The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015) Joint Conference, 14-16 December 2015, Nanjing, China FLEXURAL STRENGTHENING RC BEAMS USING A COMPOSITE REINFORCEMENT LAYER: FRP GRID AND ECC Wen-wei Wang 1 (corresponding author), Yu-zhou Zheng 2 1 Department of Bridge Engineering, School of Transportation, Southeast University, Nanjing, 210018, China Email:[email protected]2 Department of Bridge Engineering, School of Transportation, Southeast University, Nanjing, 210018, China Email: [email protected]Keywords: RC beams, Flexural behaviour, FRP Grid, ECC, Strengthening ABSTRACT In this paper, a new strengthening system for reinforced concrete (RC) beam is proposed by combination of FRP grid and Engineered Cementitious Cement (ECC) as a composite reinforcement layer. Three RC beams externally strengthened with composite reinforcement layer at the tensile faces and one reference RC beam were conducted experimentally to investigate their flexural behaviour. The thickness of FRP grid applied in test program was selected as only one test parameter. After 1 mm, 3 mm and 5 mm thickness of BFRP (Basalt Fibre Reinforced Polymer) girds were fixed on the bottom surface of RC beams through the pre-embedded short steel bars, approximately 30 mm thickness of ECC was casted along the longitudinal direction of RC beam. All test specimens have a rectangular section and the geometry dimensions are 300 mm in depth and 200 mm in width. Four beams were simply supported on two rolling bearings spaced 1700 mm centre to centre. Two point loadings separated 500 mm were symmetrically acted on the top faces of strengthened beams. Rupture of BFRP grid accompanied with concrete crushing or local debonding of composite reinforcement layer was exhibited near the mid-span section. Compared with the non-strengthened beam, the flexural stiffness and load carrying capacity of the strengthened beams were both significantly improved as expected. The test results indicate that the presented strengthened system is an effective method for flexural strengthening of RC beam and can evidently limit debonding of composite reinforcement layer. 1 INTRODUCTION There is an increasing demand for rehabilitating existing bridges due to the design requirements or deterioration of structural elements in the past few decades. FRP (Fibre reinforced polymer) as a coming new material for strengthening the concrete structures has been accepted widely over the past two decades due to its some prominent advantages, such as light weight, high strength, good corrosion resistance and durability[1-6]. However, the bonding behaviour at the FRP-to-concrete interface may be seriously affected by some environmental factors, example of ultraviolet (UV), water moisture, high temperature and fire, when using the epoxy resin as an interfacial adhesive as well as bonding matrix [7-10].Therefore, some scholars of the world tried to apply cementitious or inorganic materials to replace the epoxy resin to develop new fibre composite systems, such as dry fibre sheets bonded with cementitious materials[11-14], fibre-reinforced inorganic polymer (FRIP) composites[15-16] and textile reinforced mortars (TRM) [17-23], for strengthening of RC members. The intent for those strengthening methods is to utilize the natural advantages of cementitious material contrasted to polymer material subject to some seriously environmental factors. The engineered cement-based adhesive also has a much better material compatibility with the substrate of concrete compared with the epoxy-based one. Previous studies have shown that using the cementitious or inorganic material to strengthen concrete elements can improve their load capacities and meet the functional requirements of structures in the normal service condition. However, some deficiencies, including bad infusion of cementitious
Transcript
The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015)
Joint Conference, 14-16 December 2015, Nanjing, China
FLEXURAL STRENGTHENING RC BEAMS USING A COMPOSITE REINFORCEMENT LAYER: FRP GRID AND ECC
ABSTRACT In this paper, a new strengthening system for reinforced concrete (RC) beam is proposed by
combination of FRP grid and Engineered Cementitious Cement (ECC) as a composite reinforcement layer. Three RC beams externally strengthened with composite reinforcement layer at the tensile faces and one reference RC beam were conducted experimentally to investigate their flexural behaviour. The thickness of FRP grid applied in test program was selected as only one test parameter. After 1 mm, 3 mm and 5 mm thickness of BFRP (Basalt Fibre Reinforced Polymer) girds were fixed on the bottom surface of RC beams through the pre-embedded short steel bars, approximately 30 mm thickness of ECC was casted along the longitudinal direction of RC beam. All test specimens have a rectangular section and the geometry dimensions are 300 mm in depth and 200 mm in width. Four beams were simply supported on two rolling bearings spaced 1700 mm centre to centre. Two point loadings separated 500 mm were symmetrically acted on the top faces of strengthened beams. Rupture of BFRP grid accompanied with concrete crushing or local debonding of composite reinforcement layer was exhibited near the mid-span section. Compared with the non-strengthened beam, the flexural stiffness and load carrying capacity of the strengthened beams were both significantly improved as expected. The test results indicate that the presented strengthened system is an effective method for flexural strengthening of RC beam and can evidently limit debonding of composite reinforcement layer.
1 INTRODUCTION
There is an increasing demand for rehabilitating existing bridges due to the design requirements or deterioration of structural elements in the past few decades. FRP (Fibre reinforced polymer) as a coming new material for strengthening the concrete structures has been accepted widely over the past two decades due to its some prominent advantages, such as light weight, high strength, good corrosion resistance and durability[1-6]. However, the bonding behaviour at the FRP-to-concrete interface may be seriously affected by some environmental factors, example of ultraviolet (UV), water moisture, high temperature and fire, when using the epoxy resin as an interfacial adhesive as well as bonding matrix [7-10].Therefore, some scholars of the world tried to apply cementitious or inorganic materials to replace the epoxy resin to develop new fibre composite systems, such as dry fibre sheets bonded with cementitious materials[11-14], fibre-reinforced inorganic polymer (FRIP) composites[15-16] and textile reinforced mortars (TRM) [17-23], for strengthening of RC members. The intent for those strengthening methods is to utilize the natural advantages of cementitious material contrasted to polymer material subject to some seriously environmental factors. The engineered cement-based adhesive also has a much better material compatibility with the substrate of concrete compared with the epoxy-based one.
Previous studies have shown that using the cementitious or inorganic material to strengthen concrete elements can improve their load capacities and meet the functional requirements of structures in the normal service condition. However, some deficiencies, including bad infusion of cementitious
Wenwei Wang, Yuzhou Zheng
material into FRP sheet/textile, more amounts of FRP reinforcement and incompatibility of deformation between the fibre filament and cement-based matrix, are still existed [24-26]. Therefore, attempts are made in this paper to develop a new system, FRP grid reinforced Engineered Cementitous Composite (FRP-ECC strengthening system), in which ECC is applied as a bond agent between the FRP grid and the concrete substrate for strengthening the RC beam. This new system is expected to provide the dual strengthening effect to the original concrete RC beams due to the high strength of the FRP grid reinforcement and the strain-hardening behaviour of the ECC. Meanwhile, the ECC as a bonding adhesive is expected to be able to suppress the intermediate crack-induced debonding failure due to the multiple crack behaviour of the ECC. 2 TEST PROGRAM
2.1 Design of specimens
A total of four rectangular section reinforced concrete (RC) beams were prepared in this test program. One (BB0) was the reference beam without strengthening and three (BB1-1, BB2-3 and BB3-5) were strengthened with a composite reinforcement layer (CRL) consisted of Basalt Fibre Reinforced Polymer (BFRP) grid and ECC. All beams had identical dimensions of 200 mm×300 mm ×1800 mm and a test span of 1700 mm. Table 1 summarizes the details of all beams and Figure 1 presents the elevation and cross-section of those beams. Four ribbed steel bars (HRB335 grade) with 12 mm in diameter were longitudinally placed into the bottom and top parts of beams as the tensile and compressive steel reinforcements, respectively, as shown in Figure 1. The thickness of concrete cover was both 30 mm for top and bottom steel reinforcements. The shear steel stirrups were plain round bar (HPB235 grade) with 10 mm in diameter and spaced 100 mm to avoid the shear failure prior to flexural failure. After the BFRP gird was fixed on the bottom surface of RC beams using six embedded steel bars, the even 30 mm thickness of ECC was casted along the longitudinal direction to form the external CRL.
Table 1 Summary of test results for all experimental beams Beam
ID Concrete cracking Yielding stage Ultimate stage Failure
Note: C-concrete crushing; R-rupture of BFRP grid; D-debonding of CRL
30mm
300mm
Composite of strengthening layer
2Φ12
2Φ12
200mm
φ10
BFRP grid
600mm 500mm 600mm1500mm
1800mm
0.5P 0.5P
2φ12
2φ12
φ10@100mm
Figure 1 Details of strengthened beams
2.2 Properties of material
The designed strength grade of concrete was C30 according to the Chinese concrete structural specification [27]. The ordinary Portland cement, fine and coarse aggregates with the maximum diameter of 20 mm and water were mixed together to make the concrete. As for ECC, the ordinary Portland cement, fly ash, fine sand, polyvinyl alcohol (PVA) fibre and some admixtures were mixed together to produce strengthening layer. During the casting process of specimens, two groups of cubic
The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015)
Joint Conference, 14-16 December 2015, Nanjing, China samples were reserved to determine the compressive strength of concrete and ECC. The 28-day cube compressive strength of concrete and ECC was measured to be 35 MPa and 31 MPa, respectively.
The strength for two grades of steel reinforcement was determined by averaging the measured values of three samples. The mean values of the yielding and ultimate strengths for HRB335 grade were 560 MPa and 684 MPa, respectively, and 472 MPa and 531 MPa for HPB235 grade, respectively.
The BFRP grid used in this test program was manufactured by a company in Jiangsu province of China and its dimensions were 1500 mm in length and 200 mm in width. In this grid, the vertical and horizontal continuous basalt-based untwisted yarn as the raw material was infiltrated into the saturated resin to form the two-way grid after drying moulding resin derived. The BFRP reinforcements were arranged spaced 50 mm centre to centre along the longitudinal and transverse directions to form a rigid mesh, as shown in Figure 2. The thickness of BFRP grid was 1 mm, 3 mm and 5 mm for the beams BB1-1, BB2-3 and BB3-5, respectively. The measured elastic modulus of BFRP grid were 52 GPa, 53 GPa and 57 GPa for 1 mm, 3 mm and 5 mm thickness, respectively, and the measured ultimate strength were 357 MPa, 386 MPa and 416 MPa, respectively.
Figure 2 BFRP grid
2.3 Test setups
All beams were subjected to two-point symmetric loadings separated 500 mm. The loading was applied on the top surface of beams by using a distributive steel beam. The 0.5 kN/min loading speed was selected before concrete cracking and 1 kN/min loading speed was chosen for the rest of loading history until the failure of test beams. The load cell was connected with a 500-kN hydraulic jack to monitor the variation of loading through full processes of test. Three LVDTs were placed at two supports and mid-span section to measure their vertical displacements. Six concrete strain gauges were bonded to side surface of strengthened beam to measure the variation of strain at the mid-span section, as shown in Figure 3.
LVDT
Load CellHydraulic Jack
Steel Frame
Distributive Girder
Composite strengthening layer
LVDT
LVDT
SG1~SG6
Roller Roller
RC Beam
Figure 3 Details of setups
3. TEST RESULTS
3.1 Failure mode and load-displacement curves
The typical flexural failure mode of concrete crushing between two loading points was observed in the reference beam BB0. For the strengthened beams BB1-1 and BB2-3, the rupture of two middle longitudinal BFRP reinforcements in grid was exhibited at the mid-span section and subsequently followed by concrete crushing nearby one loading point, as shown in Figure 4a. But for the strengthened beam BB3-5, the local debonding of CRL along the ECC-to-concrete interface was observed after the fracture of BFRP grid underneath one loading point. The interfacial debonding was
Wenwei Wang, Yuzhou Zheng
across several concrete flexural cracks and the debonded length was about 192 mm, as shown in Figure 4b. Moreover, many fine cracks uniformly distributed in the ECC layer, as shown in Figure 4a and 4b. The reasons for different failure modes occurred in the strengthened beams were the interfacial stress concentration beside the main crack formed in the CRL layer (Fig.4b) and the difference of flexural stiffness between CRL and RC beam.
（a）Concrete crushing
（b）Interfacial debonding
Figure 4 Failure models of test beams Load-displacement responses for all test beams at the mid-span section are shown in Figure 5 and
the key values of the crack, yielding and ultimate stages are summarized in Table 1. For all strengthened beams, there was a remarkable increase in load carrying capacity when the external CRL was added. The increase ranged between 4% and 33% of the load carrying capacity for the strengthened beams compared with that of the reference beam BB0. As indicated in Figure 5, there was no significant difference in the section flexural stiffness of strengthened beams before the longitudinal steel reinforcement yielding. After that, the load-displacement curves showed that the section flexural stiffness of reference beam BB0 was weaker than those of the strengthened beams BB1-1, BB2-3, and beam BB3-5 until failure.
Figure 5 Load-displacement curves of all test beams
Beam BB1-1 was strengthened with 1mm thickness of BFRP grid and ECC layer. When the load applied to 54 kN (41% of ultimate load), a few of flexural cracks were formed in tension zone of concrete and numbers of additional fine cracks were appeared on the surface of CRL. When the load approached to the 121 kN (about 92% of ultimate load), new concrete flexural cracks were formed and old concrete cracks propagated toward to upper face of the beam continually. With the load further increasing, the longitudinal steel reinforcement yielded and then the mid-span displacement of strengthened beam was sharply increased. When the load of 131 kN was applied, the slight "crackling" sound from BFRP grid can be heard and two longitudinal reinforcements of BFRP grid were rupture at the mid-span section but other two longitudinal reinforcements were not still ruptured. Since then, the beam cannot carry the so big external load and then the concrete crushing finally occurred. It is
The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015)
Joint Conference, 14-16 December 2015, Nanjing, China evident that multi-point cracking phenomenon (seen from Figure 4a) in the CRL was showed up. For beams BB2-3 and BB3-5 strengthened with 3 mm and 5 mm thickness of BFRP grid and ECC layer, similar mechanical phenomenon was observed in the full loading process except the different failure mode was obtained.
By comparing with three strengthened beams BB1-1, BB2-3 and BB3-5, it can be found that the debonding failure of CRL was not obviously occurred although the flexural stiffness of strengthened beams was increased (i.e. the increasement of BFRP grid thickness). This may reflect a fact that the ECC as an interfacial adhesive could effectively limit the happening of debonding of external reinforcement layer. At the same time, the crack, yielding and ultimate loads of the strengthened beams were significantly increased by using the external composite reinforcement layer.
3.2 Strain distributions
(a) Beam BB0 (b) Beam BB1-1
(c) Beam BB2-3 (d) Beam BB3-5
Figure 6 Strain distributions at the mid-span section The distributions of strains through the depth of mid-span section of each beam are shown in Figure
6. As expected, the strain distributions were generally linear before and after concrete cracking if the concrete tensile strains were neglected. Even so, the large tensile strains (4500 με for beam BB2-3 and 5500 με for beam BB3-5) of ECC layer illustrated the strain development in ECC and well explained the phenomena of material strain-hardening. Furthermore, there was no slip strain between the CRL and the concrete substrate, which shows the good bonding performance of the interface. 4 CONCLUSIONS
A new strengthening system is proposed to strengthen RC beam by externally bonded a composite reinforcement layer consisted of BFRP grid and ECC. Three strengthened beams and one reference beam were experimentally conducted to investigate the mechanical behavior of strengthened beams. Based on the experimental observations, the following conclusions can be drawn as follows: 1) The final failure modes of three strengthened beams are all rupture of BFRP grid in spite of the local debonding of CRL in one strengthened beam. This results in an inhibiting effect on debonding of
Wenwei Wang, Yuzhou Zheng
external reinforcement layer and demonstrates in fact the effectiveness of proposed strengthening technique for RC beam. 2) The flexural stiffness and load carrying capacity of strengthened beams are greatly improved after strengthened with CRL compared with the non-strengthened beam. In addition, big strains and multi-point fine cracks in ECC layer reveal a good extension behavior of this material and dissipation of interfacial energy along the ECC-to –concrete interface.
ACKNOWLEDGEMENTS
The authors would like to express their acknowledgements to the National Natural Science Foundation of China (Project codes: 51078079 and 51578135) and Innovation Projects of Graduate Student Scientific Research in Jiangsu province（Program No. KYLX15_0142）for providing funds for this research work.
REFERENCES
[1]W.W Wang, G Li. Experimental study and analysis of RC beams strengthened with CFRP laminates under sustaining
load ,International Journal of Solids and Structures, 2006, 43(6): 1372-1387. [2] WW. Wang, JG. Dai, KA. Harries, et al. Prestress losses and flexural behavior of reinforced concrete beams strengthened
with posttensioned CFRP sheets, Journal of Composites for Construction, 2012, 16(2): 207-216. [3] S. Hashemi, R. Al-Mahaidi. Experimental and finite element analysis of flexural behavior of FRP-strengthened RC beams
using cement-based adhesives, Construction and Building Materials, 2012, (26):268-273. [4] WW. Wang, JG. Dai, KA. Harries. Performance evaluation of RC beams strengthened with an externally bonded FRP
system undersimulated vehicle loads, Journal of Bridge Engineering, 2013, 18(1): 76-82. [5] WW. Wang, JG. Dai, KA. Harries. Intermediate crack-induced debonding in RC beams externally strengthened with
prestressed FRP laminates , Journal of Reinforced Plastic and Composites, 2013, 32(23):1842–1857. [6]WW. Wang, JG. Dai, KA. Harries, et al. Prediction of prestress losses in RC beams externally strengthened with
prestressed CFRP sheets/plates, Journal of Reinforced Plastics and Composite, 2014, 33(8): 699-713. [7]JCPH. Gamage, R.Al-Mahaidi, MB Wong. Bond characteristics of CFRP plated concrete members under elevated
temperatures, Journal of Composite Structures, 2006, 75(14):199–205. [8] S. Hashemi, R. Al-Mahaidi. Flexural performance of CFRP textile-retrofitted RC beams using cement-based adhesives at
high temperature, Construction and Building Materials, 2012, (28):791–797. [9] HM. Elsanadedy, TH. Almusallam, SH. Alsayed, et al. Flexural strengthening of RC beams using textile reinforced
mortar-Experimental and numerical study, Composite Structures, 2013, (97):40–55. [10] L. Zhang, WW. Wang, KA. Harries, et al. Bonding behavior of wet-bonded GFRP-concrete interface, Journal of
Composites for Construction, 2014, doi: 10.1061/(ASCE)CC.1943-5614.0000550 , 04015001. [11] H. Kolsch. Carbon fiber cement matrix (CFCM) overlay system for masonry strengthening, Journal of Composites for
Construction, ASCE, 1998, 12(2):105-109. [12] S. Kurtz, P. Palaguru. Comparison of inorganic and organic matrices for strengthening of RC beams with carbon sheets,
Journal Structural Engineering, ASCE, 2001, 127(1), 35-42. [13] H. Toutanji, L. Zhao, Y. Zhang. Flexural behaviour of reinforced concrete beams externally strengthened with CFRP
sheets bonded with an inorganic matrix, Engineering Structures, 2006, (28): 557-566. [14] S. Hashemi, R,Al-mahaidi. Cement based bonding material for FRP strengthening of concrete structures, Proceedings of
the 9th international symposium on fiber-reinforced polymer reinforcement for concrete structures (FRPRCS-9), Sydney, Australia, 2009.
[15] JG. Dai, S. Munir, Z. Ding. Comparative study of different cement-based inorganic pastes towards the development of FRIP strengthening technology, Journal of Composites for Construction, 2014, 18(A40130113SI).
[16] Z. Ding, JG. Dai, S. Muner. Study on an improved phosphate cement binder for the development of fiber-reinforced inorganic polymer composites, Polymers. 2014, 6(11): 2819-2831.
[17] J. Zhang, CKY. Leung, Y.N Cheung. Flexural performance of layered ECC-concrete composite beam, Composites Science and Technology, 2006, (66):1501–1512.
[18] LT. Bu, J. Chen, Ch. Lu. Experiment on full-scale RC beam reinforced by polyvinyl alcohol-engineered cementitious composite mortar in flexure subjected to secondary load, Journal of Hunan University (Natural Sciences), 2011, 38(1):1-8.
[19] V. Mechtcherine. Towards a durability framework for structural elements and structures made of or strengthened with high-performance fiber-reinforced composites, Construction and Building Materials, 2012, (31):94–104.
[20] V. Mechtcherine. Novel cement-based composites for the strengthening and repair of concrete structures, Construction and Building Materials, 2013, (41):365–373.
[21] M. Tsesarsky, A. Peled, A. Katz, etc. Strengthening concrete elements by confinement within textile reinforced concrete (TRC) shells-Static and impact properties, Construction and Building Materials, 2013, (44):514–523.
[22] P. Larrinaga, C. Chastre, HC. Biscaia, etc. Experimental and numerical modeling of basalt textile reinforced mortar behavior under uniaxial tensile stress, Materials and Design, 2014, (55): 66–74.
[23]S.Verbruggen, DG. Aggelis, T.Tysmans, J.Wastiels. Bending of beams externally reinforced with TRC and CFRP
The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015)
Joint Conference, 14-16 December 2015, Nanjing, China
monitored by DIC and AE, Composite Structures, 2014, (112):113-121. [24] WY. Chen, XB. Chen, D. Yi. The shear behavior of beams strengthened with FRP grid, the 5th international conference
on FRP composites in civil engineering, Beijing, 2010. [25]SL. Xu, SP. Yin, XH. Cai. Investigation on flexural behavior of reinforced concrete beam strengthened with textile-
reinforced concrete, China Civil Engineering Journal, 2011, 44(4):23-34. [26] SL. Xu, LJ. Hou, XF. Zhang. Flexural and shear behaviors of reinforced ultrahigh toughness cementitious composite
beams without web reinforcement under concentrated load, Engineering Structures. 2012, 39(0): 176-186. [27]Ministry of Housing and Urban-Rural Construction of the People's Republic of China.Code for design of concrete
structures(GB50010-2010), China building industry press, Beijing,2010.
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