An innovative and cost affective Cathodic Protection System for the protection of reinforced concrete structures


When a detail survey is undertaken it is not unusual for only 30% of a structure to be at high risk of corrosion and often, it is well below 20%. Evaluation techniques such as Half-cell potential mapping are extremely efficient at ascertaining risk over large areas. The technique is non-destructive in nature and the readings recorded during the evaluation stage are often manipulated to be seen as colour contour plots. These can be subsequently superimposed over a technical drawing of the structure. An example is given below (Fig.3).


Fig 1. (above) MMO Titanium mesh on a bridge beam prior to concrete placement


Fig 2. (right) Discrete ceramic anodes drilled into holes of a carpark deck


BACKGROUND


The technology of Impressed Current Cathodic Protection (ICCP) has been utilised since the mid 1970’s for the protection of reinforced concrete against the onset and propagation of corrosion. The technology in its most basic form, works by passing a low level of Direct Current (DC) onto the steel reinforcement. This is achieved by the application of either a surface mounted anode system and or individual embedded units. Examples are given in Fig.1 and 2.


The anode(s) are subsequently zoned and wired to a power/rectifier and control system, which allows the scheme to be controlled and monitored over its design life. The performance requirements of a CP system are described within the relevant NACE and ISO standards and are mentioned here only for reference.


As with any repair scheme, the cost of the corrosion protection system will depend significantly upon the requirements of any given project. Publications upon the cost of ICCP per m2 m2


often quote ranges of between £150-£300 per


depending upon the system used and the project size. These figures specifically do not include installation, access and or the associated control, power/rectifier units. These units alone can cost in excess of £50k GBP, which for a large project, becomes relatively insignificant. For smaller, more isolated areas, it makes the technology cost prohibitive. For this reason, new ICCP installations are often only seriously considered when the project is of a suitable size. This is not a technical decision but one from a commercial perspective.


DETECTING CORROSION


When trying to understand the cause of corrosion, it is important to appreciate that the problem is rarely an issue affecting the whole structure uniformly, but often starts as a localised problem. This is commonly the result of a number of interrelated variables for example; a.


Changes in the concrete permeability, caused by local differences in composition, compaction and curing.


b.


Construction errors leading to areas with low concrete cover to reinforcement.


c. Variation in the exposure environment, leading to areas with elevated chloride ion content and or high carbonation depth.


Frequently these differences cannot be ascertained by visual inspections alone and often require more analytical techniques to quantify the risk of corrosion. Some of the most commonly used methods for ascertaining risk are: 1.


Delamination and Visual Surveys 2.


3. 4.


Global, open grid Half Cell Potential Surveys NB: The findings of 1 and 2 should be used to select or target representative locations for:


Screening for Chloride and other Contamination Testing for Depths of Carbonation


16


Fig 3. Half-cell corrosion potential contour map of a multi-storey carpark. Hatched areas indicate areas of concrete damage. Red areas denote a high corrosion risk in sound concrete.


Clearly for a large concrete structure such as a carpark or raised motorway section, 20-30% may still represent in excess of 1000m2


of concrete surface.


For a small inland bridge however, 20% may only represent a small section of an abutment wall and or a small number of support beams and columns that may have been exposed to chloride contaminated water. In this instance there may only be 40-60m2 be cost effective.


of concrete at risk and a full ICCP system is not going to


This concept forms part of an approach known as Targeted Protection. While CP is a proven technique for mitigating corrosion in its own right, it is not the only system that can bring benefit to a concrete structure. As an example, waterproof membranes can help reduce the general water content and therefore reduce the average corrosion rate. While this will not stop corrosion within high risk areas, it will have a prolonging effect in low to medium risk areas and limit further migration of external chlorides to the steel surface. By adopting holistic approaches to the protection of concrete structures, long lasting protection can be achieved at significantly lower costs than full scale ICCP.


PASSIVATING ACTIVE CORROSION


The traditional view of Cathodic Protection can be seen as a method of polarising steel to a point where corrosion rates are reduced to very low or negligible levels. This is achieved via the application of a relatively small direct (DC) current throughout the life of the system (2-20mA/m2


of steel surface


area). It is a well-known phenomenon that over the life of an ICCP system, the current required to achieve the performance characteristics as defined in standards often reduces. This can be attributed to two secondary effects of applying the cathodic reaction with time.


Cathodic reaction ½02 + H2 0 + 2e - 20H (Hydroxyl Ions - Alkalinity) Eq. 1


As can be seen from the equation above, the cathodic reaction generates alkalinity at the steel/concrete interface, increasing the pH with time. By artificially increasing the alkali content, the balance is tipped in favour of steel re-passivation (the creation of a thin but very dense layer upon the steel surface) and the suppression of corrosion pits.


The cathodic reaction also promotes a relative negative charge upon the steel surface. Chloride ions, being negative themselves, are repelled and distributed away from the steel surface. Both of these secondary effects can be seen in Fig 4. and are adopted by related techniques known as concrete Re- alkalisation and Chloride Extraction.


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