Multi-storey car parks

Until 1977, calcium chloride was routinely added to in-situ and precast concrete as an accelerator, either to enable rapid removal of moulds or to permit concreting in cold weather. Thus, older structures could be at risk of corrosion damage arising from cast-in chlorides coupled with increasing carbonation. For prestressed components the risk is greater as the threshold for acceptable levels is much reduced (0.1% chloride by weight of cement compared with 0.4% for reinforced concrete).

However, car park structures are also at risk from chloride salts brought in on vehicles during cold weather. Analysis of half-cell potential surveys on car park decks reveals high risk areas on ramps and traffic routes as well as the wheel positions in car parking bays. Ramps leading up (or down) to the first two levels of the car park are at risk of salt or other contaminants being brought in.

Cars are not the only problem: salt spreading on pedestrian areas can cause contamination. On the top level of a car park, the effects of rainfall wash the structure and dilute some of the salt. At lower levels, where there is no washing effect and water dries by evaporation, chlorides can become concentrated, leading to a greater risk of corrosion damage.

Chloride concentrations at the base of this ramp (in this case the second ramp from the entrance) have resulted in localised corrosion and spalling

Chloride attack is an insidious problem and can be difficult to treat unless electrochemical methods such as cathodic protection are used. Pitting corrosion is a particularly dangerous condition, particularly in prestressed or post-tensioned structures, since the reinforcement can effectively be eaten away without the formation of expansive rust. Pitting corrosion usually occurs in localised pockets of high chlorides and saturated conditions.

Half cell apparatus

Mild steel or galvanised steel fixing bolts for balustrades or vehicle impact barriers can be vulnerable to corrosion due to chlorides. Of particular concern at perimeters is the fixing of precast concrete balustrade panels. These may themselves not be intended to provide impact protection, but water that accumulates at the perimeter could find its way into what may have been poorly grouted construction joints leading to corrosion of locating dowels or gravity fixings.

Problems of vehicles crashing through perimeter balustrades are rare but not unknown. In an incident in Canterbury in 1996 a car fell some 20m from the fourth floor of the car park, seriously injuring the driver. In this case the barrier was of insufficient strength to prevent the car from plunging over the side.

Rapid corrosion of mild steel balustrade as a result of chloride contaminated water

The risks to safety of defective edge protection were addressed by SCOSS in their 10th and 11th annual reports, which highlighted in particular that many of the country's 4,000 car parks were built outside the requirements for protection that featured in the Building Regulations current at that time.

SCOSS recommended that MSCP owners should:

• commission inspections by suitably experienced engineers before commissioning repairs;
• inspect beyond the areas that have visibly deteriorated;
• identify the risk of progressive collapse;
• establish the suitability of edge barriers to resist vehicle loads and modify the barriers if necessary; and
• ensure that the height and design of barriers is appropriate to safeguard small children.

Because of the damaging effects of chloride on structures (and also the pollution hazards associated with its use), several alternative de-icing agents have been developed. Some proprietary systems include corrosion inhibitors, and others, such as urea, calcium magnesium acetate-CMA or potassium, can all be harmful to the environment and are all generally more expensive than conventional rock salt complying with BS 3247 (some 2 million tonnes of which is used annually on highway structures within the UK). (Pollution Prevention Guidelines Highway Depots PPG10, Environment Agency and SEPA) Rock salt and its substitutes effectively lower the freezing point of ice to around 210°C, although some systems are more effective than others at different temperatures.

While alternatives such as urea may not be as damaging to structures, they are salt based and when present in concentration can lead to the formation of salt crystals within the pore spaces in a concrete structure. Crystal growth eventually disrupts the concrete, leading to spalling. Salt solutions can then percolate further into the concrete, perpetuating the process and permitting side effects such as increased rates of carbonation.

Good quality, well-compacted concrete is less susceptible to salt crystalisation but the identification of affected areas serves as a good locator for core tests or chemical analysis.