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Fire spalling of ultra-high performance concrete: From a global analysis to microstructure investigations
Cement and Concrete Research 115 (2019) 207–219
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Fire spalling of ultra-high performance concrete: From a global analysis to microstructure investigations
T
L. Missemera, E. Ouedraogoa, Y. Malecota,*, C. Clergueb, D. Rogatb a b
Univ. Grenoble Alpes, CNRS, Grenoble INP, 3SR, Grenoble F-38000, France Sigma Béton, 4 rue Aristide Bergès, BP36, l’Isle d’Abeau 38081, France
ARTICLE INFO
ABSTRACT
Keywords: Temperature Ultra high-performance fibre-reinforced concrete Mercury intrusion porosimetry SEM Microstructure Pore size
This article presents the behaviour under fire of a specific ultra-high performance concrete (UHPC) called BCV. Several compositions with synthetic additions such, as polypropylene (PP) fibres and powder or acrylic (PAN) fibres, are mixed to produce these specimens. As a first step, a blowtorch test on prismatic specimens shows that small PP fibres are more efficient in resisting fire. Next, scanning electron microscopic observations after heating reveal significant behavioural differences between PP fibres and PAN fibres during their vaporisation process. Lastly, a mercury intrusion porosimetry investigation following a heating cycle at 350°C indicates that concrete porosity is not a sufficient parameter for determining whether or not a given material composition is resistant to fire spalling. A critical factor dependent on pore size distribution, is proposed herein; its threshold value, i.e. under which the material exhibits a very high probability of being prone to fire spalling, is also estimated.
1. Introduction Ultra High Performance Concretes (UHPC) represent a type of concrete with a compressive strength higher than 150 MPa. The superior performance of these products is achieved by tailoring their microstructures, that is, by maximizing the packing density with very fine minerals, quartz powder and silica fume (de Larrard and Sedran [1], Sorelli et al. [2]). Because of their low capillary, UHPC are very sensitive of explosive spalling under fire conditions which is, among the various damage mechanisms, the most dangerous for UHPC structures. As Anderberg [3], L.T. Phan et al. [4] and Kalifa et al. [5] have already demonstrated, the higher the strength of the concrete, the more intense the spalling phenomenon. Two main mechanisms are generally evoked to explain the occurrence of spalling: the presence of a thermal gradient, and water vapour motion induced by heat inside the material. The first, the thermal gradient due to fire, causes a stress gradient within the concrete (Bažant and Kaplan [6], Ulm et al. [7]) that can be so high as to reach the material failure limit. The second mechanism, known as the “moisture clog” effect, is caused by partial water saturation in the concrete (Anderberg [3], Harmathy [8]). Water in the concrete, either free or bound, is vaporised due to the temperature increase. The vapour close to the heated face of the concrete is partially drained through this heated side, whereas the other part moves in the opposite direction. Since the core of the concrete structure is colder, the
*
vapour condenses, thus increasing the saturation of unsaturated pores. An impermeable barrier obstructing gas is then assumed to be formed. Gas pressure rises and weakens the concrete; once this pressure exceeds a critical value related to the tensile strength, the concrete specimen breaks. Jansson and Boström [9] and Mindeguia et al. [10] showed however that these two phenomena of “moisture clog” and high thermal gradient both need to be present in order to rupture the concrete by spalling. Jansson suggested considering the “moisture clog” differently and more specifically as a weak area. He explained that around this area, the mechanical performance of concrete in tension as well as compression is greatly reduced as temperature rises due to the presence of water. A common solution to improving the behaviour of concrete under high temperature consists of adding cylindrical polypropylene fibres (Khoury [11], Bilodeau et al. [12], Hertz [13], Chan et al. [14], Breitenbücker [15], Harmathy [8], Malhotra [16], Liu et al. [17]). However, the actual mechanisms that serve to explain the efficiency of these fibres are not fully known. Moreover, it remains difficult to explain why polymer fibres, rather than polypropylene fibres, or the use of other shapes of polypropylene are not as efficient. A parallel can be drawn with the thawing/freezing condition of a concrete, whereby the solution to avoid high pressure from accumulating inside the concrete is for many small voids to be regularly spread. The objective of this article is to assess and improve the behaviour
Corresponding author at: Univ. Grenoble Alpes, CNRS, Grenoble INP, 3SR, Grenoble F-38000, France. E-mail address: [email protected] (Y. Malecot).
https://doi.org/10.1016/j.cemconres.2018.10.005 Received 23 April 2018; Received in revised form 2 October 2018; Accepted 8 October 2018 0008-8846/ © 2018 Elsevier Ltd. All rights reserved.
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12 mm in length - ϕ33 μm and 12 mm in length - ϕ50 μm) from the Bekaert company along with a powder, called micronal (M, diameter ϕ20 μm), from BASF company have been considered. This set-up serves to determine the influence of fibre geometry. For the PAN additions, only the 6-mm long, ϕ14-μm fibres were studied. In order to identify the thermal characteristics of PP and PAN, both Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) measurements were carried out on small samples. The TGA protocol consists of calculating the mass loss of the sample during material heating, whereas the DSC method compares the heat flux required to maintain both the sample and a reference at a similar temperature as the value of this parameter increases. The results of these tests show peaks that can generally be associated with phase changes in the material. Moreover, in order to visually observe the physical transformations of fibres with temperature, a specific experimental heating test, called the furnace test, has been developed according to the following protocol. A steel container holding a sample of fibres is, placed into a furnace heated at 10°C/min up to various target temperatures, ranging from 100°C to 500°C, and maintained at the target temperature for two hours, so as to observe the physical evolution of the material with respect to temperature. After each target temperature has been applied, a picture of the hot sample is recorded. The results of TGA and DSC tests conducted for the synthetic additions of the present study are presented in Figs. 2 and 3, while the results of the furnace test are shown in Fig. 4. Results regarding PP additions (Fig. 2) indicate an endothermic peak at 166°C during the heating phase, which reveals a phase change in the material. Since an exothermic peak was also found during the cooling phase, this would imply that the temperature of 166°C corresponds to a melting point. This finding is confirmed by the 220°C temperature setting of the furnace test, at which point the sample melted and became liquid wax. The PAN results (Fig. 3) show an exothermic peak at 311°C during the heating phase, which means that the peak temperature observed at 311°C corresponds to a degradation mechanism in the material. This result in turn has been confirmed by the furnace test temperature setting of 380°C, at which point the sample deteriorates without any liquid phase. At this high temperature, PP fibres have largely disappeared, as opposed to PAN fibres. A comparison of TGA/DSC curves reveals that mass loss occurs early for PP fibres (-65% at 300°C, -93% at 400°C) and much later for PAN fibres (-2% at 300°C, -20% at 400°C, -70% at 600°C). Accordingly, the PP polymer displays a strong tendency to volatilise between 200°C and 400° C. These significant differences between PP and PAN fibre behaviour with increasing temperature need to be highlighted for the discussion in the next part of this paper.
Table 1 Concrete composition. Constituents
kg.m-3
Mix (sand, cement and silica Fume) Superplasticizer Water W/B ratio
2085.6 28.7 194 0.19
of a specific ultra-high performance concrete (namely BCV) under fire conditions by testing concrete specimens made from several polymer fibres and a polymer powder. Another aim is to understand which parameters enable these fibres to be efficient or not. This step has entailed conducting a very high temperature study using a blowtorch test along with microscopic observations. Among the two spalling mechanisms described above, i.e. thermal gradient and moisture clog, this paper focuses on ways to reduce the effect of the latter. After a detailed presentation of materials employed, the concrete and synthetic additions, this discussion will turn to the experiments undertaken at the microstructure scale, followed by the results of each tests and the conclusion drawn. 2. Material 2.1. Studied concrete This article focuses on the specific ultra-high performance fibre-reinforced concrete called BCV. This concrete can be considered as a mortar since it is composed of siliceous sand (d/D=0/2 mm), cement, silica fume, water and superplasticizer (see Table 1). The cement paste has a low water-binder ratio, which yields ultra-high performance in terms of mechanical properties. Moreover, to enhance its use in structures, steel fibres (13 mm and 20 mm in length) have been incorporated for the purpose of increasing the material strength in tension. This configuration enables avoiding cross-reinforcing steel rods in structures. Such a material has been used to build a 50-m bridge span across the A51 motorway in France, as well as a footbridge in Lauterbrunnen (Switzerland) and a bike shelter in Switzerland (Fig. 1). The concrete material used in the present study is composed of the same constituents, except that instead of steel fibres, various synthetic additions have been incorporated to create several compositions, which are assumed to be quite resistant at very high temperature. This process enables investigating the effect of such additions on the BCV resistance to spalling without the influence of steel fibres. For this purpose, the additions are either polypropylene fibres (PP) or acrylic fibres (PAN), whose characteristics will be detailed in the following section.
2.3. Mix preparation and addition composition
2.2. Synthetic additions
Specimen production must be carefully overseen and always apply the same mixing procedure, as provided below, to ensure minimising the level of experimental scattering:
For the PP additions, three fibre sizes (6 mm in length - ϕ18 μm,
a
b
47 m
Fig. 1. Examples of constructions made of BCV: a) bridge span PS34 across the A51 motorway (France); and b) the Lauterbrunnen footbridge (Switzerland) (Photo credits: Vicat). 208
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Fig. 2. TGA and DSC curves for the polypropylene (PP) addition.
1. Premixing of the dry constituents at low speed (90 rpm) for 1 min. 2. Addition of the water at low velocity. 3. Mixing at high speed (150 rpm) for 3 min, once a change in concrete viscosity has been detected. 4. Manual mixing of the bowl in order to homogenise the concrete. 5. Mixing at high speed for another 3 min.
different values of fibres dosage. In the present study the reference parameter was specific surface area rather than fibres dosage. 3. Methods This material investigation has been undertaken using three kinds of tests, which will be described sequentially in this section: blowtorch test, scanning electron microscope, and mercury intrusion porosimetry.
Once the mix had been completed, the concrete was cast for 24 h into various moulds covered with a plastic film. To maximize the spalling risk, the specimens were then kept very humid. They were stored for 28 days in a humidity controlled room under 90% RH. Then, 24 h before the test, the specimens were held at room temperature and lower humidity (50% RH) in order to dry their faces. The nature and amount of additions introduced into the tested compositions are listed in Table 2, encompassing a range of PP fibre lengths and diameters and 1 PAN fibre length and diameter. Considering the importance of the specific surface area of fibres in concrete according to Khoury [11], the formulations were defined on the base of four specific surface areas values chosen in order to match with current PP fibres efficient dosages: 200,400,600,800(m2/m3). For a given fibre with specific length and diameter, the dosages were calculated in order to obtain the previous four values of specific surface area and the results are listed in Tables 2 and 3. Hence, the same value of specific surface for two fibres of different characteristics (length and diameter) gives two
3.1. Blowtorch test method All compositions listed in Table 2 were included in the test procedure. Prismatic concrete specimens 40 mm× 40 mm× 80 mm were produced for this testing campaign. A diagram of the test set-up is shown in Fig. 5; this is a non-standard yet highly efficient test that enabled discriminating among various concrete compositions assumed to be resistant to fire conditions. The test consisted of exposing a face of the specimen to the blowtorch flame and then recording with a camera the evolution in the target zone over a given time period. The goal here was to observe whether the material was able to withstand the heating or instead experienced certain spalling phenomena and, if so, at what intensity levels. The blowtorch was attached to its support in such a way that the operator did not need to stand in the vicinity of the experimental set-up.
Fig. 3. TGA and DSC curves for the acrylic (PAN) addition. 209
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Fig. 4. Fibre sample after a temperature sequence of: 25° C, heating at 220°C, and heating at 380°C (PP on the left, PAN on the the right).
A camcorder was placed in front of the burnt face on the 4 cm× 4 cm specimen to record the entire test. The specimen was placed at a distance “d” from the source of the flame. This initial distance “d” constituted the only control parameter and could be set at 10 or 15 cm. The test lasted 5 min at maximum possible intensity of the blowtorch (i.e. a flame temperature of 1800°C). The temperature curves (for d=10 cm or 15 cm) were determined experimentally with both a thermocouple and an infrared thermometer placed on a composition that had not experienced any spalling. The thermocouple was bent so that it touched the central zone of the heated specimen face with a slight load, in order to ensure a clear contact. Besides, the infrared temperature was measured in the same central zones and was consistent with the one of thermocouples. The corresponding temperature
measurements are displayed in Fig. 6 and compared to the French “building” (ISO 834) and “ hydrocarbure majoré ”, or “severe hydrocarbon” (HCM) standards. The “building” standard is applied in the case of building fires, whereas the “severe hydrocarbon” standard is used for tunnel fires. The heating rate reached during our experiments was either equal to (for d=15 cm) or slightly higher than (for d=10 cm) the rate indicated on the HCM curve, although for both cases (d=10 cm and 15 cm) the maximum temperature was less than the HCM standard yet higher than ISO 834 before the end of the test. Keep in mind however that these test conditions were non-standard; the thermal loadings used herein are relevant when compared to the test duration, thus creating similar conditions to those of standard tests.
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Table 2 Designations of the specimens presented in this study.
C CPP6_18_200 CPP6_18_400 CPP6_18_600 CPP6_18_800 CPP12_33_200 CPP12_33_400 CPP12_33_600 CPP12_50_200 CPP12_50_400 CPAN6_14_200 CPAN6_14_400 CPAN6_14_600 CPAN6_14_800 CM0_20_21800
Type of addition
None PP PP PP PP PP PP PP PP PP PAN PAN PAN PAN PP
Length mm
− 6 6 6 6 12 12 12 12 12 6 6 6 6 -
Diameter μm
− 18 18 18 18 33 33 33 50 50 14 14 14 14 20
Amount kg.m−3
0 0.82 1.64 2.46 3.27 1.5 3 4.5 2.3 4.5 0.81 1.61 2.42 3.22 65.73
Specific surface area m2.m−3 0 200 400 600 800 200 400 600 200 400 200 400 600 800 21 800
HCM
800 600 400 200
0
50
100
150
200
250
300
350
Time (s) Fig. 6. Time-temperature curve derived for the blowtorch test (thermal loading) for distance d=10 cm or 15 cm, with respect to the ISO 834 and HCM standards.
3.2. Scanning electron microscopic(SEM) analysis method
Composition name
Fibre amount kg.m−3
Specific surface area m2.m-3
Spalling d=10 cm
Spalling d=15 cm
C CPP6_18_200 CPP6_18_400 CPP6_18_600 CPP6_18_800 CPP12_33_200 CPP12_33_400 CPP12_33_600 CPP12_50_200 CPP12_50_400 CPAN6_14_200 CPAN6_14_400 CPAN6_14_600 CPAN6_14_800 CM0_20_21800
0.82 1.64 2.46 3.27 1.5 3 4.5 2.3 4.5 0.81 1.61 2.42 3.22 65.73
200 400 600 800 200 400 600 200 400 200 400 600 800 21 800
* * * No No * * No * No * * * * *
Yes Yes No No No Yes Yes No Yes No Yes Yes Yes Yes Yes
SEM analysis serves as one of the two experimental tests performed to investigate concrete at the microstructure scale. To generate good image quality, it is preferable to conduct observations at room temperature using the backscattered electron detector (BSED), which provides a contrast relative to the atomic number of the element: the higher the atomic number, the lighter the image. One advantage with this technique is the sharp contrast resulting in the photograph, which enables clearly visualising the cracks. The specimens were prepared by breaking 4 cm× 4 cm× 16 cm prisms into little pieces (i.e. smaller than 1 cm3). Once the pieces had been created, their observation surface was coated with a thin layer of carbon in order to improve conduction and hence image quality. It was verified that the thin layer remained present despite the thermal cycle, since the carbon was not expected to deteriorate at the eventual temperatures. Following their preparation phase, the samples were conserved in a controlled atmosphere (20°C, 50% RH) until undergoing the temperature cycles and SEM analysis. The analysis protocol consists of subjecting the specimen to different temperature cycles prior to the SEM observation. These cycles were scheduled to: heat the specimen to the specified temperatures of 25°C, 110°C, 170°C, 200°C, 280°C or 350°C, then hold the temperature constant for 2 h, and lastly cool the specimen. Upon returning to ambient temperature, the specimen was properly placed and observed under the microscope. To ensure observing the same area cycle after cycle, the specimen was fastened and a visible mark set on its support. Then, by wedging a reference disc with a mark on the specimen holder, it was possible to place the test specimen at the right location (Fig. 7).
* Test not carried out.
Marks
d distance
ISO 834
0
Table 3 Blowtorch test results for each composition.
Blowtorch
d=15 cm
1000
Temperature (°C)
Composition name
d=10 cm
1200
4x4x8 specimen
3.3. Mercury intrusion porosimetry (MIP) analysis method MIP analysis was also undertaken on various specimens made of different materials to investigate the concrete at the microstructure scale. From the previous 4 cm× 4 cm× 16 cm prisms, a number of specimens were sawed into small pieces (1 cm× 1 cm× 2 cm) and then stored for drying purposes in a furnace at a temperature of 50°C and an RH of 8% for 1 month so as to release the free water without excessive damage to the specimen. Test repeatability had been determined from reference specimens to ensure that the sawing method did not adversely affect test results. Several tests were conducted to evaluate the effect of temperature and synthetic fibres on concrete porosity. For starters, the influence of heating temperatures on MIP results was observed; for this step, the MIP analysis was carried out on specimens composed of PP fibres 12 mm long and heated for 2 h at the temperatures of 130° C, 190°C,
Fig. 5. Lateral sketch and plan view of the blowtorch set-up for a distance d=10 cm.
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the pore distribution of the material. These models however are controversial and have been criticised, especially by Diamond [18]. Since these criticisms seem relevant to our investigation, a display using pressure instead of pore size will be maintained. 4. Results and discussion 4.1. Blowtorch test results
Sample Stuck reference disc
During the blowtorch test, the distance between the blowtorch and the specimen remained constant, as long as spalling did not occur; otherwise, this distance varied. Hence, only a qualitative comparison between specimens allowed estimating the efficiency of every formulation. Table 3 summarises the blowtorch test results, while Fig. 8 shows the specimens at the end of the test. Since the test with the d=10 cm condition was more stringent than the one with d=15 cm, the d=10 cm test was assumed to be positive whenever the specimen had experienced spalling during the blowtorch test at d=15 cm. It was thus observed that the specimen able to resist test conditions was the one composed of PP fibres 6 mm long. For the other compositions, fire spalling occurred but with a different appearance. In the case of the CPP2 and CPAN specimens, small pieces of concrete were quickly, loudly and systematically ejected from the specimen, whereas for the CM specimen, a big flake of concrete was first ejected, then small pieces explode, followed by another big piece of concrete and so forth. For these three cases, the onset of spalling seemed to appear between 300°C and 350°C. Noumowé et al. [19] had already noticed a high
Sample mark
View of the sample and its support
View of the samples and the samples holder
Fig. 7. View of the specimen installation for the SEM observation stage.
350°C, 550°C, before being cooled. Alternatively, a number of tests were performed on the various compositions after heating to the same reference temperature of 350°C; a comparison of results was then carried out. The “C” formulation corresponds to the concrete paste without any synthetic additions. As far as the MIP tests are concerned, the pore distribution is usually displayed versus the applied pressure. Some simplified models based on strong assumptions actually allow converting the applied pressure into
Fig. 8. Specimen surface at the end of the blowtorch test using a distance d of 15 cm: a) CPP1, b) CPP2, c) CPAN and d) CM. 212
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Fig. 9. Evolution in drying of the heated surface for the CPP1 composition.
Fig. 10. Evolution of the cement paste and a PP fibre between 25°C and 350°C, as observed with BSED.
tests. The fact that water can evaporate from the surface probably stems from the increase in material permeability with temperature. In other tests, some water only appeared on the upper face of the specimen. Table 3 shows that concrete with PP fibres (length 6 mm, diameter 18μm) and (length 12 mm, diameter 50μm) resisted to fire spalling beyond 400m2/m3 specific surface area whereas the one (length 12 mm, diameter 33μm) resisted only for a specific surface area higher than 600m2/m3. Concrete with acrylic fibres (length 6 mm, diameter 14μm) did not resist even at 800m2/m3 as well as concrete with PP spherical balls (diameter 20μm) even with a tremendous specific surface area of 21 800 m2/m3. Hence, the link between fibres specific surface area in concrete and fire resistance, as suggested by Khoury [11], was only
performance concrete spalling around 335°C, which confirms the order of magnitude of the temperature used in our experiment. At the end of the test, some 20 mm of concrete had been ejected from the CPAN specimen, whereas the thickness was only about 10 mm for the CPP2 or CM specimen. The effect of fibre concentration was significant given that in the case of 12 mm long PP fibres, an amount of 4.5 kg/m3 of fibres was sufficient to prevent the fire spalling that had been observed at lower values. During these tests, some dark points were observed (visible in Fig. 9) to appear and disappear both on the front surface and along the lateral faces; this observation was likely due to a drying phenomenon. This particularity had also been detected with the CPP1 and the CPP2 213
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Fig. 11. Evolution of the cement paste and a PAN fibre between 25°C and 350°C, as observed with BSED.
Fig. 12. Surface state of a fibre bed for: a) a PP fibre, b) a PAN fibre.
partially observed. Some morphological characteristic of the fibres should be encountered. According to this study, the fibre specific surface area in concrete is not an intrinsic indicator for concrete resistance to fire spalling. Even though the concentrations of synthetic fibres between CPP1 and CPAN were very similar, specimen behaviour during the blowtorch test differed widely. The blowtorch test thus appears to be a relevant tool for discriminating the behaviour of various concrete compositions when exposed to fire conditions. This test however cannot explain the differences observed between the four compositions.
Figs. 10 and 11 display the evolution in SEM micrographs using the BSED option of cement paste with PP and PAN fibres, respectively, vs. an increasing heating temperature. The specimen positioning system mentioned above, Eq. (7), was successfully implemented here. The fibres are shown in black on the images because of their low atomic number (essentially composed of carbon). Several observations can be drawn from Fig. 10. First, fine cracks are clearly seen from 110°C and expand further as temperature increases. Most cracks cross the fibre bed or else surround the aggregates. From a comparison drawn between Fig. 10 and 11, it would seem that more cracks cross the fibre bed in the PP composition than in the PAN specimen. Second, the nature of the fibre bed surface is substantially different. To illustrate this point, Fig. 12 shows the surface state of the bed for both PP and PAN fibres after a 280°C temperature cycle. Whereas the crack runs perfectly free in the case of the PP fibre, in PAN fibre it appears to be covered by a plastic film. The origin of this film is questionable. We can assume that
4.2. SEM analysis results SEM observations on the CPAN and CPP2 formulations were performed in order to compare PP and PAN fibre behaviour with increasing temperature during their interaction with the cement paste. 214
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Set 1
Set 2
Set 3
Set 4
Incremental intrusion (mL/g)
0,009 0,008 0,007 0,006 0,005 0,004 0,003 0,002
a
0,001 0,000 0,001
0,01
0,1
1
10
100
1000
Pressure (MPa)
Set 1
Set 2
Set 3
Set 4
Cumulative volume (mL/g)
0,07 0,06 0,05 0,04 0,03 0,02 0,01 0 0,001
b 0,01
0,1
1
10
100
1000
Pressure (MPa) Fig. 13. Repeatability testing on specimens (reference composition without synthetic additions), after heating at 350°C: a) incremental intrusion volume, and b) cumulative intrusion volume.
it is a tendency of acrylic fibre to glue to the cement paste that can be qualified as hydrophilic tendency. The hydrophobic character of polypropylene fibres is pointed by Khoury [20,21]. Some other synthetic materials similar to acrylic like PVA are described to have hydrophilic tendency ([22,23]). It is reported for instance that PVA powder mixed to cement increases adherence of cement to supports like ceramic tiles. As far as the present study is concerned, the hydrophilic nature of acrylic fibres remains an assumption. To summarise, from this synthetic fibre analysis, as well as the blowtorch test and SEM analysis, the pressure release mechanism inside the concrete is as follows:
4.3. Mercury intrusion porosimetry analysis results The MIP investigations were carried out in order to correlate the spalling phenomenon with a microstructure parameter. In this section before doing the MIP measurements, the specimens are heated into a furnace with a heating rate of 10°C/min up to various target temperatures, and maintained 2 h at these temperatures. Whatever the compositions tested of the present study, spalling did not occur during the temperature conditioning prior to MIP tests. Fig. 13 displays the MIP results of four tests performed on the same formulation (concrete composition without synthetic additions) after heating at 350°C. It can be observed that except for set 3, which differs slightly, the various curves are nearly superimposed in the incremental as well as cumulative volume representations. This good test reproducibility indicates that the specimen is sufficiently large compared to the preparation protocol and moreover that the sawing location does not influence the porosimetry measurements. Fig. 14 shows results of the influence of heating temperature on the MIP measurement. In Fig. 14a, the concrete specimen at ambient temperature (25°C) contains two clearly separated pore areas (around
• With an increase of temperature, the synthetic fibre melts or deteriorates. • The change in material (i.e. voids in the paste) and geometry creates a local weakness where cracks appear. • Cracks make it possible to link the free volumes due to fibre disappearance. • The more open the cracks, the more easily water is transferred. 215
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25
130
190
350
550
Incremental intrusion volume (mL/g)
0,014 0,012 0,01 0,008 0,006 0,004 0,002
a
0 0,001
0,01
0,1
1
10
100
1000
Pressure (MPa)
Cumulative intrusion volume (mL/g)
25
130
190
350
550
0,1 0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02
b
0,01 0 0,001
0,01
0,1
1
10
100
1000
Pressure (MPa) Fig. 14. Porosity size distribution with temperature for a PP composition: a) incremental intrusion volume, and b) cumulative intrusion volume.
0.01 MPa and 100 MPa), while the distribution is not as widely split as the heating temperature rises, meaning that connectivity between these two pore areas begins to appear. On this same graph, a peak around 0.07 MPa is noticeable as of 190°C, which likely corresponds to the melting of PP fibres. Another maximum peak is then located around 1 MPa as of 350°C and dips slightly to 0.5 MPa at 550°C. Since fire spalling in our blowtorch tests has seemingly occurred at a temperature between 300°C and 350°C, a good reference temperature for this analysis could be 350°C. This reference temperature is then kept for the next part of the analysis. Fig. 15 focuses on the change in porosity relative to the reference concrete formulation without synthetic additions. From this figure, it is possible to confirm the existence of the two separated pore areas at ambient temperature, both of which disappear after heating at 350°C. Since this formulation explodes quickly during the blowtorch test, it serves as the reference compared to the formulations with synthetic additions. Fig. 16 shows the total porosity of various compositions (see Table 2) after heating at 350°C. This figure reveals that total porosity is not a sufficient parameter for differentiating one composition from another, as regards their resistance to spalling. In search of a better criterion, Fig. 17 compares the cumulative
intrusion volume curves of the same compositions. This graph shows that connectivity between the various pore sizes differs depending on the amount and type of fibres. The disappearance of PP fibres creates a peak (Fig. 17a) that corresponds approximatively to the fibre diameter. This melting also enables creating connectivity between the two pore areas observed at ambient temperature. The disappearance of PAN fibres is not as pronounced as that for PP fibres. The PP powder (“CM”) generates a new area of small pores corresponding to a pressure above 1 MPa, which suggests that this value provides a good limit for discriminating between formulations relative to the fire spalling phenomenon. Liu et al. [17] demonstrated the importance of the connectivity created by the melting of PP fibres, which improves the fire resistance of concrete. In our specific case, we are seeking to discriminate between the two PP fibre sizes. To define an indicator capable of representing the connectivity between macro and micro pores, we used the area located under the incremental curve and bounded by a value of 1 MPa. Put simply, this indicator reflects the value obtained on the cumulative intrusion volume graph at 1 MPa. We call this indicator the “critical factor” (denoted Fzc). The results of these values are given in Fig. 18, where a better differentiation between the formulation resistant to the blowtorch test (solid area) and the other formulations (hatched areas) is 216
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Fig. 15. Mercury intrusion porosimetry for the reference formulation without synthetic additions at ambient temperature and after heating at 350°C: a) incremental volume, and b) cumulative intrusion volume.
Fig. 16. Total porosity of various compositions (see Table 2). The solid area indicates that the formulation is resistant to the blowtorch test, while the hatched area is where the formulation is not. 217
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Fig. 17. Porosity distribution for studied compositions: a) incremental intrusion volume, and b) cumulative intrusion volume.
Fig. 18. Critical indicator value for each compositions.
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visible. We can determine a threshold value for this factor, below which fire spalling resistance cannot be guaranteed. This value would roughly equal 0.02 mL/g. This new indicator implies that the cumulative pore area must be sufficiently high at an intrusion pressure of 1 MPa and moreover that the smaller pore sizes do not influence the level of blowtorch resistance. The Fzc factor can therefore ensure whether or not a concrete composition is fire resistant to a blowtorch test. The way to obtain this value is more straightforward than carrying out a fire test on a real concrete element, and this apparently offers a great innovation in the prediction of fire spalling.
blowtorch tests, as well as L. Barnes-Davin and H. Denis for their valuable assistance with the SEM and MIP experiments. References [1] F. de Larrard, T. Sedran, Optimization of ultra-high-perfomance concrete by the use of a packing model, Cem. Concr. Res. 24 (6) (1994) 997–1009. [2] L. Sorelli, G. Constantinides, F.-J. Ulm, F. Toutlemonde, The nano-mechanical signature of ultra high performance concrete by statistical nanoindentation techniques, Cem. Concr. Res. 38 (12) (2008) 1447–1456. [3] Y. Anderberg, Spalling phenomena of HPC and PC, Int. Workshop on Fire Performance of High-Strength Concrete, NIST, Gaithersburg (USA), 1997. [4] L.T. Phan, J.R. Lawson, F.L. Davis, Effects of elevated temperature exposure on heating characteristics, spalling, and residual properties of high performance concrete, Matriaux et constructions 34 (2001) 83–91. [5] P. Kalifa, F. Menneteau, C. Galle, G. Chéné, P. Pimienta, Comportement à haute température des bétons à hautes performances : de l’éclatement à la microstructure, Cahiers du CSTB (2002). [6] Z. Bažant, M. Kaplan, Concrete at High Temperatures: Material Properties and Mathematical Models, Longman group limited ed., Longman, 1996. [7] F.-J. Ulm, P. Acker, M. Levy, The chunnel fire. II: analysis of concrete damage, J. Eng. Mech. 125 (3) (1999) 283–289. [8] T. Harmathy, Effect of Moisture on the Fire Endurance of Building Element, American Society for Testing and Materials STP, 385 (1965), p. 74. [9] R. Jansson, L. Boström, Fire Spalling: Theories and Experiments, 5th International RILEM Symposium on Self-Compacting Concrete, 2 (2007), pp. 735–740. [10] J.-C. Mindeguia, P. Pimienta, A. Noumowé, M. Kanema, Temperature, pore pressure and mass variation of concrete subjected to high temperature - experimental and numerical discussion on spalling risk, Cem. Concr. Res. 40 (2010) 477–487. [11] G. Khoury, Concrete spalling assessment methodologies and polypropylene fibre toxicity analysis in tunnel fires, Struct. Concr. 9 (1) (2008) 11–18. [12] A. Bilodeau, V. Kodur, G. Hoff, Optimization of the type and amount of polypropylene fibres for preventing the spalling of lightweight concrete subjected to hydrocarbon fire, Cem. Concr. Compos. 26 (2) (2004) 163–174. [13] K. Hertz, Limits of spalling of fire-exposed concrete, Fire Saf. J. 38 (2003) 103–116. [14] Y. Chan, X. Luo, W. Sun, Effect of high temperature and cooling regimes on the compressive strength and pore properties of high performance concrete, Constr. Build. Mater. 14 (2000) 261–266. [15] R. Breitenbücker, High strength concrete C105 with increased fire resistance due to polypropylene fibres, 4th International Symposium on Utilization of High-strength/ High-performance Concrete, 1996, pp. 571–577 Paris (France). [16] H. Malhotra, The effects of temperature on compressive strength concrete, Mag. Concr. Res. 8 (3) (1956) 85–94. [17] X. Liu, G. YE, G. De Schutter, Y. Yuan, L. Taerwe, On the mechanism of polypropylene fibres in preventing fire spalling in self-compacting and high-performance cement paste, Cem. Concr. Res. 38 (2007) 487–499. [18] S. Diamond, Mercury porosimetry an inappropriate method for the measurement of pore size distribution in cement-based materials, Cem. Concr. Res. 30 (10) (2000) 1517–1525. [19] A. Noumowé, P. Clastres, G. Debicki, J.-L. Costaz, Transient heating effect on high strength concrete, Nucl. Eng. Des. 166 (1996) 99–108. [20] G. Khoury, B. Willoughby, Polypropylene fibres in heated concrete. Part 1: molecular structure and materials behaviour. Mag. Concr. Res. 60 (2) (2008) 125–136. [21] G. Khoury, Polypropylene fibres in heated concrete. Part 2: pressure relief mechanisms and modelling criteria, Mag. Concr. Res. 60 (3) (2008) 189–204. [22] A.A.P. Mansur, D.B. Santos, H.S. Mansur, A microstructural approach to adherence mechanism of poly (vynil alcohol) modified cement systems to ceramic tiles, Cem. Concr. Res. 37 (2007) 270–282. [23] A.A.P. Mansur, H.S. Mansur, Surface interactions of chemical active ceramic tiles with polymer-modified mortars, Cem. Concr. Compos. 33 (2011) 742–748.
5. Conclusions This paper offers an innovative method for determining the ability of various BCV compositions to withstand a fire test:
• We developed a simple non-standard and efficient high temperature •
•
•
test, called the blowtorch test, which has enabled discriminating different compositions as regards their spalling resistance. From the SEM analysis and fibres heating tests, we have proven that though the wettability of concrete by melted fibres is high, a slight film remains after vaporisation of the fibres, and this process occurs with an increase in temperature. This film reduces the possibility of connectivity among the pores created by fibre disappearance. Such a phenomenon has been observed on PAN fibres, but not on PP fibres, whose composition is spalling resistant. Through the series of MIP tests, an attempt has been made to characterise the sensitivity of fibre-reinforced concrete to spalling. A new quantitative factor, referred to as the critical factor (Fzc) has been calculated; it corresponds to the area included below the pressure of 1 MPa on the curve of the cumulative intrusion volume of the test, after a heating cycle at 350°C. The value of Fzc = 0.02mL.MPa/g has been determined as the limit for discriminating one fire-resistant composition from the others. The value obtained relates to the BCV concrete composition and further studies are still needed to generalise the relevance of this factor. Besides, spalling phenomenon may be sensitive to a scale factor. The fact that spalling did not occur on tests, on laboratory size specimens, does not guarantee that it will not happen in large structures made of this material.
Acknowledgements This project has been conducted in collaboration with the Sigma Béton company, a subsidiary of Vicat, which is the manufacturer of BCV. The author would like to thank in particular J. Frécon for his tremendous help in mixing the concrete specimens and performing the
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Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres
Fire spalling of ultra-high performance concrete: From a global analysis to microstructure investigations
T
L. Missemera, E. Ouedraogoa, Y. Malecota,*, C. Clergueb, D. Rogatb a b
Univ. Grenoble Alpes, CNRS, Grenoble INP, 3SR, Grenoble F-38000, France Sigma Béton, 4 rue Aristide Bergès, BP36, l’Isle d’Abeau 38081, France
ARTICLE INFO
ABSTRACT
Keywords: Temperature Ultra high-performance fibre-reinforced concrete Mercury intrusion porosimetry SEM Microstructure Pore size
This article presents the behaviour under fire of a specific ultra-high performance concrete (UHPC) called BCV. Several compositions with synthetic additions such, as polypropylene (PP) fibres and powder or acrylic (PAN) fibres, are mixed to produce these specimens. As a first step, a blowtorch test on prismatic specimens shows that small PP fibres are more efficient in resisting fire. Next, scanning electron microscopic observations after heating reveal significant behavioural differences between PP fibres and PAN fibres during their vaporisation process. Lastly, a mercury intrusion porosimetry investigation following a heating cycle at 350°C indicates that concrete porosity is not a sufficient parameter for determining whether or not a given material composition is resistant to fire spalling. A critical factor dependent on pore size distribution, is proposed herein; its threshold value, i.e. under which the material exhibits a very high probability of being prone to fire spalling, is also estimated.
1. Introduction Ultra High Performance Concretes (UHPC) represent a type of concrete with a compressive strength higher than 150 MPa. The superior performance of these products is achieved by tailoring their microstructures, that is, by maximizing the packing density with very fine minerals, quartz powder and silica fume (de Larrard and Sedran [1], Sorelli et al. [2]). Because of their low capillary, UHPC are very sensitive of explosive spalling under fire conditions which is, among the various damage mechanisms, the most dangerous for UHPC structures. As Anderberg [3], L.T. Phan et al. [4] and Kalifa et al. [5] have already demonstrated, the higher the strength of the concrete, the more intense the spalling phenomenon. Two main mechanisms are generally evoked to explain the occurrence of spalling: the presence of a thermal gradient, and water vapour motion induced by heat inside the material. The first, the thermal gradient due to fire, causes a stress gradient within the concrete (Bažant and Kaplan [6], Ulm et al. [7]) that can be so high as to reach the material failure limit. The second mechanism, known as the “moisture clog” effect, is caused by partial water saturation in the concrete (Anderberg [3], Harmathy [8]). Water in the concrete, either free or bound, is vaporised due to the temperature increase. The vapour close to the heated face of the concrete is partially drained through this heated side, whereas the other part moves in the opposite direction. Since the core of the concrete structure is colder, the
*
vapour condenses, thus increasing the saturation of unsaturated pores. An impermeable barrier obstructing gas is then assumed to be formed. Gas pressure rises and weakens the concrete; once this pressure exceeds a critical value related to the tensile strength, the concrete specimen breaks. Jansson and Boström [9] and Mindeguia et al. [10] showed however that these two phenomena of “moisture clog” and high thermal gradient both need to be present in order to rupture the concrete by spalling. Jansson suggested considering the “moisture clog” differently and more specifically as a weak area. He explained that around this area, the mechanical performance of concrete in tension as well as compression is greatly reduced as temperature rises due to the presence of water. A common solution to improving the behaviour of concrete under high temperature consists of adding cylindrical polypropylene fibres (Khoury [11], Bilodeau et al. [12], Hertz [13], Chan et al. [14], Breitenbücker [15], Harmathy [8], Malhotra [16], Liu et al. [17]). However, the actual mechanisms that serve to explain the efficiency of these fibres are not fully known. Moreover, it remains difficult to explain why polymer fibres, rather than polypropylene fibres, or the use of other shapes of polypropylene are not as efficient. A parallel can be drawn with the thawing/freezing condition of a concrete, whereby the solution to avoid high pressure from accumulating inside the concrete is for many small voids to be regularly spread. The objective of this article is to assess and improve the behaviour
Corresponding author at: Univ. Grenoble Alpes, CNRS, Grenoble INP, 3SR, Grenoble F-38000, France. E-mail address: [email protected] (Y. Malecot).
https://doi.org/10.1016/j.cemconres.2018.10.005 Received 23 April 2018; Received in revised form 2 October 2018; Accepted 8 October 2018 0008-8846/ © 2018 Elsevier Ltd. All rights reserved.
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12 mm in length - ϕ33 μm and 12 mm in length - ϕ50 μm) from the Bekaert company along with a powder, called micronal (M, diameter ϕ20 μm), from BASF company have been considered. This set-up serves to determine the influence of fibre geometry. For the PAN additions, only the 6-mm long, ϕ14-μm fibres were studied. In order to identify the thermal characteristics of PP and PAN, both Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) measurements were carried out on small samples. The TGA protocol consists of calculating the mass loss of the sample during material heating, whereas the DSC method compares the heat flux required to maintain both the sample and a reference at a similar temperature as the value of this parameter increases. The results of these tests show peaks that can generally be associated with phase changes in the material. Moreover, in order to visually observe the physical transformations of fibres with temperature, a specific experimental heating test, called the furnace test, has been developed according to the following protocol. A steel container holding a sample of fibres is, placed into a furnace heated at 10°C/min up to various target temperatures, ranging from 100°C to 500°C, and maintained at the target temperature for two hours, so as to observe the physical evolution of the material with respect to temperature. After each target temperature has been applied, a picture of the hot sample is recorded. The results of TGA and DSC tests conducted for the synthetic additions of the present study are presented in Figs. 2 and 3, while the results of the furnace test are shown in Fig. 4. Results regarding PP additions (Fig. 2) indicate an endothermic peak at 166°C during the heating phase, which reveals a phase change in the material. Since an exothermic peak was also found during the cooling phase, this would imply that the temperature of 166°C corresponds to a melting point. This finding is confirmed by the 220°C temperature setting of the furnace test, at which point the sample melted and became liquid wax. The PAN results (Fig. 3) show an exothermic peak at 311°C during the heating phase, which means that the peak temperature observed at 311°C corresponds to a degradation mechanism in the material. This result in turn has been confirmed by the furnace test temperature setting of 380°C, at which point the sample deteriorates without any liquid phase. At this high temperature, PP fibres have largely disappeared, as opposed to PAN fibres. A comparison of TGA/DSC curves reveals that mass loss occurs early for PP fibres (-65% at 300°C, -93% at 400°C) and much later for PAN fibres (-2% at 300°C, -20% at 400°C, -70% at 600°C). Accordingly, the PP polymer displays a strong tendency to volatilise between 200°C and 400° C. These significant differences between PP and PAN fibre behaviour with increasing temperature need to be highlighted for the discussion in the next part of this paper.
Table 1 Concrete composition. Constituents
kg.m-3
Mix (sand, cement and silica Fume) Superplasticizer Water W/B ratio
2085.6 28.7 194 0.19
of a specific ultra-high performance concrete (namely BCV) under fire conditions by testing concrete specimens made from several polymer fibres and a polymer powder. Another aim is to understand which parameters enable these fibres to be efficient or not. This step has entailed conducting a very high temperature study using a blowtorch test along with microscopic observations. Among the two spalling mechanisms described above, i.e. thermal gradient and moisture clog, this paper focuses on ways to reduce the effect of the latter. After a detailed presentation of materials employed, the concrete and synthetic additions, this discussion will turn to the experiments undertaken at the microstructure scale, followed by the results of each tests and the conclusion drawn. 2. Material 2.1. Studied concrete This article focuses on the specific ultra-high performance fibre-reinforced concrete called BCV. This concrete can be considered as a mortar since it is composed of siliceous sand (d/D=0/2 mm), cement, silica fume, water and superplasticizer (see Table 1). The cement paste has a low water-binder ratio, which yields ultra-high performance in terms of mechanical properties. Moreover, to enhance its use in structures, steel fibres (13 mm and 20 mm in length) have been incorporated for the purpose of increasing the material strength in tension. This configuration enables avoiding cross-reinforcing steel rods in structures. Such a material has been used to build a 50-m bridge span across the A51 motorway in France, as well as a footbridge in Lauterbrunnen (Switzerland) and a bike shelter in Switzerland (Fig. 1). The concrete material used in the present study is composed of the same constituents, except that instead of steel fibres, various synthetic additions have been incorporated to create several compositions, which are assumed to be quite resistant at very high temperature. This process enables investigating the effect of such additions on the BCV resistance to spalling without the influence of steel fibres. For this purpose, the additions are either polypropylene fibres (PP) or acrylic fibres (PAN), whose characteristics will be detailed in the following section.
2.3. Mix preparation and addition composition
2.2. Synthetic additions
Specimen production must be carefully overseen and always apply the same mixing procedure, as provided below, to ensure minimising the level of experimental scattering:
For the PP additions, three fibre sizes (6 mm in length - ϕ18 μm,
a
b
47 m
Fig. 1. Examples of constructions made of BCV: a) bridge span PS34 across the A51 motorway (France); and b) the Lauterbrunnen footbridge (Switzerland) (Photo credits: Vicat). 208
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Fig. 2. TGA and DSC curves for the polypropylene (PP) addition.
1. Premixing of the dry constituents at low speed (90 rpm) for 1 min. 2. Addition of the water at low velocity. 3. Mixing at high speed (150 rpm) for 3 min, once a change in concrete viscosity has been detected. 4. Manual mixing of the bowl in order to homogenise the concrete. 5. Mixing at high speed for another 3 min.
different values of fibres dosage. In the present study the reference parameter was specific surface area rather than fibres dosage. 3. Methods This material investigation has been undertaken using three kinds of tests, which will be described sequentially in this section: blowtorch test, scanning electron microscope, and mercury intrusion porosimetry.
Once the mix had been completed, the concrete was cast for 24 h into various moulds covered with a plastic film. To maximize the spalling risk, the specimens were then kept very humid. They were stored for 28 days in a humidity controlled room under 90% RH. Then, 24 h before the test, the specimens were held at room temperature and lower humidity (50% RH) in order to dry their faces. The nature and amount of additions introduced into the tested compositions are listed in Table 2, encompassing a range of PP fibre lengths and diameters and 1 PAN fibre length and diameter. Considering the importance of the specific surface area of fibres in concrete according to Khoury [11], the formulations were defined on the base of four specific surface areas values chosen in order to match with current PP fibres efficient dosages: 200,400,600,800(m2/m3). For a given fibre with specific length and diameter, the dosages were calculated in order to obtain the previous four values of specific surface area and the results are listed in Tables 2 and 3. Hence, the same value of specific surface for two fibres of different characteristics (length and diameter) gives two
3.1. Blowtorch test method All compositions listed in Table 2 were included in the test procedure. Prismatic concrete specimens 40 mm× 40 mm× 80 mm were produced for this testing campaign. A diagram of the test set-up is shown in Fig. 5; this is a non-standard yet highly efficient test that enabled discriminating among various concrete compositions assumed to be resistant to fire conditions. The test consisted of exposing a face of the specimen to the blowtorch flame and then recording with a camera the evolution in the target zone over a given time period. The goal here was to observe whether the material was able to withstand the heating or instead experienced certain spalling phenomena and, if so, at what intensity levels. The blowtorch was attached to its support in such a way that the operator did not need to stand in the vicinity of the experimental set-up.
Fig. 3. TGA and DSC curves for the acrylic (PAN) addition. 209
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Fig. 4. Fibre sample after a temperature sequence of: 25° C, heating at 220°C, and heating at 380°C (PP on the left, PAN on the the right).
A camcorder was placed in front of the burnt face on the 4 cm× 4 cm specimen to record the entire test. The specimen was placed at a distance “d” from the source of the flame. This initial distance “d” constituted the only control parameter and could be set at 10 or 15 cm. The test lasted 5 min at maximum possible intensity of the blowtorch (i.e. a flame temperature of 1800°C). The temperature curves (for d=10 cm or 15 cm) were determined experimentally with both a thermocouple and an infrared thermometer placed on a composition that had not experienced any spalling. The thermocouple was bent so that it touched the central zone of the heated specimen face with a slight load, in order to ensure a clear contact. Besides, the infrared temperature was measured in the same central zones and was consistent with the one of thermocouples. The corresponding temperature
measurements are displayed in Fig. 6 and compared to the French “building” (ISO 834) and “ hydrocarbure majoré ”, or “severe hydrocarbon” (HCM) standards. The “building” standard is applied in the case of building fires, whereas the “severe hydrocarbon” standard is used for tunnel fires. The heating rate reached during our experiments was either equal to (for d=15 cm) or slightly higher than (for d=10 cm) the rate indicated on the HCM curve, although for both cases (d=10 cm and 15 cm) the maximum temperature was less than the HCM standard yet higher than ISO 834 before the end of the test. Keep in mind however that these test conditions were non-standard; the thermal loadings used herein are relevant when compared to the test duration, thus creating similar conditions to those of standard tests.
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Table 2 Designations of the specimens presented in this study.
C CPP6_18_200 CPP6_18_400 CPP6_18_600 CPP6_18_800 CPP12_33_200 CPP12_33_400 CPP12_33_600 CPP12_50_200 CPP12_50_400 CPAN6_14_200 CPAN6_14_400 CPAN6_14_600 CPAN6_14_800 CM0_20_21800
Type of addition
None PP PP PP PP PP PP PP PP PP PAN PAN PAN PAN PP
Length mm
− 6 6 6 6 12 12 12 12 12 6 6 6 6 -
Diameter μm
− 18 18 18 18 33 33 33 50 50 14 14 14 14 20
Amount kg.m−3
0 0.82 1.64 2.46 3.27 1.5 3 4.5 2.3 4.5 0.81 1.61 2.42 3.22 65.73
Specific surface area m2.m−3 0 200 400 600 800 200 400 600 200 400 200 400 600 800 21 800
HCM
800 600 400 200
0
50
100
150
200
250
300
350
Time (s) Fig. 6. Time-temperature curve derived for the blowtorch test (thermal loading) for distance d=10 cm or 15 cm, with respect to the ISO 834 and HCM standards.
3.2. Scanning electron microscopic(SEM) analysis method
Composition name
Fibre amount kg.m−3
Specific surface area m2.m-3
Spalling d=10 cm
Spalling d=15 cm
C CPP6_18_200 CPP6_18_400 CPP6_18_600 CPP6_18_800 CPP12_33_200 CPP12_33_400 CPP12_33_600 CPP12_50_200 CPP12_50_400 CPAN6_14_200 CPAN6_14_400 CPAN6_14_600 CPAN6_14_800 CM0_20_21800
0.82 1.64 2.46 3.27 1.5 3 4.5 2.3 4.5 0.81 1.61 2.42 3.22 65.73
200 400 600 800 200 400 600 200 400 200 400 600 800 21 800
* * * No No * * No * No * * * * *
Yes Yes No No No Yes Yes No Yes No Yes Yes Yes Yes Yes
SEM analysis serves as one of the two experimental tests performed to investigate concrete at the microstructure scale. To generate good image quality, it is preferable to conduct observations at room temperature using the backscattered electron detector (BSED), which provides a contrast relative to the atomic number of the element: the higher the atomic number, the lighter the image. One advantage with this technique is the sharp contrast resulting in the photograph, which enables clearly visualising the cracks. The specimens were prepared by breaking 4 cm× 4 cm× 16 cm prisms into little pieces (i.e. smaller than 1 cm3). Once the pieces had been created, their observation surface was coated with a thin layer of carbon in order to improve conduction and hence image quality. It was verified that the thin layer remained present despite the thermal cycle, since the carbon was not expected to deteriorate at the eventual temperatures. Following their preparation phase, the samples were conserved in a controlled atmosphere (20°C, 50% RH) until undergoing the temperature cycles and SEM analysis. The analysis protocol consists of subjecting the specimen to different temperature cycles prior to the SEM observation. These cycles were scheduled to: heat the specimen to the specified temperatures of 25°C, 110°C, 170°C, 200°C, 280°C or 350°C, then hold the temperature constant for 2 h, and lastly cool the specimen. Upon returning to ambient temperature, the specimen was properly placed and observed under the microscope. To ensure observing the same area cycle after cycle, the specimen was fastened and a visible mark set on its support. Then, by wedging a reference disc with a mark on the specimen holder, it was possible to place the test specimen at the right location (Fig. 7).
* Test not carried out.
Marks
d distance
ISO 834
0
Table 3 Blowtorch test results for each composition.
Blowtorch
d=15 cm
1000
Temperature (°C)
Composition name
d=10 cm
1200
4x4x8 specimen
3.3. Mercury intrusion porosimetry (MIP) analysis method MIP analysis was also undertaken on various specimens made of different materials to investigate the concrete at the microstructure scale. From the previous 4 cm× 4 cm× 16 cm prisms, a number of specimens were sawed into small pieces (1 cm× 1 cm× 2 cm) and then stored for drying purposes in a furnace at a temperature of 50°C and an RH of 8% for 1 month so as to release the free water without excessive damage to the specimen. Test repeatability had been determined from reference specimens to ensure that the sawing method did not adversely affect test results. Several tests were conducted to evaluate the effect of temperature and synthetic fibres on concrete porosity. For starters, the influence of heating temperatures on MIP results was observed; for this step, the MIP analysis was carried out on specimens composed of PP fibres 12 mm long and heated for 2 h at the temperatures of 130° C, 190°C,
Fig. 5. Lateral sketch and plan view of the blowtorch set-up for a distance d=10 cm.
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the pore distribution of the material. These models however are controversial and have been criticised, especially by Diamond [18]. Since these criticisms seem relevant to our investigation, a display using pressure instead of pore size will be maintained. 4. Results and discussion 4.1. Blowtorch test results
Sample Stuck reference disc
During the blowtorch test, the distance between the blowtorch and the specimen remained constant, as long as spalling did not occur; otherwise, this distance varied. Hence, only a qualitative comparison between specimens allowed estimating the efficiency of every formulation. Table 3 summarises the blowtorch test results, while Fig. 8 shows the specimens at the end of the test. Since the test with the d=10 cm condition was more stringent than the one with d=15 cm, the d=10 cm test was assumed to be positive whenever the specimen had experienced spalling during the blowtorch test at d=15 cm. It was thus observed that the specimen able to resist test conditions was the one composed of PP fibres 6 mm long. For the other compositions, fire spalling occurred but with a different appearance. In the case of the CPP2 and CPAN specimens, small pieces of concrete were quickly, loudly and systematically ejected from the specimen, whereas for the CM specimen, a big flake of concrete was first ejected, then small pieces explode, followed by another big piece of concrete and so forth. For these three cases, the onset of spalling seemed to appear between 300°C and 350°C. Noumowé et al. [19] had already noticed a high
Sample mark
View of the sample and its support
View of the samples and the samples holder
Fig. 7. View of the specimen installation for the SEM observation stage.
350°C, 550°C, before being cooled. Alternatively, a number of tests were performed on the various compositions after heating to the same reference temperature of 350°C; a comparison of results was then carried out. The “C” formulation corresponds to the concrete paste without any synthetic additions. As far as the MIP tests are concerned, the pore distribution is usually displayed versus the applied pressure. Some simplified models based on strong assumptions actually allow converting the applied pressure into
Fig. 8. Specimen surface at the end of the blowtorch test using a distance d of 15 cm: a) CPP1, b) CPP2, c) CPAN and d) CM. 212
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Fig. 9. Evolution in drying of the heated surface for the CPP1 composition.
Fig. 10. Evolution of the cement paste and a PP fibre between 25°C and 350°C, as observed with BSED.
tests. The fact that water can evaporate from the surface probably stems from the increase in material permeability with temperature. In other tests, some water only appeared on the upper face of the specimen. Table 3 shows that concrete with PP fibres (length 6 mm, diameter 18μm) and (length 12 mm, diameter 50μm) resisted to fire spalling beyond 400m2/m3 specific surface area whereas the one (length 12 mm, diameter 33μm) resisted only for a specific surface area higher than 600m2/m3. Concrete with acrylic fibres (length 6 mm, diameter 14μm) did not resist even at 800m2/m3 as well as concrete with PP spherical balls (diameter 20μm) even with a tremendous specific surface area of 21 800 m2/m3. Hence, the link between fibres specific surface area in concrete and fire resistance, as suggested by Khoury [11], was only
performance concrete spalling around 335°C, which confirms the order of magnitude of the temperature used in our experiment. At the end of the test, some 20 mm of concrete had been ejected from the CPAN specimen, whereas the thickness was only about 10 mm for the CPP2 or CM specimen. The effect of fibre concentration was significant given that in the case of 12 mm long PP fibres, an amount of 4.5 kg/m3 of fibres was sufficient to prevent the fire spalling that had been observed at lower values. During these tests, some dark points were observed (visible in Fig. 9) to appear and disappear both on the front surface and along the lateral faces; this observation was likely due to a drying phenomenon. This particularity had also been detected with the CPP1 and the CPP2 213
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Fig. 11. Evolution of the cement paste and a PAN fibre between 25°C and 350°C, as observed with BSED.
Fig. 12. Surface state of a fibre bed for: a) a PP fibre, b) a PAN fibre.
partially observed. Some morphological characteristic of the fibres should be encountered. According to this study, the fibre specific surface area in concrete is not an intrinsic indicator for concrete resistance to fire spalling. Even though the concentrations of synthetic fibres between CPP1 and CPAN were very similar, specimen behaviour during the blowtorch test differed widely. The blowtorch test thus appears to be a relevant tool for discriminating the behaviour of various concrete compositions when exposed to fire conditions. This test however cannot explain the differences observed between the four compositions.
Figs. 10 and 11 display the evolution in SEM micrographs using the BSED option of cement paste with PP and PAN fibres, respectively, vs. an increasing heating temperature. The specimen positioning system mentioned above, Eq. (7), was successfully implemented here. The fibres are shown in black on the images because of their low atomic number (essentially composed of carbon). Several observations can be drawn from Fig. 10. First, fine cracks are clearly seen from 110°C and expand further as temperature increases. Most cracks cross the fibre bed or else surround the aggregates. From a comparison drawn between Fig. 10 and 11, it would seem that more cracks cross the fibre bed in the PP composition than in the PAN specimen. Second, the nature of the fibre bed surface is substantially different. To illustrate this point, Fig. 12 shows the surface state of the bed for both PP and PAN fibres after a 280°C temperature cycle. Whereas the crack runs perfectly free in the case of the PP fibre, in PAN fibre it appears to be covered by a plastic film. The origin of this film is questionable. We can assume that
4.2. SEM analysis results SEM observations on the CPAN and CPP2 formulations were performed in order to compare PP and PAN fibre behaviour with increasing temperature during their interaction with the cement paste. 214
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Set 1
Set 2
Set 3
Set 4
Incremental intrusion (mL/g)
0,009 0,008 0,007 0,006 0,005 0,004 0,003 0,002
a
0,001 0,000 0,001
0,01
0,1
1
10
100
1000
Pressure (MPa)
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it is a tendency of acrylic fibre to glue to the cement paste that can be qualified as hydrophilic tendency. The hydrophobic character of polypropylene fibres is pointed by Khoury [20,21]. Some other synthetic materials similar to acrylic like PVA are described to have hydrophilic tendency ([22,23]). It is reported for instance that PVA powder mixed to cement increases adherence of cement to supports like ceramic tiles. As far as the present study is concerned, the hydrophilic nature of acrylic fibres remains an assumption. To summarise, from this synthetic fibre analysis, as well as the blowtorch test and SEM analysis, the pressure release mechanism inside the concrete is as follows:
4.3. Mercury intrusion porosimetry analysis results The MIP investigations were carried out in order to correlate the spalling phenomenon with a microstructure parameter. In this section before doing the MIP measurements, the specimens are heated into a furnace with a heating rate of 10°C/min up to various target temperatures, and maintained 2 h at these temperatures. Whatever the compositions tested of the present study, spalling did not occur during the temperature conditioning prior to MIP tests. Fig. 13 displays the MIP results of four tests performed on the same formulation (concrete composition without synthetic additions) after heating at 350°C. It can be observed that except for set 3, which differs slightly, the various curves are nearly superimposed in the incremental as well as cumulative volume representations. This good test reproducibility indicates that the specimen is sufficiently large compared to the preparation protocol and moreover that the sawing location does not influence the porosimetry measurements. Fig. 14 shows results of the influence of heating temperature on the MIP measurement. In Fig. 14a, the concrete specimen at ambient temperature (25°C) contains two clearly separated pore areas (around
• With an increase of temperature, the synthetic fibre melts or deteriorates. • The change in material (i.e. voids in the paste) and geometry creates a local weakness where cracks appear. • Cracks make it possible to link the free volumes due to fibre disappearance. • The more open the cracks, the more easily water is transferred. 215
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0.01 MPa and 100 MPa), while the distribution is not as widely split as the heating temperature rises, meaning that connectivity between these two pore areas begins to appear. On this same graph, a peak around 0.07 MPa is noticeable as of 190°C, which likely corresponds to the melting of PP fibres. Another maximum peak is then located around 1 MPa as of 350°C and dips slightly to 0.5 MPa at 550°C. Since fire spalling in our blowtorch tests has seemingly occurred at a temperature between 300°C and 350°C, a good reference temperature for this analysis could be 350°C. This reference temperature is then kept for the next part of the analysis. Fig. 15 focuses on the change in porosity relative to the reference concrete formulation without synthetic additions. From this figure, it is possible to confirm the existence of the two separated pore areas at ambient temperature, both of which disappear after heating at 350°C. Since this formulation explodes quickly during the blowtorch test, it serves as the reference compared to the formulations with synthetic additions. Fig. 16 shows the total porosity of various compositions (see Table 2) after heating at 350°C. This figure reveals that total porosity is not a sufficient parameter for differentiating one composition from another, as regards their resistance to spalling. In search of a better criterion, Fig. 17 compares the cumulative
intrusion volume curves of the same compositions. This graph shows that connectivity between the various pore sizes differs depending on the amount and type of fibres. The disappearance of PP fibres creates a peak (Fig. 17a) that corresponds approximatively to the fibre diameter. This melting also enables creating connectivity between the two pore areas observed at ambient temperature. The disappearance of PAN fibres is not as pronounced as that for PP fibres. The PP powder (“CM”) generates a new area of small pores corresponding to a pressure above 1 MPa, which suggests that this value provides a good limit for discriminating between formulations relative to the fire spalling phenomenon. Liu et al. [17] demonstrated the importance of the connectivity created by the melting of PP fibres, which improves the fire resistance of concrete. In our specific case, we are seeking to discriminate between the two PP fibre sizes. To define an indicator capable of representing the connectivity between macro and micro pores, we used the area located under the incremental curve and bounded by a value of 1 MPa. Put simply, this indicator reflects the value obtained on the cumulative intrusion volume graph at 1 MPa. We call this indicator the “critical factor” (denoted Fzc). The results of these values are given in Fig. 18, where a better differentiation between the formulation resistant to the blowtorch test (solid area) and the other formulations (hatched areas) is 216
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Fig. 15. Mercury intrusion porosimetry for the reference formulation without synthetic additions at ambient temperature and after heating at 350°C: a) incremental volume, and b) cumulative intrusion volume.
Fig. 16. Total porosity of various compositions (see Table 2). The solid area indicates that the formulation is resistant to the blowtorch test, while the hatched area is where the formulation is not. 217
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Fig. 17. Porosity distribution for studied compositions: a) incremental intrusion volume, and b) cumulative intrusion volume.
Fig. 18. Critical indicator value for each compositions.
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visible. We can determine a threshold value for this factor, below which fire spalling resistance cannot be guaranteed. This value would roughly equal 0.02 mL/g. This new indicator implies that the cumulative pore area must be sufficiently high at an intrusion pressure of 1 MPa and moreover that the smaller pore sizes do not influence the level of blowtorch resistance. The Fzc factor can therefore ensure whether or not a concrete composition is fire resistant to a blowtorch test. The way to obtain this value is more straightforward than carrying out a fire test on a real concrete element, and this apparently offers a great innovation in the prediction of fire spalling.
blowtorch tests, as well as L. Barnes-Davin and H. Denis for their valuable assistance with the SEM and MIP experiments. References [1] F. de Larrard, T. Sedran, Optimization of ultra-high-perfomance concrete by the use of a packing model, Cem. Concr. Res. 24 (6) (1994) 997–1009. [2] L. Sorelli, G. Constantinides, F.-J. Ulm, F. Toutlemonde, The nano-mechanical signature of ultra high performance concrete by statistical nanoindentation techniques, Cem. Concr. Res. 38 (12) (2008) 1447–1456. [3] Y. Anderberg, Spalling phenomena of HPC and PC, Int. Workshop on Fire Performance of High-Strength Concrete, NIST, Gaithersburg (USA), 1997. [4] L.T. Phan, J.R. Lawson, F.L. Davis, Effects of elevated temperature exposure on heating characteristics, spalling, and residual properties of high performance concrete, Matriaux et constructions 34 (2001) 83–91. [5] P. Kalifa, F. Menneteau, C. Galle, G. Chéné, P. Pimienta, Comportement à haute température des bétons à hautes performances : de l’éclatement à la microstructure, Cahiers du CSTB (2002). [6] Z. Bažant, M. Kaplan, Concrete at High Temperatures: Material Properties and Mathematical Models, Longman group limited ed., Longman, 1996. [7] F.-J. Ulm, P. Acker, M. Levy, The chunnel fire. II: analysis of concrete damage, J. Eng. Mech. 125 (3) (1999) 283–289. [8] T. Harmathy, Effect of Moisture on the Fire Endurance of Building Element, American Society for Testing and Materials STP, 385 (1965), p. 74. [9] R. Jansson, L. Boström, Fire Spalling: Theories and Experiments, 5th International RILEM Symposium on Self-Compacting Concrete, 2 (2007), pp. 735–740. [10] J.-C. Mindeguia, P. Pimienta, A. Noumowé, M. Kanema, Temperature, pore pressure and mass variation of concrete subjected to high temperature - experimental and numerical discussion on spalling risk, Cem. Concr. Res. 40 (2010) 477–487. [11] G. Khoury, Concrete spalling assessment methodologies and polypropylene fibre toxicity analysis in tunnel fires, Struct. Concr. 9 (1) (2008) 11–18. [12] A. Bilodeau, V. Kodur, G. Hoff, Optimization of the type and amount of polypropylene fibres for preventing the spalling of lightweight concrete subjected to hydrocarbon fire, Cem. Concr. Compos. 26 (2) (2004) 163–174. [13] K. Hertz, Limits of spalling of fire-exposed concrete, Fire Saf. J. 38 (2003) 103–116. [14] Y. Chan, X. Luo, W. Sun, Effect of high temperature and cooling regimes on the compressive strength and pore properties of high performance concrete, Constr. Build. Mater. 14 (2000) 261–266. [15] R. Breitenbücker, High strength concrete C105 with increased fire resistance due to polypropylene fibres, 4th International Symposium on Utilization of High-strength/ High-performance Concrete, 1996, pp. 571–577 Paris (France). [16] H. Malhotra, The effects of temperature on compressive strength concrete, Mag. Concr. Res. 8 (3) (1956) 85–94. [17] X. Liu, G. YE, G. De Schutter, Y. Yuan, L. Taerwe, On the mechanism of polypropylene fibres in preventing fire spalling in self-compacting and high-performance cement paste, Cem. Concr. Res. 38 (2007) 487–499. [18] S. Diamond, Mercury porosimetry an inappropriate method for the measurement of pore size distribution in cement-based materials, Cem. Concr. Res. 30 (10) (2000) 1517–1525. [19] A. Noumowé, P. Clastres, G. Debicki, J.-L. Costaz, Transient heating effect on high strength concrete, Nucl. Eng. Des. 166 (1996) 99–108. [20] G. Khoury, B. Willoughby, Polypropylene fibres in heated concrete. Part 1: molecular structure and materials behaviour. Mag. Concr. Res. 60 (2) (2008) 125–136. [21] G. Khoury, Polypropylene fibres in heated concrete. Part 2: pressure relief mechanisms and modelling criteria, Mag. Concr. Res. 60 (3) (2008) 189–204. [22] A.A.P. Mansur, D.B. Santos, H.S. Mansur, A microstructural approach to adherence mechanism of poly (vynil alcohol) modified cement systems to ceramic tiles, Cem. Concr. Res. 37 (2007) 270–282. [23] A.A.P. Mansur, H.S. Mansur, Surface interactions of chemical active ceramic tiles with polymer-modified mortars, Cem. Concr. Compos. 33 (2011) 742–748.
5. Conclusions This paper offers an innovative method for determining the ability of various BCV compositions to withstand a fire test:
• We developed a simple non-standard and efficient high temperature •
•
•
test, called the blowtorch test, which has enabled discriminating different compositions as regards their spalling resistance. From the SEM analysis and fibres heating tests, we have proven that though the wettability of concrete by melted fibres is high, a slight film remains after vaporisation of the fibres, and this process occurs with an increase in temperature. This film reduces the possibility of connectivity among the pores created by fibre disappearance. Such a phenomenon has been observed on PAN fibres, but not on PP fibres, whose composition is spalling resistant. Through the series of MIP tests, an attempt has been made to characterise the sensitivity of fibre-reinforced concrete to spalling. A new quantitative factor, referred to as the critical factor (Fzc) has been calculated; it corresponds to the area included below the pressure of 1 MPa on the curve of the cumulative intrusion volume of the test, after a heating cycle at 350°C. The value of Fzc = 0.02mL.MPa/g has been determined as the limit for discriminating one fire-resistant composition from the others. The value obtained relates to the BCV concrete composition and further studies are still needed to generalise the relevance of this factor. Besides, spalling phenomenon may be sensitive to a scale factor. The fact that spalling did not occur on tests, on laboratory size specimens, does not guarantee that it will not happen in large structures made of this material.
Acknowledgements This project has been conducted in collaboration with the Sigma Béton company, a subsidiary of Vicat, which is the manufacturer of BCV. The author would like to thank in particular J. Frécon for his tremendous help in mixing the concrete specimens and performing the
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