Repair; Refurb; Retrofit
While it is accurate that no single test method is proficient in determining actual risk and rate of corrosion, full half-cell potential mapping combined with other sample tests can be extremely effective. The potential map below is taken from The Institute of Structural Engineers guide to the ‘Design Recommendations for Multi-storey and Underground Car Parks’ (Fourth Edition) (4) and illustrates the information that can be gathered and subsequently used to identify invisible but at-risk areas of future corrosion.
Figure 2: Example of a Half Cell Potential Map of a Car Park deck illustrating variable corrosion potential over the complete deck (4)
Corrosion is a chemical reaction and as with any reaction, is dependent upon the presence of the right environmental conditions. With reinforced corrosion, water and oxygen are two key components that must be present in sufficient
Figure 3: Examples of premature failure on car parks due to incipient anode formation & the incorporation of galvanic anodes at the periphery to inhibit incipient anode formation
A HOLISTIC CORROSION CONTROL APPROACH
For many years Impressed Current Cathodic Protection (ICCP) was the most commonly encountered corrosion control method to address the problems caused by chloride-induced corrosion. In summary, cathodic protection is a permanent electrochemical system that applies current onto the steel reinforcement lowering its potential and reducing its corrosion rate. Such systems offer owners high levels of control and certainty and can be very effective over long periods of time (25 years plus). The ICCP process is most cost effective when large areas are protected and the costs are spread out over a long period of time. To assure their long-term effectiveness, any ICCP system requires continuous monitoring and maintenance over its active life.
As can be seen from Figure 2 however, the risk of corrosion over a single deck can vary greatly. If we multiply this to the whole car park, we can see that it is possible to utilise multiple solutions, which work in combination to enhance the protective effect of each. The remainder of this article aims to identify one such holistic approach to the corrosion protection of multi-storey car parks.
Budget constraints, reduced staffing and in-house expertise, reduced upfront cost and reduced maintenance / monitoring requirements are all major factors in the growing use of galvanic corrosion control. If the structure does not have a widespread corrosion risk, or budgetary restraints are present, then galvanic corrosion control is a suitable approach for car park decks, especially when combined with waterproof coatings and membranes.
The incorporation of galvanic anodes into patch repairs to inhibit the onset of ‘Incipient Anode Formation’ (induced new corrosion at the periphery of the patch) has been a growing technique over the past 14 years. The occurrence of incipient anode on car parks is a common sight in the UK (see Figure 3) with failure initiating in as little as 4-5 years depending upon the severity of the environment. With the incorporation of galvanic anodes however, a repair can be stabilised for up to 15-20 years, improving durability and reducing maintenance. While this type of corrosion protection is the simplest and most commonly utilised technique, it only protects areas outside of the repair area at risk from incipient anode formation. Other ‘at risk’ but currently undetected corrosion sites are not controlled.
quantities to allow propagation. The use of car park deck membranes to reduce moisture and limit further chloride contamination is a commonly encountered system in the UK. In addition to the above benefits they also provide anti-slip characteristics, improve the visual appearance and positively impact the safety of a car park. This approach, for new multi-storey car parks in particular, is very effective. In situations where chloride contamination has already taken place, other consideration must be taken into account.
For carbonated concrete, the most effective and most commonly used strategy to reduce corrosion risk is to control moisture content (8) (Relative Humidity RH). The impact of this is well documented and for those wishing to read further on the subject should consult the BRE Digest 491, ‘Corrosion of steel in concrete - A review of the effect of humidity’ (5). The BRE Digest 491 raises another point that collaborates the theme of this document. A number of studies have shown that, although some control is exercised on the rate of corrosion by reducing the internal RH of the concrete, it is not totally effective if significant chloride concentrations are present (6). If this is combined with carbonation*, the risk increases further where low levels of chloride (0.4% by weight of cement) can increase the corrosion rate at relatively low RH values to unacceptable levels (5).
In the UK (and in many other locations around the world), it is challenging to reduce the RH of concrete much below 50%, even with a waterproof coating. If we compare this value to Figure 3, we can see that corrosion is a risk and in highly chloride containing areas, (1-2% are common in UK car parks) is likely to continue underneath a coating. It is therefore important that these high-risk areas are provided with additional protection so that the concrete and coating remain stable.
The use of embedded galvanic anodes is one method that can address this risk. The use of half-cell mapping enables us to identify those high-risk areas,
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