Reinforced concrete is a widely used material within construction with an attractive range of properties and characteristics. It can be used in a variety of exposure conditions, however attention needs to be given to the potential for steel reinforcement within the concrete to corrode, that can result in staining, cracking and spalling of surrounding concrete. The porous nature of concrete allows elements such as water, chlorides and oxygen to travel into the concrete and eventually reach the reinforcement.
Reinforced concrete used within multi-storey car parks has to contend with conditions that may not be obviously apparent. Humidity is increased on a regular basis by vehicles carrying rain and snow into the car park. When this becomes mixed with pollutants deposited on cars, such as oil residues, and particularly in winter with de-icing salts, an elevated corrosive environment is created.
Under normal conditions, reinforcing steel is protected from corrosion by the alkalinity of the concrete. This inherent passivation exists at a pH value of between 10 and 13.8. However, with the introduction of moisture and carbon dioxide, alkalinity is reduced over time and depassivation occurs. This process is called carbonation. As a result, corrosion of the reinforcement entails, along with serious damage to the component. Long-term prevention of carbonation can be provided by hot dip galvanizing the reinforcing steel, as galvanizing provides corrosion protection across a range of pH values (fig. 2). The use of hot dip galvanized reinforcement for preventing carbonation-induced corrosion is advisable for exposure classes XC1 to XC4 (Table 1).
Hot dip galvanized reinforcing steel also offers protection in environments where chloride exposure is expected as galvanizing binds the low-solubility alkaline zinc chlorides, rendering them harmless. The application of galvanized reinforcing steel is recommended for structures near salt-water, concrete bridges, car parks and for thin pre-cast concrete sections. Car parks belong to exposure class XD3 (Table 2), in accordance with Eurocode 2 (EN 1992).
Corrosion is not only a major problem for car park operators in terms of structural issues. In addition to the unnecessary cost, renovation is accompanied by interruptions to business and loss of revenue. Also, crumbling concrete surfaces cause aesthetic concerns. Contaminated water dripping on parked cars may also cause costly damage to paintwork. This is particularly pertinent for car parks where vehicles are left for longer periods of time, for example, in airport car parks.