Written and reviewed by the Apolodor LTD concrete repair team

Concrete performs well in fire. It does not burn, it conducts heat slowly and it often remains standing long after other materials have failed. But surviving a fire is not the same as being undamaged. Concrete exposed to high temperatures can lose strength, crack and spall, and the reinforcement inside may be affected too.

After a fire, building owners and insurers need clear answers quickly: is the structure safe, what needs repairing and how soon can it be used again? This guide explains how fire-damaged concrete is assessed and repaired. It also explains how we work with engineers and insurers at Apolodor LTD to get fire-damaged buildings back into use.

Quick answer: Fire-damaged concrete is assessed through visual inspection, colour change mapping, hammer testing, non-destructive testing and laboratory testing of cores to establish how deep the damage goes. Weakened concrete is then removed, reinforcement is checked and treated, and sections are reinstated with structural repair mortars, micro-concrete or sprayed concrete under the direction of a structural engineer.

What Happens to Concrete in a Fire?

As temperatures rise, several changes take place:

• Moisture turns to steam. Water in the concrete pores vaporises, building pressure that can cause explosive spalling, particularly in dense, high-strength or wet concrete.
• The cement paste breaks down. Chemical changes in the hydrated cement progressively reduce strength at higher temperatures.
• Aggregates expand. Differential expansion between aggregate and paste causes microcracking.
• Reinforcement is exposed. Once spalling removes the cover, steel heats quickly and loses strength while hot.

After cooling, the concrete surface may look intact even though the outer zone has been significantly weakened.

Step 1: Make the Structure Safe

Before detailed assessment, a structural engineer should confirm whether temporary propping is needed. Elements with large deflections, extensive spalling or exposed and buckled reinforcement may be unsafe. Loose concrete must be removed to protect people working below. On our projects, make-safe works are always the first task, before any assessment or repair begins.

Step 2: Visual Inspection and Colour Mapping

Concrete made with many common aggregates changes colour as it is heated, which gives a useful indication of the temperatures reached. The Concrete Society’s Technical Report 68 describes how colour can be used in assessment:

• Pink or red discolouration typically indicates exposure to around 300°C or more, the range at which significant strength loss begins
• Whitish grey suggests considerably higher temperatures, at which concrete is usually severely weakened
• Buff colours indicate very high temperatures

Colour change depends on the aggregate type and is not visible in every concrete, so it is used alongside other evidence. The inspection also records spalling depth, cracking, deflection, exposed reinforcement and damage to joints and connections.

Step 3: Testing

Testing builds a picture of how deep the damage goes:

• Hammer surveys locate hollow or weakened areas
• Rebound hammer and ultrasonic testing compare fire-affected areas with unaffected concrete
• Core samples allow laboratory strength testing, and petrographic examination can show the depth of heat-affected concrete
• Reinforcement testing may be needed where steel has been exposed to high temperatures

Our guide to assessing concrete strength and durability explains these methods in more detail.

Step 4: Classify the Damage

Engineers usually classify each element into a damage category, ranging from cosmetic to severe structural damage. This classification decides whether an element needs only cleaning and cosmetic repair, structural repair or replacement.

Step 5: Remove Damaged Concrete

Weakened concrete is removed back to sound material using hydrodemolition or careful mechanical breaking, avoiding further damage to the remaining structure. Exposed reinforcement is cleaned, and any bars that have lost section or been distorted are assessed and supplemented if needed. The principles are similar to those used when fixing concrete around exposed rebar. We use controlled methods that remove only what is weakened, because every area of sound concrete retained reduces cost and programme.

Step 6: Reinstate and Strengthen

Repair methods depend on depth and extent:

• Hand-applied structural repair mortars for localised areas
• Flowable micro-concrete poured into formwork for deeper sections or columns
• Sprayed concrete for large areas of soffit or wall
• Strengthening with additional reinforcement, jacketing or fibre-reinforced polymer systems where capacity has been reduced

For columns and beams in particular, the repair needs a carefully planned sequence. Our article on repairing concrete columns and beams covers the practical considerations.

Step 7: Clean and Protect

Soot and smoke residues can be acidic and must be removed from surfaces that are to be retained. Once repairs are complete, protective coatings restore durability and a consistent appearance.

How Apolodor LTD Approaches Fire Damage

After a fire, clients need two things from us: a clear picture of what has to be done, and a team that can start as soon as the building is released. We work alongside the structural engineer from the assessment stage, so the repair specification reflects the actual depth of damage rather than a worst-case guess.

Repairs are carried out using structural systems from Sika, Ronacrete and Fosroc, sequenced to keep the building safe throughout, and recorded so that insurers, loss adjusters and building owners have a clear account of the work completed.

Areas we provide services in

Apolodor LTD provides fire damaged concrete repair services all across UK, including:

• London
• Manchester
• Birmingham
• Liverpool
• Leeds
• Nottingham
• Bristol
• Brighton
• Cardiff
• Plymouth
• Luton
• Reading
• Norwich
• Swindon
• Portsmouth
• Oxford
• Ipswich
• Maidstone
• Cambridge
• Southampton
• Slough
• Warrington
• Sheffield
• Leicester
• Coventry
• Milton Keynes
• Northampton
• Derby
• Stoke-on-Trent
• Wolverhampton
• Hull
• Exeter
• Gloucester
• Sunderland
• York
• Peterborough
• Chelmsford
• Newcastle
• Bradford
• Bournemouth
• Swansea
• Newport
• Middlesbrough
• Preston
• Blackpool
• Bolton
• Stockport
• Huddersfield
• Lincoln
• Worcester
• Colchester
• Canterbury

Key Takeaways

• Concrete rarely collapses in a fire, but it can lose significant strength in its outer layers
• Assessment combines visual inspection, colour mapping and testing
• A structural engineer should lead decisions on safety and repair
• Damaged concrete must be removed back to sound material before reinstatement
• Many fire-damaged structures can be repaired rather than demolished

Frequently Asked Questions

Can fire-damaged concrete be reused?

Often, yes. Where damage is limited to the outer zone, removing weakened concrete and reinstating it can return the element to full service.

How soon after a fire can assessment begin?

Once the building has been released by the fire service and made safe, an initial assessment can usually begin within days.

Does reinforcement lose strength permanently after a fire?

It depends on the type of steel and the temperatures reached. Some steels recover much of their strength after cooling while others do not, which is why testing may be needed.

Talk to Our Team

Apolodor LTD repairs fire-damaged concrete structures, working alongside engineers and insurers to return buildings to use. Call 07808 709670 or contact us here.