Coastal exposure and concrete durability, and why cover is the whole answer

Coastal exposure and concrete durability, and why cover is the whole answer

Sydney has a long coastline and a lot of concrete near it. The buildings that are still sound at fifty years and the ones that are being remediated at thirty usually differ on one thing, and it is not the strength of the concrete.

It is the thickness of concrete between the surface and the reinforcement.

How the classification works

AS 3600 assigns every concrete surface an exposure classification based on what it is exposed to. The mild internal classifications apply to protected interior surfaces. The outer classifications apply to surfaces in permanent contact with sea water or in the tidal and splash zone.

Between those, the distance from the coast does the work. A surface within a defined distance of a surf coast sits in a more severe classification than the same surface further inland, because airborne salt carries and deposits on the concrete. Surf coast and non-surf coast are treated differently, so a harbourside building and an oceanfront building are not automatically the same.

The classification then sets two things: the minimum characteristic strength of the concrete, and the minimum cover to the reinforcement.

Why cover matters more than strength

Steel inside concrete is protected by the alkalinity of the surrounding material. Chlorides destroy that protection when they reach the bar.

Chloride ingress is a diffusion process. Everything else being equal, the time it takes salt to reach the steel goes up sharply with depth, not proportionally. Adding a small amount of cover buys a large amount of time.

Higher strength concrete helps because it is denser and slows diffusion. But a high strength mix with inadequate cover fails earlier than a lower strength mix with correct cover. Cover is the variable that decides service life.

Where it goes wrong on site

Cover is a construction outcome, not a design one. The drawings can specify it correctly and the building can still be built without it.

Bar chairs at the wrong height, or at the wrong spacing so the mesh sags between them. Reinforcement stood on during the pour. Congested areas near a slab edge where the bar ends up hard against the formwork. Balcony edges and soffits, which are the most exposed surfaces on a coastal building and the most likely to be built tight.

That is why cover checks before the pour are worth the visit. It is the only point at which the problem is free to fix.

The details that carry the risk

Balcony soffits and slab edges, because they face the weather directly and are the first surfaces to show distress.

Handrail and balustrade fixings that penetrate the slab, because they create a direct path for water and salt to reach the reinforcement.

Planter boxes and set-down areas that hold water against concrete.

Junctions between the membrane and the structure, where the waterproofing terminates and the detail decides whether water runs off or sits.

What to specify beyond the minimum

The minimum is a minimum, and on an exposed coastal building it is worth exceeding it on the surfaces most at risk rather than uniformly.

Curing matters, because the surface layer is the layer doing the protecting and a poorly cured surface is porous exactly where density counts.

Supplementary cementitious materials in the mix reduce permeability and are standard practice for marine exposure.

Galvanised or stainless reinforcement is available where a detail cannot achieve the cover, and it costs less than remediating the element in twenty five years.

The link to remedial work

Everything above is why so much of our structural engineering work on older coastal buildings is remedial. Those buildings were built to the standards of their day, often with less cover than current practice requires, and the salt has had four decades to arrive.

Designing a new coastal building well is cheap. Repairing an old one is not, and the difference between the two is a decision made before the concrete was ordered.