Due to the hostile environment, it is vital to protect port and marine structures against deterioration. The damaging effects of chlorides in seawater, as well as the aggressive action of waves and currents plus carbonation attack, can all drastically reduce the design life of coastal defences, wharfs and jetties, leading to expensive maintenance bills and at worst, force premature demolition.
In the case of reinforced concrete structures, the steel reinforcement is protected against corrosion by the inherent highly alkaline environment of the concrete created by the release of calcium hydroxide from the cement hydration. This results in the formation of a passivating layer of ferric oxide on the embedded steel. As long as this surface film is maintained, the steel remains protected from corrosion. However, when concrete structures are repeatedly exposed to salt spray or submerged in saltwater, chloride ions – due to their minute size – penetrate the pores of the concrete, eventually reaching the steel, breaking down this layer and causing corrosion. Corrosion most rapidly occurs in the splash zone where the intermittently wet and dry conditions exacerbate the penetration of chlorides and there is enough oxygen to facilitate the corrosion process. There is also sufficient moisture present to increase the electrical conductivity of the concrete, leading to an aggressive form of localised corrosion called pitting corrosion; this can potentially cause rapid loss of steel section and major cracking and spalling of concrete, thereby compromising structural integrity. Identifying corrosion risks
In tidal and submerged zones, the concrete is saturated and oxygen levels are limited as the pores in the structures are constantly filled with water. Nevertheless, in areas where there is low concrete cover, corrosion can still occur, causing a challenge for its reinstatement. The depth and quality of the cover concrete is absolutely vital, as the relatively thin layer of concrete protects the reinforcing steel from corrosion by maintaining an alkaline environment and preventing the ingress of chloride ions and the other fuels for corrosion. All too often, even in new construction, elements are rejected during quality control cover checks on-site and it becomes necessary for remedial measures to ensure the design life of the structure is achieved.
As soon as low concrete cover has been identified, it is important to take swift action, otherwise the lack of protection to the re-bars will lead to premature de-passivation of the steel and subsequent corrosion. Inadequate concrete cover will not only speed up the damaging effects of carbonation but also allow even more rapid ingress of chlorides, moisture and oxygen.
Various options may be open here. These could range from the drastic and costly measures of demolishing sections that fail to meet the required specifications, or partial recasting with new concrete. This involves removal of the concrete back to behind the level of reinforcement typically using ultra high pressure water blasting techniques, repositioning the formwork to achieve the desired cover and recasting the concrete. However, in port and marine environments it can often be difficult to access the area to carry out remedial work.
Protective coatings
A more practical and cost-effective means of reinstating cover on port and marine structures is to apply a protective coating. There are many different products available on the market and it is important to assess factors such as substrate compatibility, life span and the film thickness required to provide the necessary cover, not to mention successful track record of use on similar structures and independent approvals such as CE marking in accordance with BS EN 1504.
One product which is frequently specified for reinstating effective cover on precast and in-situ reinforced concrete is Cementitious Coating 851 – a waterborne, cementitious modified polymer coating. Independent tests show that a two millimetre coating of Cementitious Coating 851 is equivalent to 100 millimetres of good quality concrete cover, as well as providing a complete barrier to water under 10 bar pressure. Being cementbased, it chemically reacts with the substrate to form an integral part and will have a design life equivalent to that of the concrete to which it is applied. 851 can be applied to green concrete by brush or spray techniques, it exhibits minimal hazard during application and is non-toxic when cured.
Preventing chloride ingress
The ability to combat chloride ingress is a critical factor on marine structures and the VINCI Construction Technology Centre has assessed the chloride ion diffusion of Cementitious Coating 851 for the past 24 years, and to date, no steady state of flux of chloride ions has been detected, whereas the control concrete achieved this in just 28 days.
Remedial options for steel
Steel structures in port and marine environments are also more prone to corrosion, not least of all due to the damaging effects of the aggressive environment but also a lack of planned preventative maintenance in many cases. There are several different products on the market for remedial works, with varying degrees of effectiveness. Traditional resin coatings are one option and whilst some products will offer protection against long-term environmental degradation, high levels of surface preparation are needed, including removal of all contaminants and corrosion by-products back to bright metal free from chlorides – a scenario which is often impossible in marine environments with restrictive tidal windows.
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