Marine Works and Submerged Structures

Because corrosion of steel in concrete depends not only on the presence of chlorides but also requires oxygen, and underwater dissolved oxygen is scarce. The steel may therefore be depassivated, that is, stripped of the chemical protection alkalinity gave it, and yet corrode very slowly because the other ingredient the reaction needs is missing. Above water the situation reverses: abundant oxygen, but a surface that stays dry most of the time, so limited chloride transport. The intermediate band brings both conditions together, and moreover brings them together cyclically: water enters the pores, evaporates, leaves the salt, and the next cycle adds more. The practical consequence is that inspection should be concentrated there, and that in assessing an existing structure the comparison between submerged and emerged parts is often the most informative datum: if the decay is concentrated in the tidal band, the mechanism is the expected one; if instead it is widespread underwater too, the cause is probably something else and must be investigated.

The right question is not about hardness but about biological resistance, and in the marine environment the main enemy is not rot but boring organisms, which tunnel inside the timber leaving the surface almost intact. It is a particularly insidious decay because it cannot be seen: a pile can look sound and be hollow within. Historically naturally durable species were used and, more recently, biocide treatments now heavily restricted for environmental reasons, which has made modified timbers interesting. Acetylation in particular acts on the timber's chemical structure, drastically reducing its ability to absorb water: the material becomes unappetising and hard to attack without adding toxic substances, which matters greatly in water because any biocide would end up in the environment. Two constructional measures remain valid for any species: avoid zones where water stagnates and details that trap water and debris, and ensure submerged elements are inspectable and replaceable, because no treatment makes timber eternal under the sea.

It is worth it where the cover strategy has exhausted its margin, and nowhere else. The reasoning is this: cover buys time, because the thickness corresponds to the years chlorides will take to cross it. As long as that time is sufficient for the required service life, increasing cover is the cheaper solution. There are, however, situations where it is not enough: slender elements where thickness cannot grow, splash zones where surface chloride concentration is extremely high, works for which a very long service life is required, or structures that realistically will not be maintained. There, changing the reinforcement material shifts the threshold instead of the time, because stainless steel tolerates far higher chloride concentrations before depassivating. The cost is considerable, so sensible use is almost always selective: it goes into the critical zones, typically the tidal band and the most exposed elements, keeping ordinary steel where passive protection suffices. What must be avoided is uncontrolled mixing of the two materials, which can trigger galvanic corrosion.

Starting from two principles that change how the campaign is set up. The first is that visual inspection alone is not enough: chloride corrosion is pitting and reduces bar section long before producing visible spalling, so a structure can look in good condition and already have compromised reinforcement. Instrumental measurement is needed, particularly a survey of reinforcement potential, which identifies zones where the steel is active, and sampling to determine the chloride profile with depth, which shows how far the front has advanced and allows the remaining time to be estimated. The second principle concerns where to look: uniform coverage of the structure is not required, the effort should be concentrated in the tidal and splash band, at the joints and at cracked points, because that is where decay concentrates. Finally, the decisive question is comparison over time: a single campaign tells you how the structure is today, two campaigns apart tell you how fast it is getting there, and it is that second piece of information that lets you plan interventions instead of enduring them.