Spalling

Spalling is the breaking away of layers or pieces of concrete from the surface of a concrete element that is exposed to high and rapidly rising temperatures, as in a fire. It can happen soon after the start of heating and can be violent and explosive. It can also happen later, when heating has weakened the concrete so much that pieces fall off as cracks develop. Spalling matters because most design methods for concrete in fire assume that the concrete stays in place. When the cover concrete falls away, the reinforcing steel is exposed to the fire and heats much faster than the design expects.

Spalling of the cover concrete exposes the reinforcement to the fire, so that it heats much faster than assumed in design. Based on Kodur and Harmathy (2002), pp. 1-163–1-164, and Fleischmann and Buchanan (2002), p. 4-255.

Two mechanisms are usually put forward. Water vapour trapped in the pores can build up a pressure that exceeds the tensile strength of the concrete. Restrained thermal expansion near the hot surface can also cause compressive stresses that are released by brittle fracture.

Normal-weight concrete members usually withstand severe fires without spalling, but spalling does sometimes occur. High-strength concrete, which is denser and less permeable, is more susceptible and often spalls explosively in tests. The best-known remedy is to add fine polypropylene fibres to the mix. These melt in a fire and leave channels through which the steam can escape.

Forms of spalling

The SFPE Handbook distinguishes two ways in which spalling develops. In the first, pieces break away soon after exposure to heat, sometimes accompanied by violent explosions. In the second, the concrete has become so weak after heating that pieces fall off the surface once cracks form. A glossary entry quoted in NUREG-1805 describes spalling in general terms as the breaking away or explosion of concrete under fire exposure. The same glossary links thermal expansion to surface chipping and scaling.

Spalling is not limited to structural concrete. It is also known to occur in fire-resistant protective coatings for steel, where asbestos or other fibre fillers are added to concrete or other cementitious materials to improve insulation. NUREG-1805 notes that cementitious sprayed coatings can spall during a fire and have had adhesion problems in practice. Effective application, complete coverage and long-term maintenance therefore have to be checked.

The extent of the damage varies widely. The consequences may be limited as long as only small areas spall. Extensive spalling, however, can lead to early loss of stability and integrity, because the reinforcement is exposed and partitions can be penetrated. The glossary lists four consequences: exposure of structural steel, reinforcement or prestressing cables, more heat transmission through a thinner section, fire passage through openings and reduced bending capacity.

Causes

The most common explanation is pore pressure. When concrete is heated, water in the cement paste turns into vapour. In dense concrete this vapour cannot escape fast enough, and the pressure rises towards the saturation vapour pressure. At 300 °C this pressure reaches about 8 MPa, while high-strength concrete has a tensile strength of only about 5 MPa. Because the heated surface is drained and the concrete has a low permeability, strong pressure gradients develop close to the surface, in what is called a moisture clog. Pieces break off when the vapour pressure exceeds the tensile strength.

Fleischmann and Buchanan state that it is generally agreed that spalling most often occurs when water vapour is driven from the cement paste. The high pore pressures then create high tensile stresses in the concrete. In some cases spalling is related to the type of aggregate, but it is more often linked to the behaviour of the cement paste. The pore pressure is considered to drive a progressive failure, and the SFPE Handbook links greater spalling to a lower permeability of the concrete.

Other researchers explain spalling with fracture mechanics. Thermal expansion close to the heated surface is restrained by the cooler concrete behind it. This creates compressive stresses parallel to the surface, which are released by brittle fracture. The NUREG-1805 glossary combines both views. It names stresses from a steep temperature gradient, expanding aggregate and trapped moisture that vaporises without a safe means of venting.

Two explanations of spalling: pore pressure from trapped water vapour (left) and restrained thermal expansion near the heated surface (right). Based on Kodur and Harmathy (2002), p. 1-163.

Factors that increase the risk

Several conditions make spalling more likely. Fleischmann and Buchanan list a high moisture content, as in fresh concrete, rapid rates of heating, slender members and high stresses at the time of the fire. The International Fire Engineering Guidelines (2005) note that spalling is more common in new concrete with a high moisture content. The NUREG-1805 glossary adds permeability, porosity, restraint and reinforcement to moisture content and the rate of temperature rise.

High-strength concrete is especially sensitive. It contains additives such as silica fume and water-reducing admixtures and reaches compressive strengths of 50 to 120 MPa. Its smaller free-pore volume fills with high-pressure vapour more quickly, and its low porosity slows the escape of that vapour. For high-strength concrete, the SFPE Handbook names strength, porosity, density, load intensity, fire intensity, aggregate type and relative humidity as the main parameters. In full-scale fire tests on loaded columns, spalling was quite significant in a high-strength concrete column compared with a normal-strength one. High-strength concrete also tends to lose strength faster than normal concrete at temperatures up to 400 °C.

The rate of heating links spalling to the choice of design fire. Introduction to Fire Safety Engineering notes that the RWS fire curve for tunnels and the hydrocarbon fire curve for oil and gas fires are more intense than the standard fire curve. Because a rapid rate of heating is one of the listed risk factors, spalling is a particular concern under these more intense exposures. Tunnels are also among the structures for which the book expects a fire safety engineering approach instead of prescriptive rules. The wider hazards of a tunnel fire are treated in a separate article.

Heating depth and structural effects

In a fire, the heat stays near the surface of a concrete element for a long time. Introduction to Fire Safety Engineering gives a rule of thumb for the depth to which heat penetrates in a given time. If an element is thicker than about twice the characteristic penetration depth, it behaves as thermally thick and its back stays relatively cool.

$$L\approx 2\sqrt{\alpha t}$$

Symbol	Description	Unit
$L$	indicative boundary thickness between thermally thick and thermally thin behaviour	m
$\alpha$	thermal diffusivity of the material	m2/s
$t$	duration of heating	s

The book stresses that this is a practical rule and not a sharp physical boundary, because a heat front has no abrupt edge. In its worked example, concrete with a diffusivity of 9.88 × 10⁻⁷ m²/s gives a depth of about 0.060 m after 15 minutes. A wall of 200 mm therefore behaves as thermally thick during the first quarter of an hour. With the same formula the depth grows to about 84 mm after 30 minutes and about 120 mm after 60 minutes. The heating, and with it the steep temperature and pressure gradients, are thus concentrated in the outer few centimetres. This outer zone includes the cover concrete that protects the reinforcement.

Indicative heat penetration depth L ≈ 2√(αt) for concrete (α = 9.88 × 10⁻⁷ m²/s). Based on Van Rossum (2027), §5.2 and §5.5.

When spalling removes this cover, deeper layers of concrete are then exposed to fire temperatures and heat reaches the reinforcement much faster. When the reinforcement is exposed directly, its temperature rises at a very high rate and the member loses strength quickly. The loss of strength in the steel, added to the loss of concrete, significantly reduces the fire resistance of the member. The strength reduction at elevated temperatures of reinforcing and prestressing steel is therefore the main reason why spalling can lead to structural failure.

The concrete itself also weakens. Concrete with siliceous aggregate keeps nearly its full compressive strength up to about 500 °C, but has lost about half of it at 650 °C. Prestressed concrete is particularly sensitive, because failure modes such as debonding, shear and spalling are more critical there.

Observations in fire tests

The fire resistance tests on World Trade Center floor assemblies, carried out to ASTM E119 in 2004, give a documented example. During the tests, loud reports were heard, often together with movement of the steel deck and dislodging of the sprayed fire protection. The investigators assumed that these signalled explosive spalling, although its exact location and extent could not be determined. The relative humidity of the concrete slabs met the requirements of the standard. All four tests produced extensive spalling on the underside of the floor slab.

The underside spalling began early. In the first three tests, the metal deck under the slab started to bulge within the first 15 minutes because concrete was spalling above it. Sections cut through the slabs after the tests confirmed delamination. The depth varied, but the remaining slab was about 2 to 3 in thick, essentially up to the double layer of welded wire fabric. In the first test the depth of the delamination spalling was of the order of 2 in, about 50 mm. The temperatures on the unexposed surface of all four assemblies were similar until significant spalling began at around 50 minutes.

Restraint made a clear difference on the top surface. The three restrained specimens showed significant cracking and explosive spalling at the top of the slab, while the unrestrained specimen of the same span showed no explosive spalling there. In the restrained 17 ft specimen with 1/2 in of fire protection, concrete fragments flew through the air and left a hole about 2 ft long through which hot gases escaped. This happened at about 55 minutes, and the opening had to be covered with ceramic fibre insulation so that the test could continue. Such a hole is the kind of integrity failure that extensive spalling can cause.

The investigators concluded that the different results of the two 17 ft specimens were due mainly to the performance of the concrete slab, through spalling and cracking, and to fire protection overspray on the deck. The thickness of the fire protection on the trusses was not the main cause. Differences in spalling were possibly due to differences in moisture content and slab cracking. The later computer models of the towers did not include concrete spalling.

Prevention and design

Polypropylene fibres are the most widely recommended measure. The fibres melt at a relatively low temperature of about 170 °C and create channels through which the steam can escape. This prevents the small explosions that cause spalling. The SFPE Handbook gives a dosage of about 0.1 to 0.25 % by volume, most effective for high-strength concrete with normal-weight aggregate. The International Fire Engineering Guidelines (2005) give 0.15 to 0.3 % and call the fibres the most economic way to reduce the likelihood of spalling.

Polypropylene fibres melt at about 170 °C and leave channels through which water vapour can escape. Based on Kodur and Harmathy (2002), p. 1-171, and National Research Council of Canada et al. (2005), p. 2.6-9.

Calculation methods need care when spalling is possible. The design methods in the concrete chapter of the SFPE Handbook assume that the concrete remains intact for the whole fire. This assumption is invalid if the cover spalls off during the fire. For high-strength concrete, Tomasson suggests a simple method that ignores concrete hotter than 500 °C in slabs and beams and hotter than 400 °C in columns. The International Fire Engineering Guidelines (2005) also note that the cross-section of a member in fire may be smaller because of charring or spalling.

Tests therefore remain important. The Australian Fire Safety Verification Method Data Sheets name materials prone to spalling among the components that are too complex to model from material properties alone without full-scale supporting data. A thermal-only method for converting fire resistance times does not directly account for spalling, shrinkage or thermal shock. This limitation is accepted only where the construction is not particularly sensitive to the heating rate.

See also

• Fire resistance criteria
• Standard fire curve
• Structural fire engineering
• Fire resistance
• Restrained thermal expansion

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Fleischmann, C., & Buchanan, A. (2002). Analytical methods for determining fire resistance of concrete members. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-239–4-256). National Fire Protection Association; Society of Fire Protection Engineers.
• Gross, J., Hervey, F., Izydorek, M., Mammoser, J., & Treadway, J. (2005). Fire resistance tests of floor truss systems (NIST NCSTAR 1-6B). National Institute of Standards and Technology.
• Gross, J. L., & McAllister, T. P. (2005). Structural fire response and probable collapse sequence of the World Trade Center towers (NIST NCSTAR 1-6). National Institute of Standards and Technology.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-155–1-181). National Fire Protection Association; Society of Fire Protection Engineers.
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.