Spalling is a phenomenon that occurs when concrete is exposed to fire. It refers to the breaking off of layers or pieces of concrete from the surface due to exposure to high temperatures. Understanding spalling is critical for engineers designing concrete buildings, as it can compromise the strength and integrity of structural members during a fire.
What causes spalling?
When concrete is heated, the water inside the concrete turns to vapor and tries to escape. This buildup of vapor pressure causes stresses inside the concrete, which can cause cracks, pop-outs, and spalling on the surface. The main factors that influence spalling are:
- Moisture content – Higher moisture levels increase spalling.
- Permeability – Low permeability traps moisture and increases spalling.
- Density – Higher density concretes are more prone to spalling.
- Aggregate type – Certain aggregates like siliceous aggregates are more susceptible.
- Heating rate – Faster heating causes greater spalling.
Additionally, poor concrete consolidation and curing can increase the risk of spalling. The depth and severity of spalling is also affected by the maximum temperature and duration of exposure.
Types of spalling
There are three main types of spalling that can occur:
Surface spalling
This involves small chips and layers coming off the surface, up to 10-20mm deep. It occurs due to buildup of pore pressure near the surface. Surface spalling does not significantly affect the strength of the member, but it can expose inner steel reinforcement to fire.
Corner spalling
On concrete members with sharp corners, the water vapor tends to concentrate in these areas as it tries to escape. This leads to severe spalling at the corners, often exposing a large area of reinforcement. Corner spalling can severely compromise strength.
Explosive spalling
This type of spalling occurs violently and suddenly, ejecting pieces of concrete at high velocities. It happens when pore pressures build up significantly inside the concrete due to rapid heating and low permeability. Explosive spalling can be very dangerous and Lead to immediate structural failure.
Factors affecting spalling
Many different factors related to the concrete mix design, member geometry, and heating conditions influence the likelihood and severity of spalling:
Concrete properties
- Moisture content – Wet concrete spalls more easily. Concrete dried to less than 3% moisture is unlikely to spall.
- Density – Higher density increases spalling potential.
- Permeability – Low permeability traps moisture and increases spalling.
- Aggregate type – Siliceous aggregates like quartz are more prone to spalling.
- Strength – High-strength concrete is somewhat more susceptible.
- Age – Older concrete is less prone to spalling.
Member geometry
- Section thickness – Thinner sections spall more easily.
- Reinforcement – More reinforcement limits spalling but increases risk of exposure.
- Corners – Sharp corners increase risk of corner spalling.
Heating conditions
- Heating rate – Faster heating increases spalling.
- Maximum temperature – Higher temperatures mean more severe spalling.
- Fire duration – Longer exposure worsens spalling.
Preventing spalling
Since spalling can compromise concrete strength and fire resistance, it is important to take measures to prevent it. Some techniques include:
- Using concrete with low moisture content (dry concrete).
- Limiting concrete density by using lightweight aggregates.
- Increasing permeability with larger aggregates.
- Using polypropylene or steel fibers to improve tensile strength.
- Adding mineral admixtures like silica fume or metakaolin.
- Applying protective coatings or polyurethane foam to insulate the concrete.
- Using thin-walled concrete filled steel tube (CFT) sections.
Proper concrete curing is also critical to reduce spalling potential. Pre-wetting concrete before fire exposure has also been found to be beneficial.
Assessing spalling risk
To assess the risk of spalling for a particular concrete member, engineers can conduct tests to quantify spalling behavior:
RILEM beam test
A small concrete beam is heated and the amount of spalled concrete that falls off is measured. This quantifies the spalling severity.
U-shaped heating test
A U-shaped concrete specimen is heated on three sides and the depth of spalling is measured, indicating moisture and pore pressure distribution.
Explosive spalling test
Cylindrical concrete samples are rapidly heated with propane burners and onset temperature for explosive spalling is identified.
Acoustic emission monitoring
Acoustic sensors on heated concrete can detect sounds from vapor pressure buildup. The measurements indicate likelihood of spalling.
Engineers use the results of such tests to design appropriate spalling prevention measures for the concrete mix and members.
Effects of spalling
Some of the major effects and risks associated with spalling during fires are:
- Loss of concrete cover – exposes internal steel reinforcement to fire.
- Reduced load capacity – loss of concrete reduces strength.
- Buckling risk – for slender columns due to loss of concrete.
- Brittle failures – spalling can occur suddenly without warning.
- Falling debris – risk to people and obstruction for firefighters.
- Fire spread – falling pieces may spread fire to lower floors.
Spalling on concrete ceilings can also cause loss of compartmentalization allowing fire to spread. Overall, spalling can cause under-capacity structural failure and collapse before required fire resistance time.
Modeling spalling in fire structural analysis
Advanced finite element models for performance-based structural fire engineering aim to capture the effects of spalling. Some approaches include:
- Applying a temperature-dependent concrete decay function to reduce strength and stiffness properties.
- Modeling moisture evaporation and pore pressure buildup in concrete.
- Simulating cracking and progressive spalling with fracture mechanics.
- Applying spatial temperature gradients across section depth.
However, accurately modeling spalling behavior is challenging due to the complex interaction of factors involved. Simplified analysis methods often just apply an assumed concrete damage depth over time.
Design methods to prevent spalling failure
Engineers use various design strategies to ensure structures have adequate fire resistance and avoid spalling-induced failures. These include:
- Prescriptive methods – Provide minimum member dimensions and concrete cover according to building codes.
- Testing – Evaluate fire endurance of concrete members in standard furnace tests.
- Calculation – Perform finite element heat transfer analysis to calculate temperature profiles.
- Addition of polypropylene fibers – Typical dosage is 3-5 kg/m3.
- Reduction of water/cement ratio – Target below 0.45 to limit moisture content.
- Vermiculite concrete – Replaces normal aggregates with vermiculite to reduce spalling.
Following relevant design standards like Eurocode 2 can help minimize spalling risks. However, performance-based approaches provide the most flexibility to optimize fire-safe concrete building designs.
Case studies of spalling failures
Some notable fire incidents where concrete spalling contributed to collapse include:
Windsor Tower fire, Madrid 2005
This high-rise office building had a severe fire on upper floors that caused major spalling of the concrete columns. The structure partially collapsed progressively over several hours.
Faculty building, Delft University 2008
A fire in this building led to spalling of precast concrete floor slabs, which then collapsed under high temperatures. The complex slab interactions were not properly considered in fire design.
Plasco Building, Tehran 2017
The structure of this high-rise was steel framing encased in concrete. A fire caused spalling of the concrete encasement leading to buckling collapse of steel columns on upper floors.
These examples demonstrate the risks of inadequate spalling prevention and the need for performance-based approaches to properly ensure fire-resistant concrete structures.
Conclusion
Spalling of concrete under fire exposure is a complex phenomenon but a critical one for structural fire engineering. Preventing explosive spalling and the associated loss of strength, fire spread risks, and potential structural collapse requires careful material selection, testing, design, and modeling. Ongoing research on quantifying spalling behavior will enable better assessment and mitigation of these risks.