The Division provides maintenance to 116 numbers of suspended marine structures all over Hong Kong. Suspended deck structures are normally built of reinforced concrete (RC) and are subject to reinforcement corrosion in the aggressive marine environment.
The durability of RC structures is largely depended on the conditions of the embedded reinforcement. In the tidal zone and the splash zone, water and oxygen contents are optimal for reinforcement corrosion to occur. The volume of corrosion products (i.e. rusts) occupies some four times the original volume of the embedded steel and the forces due to such increase in volume lead to concrete spalling and cracking. Our experience shows that reinforcement corrosion starts, normally accompany with signs of rust stains,concrete spalling and cracking of the under-deck structural elements at about 15 years after completion of a new pier.
The conventional concrete repair method (i.e. removal of defective concrete for reinstatement by patch repair, sprayed concrete or partial recasting) is short term in nature and can only restore the structural integrity temporarily and hence the next repair of such elements would be repaired in short periods of less than seven years. It is envisaged that repeated cycles of such repairs, if carried out, would inevitably impair the structural integrity of the pier-deck beam elements.
As a long-term solution to stop the reinforcement corrosion in the pier deck, we design and install impressed current cathodic protection (ICCP) systems at some selected piers. This method would significantly minimize the frequency of repair and hence, in long term, achieve cost-effectiveness in the maintenance of piers.
What is ICCP system?
Schematic diagram showing the set-up of Impressed Current Cathodic Protection (ICCP) System
Steel in concrete is protected against corrosion by a passivating gamma ferric oxide film, which forms on the steel surface because of the highly alkaline environment produced by cement hydration. In the presence of chloride ions, oxygen and water in concrete, the passive film on steel is destroyed and rusting due to anodic and cathodic reactions occurs. The theory of cathodic protection is to apply sufficient current to the steel reinforcements so that the rate of metal dissolution into the surrounding electrolyte or simply the anodic current is either stopped or reduced to an acceptable level. Thus, making the potential of reinforcement more negative and resulting in the acceleration of cathodic reaction and the reduction of anodic reaction.
A simple ICCP system includes an anode intended to distribute the cathodic protection current to the embedded reinforcement, through the concrete. The system also incorporates cables connecting the positive and negative terminals of a rectifier carrying DC current to the anode and to the reinforcement respectively.
Projects under installation of ICCP system
ICCP systems have been installed at two large sized piers and ten medium sized piers.
The ten medium sized piers are:
Hung Hom North Ferry Pier
Hung Hom South Ferry Pier
Sai Wan Ho Marine Police Base East Pier
Sai Wan Ho Marine Police Base West Pier
Sai Kung Public Pier
Tsing Yi Public Pier
Central Pier No.8
Central Pier No.9
Central Pier No.10
The two large sized piers are:
China Ferry Terminal
Macau Ferry Terminal (Inner Pier)
