MULTI-STOREY CAR PARK REPAIR AND MAINTENANCE
Dave Simpson, Chairman of the Corrosion Protection Association and Business Development Manager (Europe) for Vector Corrosion Technologies Ltd, introduces an holistic approach for corrosion control in multi-storey car parks
There are over 4,000 multi-storey car parks in operation in the UK, many of which were built in the 1960s, in response to an increased demand for shops, coupled with an increase in the use of cars. The majority of these structures were made from reinforced concrete which, at the time, was considered to be a stable medium. But a lack of understanding of the harsh environmental conditions they would be exposed to during their life, combined with flawed design features and poor construction practices has resulted in various types of deterioration within these structures and requirements for extensive and repetitive maintenance.
Chloride-induced corrosion is one of the main contributing factors in reinforced concrete deterioration. It wasn’t until 1977 that the use of chloride was restricted as an accelerating admixture for concrete in the UK. The main sources of chloride coming into contact with multi-story car parks however is in the form of de-icing salts and airborne chlorides in and around marine environments.
Poor design features also increase the probability of chloride reaching the reinforcement steel; this chloride content in the vicinity of the reinforcement is the cause of corrosion. In temperate climates where de-icing salts are used, chlorides tend to build along trafficked areas and at wheel positions in parking bays. These areas of high contamination are often most common on the first two levels of a car park, where the entrance is located and through-traffic is more pronounced.
A HOLISTIC CORROSION CONTROL APPROACH
The risk of corrosion throughout a parking structure can vary greatly depending on the location and exposure conditions. In many cases, a single system may not be completely effective to control corrosion over the entire structure and multiple solutions, which work in combination with each other, could enhance the protective effect of each system.
Since structures may not experience corrosion uniformly, corrosion testing can provide important information to enable the owner and/or their consultant to assess the extent and magnitude of existing and the risk of future corrosion. An enhanced testing programme, can enable the structure to be thoroughly
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evaluated and an economical design for corrosion control and protection of the structure to be developed.
Impressed current cathodic protection (ICCP) is one corrosion control method that addresses the problems caused by chloride-induced corrosion. ICCP is a permanent electrochemical system that applies current onto the steel reinforcement of the structure, thereby lowering its potential and reducing its corrosion rate. Such systems may offer owners high levels of control and can be effective over long periods of time (25-plus years). To ensure their long term effectiveness, ICCP systems require continuous monitoring and maintenance over their active life. If the structure does not have a widespread corrosion risk or budgetary restraints are present, then Galvanic Protection systems can be an effective approach for car park decks, expecially when combined with waterproofing 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 which can stabilise a repair for up to 15 to 20 years, thereby improving durability and reducing maintenance. However, it only protects areas just outside of the repair area from incipient anode formation. Other high- risk undetected corrosion sites are not affected by the installation.
Corrosion is a chemical reaction and is dependent on the presence of the right environmental conditions. With reinforcement corrosion, water and oxygen are two key components that must be present in sufficient quantities to allow propagation. The use of waterproofing membranes to reduce moisture and limit further chloride contamination is common practice in the UK. In addition to the sealing benefits, they may also provide anti-slip characteristics, improve the visual appearance, and potentially increase the safety of a car park. But if chloride contamination has already taken place, other protection strategies should be considered.
For carbonated concrete, corrision risk may by minimised by controlling moisture content. Moisture content in the concrete can be measured by determining the relative humidity (RH) in the slab. The
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Structural Repair
rate of corrosion may be controlled by reducing the internal RH of the concrete. However, this approach might not be completely effective if significant chloride concentrations are already present. Corrosion of reinforcing steel affected by carbonation could also be the result of minimal concrete cover over the reinforcement. For areas with minimal cover, even low levels of chloride (0.4% by weight of cement) can increase the corrosion rate at relatively low RH values.
Just reducing the RH of concrete with a waterproofing coating may not provide sufficient protection from corrosion in areas that are more likely to contain high chloride levels; corrosion could continue beneath a coating. These high-risk areas may need additional protection so that the concrete and coating remain stable. The use of embedded galvanic anodes is one method that can address this risk. Half-cell mapping can identify those high- risk areas allowing galvanic anodes to be installed in these.
Another consideration for installing galvanic anodes is that waterproofing coatings may be worn off by traffic. These areas may also be the most prone to chloride exposure and once the coating is compromised. RH and corrosion activity increases significantly. With the incorporation of galvanic anodes, the impact from the loss of the coating is controlled.
EXTENDING SERVICE LIFE OF STRUCTURES
Alkali-activated embedded galvanic anodes have been used to provide targeted corrosion control for multi-story car parks in the UK for more than a decade. Their holistic use with systems such as waterproofing membranes and properly placed high-quality concrete repair mortars provide a multi-faceted approach to controlling corrosion when aesthetic and safety improvements to parking structures are undertaken. Controlling corrosion may extend the service life of a parking structure and investigating the causes of the corrosion could assist in determining the most effective strategies to use in solving corrosion issues.
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