Why concrete breaks up
It is nearly always the same story: water reaches the reinforcement, the steel corrodes, and the rust occupies several times the volume of the original steel. That pressure blows the cover off from the inside. This is why a repair that merely fills the hole is useless: if the steel keeps corroding, it blows off again.
On the coast the process runs faster, because chlorides in the marine air attack the reinforcement even when the concrete looks sound. In Menorca this shows up very clearly on balconies and railings facing the sea.
Before you start: when to call in a professional
This guide covers local repairs to the concrete cover. There are cases where you should stop and call a surveyor or architect before touching anything:
- If the reinforcement has visibly lost section, that is, if the steel is clearly thinner or has broken through.
- If there are cracks running through a beam or a column.
- If the damage is to a structural element and covers a wide area.
- If there is visible deflection or deformation.
A balcony in poor condition is a real risk, not a question of appearance.
The method, step by step
**1. Cut back.** Break out all damaged or friable concrete and anything that sounds hollow, until you reach sound concrete. You also have to cut back behind the reinforcement, leaving it free all the way round: leave rust hidden behind it and the problem carries on. Cut the edges of the opening square, never feathered.
**2. Clean the steel.** Wire brush or grit blast until the steel is free of rust and bright. This step decides whether the repair lasts twenty years or two.
**3. Passivate the reinforcement.** Apply a passivating primer, usually two coats, which protects the steel and acts as a bonding bridge. Leave it to dry between coats for as long as the tin says.
**4. Apply the repair mortar.** Use a thixotropic structural repair mortar, which is the one that stays put vertically and overhead without slumping. Dampen the substrate first and build up in layers, respecting the maximum thickness per layer the manufacturer states. If you have a lot of depth to make up, several layers beat one thick one.
**5. Finish.** Once cured, smooth it with a finishing mortar if needed and protect it with an anti-carbonation paint or coating, which slows down the ingress of CO₂ and water. Without that final protection, the cycle starts again.
Cracks: they are not all treated the same
Before filling a crack you need to know whether it is live or dormant. A simple way to check is to put a plaster tell-tale across it and look at it weeks later: if it has split, the crack is moving.
- Dormant crack: open it out slightly, clean it and fill with repair mortar.
- Moving crack: do not fill with a rigid mortar, because it will open again. It needs a flexible sealant.
What you need
Hammer and chisel or a grinder, wire brush, passivating primer, thixotropic repair mortar, finishing mortar and anti-carbonation paint. Gloves and goggles, because it throws splinters.
Tell us the rough square metres and the depth you have to make up, and we will tell you how much mortar you need and which one suits the element you are repairing. A balcony edge and a beam are not the same job.