SHEAR RESISTANCE OF CONCRETE T-BEAMS PRESTRESSED WITH CFRP CABLES

Transcription

1 SHEAR RESISTANCE OF CONCRETE T-BEAMS PRESTRESSED WITH CFRP CABLES Payam NABIPAY B.S, EIT University o Manitoba Department o Civil Engineering 15 Gillson st., Winnipeg, Manitoba, Canada umnabipa@.umanitoba.a Dagmar SVECOVA PhD, P.Eng University o Manitoba Department o Civil Engineering 15 Gillson St., Winnipeg, Manitoba, Canada sveovad@.umanitba.a* Abstrat This paper investigates the shear apaity o onrete T-beams prestressed with CFRP strands. The eet o the shear span to depth ratio (a/d) as well as the prestressing ore on the shear strength o onrete beams reinored with FRP is disussed. Three dierent designs with a/d o 1.5, 2.5 and 3.5 were investigated with 2 samples per eah design. The experimental results along with similar data rom other researhers are ompared to theoretial values obtained rom the most reent versions o the North Amerian design odes and guidelines. The results ound the ACI guidelines most onservative or FRP reinored and espeially prestressed onrete beams. CSA-S worked best or both FRP reinored and prestressed onrete beams. From all the odes and guidelines studied, only CSA-S inluded a ator to aount or arh ation in deep beams with a/d < 2.5, whih worked reasonably well or beams tested in this projet. Keywords: ACI, Arh ation, Code equations, CSA, FRP, Prestress, Shear 1. Introdution Several ators are known to inluene the onrete ontribution to shear resistane o reinored onrete beams inluding presene o axial ore and a/d ratio. Axial ompressive ores suh as prestressing improve the beam s stiness and inrease the raking load whih will allow the beam to reah higher shear loads. The a/d ratio an inluene the load transer mehanism in the beam. While, the behaviour o slender beams with 2.5 < a/d < 6 is governed by beam ation, at a/d < 2.5 signiiant arh ation an develop whih enhanes the beam s shear apaity [1]. Studies on FRP prestressed onrete beams have shown that inreasing the prestressing ratio enhanes the shear apaity o the beams [2, 3, 4]. Also inreasing the a/d ratio was ound to redue the shear strength and inrease the dutility o the beams [2]. Although the mehanism o shear transer is the same between steel and FRP reinored onrete beams, the lower stiness o FRP reinored members is onerning. FRP reinored onrete members develop wider raks whih lead to less shear apaity in slender beams. Thereore, more researh is needed to better understand the shear behaviour o FRP reinored onrete beams. The urrent projet adds to the sare researh regarding the eet Page 1 o 8

2 o prestressing and a/d ratio on shear apaity o onrete beams ontaining FRP lexural reinorement. Various North Amerian design odes and guidelines are investigated and their eetiveness in estimating the inluene o the aorementioned parameters is disussed. 2. Experimental Work 2.1 Speimen Geometry and Testing Proedure A total o 6 T-beams were tested in this projet. All the beams inluded two layers o CFRP prestressing tendons (CFCC) with an average eetive depth o 188 mm and inluded no transverse reinorement in their shear spans. The lange o the beams ontained a grid reinorement made rom 6mm GFRP rebars. The grid reinorement was designed to ounterat the shrinkage stresses in the onrete ater asting. Also, the longitudinal bars in the lange were designed to restrit raking in the top o the beam when transerring the prestressing ore to onrete. The end zones o the beams were reinored with GFRP stirrups as well as steel spirals around eah strand to prevent raking in those regions at time o stress transer. Figure 1 illustrates a ross setion o the beam outside the end-zones CFCC φ12.5mm 150 Figure 1. Beam Cross setion geometry and test set-up All the beams were tested under our point loading at the University o Manitoba. The loading ross head was entred between the supports and had a ixed length. The shear span o the beams was varied between eah group to reate dierent a/d ratios. Table 1 outlines the speiiations o eah group o samples. Table 1. Dimensions o the beams Group ID No. Samples a/d a (mm) Span (mm) Length (mm) PR PR PR Mehanial Properties and Prestressing Proess The CFCC strands used in this researh were manuatured by Tokyo Rope Company and had 12.5mm diameter. Tension tests onduted on the strands revealed their tensile apaity o 2100 MPa and modulus o elastiity o 137 GPa whih mathed the manuaturer s speiiations. The grid reinorement in the lange was made rom 6mm GFRP rebars manuatured by Pultrall under the trade name V-Rod. The stirrups used to reinore the endzones were made rom the same rebars. The normal density onrete used in this projet had strength o approximately 47 MPa at time o testing or all the speimens. All the CFCC strands were prestressed to 53% o their ultimate apaity. The prestressing losses at time o testing were approximately 10%. The strands were pulled using a hydrauli Page 2 o 8

3 jak and a oupler system that was designed to prevent damage to the strands as a result o using steel huks. Figure 2 illustrates the oupler system. Steel CFCC 1.5 nuts Figure 2. Prestressing Coupler The CFCC and steel strands were ast separately inside threaded steel anhors with expansive grout and joined together with two welded nuts. The steel strands were o the same diameter as the CFCC strands. The oupler and the anhors were tested under tension beore appliation to ensure they ould sustain at least double the required load. The anhors were designed aording to CSA-S806 Annex B [14]. The prestressing load was monitored using a load ell plaed between the bulkhead and the jak as well as strain gauges on the strands. 3. Design Equations Among the urrent North Amerian design equations or onrete beams reinored with FRP, only the CSA-S6-10 and CSA-S onsider the eet rom prestressing. CSA-S6-10 inludes the ator β whih is a measure o the aggregate interlok aross diagonal raks and is a untion o the average longitudinal strain at mid-depth o the setion (ε x ). Inreasing prestressing in the beam will derease the value o ε x whih will in turn inrease β and the shear apaity o the beam. The ormula or V in CSA-S6-10 is shown below: V 2.5 rbd v (1) In the above ormula, r is raking strength o onrete; d v is the eetive shear depth; β is a ator used to aount or the shear strength o raked onrete and is alulated rom the ormula below: (2) x s ze M V Vp 0.5N Ap po dv x (3) 2( E A E A ) r r s ze is equivalent value o rak spaing parameter (s z ) that aounts or inluene o aggregate size; N is atored axial ore (negative or ompression); A p is area o prestressing tendon; A r is area o non-prestressed reinorement; E r and E p are modulus o non-prestressed and prestressed reinorement respetively; po is stress in prestressed reinorement when the stress in surrounding onrete is zero. CSA-S aounts or the eet o prestressing by adding the load resulting rom the deompression moment (M d ), in addition to the omponent o prestressing ore in diretion o applied shear (V p ). V r p p M dv V VsF 0. 5V p (4) M V 0.05 k k k k ( b d (5) m s a r ' ) 1/ 3 w v Page 3 o 8

4 k m represents the impat o the interation between atored shear and moment on the shear strength o the setion; k s relates to size eet; k r and k a represent the eet o reinorement rigidity and arh ation. It should be noted that the term M d V / M is the same as M d /a, or beams tested under three or our point loading and inreases as the shear span dereases. ACI 440.1R-06 guidelines reommend the ollowing ormula or design o onrete beams reinored with FRP: 2 ' V bw (6) 5 b w is width o the web; is raked transormed setion neutral axis depth; is ompressive strength o onrete. ACI 440.4R on prestressing onrete strutures with FRP tendons reommends a minimum value or V speiied below: V ' 0.17 b d (7) The ACI guidelines use the V min ormula or steel prestressed onrete beams. The ormula is not expanded due to lak o suiient researh and the at that FRP prestressed beams are likely to develop wider raks than steel ounterparts [5]. Also, the above ormula inludes no speial provision to aount or presene o prestressing in the beam. It should be noted that the ISIS design manual on FRP prestressed onrete beams ollows the same proedure as the 2006 version o the CSA-S6 that multiplied Eq.1 by the square root o the ratio between the modulus o elastiity o FRP longitudinal reinorement and ommon steel tendon with E s = MPa. Work done by other researhers suh as Matta et al. ound this ratio to be unneessary [6]. 4. Results and Disussion The prestressed beams tested in this projet are grouped as PR-1, PR-2, and PR-3 beams whih have a/d ratios o 1.5, 2.5 and 3.5 respetively. The experimental loads represent the load applied to the beams whih is hal the mahine load. 4.1 Experimental Results Figure 3 shows a typial normalised load-deletion urve or the speimens tested in this projet. The dutility o the beams inreased with the a/d ratio. This is espeially notieable between PR-1 and PR-3 samples. At servie loads o about 50% o the ultimate, PR-3 beams reahed less than 10% o their dutility while PR-1 beams reahed almost 30% o their ultimate deletion. w Figure 3. Typial normalised load-deletion urves or the design speimens Page 4 o 8

5 The higher stiness o PR-1 beams an be attributed to signiiant arh ation that develops in these beams. As a result o the arh ation the PR-1 samples reahed 80% higher shear apaity than PR-2 beams. For PR-3 beams, beam ation governs the behaviour o the beams. In slender beams without transverse reinorement suh as PR-3 beams, ailure ours as soon as the diagonal rak is ormed. However, in PR-2 and speially PR-1 speimens, the beam is able to arry additional shear passed diagonal raking through arh ation [1]. The experimental results show that at a/d ratios less than 2.5 the ontribution rom arh ation is very signiiant. It an be seen rom Table 2 that the apaity o the beams inreased as the a/d ratio dereased. Table 2. Experimental results Speimens P exp (kn) CSA-S6 ACI CSA-S806 PR-1-B1 327 PR-1-B PR-2-B1 184 PR-2-B PR-3-B1 148 PR-3-B The shear apaity o the beams almost doubled with dereasing a/d rom 2.5 to 1.5. CSA- S806 inludes a ator k a to aount or arh ation in deep beams. The results rom this omparison are shown in Table 2. For this projet, applying the CSA-S806 ormula to PR-1 beams provided a reasonable estimate with experimental values being 1.6 times the theoretial ones. It should be noted that all the beams tested in this projet ailed in diagonal tension, exept or PR-1 beams that demonstrated shear ompression ailure. Figure 4 below illustrates typial ailure or eah group o beams inside their shear span. 4.2 Disussion (a) PR-1 (b) PR-2 () PR-3 Figure 4. Typial ailure or beam speimens Tests onduted by other researhers in the past saw the shear apaity o FRP prestressed onrete beams doubled by inreasing the prestressing ratio rom 0 to 60% [3]. Similar inrease was seen in another study by prestressing the tendons to 40% o their ultimate apaity [4]. Results rom these tests are inluded in the database presented in this paper. It should be noted that all the beams in the database were made rom normal density onrete with no transverse reinorement and only FRP longitudinal reinorement. Also, all the beams in the database had a shear ailure. The number o speimens in Table 3 represents the number o beams that were inluded in the analysis. To avoid redundany, not all samples were inluded. The speimens varied by lexural rigidity o longitudinal reinorement, type Page 5 o 8

6 o FRP tendons used, a/d ratio, onrete ompressive strength, size o the beam and prestressing ratio. Table 3. Database o beams tested by other researhers Reerene a/d No. Reinored No. Prestressed speimens speimens pe [% pu ] Tureyen et al. (2002) Razaqpur et al. (2004) Mota. (2005) Zhao et al. (1995) Nishikawa et al.(1993) Whitehead et al. (2005) Matta et al. (2008) Yost et al. (2001) Sang et al. (1999) Figure 5 illustrates a omparison between experimental results rom the urrent projet and literature with theoretial values obtained rom the design equations onsidered in this study. It should be mentioned that only PR-2 and PR-3 beams with a/d > 2.5 were inluded. (a) Reinored Conrete (b) Prestressed Conrete Figure 5. Comparison o experimental and theoretial values The ACI guidelines were onservative or all the samples in the database. CSA-S and CSA-S6-10 provided more onservative values or prestressed onrete beams. This an also be seen in Table 4 that shows the average ratio o experimental loads to theoretial values or all equations. Table 4.Average ratio experimental loads to theoretial values Ratio o experimental to theoretial shear apaity a/d Reinored onrete beams Prestressed onrete beams CSA-S6 ACI 440.1R CSA-S806 CSA-S6 ACI 440.4R CSA-S806 Average St. Dev COV [%] In ase o reinored onrete beams, both CSA-S and CSA-S6-10 perormed well with CSA-S providing slightly less onservative estimate o the experimental values. The same was true or prestressed onrete beams, where CSA-S was slightly less Page 6 o 8

7 onservative ompared to CSA-S6-10. The ACI guidelines were most onservative or both reinored and prestressed beams. The ACI guidelines are espeially onservative or prestressed onrete beams with smaller ross setion and high level o prestressing. This is not surprising as these beams have a high stiness as the result o prestressing that is not aounted or in the ACI 440.4R-04 shear ormula. The high oeiient o variation (COV) in Table 4 is due to the variability o other inluential ators among the samples used in the database. As mentioned earlier, these ators inlude a/d ratio, size o the beams, lexural rigidity o longitudinal reinorement, onrete ompressive strength and type o FRP tendon used. For reinored onrete beams, this variability was the least or CSA-S In ase o prestressed onrete beams, CSA-S and CSA-S6-10 showed approximately the same variation. For both reinored and prestressed onrete beams, CSA-S and CSA-S6-10 showed less variation ompared to ACI guidelines, whih shows these odes have relatively wider range o appliability. 5. Conlusions This paper presents results rom shear tests perormed on six FRP prestressed onrete T- beams with variable a/d ratio and adds to the urrent small database o FRP prestressed beams tested in shear. Prestressing an signiiantly improve the shear apaity o FRP reinored onrete beams. From the three major North Amerian design odes and guidelines or onrete beams reinored or prestressed with FRP, the ACI guidelines were the most onservative. ACI espeially requires revision or FRP prestressed beams. CSA-S better estimated the apaity o both reinored and prestressed beams. The study also showed that the shear apaity inreases with dereasing a/d ratio, whih is espeially notieable with deep beam with a/d < 2.5 that develop signiiant arh ation. Only CSA-S inludes a ator to aount or arh ation and provided reasonable estimate or beams tested in this projet. However, more testing is needed to onirm that. This paper shows that the advantage rom prestressing and arh ation on shear apaity o FRP reinored onrete beams is underestimated. This is mainly due to onservatism assoiated with shear design. More researh is required to redue the onservative nature o design guidelines and allow or more eiient and ost eetive designs as well as more appliation or FRP material. 6. Aknowledgments The authors would like to thank the Natural Sienes and Researh Counil o Canada or their inanial support. Also the assistane o Mr. Chad Klowak and Mr. Grant Whiteside in the strutures lab at the University o Manitoba is muh appreiated. Reerenes [1] MACGREGOR, J., BARTLETT, F., Reinored Conrete Mehanis and Design Sarborough, Ontario: Prentie-Hall. (2000). [2] PARK, S., NAAMAN, A., Shear Behaviour o Conrete Beams Prestressed with FRP Tendons, PCI Journal, January 1999, pp Page 7 o 8

8 [3] NISHIKAWA, K., KANDA, M., UCHIDA, K., Strutural Behavior o Prestressed Conrete Beams Using FRP Tendons, Publi works Researh Institute, Japan 1993 [4] WHITEHEAD, P., IBELL, T., Novel Shear Reinorement or Fiber-Reinored Polymer-Reinored and Prestressed Conrete, ACI Strutural Journal, Marh 2005 pp [5] Amerian Conrete Institute (ACI), Prestressing Conrete Strutures with FRP Tendons. ACI 440.4R-04, 2004 [6] MATTA, F., NANNI, A., HERNANDEZ, T., BENMOKRANE, B., Saling o Strength o FRP Reinored Conrete Beams without Shear Reinorement, 4 th Int. Con. on FRP Composites in Civil Engineering (CICE), Zurih, July 2008 [7] TUREYEN, A., FROSCH, R., Shear Tests o FRP-Reinored Conrete Beams without Stirrups, ACI Strutural Journal, Vol. 99, No.4, [8] RAZAQPUR, A., ISGOR, B., GREENAWAY, S., SELLEY A., Conrete Contribution to the Shear Resistane o Fiber Reinored Polymer Reinored Conrete Members, Journal o Composites or Constrution, Vol.8, No.5, September 2004, pp [9] MOTA, C., Flexural and Shear Behaviour o FRP-RC members, MS. Thesis, University o Manitoba, Winnipeg, 2005 [10] ZHAO, W., MARUYAMA, K., SUZUKI, K., Shear Behavior o Conrete Beams Reinored by FRP Rods as Longitudinal and Shear Reinorement., Pro. o FRPRCS-2 Conerene, Ghent, 1995, pp [11] YOST, J., GROSS, S., DINEHART, D., Shear Strength o Normal Strength Conrete Beams Reinored with Deormed GFRP Bars, Journal o Composites or Constrution, Vol.5, No.4, pp [12] AMERICAN CONCRETE INSTITUTE (ACI), Guide or the Design and Constrution o Strutural Conrete Reinored with FRP Bars, ACI 440.1R-06, 2006 [13] CANADIAN STANDARDS ASSOCIATION (CSA), Canadian Highway Bridge Design Code, CSA-S6-10, Mississauga, Ontario, 2010 [14] CANADIAN STANDARDS ASSOCIATION (CSA), Design and Constrution o Building strutures with Fibre-Reinored Polymers, CSA-S806-10, Mississauga, Ontario, 2010 Page 8 o 8