FRP Confinement of Heat-Damaged Circular RC Columns

Transcription

1 International Journal of Conrete Strutures and Materials Vol.11, No.1, pp , Marh 217 DOI 1.17/s ISSN / eissn FRP Confinement of Heat-Damaged Cirular RC Columns Hanan Suliman Al-Nimry*, and Aseel Mohammad Ghanem (Reeived February 27, 216, Aepted November 17, 216, Published online February 7, 217) Abstrat: To investigate the effetiveness of using fiber reinfored polymer (FRP) sheets in onfining heat-damaged olumns, 15 irular RC olumn speimens were tested under axial ompression. The effets of heating duration, stiffness and thikness of the FRP wrapping sheets were examined. Two speimen groups, six eah, were subjeted to elevated temperatures of 5 C for 2 and 3 h, respetively. Eight of the heat-damaged speimens were wrapped with unidiretional arbon and glass FRP sheets. Test results onfirmed that elevated temperatures adversely affet the axial load resistane and stiffness of the olumns while inreasing their dutility and toughness. Full wrapping with FRP sheets inreased the axial load apaity and toughness of the damaged olumns. A single layer of the arbon sheets managed to restore the original axial resistane of the olumns heated for 2 h yet, two layers were needed to restore the axial resistane of olumns heated for 3 h. Glass FRP sheets were found to be less effetive; using two layers of glass sheets managed to restore the axial load arrying apaity of olumns heated for 2 h only. Confining the heatdamaged olumns with FRP irumferential wraps failed in reovering the original axial stiffness of the olumns. Test results onfirmed that FRP-onfining models adopted by international design guidelines should address the inreased onfinement effiieny in heat-damaged irular RC olumns. Keywords: RC olumns, heat-damaged olumns, repair, fiber reinfored polymers, onfinement, CFRP, GFRP, axial strength, dutility. 1. Introdution Compared to other onstrution materials, onrete provides superior fire resistane as a result of its low thermal ondutivity and inombustible nature. Proper design of typial onrete strutures for fire resistane simply requires seletion of appropriate member dimensions and onrete overs. Yet, exposure of onrete to long duration fires or elevated temperatures adversely affets its ompressive and flexural strengths, modulus of elastiity and volume stability among other mehanial properties (Chan et al. 1999; Luo et al. 2; Husem 26; Arioz 27, 29; Netinger et al. 211). Signifiant losses in the ompressive strength of onrete ranging between 5 and 7% are typially enountered at temperatures above 55 C (Georgali and Tsakiridis 25). Depending on the extent of fire-indued damages and the residual load-bearing apaities, it is often both tehnially and eonomially viable to repair and reuse onrete strutures after fire. An extensive variety of repair and strengthening tehniques an be implemented to rehabilitate fire-damaged onrete elements and strutures. Traditional repair shemes Department of Civil Engineering, Jordan University of Siene and Tehnology, Irbid 2211, Jordan. *Corresponding Author; hsnimry@just.edu.jo Copyright The Author(s) 217. This artile is published with open aess at Springerlink.om for defiient or damaged onrete olumns involve the use of onrete or steel jakets (Lin et al. 1995; Campione 212; Ramirez et al. 1997; Xiong et al. 211) whih inurs additional weight and interrupts the funtion of the struture during the somewhat lengthy repair proess. Compared to more onventional repair materials, the use of externally bonded fiber reinfored polymer (FRP) omposites offers numerous advantages inluding, but not limited to, their high strength-to-mass ratio, high stiffness-to-mass ratio, exellent orrosion resistane, adaptability, non invasive nature and ease of appliation whih minimizes funtionality interruption. As suh, the numerous appliations of FRP omposites in repairing reinfored onrete (RC) strutural elements have been widely investigated over the past three deades. Very few experimental investigations have reently examined the potential of using FRP omposites to repair heatdamaged RC olumns (Yaqub and Bailey 211a, b, 212; Yaqub et al. 211, 213; Bisby et al. 211; Bailey and Yaqub 212; Tahir et al. 213; Al-Nimry et al. 213; Roy et al. 214, 216; Al-Kamaki et al. 215). Most of the test olumns were unstressed during heating, ooling and testing despite the fat that olumns are expeted to be stressed up to 5% of their apaity during any fire inident. To this end, Al-Kamaki et al. (215) applied an axial ompressive loading of 3% of the maximum ompressive strength at ambient temperature while exposing the test olumns to elevated temperatures. Available researh results (Yaqub and Bailey 211a, b, 212; Yaqub et al. 211, 213; Bisby et al. 211; Bailey and Yaqub 212; Tahir et al. 213; Al-Nimry 115

2 et al. 213; Roy et al. 214, 216; Al-Kamaki et al. 215) indiated that jaketing heat-damaged olumns with unidiretional FRP sheets may, depending on the ross setional shape, restore the axial strength of the olumns. FRP jaketing has been found effiient in enhaning the dutility, deformation and energy dissipation apaities of heat-damaged olumns. To restore the original axial strength of square heat-damaged RC olumns; Al-Nimry et al. (213) proposed the use of externally bonded longitudinal FRP plates onfined with irumferential arbon FRP sheets over the full olumn height. Glass and arbon FRP jaketing showed no signifiant effet on the axial stiffness of wrapped heatdamaged olumns (Yaqub and Bailey 211a, b, 212; Yaqub et al. 211, 213; Bisby et al. 211; Al-Nimry et al. 213). Yaqub et al. (213) proposed the use of both ferroement and FRP jakets to restore the strength, stiffness and dutility of fire-damaged RC olumns. Compared to the onfinement strengthening systems provided by high-strength fiber-reinfored onrete, ferroement and steel plate jaketing; Roy et al. (216) found that FRP jaketing was the most effetive in restoring the ompressive strength and energy dissipation of heat-damaged RC short olumns. The present study explores the feasibility of using external FRP onfinement in restoring or enhaning the axial strength and stiffness of irular RC olumns that have been exposed to a pre-defined heat level. Effets of the heating duration/ level, stiffness and thikness of the unidiretional FRP sheets used to onfine the heat-damaged olumns were among the parameters investigated. 2. Experimental Program 2.1 General A total of 15 RC olumn speimens having a 192 mm diameter and 9 mm length were ast, wet-ured and tested under axial loading. Three unheated speimens were designated as undamaged ontrols: two idential speimens (CH-A and CH-B) were tested without jaketing whereas the third (CH-C1L) was onfined using arbon fiber reinfored polymer (CFRP) sheets before testing. The remaining 12 speimens were equally divided into two groups (CH2 and CH3), six speimens eah, and subjeted to elevated temperatures of 5 C for 2 (CH2 speimens) and 3 h (CH3 speimens). In eah of these groups, two speimens were designated as damaged ontrols (CHi-A and CHi-B) where the subsript i indiates the heating duration in hours (2 or 3 h). Four heat-damaged speimens in eah of the two groups CH2 and CH3 were repaired using FRP jakets. All repair shemes were intended to provide external onfinement for the heat-damaged olumns. The olumns were wrapped with unidiretional FRP sheets with the diretion of fibers oriented parallel to the irumferene of the ross setion. Both arbon and glass FRP sheets were used as follows: Two heat-damaged speimens in eah of the two groups CH2 (heated for 2 h) and CH3 (heated for 3 h) were wrapped using CFRP sheets: speimen CHi-C1L wrapped with a single layer and speimen CHi-C2L wrapped with two layers of the fabri sheets where the subsript i indiates the heating duration in hours. Aording to ACI 44.2R (28) the CFRP sheets, with a thikness o.131 mm, provide an FRP reinforement ratio o.273 and.546 for the single and double layer wraps, respetively. Two heat-damaged speimens in eah of the two groups CH2 and CH3 were onfined using glass fiber reinfored polymer (GFRP) sheets: speimens CHi-G1L and CHi- G2L were wrapped using one and two layers of the.17 mm thik sheets, respetively. Aording to ACI 44.2R (28), the single and double layer GFRP wraps provide an FRP reinforement ratio o.354 and.78, respetively. The speimens designations and relevant test parameters are summarized in Table Layout and Detailing of Test Speimens The general layout and reinforement details of the test speimens are shown in Fig. 1. All olumn speimens were designed as short olumns with a irular ross setion of 192 mm diameter and an unsupported length of 9 mm. Six steel deformed rebars of 1-mm diameter were used for the main olumn reinforement providing a longitudinal reinforement ratio o.17. Transverse reinforement was provided using 6-mm diameter deformed ties at a uniform spaing of 15 mm. The ties were provided with a 6 mm overlap as shown in Fig Materials Crushed oarse limestone aggregates (with a maximum aggregate size of 9.5 mm) and a mixture of rushed fine limestone and silia sands (6% fine limestone and 4% silia sand by volume) were used with ordinary Portland ement (Type I) to prepare the onrete mix for the olumns following ACI (1991) mix design proedure. The onrete mix was designed using a water-to-ement ratio of.54. A super plastiizer was used at.5% by ement weight to ahieve a slump of about 75 mm. Five onrete bathes were used to ast the olumn speimens with an average 28-day ompressive strength (f ć ) of 41 MPa. To determine onrete strength of the olumns (both unheated and heated) at time of testing; additional onrete ylinders ( mm) were prepared, wetured, heated (as appliable) and tested with their ompanion olumn speimens: ylinders tested, without heating, resulted in an average ompressive strength of about 52 MPa. Heat-damaged ylinders that were subjeted to the same regimen of elevated temperatures and air ooling experiened by the CH2 and CH3 olumns exhibited substantial redutions in the ompressive strength of onrete of 62.5 and 67.3%, respetively. Average ompressive strength values of 19.5 and 17 MPa were reorded for heating durations of 2 and 3 h, respetively. In fat, elevated temperatures of about 55 C have been reported to ause 116 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)

3 Table 1 Test speimens designations and test parameters. Speimen designation Heating duration (h) Wrapping sheme Notes CFRP layers GFRP layers Control CH-A Unheated; unwrapped Control CH-B Unheated; unwrapped Control CH-C1L 1 Unheated; wrapped with 1 layer of CFRP Control CH2-A 2 Heated (2 h); unwrapped Control CH2-B 2 Heated (2 h); unwrapped Control CH3-A 3 Heated (3 h); unwrapped Control CH3-B 3 Heated (3 h); unwrapped CH2-C1L 2 1 Heated (2 h); wrapped with 1 layer of CFRP CH2-C2L 2 2 Heated (2 h); wrapped with 2 layers of CFRP CH2-G1L 2 1 Heated (2 h); wrapped with 1 layer of GFRP CH2-G2L 2 2 Heated (2 h); wrapped with 2 layers of GFRP CH3-C1L 3 1 Heated (3 h); wrapped with 1 layer of CFRP CH3-C2L 3 2 Heated (3 h); wrapped with 2 layers of CFRP CH3-G1L 3 1 Heated (3 h); wrapped with 1 layer of GFRP CH3-G2L 3 2 Heated (3 h); wrapped with 2 layers of GFRP CH, unheated olumn; CH2, olumn heated for 2 h; CH3, olumn heated for 3 h; C1L, wrapped with 1 layer of arbon sheets; C2L, wrapped with 2 layers of arbon sheets; G1L, wrapped with 1 layer of glass sheets; G2L, wrapped with 2 layers of glass sheets; A and B, sequential numbering of idential speimens. Fig. 1 Layout and detailing of speimens. substantial redutions in onrete ompressive strength reahing up to 7% of strength at ambient temperature (Georgali and Tsakiridis 25). The average yield stress of the main steel reinforement was 451 MPa with about 18% elongation at failure. To assess the effet of elevated temperatures on the mehanial properties of the hot-rolled steel reinforement, longitudinal steel bars were extrated from two of the ontrol heated olumns (CH2-A and CH3-A) and were tested. Exposure to the 2- and 3-h heating durations of 5 C resulted in minor losses (2.2 and 7.5%) in tensile yield strength of the steel reinforement. Atually, Neves et al. (1996) showed that temperatures below 6 C have a negligible effet on the tensile strength of hot-rolled steel after air ooling. The geometri and physial properties of the arbon and glass FRP fabri sheets used for repair are summarized in Table 2. It should be noted that for the same number of FRP wrapping sheets the onfinement modulus E l (given by Eq. (1), whih is a measure of the stiffness of the onfining International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217) 117

4 Table 2 Physial properties of FRP sheets (based on manufaturer s data). Type of sheets CFRP GFRP Width (mm) 3 5 Thikness (mm) Areal weight (g/m 2 ) 23 ± ± 22 Nominal tensile strength (MPa) Nominal tensile E-modulus (MPa) 238, 76, Nominal strain at break (%) FRP jaket, of the arbon FRP jaket amounts to 2.4 times that of the glass FRP jaket. E l ¼ 2E f nt f D ð1þ where E f is the tensile elasti modulus of FRP sheets, n the number of FRP layers, t f the nominal thikness of one FRP layer and D the diameter of the olumn ross setion. 2.4 Speimen-Preparation Test speimens were ast into PVC plasti molds. All speimens were ast in a vertial position, de-molded 72 h after asting, and then wet-ured using moist anvas for 28 days. The test speimens were transferred to the open lab environment wherein superfiial defets and pores were repaired using a ommerially available dental plaster material in preparation for heating. On the average, the test speimens were heated 2 months after asting. Although the moisture ontent of the onrete was not measured at the time of heating, it is expeted that the moisture ontent of the surfae onrete was that of the ambient with a relative humidity of about 6%. Twelve speimens were subjeted to temperatures of 5 C: six for 2 h and six for 3 h using an eletrial furnae that automatially ontrols the temperature and time of exposure. The furnae size ( m) allowed heating four olumn speimens and six ylinders at a time as shown in Fig. 2. Column end surfaes were insulated, using Rokwool, to protet the end surfaes themselves as areas of diret axial loading and minimize possible heat transfer to the Fig. 2 Sample arrangement of test speimens in eletri furnae. onrete ore through the longitudinal olumn reinforement that terminates at or near the olumn bottom and top surfaes. Fire testing, even when following standard fire tests (e.g. ASTM E119), does not reflet or simulate a realisti fire senario. In a typial fire, the outer layers of onrete members are expeted to reah 5 C in a few minutes and 95 Cin1h while lower temperature levels are enountered at inner layers (Nassif et al. 1995). The heating level and duration adopted for this researh were intended, in view of the relatively large sale of the test speimens; to allow the ore onrete to reah temperature levels lose to that at the olumn surfae thereby induing signifiant redutions in the mehanial properties of the onrete (Al-Nimry et al. 213; Chen et al. 29; Jau and Huang 28) while maintaining the integrity and viability of the heated olumns for repair using FRP jaketing alone without the need to use any kind of intervention prior to FRP wrapping. Figure 3 displays the furnae temperature history implemented in this study. Logisti and safety measures ditated the long ramp period of about 1 h preeding the maximum exposure temperature. The slow heating proess adopted in the urrent study (refer to Fig. 3), in onjuntion with the extended heating for 2 or 3 h, leads to redued internal temperature gradients and allows for disarding the thermal gradient-indued stresses. To ensure safe handling of the heated speimens, the over of the furnae was slightly opened and olumns were allowed to ool for about 12 h inside the furnae before removal. The heated olumns were then removed and plaed in the lab, with an ambient temperature of about 23 C, in preparation for the repair proess whih started almost 1 month after heating. On the average, the speimens were tested 2 months after heating. Given this time and in view of the relatively large speimen sizes, negligible differenes in moisture ontent of the different olumns were expeted at the time of testing. Eight of the heat-damaged speimens were repaired using CFRP and GFRP produts aording to the repair shemes shown in Table 1. Surfae preparation and wrapping followed the manufaturer s speifiations using the dry lay-up tehnique. The FRP sheets were ut to the perimeter (plus 1 mm extra for overlapping along the irumferene onforming to manufaturer s speifiations) and height dimensions of the olumns. FRP sheets were wrapped around the olumn with the main fibers oriented in the hoop diretion. A 25 mm gap was maintained between the two 118 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)

5 Temperature ( o C) Time (hours) (a) Two hour heating duration Temperature ( o C) Time (hours) (b) Three hour heating duration Fig. 3 Temperature history. olumn ends and the FRP jaket to avoid diret axial loading of the jaket itself. Prior to testing, olumn ends were apped to ensure their vertial alignment and allow for uniform loading during testing. The onfined heat-damaged speimens were tested almost 1 month after jaketing to allow for proper uring of the impregnation resin. 2.5 Test Setup and Instrumentation On the average, olumns were 4 months of age at the time of testing. All speimens were tested under an axial onentri loading using a 4 kn (in ompression) apaity universal testing mahine. The axial loading was inreased gradually using displaement ontrol at a displaement rate o.5 mm/min. Eah of the test speimens was instrumented with four linear variable displaement transduers (LVDTs). Axial and irumferential strains were measured using a displaement measurement system (ompressometer extensometer) that was speifially devised for this test. The ompressometer inluded two aluminum rings that were fixed at a distane of 225 mm from top and bottom of the olumn speimen as shown in Fig. 4. The longitudinal displaement transduer L1, with a linear stroke of 5 mm, was mounted onto the ompressometer rings to measure axial strains over a gage length of 45 mm. The extensometer inluded an aluminum ring loated halfway between the two ompressometer rings and fixed at mid height of the test speimen. To measure hoop strains, LVDT L2, with a linear stroke of 1 mm, was plaed in the form of a hoop onto the extensometer. Two horizontal LVDTs (L3 and L4), plaed 9 apart, were used to measure the lateral displaements at mid height of the olumn. Test data was olleted at a rate of five readings per seond using an automati data aquisition system. 3. Test Results 3.1 General The harateristis of the axial load displaement (F D) urves for the ontrol, heat-damaged and repaired test Fig. 4 Test setup (dimensions in mm). International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217) 119

6 Table 3 Response parameters for test speimens. Speimen F u Seant stiffness (kn mm) D max (mm) Toughness (kn mm) Dutility Control CH-A Control CH-B Control CH2-A Control CH2-B Control CH3-A Control CH3-B (54.2) a (31.5) (46.4) a (26) CH-C1L (14) a (56) CH2-C1L (24.6) b (67.5) CH2-C2L (254.7) b (73.4) CH3-C1L (212.7) (76.3) CH3-C2L (29.6) (79.3) CH2-G1L (147.6) b (62) CH2-G2L (195.1) b (69.4) CH3-G1L (17.9) (65.6) CH3-G2L (191.1) (75.1).87 (212.2) 1.6 (258.5) 2.2 (492.7) 3.49 (41.2) 4.1 (471.3) 2.79 (263.2) 3.74 (352.8) 2.46 (282.8) 2.53 (29.8) 2.43 (229.3) 2.61 (246.2) 293 (129.1) (142.1) (923.9) 255 (87.3) 3822 (134.4) 171 (528.4) 31 (963.) 1286 (438.9) 167 (57.) 1179 (366.3) 1488 (462.2) 1.46 (122.7) 1.62 (136.1) 2.37 (199.2) 2.94 (21.4) 2.58 (176.7) 3. (185.2) 2.61 (161.1) 2.7 (141.8) 1.72 (117.8) 2. (123.5) 1.95 (12.4) a b Numbers between brakets in this row represent a perentage of ontrol unheated CH speimens. Numbers between brakets in this row represent a perentage of ontrol heat-damaged CH2 speimens. Numbers between brakets in this row represent a perentage of ontrol heat-damaged CH3 speimens. olumns are summarized in Table 3. The seant stiffness value reported in the table identifies the slope of the line radiating from the origin and interseting the F D urve at an axial load orresponding to 5% of the maximum loadarrying apaity of the olumn speimen. The tabulated D max value defines the axial displaement value orresponding to a point loated on the post-yield part of the atual urve wherein the axial load drops by 2%, indiating a state of strength failure. On the other hand, the toughness values represent the area under the F D urve up to the state of strength failure. The global displaement dutility (l) in the table represents the ratio of D max to D y where D y is the yield displaement. To determine the yield displaement value; the atual F D urve was idealized with a bi-linear urve using an iterative proedure: an initial value is hosen for the yield fore F y suh that this value does not exeed the maximum axial resistane attained during the test (F u ) and that the point with a fore level o.6f y exists on the atual F D urve. A line is then drawn from the origin to the yield point passing through the point with an ordinate of.6f y. The seond segment of the bi-linear urve extends from the yield point to the point indiating strength failure (i.e. with an ordinate o.8f u ) as shown in Fig. 5. The final value of F y is determined using an iteration proedure suh that the area under the idealized bi-linear urve approximates (with a maximum differene of 5%) that under the atual F D urve. Table 4 summarizes the axial stress, axial and hoop strain values of the different test speimens. The tabulated axial stress values are omputed using the axial load apaity (F u ) divided by the gross area of the onrete setion only. The reported axial and hoop strains represent the maximum values orresponding to F u. Axial and hoop strains are alulated using axial and irumferential displaements at mid height of the olumn, obtained from the L1 and L2 readings, divided by the relevant gage lengths. 12 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)

7 3.2 Failure Modes Conrete rushed at the top end of all olumn speimens as a result of stress onentration in the end region (as the Axial Load F y = F y.8 F u Δ y Atual urve Bilinear urve Δ max Axial Displaement (mm) Fig. 5 Idealized bi-linear fore displaement urve for speimen CH3-B. olumns were ast with no enlargement of the ross setion or additional FRP onfinement near the ends) as an be seen in Figs. 6, 7, and 8. This of ourse may indiate that higher olumn strengths may be expeted under more realisti onditions. The failure mode of the unheated speimens was almost sudden, preeded by raks at about 85% of their ultimate resistane apaities. However, in the ontrol heated speimens, raks were noted at an earlier stage at about 5% of the ultimate load apaity signifying a more dutile type of failure. As for the FRP wrapped speimens, rushing of onrete at olumn ends was followed by rupture of the FRP sheets. Rupture of the wrapping sheets was first noted at about 8% of the axial load apaity. The failure of olumn speimens wrapped with two layers of FRP sheets was more violent; explosive with a loud booming noise and without any prior warning aompanied by a sudden loss of axial strength. The failure was notably more violent in ase of CFRP jakets as ompared to GFRP jakets whih ould be assoiated with Table 4 Stresses and strains for test speimens. Speimen Axial stress (MPa) Strain at peak stress Axial Hoop Control CH-A Control CH-B Control CH2-A Control CH2-B Control CH3-A Control CH3-B 18.5 (54.1) a.145 (168.6) (46.2) a (24.7) CH-C1L (139.8) a (483.7) CH2-C1L (24.9) b (535.2) CH2-C2L (254.6) b (627.6) CH3-C1L (213.3) (351.7) CH3-C2L (291.1) (472.2) CH2-G1L (147.6) b (363.5) CH2-G2L (195.1) b (382.8) CH3-G1L (171.5) (35.1) CH3-G2L (191.8) (317.1).8 (14.4).95 (166.7).413 (724.6).466 (582.5).892 (1115.).449 (472.6).829 (872.6).518 (647.5).551 (688.8).444 (467.4).521 (548.4) a b Numbers between brakets in this row represent a perentage of ontrol unheated CH speimens. Numbers between brakets in this row represent a perentage of ontrol heat-damaged CH2 speimens. Numbers between brakets in this row represent a perentage of ontrol heat-damaged CH3 speimens. International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217) 121

8 Fig. 6 Failure modes for ontrol speimens. Fig. 7 Failure modes for CFRP wrapped speimens. 122 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)

9 Fig. 8 Failure modes for GFRP wrapped speimens. the lower dutility; higher stiffness and tensile strength of the arbon sheets as ompared to glass sheets (see Table 2). The FRP jakets exhibited satisfatory lamination, i.e. bonding between the two FRP layers did not fail. Post failure inspetion of the ruptured FRP sheets revealed satisfatory adhesion to the epoxy: this was evident through visual examination of the inner surfaes of the wraps that were overed by a thin layer of onrete. Despite the fat that ties did not open, loalized bukling of the longitudinal reinforement was evident in loations of the rushed and disintegrated onrete in speimens CH2-B and CH3-B. 3.3 Heat-Damaged Speimens Twelve speimens were subjeted to elevated temperatures of 5 C for 2 or 3 h. Elevated temperatures aused minor superfiial damage to the speimens: lateral hairline raks, parallel to transverse reinforement, were noted. In fat, these miro surfae shrinkage raks were noted in most of the speimens before heating and beame learly visible after heating. Superfiial spider raks, being more onentrated over the middle two-thirds of the olumn height, were also visible. Moreover, the intensity of surfae raking inreased with inreasing heating duration whih is believed to be aused by the development of internal stresses assoiated with differential expansion (Hertz 23). Most of the heat-indued raks healed with time and beame hardly detetable at the time repair was initiated. As expeted for this level of heating (Yaqub and Ghani 213; Hager 214), the olor of the heated onrete olumns hanged into whitish grey whih is probably assoiated with oxidization of ferri ompounds that may be found in the aggregate or sand. The general effet of heating on the axial load displaement response of the test olumns is displayed in Fig. 9. Figure 1, on the other hand, presents the axial strength, seant stiffness, toughness and dutility of the heat-damaged Axial Load CH-A CH-B CH2-A CH2-B CH3-A CH3-B Axial Displaement (mm) Fig. 9 Axial load displaement urves for ontrol speimens. speimens (both unwrapped and wrapped) as a perentage of the orresponding values of the relevant ontrol speimens. Comparison of average test results of the two ontrol speimens (CH-A and CH-B) and the four ontrol heat-damaged speimens (CH2-A, CH2-B and CH3-A, CH3-B) presented in Table 3 and shown in Figs. 9 and 1a reveals that subjeting the olumn speimens to 5 C for 2 and 3 h resulted in a redution of about 46 and 54%, respetively in their axial strength. Yaqub and Bailey (211a, b) and Yaqub et al. (211) reported ompressive strength losses of 42 and 44% in medium sale irular (/ mm) and square ( mm) RC olumns respetively, as a result of heating to 5 C. Losses of 55% in axial strength of retangular ( mm) RC olumns that have been exposed to 5 C for 3 h were enountered by Al-Nimry et al. (213). Higher losses in axial strength of irular (/ mm) RC olumns heated to 8 and 1 C for 2 h reahing up to 43 and 72%, respetively were also reported in literature (Al-Kamaki et al. 215). As expeted, differenes in speimen sizes and shapes, material properties and heating regimens have a lear effet on the residual strength of heated olumns. International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217) 123

10 3 1 % of Companion Controls 2 1 Control CH, 1 Control CH2, 54 Control CH3, 46 CH2-C1L, 25 CH2-C2L, 255 CH2-G1L, 148 CH2-G2L, 195 CH3-C1L, 213 CH3-C2L, 291 CH3-G1L, 171 CH3-G2L, 191 (a) Axial Strength, F u % of Companion Controls Control CH, 1 Control CH2, 31 Control CH3, 26 CH2-C1L, 68 CH2-C2L, 73 CH2-G1L, 62 CH2-G2L, 69 CH3-C1L, 76 CH3-C2L, 79 CH3-G1L, 66 CH3-G2L, 75 (b) Seant Stiffness % of Companion Controls Control CH, 1 Control CH2, 129 Control CH3, 142 CH2-C1L, 87 CH2-C2L, 134 CH2-G1L, 439 CH2-G2L, 57 CH3-C1L, 528 CH3-C2L, 963 CH3-G1L, 366 CH3-G2L, 462 % of Companion Controls 14 7 Control CH, 1 Control CH2, 123 Control CH3, 136 CH2-C1L, 21 CH2-C2L, 177 CH2-G1L, 142 CH2-G2L, 118 CH3-C1L, 185 CH3-C2L, 161 CH3-G1L, 123 CH3-G2L, 12 () Toughness (d) Dutility Fig. 1 Effet of heating and FRP onfinement on behavior of test olumns. The signifiant redutions in ompressive strength of the ontrol heat-damaged CH2 and CH3 olumns were aompanied by more severe redutions in initial axial stiffness of about 69 and 74%, respetively as shown in Fig. 1b. Again, major redutions (63 83%) in the axial stiffness of medium sale RC olumns with a variety of ross setional shapes, as a result of heat exposure (5 C), were also noted by other researhers (Yaqub and Bailey 211a, b; Yaqub et al. 211; Al-Nimry et al. 213). On the other hand, toughness of the ontrol heat-damaged CH2 and CH3 speimens inreased by 29 and 42%, respetively as a result of exposure to 5 C for 2 and 3 h. The notable inrease in onrete toughness upon exposure to elevated temperatures is well reognized among researhers (Zhang et al. 2). Al-Nimry et al. (213) noted an inrease of about 2% in toughness, again omputed as the area under the F D urve, of retangular RC olumns upon heating to 5 C for 3 h. Assoiated with the enhanement in toughness, an inrease in the axial deformation apaity and dutility of the ontrol heat-damaged CH2 and CH3 olumns was notied wherein the D max values inreased by about 112 and 159%, respetively and the dutility inreased by 23 and 36%, respetively. Inspetion of Table 4 reveals that substantial inreases in axial strains of 69 and 15% and in hoop strains of about 4 and 67% took plae in the ontrol olumns heated for 2 and 3 h, respetively. This notable inrease in deformation apaity of the heat-damaged olumns is attributed to the heat-indued miro-raking and the removal of water whih makes onrete soft and more porous. As suh, onrete that has been subjeted to elevated temperatures is expeted to exhibit more lateral dilation, as ompared to unheated onrete, under axial ompression. Test results of the four ontrol heated speimens indiate that the maximum axial resistane and stiffness were redued by 14.4 and 17.4%, respetively as the exposure duration inreased from 2 to 3 h. On the other hand, the maximum axial displaement attained by the olumns heated for 3 h reahed about 1.2 times that of the olumns exposed to 5 C for 2 h indiating an inrease in dutility. Dutility and toughness of the CH3 olumns inreased by 11 and 9.7%, respetively as ompared with olumns heated for 2 h. The observed derease in the axial strength and stiffness and the aompanying inrease in toughness and dutility of onrete olumns as a result of inreasing exposure duration is onsistent with available researh results (Chen et al. 29; Yaqub and Ghani 213; El-Shaer 214). It was earlier noted that a brittle type of failure ourred in the ontrol unheated speimens: the failure was of an explosive nature aompanied by sudden loss of axial resistane (Fig. 9). In fat, the test olumns were designed to experiene rushing before reahing the ritial bukling load. The signifiant redutions in ompressive strength and modulus of elastiity of the heated onrete were not large enough to indue bukling in the olumns as verified by the 124 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)

11 Table 5 Axial ompressive strength and bukling load of ontrol speimens. Control speimens CH-A CH-B CH2-A CH2-B CH3-A CH3-B F u P o E I EI P (N mm 2 ) 989 a 1484 b pffiffiffiffi 47 I g b pffiffiffiffi pffiffiffiffi I g I g a b Numbers in this olumn represent the atual axial ompressive strength (test value). Numbers in this olumn are omputed in aordane with ACI (214). nominal axial strength at zero eentriity (P o ) and the ritial bukling load (P ) values of the four ontrol heated speimens presented in Table 5. The tabulated P o and P values were omputed using ACI (214) taking into aount the heat-indued redutions in onrete and steel strengths as determined in the laboratory (see Set. 2.3). In addition, the gross moment of inertia (I g ) of the heated olumn ross setion was redued by 3% to reflet the effet of the heat-indued raks on the P value. The modulus of elastiity of heated onrete was also redued by 4% in view of reported values by Bisby et al. (211) and Tolintino et al. (22). Moreover, the near-zero readings of the lateral L3 and L4 transduers reorded during testing onfirm that olumn bukling did not take plae. Albeit, ompared to the unheated speimens, the CH2 and CH3 heat-damaged ontrol olumns exhibited a more dutile type of failure. 3.4 Repaired Heat-Damaged Columns Columns Repaired Using CFRP Sheets The axial load displaement urves for the heat-damaged ontrol olumns are ompared with those obtained for speimens wrapped with CFRP sheets in Fig. 11. Figure 11 learly shows that full wrapping with CFRP sheets enhanes the axial load resistane of heat-damaged olumns. Table 3 and Fig. 1a show that using one layer of CFPP wraps inreased the axial resistane of the CH2 and CH3 heatdamaged olumns by 15 and 113%, respetively. Using one layer of the CFRP wraps managed not only to restore the original ompressive strength of olumn speimens exposed to 2 h of heating but also exeeded the original F u value by 11%. For speimens heated for 3 h, the CFRP onfinement nearly regained the full ompressive strength of the original CH olumns. It is worth noting that the axial stiffness values of the CFRP onfined olumns were lower than those of the unwrapped heat-damaged speimens that have been subjeted to the same heating regimen: ompared to ontrol heated speimens, a redution in axial stiffness of about 33 and 24% was observed in CH2-C1L and CH3-C1L, respetively. This disrepany in the axial stiffness values is probably assoiated with the non homogeneity of the onrete and the inevitable variability between the different olumn speimens. In fat, researhers (Bisby et al. 211) onfirmed that thermal exposure aggravates non homogeneities in the onrete. Moreover, onfinement provided by the FRP wraps is not expeted to be fully ativated until reahing the maximum strength of the heated unonfined onrete after whih ontinuous pressure is applied on the onrete ore up to the failure (rupture) of the FRP wraps. Nonetheless, the overall effet of the thikness of the wrapping sheets on axial stiffness is still obvious. Within the same ategory of heating duration and type of FRP wraps, the enhanement in axial stiffness ahieved when using two layers of the FRP wraps was notable: Compared to the ase where a single layer of the CFRP sheets were used, an inrease of 8.6 and 3.9% was reorded upon inreasing the thikness of the wrap in the CH2 and CH3 speimens, respetively. Confinement provided by the CFRP jakets resulted in signifiant enhanement of toughness of the heat-damaged olumns. Toughness of the CH2 and CH3 olumns onfined with a single layer of arbon sheets was found to be 8.7 and 5.3 times toughness of the ompanion heat-damaged unwrapped olumns. Dutility of the CFRP wrapped heat-damaged olumns (both CH2 and CH3) was found to be almost 2 times that of the ontrol heat-damaged olumns as a result of the uniform onfinement provided by the FRP wraps for the miroraked heated onrete. The benefiial effet of FRP (both arbon and glass) onfinement on the dutility of heated RC olumns was noted by other researhers (Yaqub and Bailey 211a, b; Yaqub et al. 211, 213) with higher enhanement in ase of irular olumns as ompared to square olumns. The overall effet of the thikness of the wrapping sheets is obvious where using two layers of CFPR sheets had a benefiial effet on the axial resistane with an inrease of about 24.5 and 36.7% for CH2 and CH3 olumns, respetively. Upon inreasing the thikness of the CFRP jakets, stiffness of the CH2 and CH3 olumns inreased by 8.6 and 3.9%; toughness by 49.9 and 82.3%; and deformation apaity (in terms of D max ) by 17.5 and 34.1%, respetively. On the other hand, inreasing the thikness of the CFRP jakets adversely affeted the dutility of the heat-damaged speimens as an be seen in Fig. 1d. Compared to olumns repaired using a single layer of the CFRP sheets, dutility of International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217) 125

12 Axial Load CH2-A 4 CH2-B 2 CH2-C1L CH2-C2L Axial Displaement (mm) (a) CH2 Speimens Axial Load CH3-A 4 CH3-B 2 CH3-C1L CH3-C2L Axial Displaement (mm) (b) CH3 Speimens Fig. 11 Axial load displaement urves for CFRP wrapped olumns. Axial Load CH2-A CH2-B CH2-G1L CH2-G2L Axial Load CH3-A CH3-B CH3-G1L CH3-G2L Axial Displaement (mm) Axial Displaement (mm) (a) CH2 Speimens (b) CH3 Speimens Fig. 12 Axial load displaement urves for GFRP wrapped olumns. the CH2 and CH3 speimens repaired using a double-layered jaket dereased by about 12.2 and 13%, respetively. Regarding the onfinement effet provided through the CFRP wraps on the axial ompressive stress of the heated olumns, Table 4 shows that axial stresses of the onfined CH2 and CH3 speimens reahed 2 3 times the stress values of the ompanion ontrol heated speimens. Axial stresses inreased with inreasing thikness of the CFRP jakets. This was aompanied by substantial inreases in both axial (5 6 times for CH2 speimens and 4 5 times for CH3 speimens) and hoop strains (6 11 times for CH2 speimens and 5 9 times for CH3 speimens) measured at mid height of the olumns. The signifiant enhanements in axial stress, axial and hoop strains of the FRP-onfined heat-damaged olumns is learly assoiated with the inreased lateral dilation of onrete after heating whih auses higher tensile stresses in the FRP jakets and thereby provides higher restraining fores or onfinement. For the same heating duration, strains also inreased with inreasing thikness of the FRP jaket. Effet of the FRP jakets was most pronouned on the hoop strain values wherein these strains reahed 6 (for CH2-C1L) and 11 (for CH2-C2L) times those of the ontrol heated CH2 speimens. As for the onfined unheated speimen CH-C1L, wrapping the olumn with a single layer of arbon FRP sheets resulted in signifiant enhanements of axial resistane, deformation apaity, toughness and dutility. However, the axial stiffness of the CH-C1L was lower than that of the ompanion unonfined CH speimens. The axial stress, axial and hoop strains of the CFRP jaketed unheated olumn inreased signifiantly in omparison with the unonfined CH olumns. It is worth noting that CFRP onfinement inreased the axial resistane of the unheated olumn by about 4% whereas the same wrapping system indued substantially higher effets when used to onfine heat-damaged olumns. Inreases of 15 and 113% in axial resistane of the CH2 and CH3 heat-damaged olumns ompared to ontrol unheated speimens were enountered. This inreased effetiveness of the FRP onfinement when used to repair heat-damaged olumns is attributed to the enhanement in lateral dilation of onrete after heating, whih also inreases with inreased heating level or duration Columns Repaired Using GFRP Sheets The axial load displaement urves for the heat-damaged ontrol olumns are ompared with those obtained for speimens wrapped with GFRP sheets in Fig. 12. Table 3, Figs. 1a and 12 show that using one layer of GFPP wraps inreased the axial resistane of the CH2 and CH3 heatdamaged olumns by 48 and 71%, respetively. 126 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)

13 Similar to olumns jaketed with CFRP sheets, a signifiant derease of about 38 and 34% in the axial stiffness of the repaired CH2 and CH3 olumns, respetively, was noted when ompared to the unwrapped heat-damaged speimens. Again, the overall effet of the thikness of the wrapping sheets is still obvious where using two layers of GFPR wraps inreased the axial resistane of the CH2 and CH3 olumns by 32 and 11.8%, respetively. Upon inreasing the thikness of the GFRP jakets stiffness of the CH2 and CH3 olumns inreased by 11.8 and 14.4%; toughness by 29.9 and 26.1%; and deformation apaity (in terms of D max )by 2.8 and 7.4%, respetively. Inreasing the thikness of the GFRP jakets adversely affeted the dutility of the heat-damaged speimens. Compared to olumns repaired using a single layer of the GFRP sheets, dutility of the CH2 and CH3 speimens repaired using a double layer jaket dereased by about 17 and 2.5%, respetively. Comparing stress and strain values of the GFRP jaketed olumns with relevant values of the ontrol heated speimens presented in Table 4 shows that axial stresses, axial and hoop strains were signifiantly inreased due to onfinement: axial stress values were almost doubled upon using two layers of the onfinement sheets for both the CH2 and CH3 speimens. Axial strain values of 4 and 3 times those of the orresponding CH2 and CH3 speimens were ahieved. The effet on hoop strains was more pronouned; hoop strain values of 7 and 5 times those of the orresponding ontrol heated CH2 and CH3 speimens were ahieved. The axial stress, axial and hoop strain values of the GFRP jaketed olumns inreased with the inrease in thikness of the jaket. Nonetheless, using GFRP jakets with the lower onfinement modulus proved to be, ompared to CFRP jakets, less effiient in enhaning the axial behavior of the heat-damaged olumns. 4. Theoretial Strength Preditions of FRP- Confined Columns Extensive researh efforts targeting the strutural behavior of FRP-onfined onrete olumns over the past four deades have resulted in the development of design guidelines (ACI 44.2R 28; CAN, CSA-S ; fib 21; TR ; CNR 213) that provide preditive design equations for FRP-onfined olumns under ambient onditions. A large number of models, mostly empirial, orrelating the inrease in strength and dutility of FRP-onfined onrete to the passive onfining pressure provided by FRP jaketing systems have been developed (e.g. Shahawy et al. 2; Xiao and Wu 2; Harries and Kharel 22; Lam and Teng 23; Teng and Lam 24; Carey and Harries 25; Harajli 26; Jiang and Teng 27; Saenz and Pantelides 27; Teng et al. 27, 29; Youssef et al. 27; Lee and Hegemier 29; Wu and Wang 29; Benzaid et al. 21; Chastre and Silva 21; Cui and Sheikh 21; Fahmy and Wu 21; Pellegrino and Modena 21; Dai et al. 211; Liang et al. 212; Wei and Wu 212; Ozbakkaloglu and Lim 213; Lim and Ozbakkaloglu 214a, b, 215a; Pham and Hadi 214; Al Abadi et al. 216; Fig. 13 Confinement ation of FRP jakets in irular onrete setions. Lin et al. 216). The basi parameter in these models is the lateral onfining pressure (f l ) applied by the FRP jaket on the dilating onrete ore shown in Fig. 13. In fully wrapped irular olumns, the ultimate onfining pressure (f lu ) that an be exerted by the FRP jaket determines the strength gain for the onfined onrete whih is loaded in triaxial ompression as given by the general form of Eq. 2: ¼ o þ k 1f lu ð2þ where is the ompressive strength of onfined onrete, o the ompressive strength of unonfined onrete also equal to.85 and k 1 is an effiieny fator. Most of the FRP-onfining models for irular onrete setions estimate f lu in terms of the ultimate tensile strain of the fibers (e fu ) obtained from flat oupon tests whih is typially higher than the hoop rupture strain (e h,rup ) of the FRP jaket with fibers oriented in the hoop diretion (Shahawy et al. 2; Xiao and Wu 2; Lam and Teng 23, 24; Teng and Lam 24; Ozbakkaloglu and Lim 213; Lim and Ozbakkaloglu 214a, b, 215b; Wu and Jiang 213; De Lorenzis and Tepfers 23). Based on test results of 76 FRP wrapped plain onrete ylinders, Lam and Teng (23) onluded that the strain effiieny or redution fator, i.e. the ratio between the hoop rupture strain of the jaket to the material ultimate tensile strain, depends on the type of FRP material (arbon, glass or aramid). An average value o.63 was proposed when all speimens of the database were onsidered together. Wu and Jiang (213) onfirmed the vast variability in the strain effiieny values ( ) published in the literature. Using a large experimental database of irular FRP-onfined normal and high strength onrete speimens, Lim and Ozbakkaloglu (214) developed the expression given in Eq. 3 for the strain effiieny fator. The expression denotes the influene of two key parameters on the hoop strain redution fator (k e,f ) namely; the ompressive strength of unonfined onrete ( o ) and elasti modulus of onfining fibers (E f). Equation 3 an be used for FRP-onfined onretes with o up to 12 MPa and onfined by any FRP type. k e;f ¼ :9 2:3 o 1 3 :75E f 1 6 ð3þ where 1 GPa B E f B 64 GPa and with the units of the input parameters in MPa. International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217) 127

14 Table 6 Summary of FRP-onfining models for fully wrapped irular setions. Model Conrete onfined strength Confining pressure ACI 44.2R (28) ¼ þ w f 3:3f l if fl :8 w f ¼ :95 ¼ if fl \ :8 2Ef ntf efe f l ¼ D e fe ¼ :55 e fu CNR (213) Lam and Teng (23) 2=3 ¼ 1 þ 2:6 fl;eff ¼ 1 ¼ o 1 þ 3:3 fl;a o ¼ 1 o if fl;eff if fl;eff if fl;a o if fl;a o [ :5 :5 :7 \ :7 2Ef ntf efd;rid f l;eff ¼ D e fd;rid ¼ minfg a e fu = f ; :4g f l;a ¼ 2Ef tf eh;rup D for CFRP, e h;rup ¼ :586 e fu for GFRP, e h;rup ¼ :624 e fu Teng et al. (29) o ¼ 1 þ 3:5ðq k :1Þq e ¼ 1 o if q k :1 if q k \ :1 q e ¼ q k ¼ eh;rup ð 2Ef tf o =eoþd eo, e o ¼ :2 Wu and Wang (29) o ¼ 1 þ 2:23 flu o :96 f lu ¼ 2ff ntf D Fahmy and Wu (21) ¼ o þ k 1f lu k 1 ¼ 4:5flu :3 if o 4 MPa k 1 ¼ 3:75flu :3 if o [ 4 MPa Ozbakkaloglu and Lim (213) ¼ 1 o þ 3:2 f l;a f lo 1 ¼ 1 þ :58 El o f lo ¼ E l e l1 e l1 ¼ 2ff ntf f lu ¼ D :43 þ :9 El e o o and E l 1:65 o Pham and Hadi (214) ¼ :91 o þ 1:88f l þ 7:6 tf D 2Ef ntf efu f l ¼ D f f, tensile strength of FRP in hoop diretion; f l, maximum onfining pressure due to FRP jaket; f l;eff, is the effetive onfinement lateral pressure; f lo, threshold onfining pressure; e o, axial strain of FRP-onfined onrete at the unonfined onrete strength ( o ); e fe, effetive strain level in FRP reinforement attained at failure; e fu, design rupture strain of FRP reinforement determined from flat oupon tests; e l1, hoop strain of FRP-onfined onrete orresponding to axial ompressive stress at first peak; f, partial fator (1.1 for ultimate limit state and 1 for servieability limit state); g a, environmental onversion fator; e fd; rid, redued design strain of FRP reinforement; and w f, FRP strength redution fator. More reently, Lim and Ozbakkaloglu (215b) tested 36 FRP-onfined onrete ylinders (152 mm in diameter and 35 mm in height) that were speifially devised to examine the influene of onrete strength and type of FRP material on the hoop strain effiieny of FRP jakets. Test results, supplemented with 357 test results of FRP-onfined onrete olleted from the published literature, demonstrated the validity of the strain redution fator expression given in Eq. (3). Several researhers onfirmed the wide variability in strength preditions offered by the existing FRP-onfining models (Pellegrino and Modena 21; De Lorenzis and Tepfers 23; Bisby et al. 25; Chaallal et al. 26; Roa et al. 28). In a reent review of existing onfining models, Ozbakkaloglu et al. (213) assessed the performane of 88 models developed (between 1982 and 211) to predit the axial stress strain behavior of FRP-onfined onrete in irular setions. To evaluate the performane of these models, a huge test database ontaining the test results of 73 FRP-onfined onrete ylinders tested under axial ompression was established. The top performing strength enhanement models were found to be those proposed by Lam and Teng (23), Bisby et al. (25) and Teng et al. (27). The divergene in strength preditions of existing FRP-onfining models is expeted in view of the fat that these models are usually alibrated against limited sets of test data of plain onrete ylinders (rather than olumns) with wide variations in test parameters. Moreover, the strength enhanement due to the FRP onfinement is usually based on the strength of ontrol onrete ylinders rather than the onrete strength of the unonfined olumn itself. In this study, the atual axial load-arrying apaities of the FRP wrapped test olumns (F u ) are ompared with the values predited by eight FRP-onfining models inluding those proposed by ACI 44.2R (28), CNR (213), Lam and Teng (23), Teng et al. (29), Wu and Wang (29), Fahmy and Wu (21), Ozbakkaloglu and Lim (213) and a more reent model suggested by Pham and Hadi (214). As a matter of fat, the level of axial strength enhanement proposed by the ACI FRP-onfining model (ACI 44.2R 28) was reently adopted by Bisby et al. (211) for firedamaged onrete based on uniaxial ompression tests of 33 unonfined and FRP-onfined plain onrete ylinders that were heated to a range of elevated temperatures (3 686 C) for 2 4 h and ooled to room temperature. A summary of the eight seleted models is presented in Table 6 using a onsistent set of parameters whih may vary from the original model parameters. 128 International Journal of Conrete Strutures and Materials (Vol.11, No.1, Marh 217)