Designing Marine Reinforced Concrete Structures To Achieve Best Possible Up-throw Of Current From Immersed Anodes To Bars Above The Water Level

Introduction

Although provision of adequate cover may in practise prevent corrosion of bars in immersed marine based concrete structures (at least over normal lifetimes) there is much evidence that partially immersed structures are vulnerable to corrosion
Salt-induced deterioration of reinforcement in landbased bridges has been mitigated by application of retrofit Cathodic Protection (CP) systems (with anodes surface mounted or installed in holes bored into the concrete).

According to Bertolini et al1 :
- In uncontaminated concrete the use of anodes to deliver current to reinforcement aims to prevent corrosion by maintaining the passive condition of steel and simultaneously inducing chloride ions to flow away from the steel.
- The potential required to achieve such cathodic prevention is generally in the range -200 to -400 mV (SCE).

This suggests that provided CP is used throughout life, and can deliver enough current (to the areas which might be affected by chloride contamination) to maintain the steel in the passive state, there should be no corrosion in those areas.
In the context of marine structures such as pontoons, piles etc, supply of cathodic current to the steel may be made from anodes immersed in the seawater.
The “up-throw” of such current to reinforcement bars above the seawater level determines the extent over which protection may be achieved.
But how can the extent of the up-throw be assessed?

Mathematical modelling has been successfully applied to CP of steel structures immersed in electrolyte, for:
- design optimisation of new CP systems (Minimising mutual interference between anodes, avoiding shadowing, avoiding premature expiry of anodes)
- assessment of combined performance of end-of-life CP systems and retrofit CP systems proposed for life extension of the structure
- forensic investigations intended to improve understanding of unexpected observed effects (for example anodes on a structure unintentionally delivering current to another structure, return path current causing accelerated mass loss of some anodes.

The same simulation techniques can readily be applied to reinforcement of marine concrete structures, such as bridge columns or pontoons.
In many marine applications the cathode is fixed - it is simply the surface of the structure to be protected, and in this case CP system design optimisation simply involves selection of the number, size and position of anodes.
But for a new build reinforced concrete structure the design optimisation could include changing the cathode.
In other words, design for avoidance of corrosion could include selection of the arrangement of reinforcement bars which allows the best throw of current to surfaces of those bars
This paper considers for study the wall of a concrete pontoon, and investigates the effect of different arrangements of reinforcement on up-throw of current from anodes immersed in the seawater.

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