The research team, led by Daniel Paul Thanaraj and colleagues including N. Anand and Éva Lublóy, heated M20-grade concrete specimens to 200, 400, 600 and 800 degrees Celsius in a programmable electric furnace, holding each target temperature for one hour at a slow heating rate of 10 degrees per minute. K-type thermocouples embedded at the surface and at the geometric core of 150-millimetre cubes confirmed that surface temperatures tracked the furnace closely while the core lagged behind, a gap that widened with higher target temperatures. After heating, the specimens were cooled in five different ways: passive natural air cooling, water sprayed from a hose, carbon dioxide gas from a standard extinguisher, sodium bicarbonate dry-powder extinguisher, and aqueous film-forming foam.

The choice of coolant mattered far more than the researchers expected. Foam-cooled specimens consistently retained the highest residual strength across all three mechanical tests — compression, split tension and flexure — while water-quenched specimens fared worst. After exposure to 800 degrees Celsius, foam-cooled cubes kept 73.35 percent of their original compressive strength of 25.52 megapascals, compared with only 47.49 percent for water-cooled specimens. Air cooling fell in between at 53.44 percent, while carbon dioxide and sodium bicarbonate cooling produced nearly identical intermediate results of roughly 67 percent. The spread between the best and worst coolant widened dramatically with temperature: a difference of just 7.6 percent at 200 degrees grew to 25.86 percent at 800 degrees.

The physics behind these differences is subtle. Water cooling, it turns out, is doubly damaging. When cold water strikes hot, low-permeability concrete, the steep thermal gradient generates tensile stresses that crack the surface, while simultaneously reintroducing moisture into a dehydrated pore network where vapour pressure drives further microcracking and spalling. Carbon dioxide cooling, by contrast, is cryogenic — the gas leaves the extinguisher at roughly minus 78 degrees Celsius — yet it outperformed water. The researchers attribute this to its dryness and speed: the hot surface reaches equilibrium with the gas quickly, shortening the cumulative time the concrete spends under differential thermal stress, and because no liquid is involved, there is no vapour-pressure damage pathway. Thermal damage, they note, is governed by both the magnitude of the temperature difference and how long that difference persists, not by the peak gradient alone.

Sodium bicarbonate powder cooling works through an entirely different, chemically active mechanism. When the powder hits the hot surface, it decomposes endothermically between about 87 and 177 degrees Celsius, breaking down into sodium carbonate, water vapour and carbon dioxide while absorbing heat. The team observed a visible white-grey residue on treated specimens, confirming the reaction had occurred. The resulting porous sodium carbonate layer acts as insulation, buffering the surface against temperature gradients, while the released gases provide additional cooling. Foam cooling, the top performer, relies on a gentler principle still: the air trapped in the thin liquid films of its bubble structure lowers the effective thermal conductivity of the layer, so heat is extracted smoothly rather than violently. Although foam’s average cooling rate was broadly comparable to water’s, the rate of change of the surface-to-core temperature difference was far smaller, and it is precisely that rate of change — not total heat extraction — that governs thermally induced cracking.

Non-destructive testing told the same story. Ultrasonic pulse velocity values, which drop as internal cracks and voids multiply, fell from an excellent 4.78 to 5.56 kilometres per second at 200 degrees to a doubtful 0.24 to 1.54 kilometres per second at 600 and 800 degrees. Water-cooled specimens recorded the lowest velocities at every temperature above 200 degrees, while foam and sodium-bicarbonate-cooled specimens scored highest. Regression analysis revealed a strong relationship between pulse velocity and compressive strength, with coefficients of determination between 0.93 and 0.98 across all cooling regimes — meaning engineers can use a simple ultrasonic survey to estimate residual strength in a fire-damaged structure with considerable confidence.

Statistical analysis reinforced the hierarchy of effects. A two-way analysis of variance showed that exposure temperature was overwhelmingly the dominant driver of strength loss, with partial eta-squared values approaching one for flexural strength, but cooling method remained a highly significant secondary factor. Tukey post-hoc comparisons confirmed that foam cooling significantly outperformed air cooling for all three strength measures, while the largest single drop in flexural strength occurred between 200 and 400 degrees — the range in which calcium silicate hydrate gel begins to dehydrate and microcracks initiate. Water cooling also introduced the greatest variability between specimens, with coefficients of variation exceeding 15 percent at the highest temperatures, a statistical fingerprint of the chaotic, non-uniform nature of thermal shock.

Physical inspection and microscopy made the damage visible. After exposure to 800 degrees, water-cooled specimens showed the widest thermal cracks, measuring up to 0.75 millimetres, followed by air-cooled specimens at 0.62 millimetres. Foam-cooled specimens showed the narrowest cracks at just 0.16 millimetres, with carbon dioxide and sodium bicarbonate in between. Scanning electron microscopy revealed severe networks of cracks and degraded interfacial transition zones in water-cooled samples, whereas foam-cooled samples retained a relatively dense calcium-silicate-hydrate structure with good bonding between aggregate and paste. Energy-dispersive X-ray analysis showed a pronounced depletion of calcium and oxygen peaks at 800 degrees, most severe in water-cooled specimens, consistent with the breakdown of calcium-bearing hydration products.

Even the aggregates inside the concrete suffered differently depending on the coolant. Recovered coarse aggregates from water-cooled specimens showed the greatest deterioration in specific gravity, water absorption and crushing value — at 800 degrees their water absorption was 4.85 percent, nearly double the 2.48 percent measured for foam-cooled counterparts — indicating that quenching damage penetrates deep into the mineral structure of the material. Mass loss followed the same pattern, with water-cooled specimens shedding 10.28 percent of their mass at 800 degrees against 6.06 percent for foam-cooled ones, reflecting spalling and the decomposition of hydration products.

The practical implications are significant. Flexural and tensile strengths proved more sensitive to cooling damage than compressive strength, since even small microcracks disproportionately degrade tension and bending performance — water-cooled beams retained only 15.33 percent of their flexural strength after 800-degree exposure, versus 29.32 percent for foam-cooled ones. Because the relative performance of coolants diverged most sharply at high temperatures, the choice of suppression agent matters most in severe fires. The authors suggest that for structures likely to survive a fire and be repaired rather than demolished, suppression strategies that moderate the cooling rate — such as foam — could preserve substantially more structural value, while ultrasonic pulse velocity surveys paired with the regression models developed here give assessors a fast, reliable tool for triage. The team cautions that continuous temperature instrumentation during cooling was not performed in this study, and recommends future work with embedded thermocouples and controlled flow rates to quantify cooling kinetics precisely.

Subject of Research: Effect of fire suppression and cooling methods on the residual mechanical properties of heat-damaged structural concrete

Article Title: Post-fire behaviour of structural concrete: Effect of different fire suppression methods on residual strength and damage recovery

Article References: Thanaraj, D. P., G, A., Anand, N., A, D. A., & Eszter, L. E. (2026). Post-fire behaviour of structural concrete: Effect of different fire suppression methods on residual strength and damage recovery. Case Studies in Construction Materials, 25, Article e06560. https://doi.org/10.1016/j.cscm.2026.e06560

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06560

Keywords: concrete, fire, residual strength, cooling methods, foam, thermal shock, ultrasonic pulse velocity, compressive strength, fire suppression, structural assessment, microcracking, sodium bicarbonate

ABC dry powder is the workhorse of portable fire suppression across the world. The designation ABC refers to its effectiveness against Class A fires involving ordinary combustibles, Class B fires involving flammable liquids, and Class C fires involving energised electrical equipment, a versatility that explains its ubiquity in public buildings and heritage institutions alike. The dominant active ingredient in most formulations is monoammonium phosphate, typically blended with ammonium sulphate, and the powder is carried on a nitrogen propellant and formulated with silicone-treated additives to keep it free-flowing and water-repellent. When directed at flames, the phosphate salt works partly by melting and smothering the burning surface, and partly by interrupting the radical chain reactions that sustain combustion. This chemical aggressiveness, so valuable against fire, is precisely what raises concerns when the powder settles on delicate cultural materials.

The experimental core of the study involved subjecting ancient-style brass replica coins to controlled fire suppression events using standard ABC dry powder, and then characterising the resulting changes to the coin surfaces with an analytical toolkit drawn from conservation science. Replica coins are a standard and ethically sound stand-in for authentic artefacts in destructive testing, allowing researchers to quantify corrosion behaviour without sacrificing genuine heritage objects. Following exposure, the researchers documented visible alterations in colour, texture and lustre, and then interrogated the surfaces to identify the chemical species responsible for the damage and to understand how deeply the alteration penetrated the metal substrate.

The results paint a picture of rapid and chemically specific attack rather than generic soiling. The ammonium salts deposited by the extinguisher are hygroscopic, meaning they pull moisture from the ambient air and form thin films of acidic electrolyte on the brass surface. Monoammonium phosphate in particular dissolves to give acidic solutions that can react with the zinc component of the alloy, a process related to the well-known dezincification behaviour of brasses, in which zinc is selectively leached from the copper-zinc matrix and leaves behind a porous, structurally weakened, copper-rich layer. The corrosion products identified on the treated coins, including zinc phosphate phases and sulphate-bearing compounds derived from the ammonium sulphate fraction, represent a permanent chemical transformation of the surface rather than a simple deposit that could be brushed away.

The analytical characterisation behind these observations combined techniques that conservation scientists routinely deploy when assessing corroded metalwork. Spectroscopic methods were used to identify the molecular fingerprints of the corrosion products, distinguishing phosphate and sulphate phases from the copper and zinc oxides and carbonates that normally constitute the natural patina of aged brass. Microscopic examination revealed changes in surface morphology, with the powder treatment producing roughened, pitted textures where the protective patina had been chemically consumed. The distinction between deposit and corrosion proved to be central to the study: while loose powder can, in principle, be removed by cleaning, the acidic electrolyte it generates acts quickly, and once phosphate and sulphate species have reacted with the alloy, the damage is embedded in the object’s chemistry and cannot be reversed by dry brushing or gentle washing.

One of the most consequential findings concerns the interaction between the extinguisher powder and pre-existing patinas. Museum objects are rarely bare metal; centuries of slow oxidation produce patina layers that are both chemically protective and historically and aesthetically significant, often forming part of an object’s documented condition and its value to scholarship. The study indicates that acidic and ammonium-rich residues from ABC powder do not respect these layers. Instead, the hygroscopic salts can penetrate micro-cracks and pores in the patina, attack the underlying alloy, and cause localised undermining and detachment of the patina itself. For a genuine archaeological bronze or an antique brass instrument, this means that a single discharge event could strip away surface material that took centuries to form, destroying diagnostic information about manufacture, burial environment, or previous conservation treatments.

The time dimension of the damage is equally important. The experiments suggest that the worst effects are not necessarily instantaneous. Once powder residues remain on a surface, the hygroscopic salts continue to cycle with humidity, absorbing moisture in damp conditions and releasing it when the air dries, each cycle delivering a fresh pulse of acidic electrolyte to the metal. This means that a fire suppression event followed by inadequate or delayed cleaning can leave an object in a state of ongoing, self-perpetuating corrosion, sometimes described in conservation literature as an active corrosion state. Objects that appear superficially intact in the immediate aftermath of a discharge may therefore deteriorate progressively over the following weeks and months, a delayed-onset failure mode that is easy to miss in routine condition checks unless staff know precisely what to look for.

For museums, historic houses, libraries and archives, the study lands at a genuine dilemma, because fire remains one of the most catastrophic risks facing cultural heritage, and modern fire codes rightly mandate suppression systems. The research does not argue that heritage buildings should abandon fire protection; rather, it argues that the choice of extinguishing agent and the response to any discharge must account for the chemical sensitivity of collections. Alternatives exist. Water mist systems, inert gases such as nitrogen or argon, and clean agent halocarbons suppress fire without leaving corrosive solid residues, although each carries its own cost, space and engineering considerations. In spaces where ABC dry powder extinguishers remain the only practical option, the findings underline the importance of rapid post-discharge response: documented removal of residues by trained conservators, prioritisation of metal objects, and monitoring of treated surfaces for delayed corrosion activity.

The study also contributes to a broader and growing literature on collateral damage in emergency response. Previous investigations have examined the effects of water-based sprinklers on paper and textiles, smoke damage on paintings, and the chemical legacy of firefighting foams, but portable dry powder extinguishers have received comparatively little attention despite being the most likely suppressant to come into direct, close-range contact with objects during a small or incipient fire. By quantifying the specific corrosion chemistry of monoammonium phosphate and ammonium sulphate on a copper-zinc alloy, the research converts a vague institutional anxiety into a concrete, characterisable risk with testable mitigation strategies. That conversion from anecdote to measurement is what allows heritage institutions to negotiate with fire safety engineers from a position of evidence.

Ultimately, the message for the heritage sector is one of informed vigilance rather than alarm. Brass and bronze collections are not doomed by the presence of an extinguisher in the room, but the chemistry documented here shows that the aftermath of a discharge is not the end of the incident; it is the beginning of a second, slower episode of damage that unfolds on the surfaces of the objects themselves. The researchers’ characterisation of phosphate and sulphate corrosion products on powder-treated brass gives conservators a diagnostic target, gives facility managers a rationale for reviewing suppression choices in collection areas, and gives the field a reminder that fire safety and object preservation are not automatically aligned. Protecting heritage from fire, the study implies, must include protecting heritage from the means of that protection.

Subject of Research: Physicochemical corrosion damage to brass artefacts caused by ABC dry powder fire suppression

Article Title: Physicochemical damage to ancient brass replica coins during fire suppression with ABC dry powder

Article References: Physicochemical damage to ancient brass replica coins during fire suppression with ABC dry powder. (n.d.). https://doi.org/10.1038/s40494-026-03013-5

Image Credits: AI Generated

DOI: 10.1038/s40494-026-03013-5

Keywords: ABC dry powder, fire suppression, brass, corrosion, cultural heritage, conservation science, monoammonium phosphate, dezincification, patina, museum collections, Physicochemical, damage