Concrete Cover And Why It Decides Durability

Every reinforced concrete element has a specified cover: the distance from the outside face of the concrete to the nearest reinforcement.

It is a small number, usually between 20 and 65 millimetres depending on the exposure, and it decides how long the structure lasts.

What cover actually does

Steel in concrete does not rust, because the concrete around it is highly alkaline and that alkalinity keeps a protective layer on the steel.

Over time two processes eat into that protection from the outside face inward.

Carbonation. Carbon dioxide from the air reacts with the concrete and lowers its alkalinity. The carbonated zone advances slowly from the surface.

Chloride ingress. Salt, from sea air or from groundwater, penetrates and breaks down the protective layer directly. Far more aggressive than carbonation.

Both advance from the surface. Cover is the distance they have to travel before they reach the steel.

Double the cover and the time taken to reach the steel increases by roughly four times, because the ingress rate slows with depth. That relationship is why cover, and not concrete strength alone, is the governing durability parameter.

What it is specified against

Exposure classification. A slab inside a dry building, a balcony soffit in a coastal suburb and a footing in aggressive soil have very different requirements.

Concrete strength interacts with it: a higher grade concrete is denser, ingress is slower, and the required cover reduces. Which means cover and strength are specified as a pair and neither can be traded away on its own.

Fire resistance sets its own minimum cover, because the concrete protects the steel from heat as well as from moisture. On a member with a fire rating, the fire requirement occasionally governs over the durability one.

Where it goes wrong

Almost always on site, and almost always in the same ways.

Bar chairs too short, too few, or the wrong type. Chairs hold the reinforcement off the formwork. Insufficient chairs let the mesh or bars sag between supports, and the cover disappears at the sag.

Someone walks on the top mat. Top reinforcement in a slab is pushed down by foot traffic during the pour, and the top cover becomes excessive while the steel sits too low to do its job.

Mesh laid straight on the membrane in a slab on ground, which is one of the most common residential defects and leaves effectively zero cover.

Congestion at a junction. Where beams meet a column, reinforcement is dense and cover is lost at the outside of the cage.

Formwork moving during the pour, which changes cover on the face that moved.

Cover taken from the wrong face. Cover is measured to the nearest bar, including ligatures and stirrups, not to the main bars. Measuring to the main bars overstates it by the ligature diameter.

Checking it

Before the pour, which is the only time it is cheap. A pre-pour inspection checks chair type, spacing and height, the position of the top mat, and cover at edges and penetrations. It is the substance of a proper [construction supervision and pre-pour inspection](/blog/construction-supervision-pre-pour-inspections) regime.

After the pour, a cover meter reads it without breaking anything. On an existing building this is how the durability position is established, and it is usually the first measurement taken in any [concrete cancer](/blog/concrete-cancer-in-apartment-buildings) investigation.

Low cover found on a completed element is difficult to remedy. The options are a protective coating, increased maintenance, or accepting a shorter service life, and none of them are as good as getting it right before the concrete went in.

Why it matters most on balconies and facades

Exposed soffits and edges take weather on a horizontal surface with water sitting on them, and in Sydney they take salt as well.

Which is why low cover on a balcony produces visible failure in fifteen years while the same defect in an internal slab may never show. It is the mechanism behind almost every [balcony remediation](/blog/balcony-remediation-structural-assessment) project, and it connects directly to [coastal exposure](/blog/coastal-exposure-and-concrete-durability).

ACSES provides structural engineering and construction inspection across Sydney. Talk to us about a project.