Appropriate secondary containment has long been a legal requirement in many countries, particularly around tanks, storage vessels and other plant equipment containing hazardous liquids. Regulations (such as the Control of Pollution Regulations 2001 in England) are enacted to establish preventative measures. By not complying with these regulations, companies run the risk of being heavily fined, sometimes to the extent of incurring criminal proceedings. As an example, a malt producer was fined £ 20 000 for an oil fuel leak polluting the River Larkin in Suffolk, United Kingdom. The company spent a further £ 100 000 on the cleanup and maintenance costs.
Concrete bunds are commonly used as secondary containment systems to protect the environment from spills of corrosive and toxic chemicals. Concrete is cost effective and provides good structural strength, however, due to its porosity, can be easily permeated and has poor chemical resistance, making it susceptible to deterioration through chemical attack. In addition, concrete is highly prone to cracking due to substrate movement and freeze/thaw cycles. If the deterioration is not addressed early, the structural integrity of the concrete will suffer and can result in contamination of the surrounding areas and ground water.
Examples of deteriorated secondary containment
Secondary containment protection
Secondary containment is commonly protected with barrier coatings, which should be impervious to the liquid and resistant to chemical attack. In addition, the protective layer would benefit from a certain degree of flexibility. This flexibility combats the problem inherent in concrete cracking.
Rigid vs flexible coatings
Why does concrete crack?
- Plastic shrinkage is produced when fresh concrete in its plastic state is subjected to rapid moisture loss.
- Excessive loading where heavy loads cause the ground underneath the concrete to move and because the flexural strength of concrete is lower than its compressive strength, the concrete bends to its breaking point.
- Thermal expansion/contraction, significant tensile stresses in concrete are created by temperature fluctuations.
- Movement and settlement, during freeze/thaw cycles, frozen ground can lift and then settle when the ground thaws.
- Corrosion of reinforcement, corrosion from steel rebar can induce stresses, greater than the tensile strength of the concrete.
- Secondary containment protection solutions typically possess only one or two of the three properties required for continuous performance: resistance to penetration, resistance to chemical attack and resistance to movement.
Conventional chemical resistant coatings have high crosslink density. They are rigid and inflexible due to difficult to break bonds, yet offer a good chemical resistance barrier due to their relative impermeability. Flexible coatings on the other hand, with the exception of rubber, have low crosslink density and offer good flexibility, but they are permeable, thus acting as a poor chemical resistant barrier.
Recent advances in polymer raw materials allow coating manufacturers to utilise a new resin with flexible segments incorporated in the polymer chain. Such systems are able to offer crack-bridging features while not compromising on the chemical resistance, effectively protecting concrete even if cracks appear. Continuous innovation and adaptation of the new coating concepts designed for secondary containment protection can effectively minimize risks of contamination, a favourable outcome for both asset operators and the policy makers.
Edited by Claira Lloyd
Read the article online at: https://www.hydrocarbonengineering.com/petrochemicals/13112014/cracking-issues-secondary-containment/