EXPERIMENTAL TIME-DEPENDENT DEFLECTIONS OF CONCRETE BEAMS REINFORCED WITH GFRP BARS

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1 EXPERIMENTAL TIME-DEPENDENT DEFLECTIONS OF CONCRETE BEAMS REINFORCED WITH GFRP BARS Crstna MIÀS MSc Unverstat de Grona Escola Poltècnca Superor. Av. Montlv s/n. Grona. SPAIN Lluís TORRES PhD Unverstat de Grona Escola Poltècnca Superor. Av. Montlv s/n. Grona. SPAIN Albert TURON PhD Unverstat de Grona Escola Poltècnca Superor. Av. Montlv s/n. Grona. SPAIN Crstna BARRIS PhD Unverstat de Grona Escola Poltècnca Superor. Av. Montlv s/n. Grona. SPAIN Abstract Control of deflectons can usually be expected to be a lmtng factor n the desgn of fbre renforced polymer renforced concrete (FR P RC) structures. Deflectons due to creep and shrnkage can be several tmes larger than the nstantaneous values. Reported expermental data on tme-dependent deflectons of FRP RC structures are stll lmted compared to conventonal steel RC structures. Results avalable n the lterature ndcate that the prncples underlyng the formulaton used for predctng the long-term deflectons of steel RC structures can also be used for FRP RC structures. The results of an expermental study of beams renforced wth glass fbre renforced (GFRP) bars subjected to sustaned load durng 36 days are presented. Two levels of sustaned load and two dfferent renforcement ratos were tested. The expermental long-term deflectons are compared to the predctons made usng CEB-FIP, ACI 44.1R-6 and CSA-S86- procedures. Deflectons obtaned wth CEB-FIP procedure are n a good agreement wth the expermental values. However, some dfferences are found when ACI 44.1R-6 or CSA-S86- procedures are appled. Keywords: deflecton, fbre renforced polymers, long-term, sustaned load 1. Introducton Fbre renforced polymer (FRP) bars offer an alternatve to steel renforcement n concrete structures where the corroson n steel can be present. Due to the lower stffness of FRP compared to steel, deflecton control can usually be expected to be a lmtng factor n the desgn of FRP renforced concrete (RC) structures. Although nstantaneous deflectons have been wdely studed, further expermental work seems to be necessary for the case of longterm deflectons. Publshed data on short and tme-dependent deflectons of FRP RC members Page 1 of 8

2 ndcate that the prncples underlyng the formulaton used for predctng the long-term deflectons of steel RC structures can also be used for FRP RC structures [1]. Based on expermental results [,3], ACI 44.1R-6 [4] suggests a smlar approach for FRP RC structures as for steel ntroducng a reducton factor of.6 n the ACI approach [5,6] equaton. On the other hand, expermental results of GFRP RC beams [7,8] show that general CEB-FIP procedure [9,1] together wth creep and shrnkage curvature coeffcents found n Ghal and Favre [11] can predct accurately the deflectons of FRP RC members. The procedure accounts for the nfluence of the geometry, materal and envronmental condtons. In ths paper, results of expermental long-term deflectons of GFRP RC beams, subjected to a sustaned load for a perod of 36 days, are presented and analysed. The expermental data are compared wth the CEB-FIP [9-11] procedure, together wth other methodologes specfc for FRP RC structures such as ACI 44.1R-6 [4] and CSA-S86- [1].. Expermental test program.1 Beams specfcatons A total of 8 concrete beams of 14x19x45 mm (Fgure 1) wth a clear cover equal to mm were cast wth two dfferent amounts of GFRP (Ø1 or Ø 16) bars placed at the tenson sde (bottom). For comparson purposes addtonal beams were renforced w th Ø1 steel bars, havng smlar stffness to those renforced wth Ø 16 GFRP. No compresson renforcement was used. The shear-span was renforced wth steel strrups (Ø8mm/8mm) to avod shear falure, however, no strrups were provded n the central pure bendng zone. The beams are desgned as H_Lx-Gyyz (Table 1), where x denotes the level of sustaned load; yy, the dameter of the GFRP bar; and z the exstence of a notch at md-span (z=a notch, z=b no notch). Ths notch was ntroduced to ensure the propagaton of a crack at the central secton, whch was nstrumented by means of stran gauges placed on concrete and rebar. P/ P/ Steel strrups Ø8/8mm Fgure 1. Geometrc detals of the tested beams Table 1. Geometrc propertes Ø bar (mm) Renforcement ρ (%) H_L1_G1a 1 GFRP 1. H_L1_G1b 1 GFRP 1. H_L_G1a 1 GFRP 1. H_L_G1b 1 GFRP 1. H_L1_G16a 16 GFRP 1.8 H_L1_G16b 16 GFRP 1.8 H_L_G16a 16 GFRP 1.8 H_L_G16b 16 GFRP 1.8 H_L1_S1 1 Steel.7 H_L_S1 1 Steel.7 Page of 8

3 . Materal propertes The expermental compressve strength, f c, was 56MPa and the modulus of elastcty, E c, was 9GPa. Both were determned from cylnder tests. GFRP bars (ComBAR) wth nomnal dameters of 1mm and 16mm, and modulus of elastcty of 6MPa, were used..3 Test set-up The expermental program conssted of testng smply supported beams spannng mm, and subjected to two concentrated loads for a perod of tme 36days (Fgure 1). Before the applcaton of the sustaned loads, P sus, the beams were subjected to servce load and then cycled twce between the maxmum load and a mnmum of kn. The servce load was desgned to obtan a maxmum bar stran equal to με. Immedately after ths nstantaneous test, the beams were moved to the correspondng sustaned load frames (Fgure a). The sustaned loads were desgned to obtan nstantaneous concrete compressve stresses of.3f c and.45f c at the md-span secton. The magntudes of the sustaned loads are gven n Table. Table. Sustaned loads Beam seres H_L1_G1 H_L_G1 H_L1_G16 H_L_G16 H_L1_S1 H_L_S1 Load, P sus,(kn) a) b) Fgure. Set-up of (a) long-term flexural tests (b) creep tests In order to determne the creep coeffcent accordng to ASTM C51-, concrete cylnders (Ø, L=45mm) wth embedded stran gauges were loaded wth a constant pressure equal to bars (Fgure b). Addtonal specmens were left unloaded and were also montored to determne free shrnkage stran. All the specmens were placed on the same room and condtons as the beams. Page 3 of 8

4 3. Expermental results 3.1 Tme-dependent concrete propertes Obtaned results from tests of creep and shrnkage of concrete are shown n Fgure 3. Fgure 3a shows the evoluton of creep coeffcent obtaned from the stran measurements of the cylnders placed on the loadng frame shown n Fgure b. At 36days, the expermental creep coeffcent was equal to 1.86, and the free shrnkage stran was equal to 378 με (Fgure 3b). Creep coeffcent Specmen C.1 Specmen C Shrnkage stran (µ ε) Specmen C.3 Specmen C Fgure 3. Expermental free shrnkage stran 3. Tme-dependent deflectons Expermental total deflectons versus tme of tested beams are presented n Fgures 4-6. The total deflecton ncludes mmedate and long-term deflectons due to creep and shrnkage. As t can be observed, the expermental values of beams renforced wth GFRP and steel bars present the same trend. The ratos between the total deflectons and the nstantaneous deflectons at 36 days are presented n Table 3, beng equal to 1.4 for beams renforced wth Ø1 GFRP bars, and 1.5 for beams renforced wth Ø16 GFRP and steel bars, whch were desgned to have a smlar stffness Total deflecton (mm) H_L1_G1a H_L1_G1b H_L_G1a H_L_G1b Fgure 4. Total deflectons for beams renforced wth 1mm dameter bars Page 4 of 8

5 18 Total deflecton (mm) H_L1_G16a H_L1_G16b H_L_G16a H_L_G16b Fgure 5. Total deflectons for beams renforced wth 16mm dameter bars Total deflecton (mm) H_L1_S1 H_L_S Fgure 6. Total deflectons for beams renforced wth 1mm dameter steel bars Table 3. Rato between the total deflecton and the correspondng deflecton at tme of loadng, dt/d Tme(days) H_L1_G1 H_L_G1 H_L1_G16 H_L_G16 H_L1_S1 H_L_S Accordng to ACI 44.1R-6 [4], for beams contanng no compresson renforcement, the total deflecton ncludng the effect of creep and shrnkage, T ( ACI 44), of FRP RC members can be obtaned from the ntal deflecton due to sustaned load, (sus) :. 1 ( sus) T ( ACI 44) 6 where s a tme-dependent factor for sustaned loads whch s equal to 1, 1., 1.4 and. for three, sx, twelve and sxty months, respectvely. The Canadan Standard CSA-S86- [1] provdes a smlar approach but more conservatve: () ( ) T ( CSA) 1 sus On the other sde, based on the procedure presented n the CEB Manual on Crackng and Deformatons [1], descrbed and llustrated n Ghal et Favre [11], total deflectons, δ T(CEB- (1) Page 5 of 8

6 FIP), can be obtaned by nterpolatng between state 1 (uncracked transformed secton) and state (fully-cracked transformed secton): M M (3) cr cr T ( CEB FIB).5,1( t, t) T 1.5 T,( t, t) M M where the predcted total deflectons for each state (1 and ) can be obtaned as the sum of the ntal deflecton, ( t ), and the long-term deflecton due to creep and shrnkage, ( t, t) : t ) ( t, t ) (4) T, ( subscrpt takes the value of 1 for the uncracked state and for the fully-cracked state. The long-term deflecton due to creep and shrnkage are obtaned from the followng expresson: sh ( t, t) l ( t, t) ( t) ( t, t) (5), sh, d 8d where, and sh, the curvature coeffcents related to creep and shrnkage, respectvely. A detaled explanaton of the procedure for obtanng of [11]. Snce calculaton of, and sh, developed for practcal applcatons [1,11]., and sh, can be found n Appendx F s not mmedate, desgn charts have been The expermental total deflectons versus tme snce loadng are compared wth the analytcal predctons based on CEB procedure [9,1], ACI 44.1R-6 [4], CSA-S86- [1]. Snce the sustaned loads were appled after quas-statc loadng cycles, and ACI and CSA procedures do not specfcally account for ths condton, the short-term deflecton has been calculated usng Bschoff s equaton, eq. (6), wth a β coeffcent of.5 [13,14]: Icr Ie M cr I cr (6) 1 1 M g I g In Fgure 7-8 total deflectons of GFRP RC beams are shown and compared wth theoretcal values. As t can be observed, predctons usng the CEB-FIP procedure ft accurately wth the expermental results, however, ACI 44.1R-6 and CSA-S86- overestmate the total deflectons n all the cases. Total deflecton (mm) 1 5 H_L1_G1a H_L1_G1b ACI 44.1R-6 CEB-FIP CSA-S Total deflecton (mm) H_L_G1a H_L_G1b ACI 44.1R-6 CEB-FIP CSA-S Fgure 7. Total deflectons versus tme snce loadng for beams renforced wth Ø1 GFRPbars Page 6 of 8

7 5 Total deflecton (mm) 1 5 H_L1_G16a H_L1_G16b ACI 44.1R-6 CEB-FIP CSA-S Total deflecton (mm) 1 5 H_L_G16a H_L_G16b ACI 44.1R-6 CEB-FIP CSA-S Fgure 8. Total deflectons versus tme snce loadng for beams renforced wth Ø16 GFRPbars The rato between the expermental and predcted deflectons after 36 days of sustaned load s gven n Table 4. On average, the ratos between the theoretcal values and the expermental deflectons, usng ACI 44.1R-6 and CSA-S86- recommendatons for FRP RC beams, are 1.4 and 1.6, respectvely. On the other hand, ths rato s equal to.94 usng the CEB-FIP procedure. Ths last method accounts for the nfluence of geometry, materals and envronmental condtons, ncluded n the creep coeffcent and free shrnkage stran as well as the nfluence of the dfferent sectonal stffness on the long-term behavour. Table 4. Rato between theoretcal and expermental total deflecton at 36 days. Beams CEBFIB exp ACI exp CSA exp H_L1_G H_L_G H_L1_G H_L_G Mean Conclusons In ths artcle, results from an expermental program of 8 GFRP and steel RC beams subjected to a sustaned load for a perod of 36 days are presented. From the obtaned expermental results t can be concluded that: - The expermental values of beams renforced wth GFRP and steel bars present the same trend, n ther evoluton wth tme. - The rato of total deflecton ncrease to ntal deflecton was 1.4 for beams renforced wth Ø1 GFRP bars, and 1.5 for beams renforced wth Ø1 GFRP and Ø1 steel bars. Addtonally, expermental deflectons are analyzed and compared wth predcton models. CEB-FIP procedures gve accurate predctons for all the tested specmens, whle ACI 44.1R-6 and CSA S86- do not predct the long term deflectons wth the same level of accuracy for all cases. Although CEB-FIP procedure requres a greater degree of calculaton, t allows takng nto account the nfluence of geometry, materal propertes, and envronmental condtons, ncluded n the creep coeffcent, and the free shrnkage stran, as well as the nfluence of the dfferent sectonal stffnesses, on the long-term behavour. Page 7 of 8

8 5. Acknowledgements The authors acknowledge the support provded by the Spansh Government (Mnstero de Cenca e Innovacón), Project BIA1-34-C3-. The authors also thank the support of Schök Bautele GmbH for the supply of GFRP rebars. The frst author acknowledges the support from the Mnstero de Educacón y Cenca for Grant BES and the moblty grant SEST1I9XV. 6. References [1] FIB BULLETIN 4, FRP renforcement n RC structures, Task Group 9.3 FRP (Fbre Renforced Polymer) renforcement for concrete structures, Federal Insttute of Technology Lausanne, Swtzerland, 7. [] BROWN, V., Sustaned Load Deflectons n GFRP-Renforced Concrete Beams, Proceedngs of the Thrd Int. Symposum on Non-Metallc (FRP) Renforcement for Concrete Structures (FRPRCS-3), Vol., Japan Concrete Insttute, Tokyo, Japan, 1997, pp [3] BROWN, V., BARTHOLOMEW, C., Long-term deflectons of GFRP-renforced concrete beams, Frst Internatonal Conference on Compostes n Infrastructure, (ICCI-96), H. Saadatmanesh and M. R. Ehsan, eds., Tucson, Arzona, 1996, pp [4] ACI 44.1R-6, Gude for the Desgn and Constructon of Concrete Renforced wth FRP Bars, ACI Commttee 44, Amercan Concrete Insttute, Farmngton Hlls, 6. [5] BRANSON, DE., Compresson steel effect on long-tme deflectons, ACI Journal Proceedngs, Vol. 68, No. 8, 1971, pp [6] ACI Buldng Code Requrements for Renforced Concrete, ACI Commttee 318. Amercan Concrete Insttute, Detrot, Mchgan, 8. [7] HALL, T., GHALI, A., Long-term deflecton predcton of concrete members renforced wth glass fbre renforced polymer bars, Canadan Journal of Cvl Engneerng, Vol. 7, No. 5,, pp [8] AROCKIASAMY, M., CHIDAMBARAMA, S., AMERA, A., SHAHAWY, M., Tmedependent deformatons of concrete beams renforced wth CFRP bars, Compostes Part B: Engneerng, Vol.31,, pp [9] CEB-FIP MODEL CODE 199, Comté Euro-Internatonal du Béton. Fédératon Internatonale de la Précontrante, Thomas Telford House, London, England, 199. [1] Manual on Crackng and Deformatons Butlletn d Informaton nº 8-E. Comté Euro-Internatonal du Béton. [11] GHALI, A., FAVRE, R., Concrete Structures: Stresses and Deformatons. Ed. Chapman and Hall, London, [1] CSA-S86-, Desgn and constructon of buldng components wth fbre-renforced polymers, Canadan Standards Assocaton, Mssssauga, Ontaro, Canada,. [13] BISCHOFF, PH., Reevaluaton of Deflecton Predcton for Concrete Beams Renforced wth Steel and Fber Renforced Polymer Bars, Journal of Structural Engneerng, ASCE; Vol. 131, No.5, 5, pp [14] SCANLON, A., BISCHOFF, PH., Shrnkage restrant and loadng hstory effects on deflectons of flexural members, ACI Structural Journal, Vol., No. 4, 8, pp Page 8 of 8