Concrete Spalling: What It Is, What Causes It and What to Do About It

Concrete spalling is the breaking, flaking or falling away of pieces of concrete from a structure. It is one of the most visible and common signs of concrete deterioration on UK buildings, bridges, car parks and infrastructure, and in most cases it indicates a more significant underlying problem than the visible damage alone suggests.

This article explains what concrete spalling is, what causes it, when it becomes a safety concern, and how it is properly repaired.

What is concrete spalling?

Spalling is the mechanical failure of concrete at or near the surface, producing loose fragments, pitted areas, or larger sections of concrete that have broken away from the underlying structure. It can range from small surface defects the size of a coin through to large areas where structural cover has been lost across metres of exposed reinforcement.

The word “spalling” describes the outcome, not the cause. There are several different mechanisms that produce spalling, and understanding which one is at work matters enormously for how the damage is repaired. A repair specified without diagnosing the underlying cause typically fails within a few years, sometimes making the situation worse than the original damage.

What causes concrete to spall?

The most common cause of concrete spalling on UK structures is corrosion of embedded reinforcement. As steel reinforcement corrodes, it expands to occupy up to seven times its original volume. That expansion generates pressure inside the concrete that eventually cracks and breaks the surrounding material. Corrosion-driven spalling is typically preceded by rust staining on the concrete surface, and once it starts, it accelerates unless the underlying cause is addressed.

Freeze-thaw damage is another common cause, particularly on external structures exposed to the UK climate. Water penetrates small pores and cracks in the concrete surface, freezes, expands as it becomes ice, and forces the surrounding concrete apart. Repeated freeze-thaw cycles over years progressively damage the concrete surface, producing characteristic pitting and flaking. Freeze-thaw damage is worse on lower-quality concrete and on structures with inadequate protective coatings.

Fire damage causes rapid spalling through several mechanisms operating together. Extreme heat causes water inside the concrete to flash to steam, generating internal pressure that can literally explode the concrete surface. Fire also weakens the bond between reinforcement and concrete, and can cause reinforcement itself to lose strength. Post-fire structural assessment is essential for any concrete element that has been directly exposed to significant heat.

Impact damage from vehicles, plant or objects striking the concrete produces localised spalling that is usually obvious from the pattern of the damage. While the immediate visual damage is straightforward, impact events can also cause hidden cracking and delamination that only surface later as spalling in areas that appeared undamaged at first inspection.

Chemical attack from de-icing salts, industrial chemicals, or aggressive water can degrade the concrete matrix over time, producing surface breakdown that leads to spalling. This mechanism is common on multi-storey car park decks, industrial floors, marine structures and any concrete in contact with aggressive environments.

How do you know if concrete spalling is dangerous?

Not all concrete spalling is a safety risk, but some of it certainly is. Small surface spalling in a non-structural area on the ground floor of a building is a cosmetic issue that can be planned into ordinary maintenance. Large-area spalling on a high-rise façade, on a balcony edge overhanging a public route, or on a bridge soffit above traffic is a genuine safety concern requiring urgent attention.

The factors that determine whether spalling is dangerous include the location relative to people, vehicles or property below; the size and mass of the loose material; whether reinforcement is exposed (indicating deep deterioration); and whether the surrounding area is showing hidden delamination that could produce more falls without warning. Where any of these factors apply, emergency make-safe works are usually needed before the longer-term repair is planned.

How is spalled concrete repaired?

Proper concrete spalling repair follows a defined process. First, the underlying cause must be identified. This usually requires a concrete condition survey combining visual inspection with targeted testing, which establishes whether the spalling is driven by reinforcement corrosion, freeze-thaw, chemical attack, or another mechanism. The cause determines the repair specification.

Second, all loose, delaminated and damaged concrete must be removed. This often extends significantly beyond the visibly spalled area, because hidden delamination surrounding the visible damage is common. Chain drag or hammer sounding surveys identify areas that look sound but are actually debonded from the substrate.

Third, exposed reinforcement is treated according to its condition. Lightly corroded steel can be cleaned and treated with a corrosion inhibitor. Severely corroded steel needs to be cut out and replaced. Additional reinforcement may be installed where the original design allowance has been compromised.

Fourth, the concrete is reinstated using an approved repair mortar matched to the specific application. Different repair mortars are formulated for different purposes: structural repair mortars for load-bearing situations, fairing coats for cosmetic finishing, sprayed concrete for large-area reinstatement, and specialist chemical-resistant mortars for aggressive environments.

Fifth, where the underlying corrosion mechanism is still active in the surrounding structure, protection systems are installed to prevent recurrence. This may include anti-carbonation coatings, cathodic protection, corrosion inhibitors or a combination depending on the situation.

Why does patch repair sometimes fail?

Patch repair applied without addressing the underlying cause of the original spalling often fails within a few years. On corrosion-driven spalling in particular, there is a specific phenomenon called the incipient anode effect, where a localised repair can cause corrosion to accelerate in the surrounding untreated concrete. The patched area becomes cathodic and the surrounding original concrete becomes anodic, driving faster deterioration around the edge of the repair.

This is why a proper condition survey and specification matter so much. A patch repair that ignores the wider corrosion situation may look good on the day it is finished, but produce more spalling within a year or two in the areas around the patch. Repair specified from proper diagnosis addresses both the visible damage and the underlying cause together, which is what delivers durable results.

When to seek professional help

Concrete spalling should be assessed by a specialist before any repair works are specified or delivered. The right first step is an independent concrete condition survey that identifies both the visible damage and the underlying cause, quantifies any hidden delamination, and produces a specification that addresses the full scope of what needs to be repaired.

Where spalling is on an overhead element, a façade, a balcony edge, or anywhere the falling material could cause harm to people or property below, immediate make-safe works may be needed before the longer-term repair is planned. Emergency response can typically remove the immediate risk within days, buying time for proper diagnosis and specification of the permanent repair.

Closing

Concrete spalling is almost always repairable when identified early and diagnosed properly. The critical decisions are made at the survey and specification stage, not the site stage. A structure that gets the right diagnosis, the right specification and a repair delivered by a specialist contractor can be given decades of additional service life.

If you would like an initial conversation about spalling on your building or asset, get in touch with the CRS team.