The most common image of deterioration is a balcony with the concrete cover fallen off and rusted reinforcement in plain sight. But what you see is the end of the process. Corroding steel increases in volume, by two to six times depending on the type of oxide, and the resulting pressure splits the concrete from within. By the time rust appears on the surface the corrosion is already advanced, and a repair carried out without understanding the cause is repeated after a few years in the same places.
The main causes
Almost all deterioration of reinforced concrete falls into a few families. The first two, carbonation and chlorides, are covered in the article on pathologies and diagnostics. What matters here is the overall picture, because the repair strategy depends on the cause.
| Cause | Mechanism | Visible sign |
|---|---|---|
| Carbonation | carbon dioxide lowers the pH below 10 and the steel loses its protection | rust stains and spalling along the bars |
| Chlorides | local breakdown of the film that protects the steel (pitting corrosion) | rust at isolated points, high loss of section |
| Freeze-thaw | water in the pores freezes and increases in volume by 9 per cent | scaling and powdering of the surface |
| Sulfates | formation of expansive ettringite | cracks, swelling, loss of cohesion |
| Alkali-silica reaction | expansive gel forming in the aggregate | map cracking, whitish exudations |
| Fire | above 300 °C loss of strength and spalling | pinkish colour, spalling of the cover |
| Execution errors | thin cover, high w/c, missing curing, honeycombing | localised defects from the moment of placing |
In many cases the real cause is an execution defect that has shortened the timescales of all the others. A 15 mm cover instead of 35 lets carbonation reach the bars in less than a fifth of the time, because the front advances with the square root of time: time scales with the square of the thickness.
The principles of EN 1504
The EN 1504 series collects the standards on products and systems for the protection and repair of concrete. Part 9 defines eleven principles of intervention. The first six concern defects of the concrete: protection against ingress of aggressive agents, moisture control, concrete restoration, structural strengthening, physical resistance and chemical resistance. The other five concern reinforcement corrosion: preserving or restoring passivity, increasing resistivity, cathodic control, cathodic protection and control of anodic areas.
Before choosing materials the principle is decided, and before the principle the objective is set: how many years the intervention must last, what maintenance is accepted, whether the member must recover load-bearing capacity. A cosmetic repair and a structural one use different products and controls.
The sequence of a local repair
Mortar repair is the most widespread intervention and consists of phases that cannot be skipped. The deteriorated area is delimited and the unsound concrete is removed with a light hammer or high-pressure hydrodemolition, until the corroded bars are freed all the way round, with a gap of about two centimetres behind the bar so that the mortar can surround it. The bars are cleaned by abrasive blasting to grade Sa 2½ (ISO 8501-1) and protected with an anticorrosive slurry or an epoxy product conforming to EN 1504-7. If specified, a bonding bridge is applied (EN 1504-4). The repair mortar is then applied, in thicknesses compatible with its formulation, and cured.
The mortars of EN 1504-3 are classified by compressive strength: R1 (10 N/mm2), R2 (15), R3 (25) and R4 (45). For structural repairs R3 or R4 are used and a bond to the substrate, measured with a pull-off test (EN 1542), of at least 1.5 N/mm2 for R3 and 2 N/mm2 for R4 is required. The mortar must also have a modulus of elasticity and a coefficient of expansion compatible with the existing concrete, otherwise it cracks along the edge. The last phase is surface protection (EN 1504-2): anti-carbonation coatings, water repellents or impregnations, which reduce the ingress of carbon dioxide, chlorides and water.
When local repair is not enough
If chlorides remain in the concrete around the repair, corrosion restarts at the edges, because the repaired area becomes cathodic with respect to the old concrete. On important structures electrochemical systems are then used. Cathodic protection, governed by EN ISO 12696, lowers the potential of the steel with sacrificial anodes (zinc) or impressed current, and is verified with a 100 mV depolarisation criterion. Electrochemical realkalisation (EN 14038-1) restores the alkalinity of carbonated concrete without demolishing it. When load-bearing capacity has to be recovered, strengthening with carbon-fibre laminates or fabrics is used, or enlargement of the section.
Checks and maintenance
After the intervention, bond, thicknesses and the condition of the bars are checked on a sample basis, and a periodic inspection is planned. For a structure exposed to chlorides, mapping the corrosion potential (ASTM C876) repeated every few years shows whether the system is working. The cost of these checks is small compared with that of redoing the work.