1、Item No. 24223 NACE International Publication 01104 This Technical Committee Report has been prepared by NACE International Task Group 054*on Electrochemical Chloride Extraction and Realkalization of Reinforced Concrete. Electrochemical Realkalization of Steel-Reinforced ConcreteA State-of-the-Art R
2、eport April 2004, NACE International This NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or
3、using products, processes, or procedures not included in this report. Nothing contained in this NACE International report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent
4、, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subjec
5、t. Unpredictable circumstances may negate the usefulness of this report in specific instances. NACE International assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE International report are responsible for reviewing appropriate health, safety,
6、 environmental, and regulatory documents and for determining their applicability in relation to this report prior to its use. This NACE International report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment,
7、and/or operations detailed or referred to within this report. Users of this NACE International report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance
8、with any existing applicable regulatory requirements prior to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NAC
9、E reports are automatically withdrawn if more than 10 years old. Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (tel
10、ephone +1281228-6200). Foreword The purpose of this technical committee report is to present state-of-the-art information on electrochemical realkalization (ER) for conventionally reinforced concrete structures. Included are discussions of common industry practices used by design engineers to contro
11、l corrosion of reinforcing steel in portland cement concrete structures through the application of ER. This report is intended for use by engin-eers attempting to protect corroding reinforced concrete structures by use of electrochemical treatment techniques. The information presented in this report
12、 is limited to ER for atmospherically exposed reinforced concrete and is not applicable to prestressed or post-tensioned elements or concrete containing epoxy-coated reinforcing steel, galvan-ized, or other coated or nonferrous reinforcement. This report, focusing on realkalization of carbonated con
13、-crete structures, is Part II of a two-part series. Part I focuses on electrochemical chloride extraction of chloride-contaminated concrete.1Research on rebar bond strength and other issues reviewed there may be relevant to ER treatment. The reader is therefore advised to review both documents. This
14、 technical committee report was prepared by Task Group 054 on Electrochemical Chloride Extraction and Realkalization of Reinforced Concrete in collaboration with the Corrosion Prevention Association.(1)Task Group 054 is administered by Specific Technology Group (STG) 01 on Concrete and Rebar. This r
15、eport is issued by NACE Inter-national under the auspices of STG 01. _ *Chairman John Broomfield, Consulting Corrosion Engineer, East Molesey Surrey, UK. (1)Corrosion Prevention Association (CPA), Association House, 99 West Street, Farnham, Surrey GU9 7EN, UK. NACE International 2 Introduction Reinf
16、orced concrete is a versatile and widely used construc-tion material. Its excellent performance and durability rely on the compatibility of the steel with the concrete surround-ing it and the ability of the concrete to protect the steel from corrosion in most circumstances. Corrosion of the steel re
17、inforcement does not occur, despite the presence of moisture and oxygen in the concrete pores, because of the alkalinity of the concrete pore water creating a passive ox-ide film on the reinforcing steel. Unfortunately, corrosion protection is not guaranteed and can fail if sufficient chlor-ides (us
18、ually in the form of sea salt, deicing salt, or chloride contamination of the original mix) or atmospheric carbon dioxide (CO2) penetrate the concrete. This leads to the breakdown of the passive layer that protects the steel. This breakdown of the passive oxide layer leads to corrosion of the reinfo
19、rcing steel if sufficient oxygen and water are avail-able. Regardless of the cause of depassivation (chlorides or car-bonation), corrosion occurs by the movement of electrical charge from an anode (a positively charged area of steel where steel is dissolving) to the cathode (a negatively charged are
20、a of steel where a charge-balancing reaction occurs, turning oxygen and water into hydroxyl ions). One solution to carbonation-induced reinforcement corro-sion involves applying an electrochemical treatment that suppresses corrosion. Figure 1 shows the basic compon-ents of an electrochemical treatme
21、nt system for realkalyzing concrete. The components are a direct-current (DC) power source and a temporary anode distributed across the sur-face of the concrete encased in a conductive medium or electrolyte. FIGURE 1: Schematic of Electrochemical Realkalization System Electrochemical methods work by
22、 applying an external anode and passing current from it to the reinforcing steel so that all of the steel becomes a cathode. Three electrochemical techniques are used to counter cor-rosion of steel in concrete. Cathodic protection can be applied by impressed current or galvanic anodes. Electro-chemi
23、cal chloride extraction (ECE) uses a temporary anode and high current over a period of 4 to 6 weeks (see NACE Publication 011011). Realkalization is a method for treating carbonated concrete. It is similar to ECE but takes approxi-mately one week and is gaining rapid acceptance as a rehabilitation m
24、ethod for carbonation in buildings and other structures. Both ECE and realkalization use currents up to about 1 A/m2(0.1 A/ft2) of steel surface area. Anode Anode O2+ H2O + 2e-= 2OH-+ - Power Supply OH-OH-OH-OH-Na2(CO3), K2(CO3) NACE International 3 Carbonation Carbonation is the result of the inter
25、action of carbon dioxide gas in the atmosphere with the alkaline hydroxides in the concrete. The reaction is illustrated by Equations (1) and (2). Like many other gasses, carbon dioxide dissolves in water to form an acid. Unlike most other acids, the carbonic acid does not cause physical damage to t
26、he cement paste, but neutralizes the alkalis in the pore water, mainly forming calcium carbonate that lines the pores: CO2+ H2O H2CO3 (1) Gas Water Carbonic acid H2CO3+ Ca(OH)2 CaCO3+ 2H2O (2) Carbonic Pore Calcium Water acid solution carbonate There is much more calcium hydroxide in the concrete po
27、res than can be dissolved in the pore water. This helps maintain the pH at its usual level of around 12 or 13 as the carbonation reaction occurs. However, eventually all the locally available calcium hydroxide reacts, precipitating as insoluble calcium carbonate and allowing the pH to fall to a leve
28、l at which steel corrodes. This is illustrated by Figure 2. FIGURE 2: Schematic of the Carbonation Front in Concrete Showing pH Levels Corrosion caused by carbonation occurs most rapidly when there is little concrete cover over the reinforcing steel. Car-bonation can occur even when the concrete cov
29、er depth to the reinforcing steel is high. This may be caused by a very open pore structure in which pores are well connected together and allow rapid CO2ingress. It may also happen when alkaline reserves in the pores are low. These prob-lems occur when there is a low cement content, high water/ cem
30、ent ratio, and poor curing of the concrete. Carbonation is common in older, poorly constructed struc-tures (particularly buildings), and reconstituted stone ele-ments containing reinforcement that often have a low cem-ent content and are very porous. Significant carbonation is rare on modern highway
31、 bridges and other civil engineering structures. This is because water/cement ratios are low, cement contents are high, and there is good compaction and curing. There is also enough cover to prevent the car-bonation front advancing into the concrete to the depth of the steel within the lifetime of t
32、he structure. On those struc-tures exposed to seawater or deicing salts, the chlorides usually penetrate to the reinforcement and cause corrosion long before carbonation becomes a problem. Wet/dry cyc-ling on the concrete surface accelerates carbonation by allowing carbon dioxide gas into the pores
33、during the dry cycle and then supplying the water to dissolve it in the wet cycle (Equation 1). This causes problems in some count-ries in tropical or semitropical regions where the cycling Pore Water pH14 12 10 8 6 Distance into Concrete from Exposed Surface pH Range for Noncarbonated Concrete Pore
34、 Water pH Range for Carbonated Concrete Pore Water Advancing Carbonation Front with Time NACE International 4 between wet and dry seasons seems to favor carbonation, e.g., Hong Kong and some Pacific Rim countries. Carbonation is easy to detect and measure. A pH indicator, usually phenolphthalein in
35、a solution of water and alcohol, detects the change in pH across a freshly exposed concrete face. Phenolphthalein changes from colorless at low pH (carbonated zone) to pink at high pH (uncarbonated con-crete). Parrott2states that phenolphthalein changes from colorless to red between pH 8.3 and 10. H
36、e also states that the coloration becomes visible on a freshly broken concrete surface as the pH exceeds 9.0 to 9.5. BRE(2)Digest 4053recommends a solution of 1 g phenolphthalein in 50 mL alcohol diluted to 100 mL with deionized water. In an inter-nal memorandum Miller4stated that he found a visible
37、 color change at pH 11 using a solution of 1% phenolphthalein solution in 96% ethanol. Steel can corrode at pH levels below 9. If Parrott is correct then steel could corrode in an area where phenolphthalein shows it to be alkaline using some solutions of phenolphthalein. If Miller is correct, then h
38、is solution of 1% phenolphthalein solution in 96% ethanol should avoid this problem. Other approaches are described by Sergi et al.5Methods of taking carbonation measurements are described in BRE Digest 4053and in the above-mentioned sources.2,4,5Measurements can be taken on concrete cores, fragment
39、s, and down-drilled holes. The literature states that care must be taken to prevent dust or water from contaminating the surface to be measured, but the test, with the indicator sprayed onto the surface, is inexpensive and simple. Realkalization The cathodic reaction on the steel in Figure 1 shows t
40、hat when electrons are applied to the steel new hydroxyl ions can be generated at the steel surface. Locally at the rebar, electrolysis may generate a pH of 13 to 14. Using an alkali metal carbonate electrolyte, the electrolyte solution is drawn in from the surface, providing a buffer solution, main
41、taining the pH. When electrochemically treated with a 1 M solution of potassium carbonate, a pH of approximately 10.7 can be maintained even on contact with atmospheric carbon diox-ide. An electrochemical realkalization system consists of a tem-porary anode usually surrounded by an alkaline electrol
42、yte, and a direct current power supply capable of supplying up to 20 V and 1 A/m2(0.1 A/ft2) of concrete surface. Typical treatment times are 4 to 8 days. Anode and Electrolyte Retention Systems Anode types are the same as for chloride removal.1Sprayed cellulose is used in one system of electrolyte
43、reten-tion with a mild steel or coated titanium mesh anode. The steel is more likely to be used here than for ECE as the treatment time is shorter and the steel is less likely to be completely consumed. Another combination used is a tank or cassette electrolyte retention system sealed to the con-cre
44、te surface with anode and electrolyte inside it. The sprayed cellulose system can be applied to large areas with the limit determined by the required current density and the capacity of the transformer rectifier power supply. The cas-settes treat an area up to 1 m2(10 ft2). Up to 150 cassettes have
45、been used simultaneously. Electrolytes In addition to generating hydroxyl ions at the steel, it has been shown that by using a sodium carbonate electrolyte, the treatment is more resistant to further carbonation. This was demonstrated independently by Elsener et al.6Work by Andrade et al.7indicates
46、that a sodium carbonate solu-tion can move into the concrete under electro-osmotic pres-sure. However, other researchers have found no evidence of this. Sodium carbonate and other electrolytes move under capillary action and diffusion. Meitz8concluded that penetration of alkaline electrolyte from th
47、e concrete surface is controlled by diffusion and absorption. Electro-osmosis is of minor importance. Once in the pores, a certain amount of the sodium carbonate reacts with further incoming carbon dioxide preventing further carbonation. This reaction is illus- trated by Equation (3). The equilibriu
48、m is at 12.2% of 1 M sodium carbonate under atmospheric conditions. Na2CO3+ CO2+ H2O 2 NaHCO3 (3) In laboratory tests it has been shown that it is very difficult if not impossible for a specimen treated with realkalization with sodium carbonate electrolyte to carbonate again. How-ever, introducing s
49、odium ions can accelerate alkali silica reaction (ASR)(3)so in some cases other electrolytes are used. A lithium electrolyte has been proposed for ASR sus-ceptible concrete but research is still not conclusive on whether it is necessary for realkalization as the concrete starts with a low pH, and is therefore less vulnerable to ASR. A 1 M solution of potassium carbonate is now the preferred electrolyte. _ (2)Building Research Establishment (BRE), Garston, Watford WD25 9XX, U
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