Explosive Spalling of Concrete in Fire:
Novel TestING to MITIGATE DESIGN RISK
Ieuan Rickard and Luke Bisby, The University of Edinburgh,
UK
Susan Deeny, Arup, UK
SYNOPSIS
Heat-induced explosive spalling in fire poses a credible risk to
concrete structures, and has received considerable research
attention in recent decades. However, no validated guidance to
enable the design of concrete mixes to prevent spalling, nor any
established, widely verified, repeatable test methods are yet
available to confidently quantify or demonstrate spalling
resistance for a particular mix in a given application. As a result
no models yet exist that can predict spalling with sufficient
confidence to be used in design. This paper summarises contemporary
research on heat-induced concrete spalling, with particular
emphasis on design for fire of concrete-lined tunnels. The topic is
also highly relevant for modern concrete buildings. A novel,
repeatable, and economical testing method to reduce project risk by
quantifying the propensity of concrete mixes for spalling under a
range of different thermal and mechanical conditions is described.
The intent of this paper is to present the limitations of knowledge
to enable design for heat induced spalling, and to highlight
research currently underway to overcome some of the issues faced in
practice.
EXPLOSIVE SPALLING OF CONCRETE
Concrete structures have historically performed very well in
building fires1,2. Concrete is non-combustible and has a relatively
low thermal conductivity and diffusivity, so that in a fire,
provided the concrete cover to the internal reinforcement remains
in place, heat flow to the reinforcement and the inner core occurs
slowly so as to yield the necessary fire resistance. Traditional
‘fire resistance’ design of concrete structural elements is usually
accomplished by prescribing minimum overall member dimensions and
minimum concrete cover to the reinforcement. The available evidence
from real fires suggests that these simple, prescriptive approaches
have historically yielded an acceptable level of fire performance
in concrete buildings2,3 and that heat-induced concrete spalling
has not been a serious concern. However, contemporary concrete
mixes, particularly those with higher strength and/or reduced
permeability, appear more prone to heat-induced spalling, and this
has been shown by both experiences from real fires and recent
research on heat-induced spalling. This raises concerns that
heat-induced spalling may become a more prominent issue in future
fires and therefore that the potential consequences and the use of
assessment and mitigation methods ought to be considered by
structural engineers when undertaking structural fire design of
modern concrete structures.
Spalling refers to the breaking away of pieces from the surface
of concrete elements when exposed to heat1,2; it can take several
different forms and may significantly affect the load carrying
capacity of a concrete structure. This is due to reductions in
cross section, changes in the load distribution, and loss of
thermal protection to internal reinforcement. It is an extremely
complex phenomenon involving time and temperature dependent
mechanical stresses, temperature gradients, differential thermal
stresses, moisture movement, and microstructural and chemical
changes with increasing temperature4. Figure 1 below shows two
spalled samples after testing.
Figure 1: A) Extreme case of explosive spalling of ultra high
performance concrete, and B) Spalling of a concrete slab surface
exposing internal reinforcement during a standard furnace test
The available research suggests that heat-induced spalling
exhibits a stochastic nature, and experimental results are
sometimes contradictory. Whether this is due to genuine randomness
or to insufficiently controlled or instrumented testing
environments is a matter of debate2. This uncertainty hinders
design and often results in conservative and/or semi-arbitrary
mitigation measures being used. However, general trends in factors
increasing the risk of spalling can be highlighted. Spalling risk
tends to increase with increases in compressive strength, rate of
heating, moisture content, mechanical restraint, and imposed
compressive load1,2,5. An exhaustive list of factors known to
influence spalling is given by Maluk6, Bailey and Khoury1 provide a
highly accessible summary of the available knowledge on
heat-induced spalling of concrete, and a detailed and comprehensive
summary of the available literature is given by Jansson5.
BACKGROUND
The current paper is interested in a particular manifestation of
heat-induced spalling often referred to as ‘explosive spalling’;
this is violent and typically (but not always) occurs when concrete
elements experience steep in-depth thermal gradients, as in the
early stages of a severe fire. A number of notable examples of real
fires in which explosive concrete spalling has occurred are
available in the literature, including a number of tunnel fires4,7,
fires on or under bridges8,9, and building fires5.
Two key mechanisms are now acknowledged as contributing to
explosive spalling. The first mechanism is related to the
development of differential thermal stresses in the concrete
induced by thermal gradients, differential thermal expansion, and
induced restrained deformations that develop during heating10; the
second mechanism attributes spalling to a build-up of pore pressure
due to evaporation, along with transport of moisture within the
concrete microstructure11,12. Harmathy11 originally hypothesized
that the pore pressure in heated concrete is increased due to the
formation of a liquid water ‘moisture clog’ that results from
moisture transport and condensation in the cooler zones within the
concrete core. More recently other mechanisms have also been
proposed5.
Regardless of the mechanisms responsible, fire induced explosive
spalling erodes cross-sectional dimensions and removes the concrete
cover, thus exposing internal reinforcement to more severe heating
and reducing its strength and stiffness. This could ultimately lead
to structural damage, loss of structural stability, or loss of
water-tightness. The potential costs of spalling in buildings are
hard to quantify, however in tunnels the costs associated with
repairs and loss of revenue during rehabilitation are likely to be
considerable. For example, the direct repair costs following the
2008 Eurotunnel fire were estimated at £46 million, and given that
the tunnel facilitates £91.4bn of trade annually, downtime as a
result of fire could be as high as £250m per day, not accounting
for indirect losses13. There are no obvious documented cases of
large-scale structural failures or collapses that can be attributed
directly to explosive spalling, and the consequences of spalling
for structural performance of concrete structures admittedly
remains a topic of on-going research14.
The importance of preventing, or properly accounting for,
explosive spalling during design of concrete structures is related
to at least three design considerations:
1. Life safety – Explosive spalling reduces structural capacity,
and hence ‘fire resistance’, and could result in life-threatening
structural collapses, exacerbated fire spread, or loss of
water-tightness (particularly important in submerged tunnels).
2. Asset protection and financial losses – Explosive spalling
causes damage to concrete elements, raising concerns for both
direct and indirect economic losses, increased post-fire downtime
and repair, asset protection, business continuity, and so on.
3. Project risk – The predominant approach to actually assess
spalling risk for concrete mixes used in real projects (at least in
tunnelling projects where spalling mitigation often governs
concrete mix design) is by full-scale furnace testing of
representative structural elements. This approach presents serious
technical challenges, and generates project risk as a concrete may
fail at a late stage of design. It is generally not feasible to run
multiple tests, and in tunnel design and construction testing is
often performed late in the design stage after the concrete mix and
tunnel lining thickness have been fixed. Test failure would result
in redesign and project delays, thus reducing design confidence in
the early stages. Reliance on small sets of tests also places
restrictions on the Contractor, limiting their ability to change
concrete mix designs in response to supply, constructability, or
economic factors occurring after testing4. Project risk due to heat
induced spalling of concrete is usually less critical in other
areas of construction, where spalling is rarely explicitly
considered and ‘fire resistance’ is assured by relying on
historical evidence for furnace tests and real building fires.
Whether structural engineers ought to be more reflexive as regards
the potential consequences of concrete spalling in modern building
projects is open to debate.
Addressing the above design issues for modern concrete mixes
requires the ability to quantitatively predict explosive spalling –
such that the depth of spalling to be expected during a credible
design fire can be approximated and accounted for during the
structural design process – or prevent spalling – such that design
can performed confidently neglecting it’s influence. Whilst
considerable progress has been made on both fronts in recent
decades, both remain difficult challenges and are the focus of a
great deal of research internationally15.
AVAILABLE RESEARCH ON SPALLING
The available research on explosive spalling of concrete can be
classified into three broad categories, as outlined and discussed
in the following sections.
(1) Standard Furnace Testing
The ‘fire resistance’ of structural elements (of all material
types) is assessed in practice using large-scale ‘fire resistance’
tests (Figure 2). These tests subject loaded, representative, and
typically full-scale (or as close to full-scale as possible)
structural elements to a standard gas temperature versus time curve
within a fire testing furnace. Explosive spalling has been directly
observed for a large number concrete elements tested in this
manner, for more than a century16, however this spalling has
historically been mild and not thought to be of critical importance
for the overall fire resistance of concrete structures. In recent
decades, however, with the advent of engineered high-strength,
high-performance, and self-consolidating concrete mixes with
increased propensity for explosive spalling, spalling has become a
more common failure mode for concrete elements tested in
furnaces17.
Since explosive spalling depends not only on material parameters
but also on structural parameters, it is necessary to repeatably
and accurately reproduce realistic conditions (e.g. geometry,
boundary conditions, applied mechanical and thermal loads, etc)
when experimentally studying spalling of different concrete mixes.
Such control over testing parameters is challenging when testing in
furnaces, particularly during the early stages of heating which are
critical for influencing explosive spalling. Furnace tests
therefore have limited applicability for quantifying spalling risk,
although in some cases the time to spalling, final depth of
spalling, and volume of spalled material are noted in furnace
testing reports. Such testing is typically used to support design
assumptions of: (1) no spalling under a ‘standard fire’ heating
scenario (i.e. where none is observed in furnace testing); or (2) a
certain depth of spalling (sometimes assumed as the final spalling
depth observed during standard furnace testing). However, given the
known variability of spalling for a single mix under slightly
different thermal and mechanical conditions, neither assumption can
usually be confidently defended based on the available experimental
evidence.
Figure 2: A large number of samples positioned on a standard
fire testing furnace ready for spalling testing by standard fire
exposure from below
The reliance on full scale furnace testing is particularly
problematic for assessing the spalling risk of concrete tunnels or
tunnel lining segments. A number of furnace test procedures have
been applied to assess spalling of concrete for tunnelling
applications; however there has been little harmonization of these
from either a thermal or a mechanical perspective. The majority of
these test large-scale tunnel structure or lining samples exposed
to a ‘tunnel fire’ heating regime18,19 based on data from tunnel
fire tests that are assumed to adequately represent the thermal
environment during a severe fire within a tunnel.
Furnace testing of structural elements other than tunnel
segments is also challenging as it is often only possible to test
single elements, and loading/restraint cases may be hard to
replicate. Demonstration by testing that a concrete mix will
perform adequately is not as commonplace elsewhere in structural
engineering as in tunnel engineering, but may be advisable where
modern, high-strength concrete mixes are being used or where
spalling could be a critical to meet functional performance
objectives. This is the case, for example when applying slender
ultra high performance concrete slabs, pre-stressed with carbon
fibre reinforced polymers, in building applications20.
(2) Spalling Experiments
Owing to the numerous challenges associated with large-scale
furnace testing of concrete elements to assess their propensity for
spalling, a wide range of scientifically-based test methods and
research projects are presented in the technical literature5,21. A
detailed summary is avoided here, but it is noteworthy that
researchers have used a range of sample sizes (from small cubes of
only a few centimetres up to full-scale structural elements on the
scale of metres), heating conditions (including furnace
environments, electrical ovens, open pool and bonfires, direct
flame impingement, cone heaters, and electrical, optical, and
propane-fired radiant panels), and mechanical restraint and loading
conditions (including unloaded, passively restrained, and actively
restrained samples under uniaxial or biaxial, steady-state or time
varying stress states).
A shortcoming of the available data from such experiments is
that often insufficient care is taken when controlling the relevant
material, thermal, and mechanical parameters that may affect
spalling. For instance, samples have been exposed to heating
scenarios which are poorly controlled or potentially non-uniform.
Other testing has paid little attention to either mechanical or
differential thermal stress development in the samples during
heating6. As a result, there is currently no agreement or
harmonization within the spalling research community regarding the
appropriate small-scale experimental methods that should be used to
quantify spalling risk or the respective influences of parameters
influencing spalling. It is therefore difficult to compare results
from different authors. The RILEM Technical Committee on Spalling,
256-SPF, is currently seeking to address this by developing an
internationally harmonised testing method to assess spalling
risk.
(3) Predictive Numerical Modelling
A number of related coupled thermo-hygro-chemo-mechanical codes
of varying complexity are available in the literature; these
generally attempt to predict spalling time and depth by simulating
the stress state in heated concrete resulting from the relevant
thermo-hygral and thermo-mechanical processes that take place
during heating10,22. However, even the most developed of these
advanced models are, at present, unable to accurately predict
spalling for a given concrete mix in a particular application and
subjected to either standard or project-specific design fires. A
number of simplified spalling models (or spalling criteria) are
also available in the literature – for instance assuming that only
pore pressures are relevant23 or that spalling will occur at a
particular temperature in the concrete – however neither of these
approaches is easily defended based on the available experimental
evidence.
STATE-OF-PLAY
The situation described above has resulted in a state-of-play
where:
1. the mechanisms of explosive spalling, and the respective
influences of the various factors that are thought to exacerbate
explosive spalling, are not fully understood and cannot be
quantified;
2. confident prediction and/or prevention of explosive spalling,
under the full range of relevant thermal or mechanical actions for
a given mix in a given application, is not currently possible;
3. a lack of harmonization of testing methodologies has resulted
in an ad-hoc, project-specific approach to testing and
experimentation, thus somewhat muddying the waters of scientific
advancement on this topic; and
4. the potential consequences of spalling for the response of
concrete structures in fire have not been quantified in most cases
and in many applications (aside from tunnelling and nuclear
engineering) spalling is rarely explicitly considered by designers
(although again RILEM Committee 256-SPF is studying this
issue).
DESIGN APPROACHES TO ACCOUNT FOR SPALLING
Given the above state-of-play, structural designers currently
apply one of four approaches to addressing the issue of spalling in
the structural design process:
1. Assume no spalling – In most cases (aside from in tunnel
design, see below) designers assume that spalling is sufficiently
unlikely to occur, or that its consequences will be sufficiently
benign, that they need not explicitly consider it in design; this
is the approach suggested in Eurocode 224 for concrete with
compressive strength less than 80 MPa and moisture content less
than about 3%. Such an approach is only partly supported by the
available experimental data.
2. Mitigate spalling – Eurocode 224 also provides a range of
means to reduce the risk of explosive spalling. For concrete with a
strength greater than 55 MPa and with more than 6% silica fume (by
weight of cement), or for concrete with a strength greater than 80
MPa, four specific options are given: (1) adding steel
reinforcement mesh within the concrete cover at a depth of 15 mm,
(2) demonstration ‘by experience or testing’ that no spalling
occurs under fire exposure, (3) addition of supplemental fire
protection (see Point 3 below), or (4) addition of 2 kg/m3 of
polypropylene (PP) anti-spalling fibres. Polypropylene fibres have
been shown1,2,5 to reduce the risk of explosive spalling, although
the mechanism by which this is achieved remains a matter of debate.
Eurocode 2 fails to make clear, however, that the current
understanding of the phenomena of spalling is insufficient to
enable a genuine risk-based approach to design for spalling. It is
not currently possible to understand the risk of spalling for
untested concrete mixes, nor to generate specific guidance on what
would constitute acceptable spalling for the wide range of elements
and load cases that are possible.
3. Apply supplemental thermal insulation – Addition of
supplemental thermal insulation (e.g. rigid board systems or spray
applied coatings) is a common approach to protect concrete from
increased temperatures and avoid damage altogether, as well as
mitigate spalling, particularly in concrete-lined tunnels25. Such
solutions are indeed supported by the available experimental
evidence, but are likely to be conservative, to result in
significant cost increases, and will require ongoing inspection and
maintenance for the lifetime of the structure. This approach is
rarely used in normal buildings, aside from parking garages.
4. Assume a sacrificial spalling depth – In some concrete
tunnelling projects designers have assumed a sacrificial spalling
depth, which is then accounted for in subsequent heat transfer and
structural design calculations. The assumed spalling depth is
typically ‘verified’ using large-scale furnace tests on
representative structural elements. Given the stochastic nature of
spalling and its demonstrated sensitivity to thermal and mechanical
testing conditions, this approach must be considered as having
considerable uncertainty, and also being potentially
over-conservative.
It is noteworthy that if designers are sufficiently confident
that explosive spalling will not occur in a credible design fire
(using approaches 1, 2, or 3 above, for example), then fire
engineering design of concrete structures becomes possible, since
performing accurate heat transfer calculations within non-spalling
concrete is relatively straightforward with available models. If,
however, designers cannot confidently prevent or predict spalling,
then structural design calculations for concrete structures in fire
are both uncertain and rather difficult to defend. The alternative
is to take conservative and/or arbitrary actions that may result in
additional costs (i.e. the costs of supplementary fire insulation
or sacrificial concrete cover).
Novel TestiNG to MITIGATE DESIGN RISK
Current approaches to heat-induced explosive spalling design can
be described as ‘design by experiment’4 and/or use uncertain
measures to reduce risk. ‘Design by experiment’ currently relies on
large scale furnace tests which are costly, and variable. A
reliable and verifiable design tool is not feasible at present, so
this cannot be an immediate solution.
The spalling process is driven by a range of influencing
parameters and complex thermal, mechanical, physical, and chemical
processes. Some researchers6 have suggested that what is needed is
a means of experimentally characterising and quantifying the
propensity for spalling of different concrete mixes under various
credible conditions of heating and mechanical stresses, rather than
a detailed understanding of (and ability to computationally
predict) spalling. Through this approach it may be possible to
effectively guarantee (by experiment) that explosive spalling will
not occur for a given mix in a given application. This is not just
an alternative to understanding the underlying mechanisms, but a
necessary precursor to properly test hypotheses and models.
Fire engineering has frequently sought to address knowledge gaps
through the establishment of test methods and sets of empirical
data. The success of this approach is dependent upon the ability of
those test methods to characterise the key parameters affecting
behaviour. The University of Edinburgh is working with Arup to
address some of the key issues facing design for fire induced
spalling by adapting a novel fire testing method and
apparatus9,21,26 which is shown in its current form in Figure 3.
The approach is to undertake tests on concrete under a range of
carefully controlled conditions of thermal exposure and sample
loading/restraint.
Figure 3: The current H-TRIS spalling test setup at The
University of Edinburgh
Thermal Exposure
The temperature-time curves used in current structural and
tunnel fire testing are notionally based on experimental data;
however furnace testing is known to provide variable thermal
exposures due to its approach of matching temperatures inside the
furnace to a prescribed ‘standard fire’ temperature-time curve
(within acceptable limits). Spalling can be particularly sensitive
to this variation20. Applying the thermal load as a prescribed
received thermal exposure (or incident heat flux) is potentially
more consistent in that amount of energy received by each sample is
directly controlled. Maluk26 developed a novel test method and
apparatus named H-TRIS (Heat-Transfer Rate Inducing System), which
heats prismatic concrete samples using a computer controlled array
of radiant panels, and has outstanding repeatability and economy
when compared to conventional furnace testing, particularly as
regards spalling testing.
The H-TRIS thermal test method can produce exposures that are
equivalent to the temperature-time curves currently used in furnace
testing and design. Alternatively, this method is also capable of
replicating realistic design fire exposures; for example determined
on the basis of CFD or zone fire modelling, and thus offering
greater flexibility in support of functionally based fire
engineering design.
Loading and Restraint
Differential thermal stress is one of the key factors known to
contribute to spalling in some cases. To impose representative
mechanical stress conditions during testing, the new test method
replicates the pre-fire stress state by externally pre-stressing
the sample using a bespoke 3 MN uniaxial loading frame. This
loading frame has been developed with tunnel testing applications
in mind, but different loading frames have been used to provide
general insights into the spalling phenomenon – applicable to
essentially all areas of construction.
As the sample is heated, the internal sample self-restraint
changes but the total external stress is maintained constant in the
H-TRIS method. While this may not represent the true time-dependent
loading and restraint conditions in a given concrete structure, it
generates a known external mechanical stress condition and allows
multiple tests to be quickly and inexpensively performed under a
range of mechanical conditions, to highlight the potential
influence of loading on spalling risk for a given concrete mix.
Some typical images from recent H-TRIS testing are shown in Figure
4. Both samples shown have the same geometry and were tested to the
same thermal exposure. A uniaxial compressive load of 10 MPa
(Sample B) was observed to be sufficient to cause severe spalling
after 7 minutes 49 seconds, compared to no spalling occurring
during a 30 minute exposure when no load was applied (Sample
A).
Figure 4: Before (top) and after (bottom) images of the heated
surface of two identical samples tested using H-TRIS - (A)
Unloaded, and (B) loaded to a sustained compressive stress of 10
MPa uniaxial compression
The H-TRIS test method is currently based on medium scale
samples21 (500 x 500 mm in plan). At this scale, tests with H-TRIS
address many of the design confidence and risk issues associated
with spalling prevention. Tests are less expensive and faster,
allowing more candidate mixes to be tested repeatedly, testing
earlier in the project programme, and rapidly retesting to allow
mix optimisation. The H-TRIS test method has been validated in
terms of the thermal exposures it applies by comparison with
in-depth temperatures measured in full scale furnace tests of
identical samples6,27. The authors are actively seeking research
partners interested in undertaking a programme of validation
testing in parallel with a conventional spalling assessment
involving large scale furnace tests on loaded samples (most likely
involving tests on precast concrete tunnel lining segments).
THE STRUCTURAL ENGINEERS’ RESPONSIBILTY
Reinforced concrete has a good record of performance in real
fires, and the information presented in this short paper should not
be construed as a criticism of concrete as compared with other
candidate construction materials. All construction materials
present risks in fire; how the structural engineering community
manages these risks will differ between materials and must
necessarily change as construction materials continue to evolve,
presenting new and different fire risks.
However, the available research suggests5,6 that many modern
concrete mixes, used across all areas of construction, are more
prone to spalling than has historically been the case. The
structural engineering community therefore ought (1) to consider
whether spalling is likely for a given concrete mix and credible
design fire conditions for a particular design scenario, and (2)
account for the potential impact of spalling on the structure’s
ability to meet the agreed functional performance objectives for
the structure in fire. Mitigating actions, which will depend on the
particular circumstances of a given application, should be taken if
necessary.
The available guidance24 on concrete spalling in fire is based
in research but is not fully supported by the available
experimental evidence, posing a challenge for designers. Additional
research on this issue is therefore needed, particularly as
advances in concrete technology continue to generate novel high
performance concrete mixes for use in construction. Designers need
to ensure that potential for spalling is not overlooked.
A novel experimental method and apparatus such as H-TRIS
presents an opportunity to advance current approaches. It can
address a number of the necessary issues, both technical and
financial, in the assessment and mitigation of explosive concrete
spalling. Its basis on heating specimens by directly controlling
the thermal exposure received also offers flexibility in its
capability to replicate thermal exposures predicted using e.g. CFD
analysis in performance-based structural design for fire.
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