Patching for cathodic protection

If patching is required due to chloride corrosion then the usual specification is to remove concrete to about 25 mm behind the rebar ensuring that all corroded steel is exposed around delaminated areas and the rebar is cleaned to a near white finish to remove all rust and chlorides. Figure 6.1 shows three types of repair. A bad repair, a good chloride repair and a patch for cathodic protection. The cut edges and faces of the concrete must be square and clean of all dust and debris. 

Patching for cathodic protection is merely to ensure that there is ionic continuity between the steel and the anode (Figure 7.1). This means that the patch repair material must have the following properties ... 

Sprayed concrete, shotcrete (dry sprayed) or gunite (wet sprayed) are methods of rapidly applying concrete to soffits or vertical surfaces. It can be used over patches and to overlay metal mesh anodes for cathodic protection as described in Section 7.6. It is sometimes applied as a temporary cosmetic repair in the Northern USA and Canada when concrete has spalled or is in danger of spalling. As it does nothing to slow the corrosion rate it is comparable in effectiveness to patch repairing, and may suffer from the same incipient anode problem. 

Some degree of patching will probably be required whatever other repair is required. The merits and limitations of patch repairs have been discussed in the previous chapter along with the needs for different depths of repair for cathodic protection or chloride removal. It may also be possible to use less stringent repairs if corrosion inhibitors are being applied in the patch repair material and to the concrete. The issue of corrosion induced around the patch has also been discussed (the incipient anode. Figure 5.5). 

The reason for choosing cathodic protection is almost always cost effectiveness. The cathodic protection system prevents corrosion across the whole of the protected area of the structure, unlike localized patch repairs. If repeated cycles of patch repairs are too expensive or unacceptable, then... 

One of the major issues facing any consultant or owner of a structure suffering from chloride or carbonation induced corrosion is what form of repair to undertake. As we have seen from the previous sections there are coatings, sealants, membranes and enclosures, specialized patch repair materials, options for total or partial replacement, impressed current and galvanic cathodic protection, electrochemical chloride removal, realkalization, electro-osmosis and corrosion inhibitors. These can be applied to structures suffering different degrees of corrosion due to chloride attack or carbonation or a combination of these two. Each treatment will have implications for the future maintenance requirements, time to next major intervention and ultimate service life of the structure. 

The author has been involved with repairs to several structures where there was a risk of bars bending when the concrete was removed for patching due to cast in chloride induced corrosion. This led to either an inferior patch repair (because chloride laden concrete was not removed from behind the rebar) or a decision to cathodically protect the structure to avoid the need to carry out such strenuous repairs. 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

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