Monitoring chloride levels, through core sample testing, allows a proactive approach.
All asset managers should get to know the chloride and carbonation profile of their concrete better, particularly if that concrete is aging and located in coastal environments. “Without a proactive approach, the first sign of a problem with a structure is typically when a piece of concrete falls off due to corrosion,” said Edelman. “At that point it may be too late for a coating to protect the remainder of the structure, and you may be up for some very large repair bills.”
A number of QSL’s assets have experienced significant corrosion and spalling of concrete over the years due to chloride ingress. Traditional patch repair, in many cases with replacement of corroded reinforcement, has been used, but with inconsistent results. “We have some patch repairs that are pushing 30 years old and remain in great condition,” Edelman said, “but others are beginning to crack and fail after less than 10 years.” However, one of the limitations of patch repairs is that it is often necessary to remove large quantities of sound concrete to solve the problem, causing extensive disruption and costing approximately $3000 per square metre.
One of the alternative methods of protection used on concrete, especially in marine environments, is Cathodic Protection. One type, Impressed Current Cathodic Protection (ICCP), is a technique whereby a small, permanent current is passed through the concrete to the reinforcement in order to virtually stop the corrosion of the steel.
Cathodic protection is relatively simple in theory. Anodes are inserted into the concrete at set spacing attached to the positive terminal of a DC power supply and connect the negative terminal to the reinforcing steel. Large amounts of cabling and permanent power supplies are required, making the technology really only suitable for commercial infrastructure. The initial CP current totally passivates the steel reinforcement, migrating chloride away from the bars and restoring an alkaline (high pH) environment in the concrete around the steel reinforcement.
Well designed and installed CP systems can achieve a 30 year or longer operational life.
One of the QSL conveyor tunnels has already had an ICCP system installed and the company is preparing to add the technology to a particular section of the Lucinda Jetty that is subject to near-constant wetting from waves.
“In this section, chlorides have reached a level where corrosion has begun and some spalling has occurred. Cathodic protection is a more cost-effective option compared to allowing the corrosion to continue and having to carry out constant repairs,”
Edelman stated. “There will be long term cost savings, which helps a lot – with the total annual spend for concrete repair and protection of around $1 million across the six terminals.”