Reinforced concrete repair

Why concrete degrades

The reinforcing steel is protected by the strongly alkaline environment of the concrete, which forms a passivating layer on its surface. Two phenomena destroy this protection: carbonation, i.e. the reaction of atmospheric carbon dioxide with the lime in the cement, which lowers the pH; and chloride penetration, typical of marine environments and structures treated with de-icing salts.

Once the passivating layer is lost, the steel oxidises. Rust occupies a volume many times greater than the steel it comes from, and that expansion pops the cover concrete off from the inside. By the time the crack is visible, the process is already well advanced.

The standard's pathway

UNI EN 1504 organises the intervention into principles and methods: once the degradation mechanism is identified, the principle of intervention is chosen, and from that the method and the product class. This is not bureaucracy: it is what makes the repair verifiable and what a site management team can demand.

Repair mortars are classified by strength into R1, R2, R3 and R4, where R4 indicates the structural class. Subsequent surface protection falls under part 2 of the standard and is what prevents the degradation from starting again.

The correct sequence

All degraded concrete is removed down to sound material, exposing the reinforcement around its full perimeter. The bars are wire-brushed to bright metal and passivated. The section is rebuilt with a structural mortar of the required class. Finally, an anti-carbonation protective coating is applied: without it, the cycle starts again, more slowly but just as inexorably.

If the substrate is damp — an ordinary condition on below-grade or exposed structures — a moisture-tolerant epoxy primer is needed before any coating.