig. 1. As there were identical corbels in each group, only one representative corbel from each pair was taken for comparison. Table 3 contains the details of the RC corbels which divided into three groups.
Table 1 Details of mechanical properties.
f (mm)
fy (MPa)
fu (MPa)
eu (%)
8 10 12
742 518.5 537
810 617.3 660
11.7 21.2 17
Table 2 Material manufacturer’s characteristics of CFRP sheet & adhesive epoxy. Characteristics
CFRP sheet 0.131 mm thickness
Adhesive epoxy
Ultimate tensile strength (MPa) Elastic modulus (GPa) Tensile strain capacity %
4300 238 1.8
30 3.8 0.9
Table 3 Details of RC corbels. No.
Group
Corbel Symbol
CFRP Layers
Description
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12
1
CN-WO CN-WO CN-W CN-W INC-WO INC-WO INC-W INC-W HOR-WO HOR-WO HOR-W HOR-W
0 0 0 0 1 1 1 1 1 1 1 1
Control corbels without horizontal stirrups
2
3
C = corbel, CN = control, WO = without, W = with, INC = inclined, HOR = horizontal.
Control corbels with horizontal stirrups Corbels with 1 inclined CFRP layer & without horizontal stirrups Corbels with 1 inclined CFRP layer & with horizontal stirrups Corbels with 1 horizontal CFRP layer & without horizontal stirrups Corbels with 1 horizontal CFRP layer & with horizontal stirrups
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Fig. 2. Shows various aspects of the pouring operations.
Fig. 3. Corbels CFRP configurations.
2.3. Casting operation The molds were made of wood and they were placed horizontally, oiled then the concrete was poured, all corbels were cast at the same time along with six standard cylinders, three cylinders to measure the compressive strength of concrete and three for splitting test. Each corbel was cast by three concrete layers and each layer was vibrated about 20 s and the final layer was vibrated until no air bubble appear on the concrete surface and the surface troweled to a smooth finish. After 24 h, the corbels were demolded and covered by wet cloth for about 28 days while the cylinders were kept in curing tank for the same period. Fig. 2 shows the molds, the steel reinforcement cages and specimens after pouring operations. 2.4. Bonding of CFRP to corbels After curing time, the specimens were strengthened with one layer and with two different CFRP orientation (i.e. two configurations), horizontal and diagonal 45 configurations as shown in Fig. 3. Prior to bonding CFRP to the specimens, the corners of all specimens were rounded then the locations were wrapped by CFRP cleaned from dirt. CFRP was cut to the required width and length and the surface of CFRP strips were kept clean. A thin coat of a prime was applied to the clean corbel surface and allowed to cure for at least of one hour. Resin and hardener were weighed as ratio (4:1) due to manufacturer recommendations and mixed until the color was gray then applied to both concrete surface and CFRP strips. Finally, CFRP strips were fixed to their place by brush. 2.5. Testing technique The corbels were tested to their ultimate load (Vu) by Universal computerized testing machine Walter + Bai AG/ Switzerland type, 3000 kN capacity as shown in Figs. 4 and 5 respectively.
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Fig. 4. Detail of corbel supports and measurement instrumentations.
Fig. 5. View of specimen showing the measurement units.
The test samples were inverted column with two corbels and the corbels were seated on steel roller supports. The position of the support from the faces of the column were fixed (a = 110 mm) yielding a/d = 0.846. The displacement in each corbel was measured by using linear variable differential transducer (LVDTs reading accuracy of 0.001 mm) supported as shown in Figs. 4 and 5 respectively. Mechanical foil strain gauges were used to measure strain, one strain gauge was placed on one of the two main bars near to the column-corbel junction. As for the stirrups, the strain gauges were mounted at the corbel column interface. It should be noted that strain gauges were also used on strengthened samples and placed on the diagonal line from centre of support to the column-corbel junction to the direction of CFRP. All the specimens were loaded monotonically in small increment (1 kN/s) up to ultimate load. At each increment, the readings were recorded and the development of cracks were observed and marked later on the specimens.
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Table 4 Test results of ultimate load, ultimate deflection and ultimate strains. No.
Group
Corbel Symbol
Pu kN
Aver. du (mm)
eu in steel micron
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12
1
CN-WO CN-WO CN-W CN-W INC-WO INC-WO INC-W INC-W HOR-WO HOR-WO HOR-W HOR-W
90.65
1.33
3557
95.45
1.56
93.45
1.82
6041
115.35
1.25
8435
104.25
1.22
115.1
1.31
2
3
eu in CFRP micron 5 cm
3470 4451
C = corbel, CN = control, WO = without, W = with, INC = inclined, HOR = horizontal.
3. Results and discussion In the following sections, the observed behaviour and the failure mode results of RC corbels will be discussed. The specimen test results are listed in Table 4 in the terms of ultimate load, the corresponding deflections and foil gauges strain measurements. By comparing Table 4 with Table 2, it is obvious that the strain in the CFRP at failure is typically less than the strain data provided by the manufacturer. This could be due to the difference in testing a single fibre (manufacturer’s data) and testing a practical specimen. The variation in such data could also be due to the fact that the strain were gained using conventional strain gauges. In some cases, the foil gauges de-bonded prior to failure, and these data points are therefore being lost. 3.1. Load versus deflection response Load-deflection curves for tested representative pairs of corbels described in Table 4 are shown in Fig. 6. The curves are for both control and CFRP bonded specimens with 1-layer. Form each group of corbels labeled in Table 4, two corbels were reinforced with longitudinal steel bars in addition to stirrups. The average experimental deflections of the samples were taken from LVDTs movements under load relative to supports. From the trend of the relationships it can be detected that the overall response of the corbel was quite comparable in terms of both maximum load, initial stiffness and the failure. The first segment is very steep, tracked by a kink. A reasonable increase in the fracture toughness, ductility and ultimate strength resulted when strengthen corbels with CFRP sheet. 3.2. Ultimate load capacity and ductility enhancement due to CFRP reinforcement Table 4 shows the results for the ultimate capacity of the tested corbels. It can be seen from the table that the maximum enhancement in the ultimate load capacity of RC corbels is achieved when external CFRP overlays are applied. CFRP can increase the ultimate load, ductility and enhances the stiffness of RC corbels. The level of improvement depends on the orientation of CFRP fabrics (configuration) and presence of main bars plus stirrups. In general, the ultimate load was larger in corbels strengthened with CFRP with the presence of stirrups than those of unstrengthen corbel specimen (control corbel). The tougher corbel failed at an ultimate load of 115.3 kN with an increase in strength of about 27% compared to control specimen, see Fig. 7. It can be noticed that the percentage of increase for inclined configuration is almost similar (i.e. 27%) to horizontal scheme compared to control corbel. This is almost resulted from insufficient CFRP overlap, but if sufficient overlap was provided, the enhancement rate will be higher. The axial toughness exhibited by a RC corbel can be obtained to be reflected by the area under the respective loaddeflection curves. These are shown in Fig. 8 for all tested specimens and were calculated using Grapher [29] software. The Figure shows that there is a steady increase in axial toughness with increasing CFRP layers with the presence of stirrups. It also illustrates that the presence of CFRP improves the axial toughness significantly with the presence of stirrups. In general, the axial toughness for inclined CFRP configuration with only one layer of CFRP is higher than that of the horizontal outline. 3.3. Effect of secondary reinforcement In the presence of transverse reinforcement (stirrups), finer cracks formed and the ultimate load capacity measured was higher than that of corbels with main steel reinforcement only, see Fig. 7. Because of the bridging action of transverse reinforcement against the principal cracks, more ductile behaviour was observed. Test results revealed that providing stirrups increase the level of load enhancement of corbels in ranges between 6% and 24%, for both control CFRP overlay attached corbels.
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Fig. 6. Load-deflection curves.
Fig. 7. Ultimate load enhancement.
3.4. Effect of CFRP orientation Test results revealed that the level of strengthening of corbels with CFRP sheets ranges between 13% and 17% (see Fig. 7), depending on the orientation of the CFRP material compared with control specimens without and with secondary reinforcement respectively. The resistant mechanism involved in the CFRP bonded corbels was also categorized by the contact phenomena at the interface between the CFRP overlay and concrete surface. The external strengthening of a corbel using CFRP can enhance the corbel capacity when the CFRP is adequately arranged. However, in this study the inadequate overlap was observed and thus the diagonal CFRP configuration increased the ultimate load carrying capacity of the corbel by 27% of the control specimens similar to that of horizontal arrangement.
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Fig. 8. Axial toughness enhancement.
Fig. 9. Crack pattern and failure mode for specimens.
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3.5. Failure modes The modes of failure were identified as follows: For corbels without CFRP strengthening (i.e. control samples), the first predicted bending crack patterns were vertical cracks appearing at the corbel faces close to the column side. The other, major cracks were two diagonal cracks at an angle of almost 45 when the load increased, this crack started to widen and propagate, leading to collapse. For samples strengthened with an externally bonded horizontal strip of CFRP fabric, only one major diagonal crack started at the bearing strip and propagated toward the junction of column and face of corbel. The carbon fibre fabric is broken (fabrics peeling off) in this case, as in Fig. 9. The external CFRP composite limited widening and growth of the flexural crack developed at the junction of the tension face of the corbel and the face of the column. The inclined shear cracks developed in the corbels with external longitudinal composite strengthening were more curved toward the outer surface of the corbels, relative to the cracks developed in the corresponding specimens with internal steel reinforcement only. It was noted that the mode of failure of most tested strengthened corbels was shear failure represented by diagonal tension cracks. In the other word strengthening against diagonal tension failure cannot be fully controled, to prevent shear type of failure. However, such type of failure in strengthened corbels is not differ widely from the flexural failure because the corbel can withstand moderately high deformation before final collapse as discussed before and well observed from the load-deflection relationships. It was also noticed that the provision of secondary reinforcement reduces crack widths, improves ductility, and for beams failing in compression may change the failure mode from diagonal splitting to compression strut crushing. Therefore, they advised the provision of a minimum quantity of horizontal stirrups, similar to that for normal-strength concrete, 4. Conclusions The following conclusions can be drawn based on the results of the present study: 1 2 3 4
There is an enhancement in both load bearing capacity and ductility due to CFRP enhancement. The first segment of load-deflection is very steep, followed by a kink. Test results showed that providing secondary reinforcement increase the level of load enhancement of corbels up to 24%. Due to inadequate CFRP overlap, the diagonal CFRP configuration increased the ultimate load carrying capacity of the corbels by 27% of control specimen similar to that of horizontal arrangement. 5 All wrapped RC corbels failed by tensile rupture of the CFRP wrap.
Conflict of interest None Acknowledgements The authors would like to express their gratitude to the staff of Bekhal Concrete for their assistance. References [1] O.Q. Aziz, Shear strength behavior of crushed stone reinforced concrete corbels, 26th Conference on Our World in Concrete & Structures, Singapore, 2001, pp. 767–775. [2] M.A. Elgwady, M. Rabie, M.T. Mostafa, Strengthening of Corbels Using CFRP an Experimental Program, Cairo University, Giza, Egypt, 2005, pp. 1–9. [3] ACI-318, Building Code Requirements for Structural Concrete American Concrete Institute, (2011) . [4] A.M.A. Hafez, M.M. Ahmed, H. Diab, A.A.M. Drar, Shear behaviour of High strength fiber reinforced concrete corbels, J. Eng. Sci. Assiut Univ. 40 (2012) 669–987. [5] R.M.F. Canha, D.A. Kuchma, M.K. El Debs, R.A.D. Souza, Numerical analysis of reinforced high strength concrete corbels, Eng. Struct. 74 (2014) 130–144. [6] W. Kassem, Strength prediction of corbels using strut-and-tie model analysis, Int. J. Concr. Struct. Mater. 9 (2) (2015) 255–266. [7] L.D.M.A. Attiya, A.A. Mohamad-Ali, Experimental behaviour of reinforced concrete corbels strengthened with carbon fibre reinforced polymer strips, Basrah J. For. Eng. Sci. (2012) 31–45. [8] L.B. Kriz, C.H. Raths, Connections in precast concrete structures: strength of corbels, PCI J. (1965) 16–61. [9] F. Torres, Theoretic-Experimental Analysis of Reinforced Concrete Corbels, Dissertation (Master Science in Structural Engineering)-Engineering School of São Carlos, São Paulo University, São Carlos, (In Portuguese), 1998. [10] G. Russo, R. Venir, M. Pauletta, G. Somma, Reinforced concrete corbels - shear strength model and design formula, ACI Struct. J. 103 (1) (2006) 3–10. [11] A. Yousif, Prediction of ultimate load capacity of high-strength reinforced concrete corbels, Al-Rafidain Eng. J. 17 (4) (2009) 12–27. [12] L.A.G. Yassin, Q.A.M. Hasan, Reinforced concrete Corbels–State of the art, J. Mater. Eng. Struct. 2 (4) (2016) 180–205. [13] D.L. Araújo, S. Azevedo, E. Muniz, E.O. Silva, L. Oliveira Júnior, Strength evaluation of concrete corbels cast in a different stage from the column, Revista IBRACON de estruturas e Materiais 10 (2) (2017) 509–546. [14] A.H. Mattock, K.C. Chen, K. Soongswang, Behavior of reinforced concrete corbels, J. Prestressed Concr. Inst. 21 (2) (1976) 52–77. [15] N.I. Fattuhi, Strength of FRC corbels in flexure, J. Struct. Eng. (United States) 120 (2) (1994) 360–377. [16] N.I. Fattuhi, B.P. Hughes, Ductility of reinforced concrete corbels containing either steel fibers or stirrups, ACI Struct. J. 86 (6) (1989) 644–651. [17] N. Fattuhi, Corbels with shear reinforcement in the form of stirrups or fibres, Proceedings of the 3rd RILEM International Symposium on Developments in Fibre Reinforced Cement and Concrete, University of Sheffield, (1986) paper No. 8.
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