Bond degradation due to the desalination process

Bond degradation due to the desalination process

Construction and Building Materials 17 (2003) 281–287 Bond degradation due to the desalination process J.J. Chang* Department of Harbor and River Eng...

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Construction and Building Materials 17 (2003) 281–287

Bond degradation due to the desalination process J.J. Chang* Department of Harbor and River Engineering, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, 202, Taiwan, ROC Received 12 March 2002; received in revised form 20 October 2002; accepted 25 October 2002

Abstract In this paper, bond degradation due to the desalination process of chloride-contaminated reinforced concrete is investigated. Four different constant voltages were applied to study the effect of the current on the desalination. Two constant current densities were applied for comparison. Two different electrolytes, NaOH and Ca(OH)2 , were used to examine the effect of electrolyte on the bond characteristics. Pullout specimens were cast and tested, using a single rebar pullout test. Three bond parameters of the bond strength, the critical debonding shear load per unit embedded rebar length and the shear stiffness per unit embedded rebar length were obtained to evaluate the bond quality after desalination. It is found that the bond characteristics decrease dramatically as the duration andyor the voltage of desalination increase. Using Ca(OH)2 solution as the electrolyte, less bond loss is observed than using NaOH solution. A micro-hardness experiment shows that softening of concrete near the rebar becomes more apparent as the desalination duration andyor desalination voltage increase. A chemical titration experiment indicates that the accumulation of potassium and sodium ions increases as the desalination duration andyor desalination voltage increase. A pH value measurement shows that the desalination current increases the pH value of concrete near the rebar. More than two-thirds of chloride ions migrate out of the rebar–concrete interface in two weeks of desalination with desalination voltage greater than 10 V. Bond strength decreases approximately 40–60%. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Desalination; Bond strength; Electrolyte; Chemical titration

1. Introduction It is reported that chloride ions could induce catastrophic failure in reinforced concrete structures w1x. The use of deicing salts during winter and intentional addition of chloride-based admixtures to concrete, lead to substantial amounts of this aggressive species. Furthermore, other sources include the use of contaminated aggregates or mixing water, and the exposure to industrial brine and seawater. Chloride ions destroy the passive film of rebar and result in corrosion damage of concrete. It is reported that after reaching a threshold of chloride ions, the passive film collapses. The threshold wClyx G0.6 w2x. chloride concentration is expressed by: wOHyx Consequently, chloride removal becomes a very important issue in engineering practice. *Corresponding author. Fax: q886-2-24-632-932. E-mail address: [email protected] (J.J. Chang).

Since chloride removal in concrete was first investigated w3,4x, the application of electrical fields to concrete as non-destructive repair method has been increasingly conducted w5,6x. The procedure of desalination or electrochemical chloride extraction (ECE) involves mounting an anode surrounded by a liquid electrolyte on the surface of the concrete and driving direct current into the embedded reinforcement, which acts as a cathode. The current extracts chloride ions towards the anode. Once reaching the surface, the ions eventually pass into the anolyte and are thereby removed from the concrete. During the extraction process, the following chemical reactions occur at the cathode, 2H2OqO2q4ey™4OHy

(1)

2H2Oq2ey™2OHyqH2

(2)

0950-0618/03/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 2 . 0 0 1 1 3 - 7

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Table 1 Mix design for concrete (wycs0.66) Material

Control (kgym3)

Contaminated (kgym3)

Water Cement Sand Coarse aggregate NaCl

169 307 765 1117 0

169 307 765 1117 9.21

Anodic reactions can be expressed as the following equations, 4OHy™2H2OqO2q4ey

(3)

2H2O™4HqqO2q4ey

(4)

2Cly™Cl2q2ey

(5)

Arya et al. w7x investigated several parameters that influenced electrochemical removal including the applied potential gradient, reinforcement arrangement, initial chloride content, depth of chloride contamination with respect to the cover reinforcement, use of an external cathode and soffit desalination, i.e. chloride removal from the source of the contamination. Ihekwaba et al. w8x reported that desalination was considerably retarded by the presence of a carbonation front. Ihekwaba et al. w9x also reported that the desalination current reduced the concrete compressive strength, especially for the concrete near the cathode. By investigating the rehabilitation of several vertical structures, Ihekwaba et al. w10x concluded that circular columns containing spiral reinforcements showed better ECE performance than structures with planar surfaces. They w11x also reported that pullout bond degradation of steel rebars in ECE concrete could be a major side effect of this technique. In their experiments a 44% bond degradation was observed in some specimens. Macrotte et al. w12x reported that although a desalination current halted chlorideinduced pitting corrosion, it increased the overall corrosion rate for all specimens studied because of the passive film reduction and significant changes in the pore solution and cement chemistry adjacent to the reinforcements. They w13x also reported that calcium– silicon-rich phases existing in untreated concrete were not detectable after desalination. Instead, sodium-rich, calcium-rich, iron-rich and calcium–aluminum-rich phases were observed. Andrade et al. w14x proposed a mathematical model to describe a desalination process in which a Nernst–Plank equation was used. Sa’idShawqi et al. w15x later used a numerical model of electrochemical chloride removal from concrete based on Nernst–Plank and the Laplace equations. Castellote

et al. w16x used a simplified mathematical model to investigate the desalination efficiency. Polder w17x applied the desalination of an electrochemical chloride removal technique for 39 days to treat concrete with a 16 years’ submersion in the North Sea. It was reported that 70–90% of the chloride content was removed. In this study, the desalination efficiency and the corresponding bond degradation are examined at the same time. The desalination technique, which is in principle similar to a cathodic protection, applies a high current to the reinforcements. This current does not only change the microstructure of the hydrates, but also significantly reduces the bond characteristics. Traditionally, chloride ion removal is defined as chloride ions passing into the anolyte and thereby removed from the concrete. When the cover thickness is large, this process may result in catastrophic failure due to bond degradation. In this study, we propose to define chloride ion removal by pushing the chloride ions away from the rebar. Although the chloride ions are not totally removed from the concrete, they may not attack the passive film of rebar in a short period, depending on the diffusion process of chloride ion and how far we have pushed them away. Based on this idea, we study a possibility to produce a compromise in which the reinforcements can be made safe from chloride attack through a desalination process that will not harm the interface bond quality. 2. Experimental scheme 2.1. Materials Type I cement, local river sand with a fineness modulus of 3.24 and coarse aggregate with a fineness modulus of 6.88 were used. A single batch of concrete mix design with a wyc ratio equal to 0.66 was used. Details of the concrete mix are tabulated in Table 1. To simulate chloride contamination, 3% chloride ions (by cement weight) were added for contaminated specimens. Number 4 medium-carbon deformed rebar was used. The chemical composition of the steel rebar is listed in Table 2 and its geometrical layout is shown in Table 3. The Young’s modulus for the rebar is 203 GPa (29 435 ksi) and the yield strength is 410 MPa (59.5 ksi). A titanium mesh was used as the anode, and 0.1 M NaOH and Ca(OH)2 solutions were used as the analyte. Table 2 Chemical composition of rebar Element

C

Cu

Si

Mn

P

S

Weight percentage (%) Element Weight percentage (%)

0.36 Ni 0.11

0.23 Cr 0.12

0.20 Mo 0.01

0.61 Sn 0.02

0.04 Fe Balance

0.03

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Table 3 Configuration of 噛4 rebar Area of cross section (cm2)

1.19

D (mm) d (mm) Lr(mm) Lb (mm) H (mm) a (degree) Sr (mm)

12.30 3.99 1.36 2.24 0.74 66 8.23

Fig. 1. Experimental setup for pullout tests.

2.2. Variables in experiments In previous studies, two kinds of electrical fields were applied, which were constant potential gradient and constant current density. The current density from 1 to 10 Aym2 was suggested as an appropriate current density for desalination. However, the relative humidity changes the electrical impedance of concrete. In order to keep constant current density, under dry weather, high voltage should be applied between electrodes, and thus a lot of energy is consumed. Therefore, some researchers employed constant voltage method w7,18x. In this study, four different constant voltages (5, 10, 20, 30 V) were used. For comparison, two constant currents, 3 and 30 mA (1.88 and 18.8 Aym2 correspondingly), were also used. The reason why we selected 30 mA as the constant current is that the steady current was observed for 5 V desalination. The desalination periods for each electrical field applied were 1, 2, 3, 4, 5 and 6 weeks. Two electrolytes, 0.1 M NaOH and Ca(OH)2 solutions, were used.

Fig. 1, the average displacement obtained from two LVDTs actually represents the slip displacement at the point where the rebar was clipped. This location may vary from one experiment to another. To determine displacements at the same position, one can use the following modified formula, UcalsUmeasuredy

PLoff EfAf

(6)

where Ucal is the calculated displacement of point A on the rebar, which is near the concrete surface, as shown in Fig. 1. Umeasured is the measured displacement at the point of clipping B in the figure. P is the pullout load. Loff is the offset length, i.e. the distance between points A and B. Ef is the Young’s modulus of the rebar and Af is the nominal cross-sectional area of the rebar. A typical pullout force vs. displacement curve is sketched in Fig. 2. Three stages exist in the pullout test. In the first stage, the load and displacement maintain a linear relationship up to the critical load Pcr and no

2.3. Experiments Cylindrical concrete specimens with 10 cm diameter and 20 cm height were cast and 噛4 rebar was placed in the axial direction with an embedded length of 4 cm. Two groups of specimens were prepared. The first was the uncontaminated group and the second is the contaminated group. After demolding, specimens were cured in a lime-saturated water for 28 days. After curing, specimens were connected to the desalination currents for various desalination periods. As the designated desalination time was achieved, six specimens were loaded into a universal-testing machine according to ASTM C234-91a. The difference from ASTM C234-91a was that the shape of concrete, of which was a cylindrical specimen, not prismatic. During the pullout tests, the pullout loads and slip displacements of the rebar were recorded. As shown in

Fig. 2. A typical pullout load diagram vs. displacement curve.

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Fig. 3. Illustration for sampling concrete near the rebar.

debonding occurs in this stage. The slope of PyU is then called the shear stiffness, Ks. The shear stiffness increases as the embedded rebar length increases, and thus is not a constant. In the reference w19x, a parameter called the shear stiffness per length is defined as Ks

Ks Lemb

(7)

where K is the shear stiffness per unit embedded rebar length and Lemb is the embedded length of the rebar. It is reported that this parameter, K, is constant as long as the rebar geometry is unchanged. After the critical load Pcr, the curve becomes so non-linear that the stiffness decreases. This critical load also depends on the embedded length. For the same reason, the reference w18x is applied to define the critical shear debonding load per unit embedded length qy as, qys

Pcr Lemb

Fig. 4. Bond strength decrement due to constant voltage desalination.

titration of potassium and sodium ions w11x was conducted. Furthermore, measurements for the pH values were made to evaluate the concrete realkilisation. The remaining chloride contents in this region were also examined, according to ASTM C114. 3. Results and discussions From the results, it is found that the bond strength decreases as the desalination voltage andyor desalination time increase as found in Fig. 4. After one-week desalination, more than 50% decrease in bond strength is found for the voltages higher than 10 V, and a 20% bond loss was observed for 5 V. After two-weeks of desalination, the bond loss for applied voltages higher than 10 V was approximately 60 and 40% is observed

(8)

The second stage is called the bend-over stage due to its shape, and the load then reaches the maximum, Pmax. From the maximum pullout load, one can define the bond strength, dividing it by the nominal embedded rebar area. After the maximum pullout load, the curve descends as shown in Fig. 2. This stage is called the descending stage. Such three parameters as K, qy, and the bond strength were used in this study to evaluate the bond characteristics. After the pull-out tests, several additional experiments were conducted. A micro-hardness experiment was carried out to examine the mechanical properties of the concrete adjacent to the reinforcement. To examine the chemical properties of the concrete adjacent to rebar, a square region of concrete with an edge length 3-cm shown in Fig. 3 was taken and powdered. A chemical

Fig. 5. Bond strength decrement due to constant current desalination.

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Fig. 6. Decrement in shear stiffness per unit length vs. desalination time.

for the 5 V group. From Fig. 5, it is observed that the bond strength at 30 mA constant current is similar to that of the 5 V group. After two-weeks desalination, not much difference could be found. Within the first 2 weeks, in contrast the 30 mA group shows greater loss than the 5 V group. When a constant voltage is applied to the concrete, the current in the beginning is low due to the high resistance of concrete. When the desalination time increases, microstructures in the concrete change and the concrete resistance decreases to a stable value. The bond loss results from the accumulated potassium and sodium ions, which attack the CSH gel to form a worse phase w11,20x and increase as the current density increases w20x. The total charge of the 30 mA group was larger than the 5 V group for the above-mentioned

Fig. 7. Critical debonding shear force per unit length vs. desalination time.

285

reason. As a result, the larger loss in bond strength occurred for the 30 mA group than the 5 V group. It was also observed that the loss of bond strength in the 3 mA group was smaller than that expected because the applied electrical field was weaker. As shown in Figs. 6 and 7, significant losses in shear stiffness per length (K) and critical debonding shear force per length (qy) are found. The shear stiffness per length indicates the elastic stiffness of the bond. It can be concluded from our experiments that the desalination current softened the bond. The critical debonding shear force per length represents the chemical bonding quality. The lower debonding shear force per length is, the worse chemical binding characteristics occur. Thus, it is found that desalination current might also change the chemical composition of the concrete interface. In a previous study w13x, calcium–silicon-rich phases (CSH gel) were no longer observed after desalination. Instead, calciumrich phases, sodium-rich, calcium-rich, iron-rich and calcium–aluminum-rich phases were observed. From this evidence, it is obvious that the attacks by sodium

Fig. 8. (a) Cumulative sodium ions. (b) Cumulative potassium ions.

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Fig. 9. Effects of different electrolytes on the bond strength degradation.

Fig. 11. Enhancement of pH value due to desalination current.

and potassium ions change the chemical composition of the binding materials, thereby reducing mechanical properties: stiffness and bond strength. In Fig. 8a,b, the accumulated amount of sodium and potassium ions are illustrated. Both sodium and potassium ions increase as the desalination time andyor desalination voltage increase. Furthermore, the sodium ion concentration is much higher than that of potassium ions due to the contamination salt (NaCl) used in specimens. In Fig. 9, the losses of bond strength using different electrolytes are evaluated. Using the Ca(OH)2 solution as the electrolyte shows a smaller loss in bond strength than using the NaOH solution, although the difference is minor. This implies that selection of electrolytes may change the bond strength, but the essential

factor is the contaminated salt of potassium or sodium ions. Micro-hardness experiments were conducted to determine the degree of concrete softening near the rebar. Results are illustrated in Fig. 10. It is found that higher desalination voltage andyor longer desalination time result in lower microhardness. The higher the microhardness is, the denser concrete microstructure exits. The reduction in microhardness occurs primarily in the first two weeks. After 2 weeks, the hardness values are not much changed. This implies that destruction of the concrete interface occurs in the first two weeks. The pH value measurements were conducted and results are shown in Fig. 11. From the figure, higher voltage andyor longer desalination time lead to higher alkaline environment. The desalination current not only takes chloride ions from the rebar but also increases the

Fig. 10. Interface hardness changes due to desalination current.

Fig. 12. Chloride removal of concrete nearby rebar by desalination.

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pH value, which could help in the recovery of passive film. The remaining chloride contents near the rebar were measured and results are shown in Fig. 12. It is observed that after two weeks of desalination nearly two-thirds of the chloride ions are taken away from the adjacent region where the applied voltages are higher than 10 V. To obtain similar results, a 6-week desalination is required for the 5 V group. According to Fig. 4, in the case that two-thirds of the initial chloride ions are required to be removed from the adjacent region, the reinforced concrete suffers approximately 60% bond strength loss regardless the desalination voltage. Therefore, removing chloride ions from concrete results in a high loss in bond strength. From engineering practice, such a high loss in bond strength may result in sudden failure in a reinforced concrete structure due to the debonding effect. From our experimental results, it can be concluded that the desalination or electrochemical chloride removal process should be carefully considered. For example, using reinforcements as the cathode in the current approach may result in a tragedy. The challenge is to develop a technique that can remove chloride ions and produce few changes in the bond characteristics. 4. Concluding remarks In this study, the desalination of chloride-contaminated concrete was examined. It is found that although desalination current successfully removes the chloride ions and increases the pH value near the rebar, the bond characteristics dramatically decrease. As a result, 60% bond strength loss is observed when two-thirds of the chloride ions are removed from the defined adjacent region. It is concluded that a new technique for removing chloride ions from concrete without using the reinforcement as a cathode is necessary. Acknowledgments The author would like to express his thanks for the financial support from the National Science Council, Taiwan under contract number NSC. 89-2211-E-019026. References w1x Isecke B. Corrosion of reinforcement in concrete construction. In: Crane AP, editor. Society of Chemical Industry. London: E&FN Spon, 1983. w2x Ethesham HS, Al-Sandoun SS. Effect of cement composition on chloride binding and corrosion of reinforcing steel in concrete. Cem Concr Res 1991;21 (January):777 –94.

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