fire protection of concrete structures exposed to fast fires

Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010 

FIRE PROTECTION OF CONCRETE STRUCTURES EXPOSED TO 

FAST FIRES 

Pierre Pimienta *, Octavian Anton **, Jean-Christophe Mindeguia *, Romuald Avenel * 

Heidi Cuypers **, Eric Cesmat * 

* CSTB – Scientific and Technical Center for Construction, University of Paris-Est - 84 Avenue Jean 

Jaurès, 77447 Marne la Vallée Cedex, France (contact: pierre.pimienta@cstb.fr) 

** PRTC N.V. - Bormstraat, 24 B-2830 Tisselt, Belgium (contact: h.cuypers@prtc.be) 

KEYWORDS: Concrete spalling, fire protective panels, calcium alumina silicates engineered matrix 

for high performing fire protection of concretes. 

ABSTRACT 

The aim of this research is to assess, through a scientific experimental program, the behaviour under 

the increased hydrocarbon temperature/time curve, of concrete structures protected by calcium 

alumina silicates panels developed and produced by a special mineral and matrix engineering 

technology for a resistance to these conditions of temperature. 

The principal investigations on this research are focussed on: the influence of the thickness of the 

protection on the temperature gradient and behaviour of the concrete; the nature of the concrete and in 

particular the nature of the aggregates which impacts on the thermal diffusivity of the concrete and the 

sensitivity with respect to spalling, the implementation or not of a compressive stress on bottom fibre 

of the slab exposed to fire. 

1. Introduction 

The construction professionals pay a particular interest to the behaviour of concrete structures in fast 

fires conditions and particularly in tunnels. This interest was jumping at a higher degree of concern 

following dramatic events encountered in road and railways tunnels in the recent decade and largely 

informed by different media [1]. Numbers of research studies concerning the behaviour of different 

concrete structures were performed, experimentally from small to big scale full systems testing in real 

exposure conditions [2], [3] or by mathematical modelling [4], [5]. Meanwhile, the exact mechanisms 

controlling the behaviour of concrete in fast fire conditions are still not well known; and this is due to 

the complexity of parameters playing a role in these scenarios. Still today, experimentation is then the 

only reliable way of assessing the fire resistance of a concrete structure. 

The most apprehended phenomenon is the spalling of the concrete when exposed to a fast firing and 

particularly in tunnels where temperatures grow very fast, reaching in few minutes high values as 

1300°C and even more [6]. Spalling can take several aspects but always leading to the loss of a part of 

the structure and the loss of the mechanical resistance for the remaining concrete [7]. Between the 

hypotheses on the process leading to spalling, two hypotheses are largely considered today: the 

thermo-mechanical one that is essentially based on the differential dimensional constraints developed 

during exposure to high temperature [8], [9], and the thermo-hydric one that considers the pores water 

vapour pressures developed when the structure is exposed to a fast firing as responsible for the 

physical destruction of the concrete [10], [11]. 

In this study we chose to focus the spalling study on the analysis of the thermal gradients into the first 

centimetres of the concrete slabs. One of the main objectives of this study was to define the critical 

thermal gradient for the type of concrete investigated and test configuration (with or without 

constraint). 

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2. The fire protection of concrete: the PROMATECT® 

In view to better understand the transfer of temperature through the concrete structure and the relation 

to the appearance and development of spalling, non protected and protected concrete constructions 

were investigated. The type and thickness of the protection was between the key tools for a scientific 

investigation and definition of effective protection. This study investigated on the behaviour of 

concrete protected mainly by a calcium alumina silicate board, PROMATECT®T, developed and 

industrially produced following a special approach – the Mineral and Matrix Engineering – for the 

best fire protection performances for flat as well as curved concrete surfaces. Known also as 

PROMAXON® Technology (see Figure 1), the manufacturing process of the product starts from the 

atom level of selected raw materials playing with their chemical affinities in view to build synthetic 

mineral crystal structures and aggregate of crystals best fitted to build the full matrix of the product 

with the desired texture and structure for the best performances. In this program different thicknesses 

were voluntary applied in order to involve development of different thermal gradients into the 

concrete slab during the fire. 

_ 3 µm 

Figure 1. Controlled crystal growth and aggregation for specially designed porosity – PROMAXON® 

Technology. 

In the construction of the matrix by this technology, special endothermic phases are integrated in the 

crystallization process to bring at the desired moment in a fire the most effective cooling of the 

interface with the concrete to protect. Two type of protection boards were used in this study - 

PROMATECT® H for some particular constructions and PROMATECT® T. 

PROMATECT® -H is an autoclaved calcium silicate board with nominal dimensions of 2.5 x 1.25 m, 

thickness from 6 to 27 mm and a nominal density of 870 kg/m³. The matrix of H type was developed 

to satisfy different fire protection construction requirements. 

PROMATECT® -T is a product specially designed for fire protection applications of concrete 

structures in tunnels, board with nominal dimensions of 2.5 x 1.2 m, thickness from 15 to 40 mm and 

a nominal density of 900 kg/m³. The matrix is based on calcium alumina silicates developed under 

controlled crystal growth technology. Mineral and Matrix Engineering concept was used to design a 

matrix with an optimized cooling action at the right moment during the fire. 

Between the physical characteristics of interest for tunnel applications, PROMATECT® H has 

thermal conductivity of 0.175 W/mK, 0.212 W/mK for PROMATECT® T. The densities being 

similar, the differences are on the textural detail of the matrix: H type has a laminar texture when T 

type has a monolithic one. Another important characteristic is the specific energy (Cp) that is also a 

parameter largely used particularly by those using mathematical modelling approach for the definition 

of the necessary protection(type and thickness) for safe constructions. The value given by 

scientific/technical literature is around 1.00 J/gK for calcium silicates or calcium alumina silicates. 

Important to note is that the value is measured after heating the product up to 500 °C and cooled 

down. But by such method of measuring, all eventual advantages that a specific product can bring for 

an efficient and cost advantageous fire protection can not be seen and exploited (Figure 2, left second 

run). This study paid a particular attention to this and proposed, by a rigorous parallel experimental 

and mathematical modelling, a better procedure for assessing reliable Cp values (Figure 2, right). 

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specific energy [J/gK] 

12 

10 

8 

6 

4 

2 

0 

0 100 200 300 400 500 600 

CP on sample after 

Temperature 

stabilisation (second 

[°C] 

run) 

Cp on sample as received (first run) 

Specific energy [kJ/kgK] 

FEM specific enery PT-T 

50000 

45000 

40000 

1st endotherm 

35000 

30000 

25000 

2nd endotherm 

20000 

15000 

10000 

5000 

0 

0 100 200 300 400 500 600 

Temperature [°C] 

Figure 2. On the left, specific energy (Cp) measured after a first heating to 500 °C (generally used by modelling 

people) and for virgin sample (first run, i.e. with no pre heating). On the right, the endotherms that are 

considered in this study for the finite element modelling. 

3. Experimental settings 

3.1 Testing program 

The experimental program of this study was jointly carried out at the PRTC laboratory (Promat 

Research and Technology Center, Belgium) and at the CSTB laboratory (France). The number of 

tests, depending on the thermal protection thickness, type of concrete and type of loading are 

summarized in the Table 1. Tests in the two laboratories were done on the same slabs concrete and 

protection systems. The main difference is that the tests that were made at PRTC consisted in 

unloaded slabs while tests in CSTB consisted in loaded slabs. 

Number of tests 

Unloaded slabs 

(Promat Research and Technology 

Center) 

Loaded slabs (CSTB) 

Protection 

thickness [mm] 

Calcareous 

concrete 

Silico-calcareous 


Calcareous 




/ = no test 

0 1 1 1 1 

8 1 1 1 1 

12.5 1 1 1 / 

15 1 1 1 / 

20 / / 1 / 

25 / / 1 / 

3.2 Concrete mixes 

Table 1. Summary of the experimental program 

For this study, two types of concrete have been tested. Both of them are ordinary concretes 

(compressive strength lower than 60 MPa). The only difference between the two formulas comes from 

the nature of the aggregates. The first concrete, that we called B40, is fabricated with calcareous 

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aggregates. These aggregates are crushed rocks with a content of almost 99 % of CaO. The second 

concrete, that we called B40SC, is fabricated with a mix of calcareous and siliceous aggregates. These 

silico-calcareous aggregates are composed by alluvial semi-crushed sand and limestone gravels. 

Testing two types of aggregates allows analysing the fire behaviour of the most common concretes 

that one may use in France. Moreover, previous studies have shown that the behaviour of concrete at 

high temperature is strongly dependant on the nature of the aggregates [13]. 

The composition of the two types of concretes used for the study is given in Table 2. It is in 

conformity with standard NF EN 206. 

kg Calcareous concrete Silico-calcareous concrete 

CEM II 42.5 R cement 350 350 

8/12.5 calcareous gravel 330 

12.5/20 calcareous gravel 720 

4/8 silico-calcareous gravel 150 

8/20 silico-calcareous gravel 890 

0/2 sand 845 845 

Water 188 188 

Superplasticizer 4.31 5.25 

W / C ratio 0.54 0.54 

28 days Rc [MPa] 39 40 

Table 2. Composition of the two concretes of the study 

3.3 Unloaded tests 

During the same fire test, both concrete types (calcareous and silico-calcareous) were tested (see 

Figure 3). In parallel with the thermal data collection, a video camera placed inside of the furnace was 

capturing all moments and brought very useful information of the moment and the way concrete 

spalling occurred. 

Figure 3. The testing furnace in Promat laboratory (left) and the design of the construction for testing (right). 

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3.4 Loaded tests 

Generally, most of the fires tests carried out on concrete slabs consist in applying a mechanical load 

that results in the bending of the slabs (involving an initial tensile stress in the slab bed that is exposed 

to fire). However, previous studies shown that concrete spalling can be more severe for compressed 

elements. In this study, the influence of an initial compressive stress in the slab bed exposed to fire 

has then been analysed. 

For that, an innovative testing procedure has been developed at the CSTB. This facility consists in 

bending the slab in such a way that a compressive stress of 10 MPa is initially applied to the slab bed 

that is exposed to fire. Firstly, the concrete slab is horizontally placed upon a gas burners furnace. The 

geometry of the slab is shown in Figure 4. The exposed surface of the slab is 4490 x 1000 mm² and its 

thickness in the "fire zone" is 200 mm. Then stiff metal beams (length = 4000 mm) are fixed on each 

end of the slab. The ends of the slab have a greater thickness than in heated zone (350 mm) and are 

rigidified by a dense reinforcement. A dead load of 1 ton is suspended at the end of the metal beams. 

This system involves the bending of the slab in a direction at the opposite of the fire. This induces 

then an initial compressive stress of 10 MPa in the inferior bed of the slab. 

Figure 4. Geometry and principle of the "compressive" fire test. 

3.5 Fire curve 

All the tests of the study (loaded and unloaded tests) are carried out under the so-called temperature/ 

time curve "HCM" (for Modified HydroCarbon fire, in French), defined by the Technical Instruction 

annexed in the circular n° 2000-63 of August 25th, 2000. 

The thermal program is described by the following relation: 

T = 1280. (1-0,325e (-0,167t) -0,675 e (-2,5t) )+ 20 

Where T is the furnace inner temperature in [°C] and t the time in [min]. The evolution of the 

temperature, according to the HCM curve is shown in Figure 4 for a duration of 3 hours. 1200 °C are 

reached in only 10 minutes and 1300 °C are reached after 30 minutes. 

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1400 

Modified HydroCarbon (HCM) fire curve 

1400 

Temperature (°C) 

1200 

1000 

800 

600 

400 

200 

0 

0 20 40 60 80 100 120 140 160 180 200 

Time (min) 

Temperature (°C) 

1200 

1000 

800 

600 

400 

200 

0 

0 2 4 6 8 10 12 14 16 18 20 

Time (min) 

Figure 5. Evolution of the inner furnace temperature under a HCM fire curve (on the right, focus on the 20 first 

minutes of fire). 

4. Results 

In this paragraph, the main results of the study are presented. Firstly, we present the analysis of the 

concrete spalling risk depending on different parameters (thermal protection thickness, type of 

aggregates, mechanical loading). Then we present an experimental and numerical study attending to 

establish the link between concrete spalling and the thermal gradients that is induced in the first 

centimetres of the fired slab. At last, we present the results of the assessment of the residual 

compressive strength of concrete in the fired slabs, from drilled cores. 

4.1 Concrete spalling 

The Table 3 summarizes the results of the maximal concrete spalling depths that were measured 

depending on different parameters such as the thickness of the thermal protection, the type of 

aggregates and the type of mechanical loading. The main observations are as following. 

When concrete slabs are not thermally protected, concrete spalling occurs for any type of our tested 

concretes (i.e. for both types of aggregates) and for any type of mechanical loading (i.e. with no 

loading or for compressive loading). In that case, concrete spalling involves the disappearance of 

concrete cover and then the direct exposure to fire of the reinforcement. 

For the under sized thermal protection boards ( from 8 to 15 mm), concrete spalling can also occur. 

This has been particularly observed during loaded tests. It is very important to note that when concrete 

spalling occurred for protected slabs, concrete spalling was deeper than for slabs with no protection 

(see Figure 6). This unexpected result clearly indicates that an under sizing of the thermal protection 

can involve an important damage of the concrete structure (with in particular an important exposure of 

the reinforcement to fire). 

For thick enough thermal protection (20 and 25 mm in our study), concrete spalling is avoided in the 

case of loaded slabs. This results in a continuous protection of the reinforcement during the fire and 

good residual mechanical performances (see § 4.3). 

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Maximal spalling 

depth 

Protection 

thickness [mm] 

Calcareous 


Unloaded slabs 



Calcareous 


Loaded slabs 



0 55 30 49 85 

8 50 0 63 77 

12.5 0 0 137 / 

15 0 0 130 / 45 / 

20 / / 0 / 

25 / / 0 / 

/ = no test; 0 = no spalling 

Table 3. Summary of the measurement of the maximal spalling depth. 

In our study, the influence of the type of aggregates on concrete spalling is not clear. Indeed, for tests 

with no mechanical loading, concrete spalling is deeper for calcareous concrete. Moreover, we 

observed for slabs protected with 8 mm boards that concrete spalling only occurs for calcareous 

concrete. On the contrary, for tests with mechanical loading, concrete spalling is deeper for silicocalcareous 

concrete. One important aspect of fire concrete behaviour is the spalling that can occur 

during the cooling down of the structure and even for a long period after the fire (actually, it can be 

better seen as a concrete departure or a concrete fall than a real spalling). In the § 4.3, we will see that 

the type of aggregates can influence this particular type of concrete departure. 

Observing the results in Table 3, we can say that the initial presence of a compressive stress at the 

inferior bed of the slab is unfavourable regarding to concrete spalling. 

However, for slabs with no protection, the influence of loading is not that clear: the presence of load 

only involves deeper spalling for silico-calcareous concrete. In the case of the calcareous concrete, 

spalling depth does not seem to be influenced by the loading. 

In the case of under sized thermal protection boards (from 8 to 15 mm), the presence of the loading 

leads to deeper spalling. In particular, for thermal protection of 12.5 and 15 mm, concrete spalling 

only occurs in the case of loaded slabs. However, we want to emphasize that making a direct link 

between concrete spalling risk and the presence of a compressive loading is not that easy. Indeed, 

observations during the fire tests led us to distinguish two modes of rupture resulting on concrete 

spalling: 

- 1- concrete spalling firstly occurs leading to the fall of the protection board. In such a case, 

we could directly link concrete spalling to the presence of a mechanical load, 

- 2- thermal protection board first falls resulting in a sudden exposure of "virgin" concrete 

zones to fire. This mechanism also leads to concrete spalling but the influence of the 

mechanical loading seems to be more linked with the mechanical resistance of the thermal 

protection system (board and anchorages) itself. This behaviour has been also observed by 

[13]. 

One main difficulty of the study is that these both types of rupture mode can occur during the same 

fire test. Further studies have to be performed in order to better analyse the influence of the 

mechanical loading on concrete spalling (e.g. by using a high velocity camera during fire tests). 

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0 mm 

(49 mm) 

8 mm 

(63 mm) 

12.5 mm 

(137 mm) 

Figure 6. Mapping and pictures of the exposed surface after HCM fire tests of loaded slabs (calcareous 

concrete) with thermal protection of 0 (unprotected), 8 and 12.5 mm. Between brackets is the maximal spalling 

depth. 

4.2 Thermal gradient analysis 

The thermal gradients developed in the concrete slabs of different constructions were determined 

experimentally by measuring temperature via thermocouples inserted from the surface into the 

concrete at every 20 mm up to the cold side, and also calculated by finite elements modelling method. 

In a first step of the study, these values were analysed and compared for each construction tested by 

the two laboratories. The thermal properties – thermal conductivity, specific heat and density 

evolution with temperature – of the concrete, were determined following the Eurocode 2 documents 

[14]; those of the protection boards were established by Promat laboratories. The thermal gradient 

developed near the concrete surface is considered being the most critical one; the values were 

expressed as: 

Gradient = (T 20mm – T 0mm ) / 20 mm [°C/cm] 

Experimental thermal gradient: first of all, the values of temperatures measured in the two 

laboratories were used to verify the similarities between the two furnaces as temperature 

developments and distribution during each trial (and these in comparison with the common calibration 

values of furnaces from the two laboratories). Then, the values from the interface (or surface for the 

non protected constructions) through the concrete were used to search the correlation between the 

thermal gradients and the presence or absence of spalling. It was observed that the temperature values 

measured in the concrete in both laboratories were in good agreement; on the contrary, the values 

measured at the interface by the thermocouples scales (depth zero) were slightly different. The 

interface temperatures measured at PROMAT are in better accordance with simulated values (see next 

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paragraph). Then, they were used for the establishment of the thermal gradients for the different tests. 

The gradients experimentally determined by this way are presented in Figure 7. 

Numerical thermal gradient: calculated thermal values were compared with the measured ones. 

Best fitting between the calculated and the measured experimental values was found when the FEM 

took into consideration the dynamic change in thermal properties of materials with temperature 

increase and particularly the endothermic events (see § 2). The numerical simulations were done 

separately for both laboratories and are related with the testing observations (spalling or no spalling 

and time of spalling). The Figure 7 presents the appearance of spalling (or absence of spalling) in a 

graph giving the relation between the thermal gradients and the thickness of the thermal protection 

(including the tests without protection). Three distinct cases/zones were defined (Figure 8): zone 1 

including the case of non protected concrete or concrete protected with 8 mm PROMATECT® H 

developed spalling in both laboratories; a zone 3 includes the cases of concrete protected with 20 and 

25 mm PROMATECT® T boards where no spalling was observed; and in between, a zone 2 that 

concerns slabs protected by 12.5 and 15 mm thick PROMATECT® T boards. In this zone, unloaded 

experiments demonstrated no spalling while loaded experiments have shown spalling. This is 

particularly in this zone that the two different modes of failure described at the § 4.1 were observed. 

Gradient [°C/cm] 

300 

250 

200 

150 

100 

50 

spalling after 2/1.5 min 

at CSTB/PRTC 

spalling after 20/21 min 

at CSTB/PRTC 

no spalling at PRTC 

damage at CSTB 

(32min) 

Exp. Gradient at failure time determined by CSTB 

FEM gradient at failure time determined by CSTB 

Exp. Gradient at failure time determined by PRTC 

FEM gradient at failure time determined by PRTC 

no spalling at PRTC 

no spalling at CSTB 

0 

damage at CSTB (19min) 

no protection 

8mm PT-H 

12.5mm PT-T 

15mm PT-T 

20mm PT-T 

25mm PT-T 

Type of thermal protection 

Figure 7. Relation between thermal gradient, thickness of the protection and development of concrete spalling. 

Gradient [°C/cm] 

300 

250 

200 

150 

100 

50 

Zone 1 

0 

no protection 

8mm PT-H 

12.5mm PT-T 

15mm PT-T 

20mm PT-T 

25mm PT-T 

Exp. Gradient at failure time determined by CSTB 

FEM gradient at failure time determined by CSTB 

Exp. Gradient at failure time determined by PRTC 

FEM gradient at failure time determined by PRTC 

Zone 2 

Zone 3 

Type of thermal protection 

Figure 8. Definition of risk and safety zones; the intermediate zone 2 defines a questioning situation. 

4.3 Residual compressive strength of concrete 

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The assessment of the residual mechanical performance of concrete slabs has been carried out on 

drilled cores. The cores were drilled at different locations of the slabs several months after the fire test 

(see Figure 9). The first operation consisted in measuring the height of the core, allowing us to 

determine the concrete spalling depth (operation in Figure 9). Small disks were then cut from the 

cores; the distance "x" from the interface between concrete and thermal protection board to the upper 

side of the cut disk was noted (operation in Figure 9). At last, a compression test was carried out in 

a direction parallel to the main heat flow in the slab (operation in Figure 9). The obtained 

compressive strength was assumed to be representative of the residual mechanical performance of the 

concrete at a distance "x" from the interface. 

Figure 9. Schematic representation of the testing procedure for the assessment of the residual compressive 

strength on drilled cores from fire tested slabs (blue zones correspond to ejected concrete). 

The different spalling depths that were measured on drilled cores (for a same slab, we present the 

minimal, maximal and mean measured spalling depths) are presented in Figure 10. The results are 

presented for different thermal protection board thicknesses. The main observations are as following: 

- by comparing the measurements of concrete spalling that were done just after the fire tests 

(red curves in Figure 10) and the measurements of concrete spalling that were done on drilled 

cores (i.e. several month after fire test), we can observe that the departure of cover concrete 

can continue even after a long period following a fire. This phenomenon is particularly true in 

the case of the silico-calcareous concrete. This "post-fire" concrete departure can be explained 

by different mechanisms (inverse thermal gradient during cooling down that induces tensile 

stresses in the cover concrete zones, "re-hydration" or "re-humidification" of the concrete that 

can result in swelling and falling of the material…) and appears as a main factor to take into 

account for the post-fire assessing of a concrete structure. Indeed, this cover concrete 

departure can play a role for the residual mechanical performance of the structure (decrease of 

the resistive cross-section) as well for its durability (decrease of reinforcement protection 

regarding to aggressive factors), 

- the measurements confirm the tendencies observed just after the fire test (see § 4.1): the 

absence, or the under sizing of the thermal protection (from 8 to 15 mm in our study), 

involves and important spalling of cover concrete. Moreover, the measurements confirm the 

fact that in the case of an under sized thermal protection, concrete spalling can be deeper than 

for a slab without protection, 

- a thermal protection that is correctly designed (20 and 25 mm in our study) allows avoiding 

concrete spalling, even after a long period after the fire. 

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Spalling depth [mm] 

120 

100 

80 

60 

40 

t0 spalling - calcareous 

t0 spalling - siliceous 

tmini spalling - calcareous 

tmini spalling - siliceous 

tmean spalling -calcareous 

tmean spalling -siliceous 

tmaxi spalling - calcareous 

tmaxi spalling - siliceous 

20 

0 

0 5 10 15 20 25 30 

Thermal protection boards thickness [mm] 

Figure 10. Measurements of the spalling depth of concrete on drilled cores from fires tested slabs according to 

the thermal protection board thickness and the type of concrete (blue curves). The red curves (t0 spalling) 

correspond to the measurements on concrete slabs just after the fire test. 

The measurements of the residual compressive strength of concrete at different depths of the fired 

slabs are presented in 

Figure 11. The main observations are as following: 

- in the case of slabs without thermal protection and in the case of slabs with under sized 

thermal protection (from 8 to 15 mm in our study), the residual mechanical performance 

decreases when approaching the interface between the concrete and the protection board. 

Indeed, residual compressive strengths are twice or three times lower close to the interface 

than in a zone where we can consider that concrete is still "healthy" (at more than 100 mm 

from the interface), 

- for well designed thermal protection (20 and 25 mm in our study), the residual compressive 

strength does not vary along the depth of the slab. Indeed, the strengths that are measured 

close at the interface (these measurements are only possible in that case of thermal protection 

since concrete spalling does not occur) are quite similar with the ones that are measured in the 

"healthy" zone. 

60 

Residual compressive strength vs concrete depth 

Compressive stregth [Mpa] 

50 

40 

calcareous_no protection 

30 

siliceous_no protection 

calcareous_8 mm 

20 

siliceous_8 mm 


10 



0 

0 20 40 60 80 100 120 140 160 180 200 

interface concrete slab / 

thermal protection board 

Concrete depth [mm] 

Figure 11. Residual compressive strength of concrete measured on cut drilled cores at different depth of the 

concrete slab. Results for both types of concretes and different thicknesses of thermal protection. 

245


5. Conclusions 

This study was carried out thanks to the joint action of two laboratories: PRTC in Belgium and CSTB 

in France. Its main goal was the optimization of the calcium alumina silicates thermal protection 

PROMATECT® for concrete structures that are exposed to fast fires (HCM fire in this study). 

Fire tests were then carried out in both laboratories in order to analyse the influence of several 

parameters on concrete spalling: thickness of the thermal protection boards, type of concrete (2 types 

of aggregate) and type of mechanical loading (no loading and compressive loading). The main 

conclusions are as following: 

- Under the HCM fire curve, all the slabs that were not protected showed important spalling. 

One consequence of this spalling is the direct exposition to fire of the reinforcement, which 

can imply failure of the structure. In the same way, if the thermal protection is under sized 

(from 8 to 15 mm in our study), concrete spalling can also occur. An important result is that 

concrete spalling for under sized thermal protections can be more severe than unprotected 

slabs. 

- A correct design of the thermal protection (20 and 25 mm in our study) allows avoiding the 

concrete spalling. 

- No clear influence of the type of aggregate has been observed. Calcareous concrete showed 

more pronounced spalling than silico-calcareous concrete for fire tests without loading. The 

inverse tendency was observed for loaded fire tests. 

- The influence of the type of loading on concrete spalling was not easily interpreted. Indeed, 

two types of failure have been observed during the fire tests and both of them can occur 

during the same test: either concrete first spalls involving the fall of the thermal protection, or 

the thermal protection system (board + anchorages) falls involving a sudden exposure to fire 

of virgin concrete (and then spalling). Further observations are needed to better analyse the 

influence of the type of loading. 

In this study, we made a link between concrete spalling and the induced thermal gradient in the first 

centimetres of the slab. This approach is assumed to be reliable if one regards the severity of such a 

fast fire (HCM curve). Thermal gradient was first assessed in an experimental way, by taking into 

account the temperature measurements of both laboratories at the interface between the concrete slab 

and the protection board and the measurement of the temperature at 20 mm into the concrete slab. 

Temperatures that were measured inside concrete appeared very similar between both laboratories 

while the interface temperature showed slight differences. The use of numerical simulations, by mean 

of finite elements software, allowed us to better evaluate the interface temperature and then to assess 

the thermal gradient in concrete at the moment just before concrete spalling. By this way, three zones 

have been identified: 

- Zone 1: in this zone, concrete spalling risk is high. In our study, this zone concerns the slabs 

without protection and the slabs protected with an 8 mm thick board. 

- Zone 2: this zone appears to be a "grey" zone i.e. no clear conclusion can be made concerning 

concrete spalling risk. More observations are needed. In our study, this zone concerns the 

slabs protected with a 12.5 and 15 mm thick board. 

- Zone 3: this zone can be considered as a "safe" zone. No concrete spalling is observed during 

the fire. In our study, this zone concerns the slabs a 20 and 25 mm thick board. 

A major aspect of the study of the behaviour of fired concrete structure is the assessment of the 

residual mechanical properties (i.e. just after cooling and after a long period following the fire). In this 

study, the residual compressive strength of concrete has been measured on drilled cores that were 

taken out from the slabs several months after the fire test (only for loaded slabs). An original 

experimental method allowed us evaluating the residual mechanical performance of the slabs 

depending on the thermal protection thickness and on the type of concrete (nature of the aggregates). 

The main observations are as following: 

- An under sizing (from 8 to 15 mm in our study), or an absence, of the thermal protection 

involves an important decrease of the residual mechanical performance of the concrete. This 

is particularly true for cover concrete. This loss of compressive strength, that we assume to be 

246


representative of the damage state of the structure, implied a decrease of the whole structure 

resistance and a decrease of its durability. 

- A correct design of the thermal protection (20 and 25 mm in our study) allows minimizing the 

mechanical damage of the whole concrete slab during fire (even for the cover concrete). Then 

the advantage of these protections not only comes from the suppression of concrete spalling 

risk but also from the preservation of the mechanical performances of a fired concrete 

structure. This is particularly an advantage in the case of fired tunnels, in terms of financial 

and time needs for the structure repairing. 

To conclude, the importance of the fire protection relies on its quality and its thickness. These aspects 

are strictly dependent on the product design. In particular, the results that are presented in this paper 

can not be directly extended to other similar thermal protection products, other types of concrete or 

other fire curves. 

6. Acknowledgments 

The authors acknowledge the staff of both laboratories for their participation to this experimental 

research. 

7. References 

[1] Haack, A. Catastrophic Tunnel Fires - What have we learnt? International symposium on catastrophic tunnel 

fires (CTF). Boras, Sweden, 2003. 

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fire protection of concrete structures exposed to fast fires

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