ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING) VOL. 6, NO. 3 (2005) PAGES

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1 ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING) VOL. 6, NO. 3 (25) PAGES STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION AS A NON-DESTRUETIVE REPAIR METHOD: PART I. DISCRETE TEST SAMPLES A. Hosseini a and Ali R. Khaloo b a Department of Civil Engineering, University of Zanjan, Zanjan, and BHRC of Iran, b Department of Civil Engineering, Sharif Univ. of Technology, P.O. Box , Tehran, Iran, ABSTRACT The representative parameters of ECE and the side effects were determined experimentally. The experimental program investigates chloride and alkali concentration, chloride removal efficiency, transference number, half-cell potential and possible bond strength degradation which is determined by pull-out test. Test variables were percentage of chloride concentration to mass of cement (i.e., %, 1%, 1.5% and 2%) and duration of application of constant electric current (i.e., 2, 4, 6 and 8 weeks). Tests results indicate that chloride concentration around rebars reduces to the threshold concentration level of about.4 after 6 to 8 weeks of treatment. Half-cell potential shows that corrosion activities of the steel reinforcement were stopped 8 weeks after the end of treatment. Chloride removal efficiency is hardly reached to 6% after 8 weeks of ECE treatment. Bond strength in specimens with and without transverse reinforcement was reduced by 3-44% and 19-27% at the end of the treatment, respectively. Bond strength in specimens with and without transverse reinforcement was increased by 1-16% and 8-1%, 8 weeks after end of treatment, respectively. Test results on a reinforced concrete slab will be presented in the subsequent paper. Keywords: corrosion; electrochemical properties; durability; chloride; pull-out strength. 1. INTRODUCTION Carbonation of concrete reduces the ph of its pore solution to the level lower than required to maintain the passivity of steel. Chloride ions introduced to concrete, directly attack and break down the passive film, even though the ph is high enough to maintain otherwise a passive layer on the steel surface. If the concentration of chloride ions is higher than the threshold level of corrosion or the ph of the pore solution is lowered by the carbonation, the passive layer on the reinforcement may be destroyed and the reinforcement may corrode. -address of the corresponding author: abdi_hosseini@yahoo.com

2 168 A. Hosseini and Ali R. Khaloo Two electrochemical repair methods for reinforced concrete damaged by carbonation and chloride attack have been developed. The first method is called "Realkalization" and the second "Desalination" [1, 2]. The mechanism of desalination is shown in Figure 1. Placing the anode and electrolyte on concrete surface and passing direct current between this anode and the reinforcing steel acting as a cathode accomplish the electrochemical removal of chloride ions. Since anions migrate toward the anode, it is possible to take Cl out of the concrete structure and at the same time, cations (Na +, K + ) migrate toward the reinforcing steel. Electrolyte Media (Saturated Ca(OH) ) 2 Concrete + Na + K OH CL Steel bar - 2H 2O +2e 2H + H2 Titanium Mesh Figure 1. Mechanism of desalination A DC Power supply + - Ampere meter Variable resistance to adjust electrical current density A number of concerns about desalination including the possible reduction in bond strength [3,4,5,6], micro cracking [4], the possible instigation of Alkali Silica Reaction (ASR) [7] and hydrogen embrittlement [4] have been studied. Research has been conducted on the system efficiency [6,8], composition of the pore solution [1], and more recently modeling of the extraction process [9] as well as practical application on structures. Although extensive research have been conducted on the feasibility of applying electrochemical desalination in particular cases, however, several aspects in relation to design and potential detrimental side effects of the treatment require clarification. The present investigation was aimed at developing representative parameters and design basis of electrochemical desalination. Furthermore, it provides a methodology for monitoring the effectiveness of the process to existing structures, before, during, and after treatment. Polder et. al., [1] studied the effect of external electrolyte and concluded that the efficiency of desalination process is much higher when saturated calcium hydroxide was used. The choice of current density and period of treatment has varied widely in the range of.6 to 4 A/m 2 for 3 to 6 weeks [1], 1 to 8 A/m 2 for 4 to 12 weeks [7], 2.5 to 5. A/m 2 for 4 to 16 weeks [11] and.2 to.75 A/m 2 for up to 32 weeks [3]. Typically, ECE involves current density of around 1 A/m 2 of concrete surface, which normally corresponds to a current density between.5 and 2 A/m 2 of steel surface [3]. In this paper, current density is expressed in terms of steel surface. Several studies considered high current densities of 4 or 5 A/m 2 [3]. In present research current density of 1 A/m 2 with respect to concrete surface

3 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION corresponding to 6 A/m 2 with respect to steel surface and saturated calcium hydroxide electrolyte were used. 2. TEST PROGRAM This study consists of chemical analysis and measurement of half-cell potential and bond strength. The test program is designed in three time intervals as shown in table 1. Chemical analysis for total chlorides (acid soluble) was carried out according to BRE [13]. Chemical analysis for determination of equivalent alkali ions (R 2 O=Na 2 O+.658K 2 O) was carried out by Rapid Alkali Test (RAT) based on method of potentiometery. Electrical potential measurements for determining corrosion activity of rebars was performed by half-cell potential test method with Ag/AgCl reference electrode according to ASTM C 876. Bond testing was carried out 24h after disconnection. Figure 2 shows the bond test arrangement. The displacement was applied at a rate of around.1 mm per minute to the end of the rebar with respect to bottom surface of concrete. For determination of chloride and alkali content at different depths of concrete cover, the specimens were cut in slices of 15 mm thickness parallel to current passing surface. b Figure 2. Pull-out test arrangement 2.1. Test specimens Specimens were 15x15x15 cm concrete cubes with a deformed steel bar Ф16 (d b =16 mm) embedded at center. Bond length was 8 cm that equals to 5 d b. Mix proportions of concrete is shown in table 2. Portland cement type II was used and refined NaCl with 99% purity was dissolved in the mix water as premixed chloride. When sodium chloride was used, an equal weight of fine aggregate was reduced.

4 17 A. Hosseini and Ali R. Khaloo Table 1. Test Program Time Step I (Pre ECE) II (ECE) III (Post ECE ) Concrete mix Duration 8 weeks 8 weeks 8 weeks Current Density - 6 A/m 2 - Tests Chloride profile: At the end of step Alkali profile: At the end of step Bond strength¹: At the end of step Half-cell potential: Once per week after 28 days Chloride profile: Once per two weeks Alkali profile: Once per two weeks Bond strength¹: Once per four weeks Half-cell potential: Once per week 1-Bond strength test was carried out 24h after electrical disconnection Ratio of total chloride to mass of cement (%) Water (Kg/m 3 ) Table 2. Concrete mix proportions Cement (Kg/m 3 ) Sand (Kg/m 3 ) Gravel (Kg/m 3 ) Salt (Kg/m 3 ) Chloride profile: Once per two weeks Alkali profile: Once per two weeks Bond strength¹: At the end of step Half-cell potential: Once per week Slump (mm) Density (Kg/m 3 ) Compressive strength of 28 day (MPa) ZC LC MC HC Procedure of passing electric current All the specimens were demolded one day after concrete casting. After moist curing for 8 weeks under polyethylene cover, electrodes were placed around the specimens. The specimens were submerged in electrolyte liquid and direct electric current was applied. A saturated solution of Ca(OH) 2 was used as electrolyte and titanium mesh was used as the anode. The electric current was passed through two sides of the specimens for the required period while the other faces were insulated by epoxy resin coating as shown in Figure 4 and 5.

5 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION Current passing surface Figure 3. Bond test equipment Coated Cube 15x15x15 16 Coated Figure 4. Test Specimens (bottom surface was also coated) Figure 5. Specimens during ECE treatment

6 172 A. Hosseini and Ali R. Khaloo 3. TEST RESULTS AND DISCUSSIONS 3.1. Chloride extraction Migration of Cl The average value of total chloride content in concrete at locations of 15, 3, 45, 6 and 75 mm from each face of specimen before and after treatment for 2, 4, 6 and 8 weeks corresponding to total current charges of 18, 36, 54 and 72 A.h/m² of steel surface, respectively, are shown in Figures 6a to 6c. Since the current density is inversely proportional to the distance from reinforcement, the efficiency of chloride extraction is higher near the reinforcement than that at the surface of concrete. Figure 6 shows that the efficiency of chloride extraction reduces 6 weeks after treatment (i.e., total charge passed is 54 A.h/m²). The change in chloride content of concrete at rebar location is shown in Figure 7. The chloride content in concrete around rebar decreases to.4% after 6 to 8 weeks. It is known that fixed chlorides in concrete in the form of Friedel's salt make up about.4% of the cement mass. Chloride content of.4% is known as threshold value for corrosion initiation in existing concrete structures [11] Transference number of Cl Transference number of Cl (i.e., t Cl ) is calculated by the following equation: Cl A M.I.t = 1, T = T Z. F A = Amount of extracted chloride (gr) T = Amount of extractable chloride calculated by passing electric charges (gr) M = Atomic weight (35.5 for Cl ) F = Faraday constant (965) Z = Chloride valance =1 I.t = Charge passed (colomb) The relation between transference number of Cl and passing of electric charge during treatment is shown in Figure 8. The transference number of Cl increased for larger amount of chlorides at the early stage of treatment. When concrete contains a large number of extractable Cl ions, the rate of Cl to total ions contained in concrete increases, consequently, the electric charges carried by Cl drops to a certain level, and therefore the transference number of Cl decreases. (1)

7 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION Total chloride to mass of cement (%) Total chloride to mass of cement (%)... Total Chloride to mass of cement (%) Thershold value Time (week) Thershold Value.2 (a) LC specimens Time (week) (b) MC Specimens Thershold value Time (week) 1.5cm 3cm 4.5cm 6cm 7.5cm 1.5cm 3cm 4.5cm 6cm 7.5cm 1.5cm 3cm 4.5cm 6cm 7.5cm (c) HC Specimens Figure 6. Distribution of chlorides in cover concrete

8 174 A. Hosseini and Ali R. Khaloo Total chloride to mass of cement (%) Transference number of cl - Chloride Removal Efficiency Thershold value LC(1%) MC(1.5%) HC(2%) Time (week) Figure 7. Distribution of chlorides around the steel bar 1. LC(1%) MC(1.5%) HC(2%) Time (week ) Figure 8. Transference Number of chloride LC(1%) MC(1.5%) HC(2%) Time (week) Figure 9. Chloride Removal Efficiency

9 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION Migration of alkali The relation between equivalent alkali (R 2 O= Na 2 O+.658 K 2 O) and the passed electric charge is shown in Figure 1. The larger the amount of chlorides in the specimens, the larger is the amount of alkali concentration around the steel bar due to desalination. As shown in Figure 11, the value of alkali around the steel bar increases with increase in amount of premixed Cl. Furthermore, accumulation of alkali increases with increase in the treatment period; however, the rate of increase reduces with time. The increase in alkali content reduces bond between the concrete and steel bar and may accelerate the potential of alkali aggregate reaction. There is a gradient of alkali distribution at the end of treatment, which is expected to diffuse and relatively balance with elapsed time. Alkali concentration around steel bar decreased by about 3% in 8 weeks after desalination (Figure 1). Alkali content (kg/m 3 ) Alkali content (kg/m 3 ) Initial alkali conten Bar Location (cm) Immidiately after 8 weeks ECE 8 Weeks after 8 weeks ECE Figure 1. Redistribution of alkali ions 8 week after treatment Time (week) LC MC HC Figure 11. Accumulation of alkali ions around steel bar during treatment

10 176 A. Hosseini and Ali R. Khaloo 3.2. Half-cell potential Changes in half-cell potential of the steel bars embedded in concrete 4 weeks before, during and 8 weeks after treatment with 6 A/m 2 of current density is shown in Figure 12a. The four areas follow ASTM criterion according to NASE standard cathodic protection area [12]. All half-cell potentials of non-treated specimens (controls) were in the corrosion area. On the other hand, those of treated specimens during and just at the end of treatment were in the protection area. The specimens corrosion activities were almost changed to the noncorrosion area 8 weeks after the end of treatment. The repair effect of desalination is confirmed with the recovery of passivation layer of steel bars. Half-cell potentials of specimens before and 8 weeks after treatment are shown in Figure 12b. Half Cell Potential (mv., Ag/Agcl) Treated Non-Treated Non-corrosion Uncertain Corrosion Protection PRE ECE LC(lc=1%) MC(lc=1.5%) HC(lc=2%) LC(lc=1%) MC(lc=1.5%) HC(lc=2%) ECE Period (days) POST ECE Figure 12a. Half-Cell potential changes before, during, and after treatment 3.3. Pull-out test Two types of ultimate bond failure in tension steel bars may occur. The first is direct pull-out of the bar, which occurs when ample confinement is provided by the surrounding concrete or transverse reinforcement. This could be excepted when relatively small diameter bars are used with sufficiently large concrete cover and bar spacing (Figure 13a). The second type of failure is splitting of the concrete along the bar. This occurs when cover confinement or bar spacing is insufficient to resist the lateral tension in concrete (Figure 13b) [14].

11 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION Untreated 8 Weeks after treated Half Cell Potential (mv., Ag/Agcl) Noncorrosion Uncertain Corrosion LC MC HC Concrete specimens characteristics Figure 12b. Effect of desalination in half-cell potential 8 weeks after treatment (a) Direct pullout (b) Splitting Figure 13. Bond failure mechanism In the first bond failure mechanism (direct pull-out) lateral reinforcement and concrete

12 178 A. Hosseini and Ali R. Khaloo compression strength adjacent to the bar interface and bar deformations configuration and its ribs height are dominant factors. The effectiveness of transverse reinforcement depends on its yield strength, cross section area, and spacing along the development length. For the second bond failure mechanism (splitting) the tensile strength of concrete and cover thickness are the most effective factors Effect of ECE on bond strength Softening of cement paste: The accumulation of Na + and K + ions around the rebar causes softening of the cement paste around the rebar. In highly concentrated solutions, alkali hydroxides attack Portland cement paste by dissolving the aluminates and to a lesser extent the silicates. Such softening associated with ECE occurs where high current densities are used. Rosa et. al., [3] observed up to 6 mm softening of concrete around the rebar of specimens subjected to 4.3 A/m² for one year. They observed hardening after drying of previously softened cement paste and suggested that softening is reversible. This affects both bond failure mechanisms with greater effect on the direct pull-out [3]. The specimens were disconnected and removed from the water tank. The bearing surface was capped, and pull-out test was carried out 24h after disconnection. Pull-out test arrangement is shown in Figure 3. The load was applied at a rate of 1 KN/min. The slippage of rebar in relation to concrete was measured by a sensitive LVDT. Required bond strength calculated for unconfined and confined specimens (with spiral reinforcement 2 mm) was f b,req =5. MPa and f b,req = 8. MPa according to ACI with a safety factor of 2, respectively [15] Bond test results and discussion Test results for bond strength during and after ECE are shown in table 3. The variation of bond strength versus time for specimens with transverse reinforcement and required bond strength are shown in Figure 14a. Ultimate bond failure was in the form of direct pull-out in all the specimens. Prior to ECE the bond strength of specimens containing chloride was about 4 to 12% higher than that of specimens not containing chloride (ZC). This is attributed to corrosion enhancing bond due to expansion of corroded steel bar. According to half-cell potential measurement corrosion was initiated in MC and HC specimens prior to ECE. Untreated specimens showed a small gradual increase in bond strength during 8 weeks of ECE. This is due to further hydration and strength development, except for HC and MC specimens that show decrease in bond strength due to formation of radial longitudinal cracks caused by corrosion. The specimens with higher chloride show bond strength reduction. This is attributed to build up of sodium and potassium ions at the regions near the rebar and softening of cement paste. At the end of 8 weeks of treatment the bond strength reduces between 2 and 44% for LC, MC and HC specimens. Bond strength increased by about 1 to 16% eight weeks after the end of treatment (post ECE). Therefore it can be concluded that reduction in bond strength is partially recoverable.

13 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION Bond strength MPa Bond strength MPa required bond strength (ACI) Time (week) required bon 5 strength (ACI) 4 (a) Specimens with spiral reinforcement Time (week). (b) Specimens without transverse reinforcement Figure 14. Variation of bond strength versus time (charge passed) Treated Untreated ZC LC Treated Untreated The variation of bond strength versus time for specimens without transverse reinforcement and the required bond strength are shown in Figure 14b. Ultimate bond failure was in the from of splitting in all the specimens. Prior to ECE the bond strength of LC, MC and HC was between 7 and 12% lower than that of ZC, that is attributed to tensile tangential strain induced by expansion of corrosion products. Untreated specimens (ZC) showed a small increase in bond strength during 8 weeks of ECE, which is due to further hydration and strength development and HC and MC specimens cracked due to corrosion of embedded rebar. ZC LC MC HC MC HC

14 18 A. Hosseini and Ali R. Khaloo Table 3. Test results for bond strength during and after ECE (MPa) All specimens were cured for 8 weeks before ECE Total charge passed (A.h/m 2 ) Time during ECE (week) Time after treatment (week) * ZC (+4%) (+8%) (+11%) LC (-11%) (-29%) (-19%) MC (-14%) (-34%) (-26%) HC (-17%) (-44%) (-28%) ZC (+9%) - LC (+2%) - MC (-9%) - HC (-14%) - ZC (+2%) (+7%) (+8%) LC (-8%) (-19%) (-1%) MC (-11%) (-26%) (-21%) HC (-1%) (-27%) (-18%) ZC (+3%) - LC (-11%) - MC Cracked - HC Cracked - * Reference bond strength for calculation of percentage of variation. Specimens with spiral reinforcement Specimens without Confinement Treated Untreated Treated Untreated F b,required, ACI (MPa) After 8 weeks of treatment, bond strength reduced due to cement paste softening by 19%, 26% and 27% for LC, MC and HC, respectively. Bond strength was increased by 1% eight weeks after the end of treatment with respect to the end of ECE (post ECE). This indicates that reduction in bond strength is partially recoverable. The results of Rasheeduzzafar et. al., [5] are broadly consistent with these findings. They exposed specimens to.54 A/m 2 for 14 months (5443 A.h/m 2 charge passed) and observed 19% and 33% reduction in bond strength in specimens containing.36% and 1.45% chloride by weight of cement, respectively

15 STUDY OF ELECTROCHEMICAL CHLORIDE EXTRACTION CONCLUSIONS Based on the results of tests conducted in this study, the following conclusions are reached. 1. Cl ions in concrete could be extracted by applying desalination. At early stages of treatment Chloride Removal Efficiency (CRE) was improved with increase of passed electric charges; however, it did not reach over 6% and became almost constant after 8 weeks of treatment. At early stages of treatment, the transference number of Cl (t Cl ) was increased with increase in amount of premixed chlorides; however, it decreased with increase of electric charges passed. These two parameters (CRE and t Cl ) demonstrated that efficient ECE duration is between 6 and 8 weeks that corresponds to A.h/m² total charges passed. 2. The ECE treatment caused the alkali metal ions (i.e., Na +, K + ) to accumulate around the steel bars. With a larger amount of premixed chlorides and passing electric charges the amount of alkali accumulation also increased. However, when the amount of alkali reached a certain level, the increase of alkali accumulation ceased. Alkali accumulation around rebar causes cement paste softening [3]. 3. Half-cell potential of steel bars embedded in specimens, just after completion of treatment were classified in the cathodic protection area. However, these potentials changed to nobler potentials with elapsed time and they were classified in the noncorrosion area after 8 weeks of completion of treatment. 4. Bond strength for direct pull-out and splitting failure mechanisms decreased by 44% and 27% in HC specimens, respectively. These reductions correspond to 8 weeks of treatment (equivalent to 72 A.h/m 2 total charges passed). Eight weeks after the end of treatment, bond strength increased by about 1 to 16% with respect to bond strength at the end of treatment. 5. For design purposes it is recommended to provide the required bond strength (according to ACI (318-2) with safety factor of 2) by restriction of total charges in one stage of treatment to 45 A.h/m 2, that is almost adequate for chloride extraction. Acknowledgements: Partial support of Sharif University of Technology in conducting this research study is gratefully appreciated. The supports of Building and Housing Research Center (BHRC) under National Research Project Performance of Chloride Extraction and Realkalization of Reinforced Concrete in Stopping Steel Corrosion were instrumental in carrying out the study. The assistance of laboratory staff of Concrete Division of BHRC is greatly acknowledged. REFERENCES 1. Emmanuel, E.V., Henriksen, S.K. and Whitmore, D., Chloride extraction and realkalization of reinforced concrete stop steel corrosion, ASCE Journal of Performance of Constructed Facilities, No. 2, 12(1998) Ishibashi, K., Ashida, M., Udagawa, Tomosawa, H. F., Shimizu, A., Masuda, Y., Abe, M., and Kage, T., Basic study of electrochemical rehabilitation of carbonated and

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