1、 1 ACI 364.13T-15TechNoteRepaiRs foR ReinfoRcement with shallow coveRKeywords: cathodic protection; concrete cover; corrosion; corrosion inhibitor; deterioration; epoxy; membrane; protection; reinforcement.IntroductionA reinforced concrete structure needs a surface repair due to corrosion of the rei
2、nforcement, which resulted from an inadequate concrete cover (less than code minimum).QuestionWhat are the recommended remedies to extend the service life of this type of surface repair?AnswerMany surface repairs have limited service life due to inadequate protection against corrosion-related deteri
3、o-ration of embedded steel reinforcement. A number of protection options are available to extend the service life of surface repairs, such as increasing cover thickness, enhancing the concrete performance using admixtures or coatings, coating the reinforcement, installing cathodic protection measure
4、s, or a combination of these. The degree of corrosion protection required is a function of the exposure conditions; the actual concrete condition; and the type, layout, and service use of the structure. Therefore, final selection and design of a remedial measure should consider these factors. The de
5、velopment of the reinforcement must also be considered to maintain the structural integrity of the structure. Furthermore, in accordance with ACI 562, the repaired elements should comply with applicable building code requirements and relevant fire regulations valid at the project location.Discussion
6、Prior to designing or implementing repair, the contributing cause(s) for the noted deterioration should be identified. To offer appropriate and cost-effective remedial actions, one needs to identify the existing condi-tions, such as chloride levels, state and characteristics of the concrete, actual
7、concrete cover thickness, and the required level of protection for the structures. Geographic location and surface orientation are two key controlling factors.Geographic locationACI 362.1R lists five exposure groups used to describe the different levels of exposures for parking structures, as shown
8、in Fig. 1. Some reinforced concrete structures in the regions with northern climates (Zone III) are exposed to deicing salt and freezing-and-thawing cycles. Corrosion protection is critical for these elements. By the same token, structures in regions with southern climates (Zone 1 and part of Zone I
9、I6), with the exception of coastal exposures, typically need a lower level of corrosion protection because of the differing exposure.Surface orientationOne consideration for having the appropriate corrosion protection level provided by concrete cover is the orientation of the surface (horizontal or
10、vertical). Vertical surfaces are not usually as prone to severe corrosion-related deterioration as horizontal surfaces; an exception is where calcium chloride was added to concrete to facilitate winter construction. Such practice was commonly done before the 1970s. The primary reason is that water i
11、s not likely to stay on the vertical surfaces to penetrate through the concrete cover. As a result, protection requirements for elements with vertical surfaces may not need to be as stringent as that for horizontal surfaces.Cracking and low-quality concrete can contribute to corrosion-related damage
12、. Therefore, to select the proper level of surface protection, one should consider the physical condition and material properties of the structural element.American Concrete Institute Copyrighted Material www.concrete.org2 REPAIRS FOR REINFORCEMENT WITH SHALLOW COVER (ACI 364.13T-15)Once the corrosi
13、on protection that is needed for the structural elements is defined, different remedial options can be selected to achieve the target levels of protection. There is a variety of protection remedies that can be implemented to minimize the impact of corrosion-related deterioration, thereby extending t
14、he service life of the concrete surface repair (Table 1) (ACI 546R; NACE SP0187).Corrosion protection optionsSix protection options are identified and discussed as follows:1. Use of corrosion inhibitorsA schematic drawing of using corrosion inhibitor in repair concrete is shown in (b) of Table 1. Th
15、e most commonly used corrosion inhibitor is calcium nitrite, which is added to the repair concrete mixture as an addi-tive. The inhibitor protects the steel from chloride-induced corrosion by reacting with ferrous ions to repas-sivate the reinforcing steel surface. This method has good performance h
16、istory and has been widely used, provided the nitrite and chloride ratio is in an appropriate range per manufacturers recommendations.Another form of applying corrosion inhibitor is to use a penetrating corrosion inhibitor (also known as surface-applied corrosion inhibitors or migrating corrosion in
17、hibitors). These materials are applied over existing concrete surfaces and intended to penetrate into the concrete. Effectiveness varies depending on the applica-tion condition and location, how the product is applied, the particular proprietary product, the actual concrete cover thickness, and the
18、in-place concrete properties. Penetrating corrosion inhibitors remain controversial, with some studies demonstrating effectiveness while other studies show little or no effectiveness (EFC 38).2. Protective barrier coating or waterproofing membrane on concrete surfacesApplying a coating or waterproof
19、ing membrane on the concrete surface, as shown in (c) of Table 1, is an effective means to reduce moisture and chloride ion penetration into concrete, thereby minimizing corrosion of the embedded reinforcement. There are different types of commercially available membranes, ranging from the commonly
20、used polyurethane to polyurea and other systems such as a composite systems that uses an epoxy top coat with a polyurethane main coat (ACI 546.3R). These products should be selected with input from licensed design professionals and the material manufacturers. Appropriate maintenance of these barrier
21、 systems is required to prevent the penetration and accumulation of water in damaged areas.3. Protective coating on the exposed surfaces of the reinforcementZinc-enriched coating, epoxy (ACI 364.3T), and cementitious products are commonly used coatings that can be applied over the reinforcing bars o
22、r tendons, as shown in (d) of Table 1. The coating forms a barrier, stop-ping the chloride ions, moisture, and oxygen from reaching the steel and isolates the steel electrically from the surrounding concrete, thereby reducing the corrosion potential.Fig. 1Geographic weather exposure zones per ACI 36
23、2.1R-12. (Zone III and Zones CC are the most demanding condition, and Zone I is the least. Zone CC is coastal exposure condition.)American Concrete Institute Copyrighted Material www.concrete.orgREPAIRS FOR REINFORCEMENT WITH SHALLOW COVER (ACI 364.13T-15) 3Table 1Different corrosion protection opti
24、ons(a) Insufficient concrete coverThis shows the condition where the concrete cover is less than required to provide adequate protection from the environment, such as chlorides and carbon dioxide (carbonation). The various methods listed in the following may be used singularly or in combination, dep
25、ending on how aggressive the corrosive environment is.(b) Corrosion inhibitorInhibitors protect the bar from corrosion. Costs are generally relatively low as compared to most alternative methods listed in the following. Most often, corrosion inhibitors are used as admixtures added to the repair mate
26、rial(s). Migrating corrosion inhibitors can also be applied directly onto the concrete surface to be protected. (c) Protective barrier (coating or waterproofing membrane)Coatings and membranes are effective in eliminating chloride penetration. Special coating systems are required to prevent carbonat
27、ion of concrete. Consult with coating manufacturers for specific installation recommendations and surface preparation requirements.(d) Protective coating applied on the reinforce-mentBar coatings protect the coated section of the bar directly from chlorides and mois-ture. Surface condition of the ba
28、r shall conform to the requirements stipulated by the coating manufacturer.(e) Increasing the concrete cover thicknessIncreasing the repair layer thickness is a simple and effective way to add cover. Review aesthetic considerations as well as headroom, egress, and drainage effects if this method is
29、considered.(f) Repositioning of the reinforcementRemove and relocate existing bars to code required depth within the repair. This method is very expensive if relocation of bars involves doweling into existing concrete.(discrete anode connected to the reinforcement)(distributed anode embedded within
30、the cover and connected to the reinforcement)(g) Galvanic cathodic protection with embedded anodesEmbedded galvanic anodes are either of discrete or distributed types. Discrete galvanic anodes are used in concrete rehabilitation to prevent the formation of new corrosion sites adjacent to completed s
31、urface repairs (known as the ring anode, incipient anode, or halo effect). The discrete anode consists of a sacrificial zinc core that is activated by the surrounding, specially formulated, mortar to keep the zinc active. Discrete anodes are quickly and easily fastened to the exposed reinforcing ste
32、el, usually using metal ties. Distributed galvanic anode systems are installed to provide galvanic cathodic protection over a large area and typically consist of specially configured zinc mesh or rods designed to prevent passivation when embedded in the recommended concrete or mortar. Once installed
33、, the zinc core of both discrete and distributed galvanic anodes corrodes preferentially to the surrounding reinforcing steel, thereby providing galvanic corrosion prevention to the adjacent reinforcing steel.American Concrete Institute Copyrighted Material www.concrete.org4 REPAIRS FOR REINFORCEMEN
34、T WITH SHALLOW COVER (ACI 364.13T-15)Advantages of using a protective barrier coating include:a) It provides additional equivalent cover to trouble spots. Equivalent cover corresponds to the actual concrete cover thickness that would provide the same level of protection.b) It provides another layer
35、of protection to the steel if cracks develop in the repair material. Popovic (1998) reported that some protection can be provided even with partially coated bars.c) It provides additional resistance to the ingress of oxygen, moisture, and chlorides to the surface of the rein-forcing steel. Such addi
36、tional protection is critical in situations where thin concrete cover over the reinforce-ment cannot be avoided.Disadvantages of using a protective barrier coating may include:a) The application of some protective barrier coatings on reinforcing steel requires a certain amount of time (application i
37、n layers, drying period after each layer). Besides, many materials, especially those containing epoxy, have a finite open time beyond which they reduce bond or even become bond breakers.b) Because it is difficult to coat the entire bar, the effectiveness of the coating may be limited or, at times, i
38、ts use can be detrimental.4. Increased or modified concrete coverIncreasing the concrete cover (e) of Table 1) is one of the most cost-effective methods to protect embedded reinforcement against steel corrosion. Alternatives to increasing the repair concrete layer thickness include the use of higher
39、-quality concrete with a low permeability and repositioning the reinforcing bars or tendons into the concrete element, as shown in the latter case in (f) of Table 1. Any repositioning of reinforcement should involve an engineering evaluation. For larger areas, a concrete overlay with the added cover
40、 can be used.Depending on the extent and detailing of exposed reinforcing bars, it may not be feasible to reposition the bars to achieve the specified concrete cover. Increasing concrete cover could be unacceptable because of the surface discontinuity that may result in more than one location, which
41、 could be aesthetically unacceptable as well as creating potential tripping hazards. To avoid ponding in floor slabs, the impact to the overall drainage profile should also be evaluated before concrete cover is increased locally.5. Use of galvanic cathodic protection for reinforcementThere are diffe
42、rent forms of galvanic cathodic protection. A commonly used system is sacrificial anodes that are positioned and tied to the reinforcing bars, as shown in (g) of Table 1. A sacrificial anode may also be spray-applied on the concrete surface, as shown in (h) of Table 1. By connecting steel to a sacri
43、ficial metal that is more electro-negative, corrosion is directed away from the reinforcing bars to the sacrificial anode (NACE 01105).There are a number of products available on the market, and suppliers installation recommendations should be followed.(h) Galvanic cathodic protection with superfi-c
44、ial anodes connected to the reinforcementAnother method for galvanic cathodic protection of reinforcing steel in concrete uses zinc metallizing. After sandblasting the concrete surface, the zinc is flame- or arc-sprayed over the surface to be protected. The zinc is connected to the steel either thro
45、ugh surface plates that are in contact with embedded steel, or spalled concrete is removed and zinc is sprayed directly onto the steel (ACI 362.1R).(i) Impressed current cathodic protectionImpressed current cathodic protection systems (ICCPs) consist of inert or sacrificial anodes positioned near (b
46、ut not in electrical contact) to the reinforcing steel that are powered by an external DC electrical source and embedded in a material that is conductive to ions. The reinforcing steel to be protected is electrically connected as the cathode. ICCP, if properly designed, installed, monitored, and mai
47、ntained, can prevent and mitigate corrosion of reinforcing steel. Electrical continuity between all reinforcing steel and metallic embedment is necessary for cathodic protection to be effective. Other metals in concrete shall receive additional attention. A corrosion monitoring system should be prov
48、ided for the reinforced concrete structure to allow an estimate of the extent or degree of corrosion activity on the structure as well as to determine the effectiveness of the corrosion control method used.American Concrete Institute Copyrighted Material www.concrete.orgREPAIRS FOR REINFORCEMENT WIT
49、H SHALLOW COVER (ACI 364.13T-15) 56. Use of impressed current cathodic protection (ICCP) for reinforcementThis method, illustrated in (i) of Table 1, involves actively applying electric current to counteract the current of the corrosion cell. This method can effectively combat active steel corrosion in a reinforced concrete struc-ture, depending on the design, installation, monitoring, and maintenance of the system. The total installation and operating costs for such systems are relatively high because the applied electric current will need to be maintained as l
