fire resistance of ductal ultra high performance concrete - brutdebeton

Session 7
Development of new materials
FIRE RESISTANCE OF DUCTAL ® ULTRA HIGH PERFORMANCE CONCRETE
M. Behloul G. Chanvillard P. Casanova G. Orange
VSL Lafarge Central Research Laboratory Lafarge SA Rhodia CRA
France France France France
Keywords: fire resistance, ultra high performance concrete, material, structure, simulation
SUMMARY
Ductal ® is a range of ultra-high performance concretes (UHPFRC), developed by BOUYGUES-LAFARGE-
RHODIA co-operatively. Ductal ® is a technological breakthrough offering compressive strength of 160 to
240 MPa and flexural strength of over 40 MPa, with true ductile behaviour. This technology offers the
possibility to build structural elements without passive reinforcements in structural elements and to
combine innovation, lightness, and extreme durability. Nevertheless, a key question for using ultrahigh
performance concrete for building and housing is fire resistance of the material and safety of the
structure during a long fire. This paper synthesises scientific analyses and experimental results on this
topic. Scaling features and stability during a standard fire were measured on structural elements of
various shapes (shells, girders, columns) submitted to a standard fire (ISO 834). The complete
mechanical characterisation of the material as a function of the temperature provided data for a
numerical computation of temperatures, stress field, and damage of these pieces during a simulated
fire. Results are very positive in comparison with ordinary concrete when using French rules for fire
safety ("DTU Feu"). Numerical simulations are particularly demonstrative.
1 INTRODUCTION
Overall, buildings made of concrete are the safest, in terms of keeping people and property safe, as
concrete does not feed a fire in any way, slows its spread by consuming a significant amount of heat
energy (albeit less than plaster), plays an important part in the “firestop“ function (which aims to
contain a fire and control its development), and yields structures with the longest fire stability times.
Metal structures have a major handicap inasmuch as the load-bearing elements are always unstable
and must be insulated against fire; this is no easy matter, as metal conducts heat very efficiently,
which means that a localised defect can lead to catastrophic consequences. Concrete structures, on
the other hand, are more “robust“ (in the sense that they are better able to withstand localised defects
or errors), because any degradations cannot spread any faster than the rate at which heat penetrates
the material, which at 2-3 centimetres per hour is very slow.
The fire behaviour of High Performance Concrete depends on a number of parameters that
characterise the actual material (aggregate type and calibre, total porosity and pore size, free and
bound water content), the structure (element size, geometrical form, reinforcement) and the load
applied during the fire [1]. When subjected to ISO fire conditions, ordinary concretes produce little
spalling, and ultimate failure occurs through the loss of thermo-mechanical balance. With HPCs, the
key objective is to control the spalling phenomenon. The main factors that cause spalling are the
concrete's low porosity, the dimensions of the structural part and the mechanical load applied. All
these effects are inter-related, and individually identifying their respective influences is difficult.
Analysing the degradation mechanisms of HP concretes enables a distinction to be drawn between
stresses caused by steam pressure within the pores and stresses having mechanical (load) or thermal
(constrained expansion) causes. An improved understanding of these phenomena recently enabled
the Fire French DTU to be extended to concretes with compression strengths of up to 80 MPa [2].
After a joint research programme lasting more than five years, Bouygues, Lafarge and Rhodia have
for the first time launched a range of ultra high performance fibre-reinforced concretes: Ductal ® . These
products are suitable for use across the whole construction industry and are now being used in civil
engineering applications (e.g. bridges and footbridges, anchor plates and sewage works), building
works (e.g. wall cladding, acoustic panels and sun screens), industrial structures (e.g. beams and
large-span roofing sections) and other technical applications. Fire resistance is one of the main issues
for development in the building industry. Fire resistance is determined both by the choice of materials
used and by the structure's design. To address this issue, Bouygues, Lafarge and Rhodia have
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Proceedings of the 1st fib Congress
developed the Ductal ® AF range, which possesses excellent fire resistance properties, and have
validated this behaviour on various configurations of load-bearing elements.
Like all concretes, UHPCs are M0 classified and slow the spread of fire. However, the very low
porosity of UHPCs leads to greater internal stresses. In these materials, the porosity is totally
enclosed, which prevents water vapour (steam) from escaping. This increases the pressure within the
material and causes the spalling phenomenon. Part of the solution for eliminating spalling has been to
use organic fibres. This approach is now becoming outdated in the field of HPC fire behaviour. In fact,
various northern European countries and Japan recommend the use of polypropylene fibres in their
national legislation. Above 150°C, polypropylene fibres begin to soften and melt, thereby providing
escape routes for trapped steam.
Using organic fibres does however lead to a loss of rheology and strength that must be countered
by fine-tuning the formulation. The Ductal ® AF range was developed with this end in mind. It offers
outstanding fire behaviour while still offering high slump flow and strength (200 MPa in compression).
This article describes the extensive campaign of tests that were conducted at both material and
structural level in order to validate the behaviour of these products. The formulas were tested in ISO
fire conditions by independent laboratories (CSTB, SFC and VTT). Some elements had received heat
treatment as standard with the Ductal ® range.
For the purposes of this article, the terms "cured" or "heat treated" are used to describe parts or
specimens exposed to saturating steam at a temperature of 90°C for 48 h, at least 24 h after setting.
2 MAIN PRODUCT CHARACTERISTICS
The table 1 shows the main characteristics of Ductal ® CS1000F, a product in the Ductal ® AF range
which was used to produce a footbridge in Sermaises (Loiret, France). As with all the products in the
AF range, it contains a mixture of steel fibres and organic fibres. Rheological behaviour can be
characterised using the Abrams cone spreading test, which yielded a value in excess of 50 cm in tests
conducted at the precast plant.
Table 1 – Ductal® CS 1000 F characteristics
Characteristic
Value after heat
treatment
Value without
heat treatment
Typical compression strength of a 14 cm high,
∅ 7 cm cylinder (and mean value)
180 MPa
(200 MPa)
140 MPa
(160 MPa)
Typical flexural strength of 7x7x28 prisms (and mean
value)
24 MPa
(32 MPa)
20 MPa
(26 MPa)
Typical direct tensile strength (and mean value)
9 MPa
(12 MPa)
7 MPa*
(10 MPa)*
Elastic modulus 50 GPa 45 GPa
Total shrinkage 0 500 µm/m
Creep coefficient 0.2 0.8
Abrasion resistance (CNR test) 1.1 1.2
Freeze-thaw resistance (modulus after 300 cycles) 100 % 100 %
Chlorine ion diffusion (AASHTO T 259) 2.10 -14 m 2 -
Carbonation (CEN standard)

Session 7
Development of new materials
were performed after maturing them under water for four months. For the purposes of this
document, we shall refer to these tests as the SFC campaign.
The various mechanical properties relevant to a concrete’s fire behaviour are discussed in this
article, with references to the standardised characteristics defined in the French experimental standard
XP P 92-701/A1 which incorporates the guidelines set out in the French DTU [2].
3.1 Tests conditions
The samples were heated at a rate of 2°C/minute to the required test temperature, which was then
maintained for one hour. Performances were measured hot and after cooling in the case of the HITECO
campaign, and exclusively while hot in the SFC campaign. Figure 1 shows the thermal and mechanical
loading cycles applied to the specimens
Figure 1. Thermal and mechanical specimen loading cycles
3.2 Evolution of compression strength according to temperature
The table 2 summarises the characteristics of the specimens used to monitor the compression
performances at various temperatures.
Table 2. Specimen description
Test campaign HITECO SFC
Diameter (mm) 60 32
Length (mm) 180 80
Specimen manufacturing
technique
Cast
Cored and centre-drilled
Instrumentation
Load 20% or 25%
20 o C
Start of
Test
Heating
o
Test Temperature
Thermocouples embedded in
centre
Thermocouple inserted in
centre
Before applying the thermal stress to the specimens, preliminary loads representing 20%, i.e. 40 MPa
(HITECO) and 25%, i.e. 50 MPa (SFC) of the nominal compression strength capacity at 20°C were
applied.
1 Hr
Loading at
Failure “Hot”
End of
“Hot” test
Cooling
End of
“Residual”
test
Loading at
Failure “Residual”
Test temperature
“Residual” 20 o C
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Proceedings of the 1st fib Congress
Normalized strength
1.4
1.2
1
0.8
0.6
0.4
0.2
0
reference DTU B80
SFC cured 25% Hot test
SFC non cured 25% Hot Test
HITECO 20%, Hot Test
HITECO 20% Cold Test
0 100 200 300 400 500 600 700 800
temperature (°C)
Figure 2. Evolution of standardised strengths as a function of temperature
We noted that the variation in mechanical compression performances according to temperature
complied with the specifications in the French DTU [2] (Figure 2). Furthermore, with the tests
conducted after cooling, the measured mechanical performances exceeded those measured hot. This
can be partially accounted for by the internal overpressure resulting from the dehydration mechanisms
that are only active when the concrete is hot. Lastly, in the case of the uncured Ductal ® , improved
mechanical performances were obtained at temperatures above 100 °C, which could be explained by
additional hydration of the microstructure's residual clinker, possibly in a hydrothermal condition, owing
to the material's very low permeability.
3.3 Evolution of tensile strength according to temperature
12
Tensile strength can be characterised in several
HOT
different ways. As part of the HITECO programme,
f RESIDUAL
ct
tensile tests were conducted directly on notched
[MPa]
cylindrical specimens 150 mm long and 50 mm in
8
diameter (with a 5 mm deep circumferential notch).
The SFC programme included bending tests on
prisms cut from a solid block measuring 150*27*25
mm (three-point deflection tests on a 125 mm
4
span).
With the direct tensile tests, a very gradual
reduction in the material’s tensile capacity was
noted in both the hot and residual performance
0
tests. Thus, at 600°C, Ductal ® still delivers 50% of
0 100 200 300 400 500 600 its nominal tensile performances. (Fig. 3).
T [°C]
Figure 3. Evolution of tensile strength according to temperature [4]
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Session 7
Development of new materials
3.4 Evolution of the elastic modulus according to temperature
Relative elastic modulus
1.2
1
0.8
0.6
0.4
0.2
reference
20%, residual test
20%, Hot Test
0
0 100 200 300 400 500 600 700 800
temperature (°C)
In the HITECO campaign, the elastic modulus
was characterised as a function of temperature.
Figure 4 shows the results. The specimens were
identical to those used for the compression tests
and the heat and mechanical cycles.
Again, the residual performances were seen to be
better than those measured while hot. A
comparison with the curve representing the
average of the French standardised recommended
values [2] reveals the performances of Ductal ® AF
to be more sustainable.
Figure 4 Evolution of the elastic modulus according to temperature (HITECO)
3.5 Thermal conductivity
thermal conductivity (J/m/s/K)
3
2.5
2
1.5
1
0.5
0
Ductal AF
reference DTU
0 200 400 600 800
temperature (°C)
Figure 5. Evolution of thermal conductivity
according to temperature (HITECO)
This was evaluated as part of the HITECO
programme using the thermal shock probe
technique, in accordance with a protocol
similar to the one described in the standard
ISO 8894-1. Figure 5 shows the results
obtained. The thermal conductivity of
Ductal ® AF is reflected in values higher
than those recommended in the French
DTU [2] for standard concretes. The
presence of metal fibres partially accounts
for this trend. However, this parameter
evolved with temperature according to the
same profile as for standard concretes.
3.6 Specific heat
This parameter was also measured during the HITECO programme, simultaneously with the
thermal conductivity characterisation tests. As Figure 6 shows, the specific heat was relatively
unaffected by the concrete’s composition, and the test produced values very similar to those given in
the DTU.
specific heat (J/kg°C)
1200
1000
800
600
400
200
0
Ductal AF
reference DTU
0 200 400 600 800
temperature (°C)
Figure 6. Evolution of specific heat according
to temperature (HITECO)
dilatation (0/00)
14
12
10
8
6
4
2
0
HITECO
SFC cured
SFC non cured
1
12e-6
0 200 400 600 800 1000
temperature (°C)
Figure 7. Dilatometric tests
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Proceedings of the 1st fib Congress
3.7 Thermal expansion coefficient
Dilatometric tests were used to determine the thermal expansion coefficient and the microstructural
transformations that occur as a function of temperature. Figure 7 shows the results obtained in the HITECO
and SFC campaigns. The trends observed were very similar. Up to temperatures of about 500°C, extension
was essentially linear, representing nothing more than thermal expansion. The slope in this segment was 12 e -
6 /°C which corresponds to the thermal expansion coefficient. The extension curves then "bulge" as a direct
result of the quartz's highly expansive alpha/beta transformation (at 573°C).
3.8 Load-induced thermal strain
There were two groups of Load Induced Thermal Strain (LITS) results: HITECO and SFC. Graph 8
shows all the results. The LITS (total strain under load minus free thermal strain and elastic strain)
divided by the load rate (ratio of the applied force to the strength at 20°C) is shown along the Y-axis.
load-induced thermal strain / load rate
(0/00/%charge)
0
0
-10
100 200 300 400 500 600
-20
-30
-40
-50
-60
-70
HITECO, 10%
HITECO, 20%
HITECO, 30%
SFC non curé, 29%
SFC curé, 22%
temperature (°C)
Figure 8. Evolution of thermal strain according to temperature
Température LITS en 0/00/q
20 0
350 0
1000 -80
Tableau 3. LITS calculation
data
A simplified LITS description was used for the calculation (table 3). It should be noted that only LITS
occurring above 350°C was considered to account for the differential in kinetis between material and
structuralk tests (compare fig. 1 and fig. 9).
4 TESTS ON STRUCTURAL ELEMENTS
Columns and beams, loaded or unloaded, with or without thermal treatment, were submitted to an
ISO 834 fire. The tests were performed at CSTB laboratory [5] for unloaded specimens and at VTT
laboratory [6] for loaded specimens. Figure 9 below shows the ISO 834 fire curve used for the tests,
i.e. ∆T = 345log(8t+1).
Temperature (°C)
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
0 60 120 180 240
Time (min)
Figure 9. ISO 834 Fire Curve used during the tests
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Session 7
Development of new materials
4.1 Tests on columns
The sizes of the columns are 200*200*900 mm. The columns do not include any reinforcement.
Each column is instrumented with 21 thermocouples. The extremities are protected against high
temperatures using rock wool.
4.1.1 Unloaded column test
The columns remained intact until the end of the test after 132 minutes’ exposure. No spalling was
observed on the heat-treated columns. Slight spalling was observed in places on the untreated
columns. After cooling, the vertical surfaces were crazed and fibres near the surface were blackened.
The edges remained well-defined. A few minor edge cracks were visible.
4.1.2 Loaded columns tests
même excentricité
e aux deux extrémités
900
600
Figure 10. Test of the columns
Isolation par laine de roche
Four opening
Brûleurs au
gaz
4 tests were made in VTT's
laboratory using an ISO fire curve:
. 2 tests on loaded heat treated
Ductal®-AF columns
. 2 tests on loaded non-heat
treated Ductal ® AF beam
Before testing, all the elements were
stored in the VTT’s lab hall in at an
ambient temperature of 23±2°C and
relative humidity of 50±5%.
2 steel plates of 40 mm in thickness
were fixed at both ends. During the
test, a load of 2000 kN was applied
with an eccentricity of 14 mm. The 4
vertical faces of the column were
submitted to an ISO fire.
4.1.2 a) Control tests
During the fabrication of the columns, specimens were prepared. For compression cylinders of 11
cm in diameter and 22 cm in length are used. For bending prismatic 40*40*160 mm specimens are
used. The table 4 hereafter presents the mean results obtained.
Table 4 – Control tests results
With heat treatment
Without heat
treatment
Compression (BY lab) 214.9 MPa 162.7
Bending (BY lab) 43.5 MPa 38.1
Compression test the day of fire
test (in VTT)
205.8 MPa 181.0
4.1.2 b) Tests results
Table 5 presents the time at failure of the columns and the external temperature reached.
Table 5 – Columns fire duration under loading
Reference Fire duration External temperature reached
BY-Ductal#1 : Heat treated column 89 min 984 °C
BY-Ductal#2 : Heat treated column 82 min 972 °C
BY-Ductal#17 : non-heat treated 95 min 994°C
column
BY-Ductal#18 : non-heat treated
column
85 min 977°C
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Proceedings of the 1st fib Congress
4.2 Beam tests
4.2.1Geometry
40 54 20 30
Chanfreins 10 x 10
50
38
150 mm
Y
150 mm
38
Z
240
Figure 11. Cross section of the beam
2 Torons
T15S
3 prestressed beams were fabricated. One
beam is tested unloaded under ISO fire, the
second beam is tested loaded under ISO fire
and the third one is teested at room
temperature to determine the ultimate capacity.
The length of the beam is 6.15 m. The
transversal section has an I-shaped form. The
height of the transversal section is 24 cm, the
width of the flanges is 15 cm. The beams are
prestressed by 2 0.6-inch tendons positioned
at the lower part. The initial force in each
tendon was 18.1 t.
4.2.2 Unloaded beam test (CSTB test)
The beams were heat-treated at 90°C for
48 hours with steam.
The beam remained intact throughout the
test. No significant material loss was
observed during the 2 hours test duration,
apart from slight spalling in the centre on
the bottom surface. After cooling a few
horizontal and vertical cracks were visible
on the surface.
Photo 1. Unloaded beam after a 2 hours
exposure to an ISO 834 (CSTB)
4.2.3 Loaded beam tests (VTT tests)
The beam was loaded for the fire test. Two jacks located 1.5 m from the bearing points each
applied a 25 kN downward force. The maximum bending moment was 42 kN.m. This value includes
the applied load (37.5 kN.m) and the moment generated by the beam’s own weight. The breaking
strain measured at room temperature is 88.83 kN.m: the loading rate applied during the fire test was
therefore 47%.
During the test, the top of the beam was protected, and the other faces were exposed to ISO fire
conditions.
The test lasted 36 minutes. The photo below shows the state of the beam after the test, during
which a bending failure occurred. No spalling was observed. The photo shows that failure was caused
by the pre-stressing cables: the crack was straight and the upper part of the beam was undamaged.
F = 25 kN
F = 25 kN
1.50 2.90
1.50
5.90
6.15
Figure 12. Loading conditions
Photo 2. Central zone after cracking
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Session 7
Development of new materials
5 FIRE CALCULATIONS: MATERIAL DATA AND SIMULATIONS
Full-scale tests (both with and without applied loads) proved Ductal-AF’s good behaviour in ISO fire
conditions. This good behaviour was obtained because the material does not suffer from spalling. The
material’s uniform fire resistance/non-spalling behaviour allowed a conventional thermo-mechanical
method to be used to determine the fire resistance of the studied structural elements. The FIREXPO
computational tool can be used to study the fire behaviour of structures. This software has two main
parts:
- Temperature field calculations
- Mechanical calculations.
At each time interval, the mechanical calculation identifies the state of equilibrium, taking into
account the calculated temperature field and the material’s mechanical properties at the relevant
temperature.
The mechanical properties incorporate the non-linear behaviour law corresponding to the material’s
temperature. To this end, the behaviour law is configured with the material’s compressive strength and
Young’s modulus. The change in compression strength and Young’s modulus according to
temperature must then be entered. The material’s free thermal strain and load-induced thermal strain
must also be specified.
5.1 Columns
We modelled a 20*20*90 cm column, with a 2 MN load applied 14 mm off-centre. Figure 13 shows
the mesh of the 20*20 cm cross-section. This graph shows the load rate within the column at the
moment of failure. Each point shows the applied force at temperature T divided by the compressive
strength at temperature T.
The calculation results yield a fire resistance of 82 minutes, as compared with the four experimental
values of 82, 85, 89 and 95 minutes. We performed the thermo-mechanical calculations and produced
the mean strain/temperature curves. Graph 14 shows a comparison between a representative
experimental curve and the curves calculated with the simplified LITS law.
2,00
1,00
Experimental curve
Calculated curve
Strain (0/00)
0,00
0 20 40 60 80
-1,00
-2,00
Figure 13. Load rate within a 20x20 cm
column at the point of failure
Times (minutes)
Figure 14. Comparison of measured and
calculated strain in a column
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Proceedings of the 1st fib Congress
6 CONCLUSIONS
This work enabled the fire behaviour of Ductal ® AF to be clearly identified. This range of product is
perfectly suitable for use in fire-resistant applications. This formula is not susceptible to spalling. The
metal fibres ensure high strength even at high temperature. Because there is no spalling, a
calculation-based approach can be adopted to determine the fire resistance of Ductal-AF parts. This
calculation should be performed using the variations in the compression and direct tensile strengths
measured for the material.
- The temperature field can be calculated using the assumptions given in the Fire French DTU [2]
(boundary conditions and flow formulae).
- Compressive strength, Young’s modulus and free strain data for different temperatures are given in
3.4, 3.5, 3.6 and 3.7 of this document.
- The load-induced thermal strain can be approximated using a bilinear law:
Temperature LITS in 0/00/q
20 0
350 0
1000 -80
All these data can be used together to simulate the fire behaviour of Ductal ® AF parts, in terms of
both their fire resistance times and their strain characteristics. The simulations compare well with
experimental results.
REFERENCES
[1] European project BRITE EURAM III - BRPR-CT95-0065 HITECO
[2] "Méthode de prévision par le calcul du comportement au feu des structures en béton" (Predicting
the fire behaviour of concrete structures – a calculation-based method), French code of practice DTU
P 92-701, 2000
[3] Tests report year 2001, Société Française de Céramique, Paris, France
[4] R. Felicetti, P.G. Gambarova, M.P. Nathali Sora and G.A. Khoury : Mechanical behaviour of HPC
and UHPC in direct tension at high temperature and after cooling, Proceedings Fibre Reinforced
Concretes BEFIB’ 2000, Ed. P. Rossi and G. Chanvillard.
[5] Tests report year 2000, Centre Scientifique et Technique du Bâtiment, Marne la Vallée, France
[6] Tests report year 2001, VTT BUILDING TECHNOLOGY, Finland
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