An Experimental Study on Shear Strengthening of RC Lightweight Deep Beams Using CFRP

Transcription

1 Journal o Rehabilitation in Civil Engineering - (014) 9-19 journal homepage: An Experimental Study on Shear Strengthening o RC Lightweight Deep Beams Using CFRP A.A. Asghari 1, Z. Tabrizian *, M.H. Beygi 3, G. Ghodrati Amiri 4 and B. Navayineya 5 1. PhD Student, College o Civil Engineering, Babol Noshirvani University o Tehnology, Babol, Iran.. PhD andidate, College o Civil Engineering, Babol Noshirvani University o Tehnology, Babol, Iran. 3. Assistant Proessor, College o Civil Engineering, Babol Noshirvani University o Tehnology, Babol, Iran. 4. Proessor, Center o Exellene or Fundamental Studies in Strutural Engineering, Shool o Civil Engineering, Iran University o Siene & Tehnology, Tehran, Iran. 5. Assistant Proessor, College o Civil Engineering, Babol Noshirvani University o Tehnology, Babol, Iran. * Corresponding author: zahra_tabrizian@stu.nit.a.ir ARTICLE INFO Artile history: Reeived: 9 Otober 01 Aepted: Otober 014 Keywords: Lightweight onrete, Deep beams, Shear enhanement, Analytial modeling, CFRP omposite. ABSTRACT This paper presents the results o an experimental investigation on shear strength enhanement o reinored onrete deep beams externally reinored with iber reinored polymer (FRP) omposites. A total o six deep beam speimens o two dierent lasses, as-built (unstrengthened) and retroitted were tested in the experimental evaluation program. Two omposite systems namely arbon/epoxy laminates and arbon/epoxy sheets were used or retroit evaluation. A omparative study o the experimental results with published analytial models, inluding the ACI 440 model, was also onduted in order to evaluate the dierent analytial models and identiy the inluening ators on the shear behavior o FRP strengthened reinored onrete deep beams. Experimental results indiated that the omposite systems provided substantial inrease in ultimate strength o strengthened beams as ompared to the as-built beam speimen. The strength gain aused by the CFRP sheets was in the range o 30-58% Analytial Comparison indiated that the shear span-to-depth ratio (a/d) is an important ator that atively ontrols the shear ailure mode o beam and onsequently inluenes on the shear strength enhanement. 1. Introdution Behavior o a reinored onrete beam in shear is very omplex in nature and diiult to predit aurately. For this reason, the shear apaity o beams in most odes o

2 10 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) 9-19 pratie is empirial or semi-empirial. The shear behavior o RC beams depends on the size o the speimen, reinorement type and detailing as well as the type and position o applied loading. Besides the onrete shear resistane apaity, additional ontributions ome rom external and/or internal reinorements o a RC beam speimen. Provided reinorements help in improving the ailure nature, brittle to dutile, to some extent. External shear strengthening using FRP omposites gained popularity over steel beause o several reasons inluding material ost, lightweight eature and ease o appliation. Moreover FRP has more reliable bond line strength as ompared to steel where orrosion at the interaes unavoidable in the presene o moisture [1]. Numerous studies reported a remarkable inrease in the shear strength o shallow RC beams when strengthened with externally bonded FRP system [-4]. Triantaillou reported that the gain in shear strength aused by FRP was primarily dependent on the number o FRP layers and the amount o internal shear steel reinorement. The FRP shear strengthening system was ound more eetive when the ibers were oriented in a diretion perpendiular to the potential diagonal shear raks. Deniaud and Cheng presented a review summary o several shear design methods or reinored onrete beams strengthened with omposites Very ew researhers investigated the eetive o externally bonded FRP system as an external reinorement to strengthening RC deep beams [5-7]. Zhang and Moren reported that the ontribution o the externally bonded FRP system to the shear strength dereased as the shear span-to-depth ratio was redued. Vertial FRP shear strengthening resulted in a 79% inrease in the shear strength at a shear span-to-eetive depth ratio o whereas only a 46% strength gain was reorded at a shear span-to-eetive depth ratio o 1.5. This indiates that the eetiveness o the FRP system dereases as the beam behavior hanges rom a shallow beam ation to a deep beam ation. Islam, Mansur and Maalej reported that reported that use o externally bonded FRP systems leads to enhane o shear strengthening up to 40%. Maaddawy and Sheri reported that strutural response o RC deep beams with opening was primary dependent on the degree o the interruption o the natural load path. Over past ew years, a number o theoretial investigations to predit the shear apaity o reinored onrete beams strengthened externally with FRP wet layup laminates or preured strips were done and analytial models were developed based on dierent theoretial assumptions and experimental observations. Triantaillou and Antonopoulos proposed theoretial model to ompute the shear strength apaity o a beam strengthened with externally applied FRP [8]. Lateran, this proposed model was alibrated using seventy-ive published experimental test results [8] whih resulted in an expression to predit the eetive strain in FRP laminates. In year 001, Matthys and Triantaillou [9] made slight modiiation o Triantaillou and Antonopoulos [8] expression o eetive strain and indiated that in addition to all other inluential ators, eetive strain in FRP is also a untion o the beam shear span-to-eetive depth ratio. However, Matthys and Triantaillou [9] made this omment on the basis o the least square itting o experimental results olleted rom past literatures. Colotti, Spadea and Swamy developed a theoretial model based on the truss analogy method in onjuntion with

3 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) the theory o plastiity [10]. They reported that the theoretial model provided good orrelation with past experimental results. The reent edition o the ACI 440 [11] proposed a theoretial model to ompute the enhanement o shear strength o RC beams using external FRP laminates. They oused on the developed eetive strain in omposites at ailure whih varies depending upon the variability o omposite material properties, dimensions and the appliation tehniques. Thereore, it is extremely diiult to make any generalize solution or the shear strength enhanement o strutural members using external FRP omposite reinorements.. Test Program.1. Details o the struturally deiient beam The six deep beams onstruted were idential in every respet. Eah beam was 1500mm long with a retangular rosssetion, 10(mm) wide and 500(mm) in overall depth (500mm). As shown in Fig.1, the lexural reinorement onsisted o 4T16 deormed bars o yield strength 400MPa. These bars were plaed in two layers and were welded to 10mm thik steel plates at both ends to provide the neessary anhorage. The shear reinorement onsisted o one layer o deormed bars having 150(mm) square openings. The bars diameter was 6(mm). the minimum web reinorement requirements by ACI ode [1] are not available by this value. A 0(mm) thik and 100(mm) long steel plate was used at eah loading and reation points overing the ull width o the beams (Fig.1). Conrete having average 8 days ube strength o 30 MPa was made rom ordinary Portland ement, river sand, rushed gravel o 10mm maximum size and lea (Light Expanded Clay Aggregate) o 3-10mm size. The aggregate ement ratio 5.1 by weight and the water-ement ratio were 0.49 by weight. Fig. 1. Dimension and internal reinorement details o reerene deep beam speimen

4 1 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) 9-19 Row 1 Name REF-a Table 1. Strengthening Sheme o the Tested Deep Beams Strengthening Type None S Fibers Strengthening Coniguration Angle - Without strengthening CONa- VW(C) CFRP Sheet _ 90 3 CONa- DW(C) CFRP Sheet CONa-VS1 CFRP Laminae 15 m 90 5 CONa-VS CFRP Laminate 15 m 90 6 CONa-DS CFRP Laminate 15 m 45.. Strengthening Materials and Method Two dierent FRP types have been used to strengthen the basi beams. These are: CFRP sheets and CFRP laminates. The parameters investigated were the angle o ibers respet to longitudinal axis o beam and type o FRP. The dierent strengthening sheme or eah beam presented in the Table I. Instead o providing the strengthening material (FRP) throughout the beam surae, strengthening materials was used only in the shear span aross the potential ailure region. Table. Properties o strengthening materials Type o FRP Thikness (mm) Width(mm) Tensile Strength(MPa) Tensile modulus o Failure strain)%( elastiity(gpa) CFRP laminate CFRP sheet GFRP sheet

5 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) The mehanial properties o the strengthening materials and epoxy resin, as reported by the manuaturers are shown in Table II and III, respetively..3. Instrumentation and Test Proedure The beams were simply supported having a lear span o 1350mm and tested under two symmetrial point loads, thus given a ratio o.7 and an ratio o 0.95, where is the shear span (as shown in Fig.1). The load was applied using a servo ontrolled hydrauli atuator with a maximum apaity o 1000kN. Mid-span deletion o the beams was measured by LVDTs. 3. Test Results 3.1. Load Deletion Behavior The load vs. mid-span deletion o the beams plotted in Figs. -3 an be seen to be typial o those generally observed or a deep beam with a small ratio. All deep beams demonstrated a nearly linear response up to ailure beause the lexural reinorement didn t yield. It was noted that strengthening by externally bonded o CFRP sheets and laminates resulted in an inrease in the stiness when omparing with the reerene beam. The highest stiness being exhibited by deep beams that strengthened with 45 angle o ibers respet to longitudinal axis o beam. Fig.. load-deletion behavior o deep beams strengthened with CFRP sheets. Fig. 3. load-deletion behavior o deep beams strengthened with CFRP Laminates 3.. Ultimate Strength and Mode o Failure All the beams ailed in shear. The parent beam REF-a ailed by diagonal splitting between the load point and support point in shear-span as an be seen in Fig.4a. Beam CONa-VW(C) whih was strengthened by CFRP sheets in normal orientation, ailed at a load %46 higher than the parent beam. Failure ourred by rushing o the onrete under the sheets (Fig.4b). In beam CONa- DW(C) whih was strengthened by CFRP sheets in diagonal diretion, the ailure ourred by delamination o CFRP sheet and rushing o the onrete under the sheets. Beam showed a %58 inrease in ultimate strength (Fig.4). In beam CONa-VS1, ailure due to the delamination o the seond laminate rom let. The ailure load o 330kN was lose to that o the parent beam beause the CFRP laminates were not ativated (Fig.4d). Beam CONa-VS showed a %30 inrease in ultimate strength. The ailure o this beam was initiated due to the separation o the lower end o the bottom laminate rom the onrete surae. Shortly aterwards, the other laminates gave way one ater another in suession starting rom the lower end. Complete ailure ourred by splitting diagonal and delamination o seond and third laminates rom right (Fig.4e). Closer observations revealed that the separation o the FRP laminates was assoiated with

6 14 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) 9-19 Table 3. Properties o epoxy Resin Hardener Density (g m -3 ) Speial gravity Flexural strength (Kg m - ) The beam CONa-DS was strengthened with the CFRP laminates in diagonal orientation, ailure along a shear plane at the out o the strengthening onine, end o laminates, ourred. At ailure, delamination o the laminate o up o right support was noted (Fig.4). It may be noted that CFRP sheets in normal orientation perormed better than CFRP laminates in normal orientation. This beomes obvious when the results o beams CONa-VW(C) and CONa-VS with an additional web reinorement ratio o 0.3% and 0.40%, respetively (alulated rom the volume o additional web reinorement divided by the volume onrete in shear span), as presented in Table 4, are ompared. Also, FRP ibers in diagonal orientation showed signiiant inrease in ultimate apaity over that o FRP ibers in normal orientation. However, CFRP sheets and CFRP laminates all an be onsidered suitable or shear strengthening a deep beam. The ultimate load arrying apaity o all the beams along with the nature o ailure, load ausing the initial raks and shear enhanement are given in Table IV. (a) (b) () (d)

7 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) (e) (a) () Fig. 4. Failure mode o test beams (b) Fig. 5. Delamination o FRP omposites a) CFRP sheets b) CFRP Laminates. Table 4. Test Results Beam REF-a CONa-VW(C) CONa-DW(C) CONa-VS1 CONa-VS CONa-DS Load at Initial Crak (kn) Ultimate Load (kn) Shear Enhanement (%) Nature o Failure Shear Failure Crushing o Conrete under CFRP Sheets Shear Failure +Crushing o Conrete under CFRP Sheets Shear Failure + Delamination o CFRP Laminates Shear Failure + Delamination o CFRP Laminates Shear Failure + Delamination o CFRP Laminates

8 16 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) Analytial Study In the reent past, ew analytial models are developed by researhers to theoretially predit the shear strength apaity o an externally FRP reinored beam speimen. In this paper, three models are used to study the predited shear strength apaity o the strengthened deep beam speimens and ompare those with experimental result. The analytial model is desribed in the ollowing part o this paper ACI 440 Model The model proposed by ACI 440 [11] is appliable to reinored onrete beams with externally applied FRP reinorement. Aording to this model, the nominal shear strength o onrete is. V n V V V (1) s Where is the additional redution ator given in Table 10.1 o the ACI 440 [11] doument. For omparison purpose, as there is no redution ator onsidered rp during experiment, Eq.(1) is modiied as V n V V V () s rp Where the shear strength ontribution rom FRP is (sin os ) (3) A ve V ed S The eetive strain, e in FRP is assumed to be smaller than the ultimate strain, u. This an be omputed as u or ompletely wrapped beams e k vu or beams with twoor three (4) sides laminated k1k le (5) k v u L 3300 e (nt E 3 1 ( ) 7 ) 0.58 (6) k (7) k d Le d d L d e or U wraps or two sides laminated Where, and E are in MPa 4.. Colotti et al. model (8) Colotti et al. [10] proposed that the total ontribution o shear reinorement an be expressed as i e (9) The internal shear strength ontribution oming rom the internal steel reinorement is Ast y (10) i bs Where as, the external ontribution orm FRP is w t e e bs or ompletely wrapped beams w d w t e min( u ; ) bs bs (11) or beams with two or three sides laminated V ( )bd v Where obtained the ollowing ailure ases: From the ollowing ailure ases : Failure ase (1) (1) is the minimum o that (4)

9 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) (1 ) Failure ase (1) For 0 1 4(1 ) For 0 (13) (14) (15) Ast ly a Where and. bdv d v 4.3. Triantaillou and Antonopoulos [8] Model Aording to this model, V V V V V 0.45 bd (16) n Where A V v s e d rp S R o (sin sin ) (17) / 3 Tensile strength o onrete,.3( ), onrete shear resistane, k 1.6 d 1(d is in m), t R 0 0.5, t A sl l, bd A bs o and 00 ( is in MPa). The eetive strain in this model expeted was / e.17( ) E 0 For beams ully wrapped with CFRP min ( E / 3 / e ) ;0.17( ) E For beams ully wrapped with AFRP (15) (18) u u For two or three sides with CFRP laminated / e.048( ) E 0 5. Comparison u Results rom both analytial and the experimental studies are shown in Table V and Fig.6. Shows the omparison o total shear apaity o deep beam speimen strengthened with CFRP and GFRP omposites. As this igure indiates, the ACI 440[11] model is always providing shear strength o a FRP strengthened deep beam higher than these obtained rom the experimental program. This an be attributed to the at that this model has not onsidered the shear-to-depth ratio ( a ) o D the deep beam in alulation o the shear strength. Hene, rom this results it is evident that shear-to-depth ratio has an important eet on the shear strength apaity o RC FRP strengthened deep beams.

10 Shear Fore (kn) 18 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) 9-19 Experimental results ACI-440 Model Triantaillou et al Model Fig. 6. Comparison o experimental results with theoretial values o strengthened beams Table 5. Summary o Comparison o Experimental and Theoretial Results Beam CONa-VW(C) Experimental results V u (kn( 44.5 V u (kn) 84.3 ACI-440 Presene o dierent 16 Triantailou et al V u (kn) 84.3 Presene o dierent 7.5 V u (kn) 31.3 Colloti Presene o dierent 5.4 CONa-DW(C) CONa-VS1 CONa-VS CONa-DS Conlusion Based on testing eight deep beams and the analytial study, the ollowing onlusions an be drawn: 1) Use o CFRP sheets or laminates and GFRP sheets or strengthening reinored lightweight onrete is an eetive method to inrease its ultimate arrying apaity. An enhanement o shear strength was up to 58%. ) The dutility o deep beams that strengthening with CFRP sheets or laminates is signiiantly redued. 3) The best sheme used to strengthen lightweight onrete deep beams is to attah FRP sheets or laminates in the diagonal orientation. 4) The ACI 440[11] model does not onsider the shear-to-depth ratio in predition o the shear strength enhanement o a reinored lightweight onrete deep beam strengthened with externally bonded FRP omposites. For Lesser ratio (e.g. a 0. 95, D as it is taken in the present experimental program), the experimental shear strength apaity is always less than those predited by the ACI 440[1] model. 5) Similar to the ACI 440[11] model, Triantaillou and Antonopoulos[9] model always results in higher shear strength. This an be attributed to the at that this model

11 A.A. Asghari et al./ Journal o Rehabilitation in Civil Engineering - (014) does not onsider the shear span-to-depth ratio(a/d). 6) Colotti et al.[10] model provides nearly the same estimation o the shear apaity o FRP strengthened beams as ompared to experimental result in all two lamination shemes reported in this paper. REFERENCES [1] Mosallam A. (00), Composites in onstrution In: Kutz M, editor. Handbook o materials seletion. New York: John Wiley & Sons, [hapter 45]. [] Triantaillou, T.C. (1998), Shear strengthening o reinored onrete beams using epoxy-bonded FRP omposites, ACI Strutural Journal, Vol. 95 (0), pp [3] Deniaud, C. and Cheng, JJR. (004), Simpliied shear design method or onrete beams strengthened with FRP, ASCE Journal o Composites or Constrution, Vol. 8(5), pp [4] Monti, G. and Liotta, MA. (007), Tests and design equation or FRPstrengthening in shear, Constrution and Building Materials Journal, Vol. 1(4), pp [5] Zhang, Z. Hsu, C. and Moren, J. (004), Shear strengthening o reinored onrete deep beams using arbon iber reinored polymer laminates, ASCE Journal o Composites or Constrution, Vol. 8 (5), pp [6] Islam, M., Mansur, M. and Maalej, M. (005), Shear strengthening o RC deep beams using externally bonded FRP systems, Cement & Conrete Composites, Vol. 7 (3), pp [7] Matthys, S. and Triantaillou, T.C. (001), Shear and torsion strengthening with externally bonded FRP reinorement, In: Reality A, Cosenza E,Manredi G, Nanni N, editors. Proeedings o the international workshop on omposite in onstrution, Capri, Italy, July 0-1, pp [8] Triantaillou, T.C. and Antonopoulos, C.P. (000), Design o onrete lexural members strengthened in shear with FRP, ASCE Journal o Composites or Constrution, Vol. 4(4), pp [9] Maaddawy, T.EI and Sheri, S. (009), FRP omposites or shear strengthening o reinored onrete deep beams with opening, Composite Strutures, Vol.89, pp [10] Colotti, V. Spadea, G. and Swamy, R.N. (004), Analytial model to evaluate ailure behavior o plated reinored onrete beams strengthened or shear, ACI Strutural Journal, Vol.101(6), pp [11] ACI Committee 440, (003), Guide or the design and onstrution o externally bonded FRP systems or strengthening onrete strutures (ACI 440.R-0), Amerian Conrete Institute, Farmington Hills, Mihigan, USA, 45p. [1] ACI Committee 318-0, (1999), Building ode requirements or Strutural onrete (ACI 318-0), Amerian Conrete Institute, Detroit, 111 pp.