Spall and Spalling: How Solids Shed Fragments

Spall is the name for flakes, chips or slabs of material that break away from the surface of a larger solid. Spalling is the process that produces them. The same words turn up in fields that otherwise have little in common: civil engineering, armoured vehicle design, bearing maintenance, geology, fire safety, emergency medicine and nuclear physics. In each case something has pushed the inside of a material harder than its surface layer can tolerate, and a piece comes off.

Wikipedia's article on the subject lists the main causes as projectile impact, corrosion, weathering, cavitation and excessive rolling pressure, as in ball bearings. That list hides a common thread. Almost all spalling comes from tension or shear that develops just beneath a surface. It might be a stress wave bouncing off the back face of a steel plate, steam pressure building inside hot concrete, rust swelling around a reinforcing bar, water freezing in a crack in stone, or repeated rolling loads fatiguing the metal a fraction of a millimetre under a bearing raceway.

This article explains how spalling works in each of these settings, gives real examples including the 1996 Channel Tunnel fire, describes how engineers prevent and manage it, and clears up some common confusions, especially the difference between mechanical spalling and nuclear "spallation."

Words and meanings

Spall, spalling and spallation

The three words overlap and are often used interchangeably.

Spall is the fragment itself, or the collection of fragments.
Spalling is the process of shedding them, used especially in civil engineering, geology and bearing analysis.
Spallation is used for the same mechanical process in impact physics and, separately, for a nuclear process in which a high-energy particle knocks many particles out of an atomic nucleus.

Wikipedia notes that its "Spall" article is slated to be merged into "Spallation," which reflects how tangled the terminology has become.

A borrowed word in physics

According to Wikipedia, particle physicists borrowed the term. Neutron sources for scientific research produce neutrons by bombarding a heavy target, and the ejected neutrons are described as "spall." The Spall article credits Glenn T. Seaborg with coining "nuclear spallation" in his 1937 doctoral dissertation, while the Spallation article says Seaborg coined the term in 1947, the same year the process was first observed with accelerators. The two pages disagree on the year, so the safest statement is that the word was taken into nuclear physics from its older mechanical meaning around the middle of the twentieth century, with Seaborg credited for it.

The core mechanism: why things break near surfaces

Tension is the weak point

Most brittle materials, including concrete, stone, glass and many ceramics, are much stronger in compression than in tension. Metals are more balanced, but even they fail when tensile stress exceeds a critical value long enough. Spalling happens when a process creates tension, or high shear, in a thin layer near a surface. The surface layer then separates along a plane more or less parallel to the face.

Stress waves and free surfaces

The clearest physical picture comes from impact. When something strikes a plate, it sends a compression wave into the material. When that wave reaches the opposite face, which is free to move, it reflects as a tension wave travelling back. Wikipedia describes how, in plate impact tests, these reflected waves interact inside the plate and create a zone of high tensile stress. Where that stress exceeds the material's local tensile strength, small internal voids or cracks form, a process the Spallation article calls internal cavitation. These grow and join until a layer breaks away from the back face.

The Spallation article adds that the ejected fragment acts as a secondary projectile and can reach speeds of up to about one third of the stress wave speed. That is why spall can be dangerous even when the original projectile did not pass through the plate.

Worked example: where does the spall plane form?

This is a simplified, hypothetical illustration using idealised one-dimensional wave theory, not data for any real material or weapon.

Imagine a compression pulse of fairly uniform strength travelling through a thick plate. Assume, for illustration:

Wave speed in the plate: 5,000 metres per second
Duration of the pulse: 2 microseconds (0.000002 seconds)

The physical length of the pulse is wave speed multiplied by duration: 5,000 × 0.000002 = 0.01 metres, or 10 millimetres.

When the front of the pulse reaches the free back face, it reflects as tension and starts travelling back into the plate, while the rear of the incoming compression pulse is still arriving. In this idealised picture, the reflected tension becomes the dominant stress at a depth of roughly half the pulse length from the free face, about 5 millimetres here. If the net tension there exceeds the material's tensile strength, a layer roughly that thick breaks away.

Two practical lessons follow from even this crude picture. First, short, sharp pulses tend to produce thin spall layers, and longer pulses thicker ones. Second, because the critical tension develops near the back face, the visible damage can be on the side away from the impact. Wikipedia's figure that a spall fragment can reach up to about one third of the stress wave speed would put an upper bound of roughly 5,000 ÷ 3, or about 1,700 metres per second, on fragment speed in this hypothetical plate. Real fragments are usually much slower than that bound, but even a small fraction of it is enough to injure.

Other routes to near-surface stress

Not all spalling involves waves. Other mechanisms create the same near-surface tension more slowly:

Mechanism	What creates the stress	Typical setting
Stress wave reflection	Impact or explosion	Armour, plates, blast-loaded structures
Rolling contact fatigue	Repeated contact loads with maximum shear just below the surface	Ball and roller bearings, gears, rails
Cavitation	Collapsing vapour bubbles producing local high pressure	Pumps, turbines, propellers, piping
Thermal gradient	Hot surface expands faster than the cooler interior	Fire-exposed concrete, refractory linings, rock in wildfires
Internal pore pressure	Water turning to steam inside a dense material	Fire-exposed concrete
Expansive corrosion	Rust occupying more volume than the steel it replaces	Reinforced concrete
Freeze-thaw	Water expanding as it freezes in cracks and pores	Stone, brick, concrete, terracotta
Salt crystallisation	Salts crystallising and growing near the surface	Brick, stone, tile, concrete
Unloading	Release of confining pressure when overlying rock is removed	Rock outcrops, mines, quarries
Oxide shedding	Corrosion products that expand and do not adhere	Some reactive metals

The rest of this article goes through these settings one at a time.

Spalling in armour and anti-tank warfare

High-explosive squash head

Spall became a military design principle in the twentieth century. Wikipedia describes the high-explosive squash head (HESH) shell, which flattens against the outside of armour before detonating. The detonation sends a compression wave into the plate, which reflects off the inner face as a tension wave and breaks pieces of metal off the inside. The armour is not necessarily pierced, but the crew compartment is showered with fragments.

Wikipedia also cites the Polish wz. 35 anti-tank rifle as an early example of a weapon intended to cause spallation rather than penetration. It illustrates the effect of large-calibre fire on armour with a photograph of armour plating gouged from the X turret of HMS New Zealand at the Battle of Jutland in 1916.

Penetrators also cause spall

Spalling is not limited to weapons designed for it. According to Wikipedia, kinetic energy penetrators that defeat armour generally also cause internal spalling as they exit the inner face. The fragments add to the damage done by the penetrator itself.

Spall liners

The standard defence is a spall liner, a layer of tough material fixed to the inside of the armour. A United States patent on armoured vehicle mounting systems describes the purpose of such liners as protecting occupants from flying metallic debris, called spall, that results when a projectile penetrates the hull. Common liner materials include aramid fibres such as Kevlar and Twaron, glass fibre composites, and ultra-high-molecular-weight polyethylene.

Independent test evidence exists for some of these choices. A United States Army report held by the Defense Technical Information Center found that Kevlar and S-2 glass spall liners for the M113A3 armoured personnel carrier provided comparable ballistic protection from spall when the vehicle was attacked by shaped charge or penetrator munitions, and that neither created an additional hazard from breathable liner particles.

Some figures in this area come from manufacturers rather than independent testing. The Israeli armour company Plasan states that traditional steel armour typically produces spall cones with angles of 30 to 60 degrees, and the British composites maker Permali lists "cone angle reduction" among the benefits of its liners. These are vendor statements and should be read as such. The underlying idea is simple, though: a liner catches fragments and narrows the cone they spread into, so fewer people and components inside are hit.

Spalling inside the body: blast injury

The same physics applies to the human body. Wikipedia's Spall article, citing a 2006 paper on blast lung injury in Prehospital Emergency Care and a 2023 review of blast injuries in the American Journal of Emergency Medicine, explains that blast overpressure can cause spalling where a pressure wave passes from denser tissue into less dense tissue. Examples include the boundary between the bowel wall and gas inside the bowel, and between lung tissue and air spaces.

Medical sources list spallation as one of three primary mechanisms of blast injury, alongside implosion and shearing. This is one reason gas-filled organs, especially the lungs, are particularly vulnerable to blast. Anyone exposed to a blast needs medical assessment even if they look uninjured, because these internal injuries may not be obvious at first.

Spalling in bearings, gears and machinery

Rolling contact fatigue

In machinery, spalling is a well-known failure mode of rolling bearings. When a ball or roller presses against a raceway, the contact stress is concentrated in a small area. Wikipedia's Spall article notes that the maximum shear stress in such contacts lies just below the surface, and that under repeated loading the material can fail there. Over millions of load cycles, cracks start below the surface, grow, and eventually release flakes of metal, leaving pits in the raceway. Wikipedia contrasts this with brinelling, which is a different failure involving permanent dents.

The ISO 15243 classification

The international standard for classifying bearing damage, ISO 15243, groups rolling contact fatigue into two subcategories, as explained by the bearing manufacturer SKF:

Subsurface initiated fatigue. Caused by cyclic stresses just beneath the raceway surface. SKF describes it as closely related to the basic rating life of a bearing, written L10mh. It can be accelerated by high stresses, for example from temporary overload or anything that weakens the material, in which case SKF says the fatigue life can be as short as 5 to 10 percent of the L10mh rating life.
Surface initiated fatigue. Starts on the rolling surface itself and is typically caused by surface distress due to poor lubrication or poor cleanliness, such as contamination by particles.

Both can end in spalling. SKF training material notes that if a bearing failing from subsurface fatigue is monitored carefully, the first sign is typically a small spalled area. If the bearing keeps running, the area grows and more areas appear. Different sources cite the 2004 and 2017 editions of ISO 15243, so anyone using it for formal analysis should check which edition applies.

Why spalling matters for maintenance

Spalling in a bearing is progressive. Flakes of hardened steel circulate through the lubricant and damage other surfaces. The pitted raceway produces vibration, noise and heat. This is why vibration monitoring is common on critical rotating machines: a small spall can often be detected long before the bearing seizes.

Cavitation spalling

Wikipedia also describes spalling caused by cavitation. When liquid pressure drops locally, vapour bubbles form. When they move into higher pressure and collapse, they produce intense local pressure that can spall adjacent surfaces. Pumps, hydraulic turbines, ship propellers and piping are all affected. The general literature on water turbines lists pitting from cavitation as a normal form of wear, usually repaired by welding with stainless steel filler.

Spalling in concrete

Concrete is where most people meet the word. Concrete can spall for several different reasons, and telling them apart matters because the cures are different.

Reinforcement corrosion

Most concrete structures contain steel reinforcing bars. Fresh concrete is strongly alkaline, and that chemistry normally protects the steel. When chloride ions from de-icing salts or seawater reach the steel, or when the concrete near the surface loses its alkalinity through reaction with carbon dioxide, the protection breaks down and the steel corrodes.

Rust takes up more space than the steel it replaces. As it forms, it pushes outward on the surrounding concrete and creates tensile stress. Concrete is weak in tension, so it cracks, then delaminates, and eventually the cover layer over the bar falls away as spall. Published sources differ on exactly how much bigger rust is than the original steel, with figures ranging from two to four times in some technical articles and higher in some industry brochures, but they all agree the expansion is large enough to crack concrete.

Some numbers from the research history help show how sensitive this is. A frequently cited Federal Highway Administration study by Clear in 1976 found that a chloride level of about 0.20 percent total acid-soluble chloride by weight of cement could induce corrosion of reinforcing steel in bridge decks. This is a 1970s finding, and current design guidance should be checked for any specific project.

The most important defence is cover, the thickness of concrete between the steel and the surface. Technical literature describes the depth of cover as perhaps the single most important design and construction factor in delaying the onset of corrosion spalling. The Portland Cement Association also stresses that a low water to cement ratio and suitable materials reduce the permeability of concrete, which slows the entry of corrosive substances, and that sealers and coatings can help repel them.

Freeze-thaw and salt

Wikipedia describes freeze-thaw spalling in stone, masonry and terracotta, and the same process affects concrete. Water in cracks and pores expands when it freezes, pushing the surface layer outward. Repeated cycles flake it away. Salt spalling is related: dissolved salts crystallise near the surface of porous materials such as brick, stone, tile and concrete, and the growing crystals break the surface layer off.

Fire spalling

Fire spalling is the most dramatic form, and the one that most concerns tunnel engineers. When concrete is heated quickly, two effects combine:

Thermal stress. The hot outer layer tries to expand while the cooler interior holds it back, producing compression at the surface and tension just behind it.
Pore pressure. Water in the concrete's pores turns to steam. In dense, low-permeability concrete, the steam cannot escape fast enough, so pressure builds just behind the heated face.

Wikipedia's Spall article, discussing refractory concrete, names the same two drivers, rapid-heating thermal strain and internal pressure from water removal, and warns that explosive spalling can throw fragments of 1 to 10 kilograms over many metres, creating serious hazards and costly repairs.

A 2020 Tunnels and Tunnelling article summarising a talk by Tom Lennon of the Building Research Establishment (BRE) listed the conditions that make fire spalling more likely: high moisture content, high concrete strength, rapid temperature rise, and high levels of restraint or applied load, alone or in combination. It explained why high-strength concrete is especially vulnerable: its low porosity and permeability trap pore pressure and thermal stress that cannot be relieved. Spalling can be explosive, or it can be gradual, sometimes stopping when it reaches a layer of reinforcement.

How fire tests represent tunnel fires

Ordinary building fire tests do not represent fuel fires in tunnels well. The BRE talk described several fire curves used in tunnel work:

Fire curve	Description given
RWS curve	Rises to about 1,360 °C
EUREKA curve	Reaches about 1,200 °C after 5 minutes, holds for about 60 minutes, then cools
Commercial test regime described by BRE	Furnace required to reach 1,200 °C in 5 minutes and cool to ambient after 170 minutes

BRE's own large-scale tests found clear differences between mixes. In tests at its Cardington facility, high-strength concrete containing polypropylene fibres showed no spalling, while ordinary concrete without fibres spalled significantly. In an earlier column series of 30 unreinforced specimens, only 4 survived, 3 of which contained polypropylene fibres.

Polypropylene fibres

The main mix design defence against explosive spalling is to add fine polypropylene fibres. The usual explanation is that they melt when the concrete gets hot, leaving tiny channels through which steam can escape, so pore pressure does not build up.

How much fibre to use is debated. A Sika product document states that Section 6.1 of EN 1992-1-2, the fire design part of Eurocode 2, cites 2 kilograms per cubic metre of polypropylene fibres for controlling explosive spalling in high-strength concrete. Comments reported in the Tunnels and Tunnelling article give other figures: London Underground specified 1 kilogram per cubic metre, an engineer recalled about 1 kilogram of fibres under 32 micrometres in diameter being used on the Channel Tunnel, and one speaker argued that fibre count matters more than mass, since an 18 micrometre fibre at 1 kilogram gives far more fibres than a 32 micrometre fibre at 2 to 3 kilograms. A Croatian study described the usual effective range as about 1 to 3 kilograms per cubic metre, depending mainly on fire severity and moisture content, and Sika notes that dosages of 1.5 kilograms or less have been used after careful trials.

The point on which all these sources agree is that spalling resistance should be proven by large-scale fire testing of the specific concrete mix. Lennon also cautioned that fibres are not a complete solution on their own. Other options include fire protection boards, sprayed fire protection, fire barriers and spalling-resistant mix designs.

Case study: the 1996 Channel Tunnel fire

What happened

On 18 November 1996, at 21:48 Central European Time, a fire was seen on a heavy goods vehicle shuttle train, number 7539, as it entered the Channel Tunnel from the French portal. The train was carrying 15 vehicles on open lattice wagons, with 31 passengers and 3 crew. The lorry drivers travelled in an amenity coach. The train stopped at 21:58, about 19 kilometres into the tunnel. The cause of the fire was never confirmed, and arson was considered possible. Nobody died. Passengers and crew suffered smoke inhalation and were taken to hospital in France for observation.

Damage to the lining

The fire caused severe spalling of the tunnel's concrete lining. According to the Wikipedia account of the fire:

Over a 50 metre length, the normally 40 centimetre thick lining was reduced to a mean thickness of 17 centimetres, with a minimum of just 2 centimetres.
Over a 240 metre length, damage extended as far back as the first layer of reinforcing bars.
Superficial damage to the concrete segments was evident along a further 190 metres.

An MIT researcher quoted at the time described the walls as having peeled away layer by layer like an onion. That is exactly what progressive fire spalling looks like: the heated face breaks off, exposing fresh concrete to the fire, which then breaks off in turn.

Worked example: how much lining was lost?

Using the figures above:

Mean loss in the worst section: 40 cm minus 17 cm = 23 cm. As a fraction of the original thickness, 23 ÷ 40 = 0.575, or about 58 percent.
At the thinnest point: 40 cm minus 2 cm = 38 cm lost, which is 38 ÷ 40 = 0.95, or 95 percent.

Some technical sources quote a nominal lining thickness of 450 millimetres rather than 40 centimetres, and describe repairs needing more than 1,300 tonnes of fibre-reinforced sprayed concrete. Figures differ between sources, and the official inquiry reports are the authority for exact values.

Repair and reopening

As a precaution, colliery arches were installed to support the damaged section. Track, overhead line, signalling and other services were replaced, including 500 metres of track and 800 metres of overhead line. The French contractor Freyssinet completed the civil engineering repairs in 60 days. The tunnel reopened in stages: freight on 29 November 1996, Eurostar passenger trains on 4 December, car shuttles on 10 December, coach shuttles on 6 January 1997, and heavy goods vehicle shuttles on 15 May 1997. The infobox in the Wikipedia article gives the cost as £200 million in 1997 money.

What the inquiries found

Three inquiries examined the fire. Their findings were mostly about operations rather than concrete. An alarm was not treated seriously at first; doors that should have closed to keep smoke out of the other running tunnel did not close properly; control centre staff were overwhelmed; and the supplementary ventilation was brought into action late. Eurotunnel abandoned the earlier approach of driving a burning train out of the tunnel and instead stops trains and evacuates people into the central service tunnel.

The spalling lesson was taken up by the wider tunnelling world. The Channel Tunnel had further fires in 2006 and 2008, and the BRE talk listed it with the Great Belt tunnel fire in Denmark in 1994, which began in one of the tunnel boring machines during construction, and the 1999 Mont Blanc and Tauern road tunnel fires, as key events behind modern tunnel fire testing.

Spalling in rock and natural weathering

Rock spalls for the same reasons concrete does, usually over longer timescales.

Unloading

Wikipedia describes spalling from unloading. When overlying rock is removed, by erosion or by excavation, the rock below is suddenly relieved of the pressure that was squeezing it. It expands, and stress concentrates near the surface, which can cause slabs to break away.

Exfoliation

Exfoliation, sometimes called onion-skin weathering, is a form of spalling in which thin sheets peel off rock surfaces. Wikipedia attributes it to differential thermal expansion from heating or repeated temperature cycles, including forest fires, and gives the granite domes of places like Yosemite as an example.

Research has made the role of heat more precise. In a 2016 paper in Nature Geoscience, "Rockfall triggering by cyclic thermal stressing of exfoliation fractures," Brian Collins of the US Geological Survey and Greg Stock, the park geologist at Yosemite National Park, reported 3.5 years of monitoring of a near-vertical exfoliation sheet of granodiorite in Yosemite Valley. The sheet was about 19 metres tall, 4 metres wide and 10 centimetres thick. They measured temperature and the width of the crack behind the sheet. They found that daily, seasonal and annual temperature changes drive repeated and cumulative opening of the fracture, which can extend the crack and eventually detach the rock. They concluded that the warmest parts of the day and year are especially likely to trigger rockfalls, which explains some rockfalls that happen without any obvious trigger such as rain, earthquakes or freezing. Press coverage reported that the crack opened by almost a centimetre during the hottest part of the day.

A separate model by the geologist Martel, discussed in Physics Today, argued that curvature and slope, rather than overall relief, are the key geometric factors in where sheeting joints form, and that they open on convex surfaces.

Freeze-thaw and salt in stone

Freeze-thaw and salt spalling, described above for concrete, are major causes of decay in natural stone, brickwork and terracotta on buildings and monuments. Water gets into cracks, freezes, expands, and flakes off the surface. Over many winters, carved details are lost.

Spalling from corrosion of metals

Most metals form an oxide layer when exposed to air. In aluminium and stainless steel, that layer sticks and protects the metal underneath, a process called passivation. In other cases the corrosion products occupy a very different volume from the metal and do not adhere. Wikipedia describes these products as spalling off as particles instead of forming a protective film.

Its example is depleted uranium, which expands strongly when exposed to air and sheds a fine layer of oxide. Because actinide metals are pyrophoric, toxic and in some cases radioactive, Wikipedia notes they are often handled in gloveboxes filled with an inert atmosphere. The oxide that spalls off is a fine powder, which is a hazard in itself.

Nuclear spallation: a different process with the same name

What it is

Nuclear spallation has nothing to do with cracks. According to Wikipedia, it happens when a high-energy particle strikes a heavy nucleus, which then emits many nucleons and loses much of its mass. The ejected neutrons are what physicists call spall.

Spallation neutron sources

Spallation is used to make neutrons for research. Wikipedia's Spallation article describes the usual arrangement: a proton beam with an energy of around 1 GeV strikes a heavy target such as mercury, tantalum or lead, and each target nucleus releases roughly 20 to 30 neutrons. It gives an energy cost of about 30 MeV per spallation neutron, compared with about 200 MeV released per fission event. Because spallation does not rely on a chain reaction, the process is not critical in the reactor sense, and the beam can be pulsed relatively easily.

Facility	Location	Details given by Wikipedia
ISIS	Harwell, United Kingdom	Synchrotron producing about 200 microamperes at 0.8 GeV, pulsed at 50 Hz
Nimrod (ISIS predecessor)	Harwell, United Kingdom	About 2 microamperes at 7 GeV
SNS	Oak Ridge, United States	Spallation Neutron Source
ESS	Sweden	Under construction, linear accelerator design
SINQ	Paul Scherrer Institute, Switzerland	Spallation neutron source
J-PARC, LANSCE, CSNS	Japan, United States, China	Other spallation sources

Wikipedia notes that tantalum or tungsten targets are typical, that depleted uranium gives the most intense beams but has the shortest target life, and that spallation has been proposed as the neutron source for subcritical reactors such as MYRRHA, intended to study transmuting high-level nuclear waste.

Cosmic ray spallation

Spallation also happens in nature. Cosmic rays striking nuclei in the atmosphere, on meteorites and on the Moon break them apart. Wikipedia explains that lithium, beryllium and boron are much more common in cosmic rays than in average cosmic matter, and attributes this to spallation of heavier nuclei such as oxygen, nitrogen and carbon. On Earth, spallation produces cosmogenic isotopes of elements including aluminium, beryllium, chlorine, iodine and neon. Measuring them in rock surfaces lets geologists estimate how long those surfaces have been exposed, a technique called surface exposure dating. Wikipedia records that cosmic ray spallation was observed in the 1930s and the first accelerator observations came in 1947.

Worked example: a rough look at thermal stress

This example is hypothetical and simplified. It shows why heating alone can break concrete, without claiming exact values for any real material.

If a surface layer is heated by an amount ΔT but is fully restrained from expanding, the stress that develops is roughly:

stress = E × α × ΔT

where E is the stiffness (Young's modulus) and α is the coefficient of thermal expansion.

Assume, for illustration only:

E = 30 gigapascals
α = 10 millionths per degree Celsius (0.00001 per °C)
ΔT = 100 °C

Then stress = 30,000,000,000 × 0.00001 × 100 = 30,000,000 pascals, or 30 megapascals.

Now assume a hypothetical tensile strength for the concrete of a few megapascals. Even if only a fraction of that thermal stress appears as tension just behind the heated layer, it can exceed the tensile strength. Add steam pressure in the pores, and the layer has every reason to separate. Real behaviour is far more complicated, because restraint is partial, properties change with temperature and cracks relieve stress, but the order-of-magnitude result explains why fire spalling is so common in dense concrete.

Detecting, preventing and repairing spalling

Concrete structures

Design. Adequate concrete cover over reinforcement, low permeability mixes and suitable materials slow the corrosion that causes most everyday spalling.
Protection. Sealers, coatings and drainage reduce the water and salt that drive corrosion and freeze-thaw.
Fire resistance. For tunnels and high-strength concrete, polypropylene fibres, protective boards or sprays, and mix designs proven in large-scale fire tests reduce explosive spalling.
Repair. Damaged concrete is removed back to sound material, corroded steel is cleaned or replaced, and new concrete or sprayed concrete is applied. After the Channel Tunnel fire, fibre-reinforced sprayed concrete was used.

Bearings and machinery

Correct selection and lubrication. Overloading and poor lubrication shorten fatigue life and promote spalling.
Cleanliness. Contamination is a common cause of surface-initiated fatigue.
Condition monitoring. Vibration analysis and lubricant debris monitoring can catch spalling early.

Armour

Spall liners. Aramid, glass fibre or polyethylene liners catch fragments and reduce injuries inside vehicles.

Rock

Monitoring. In places like Yosemite, measuring how cracks behind exfoliation sheets move with temperature helps scientists understand when rockfalls are more likely.

Summary of causes and responses

Setting	Main driver	Typical response
Reinforced concrete in service	Corrosion of steel from chlorides or loss of alkalinity	More cover, low permeability concrete, sealers, repair of corroded areas
Concrete in cold climates	Freeze-thaw and de-icing salts	Durable mix design, drainage, surface protection
Concrete in tunnel fires	Pore pressure and thermal stress	Polypropylene fibres, fire boards or sprays, mixes proven by fire testing
Refractory linings	Rapid heating and water removal	Controlled heating and drying, suitable materials
Armoured vehicles	Stress waves from HESH or penetrators	Spall liners of aramid, glass fibre or polyethylene
Rolling bearings	Subsurface or surface-initiated fatigue	Correct loading, lubrication, cleanliness, condition monitoring
Pumps, turbines, propellers	Cavitation bubble collapse	Hydraulic design to avoid low pressure, resistant materials, weld repair
Natural rock	Unloading, heating cycles, frost, salt	Monitoring, hazard assessment
Reactive metals	Expansive, non-adherent oxide	Handling in inert atmosphere

Common misconceptions

"Spalling means the material was weak"

Not necessarily. High-strength concrete is more vulnerable to explosive fire spalling than ordinary concrete because its low permeability traps steam. Hardened bearing steel spalls because rolling contacts concentrate enormous stress in tiny areas.

"Armour that isn't penetrated has done its job"

HESH rounds were designed to injure crews by spalling the inside of armour without penetrating it. Spall liners exist because stopping the projectile is not enough.

"Concrete spalling is always caused by rust"

Corrosion is the most common cause in everyday structures, but freeze-thaw, salt crystallisation, fire, impact and overloading can all cause spalling. The repair depends on the cause.

"Nuclear spallation is the same thing as concrete spalling"

They share a name and a loose metaphor of chipping pieces off something larger. Mechanically, they are completely different. One is fracture of a solid; the other is a nuclear reaction.

"Rockfalls always have an obvious trigger"

Collins and Stock showed that ordinary daily and seasonal heating can progressively open fractures and trigger rockfalls without rain, earthquakes or frost.

"Polypropylene fibres make concrete fireproof"

Fibres reduce the risk of explosive spalling, but they do not stop concrete from losing strength at high temperature, and BRE's Tom Lennon cautioned that they are not a complete solution on their own.

Frequently asked questions

What is spall?

Spall is a fragment of material that has broken away from the surface of a larger solid. Spalling is the process.

Why does concrete spall in fires?

Rapid heating creates thermal stress near the surface and turns water in the pores into steam. In dense concrete the steam cannot escape, so pressure and stress build until the surface layer breaks off, sometimes explosively.

How bad was the spalling in the Channel Tunnel fire?

Over a 50 metre length, the 40 centimetre lining was reduced to an average of 17 centimetres and in one place to 2 centimetres. Damage reached the first layer of reinforcement over 240 metres.

What causes spalling on old concrete balconies, bridges and car parks?

Most often, corrosion of reinforcing steel caused by chlorides from salt or by loss of alkalinity near the surface. Rust expands, cracks the concrete and pushes the cover off. Freeze-thaw damage is also common in cold climates. A qualified engineer should assess spalling on any structure people use, because falling concrete can injure people and spalling can indicate loss of reinforcement.

How do polypropylene fibres reduce spalling?

The usual explanation is that they melt in a fire and leave channels that let steam escape, relieving pore pressure.

What does spalling look like in a bearing?

Pits or craters on the raceway or rolling elements where flakes of metal have broken away. It usually starts small and grows if the bearing keeps running.

What is a spall liner?

A layer of tough material, often aramid fibre such as Kevlar, fixed to the inside of armour to catch fragments knocked off the inner face.

What is HESH?

High-explosive squash head is a type of shell that spreads against the outside of armour and detonates, sending a stress wave that spalls metal off the inside.

Is spallation in particle physics dangerous?

The process itself is a nuclear reaction inside a shielded target at an accelerator facility. It has nothing to do with mechanical spall, and it does not involve a chain reaction, which is one reason Wikipedia describes it as non-critical.

Can rock spall without frost?

Yes. Heating and cooling cycles, unloading after erosion, salt crystallisation and fire can all cause rock to spall.

Summary

Spalling is what happens when a solid is pushed harder just beneath its surface than its outer layer can withstand. Impact waves reflecting from a free face, steam and thermal stress in burning concrete, rust swelling around steel, ice and salt growing in pores, fatigue under rolling contacts, and the slow breathing of granite in the sun all lead to the same outcome: a piece breaks off. Understanding which mechanism is at work matters, because each has its own prevention. Engineers add cover and lower permeability against corrosion, fibres and fire protection against explosive spalling, liners against armour spall, and clean lubrication against bearing fatigue. The 1996 Channel Tunnel fire, where parts of a 40 centimetre lining were reduced to a few centimetres, showed how quickly fire spalling can remove a structure's protective material. And in nuclear physics, the same word has been borrowed for something quite different, the breaking apart of atomic nuclei to produce neutrons for research.