Durability evaluation of retrofitted corroded reinforced concrete columns with FRP sheets in marine environmental conditions

Construction and Building Materials 151 (2017) 520–533

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Durability evaluation of retrofitted corroded reinforced concrete columns with FRP sheets in marine environmental conditions Amin Kashi a,⇑, Ali Akbar Ramezanianpour a, Faramarz Moodi b a b

Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran Concrete Technology and Durability Research Center (CTDRc), Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Three different retrofit methods for

rehabilitation of corroded RC columns with FRP sheets were evaluated.
A marine simulator environment was designed and constructed similar to the real conditions.
In harsh environmental condition, protecting the exterior surface of the FRP sheets should be considered.

a r t i c l e

i n f o

Article history: Received 2 December 2016 Received in revised form 19 June 2017 Accepted 21 June 2017

Keywords: Corroded RC column Wrapping by FRP Marine exposure Durability Distance based approach

a b s t r a c t The purpose of this paper is to investigate the effect of marine environmental condition on durability of RC-corroded columns strengthened with FRP sheets. In this study, three retrofit methods based on substrate repairs were evaluated. There are two common retrofit methods for the corroded RC columns, which are directly wrapping FRP (method 1) and wrapping FRP after replacing damaged concrete with repair mortar (method 2), respectively. Also, a durable retrofit method was developed in this study (method 3). In this method, after removing damaged concrete, wrapping was carried out, and then repair mortar was applied around GFRP sheet. After strengthening, specimens were stored in a marine simulator for 3000 and 9000 h. In retrofit method 1, ultimate strength of retrofitted columns by one layer of GFRP and CFRP sheet decreased by 27.4% and 19.7%, respectively, after 9000 h marine exposure. The decrease in ultimate strength of columns wrapped by one layer of GFRP and CFRP in retrofit method 2 was 25% and 18.3%, respectively. In retrofit method 3, 9000 h marine exposure caused 8.8% and 2.2% increase in ultimate strength of strengthened columns with one and two layers of GFRP, respectively. The failure mode of strengthened columns by methods 2 and 3 was brittle. Finally, Distance Based Approach (DBA) was used in order to compare different retrofit methods. It was found that the performance of retrofitted columns in method 3 would be ideal in marine environmental conditions. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (A. Kashi). http://dx.doi.org/10.1016/j.conbuildmat.2017.06.137 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

Reinforced concrete (RC) columns in marine environment are continuously exposed to chloride attack, which lead to corrosion

A. Kashi et al. / Construction and Building Materials 151 (2017) 520–533

of the reinforcing steel and degradation of the concrete [1–5]. Traditionally, corroded RC columns have been strengthened with steel plates. Steel jacketing has difficulties in application. Moreover, corrosion of steel plates is another problem. In the last decade fiber reinforced polymers (FRP) have been recognized as an effective technique for strengthening corrosion-damaged RC columns. The remarkable properties of FRP, such as lightweight, resistance to electro-chemical corrosion, and high tensile strength, allow them to be used for rehabilitation of corroded structures without serious difficulties [6]. While wrapping by FRP sheets is known as an effective rehabilitation technique, the durability of this method especially in harsh environmental conditions such as marine environment is still under investigation [7,8]. A number of researches on the durability of FRP system in marine environments are represented in the following: While wrapping by FRP sheets is known as an effective rehabilitation technique, the durability of this method especially in harsh environmental conditions such as marine environment is still under investigation Generally, hydrothermal effects (the combination of humidity and heat) are dominant in the durability degradation of FRP materials [9]. Along with moisture absorption, the presence of salt in water may exacerbate deleterious effects [10]. Karbhari and Zhao investigated the behavior of FRP sheets subjected to three different environmental conditions consisted of immersion in fresh water, immersion in salt water, and freeze-thaw cycles. Salt water exposure caused maximum degradation in mechanical properties of FRP sheets, especially in GFRP, in comparison to other environmental conditions [11]. Having put wrapped concrete columns by FRP layers in 15% saline solution, Micelli et al. observed that the ultimate strength reduced by 27% and 10% in GFRP and CFRPwrapped columns, respectively, after 120 days [12]. In a study conducted by Bae and Belarbi on concrete specimens confined by CFRP and GFRP sheets, the salt water had deteriorate effect on GFRPwrapped columns, whereas CFRP confined columns only experienced a minor decrease in ultimate strength [13]. An experimental program was conducted by Gharachorlu and Ramezanianpour to evaluate the strength and durability of concrete cylinders strengthened with FRP sheets subjected to marine environmental condition. The highest decrease in strength was observed in GFRPwrapped specimens after one year exposure to the wet-dry cycles of saline solution at 38 °C. They also found that the durability of wrapped specimens could be improved by increasing the number of the layers [14]. Cromwell examined the performance of FRP sheets when subjected to different environmental conditions, such as water, saltwater, alkaline, dry heat, diesel fuel, and freeze/heat exposure. In this research degradation was particularly observed in salt water and alkaline environments, especially in GFRP sheets [15]. A report by Böer et al. presented results from environmental effects on durability of FRP-wraps in civil engineering. They found that the exposure to saltwater reduced both the ultimate strength and ductility of the columns which were strengthened with GFRP sheets. The CFRP-wrapped RC columns only experienced slight decrease in ultimate strength. Also, humidity was found to have the primary destructive effect, while salt crystals expanding over time in the micro cracks exacerbated the deteriorating effect on the sheets [16]. In a study which was conducted by Silva et al., accelerated environmental condition was imposed by immersing the GFRP sheets in a 5% NaCl solution. The saltwater was kept at three different temperatures, 35 °C, 50 °C and 65 °C. Significant decrease in mechanical properties of GFRP sheets, namely, on their tensile strength was observed at different temperatures. Furthermore, the solution absorption by the GFRP increased with water temperature as well as their mechanical properties degradation [17]. Also, Kashi et al. found that marine environment caused a

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43% and 34.3% reduction in the ultimate strength of the specimens wrapped by one and two layers of GFRP sheets, respectively [18]. Although numerous researches have been devoted to the performance of FRP sheets in an aggressive environment, not much information is available on the post-repair structural behavior of corrosion-damaged columns wrapped with FRP sheets in marine environments where prolonged exposure to high humidity and temperature, wet/dry cycles, salt water, and ultraviolet-radiation. The main objective of this paper is to survey the performance of corroded RC columns after strengthening by FRP sheets, in the marginal region of the southern sea of Iran, which is known to have extraordinarily harsh conditions due to the presence of chloride ions in seawater, high temperature and humidity, tidal phenomena, and solar UV radiations. In this study, a durable retrofit method was developed. In this method, after removing damaged concrete, wrapping was carried out, and then repair mortar was applied around FRP sheet. To compare the durable method with two common retrofit methods, which are respectively bonding directly FRP on damaged concrete and bonding FRP after replacing damaged concrete, uni-axial compression tests were conducted on a series of corrosion- damaged RC columns which wrapped by FRP sheets. Because of lower durability of GFRP sheets, the effect of durable retrofit method was investigated only in GFRP-repaired columns. 2. Experimental program 2.1. Material properties There were totally 60 RC columns constructed in this study. The height and the diameter of the specimen were 300 mm and 150 mm, respectively. Specimens reinforcement consist of four U10 longitudinal steel bars at 25 mm cover. The rational for the cross section of the steel bars as a fraction of the gross sectional of the specimen was 1.7%. Two U6 circular hoops at the top and bottom of specimens supported the longitudinal reinforcement. Columns preparation, typical geometry and reinforcement are shown in Fig. 1. In some cases, damaged concrete cover was replaced by self-compacted mortar. Mix properties of the concrete and self-compacted mortar are summarized in Tables 1 and 2, respectively. Slump flow test was carried out on the selfcompacted mortar according to the EFNARC guideline [19]. Unidirectional carbon and glass fibers sheets were used for strengthening of the specimens which installed with the wet layup technique. Tensile strength parallel to the fiber was determined in accordance with the ASTM D3039 standard [20]. Mechanical properties of the FRP sheets are shown in Table 3. 2.2. Accelerated corrosion Since the main goal of present research was to evaluate different retrofit methods for the corrosion-damaged RC columns in marine environment, 57 columns were firstly corroded by an accelerated corrosion process, and three columns remained uncorroded. In many researches, accelerated corrosion process and calculation of the time required to specific corrosion rate (mass loss in reinforcing bars) have been carried out according to Faraday’s law, which is expressed by the following equation [21–26]:

t¼

mnF aI

ð1Þ

where t = time of the corrosion process (s); m = mass loss of iron (gr); n = valence of the reacting electrode for the material (nsteel = 2); F = Faraday’s constant (96500 A.s); a = atomic mass of iron (55.85 gr); I = corrosion current (A).

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Fig. 1. (a) Preparation of RC columns (b) RC columns-geometry and reinforcement details (dimension in mm).

Table 1 Mix properties of the concrete. W/C

Cement (kg/m3)

Coarse aggregate (kg/m3)

Fine aggregate (kg/m3)

Compressive strength (MPa)

0.51

341

804

974

29.8

Table 2 Mix properties of self-compacted mortar.

*

W/C

Cement (kg/m3)

Sand (kg/m3)

Limestone powder (kg/m3)

Superplasticizer by weight of cement (%)

Slump flow (cm)

Compressive strength (MPa)*

0.45

450

1469

130

0.8

26

60

compressive strength test was carried out on 5  5  5 cm cubic specimens.

Table 3 Mechanical properties of FRP sheets. FRP type

Fiber volume ratio (%)

Tensile strength (MPa)

Ultimate strain (%)

Tensile modulus of elasticity (GPa)

GFRP CFRP

35 30

190 342

1.21 0.76

15.6 44.7

In this study, the RC columns were immersed up to 2/3 of their height in a plastic tank which had saline solution with a concentration of about 3.5% by weight of water; then the specimens were subjected to 1(A) external current which continued up to 15% mass loss of steel bars (see Fig. 2). 2.3. Retrofit methods In this study, three different types of repair were considered for corrosion-damaged RC columns. Method 1 was conventional repair method which was directly bonding FRP sheets on the corroded concrete. In this method, after cleaning any staining and rust deposits from the specimen surface, cracks were filled by primer (see Fig. 3), then specimens were strengthened by one layer of GFRP and CFRP, respectively.

In retrofit method 2, damaged concrete was removed and replaced by repair mortar. The replacement mortar was a selfcompacted mortar. In this method, firstly, the corroded concrete was cut up to the level of longitudinal steel bars, and then it was removed and repair mortar was applied around sound concrete (see Fig. 4). Cutting corroded concrete before removing process caused less destructive effect on the sound concrete. The concrete cylinders were kept in mold at laboratory temperature (20 °C) for 24 h. After demolding, specimens were stored in lime-saturated water for 28 days. Afterwards, specimens were kept for 14 days at laboratory temperature, and then wrapping was carried out with one layer of GFRP and CFRP, respectively. Retrofit method 3 was developed due to the improving the durability of wrapped columns with GFRP sheets. This strengthening method was carried out in the following steps:

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Fig. 2. (a) Schematic presentation of the accelerated corrosion process (b) Accelerated corrosion process in the laboratory.

Fig. 3. Preparing specimens for strengthening in method 1 (a) Cleaned specimens (b) Applying primer.

Fig. 4. Preparing specimens for strengthening in method 2 (a) Cutting corroded concrete (b) Removing corroded concrete (c) Replacing corroded concrete by self-compacted mortar.

a) Corroded concrete cover was removed similar to the retrofit method 2. b) The surface preparation was carried out before wrapping by GFRP sheets. For this purpose, sound concrete was placed in cylindrical mold with dimensions 120 mm in diameter by 300 mm in height and then self-compacted mortar was casted around it (Fig. 5a). Curing process for repair mortar was similar to the retrofit method 2. c) 120  300 mm columns were wrapped by one and two layers of GFRP, respectively. d) In this

retrofit method, GFRP sheets were protected by self-compacted mortar. For this purpose, firstly, the silica sand was spread onto wet resin immediately after confining. It was due to the roughening the surface of GFRP sheets and provide a better bonding with protective mortar (see Fig. 5b). e) After curing of GFRP sheets, self-compacted mortar was applied around them (see Fig. 5c). Repair mortar was cured for 28 days in lime-saturated water.

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Fig. 5. Strengthening in method 3 (a) Surface preparation of specimens before wrapping (b) Sanded confined specimens (c) Casting self-compacted mortar on the sanded resin.

After strengthening, epoxy resin was applied on the unwrapped portions of the columns. So, moisture and chloride ions couldn’t ingress through the top and bottom of the wrapped specimens. 2.4. Environmental conditions After preparation of the specimens, they were placed in two different environmental conditions: 1) normal condition in laboratory and 2) marine simulator. In this study, a marine simulator has been used at the Concrete Technology and Durability Research Center (CTDRC) at Amirkabir University of Technology for accelerated simulation of coastal region in the south of Iran which was selected as a case study in this research. In this area, many RC structures show signs of deterioration, mainly due to the chloride-induced corrosion of steel bars. Weather conditions of coastal region in the south of Iran- including high temperature and humidity, tidal phenomenon of seawater and UV radiations- are simulated in an accelerated manner on RC columns exposed to weathering. The exterior view of the marine simulator and also the RC columns which were placed in the tidal pool are shown in Fig. 6. In order to simulate the real environmental condition, the temperature of the interior simulator was set to 40 °C, its humidity to 68%, and the salt concentration of water to 36.6 grams per liter. The specimens were stored in the simulator for 3000 and 9000 h and experienced 250 and 750 cycles, respectively. One cycle lasted 24 h and consisted of 12 h drying (Fig. 6b) and 12 h immersion in salt water (Fig. 6c). UV radiation was applied in correspondence with dry cycles (when the tidal pool was empty). The temperature and humidity of the normal condition in laboratory was 20 °C and 40%, respectively 2.5. Test methods After completion of the accelerated marine conditioning, uniaxial compression tests were carried out in order to evaluate the durability of different retrofit methods. For this purpose, the mechanical properties of RC columns which were stored in marine simulator compared with similar specimens which were kept in normal environmental condition. Concrete cylinders were tested under monotonically increasing axial displacement (stroke control) by the rate of 1 mm/min. The axial deflection of specimen was measured base on the relative displacement between the top and bottom loading platens of the test machine. A compressive strength test was carried out by DARTEC machine in Rock Laboratory of AmirKabir University.

The test specimens are summarized in Table 4. Three identical columns were prepared for each type of specimens. As shown in Table 4, columns C-C and CA-C are non-corroded and corroded columns, respectively, without any exposure. Columns C15A1G1-C, C15A1C1-C, C15A2G1-C, C15A2C1-C, C15A3G1-C and C15A3G2-C, are the control specimens (without exposure) wrapped with one layer of GFRP in retrofit method 1, one layer of CFRP in retrofit method 1, one layer of GFRP in retrofit method 2, one layer of CFRP in retrofit method 2, one layer of GFRP in retrofit method 3 and two layers of GFRP in retrofit method 3, respectively. Columns C15A1G1-M3, C15A1C1-M3, C15A2G1-M3, C15A2C1-M3, C15A3G1-M3 and C15A3G2-M3, are the weathered specimens for 3000 h, wrapped with one layer of GFRP in retrofit method 1, one layer of CFRP in retrofit method 1, one layer of GFRP in retrofit method 2, one layer of CFRP in retrofit method 2, one layer of GFRP in retrofit method 3 and two layers of GFRP in retrofit method 3, respectively. Also, Columns C15A1G1-M9, C15A1C1-M9, C15A2G1-M9, C15A2C1-M9, C15A3G1-M9 and C15A3G2-M9, are the weathered specimens for 9000 h, wrapped with one layer of GFRP in retrofit method 1, one layer of CFRP in retrofit method 1, one layer of GFRP in retrofit method 2, one layer of CFRP in retrofit method 2, one layer of GFRP in retrofit method 3 and two layers of GFRP in retrofit method 3, respectively. 3. Experimental results and discussion 3.1. Effect of accelerated corrosion on compressive behavior The stress-strain curves of non-corroded and corrosiondamaged columns are shown in Fig. 7a. Corroded columns showed a reduction of 29%, 14.5% and 22.1% in ultimate strength, ultimate axial strain and the modulus of elasticity, respectively. Degradation in mechanical properties of RC corroded columns was mainly due to the reduction in cross-sectional area of steel bars and consequently propagation of the cracks in concrete. The area under the stress-strain curves is related to the energy absorption. The energy absorption of corroded RC columns decreased when compared to the non-corroded ones. This indicated that the failure mode of corroded columns was more brittle than the non-corroded columns (see Fig. 7b and c). 3.2. Effect of retrofit method 1 on compressive behavior Fig. 8 presents the axial stress vs. axial strain curve of the corroded columns which were strengthened in retrofit method 1.

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Fig. 6. (a) Exterior view of the marine simulator (b) Specimens placed in tidal pool (dry cycle) (c) Specimens placed in tidal pool (wet cycle).

Mechanical properties which obtained from these curves are summarized in Table 5. As shown in Table 5, retrofit method1 with CFRP and GFRP increased ultimate strength by 87% and 40.3%, respectively, in normal environmental condition. After 3000 h marine exposure, ultimate strength of columns retrofitted by method 1 decreased by 4.5% and 26.3% in CFRP and GFRP wrapped columns, respectively. Also, after 9000 marine exposure, 19.7% and 27.4% reduction in ultimate strength of strengthened columns by this retrofit method was observes in CFRP and GFRP-wrapped specimens, respectively. The second slope of the stress-strain curve is an important indicator in the design of FRP-wrapped RC columns and is referred to as axial plastic modulus (E2), throughout this paper. E2 in RC columns retrofitted by method 1 decreased by 20.7%

and 31.2% in CFRP and GFRP-wrapped columns, respectively, after 3000 h marine exposure. Also, after 9000 h exposure in marine simulator, 44.6% and 31.5% reduction in axial plastic modulus of CFRP and GFRP-wrapped columns was observed, respectively. It was found that exposure to marine environment was the most extreme condition for the GFRP-wrapped columns. While CFRP-wrapped column was more durable than GFRP-wrapped one, marine exposure reduced both the compressive strength and axial plastic modulus of CFRP-wrapped columns. After confining with CFRP sheets, moisture and chloride ions were trapped in the wrapped columns, and hence, internal cracks developed due to the corrosion of steel bars. So, increasing in ultimate strain was observed in CFRP-wrapped columns after exposure. Because of

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Table 4 Test specimens. Specimen ID

Accelerated Corrosion

Retrofit Method

FRP type

Number of layers

Environmental conditioning

C-C CA-C C15A1G1-C C15A1C1-C C15A2G1-C C15A2C1-C C15A3G1-C C15A3G2-C C15A1G1-M3 C15A1C1-M3 C15A2G1-M3 C15A2C1-M3 C15A3G1-M3 C15A3G2-M3 C15A1G1-M9 C15A1C1-M9 C15A2G1-M9 C15A2C1-M9 C15A3G1-M9 C15A3G2-M9

No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

– – 1 1 2 2 3 3 1 1 2 2 3 3 1 1 2 2 3 3

– – GFRP CFRP GFRP CFRP GFRP GFRP GFRP CFRP GFRP CFRP GFRP GFRP GFRP CFRP GFRP CFRP GFRP GFRP

– – 1 1 1 1 1 2 1 1 1 1 1 2 1 1 1 1 1 2

Control Control Control Control Control Control Control Control Marine Simulator-3000 h Marine Simulator-3000 h Marine Simulator-3000 h Marine Simulator-3000 h Marine Simulator-3000 h Marine Simulator-3000 h Marine Simulator-9000 h Marine Simulator-9000 h Marine Simulator-9000 h Marine Simulator-9000 h Marine Simulator-9000 h Marine Simulator-9000 h

Fig. 7. (a) Axial stress-strain curves of non-corroded and corroded RC columns before exposure (b) Failure mode of non-corroded RC columns (c) Failure mode of corroded RC columns.

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Fig. 8. Axial stress-strain curves of retrofitted columns in method 1 (a) wrapped with CFRP (b) wrapped with GFRP.

Table 5 Mechanical properties of the strengthened specimens in methods 1, 2 and 3 for both control and marine environmental conditions.

y à

Environmental condition

Specimen ID

,u (MPa)*

єu (%)*

E2 (GPa)*

Control

C-C CA-C C15A1G1-C C15A1C1-C C15A2G1-C C15A2C1-C C15A3G1-C C15A3G2-C

37.1 26.3 36.9 49.2 54.7 66.5 42.1 47.4

0.738 0.631 0.945 1.137 0.906 1.075 0.819 0.861

(49.7%)y (80.1%)y (43.5%)y (70.4%)y (29.8%)y (36.5%)y

– – 1.98 2.72 – – – –

Marine Simulator after 3000 h

C15A1G1-M3 C15A1C1-M3 C15A2G1-M3 C15A2C1-M3 C15A3G1-M3 C15A3G2-M3

27.2 (3.4%)y (26.3%)à 47 (78.7%)y (4.5%)à 42.4 (61.2%)y (22.4%)à 63.7 (142.2%)y (4.2%)à 44.7 (70%)y (6%)à 48 (82.3%)y (1.3%)à

0.831 1.259 0.578 0.919 0.754 0.729

(31.7%)y (12%)à (99.5%)y (10.7%)à (8.4%)y (36.2%)à (45.6%)y (14.5%)à (19.5%)y (8%)à (15.5%)y (15.3%)à

1.36 (31.2)à 2.16 (20.7%)à – – – –

Marine Simulator after 9000 h

C15A1G1-M9 C15A1C1-M9 C15A2G1-M9 C15A2C1-M9 C15A3G1-M9 C15A3G2-M9

26.8 (1.9%)y (27.4%)à 39.5 (50.2%)y (19.7%)à 41 (55.8%)y (25%)à 54.3 (106.3%)y (18.3%)à 45.9 (74.3%)y (8.8%)à 48.4 (84%)y (2.2%)à

0.746 1.356 0.604 0.812 0.740 0.703

(18.2%)y (21%)à (115%)y (19.1%)à (4.3%)y (33.4%)à (28.7%)y (24.5%)à (17.3%)y (9.6%)à (11.4%)y (18.3%)à

1.36 (31.5)à 1.50 (44.6)à – – – –

(40.3%)y (87%)y (108%)y (152.8%)y (60.1%)y (80%)y

* ,u: The ultimate compressive stress at failure, єu: The ultimate axial strain at failure, E2: Axial modulus in the plastic region. Values in parentheses show the increase/decrease percentage compared to CA-C. Values in parentheses show the increase/decrease percentage compared to similar specimens which were placed in normal condition.

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severe degradation in mechanical properties of GFRP sheets, this phenomenon was not seen in GFRP-wrapped columns. Failure modes of retrofitted columns in normal condition revealed in both GFRP and CFRP-wrapped columns FRP sheets ruptured perpendicular to fibers orientation (see Fig. 9a and b). This failure mode was due to matrix cracking and fibers fracture together. After exposure, in CFRP-wrapped columns FRP sheets ruptured mainly parallel to fibers orientation. It was due to the degradation of fiber–matrix interface. On the other hand because of lower durability of glass fibers, GFRP sheets ruptured by fibers fracture (see Fig. 9c and d). 3.3. Effect of retrofit method 2 on compressive behavior The axial stress-strain curves of the corroded columns which were strengthened by retrofit method 2 are presented in Fig. 10. Also, mechanical properties which obtained from these graphs are summarized in Table 5. The linear axial stress–strain curve of the FRP-confined column has been verified in this retrofit method. It is important to note that this behavior is different from that of FRP–wrapped concrete. It was due to the elastic-brittle behavior of the repair mortar. As shown in Table 5, in normal environmental condition, retrofit method 2 with CFRP and GFRP sheets increased the compressive strength of the corroded column by 152.8% and 108%, respectively. In this retrofit method, after 3000 marine exposure, 4.2% and 22.4% reduction in ultimate strength of CFRP and GFRP-wrapped columns was observed, respectively. Also, 9000 h marine exposure decreased the ultimate strength of wrapped columns with CFRP and GFRP by 18.3% and 25%, respectively. These results indicate, compared with retrofit methods 1, the retrofit method 2 could provide higher compressive strength. It was due to the higher strength of repair mortar when compared to the damaged concrete. In term of durability, degradation in ultimate strength of retrofitted columns by method 2 was almost similar to the corresponding strengthened columns in method 1. It was due to the similar exposure condition, which in both retrofit methods, FRP system was exposed during its service life. It should be noted that prevention of internal cracks propagation in retrofit method 2, caused minor enhancement in durability of wrapped columns. Moreover, because of lower strength and durability of GFRP sheets, internal cracks expansion had more destructive effect in GFRP-wrapped columns. Fig. 11 shows failure modes of columns retrofitted by method 2. Similar to the retrofit method 1, GFRP-wrapped columns failed due

to the matrix cracking and fibers fracture together which was more evident in the weathered specimens. In CFRP-wrapped columns, fiber–matrix interface fracture was seen in normal environmental condition. Also, fiber-matrix interface degradation was more evident after marine exposure. 3.4. Effect of retrofit method 3 on compressive behavior The axial stress-strain curves of columns retrofitted by method 3 are shown in Fig. 12. Also, mechanical properties of these columns are summarized in Table 5. Similar to the retrofit method 2, linear axial stress–strain curve was observed in this retrofit method. It can be due to the elastic behavior of mortar, and also the effect of mortar confinement around the GFRP sheets. As shown in Table 5, in normal environmental condition, retrofitted columns by method 3 increased ultimate strength by 60.1% and 80% after confining with one and two layers of GFRP, respectively. Different durability performance was seen in retrofit method 3. Unlike the retrofit methods 1 and 2, not only the durability of the retrofitted columns with method 3 was not affected, but also a minor increase was observed in ultimate strength of the columns after marine exposure. It was due to the post-curing of the repair mortar in marine environmental condition. In retrofit method 3, after 3000 h marine exposure, ultimate strength of columns wrapped with one and two layers of GFRP, increased by 6% and 1.3%, respectively. Also, in this retrofit method, 8.8% and 2.2% increase in ultimate strength of the wrapped columns with one and two layers of GFRP was observed, respectively, after 9000 h exposure. The failure modes of retrofitted columns by method 3 are shown in Fig. 13. In these specimens, loading was continued up to the rupture of GFRP. Concurrent with the rupturing of the sheets, some longitudinal cracks appeared on the protective mortar and they failed simultaneously. Moreover, in this retrofit method, the failure of the columns occurred with a lower sound and a less explosive manner, in comparison with other retrofit methods. In normal environmental condition, after failure, cracked mortar cloud be removed easily from the surface of FRP. On the other hand, after marine exposure, cracked mortar was departed together with some parts of sheets. Therefore, it can be found that bond strength between FRP and protective mortar was improved after marine exposure. In some specimens, steel bar buckling was observed behind of ruptured sheets (see Fig. 13d). It seems in this retrofit method, pre-

Fig. 9. Failure mode of (a) C15A1G1-C (b) C15A1C1-C (c) C15A1G1-M9 (d) C15A1C1-M9.

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Fig. 10. Axial stress-strain curves of retrofitted columns in method 2 (a) wrapped with CFRP (b) wrapped with GFRP.

Fig. 11. Failure mode of (a) C15A2G1-C (b) C15A2C1-C (c) C15A2G1-M9 (d) C15A2C1-M9.

mature failure of FRP occurred by buckling of longitudinal reinforcement. It was due to the closer distance between longitudinal steel bars and FRP sheets. 3.5. Comparison of retrofit methods Comparing with different retrofit methods, method 1 was easier to do, but could not prevent the corrosion of steel reinforcements. On the other hand, both retrofit methods 2 and 3 could protect steel bars from corrosion. Also, they were more costly and increased the difficulty of strengthening. Thus, retrofit methods 2

and 3 better to use for the corroded columns undertaken high level of corrosion. In term of durability, comparing with different retrofit methods, retrofit method 3 was more durable, which was due to the protection of the sheets against marine exposure. In term of strength, replacing confined damaged concrete by self-compacted mortar, resulting in the increasing of the ultimate strength in retrofit methods 2. In this study, Distance Based Approach (DBA) [27–29] has been utilized in order to evaluation of three retrofit methods. In this technique, the first step was to establish the decision criteria. For this purpose, strength, durability and cost have been selected as

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Fig. 12. Axial stress-strain curves of retrofitted columns in method 3 (a) wrapped with two layers of GFRP (b) wrapped with one layer GFRP.

Fig. 13. Failure mode of (a) C15A3G1-C (b) C15A3G2-C (c) C15A3G1-M9 (d) C15A3G2-M9.

criteria for comparing different retrofit methods. Strength criterion was calculated by comparing ultimate strength of each specimen with specimen CA-C (see Table 5). Also, durability criterion was calculated by comparing ultimate strength of weathered specimens with corresponding specimens in control condition (see Table 5). A repair cost included, FRP, primer, repair mortar and worker wage cost, which was calculated based on each specimen

in different retrofit methods. After specifying decision criteria, they should be standardized. Higher standardized values are nearer to the ideal state. Standardized criteria for different retrofitted specimens are presented in Table 6. For each environmental condition, individual standardization was carried out. Finally, the distance between each alternative method to the optimum state is derived from the following equation:

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A. Kashi et al. / Construction and Building Materials 151 (2017) 520–533 Table 6 Standardized criteria. Environmental condition

Specimen ID

Strength criterion

Durability criterion

Cost criterion

Control

C15A1G1-C C15A1C1-C C15A2G1-C C15A2C1-C C15A3G1-C C15A3G2-C

1.22 0.01 0.51 1.64 0.71 0.21

– – – – – –

1.62 0.03 0.60 0.99 0.32 0.96

Marine Simulator after 3000 h

C15A1G1-M3 C15A1C1-M3 C15A2G1-M3 C15A2C1-M3 C15A3G1-M3 C15A3G2-M3

1.56 0.13 0.26 1.55 0.06 0.21

1.37 0.29 1.08 0.32 1.1 0.74

1.62 0.03 0.60 0.99 0.32 0.96

Marine Simulator after 9000 h

C15A1G1-M9 C15A1C1-M9 C15A2G1-M9 C15A2C1-M9 C15A3G1-M9 C15A3G2-M9

1.68 0.33 0.17 1.24 0.34 0.61

0.94 0.43 0.78 0.34 1.46 1.03

1.62 0.03 0.60 0.99 0.32 0.96

Dk ¼

( X

wj  ðzkj  zJ Þ

2

)1=2 ð2Þ

j

where k = number of alternatives; j = number of criteria; Dk = distance index for kth alternative; wj = weight of jth criterion; zkj = standardized value of the kth alternative with respect to the jth criterion; z⁄j = the higher value of jth criterion. In this study distance index was calculated based on the assumption of equal weights. The results of the DBA methodology for each retrofit method in different environmental conditions are presented in Fig. 14. Based upon the concept, the chosen alternative should have the shortest distance index from the ideal solution. Results showed, regardless of durability (in normal environmental condition), strengthening with one layer of GFRP in method 2 was the best retrofit method. It should be noted that retrofit method 3 was not appropriate in this condition. After 3000 marine exposure, specimen which was wrapped by one layer of CFRP in retrofit method 1 showed better performance. It was due to the higher strength and durability of CFRP sheet, and also lower cost of retrofit method 1. Columns retrofitted by method 3 showed the best performance after 9000 marine exposure. It was due to the higher durability of these specimens. On the other hand, comparing with retrofit method 3, in retrofit methods 1 and 3, which FRP sheets were exposed to the marine environment, the performance of the retrofitted columns was not acceptable. Hence, it can be concluded that durability is a significant parameter which must be considered in strengthening of the corroded columns in marine environment. In a short period of time, applying durable type of FRP sheets can be improved the performance of wrapped members, but in long term exposure, protecting FRP sheets seems to be necessary.

2)

3)

4. Conclusion 4) In this paper, an experimental investigation was carried out in the rehabilitation of corroded RC columns with FRP sheets. The following conclusions are drawn from this study: 1) In the retrofit method 1, regardless of environmental condition, wrapping corroded columns by one layer of GFRP and CFRP increased ultimate strength by 40.3% and 87%, respectively. After 9000 h marine exposure, ultimate strength of these columns decreased by 27.4% and 19.7%,

5)

respectively. So, with regard to the long-term durability of RC columns wrapped with FRP sheets, this retrofit method was not appropriate, especially in GFRP-wrapped columns. In the retrofit method 2, confining RC columns increased ultimate strength by 108% and 152.8% in GFRP and CFRPwrapped columns, in normal environmental condition. Regarding to the long-term durability, 25% and 18.3% reduction in ultimate strength of GFRP and CFRP-wrapped columns was observed, respectively, after 9000 marine exposure. Hence, it can be concluded that bonding FRP sheets after replacing corrosion-damaged concrete didn’t have any significant effect on durability of retrofitted columns. Compared with the retrofit method 1, minor improvement in durability of retrofitted columns by method 2 was observed, which was due to the prevention of further steel corrosion. In the retrofit method 3, strengthening RC columns with one and two layers of GFRP increased ultimate strength by 60.1% and 80%, respectively, in normal environmental condition. Generally, in retrofitted columns by methods 2 and 3, brittle manner was observed, which was mostly due to the brittle behavior of self-compacted mortar. In the retrofit method 3, regarding to the long-term durability of strengthened columns, 8.8% and 2.2% improvement in ultimate strength of wrapped columns by one and two layers of GFRP was observed, respectively after 9000 h exposure. Therefore, in this retrofit method not only any degradation was not observed, but also ultimate strength was slightly improved. This was due to the protection of GFRP sheets by repair mortar, and also better curing of repair mortar in marine environments. Furthermore, repair mortar showed better bonding to GFRP sheet after marine exposure. Failure modes of specimens revealed that in RC-wrapped columns which FRP sheet was exposed during its service life, CFRP sheets mostly ruptured because of degradation of fiber–matrix interface. On the other hand, degradation of glass fibers was more evident in GFRP sheets. DBA analysis based on ultimate strength, durability and cost criteria was carried out. Results showed that in marine environmental condition, short-term performance of retrofitted column by one layer of CFRP in the method 1 was ideal. It was also found that strengthened columns by one layer of

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Fig. 14. Distance indicator for specimen retrofitted by methods 1, 2 and 3 which stored in (a) Control environmental condition (b) Marine simulator for 3000 h (c) Marine simulator for 9000 h.

GFRP in the retrofit method 3 performed better than other specimens. Thus, it can be concluded that in harsh environmental condition, wrapping by durable type of FRP sheets would not be sufficient, and protecting the exterior surface of the FRP sheets should be considered.

Acknowledgments The authors acknowledge the financial support provided by the Concrete Technology and Durability Research Center (CTDRC) of Amirkabir University of Technology.

A. Kashi et al. / Construction and Building Materials 151 (2017) 520–533

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