Partial versus full wrapping confinement systems for concrete columns

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1 Partial versus ull wrapping oninement systems or onrete olumns J. A. O. Barros Assistant Pro., Dep. o Civil Eng., Shool o Eng., Univ. o Minho, Azurém, Guimarães, Portugal D. R. S. M. Ferreira PhD Student, Dep. o Civil Eng., Shool o Eng., Univ. o Minho, Azurém, Guimarães, Portugal ATRACT: The present work aims to ompare the oninement eiay o ull and partial wrapping o onrete elements under ompression loads. The Mander et al. analytial model was modiied to predit the ompression stress-strain behaviour o onrete olumn elements partially onined by strips o CFRP lay-up sheets. The main results o the experimental program are presented and analysed. The model perormane is assessed using the experimental results. 1 INTRODUCTION Strutural elements suh as beams, slabs, and olumns may require strengthening during their servie lie period. The need or strengthening and rehabilitating o existing strutures is, in general, aused by the ollowing main reasons: inrease o servie load levels; material degradation; design/onstrution deets; new ode requirements. Carbon and Glass ibre reinorement polymer (CFRP, GFRP) sheets have been used as the reinoring system on strutural rehabilitation and strengthening. These materials are opportune alternatives to the use o onventional materials like onrete and steel in the strengthening pratie, sine they have high tensile strength, they are lightweight, whih makes its installation osts low in omparison to onventional steel systems, and they have high resistane to orrosion. The ull wrap o the onrete olumn with CFRP or GFRP sheets is a general pratie to inrease the load arrying apaity, the dutility, and the shear strength o this type o strutural elements, Mirmiran and Shahawy (1997). The present work aims to ompare the oninement eiay o ull and partial wrapping o onrete elements under ompression loads. For this purpose, series o ylinder onrete elements, onined by distint arrangements o CFRP wet lay-up sheets, were tested under diret ompression loading oniguration up to its rupture. The experimental program was designed to evaluate the inluene o the onrete strength lass, the stiness o the CFRP sheet, the number o strips, the width o the strip, and the number o layers per eah strip. The Mandel et al. analytial model was modiied in order to predit the ompression stress-strain responses o the onrete elements partially onined, Mander, Priestley and Park (1988). 2 CONFINEMENT ARRANGEMENTS The oninement systems are omposed by strips o CFRP sheet bonded to onrete and to subjaent layers by epoxy resin. Eah speimen is designated by WiSjLk, where Wi is the strip width, Sj is the number o strips along the speimen and Lk is the number o CFRP layers per eah strip. s'/2 W W s' W s'/2 s' a) b) ) d) Figure 1 - Generi oninement system and photos o some adopted oninement systems. Figure 1a) shematizes the partial oninement system, and Figure 1b) to Figure 1d) inludes photos o some o the adopted oninement systems. A detailed desription o the oninement arrangements and proedures are given elsewhere Ferreira and Barros (23). 3 MATERIALS SG SG1 SG2 To evaluate the inluene o the onrete strength lass and the stiness o the CFRP sheet on the on-

2 inement eiay provided by the distint CFRP arrangements, a moderate and a low strength onretes and two CFRP sheets o distint iber ontent were used in the experimental program. From uniaxial ompression tests arried out at 28 days with onrete ylinder speimens o 15 mm diameter and 3 mm height, average ompression strength o 23 MPa and 16 MPa was obtained or the moderate and low strength onretes, respetively. The CFRP sheets used has the trade name o Mbrae CF-13 (3 g/m 2 o ibers) and CF-12 (2 g/m 2 o ibers). Aording to the supplier, the Mbrae CF-13 and CF-12 sheets have a thikness o.167 mm and.117mm, respetively, and an attain a tensile strength higher than 37 MPa, and an elastiity modulus and an ultimate strain in the ibre diretion o about 24 GPa and 15, respetively, Mbrae (23). To evaluate these properties, samples o CFRP were tested aording to ISO reommendations (23). The obtained results are presented in table 1. Table 1 CFRP properties (average o ive tests) CFRP Ultimate Sheets strain (%) Tensile strength (MPa) Elastiity modulus (GPa) CF-12 (S&P) CF-13 (S&P) TEST SETUP Three displaement transduers were positioned at 12 degrees around the speimen and registered the displaements between the loading steel plates o the equipment. This test setup avoids that the deormation o the test equipment are being added to the values read by the LVDTs. Taking the values reorded in these transduers, the displaement at the speimen axis was determined or eah san reading, and the orresponding strain was obtained dividing this displaement by the measured speimen s height. To derease the oninement eet on the speimen introdued by the mahine load platens, a telon system was applied in-between the platens o the testing rig and the speimen extremities. Strains in the ibre diretion o the CFRP strips were measured by strain gauges (SG) plaed at hal width o the strip, aordingly to the arrangement represented in Figure 1. A detailed desription o the test equipment and test proedures an be ound in Ferreira and Barros (23). 5 EXPERIMENTAL RESULTS Figure 2 to Figure 5 show the relationships between onrete stress and both the onrete axial strain and the CFRP strain in the iber diretion or the groups o tests C23S3, C23S2, C16S2 and C16S3 onined with strips o 45 mm, 6 mm and 3 mm o width. In the designation attributed to the our groups o tests, C16 and C23 means speimens onstituted by a onrete average ompression strength o 16 and 23 MPa, respetively, while S2 and S3 indiates the type o CFRP sheet, 2 g/m 2 and 3 g/m 2, respetively. Eah urve represents the average response registered in the three speimens that ompose eah series. The onrete stress is the ratio between the applied load and the speimen ross setion. From the analysis o the results o group C23S3 it is veriied that it is not eetive to apply a number o layers higher than 5, sine the speimen load arrying apaity and its energy absorption apaity are not signiiantly inreased. Thereore, in the remaining groups o tests, the number o layers was 3 and _ext _ext1 _ext2 W45S4L7_ext2 W45S4L7_ext1 _ext1 W45S4L Axial strain (mm/mm) 5 W6S3L7 W6S3L5 W6S3L3 W6S3L7 W6S3L5 W6S3L W3S1L5_ext1 W3S1L3_ext1 W3S1L5 W3S1L Figure 2 Test group C23S3

3 1 1 _ext1 5 _ext2 _ext2 _ext W6S3L3_ext1 W6S3L5_ext1 W6S3L5 W6S3L Axial Strain(mm/mm) W3S1L5_ext1 W3S1L3_ext1 W3S1L5 W3S1L Figure 3 Test group C23S2 The obtained urves show that the speimen load arrying apaity inreases signiiantly when CFRP oninement ratio ( ρ ) is augmented. The stressstrain relationship up to the ompression strength o the plain onrete (PC) speimens is pratially unaeted by the presene o the CFRP. In general, the σ ε relationship o the onined speimens is omposed by two quasi-linear branhes. The stress orresponding to the transition point between these two branhes is higher than the ompression strength o the orresponding PC speimens. This eet is more pronouned in the groups o C23 onrete. The stiness o the seond branh (inlination) is higher in the group C16S3 (lower strength onrete onined by the stier CFRP sheet). The values o / o, ε /ε o and U /U o ratios (see Figure 6) or the tested series are indiated in Table 2 to 4. The onept o, o, ε, ε o, U and U o is shematially represented this Figure. Note that U o and U are the energy dissipated in the sotening phase o the unonined and onined speimen, respetively. The values o these ratios indiate that the eetiveness o a oninement system, in terms o inreasing the speimen load arrying apaity, deormability and energy absorption apaity, inreases when the onrete ompressive strength dereases and when the stiness o the CFRP sheet inreases. In series o equal ρ, suh is the ase o series W45S4 and W6S3, the oninement was more eetive in the W45S4 series sine the ree spae between the CFRP strips is smaller in this series, whih means that more volume o onrete is eetively onined. A high satter was registered on the maximum strain values in the CFRP, sine the reorded values only represent the areas where the strain gauges are plaed, and are too dependent on speimen ailure mode oniguration. In the series W3S1L5 o test group C23S3, the maximum apaity o the mahine was ahieved without the ourrene o the rupture o the speimens o this series _ext2 _ext1 _ext1 _ext W6S3L3_ext1 W6S3L5_ext1 W6S3L3 W6S3L W3S1L3_ext1 W3S1L5_ext W3S1L3 W3S1L Figure 4 Test group C16S2

4 1 1 5 _ext2 _ext W6S3L3 W6S3L5 S6S3L3 W6S3L W3S1L3_ext1 W3S1L5_ext1 W3S1L3 W3S1L Figure 5 Test group C16S3. σ o onined onrete Uo U FRP ailure unonined onrete εo 5.5 ε ε Figure 6 Stress-strain diagram or the evaluation o U. Table 2 - / o values or the group o tested series (see also Figure 6) Series Group C16S2 C16S3 C23S2 C23S W6S3L W6S3L W3S1L W3S1L Table 3 - ε /ε o values or the group o tested series (see also Figure 6) Series Group C16S2 C16S3 C23S2 C23S W6S3L W6S3L W3S1L W3S1L Table 4 - U /U o values or the group o tested series (see also Figure 6) Series Group C16S2 C16S3 C23S2 C23S W6S3L W6S3L W3S1L W3S1L MODIFIED MANDER ET AL. MODEL To evaluate the behaviour o ull wrapped onrete speimens with CFRP sheets several analytial models have been proposed, Saaman, Mirmiran and Shahawy (1998), Toutanji (1999), Xiao and Wu (2), Untiveros (22), Lam and Teng (23). Most o these models are based on stress-strain equations deined or modelling the oninement provided by steel jakets, Untiveros (22). To simulate the partial oninement systems o the present work, the model developed by Mander et al. (1988) seems to be the most appropriate. The Mander et al. model will be modiied to take into aount that the oninement is now provided by strips o CFRP sheet that has a tensile behaviour distint o the steel hoops onsidered in the Mander et al. original model. Aording to the Mander et al. model, the stress in the onined onrete ( ) is determined by the ollowing expression: xr = (1) r r 1+ x where 7.94 l l = (2) o o ε x = ; ε = ε o 1+ 5 ε 1 o (3) E r = ; E se = E ε (4) l E se 1 s' 4A = keρ ; = k e ; ρ = ; d s d s ( W + s' ) = ε (5) E, e 2

5 In eq. (2), is the maximum stress o the onined onrete, o is the maximum stress o the orresponding unonined onrete and l is the oninement pressure exerted by the CFRP, see Figure 7 and Figure 8. In eq. (3), ε is the onrete axial strain, and ε is the axial strain orrespondent to. In eq. (4), E is the onrete Young s modulus that, aording to Mander et al., an be determined by E =15( o ) (1/2). However, sine in the present work the strain was obtained rom the displaements measured between the mahine load platens, the E values are smaller to those values determined rom Mander et al. equation. Thereore, E was onsidered as being the initial slope o the σ -ε relationship reorded in the tests. In eq (5) l is maximum pressure applied by the CFRP, k e is a oeiient that depends on the oninement s oniguration, s is the distane between strips (see Figure 1), d s is the diameter o the speimen (15 mm), ρ is the CFRP oninement ratio, A is the CFRP ross setion area per unit o volume o onrete (in the present oninement system, A =W e L, see Figure 1, where e is the thikness o the CFRP sheet), is the maximum stress in the CFRP and E is the elastiity module o the CFRP. In the original Mander el al. model, is the yield stress o the steel hoops. For the CFRP disrete oninement proposed in the present work, represents the eetive stress installed in the CFRP strips. This eetive stress is obtained rom the onept o eetive strain, ε,e, whih was determined rom bakitting analysis, using the stress-axial strain urves registered in the experimental program. Sine the strain in the CFRP is not uniorm in the perimeter o the strip, ε,e was determined multiplying the maximum reorded strain, ε máx, by an eetive oeiient, k, resulting = E k ε máx. The k values were determined in order to approximate with the minimum error the analytial and the experimental ε relationships. For the speimens onined with three layers per strip k =.6, while in speimens with ive layers k =.4, whih means that k dereases with the inrease o the stiness o the oninement system. σ σ o Analyti (Mander Model) Behaviour elasti-plasti o steel bars Behaviour linear elasti o CFRP Experimental Unonined onrete ε o ε ε Figure 7 Stress-strain diagram or the onined onrete. CFRP ds Figure 8 - Sheme o oninement ation. Tensão axial () Tensão axial () Tensão axial () Tensão axial() l _C23S Extensão axial (ε ) _C23S2 Mander_modiiado Mander_modiiado Extensão axial (ε) W6S3L3_C23S Extensão axial (ε) W6S3L5_C23S2 Mander_modiiado Mander_modiiado Extensão axial (ε) Figure 9 Curves axial stress versus axial strain.

6 The ε relationship o the series W45S4 and W6S3 or the group C23S2 was predited by this modiied Mander el al. model. Using the E =21GPa obtained in the tensile tests (see Table 1), it was determined the analytial ε urves that are ompared with the orresponding experimental ones, in Figure 9. The analytial urves it quite well the experimental ε relations. In the remaining series a level o auray similar to the one o C23S2 group was obtained. The larger deviations our or strain levels above ε o (see also Figure 7). Sine the Mander et al. model was developed to simulate the oninement provided by steel hoops, the shape o the branh ater ε o has a nonlinear proile. This nonlinear shape is not so pronouned in the experimental ε relations beause the oninement is provided by CFRP materials that has linear-elasti behaviour up to ailure. 7 CONCLUSIONS In the present work, the behaviour under diret ompression o onrete speimens onined by disrete and ontinuous CFRP systems is analysed. The disrete oninement system is omposed by strips o CFRP wet lay-up sheets while the ontinuous oninement system orresponds to ull wrapping the onrete speimen. The inluene o the strip s width, the number o strips along the speimen, the number o CFRP layers per strip, the onrete strength lass and the stiness o the CFRP sheet, was analyzed. The speimen load arrying apaity has inreased with the CFRP oninement ratio, ρ. In series o equal ρ, the most eetive oninement system was the one o lower distane between strips o CFRP, sine it orresponds to the oninement oniguration where the wrapping material is distributed more uniormly along the length o the speimen. Amongst the groups o series o tests, the most eetive was the one o speimens manuatured by the lower onrete strength and onined by the highest sti CFRP sheet (C16S3). For this series the ratio between the ultimate load o onined speimens and the ompression strength o its orresponding unonined speimens have varied between 2.62 (series W6S3L3) and 6.58 (W3S1L5). In omparison to the ull wrapping oninement system, the partial oninement arrangements are easier and aster to apply, and onsume ew CFRP and epoxy adhesive materials. The analytial model developed by Mander et al. to simulate the stress-strain relationship o onrete speimens onined with steel hoops was modiied in order to take into aount that the oninement systems are now made by CFRP material that has linear-elasti behaviour up to its ailure. The modiied Mander s analytial model has predited with good auray the experimental responses. 8 ACKNOWLEDGMENTS The authors o the present work wish to aknowledge the generous support provide by MBT Bettor Portugal and S&P Clever Reinorement. The seond author would like to thank the inanial support by PRODEP ation 5.3/N/199.14/1. 9 REFERENCES Bettor(23). Master Building Tenologies, Tehnial and ommerial doument. Ferreira D., Barros J., 23. Strategies or onrete oninement with strips o CFRP sheets. CCC23-Composites in Constrution International Conerene, University o Calabria, Italy, o September, ISO TC 71/SC 6 N (23). Non-onvenional reinorement o onrete-test methods-part 2: Fiber reinored polymer (FRP) sheets. Lam, L., Teng. J. G., 23. Design-oriented stress-strain model or FRP-onined onrete. Constrution and building materials, Elsevier, vol. 17, p Mander, J. B., Priestley, M. J. N. e Park, R. (1988). Theoretial stress-strain model or onined onrete. Journal o Strutural Engineering, ASCE, 114(8), Mirmiran, A.& Shahawy, M.,1997. Behavior o onrete olumns onined by iber omposites. Journal o Strutural Engineering, ASCE, May, 123(5), Samaan, M., Mirmiran, A., Shahawy, M Model o onrete onined by iber omposites. Journal o Strutural Engineering, ASCE, 124(9), 1-131, Saai, M., Toutanji, H. A., e Li, Z Behavior o onrete olumns onined with iber reinored polymer tubes. ACI Material Journal, 96(4), 5-59, Spoelstra, M., R. e Monti, G., FRP-onined onrete model. Journal o omposites or onstrution, ASCE, 3(3), , Toutanji, H. A Stress-strain harateristis o onrete olumns externally onined with advaned iber omposites sheets. ACI Material Journal, 96(3), Xiao, Y., Wu, H. (2). Compressive behavior o onrete onined by arbon iber omposite jakets. Journal o Material in Civil Engineering, ASCE, 1(3), 5-264, Untiveros, Carlos, 22. Estudio experimental del omportamiento del hormigón oninado sometido a ompresión. Tesis dotoral, Universitat Politènia de Catalunya.