Corrosion of reinforced concrete structures

Reinforced concrete is the most widely used construction material in the world today. If concrete has the ability to prevent corrosion of reinforcement, it's important to know under what conditions.

Reinforcement corrosion occurs as a result of carbonation of the concrete and/or penetration of chloride ions. The corrosion process is an electrochemical phenomenon that takes place within the concrete. Deterioration takes place in two stages:

• in the first, so-called "priming" phase, the steel is initially protected by the high alkalinity of the interstitial solution inside the concrete: a thin passive layer of protective oxides is formed. Carbonation of the concrete, by reducing the pH and/or a sufficient quantity of chloride ions, can destroy this passivity and initiate the second stage;

• in the second phase, corrosion propagates. Oxygen supply, and above all ambient humidity, control the rate of corrosion.

Corrosion propagation gradually leads to crack formation and detachment of the concrete cover. To improve the durability of reinforced concrete structures, the priming period should be extended as far as possible, using compact, low-permeability concretes (in the presence of chlorides, the addition of slag, fly ash or silica fume can be beneficial). The thickness of the coating must also be sufficient. Compliance with current regulations (European standards), or with performance-based approaches, enable us to target a given service life. Numerical models can be used to predict the duration of onset periods, as well as corrosion development rates, taking uncertainties into account.

Structural diagnostics methods enable us to quantify the state of deterioration caused by corrosion, using a combination of different techniques. Depending on the state of damage, preventive, rehabilitation or even replacement measures will be implemented to guarantee the safety of the structure or, where applicable, its requalification as part of a maintenance program.