Once the fire is out, the owner's first question is whether the structure is still usable. Black soot and spalled concrete seen by eye do not reveal the true residual strength. A building that looks badly damaged may be repairable, while one that looks tidier may have reinforcement that has already lost strength permanently. This article explains what a post-fire structural inspection (NDE) covers and the criteria used to decide.

Concrete after a fire: temperature drops quickly with depth, surface color indicates the temperature range reached, and the 300 °C line sets the depth to be broken out for repair The left image is a cross-section of a reinforced concrete beam under a slab exposed to fire from below. Heat enters from the beam soffit, the beam sides and the slab soffit as thin layers along the surface. The outermost layer reached over 900 degrees, followed by a whitish-grey layer at 600 to 900 degrees and a pink layer at 300 to 600 degrees, while the inner core is still normal grey. The red dashed line is the 300-degree line, which TR68 uses as the depth limit of concrete to be removed during repair. The bottom reinforcement lies within the cover zone and so received high temperatures. The right image is a conceptual graph of temperature reached versus depth from the fire-exposed surface; temperature drops rapidly within a few centimeters. Values in the graph are hypothetical. Below the graph it notes that hot-rolled bars regain nearly their original strength if not heated beyond about 500 to 600 degrees, while cold-worked steel and prestressing strand lose strength permanently from about 300 to 400 degrees. The bottom strip relates temperature ranges to surface color and damage level, which is only a preliminary indicator because it depends on the aggregate type and must be confirmed by testing. Fire-damaged concrete: how deep does heat go? Not to scale Beam-slab section exposed to fire from below Core stays cool < 300 °C normal grey Pink zone 300–600 °C 300 °C line Repair break-out limit (TR68) Bottom bars in the hot zone Concrete conducts heat poorly → only outer layer damaged Temperature drops fast with depth 300 600 900 0 25 50 75 100 Depth from exposed face (mm) Temp. (°C) → Remove Rebar level Hypothetical Hot-rolled bar: recovers if ≤ ~500–600 °C Strand/cold-worked: lost ≥ ~300–400 °C Confirm by core (ASTM C42) + C856 < 300 °C Normal grey · minor loss 300–600 °C Pink/red · clear loss 600–900 °C Whitish grey · friable > 900 °C Buff · crumbling
Indicative relationship between the temperature reached by the concrete, surface color and damage level — used for preliminary screening to plan test locations, not as a conclusion on residual strength.

What Does Heat Do to Concrete?

Heat reduces the compressive strength and stiffness of concrete according to the temperature reached. As temperature rises, pore water and chemically bound water in the cement paste evaporate, the microstructure of the cement paste deteriorates, and the aggregates expand while the paste shrinks, causing microcracking throughout the concrete.

  • Below about 300 °C — little loss of compressive strength; most of the structure can remain in service after surface repair
  • About 300–500 °C — compressive strength starts to drop significantly; a structural assessment is needed
  • Above about 500–600 °C — residual compressive strength falls to only a fraction of the original; the concrete is severely deteriorated

In practice, TR68 makes the conservative assumption that concrete heated above about 300 °C contributes no strength, and uses the boundary of the zone where the surface turns pink or red (the 300 °C line) as the depth limit for concrete to be broken out during repair. The 500–600 °C figure is therefore the level of clearly severe damage, not the threshold for starting break-out repair.

Heat does not penetrate the whole section. Concrete conducts heat poorly, so temperature drops very quickly with depth. In a typical fire, the heavily heated layer is usually only a few centimeters thick from the surface. The assessment therefore focuses on finding “how deep the heat went” rather than concluding that the whole member is damaged.

A phenomenon that needs special attention is explosive spalling, caused by steam pressure inside very dense concrete. It is more common in high-strength concrete and in concrete that is still moist. Once the cover concrete is lost, the reinforcement is exposed directly to heat and is damaged much faster.

📚 Key points

The surface color of concrete is a useful preliminary indicator, but it depends on the aggregate type. A pink surface usually indicates heating above 300 °C, but this must be confirmed by testing before drawing conclusions on residual strength.

Is Fire-Exposed Reinforcement Still Usable?

It depends on the steel type and the temperature reached. The key question is whether the steel regains its strength after cooling, and the two groups of steel give different answers.

  • Hot-rolled reinforcing bars — generally regain nearly their original strength if not heated beyond about 500–600 °C
  • Cold-worked steel and prestressing strand — lose strength permanently from about 300–400 °C upward, because heat destroys the effect of cold working

Prestressed concrete structures therefore always require a stricter assessment than ordinary reinforced concrete. Besides strand strength, loss of prestress and the bond between strand and concrete must also be considered.

Another often overlooked issue is the bond between reinforcement and concrete. Even if the steel regains its strength, bond strength drops if the cover concrete has deteriorated or spalled, which directly affects member behavior.

What Is Inspected After a Fire?

The assessment starts with an overall survey, then in-depth testing at selected locations. The commonly used test set has 6 items.

← Scroll to see the full table →

Inspection itemWhat it answersReference standards
Visual inspection and hammer tapping (sounding)Extent of damage, delaminated areas, surface colorDPT 1501-51 · TR68 / ACI 562
Ultrasonic Pulse VelocityUniformity of concrete, comparing fire-exposed and unexposed areasASTM C597 · DPT 1504-51
Rebound HammerScreening surface hardness to select core locationsASTM C805 · DPT 1502-51
Concrete core drillingActual residual compressive strength and internal cracking with depthASTM C42
Microstructure examinationDepth of heat penetration and degree of paste deteriorationASTM C856
Cover and rebar location measurementWhether the reinforcement was directly exposed to heatBS 1881-204 · DPT 1505-51

What makes the results reliable is comparison with a reference area: always collect data from unheated areas of the same building as well, because UPV and rebound hammer values are more meaningful as comparisons than as single readings, while core drilling remains the only method that gives actual compressive strength.

3D model of a post-fire concrete column assessment: testing the fire-exposed column against an unexposed reference column in the same building, and coring to see how deep the heat reached A conceptual 3D view of a concrete floor with two columns. The rear column is an unexposed reference column with normal grey surface and marked measurement points. The front column was exposed to fire: its surface is sooty black, partly turned pink, a corner has spalled exposing the longitudinal bars and ties, and there is map cracking on the surface. Ultrasonic transducers attached to the column surface are cabled to a reader on the floor. Another face has a core hole, and a core lies on the floor with its fire-exposed end dark and pink. The bottom-left inset enlarges the core to show the color layers with depth: the outermost layer is soot and friable concrete, next a pink layer indicating heating above about 300 degrees Celsius, then normal grey concrete inside. A dashed line marks the repair break-out limit per TR68. The bottom-right inset shows the concept of comparing pulse velocity and rebound values of the fire-exposed column with the reference column, with lower values for the exposed column, and warns that the rebound hammer reads only the surface and must not be used alone to decide. The right panel summarizes the five-step sequence after the fire is out. 3D model: fire-exposed column vs. reference column Not to scale Reference (no fire) Same method Compare columns Values mean more when compared than alone UPV probes + reader Velocity drops in damaged zones Surface turned pink Usually > ~300 °C Core hole ASTM C42 Spalled corner Exposed rebar Core sample Color layers by depth Core: read color by depth Pink Grey ~300 °C line from exposed face Remove to this line (TR68) · confirm by C856 Compare (concept) UPV Rebound Ref. Fire Rebound: surface only, never alone Steps after the fire 1 Cordon off hazards Loose falling pieces 2 Engineer's initial survey Visual + tap sounding 3 NDT vs. reference UPV · Rebound · Cover 4 Cores + microscopy ASTM C42 · ASTM C856 5 Calculate, then decide Patch · Strengthen · Replace Don't demolish or plaster before inspection — assessment evidence will be lost Color depends on aggregate Confirm by testing Conceptual, not actual design
3D model of inspecting a fire-exposed column against an unexposed reference column in the same building: UPV measured the same way on both columns, a core drilled from the damaged column, and the color layers read by depth to locate the approx. 300 °C line, which is the repair break-out limit per TR68 (conceptual illustration).
⚠️ Cautions

The rebound hammer measures surface hardness only — and the surface is the most damaged layer after a fire — so it usually reads far below the true strength of the inner concrete. Never use rebound hammer results alone to decide that a building must be demolished.

How Is Damage Classified?

The most widely referenced guidance is the Concrete Society's Technical Report TR68, which classifies member damage into 5 classes: from class 0, with practically no structural effect, through intermediate classes requiring surface or structural repair, up to the highest class, where damage is so severe that the member usually has to be replaced.

Four main references are used together in this type of work.

  • DPT 1902-62 — Standard for inspection and strength evaluation of old and damaged building structures (Department of Public Works and Town & Country Planning). It covers fire damage directly, with a section on exposure to high temperature (clause 4.2.3.8) and a section on fire (clause 5.3.7), and is the main framework in the Thai context
  • ACI 216.1 — requirements for determining the fire resistance of concrete and masonry construction, used as the basis for understanding member behavior under fire
  • ACI 562 — code requirements for the assessment, repair and rehabilitation of existing concrete structures, used as the framework for the assessment process and repair design
  • Concrete Society TR68 — specific guidance on the assessment, design and repair of fire-damaged concrete structures, including a damage classification system

RILEM recommendations and fib Bulletin No. 46 are additional references on the mechanical properties of concrete at high temperature. In practice, post-fire building assessments in Thailand follow DPT 1902-62 as the main framework, supplemented with technical detail from the international guidance above.

How Do You Decide Between Repair and Demolition?

The decision rests on 3 engineering questions: is the residual strength sufficient for the intended use, is the damage practically repairable, and are the repair cost and time more worthwhile than demolishing and rebuilding?

Cases that are usually repairable are those where damage is limited to the surface, the reinforcement was not heated beyond the range in which it regains strength, and the main members show no permanent deformation. Cases likely to require member replacement include columns or main beams with clear deformation, prestressed structures whose strand was heated to high temperatures, and members with concrete deteriorated so deeply that, after break-out, the remaining section is insufficient.

2D decision diagram after a fire: five damage classes per TR68, the temperature ranges within which reinforcement regains strength, and three questions used to decide whether to repair or replace a member A four-part diagram. The top shows five column cross-sections from class 0 to class 4 per Concrete Society TR68, with damage increasing from left to right. Class 0 has only surface soot and practically no structural effect. Classes 1 to 3 are intermediate, with map cracking, a pink layer and spalled corners exposing the reinforcement, requiring anything from surface repair to structural repair. Class 4 is severe, with deformed reinforcement, and the member usually has to be replaced. The sections are conceptual. The middle left is a 0 to 900 degree Celsius temperature bar for the two steel groups: hot-rolled bars generally regain nearly their original strength if not heated beyond about 500 to 600 degrees, while cold-worked steel and prestressing strand lose strength permanently from about 300 to 400 degrees upward. The middle right lists three questions: is the residual strength sufficient, is it really repairable, and is it more worthwhile than demolishing and rebuilding. The bottom compares cases that are usually repairable — damage limited to the surface, steel not beyond the recoverable range and no permanent deformation — with cases likely to require replacement — columns or main beams clearly deformed, prestressing strand heated to high temperatures, and insufficient remaining section after break-out. 2D diagram: damage class → repair or replace Not to scale Member damage classes (TR68 guidance) Column section · conceptual Class 0 Class 1 Class 2 Class 3 Class 4 Practically no structural effect Intermediate: surface → structural repair Severe; usually replace member Rebar: strength recovery limit (approx.) Hot-rolled bar · approx. 500–600 °C Test Recovers Cold-worked / strand · approx. 300–400 °C Permanent loss Recovers 0 300 600 900 Steel temperature (°C) 3 questions: repair or replace? 1 Enough residual strength? Core results + engineer's calc 2 Really repairable? Enough section after break-out 3 Better than rebuilding? Repair cost and duration Usually repairable Damage limited to surface Steel within recoverable range No permanent deformation Likely needs replacement Column/main beam clearly deformed Strand heated to high temp. Too little section after break-out
2D decision summary: damage classes 0–4 per TR68 (sections are conceptual), the approximate temperature ranges within which each steel type still regains strength, the 3 questions before deciding, and the characteristics of cases usually repairable versus cases likely to require member replacement.

The final conclusion must come from calculations by the responsible engineer based on actual inspection results. This article provides a framework for understanding and planning, not decision criteria that replace on-site assessment. In many cases this work is carried out together with a structural inspection of the whole building to assess the condition of unexposed areas at the same time.

✅ What to do after the fire is out

First cordon off areas with pieces that may fall, then have an engineer carry out an initial survey to assess whether it is safe to enter, and only then plan the test locations. Rushing to demolish or plaster over before inspection destroys the evidence needed for the assessment.

How Much Does a Post-Fire Building Assessment Cost?

The fee depends on the building size, the number of members to be assessed, the number of test points and cores, and site access. Contact us for a free quotation within 24 hours. Prices exclude travel and equipment mobilization, which are charged extra according to the distance to site. Reports are certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number, and can be submitted to the relevant authorities.