What spalling concrete looks like
Spalling is concrete breaking away from the surface. It can be a thin flake at the edge of a step, or a chunk the size of your hand from the underside of a balcony. Often there is rusted steel visible in the hole.
It rarely appears without warning. The early signs are:
- Rust staining running down from a crack or a corner
- Fine cracks along a straight line, which usually follows a reinforcing bar just under the surface
- Bulging or a hollow sound when the concrete is tapped, where the cover has already lifted off the steel but not yet fallen
- Exposed bars at the edges of slabs, nibs and lintels
The hollow areas matter most. In a survey it is common to find that the damaged area behind the surface is several times larger than the part that has already fallen.
Why the steel inside rusts
Reinforced concrete is designed to protect its steel. Fresh concrete is strongly alkaline, and in that alkaline environment a thin protective film forms on the steel and stops it rusting. Two things break that protection down.
Carbonation
Carbon dioxide in the air slowly reacts with the concrete from the surface inwards and lowers its alkalinity. This is carbonation. It moves faster in porous or poorly compacted concrete and where the concrete is thin. When the carbonated layer reaches the steel, the protective film is lost, and with water and oxygen present the steel starts to rust.
Many buildings from the 1950s to the 1970s have less concrete cover over the steel than would be specified today. On those buildings carbonation has often reached the steel already, which is why balconies and walkways of that age are where spalling is most common.
Chlorides from salt
Chlorides attack the protective film even where the concrete is still alkaline. They come from de-icing salt spread on steps, ramps, walkways and car park decks, and from sea spray on coastal buildings. Dundee, Aberdeen, the Fife coast and the Firth of Forth all see plenty of both. Chloride corrosion tends to be concentrated in pits, so it can take a lot of steel section away before much shows on the surface.
What happens next
Rust takes up several times the volume of the steel it forms from. As the bar corrodes, it pushes against the concrete around it until the cover cracks and lifts off. That opens a path for more water, oxygen and salt to reach the steel, so the corrosion speeds up.
Frost damage and other causes
Not all damaged concrete is spalling from corrosion. Water expands by about nine per cent when it freezes. In concrete that is porous or saturated, repeated freezing and thawing breaks up the surface layer by layer. Scottish winters bring many freeze-thaw cycles, so steps, sills and copings that hold water wear quickly.
Other causes include:
- Poor original construction: thin cover, honeycombed concrete that was not compacted properly, or a weak mix
- Impact: vehicles, trolleys and forklifts hitting kerbs, columns, ramps and slab edges
- Previous repairs that failed: cement patches over rusted steel, or repairs in a mortar much harder or weaker than the concrete around them
The stair above shows frost and wear rather than corrosion. The nosings had rounded off and the treads had worn hollow, but there was no steel showing. The repair was different too: timber formwork fixed to each step and the profiles rebuilt, rather than breaking out round reinforcement. Telling the two apart is one of the first jobs of a survey.
Where spalling shows up on Scottish buildings
- Balconies and walkways on 1960s and 1970s flats and deck-access housing, especially the slab edges and the underside
- External stairs and steps, at the nosings and where salt is spread in winter
- Concrete lintels and sills over windows and doors, common on tenements and post-war houses
- Canopies and porches over entrances
- Car park decks and ramps, from salt carried in on vehicles
- Columns, beams and slab edges on industrial and commercial buildings, often with impact damage as well
- Retaining walls and copings, where water sits and freezes
Why it gets worse if it is left
Spalling does not stop on its own. Each winter, more water and salt reach the steel through the cracks, and frost widens them. The steel loses section as it rusts, which on a balcony, lintel or beam means it is losing strength. The repair gets bigger and more expensive with every year it waits.
There is also the risk to people. Pieces falling from a balcony edge or canopy onto a path below are the most urgent reason to act. If concrete has fallen or looks loose over somewhere people walk, fence the area off and have it checked.
How a lasting repair is carried out
Concrete repair in the UK follows the principles in BS EN 1504, the standard for the protection and repair of concrete structures. In practice, a repair that lasts goes through these stages.
Survey
Tap-test the concrete to find hollow areas, locate the bars and measure the cover with a cover meter, and test the depth of carbonation and, where salt is a risk, the chloride content.
Specification
Write down the repair method, materials and the extent of each repair, so the work can be priced and checked against the same document.
Break out
Cut back all loose and contaminated concrete, past the edge of the visible damage and behind the corroded bars, so the new mortar can wrap right round them.
Treat the steel
Clean the bars back to bright steel, replace or add steel where too much section has been lost, and prime them with a corrosion-inhibiting coating.
Rebuild
Reinstate the concrete profile with a repair mortar matched to the job, built up in layers by hand or placed into formwork for larger sections.
Protect
Finish with an anti-carbonation coating or a sealer, so carbon dioxide, water and salt are slowed from reaching the steel again.
On the Aldi access ramp in Carluke, for example, the worn concrete was repaired with Sika Monotop 630 Rapid before a three-coat anti-slip MMA resin was laid over it. The repair mortar is chosen for the job: its strength, how fast it needs to cure, and how thick it has to be laid.
Why quick patch repairs often fail
The most common failed repair is a patch of cement or filler over rusted steel. The rust carries on underneath, the patch cracks and drops off, and the hole is bigger than before.
There is a less obvious failure too. When one area of corroded steel is repaired properly but the concrete around it is still carbonated or full of chlorides, the steel just outside the patch can start to corrode faster. This is sometimes called the halo or incipient anode effect. It is why a survey looks at the condition of the concrete around the visible damage, and why the repair and protection need to cover the whole affected area, not only the holes.
When to get a survey
Get spalling looked at if you see rusted steel, rust stains, cracks running in straight lines, or concrete that sounds hollow when tapped. Get it looked at quickly if pieces have fallen anywhere near a path, doorway or car park.
For factors and property managers, a survey with photos, the likely cause and the recommended repair gives owners what they need to agree the work. On commercial and industrial sites it lets the repairs be phased around the business.
We survey, specify and repair spalling concrete from our base in Dundee and across Scotland. See our concrete and structural repair service for the defects we repair, or send us photos of what you are seeing.
