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On the effect of pore-space properties and water saturation on explosive spalling of fire-loaded concrete
Construction and Building Materials 231 (2020) 117150
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On the effect of pore-space properties and water saturation on explosive spalling of fire-loaded concrete Marcus Maier ⇑, Matthias Zeiml, Roman Lackner Material Technology Innsbruck (MTI), University of Innsbruck, Technikerstrasse 13, A-6020 Innsbruck, Austria
h i g h l i g h t s A novel Two-Chamber-Fire-Furnace is presented that allows an improved reproduction of temperature histories. Relevant parameters causing explosive fire-spalling of concrete are identified. Correlations between governing parameters to evaluate the spalling behavior of concrete mixtures are presented. An assessment method to evaluate the spalling risk is introduced.
a r t i c l e
i n f o
Article history: Received 25 April 2019 Received in revised form 30 September 2019 Accepted 3 October 2019
Keywords: Mechanical properties Permeability Transport properties Concrete Fire spalling Assessment method
a b s t r a c t In this paper, the influence of concrete properties such as permeability and environmental conditions (water content) on the spalling behavior of concrete subjected to fire loading is investigated. For this purpose, a special fire-test setup is presented, allowing an improved reproduction of temperature histories observed, e.g., in tunnel fires. Moreover, the developed test setup enables a continuous monitoring of spalling, giving access to the spalling history and the final level of damage of concrete specimens. The obtained results, considering different water/cement-ratios and saturation degrees, are related to the required initial tensile strength determined by means of a numerical assessment tool. Finally, correlations between the identified parameters governing fire-spalling were established and allowed an evaluation of the spalling risk of concrete mixtures. These correlations revealed that – in contrast to requirements given in national and international standards – the combination of water content and permeability may serve as proper key and design parameter, defining the risk of spalling. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Fire accidents in tunnels lead to severe damage to tunnel linings caused by thermomechanical and/or thermohygral processes inducing spalling. In the past, extended research on concrete was carried out to identify the governing parameters causing explosive spalling associated with the aforementioned thermohygral processes (see, e.g., [1–4]). In addition to the mechanical properties of concrete, heating rate, pore volume, moisture content and permeability were identified as main parameters influencing the occurrence and extent of spalling [5–7]. As regards explosive spalling, the increasing water-vapor pressure within the concrete pore system is assumed to be the governing force causing spalling in case the transport properties of concrete are insufficient to evacuate the water-vapor fast enough.
⇑ Corresponding author. E-mail address: [email protected] (M. Maier). https://doi.org/10.1016/j.conbuildmat.2019.117150 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
As regards transport towards the ‘‘cold side”, water-vapor transport results in condensation and hence accumulation of water, forming the so-called moisture clog (see, e.g., [8,9]). This effect and the associated increase in water-vapor pressure was investigated experimentally and numerically by several authors [2,10– 13]. In order to avoid explosive spalling, poly-propylene (PP) fibers were added to the concrete mix, increasing the transport properties of concrete because they provide additional space for the water-vapor in the course of temperature loading. The positive effect of poly-propylene (PP) fibers on the transport properties and fire-protection properties was reported, e.g., in Ref. [4] (see Fig. 1). In case no PP-fiber reinforced concrete is employed, the behavior in case of fire as well as the potential need for fireprotection measures is standardly linked to the water content of concrete, e.g., according to the Eurocode [14], spalling of concrete is not expected for concrete with a water content less than 3 M% [14]. In addition, national appendices to the Eurocode may define different threshold values such as, e.g., the Austrian appendix
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150
Fig. 1. Effect of transport properties of concrete on occurrence and extent of explosive spalling: (left) ordinary concrete and (right) concrete with PP fibers [4].
giving a threshold value of 2 M% [15]. Actually, the high influence of water saturation and the pore volume (defining the water content for a certain saturation) was also confirmed numerically [1]. This paper is devoted to the experimental verification and quantification of the effect of water saturation and pore volume on the progress and final extent of explosive spalling. In this paper, real-life situations and fire scenarios are considered which resulted in two challenges to deal with: 1. The application of thermal loading in fire tests is of crucial importance, as the temperature history strongly affects the risk and extent of explosive spalling. e.g. fire tests on samples subjected to a moderate heating rate show different spalling results compared to samples subjected to high heating rates. Furthermore, single-chamber furnaces face the challenge to accurately follow the required temperatures within the first minutes of the fire tests where already spalling occurs. To provide remedy to this challenge, a novel Two-Chamber Fire Furnace (TCFF) is presented, providing an improved reproduction of temperature loading. 2. The range of material and environmental properties present in real-life situations needs to be captured, setting conditions on the experimental program covering different types of concrete and different extents of initial saturation prior to fire testing. In addition to the fire tests, temperature-dependent material properties (mechanical and transport properties) were determined experimentally. Based on the obtained experimental results, the following questions shall be answered: 1. How do initial water content and permeability influence spalling? 2. Is it possible to identify key parameters to assess the spalling risk? 3. How to proceed in risk assessment of spalling? This paper is organized as follows: In the following chapter, the employed types of concrete as well as specimen preparation and conditioning are described. The permeability-testing procedure and the developed fire furnace are presented in Section 3. Section 4 contains the obtained experimental results, dealing in detail with the progress and extent of spalling as a function of saturation and transport properties. Section 5 includes the discussion of the obtained results and finally, Section 6 closes with concluding remarks. 2. Materials In order to provide different types of concrete characterized by varying mechanical and permeability properties, normal-weight
concrete with a water/cement-ratio (w/c-ratio) of 0.4, 0.5, and 0.6 was produced. The underlying mix designs are given in Table 1. Compressive strength, Young’s modulus, and tensile strength were measured after 28 days of storage in water on cylindrical specimens with a diameter of 100 mm and a height of 300 mm. The tensile strength was obtained indirectly by conducting cylinder-splitting tests (Brazilian test). The obtained properties for the employed types of concrete are listed in Table 2 (identification of residual permeability parameters and porosity is described in detail in the following section). 2.1. Concrete specimens for fire testing The influence of saturation and transport properties on spalling was investigated on slabs of 500 800 120 mm3 (L W H). All concrete slabs were stored under water for 28 days for curing. It should be noted that this curing condition differs from construction practice where the concrete members will be demolded earlier and are exposed to the environmental conditions. Nevertheless, this curing condition was chosen to ensure fully saturated samples as a starting point for the investigation on the fire spalling behavior. The weight of the fully saturated samples served as initial mass to control the partly saturated stages of the specimens by monitoring its mass loss during drying. Fully saturated samples were therefore fire-tested right after they were removed from the water whereas partly saturated specimens were first dried in a temperature chamber at 60 °C till a desired mass was achieved. During fire testing, the bottom surface of the specimen (500 500 mm2) was exposed to a pre-specified temperature loading (hydrocarbon fire curve, HC1200) for 90 min with a maximum temperature of 1200 °C. In total, six slabs with different initial moisture contents (ranging from saturated to dry) were tested for each w/c-ratio (0.4, 0.5 and 0.6), giving a total of 18 specimens for fire testing. The specimens for fire testing and their saturation degree measured at a depth of 0–2.5 cm are given in Table 3. During fire testing, the temperature distribution within the slab was recorded by two sets of Type K thermocouples positioned at a distance of 5/10/20/30/40/50/70/90 mm from the heated surface.
Table 1 Mix designs of considered types of concrete. Cement: CEM I 42.5R Fly Ash (Hydraulit M) Water w/c-ratio Aggregates: siliceous Agg. size 0–4 mm Agg. size 4–8 mm Agg. size 8–16 mm Agg. size 16–32 mm
kg/m3 kg/m3 kg/m3 – kg/m3 Mass % Mass % Mass % Mass %
290 50 132/165/198 0.4/0.5/0.6 1888 30 30 20 20
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150 Table 2 Properties of concrete with w/c-ratio = 0.4/0.5/0.6. Water-cement ratio Young’s modulus Compressive strength Tensile strength porosity Permeability at 105 °C
E fc ft n kint b
0.4 40.1 63.7 4.8 10.2 16.9 1.6
GPa MPa MPa – nm2 bar
0.5 35.2 52.2 4.1 11.9 30.8 1.6
0.6 29.5 38.7 2.7 14.1 52.6 1.6
Table 3 Water/cement ratio and saturation degree of specimens prepared for fire testing. w/c = 0.4
w/c = 0.5
w/c = 0.6
Specimen [–]
Saturation [%]
Specimen [–]
Saturation [%]
Specimen [–]
Saturation [%]
WCR04-1 WCR04-2 WCR04-3 WCR04-4 WCR04-5 WCR04-6
58 57 35 42 21 18
WCR05-1 WCR05-2 WCR05-3 WCR05-4 WCR05-5 WCR05-6
87 90 45 52 7 10
WCR06-1 WCR06-2 WCR06-3 WCR06-4 WCR06-5 WCR06-6
90 73 61 75 4 6
These thermocouples consist of two 0.2 mm wires made of NickelChromium and Nickel-Alumel insulated with glass silk. As shown in Fig. 2, the thermocouples are positioned from the top of the mold at the required depth, hence, not affecting the temperature ingress from the heated surface during fire testing. The location of the thermocouples is shown in Fig. 3. The numbers shown refer to the depth from the heated surface (in [cm]). Additionally, a video camera recording the bottom surface was employed, to record spalling events and the damage in the course of the fire test. To investigate the influence of saturation on the spalling behavior, the concrete slabs were dried at 60 °C for various durations. The slabs were stored in a temperature chamber and weighed periodically until the designated mass loss was achieved. The saturation degree of the partly saturated samples was controlled by monitoring the mass loss of the initially fully saturated samples. This was achieved by weighing the samples daily till a designated mass loss was achieved and therefore varying saturation degrees could be obtained. To avoid dehydration of the concrete samples, 60 °C was chosen for drying as temperatures above 60 °C will indicate ettringite dehydration and therefore to a change in the microstructure [16]. To identify the initial saturation degree on the day of fire testing, drill dust was collected from each slab and
Fig. 3. In-plane position of the thermocouples within the concrete slab. The numbers refer to the depth in cm of the measurement points measured from the heated surface.
dried at 105 °C before testing. Concrete structures, especially tunnel linings exhibit different moisture contents e.g. throughout
Fig. 2. Mounting of thermocouples before/during casting.
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150
the tunnel length as well as throughout the thickness of the concrete member. Inhomogeneous moisture distributions within a concrete member result in different amounts of water vapor during heating. This results in varying vapor pressures within the concrete matrix that has to be released by the permeability and/or the forming cracks to avoid spalling. Due to the build-up vapor pressure, water vapor also moves deeper inside the concrete member to colder regions, condensates and, depending on the available evaporable water, forms the so called ‘‘Moisture Clog”. At this steamtight zone, vapor pressure increases over time till the actual tensile strength of the concrete is met and spalling might occur. To consider this effect the inhomogeneous moisture distribution within the specimens was evaluated within two different depths. The depth of 0.0–2.5 cm and 2.5–5.0 cm was chosen as this was the expected spalling depth. Drill powder samples were taken from both sides of the slab giving a total of 4 samples for each test specimen. Based on the obtained change in density, the saturation degree Sr was determined as
Sr ¼
qs qd nqW
where qs and qd represent the density before and after drying at 105 °C, respectively, n the porosity (see the following section for description of the test method for determination of n) and qw the density of water. Figs. 4 and 5 present the obtained water content and saturation degree for the 18 slabs right before fire testing. 2.2. Concrete specimens for material characterization In addition to fire testing, the employed types of concrete are characterized as regards their (temperature-dependent) mechanical and transport properties (see Table 2). For this purpose, discs with a diameter of 100 mm and a thickness of 30 mm served as
specimens. In addition to the initial properties at room temperature, residual permeability and porosity tests were performed on specimens subjected to heating levels of 80/105/300/600 °C. For this purpose, the specimens were heated with a temperature increase of 2 °C/min in order to avoid cracking due to thermal gradients within the specimens. Each temperature level was kept for 12 h to ensure a uniform temperature distribution before the specimens were naturally cooled down for testing. 3. Methods 3.1. Permeability and porosity As highlighted by many authors, transport properties as well as the water saturation have a significant influence on the spalling behavior of concrete [17–19]. Therefore, in addition to the fire tests, residual air permeability and open porosity tests were performed within the presented research work. Permeability was determined using the experimental setup given in Fig. 6, giving access to the so-called intrinsic residual permeability kint [nm2] and Klinkenberg’s parameter b [bar]. To conduct the residual airpermeability test, the test specimen is placed between two steel pipes and encased with a rubber membrane (Fig. 6). Pressure is applied to the membrane to seal the lateral surface of the specimen and hence to ensure a one-dimensional air flow through the specimen as illustrated in Fig. 6 (right). More details on measurement equipment and calculation of the permeability parameters can be found in Refs. [20,21]. For the open-porosity tests, the air-permeability specimens were reused for testing according to Ref. [22]. Hereby, the open porosity n is obtained by
n¼
ms md 100 ms mh
Fig. 4. Water content by weight loss of WCR 0.4/0.5/0.6 measured right before fire testing (Numbers in Graphs refer to the specimen number).
Fig. 5. Saturation degree for concrete with WCR = 0.4/0.5/0.6 measured right before fire testing (Numbers in Graphs refer to the specimen number).
M. Maier et al. / Construction and Building Materials 231 (2020) 117150
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Fig. 6. (left) Permeability setup and (right) air flux through the disc-shaped specimen [23,24].
where ms is the partly-saturated mass of the specimen, md the dry mass and mh the mass of the specimen submerged in water. 3.2. Fire-test chamber For the presented experimental program, a novel TwoChamber-Fire-Furnace (TCFF) was developed, consisting of four main parts (see Fig. 7): (i) (ii) (iii) (iv)
oil-burner, combustion chamber, temperature-control unit, and test chamber.
The oil-burner is placed at the front wall of the furnace and feeds the combustion chamber with temperatures up to 1400 °C. This chamber serves as temperature reservoir and is connected to the temperature-control unit. This unit allows an automatic and precise control of the thermal flux into the (adjacent) test chamber (L W H = 1400 500 400 mm3) following any desired time-temperature curve. The temperature-control unit adjusts the burner capacity as well as the thermal flux by taking the mean value of two 3 mm insulated Type K thermocouples placed in the test chamber. The benefit of the proposed two-chamber setup is the separation of the flame (oil-burner) from the test chamber. Accordingly, the thermal flux to and hence the temperature loading within the test chamber does neither depend on the capacity of the
oil-burner nor on the time span of temperature loading. Standardly, single-chamber oil furnaces tend to significantly overdrive moderate heating rates as shown in Fig. 8 (left) or underdrive the prescribed chamber temperature, in case of high heating rates (e.g., hydrocarbon fire curve) are required. As indicated in Fig. 8, the presented TCFF is able to capture both moderate heating rates as specified for the ISO 834 fire curve [25,26] (see Fig. 8 (left)) and high heating rates (hydrocarbon fire curve HC1200 [27], see Fig. 8 (right)) as employed within the experimental program. In fact, the achieved temperature in the TCFF meets the accuracy requirements of RVS 09.01.45 (Austrian Research Association for Roads, Railways and Transport) [28] given by the grey curves (lower and upper limit) in Fig. 8(right). 4. Results 4.1. Concrete properties 4.1.1. Permeability Within this study the residual permeability of the concrete mixtures was evaluated, meaning that the samples were heated up to the designated temperature to cause thermal damage, then cooled down and the residual permeability was measured at room temperature. This procedure was chosen due to the challenges which appear for measuring the so called ‘‘hot permeability” and the lack of available ‘‘hot permeability” instrumentation. It should be noted that the residual permeability might underestimate the permeability measured at the designated temperature due to the fact that the
Fig. 7. Design of Two-Chamber Fire Furnace (TCFF) (dimensions in [cm]).
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Fig. 8. Temperature loading provided by TCFF: (left) ISO 834 fire curve and (right) hydrocarbon fire curve HC1200 (SCFF = Single-Chamber-Fire-Furnace; TCFF = Two-ChamberFire-Furnace).
heated specimen expands resulting in expanded cracks and air pathways. Results of the temperature-dependent residual permeability (mean value and standard deviation) are shown in Fig. 9 and given in Table 4. For each data point at least 4 samples were testet. The highest residual permeability was observed for w/ c = 0.6, the lowest for w/c-ratio 0.4. Considering the results at 105 °C (kint = 16.9, 30.8 and 52.7 nm2 for w/c = 0.4, 0.5 and 0.6), a 1.8 and 3.1 times higher permeability for w/c = 0.5 and 0.6 is observed compared to w/c = 0.4. Results of the 80 °C tests show lower residual permeability than at 105 °C except for w/c = 0.6. It should be noted that at 80 °C the specimens still contain free water which hinders the air flow through the specimen. All specimens
tested at 105 °C are considered to be dry with no physicallybound or free water present within the specimen. Temperature loading above 105 °C results in decomposition of the cementpaste matrix associated with micro- and macro-cracking, leading to an increase of the residual permeability. Due to the high extent of decomposition for specimens heated up to 600 °C, no permeability could be measured for w/c = 0.4. 4.1.2. Porosity The open porosity results, obtained according to ÖNORM EN 1936 [22], as a function of temperature loading are listed in Table 5, showing a continuous increase of the open porosity from w/c = 0.4
Fig. 9. Intrinsic residual permeability kint and b as a function of temperature.
Table 4 Intrinsic residual permeability kint [nm2] and b [bar] for concrete with w/c-ratio = 0.4/0.5/0.6 (mean value and standard deviation SD in [%]). 80 °C
105 °C
300 °C
600 °C
kint
b
kint
b
kint
b
kint
b
w/c = 0.4
Mean value SD[%]
6.55 15,1
1.21 40.4
16.9 14,5
1.59 55.3
295.9 12,0
0.75 7.1
– –
– –
w/c = 0.5
Mean value SD[%]
20.2 25.9
1.17 34.0
30.8 44.4
1.59 52.3
316.9 13.6
0.66 26.7
12,083 16.2
2.21 27.9
w/c = 0.6
Mean value SD[%]
54.8 13.4
1.19 42.0
52.7 18.3
1.59 9.5
771.3 23.69
0.82 23.6
19,418 28.9
6.33 11.0
M. Maier et al. / Construction and Building Materials 231 (2020) 117150 Table 5 Open porosity [%]. Temperature level
80 °C
105 °C
300 °C
600 °C
w/c = 0.4 w/c = 0.5 w/c = 0.6
9.6 10.6 13.7
10.2 11.9 14.1
11.6 13.1 16.4
16.5 16.8 19.7
to 0.6. The open porosity defines the fraction of volume that can be occupied by water within an interconnected network of the concrete matrix. These pores and channels are connected to the concrete surface and interact with the surrounding moisture resulting in a change of the saturation of a concrete member. Hence, the open porosity is a parameter to conclude to the maximum amount of water that can be contained within a concrete matrix. Drying and rewetting of concrete samples, as done for the open porosity measurements, results in de-, and rehydration and therefore to a change in the microstructure especially for temperatures above 100 °C. Below 100 °C, ettringite dehydration starts at around 60 °C and the C-S-H dehydration initiates at around 100 °C [16]. To fully remove the evaporable water within the matrix before measuring the open porosity, drying of samples at 105 °C till constant mass is commonly used for concrete specimens. To minimize the effect of de-, and rehydration, specimens were kept in the thermal chamber as short as possible and only specimens dried at 105 °C were further considered within this study. Results of the 80, 300 and 600 °C serve as indicator. The decomposition of the cementpaste matrix at temperatures above 105 °C results in a higher porosity at 300 °C and 600 °C while the presence of free and physically-bound water for 80 °C reduces the porosity. Considering specimens dried at 105 °C, an increasing porosity within the considered w/c-ratios ranging from 10.2% and 11.9% to 14.1% for w/ c = 0.4, 0.5 to 0.6 is obtained. Those results served as input for determination of the saturation degree for the specimens used for fire testing (see Section 2).
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and 11. These two slabs differ in w/c-ratio, initial residual permeability and saturation degree (see Table 6). The most striking result to emerge from the obtained data is that WCR04-1 exhibited severe spalling with a resulting damaged area of 78% within a short period of time (80 s) whereas WCR06-1 shows a damaged area of only 32% during 120 s of spalling. This is remarkable because the initial saturation degree of WCR04-1 is significant lower (58%) than the saturation degree of WCR0.6-1 with 90%. This behavior highlights the influence of the transport properties, with the intrinsic residual permeability being about 3 times higher for WCR06-1 than for WCR04-1 (see Table 6). Summarizing all fire tests (see Fig. 12), the spalling start was detected between 30 and 139 s with spalling durations from 75 to 120 s. Assessing the contribution of the initial saturation degree on spalling (see Fig. 5) the following trend could be identified: a lower saturation degree generally caused a time lag in the start of spalling (see Fig. 12) whereas the permeability governed the mode of spalling: a low permeability results in explosive spalling of small pieces with a size of a few millimeter, so-called popping of concrete (as shown for w/c = 0.4 in Fig. 10), whereas explosive spalling of bigger pieces was observed for higher permeability (as shown for w/c = 0.6 in Fig. 11), with the size of pieces reaching several centimeters. Indeed, the mode of spalling is found to depend on the saturation degree as well, with a combination of popping and explosive spalling being observed for specimens with a low saturation degree, e.g., for WCR05-3 and WCR05-4. Approximately 18 h after fire testing, the mass of the spalled-off concrete and the damage level were evaluated on the cooled-down slabs. The mass of spalled-off concrete was obtained from the concrete flakes and pieces collected from the bottom of the test chamber after fire testing. The obtained results are given Fig. 13. Additionally the spalling depth at the fire-exposed surface was recorded at given grid points (20 mm distance between adjacent grid points), with the obtained results summarized in Table 7. The maximum spalling depth ranges from 5 to 10 mm with a mean value between 1.6 and 6.5 mm considering all conducted tests.
4.2. Fire tests 5. Discussion In order to highlight the significant influence of transport properties and the saturation degree on the extent of spalling, two concrete slabs (WCR04-1 and WCR06-1) were chosen for a detailed presentation, with their spalling histories given in Figs. 10
In the following, the influence of the identified governing parameters, the water content and the permeability, on spalling is discussed and a method to address the risk of spalling is intro-
Fig. 10. Spalling history of concrete with w/c-ratio = 0.4 (first slab, referred to as WCR04-1) with initial saturation of 58%.
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Fig. 11. Spalling history of concrete with w/c-ratio = 0.6 (first slab, referred to as WCR06-1) with initial saturation of 90%.
Table 6 Initial properties and spalling results for WCR04-1 and WCR06-1.
w/c-ratio Saturation degree Intrinsic permeability Spalling start Spalling duration Damage level
WCR04-1
WCR06-1
0.4 58% 16.9 nm2 43 sec 80 sec 78%
0.6 90% 52.6 nm2 44 sec 120 sec 32%
duced. Currently, international standards and national appendices conclude to a spalling criteria of a concrete mixture by limiting the water content to a certain amount e.g. 2 M% (see Refs. [14,15]), to classify if spalling occurs or not. The presented method aims to extend this criteria by involving additional parameters responsible for spalling as well as the water/cement ratio to separate between concrete mixtures to evaluate their spalling risk more accurately. The influence of the water content on spalling is shown in Fig. 14 where the trend of each mixture (w/c = 0.4/0.5/0.6) is indi-
Fig. 12. Spalling start, duration and saturation degree for w/c = 0.4/0.5/0.6.
Fig. 13. Spalled-off mass and damage level for w/c = 0.4/0.5/0.6.
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150 Table 7 Maximum, mean and standard deviation (SD) of spalling depth [mm]. Test Nr.
w/c = 0.4 2
Sr [%] 1 2 3 4 5 6 1 2
58 57 35 42 21 18
w/c = 0.5 Max. depth 9 10 5 8 0 0
Mean 1.6 2.6 2.3 4.4 0.0 0.0
SD 1.4 2.1 1.1 1.8 0.0 0.0
2
Sr [%] 87 90 45 52 7 10
w/c = 0.6 Max. depth No meas 10 5 8 0 0
1
Mean
SD
Sr2 [%]
Max. depth
Mean
SD
– 1.6 3.9 3.6 0.0 0.0
– 1.6 1.2 0.6 0.0 0.0
90 73 61 75 4 6
10 8 No meas1 5 0 0
6.5 5.1 – 2.8 0.0 0.0
1.6 1.3 – 1.1 0.0 0.0
No measurement could be made due to failure of the specimen. Sr. . . saturation degree measured at a depth from 0 to 2.5 cm.
Fig. 14. Influence of water content on damage level for w/c = 0.4/0.5/0.6.
cated by a grey area. Up to a water content of 1.0 M% none of the specimens showed spalling and therefore no damage level was recorded. When considering each mixture separately, damage increased with increasing water content. In addition to the water content, the permeability has a significant influence on spalling. For instance, the specimens with w/c = 0.6 show the highest water content (between 4.6 M% and 5.6 M%) but exhibited damage levels below 34%. Furthermore, the damage level is found to be very sensitive to the initial water content and shows a rapid increase within a small range of M%. The highest sensitivity is observed for w/c = 0.4, where a 1.0 M% increase in water content (from 1.5 to 2.5 M%) results in an increase of 64% of the damage level (from 14 to 78%). A lower sensitivity was observed for w/c = 0.5 where a 2.1 M% change in water content showed an increase of 82% of the damage level. The specimens with a w/c = 0.6 showed the lowest sensitivity resulting in an increase of the damage area of 30% within a 1.0 M% increase in water content. In addition to the water content, the permeability is found to strongly affect the damage level. While permeability and tensile strength are directly linked to the underlying mixture (via the w/ c-ratio) the water content may vary depending on environmental condition. As the underlying mixture affects permeability and tensile strength differently i.e., higher w/c ratio results in an increase of permeability but, on the other hand, in a drop of tensile strength a clear correlation between mixture and damage level is not straightforward. Following the approach presented in [29], the risk of spalling may be assessed by means of the so-called ‘‘required RE
initial tensile strength” f t;0 , being a function of permeability (i.e., concrete mixture) and water content (see Fig. 15). In the work of
RE
Fig. 15. Numerically-determined required initial tensile strength f t;0 for w/c = 0.4/ RE 0.5/0.6 as a function of permeability and water content; f t;0 obtained from [29].
Zhang et al. [29] the authors propose an estimation method for a fast assessment of the fire spalling risk of tunnel linings. The estimation method is developed, based on a well-established coupled thermo-hydro-chemo-mechanical model, considering specific boundary condition of tunnel linings (i.e., plate-like structure subjected to one-side heating). Based on parameter studies which cover a large range of concrete mixes and environmental conditions, empirical functions are developed to estimate the so-called RE
‘‘required initial tensile strength” f t;0 , to quantifying the spalling RE
risk. This required initial tensile strength, f t;0 , describes the minimum numerically obtained tensile strength of the concrete required to withstand spalling during a fire event taking material properties, transport properties as well as saturation of the concrete into account. Therefore, concrete structures with a tensile strength, ft,0 > fRE t,0, have a low risk of fire spalling, structures with a tensile strength, ft,0 < fRE t,0, exhibit a high risk of fire spalling The properties of the underlying concrete mixture were considRE
ered during determination of f t;0 (according to Ref. [29]) but, however, are not included in the presented correlation (water content RE
vs. required strength) in Fig. 15. e.g., f t;0 may be scaled by the actual tensile strength of the mixture f t , as shown in Fig. 16 (left). Alternatively, the water content may be scaled by the permeability of the respective concrete mixture, leading to Fig. 16 (right). RE
By scaling the numerically obtained tensile strength f t;0 [29] by the experimentally obtained tensile strength f t , on the one hand, and the water content by the permeability, on the other hand, the influence of all three parameters (i.e., tensile strength,
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150
RE
RE
Fig. 16. Required tensile strength f t;0 and water content scaled by mixture properties, i.e., initial tensile strength f t (left) and permeability (right), respectively, f t;0 obtained from [29].
testing device was proposed, providing temperature loading within the accuracy demanded by standards. The presented testing procedure was applied to identify the influence of transport properties (triggered by the water/cement-ratio) and the initial saturation degree. The obtained results were compared to the required initial tensile strength obtained from a numerical assessment method recently published in Ref. [29]. Based on the obtained results, the three questions stated in the introduction of this paper are addressed as: 1. How do initial water content and permeability influence spalling?
Fig. 17. Influence of water content scaled by permeability on damage level for w/c = 0.4/0.5/0.6.
permeability, and water content) on spalling becomes apparent. Interestingly, a linear relation for the required tensile strength as a function of the water content/permeability-ratio was encountered (see Fig. 16 (right)). For all investigated concrete mixtures, neither a critical permeability nor a critical water content could be specified exclusively to determine the risk of spalling. On the other hand, the water content/permeability-ratio shows a clear correlation, giving higher required tensile strengths as the ratio increases. The same linear correlation is encountered for the damage level shown in Fig. 14 when scaling the water content with the permeability, (see Fig. 17). As a result, the water content/ permeability-ratio can be identified as the critical parameter for the assessment of the risk of spalling. That means that, for a given water content and permeability, the required tensile strength may be specified in order to avoid spalling. 6. Concluding remarks In this paper, a testing procedure for the assessment of the risk and extent of spalling was presented. This procedure comprises the specification of key material parameter as well as recommendations for fire tests. As regards the latter, a novel two-chamber
As a matter of fact, the presented results highlighted the strong influence of both parameters on spalling. A change in permeability, triggered by the w/c-ratio, also affects the tensile strength of concrete. This change in tensile strength leads to a trend opposite to the beneficial effect of the permeability on spalling. Only for water contents below 1 M% no tested specimen showed spalling. Above 1 M%, a small increase of the water content resulted in a significant increase of damage of concrete, especially for mixtures with low permeability. But also concrete with a high permeability experienced spalling for a water content of 5.6 M%. Accordingly, neither the permeability nor the water content alone could be specified as critical parameter causing spalling. 2. Is it possible to identify key parameters to assess the spalling risk? Observations within the presented study highlighted that no clear correlation between water content and damage level exists but also concrete properties such as permeability and tensile strength need to be taken into account since they clearly affect the damage level. In fact, the water content/permeability-ratio was identified as critical parameter showing a linear relation with the required initial tensile strength which may be used for quantification of the spalling risk of a concrete mixture. Fire exposure of one side of the specimen causes a temperature gradient within the specimen which induces additional stresses to the concrete matrix. These stresses combined with the built-up vapor pressure result in a higher stress level within the matrix and therefore spalling might occur sooner. Due to the selected test setup, where dilatation of the specimen is allowed, the contribution of the stresses due to the thermal gradient is minimized and the vapor pressure governs the stress state within the specimens.
M. Maier et al. / Construction and Building Materials 231 (2020) 117150
3. How to proceed in risk assessment of spalling? In contrast to the critical water content of 3 M% or 2 M% currently used in international standards and national appendices (see e.g. [14,15]), the use of the required initial tensile strength obtained from the water content/permeability-ratio is proposed in this paper. While below a water content of 1.0 M%, no spalling may occur, concrete is certainly at risk of spalling with increasing water content – even for concrete mixtures with high permeability. This can be properly assessed by the water content/permeabilityratio in combination with the required tensile strength according to Ref. [29]. The basis for the combined assessment of the spalling risk employing the water content/permeability-ratio and the required tensile strength [29] is provided by the experimental procedure outlined in this paper, comprising identification of key material properties, such as the mechanical properties (Young’s modulus, compressive and tensile strength), moisture content (measured at different depths), the open porosity, and the permeability as well as the fire-test itself. As regards the latter, a Two-ChamberFire Furnace was designed which allow to accurately follow the time-temperature history required by international standards and recommendations especially at the beginning of the fire test. Special emphasis was laid on the correct representation of the temperature loading, as this affect the risk and extent of spalling. Restraining the thermal dilation during the fire test by, e.g., external pre-stressing of the concrete slabs as performed in [30,31] may be included within the presented fire-test setup and is topic of ongoing work. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support of the Austrian Ministry for Transport, Innovation and Technology (bm. vit) within the KIRAS-project (Austrian security research program) 813794 ‘‘Sicherheit von Hohlraumbauten unter Feuerlast” (‘‘Safety of underground structures under fire loading”). References [1] M. Zeiml, D. Leithner, R. Lackner, H.A. Mang, How do polypropylene fibers improve the spalling behavior of in-situ concrete?, Cem. Concr. Res. 36 (5) (2006) 929–942, https://doi.org/10.1016/j.cemconres.2005.12.018. [2] P. Kalifa, G. Chéné, C. Gallé, High-temperature behaviour of HPC with polypropylene fibres: From spalling to microstructure, Cem. Concr. Res. 31 (10) (2001) 1487–1499, https://doi.org/10.1016/S0008-8846(01)00596-8. [3] J.C. Mindeguia, H. Carré, P. Pimienta, C.L. Borderie, Experimental discussion on the mechanisms behind the fire spalling of concrete, Fire Mater. 39 (7) (2015) 619–635, https://doi.org/10.1002/fam.2254. [4] W. Kusterle et al., Brandbeständigkeit von Faser-, Stahl- und Spannbeton [Fire resistance of fiber-reinforced, reinforced, and prestressed concrete], Bundesministerium für Verkehr, Innovation und Technologie, Vienna, 2004. vol. Tech. Rep. 544. [5] K.D. Hertz, Limits of spalling of fire-exposed concrete, Fire Saf. J. 38 (2) (2003) 103–116, https://doi.org/10.1016/S0379-7112(02)00051-6. [6] 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, Mater. Struct. 34 (2) (2001) 83–91, https://doi.org/10.1007/ BF02481556.
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[7] T.P. Long, J.C. Nicholas, Effects of test conditions and mixture proportions on behavior of high-strength concrete exposed to high temperatures, Mater. J. 99 (1) (2002), https://doi.org/10.14359/11317. [8] R. Jansson, L. Boström, Fire spalling - the moisture effect, Presented at the First International Workshop on Concrete Spalling due to Fire Exposure, MFPA, Leipzig, 2009. [9] G.H.A. van der Heijden, H.P. Huinink, L. Pel, K. Kopinga, One-dimensional scanning of moisture in heated porous building materials with NMR, J. Magn. Reson. 208 (2) (2011) 235–242, https://doi.org/10.1016/j.jmr.2010.11.010. [10] F. Pereira et al., Monitoring of fire damage processes in concrete by pore pressure and acoustic emission measurements, Presented at the 2nd International RILEM Workshop on Concrete Spalling due to Fire Exposure, 2011. [11] G.H.A.V. der Heijden, L. Pel, O.C.G. Adan, Fire spalling of concrete, as studied by NMR, Cem. Concr. Res. 42 (2) (2012) 265–271, https://doi.org/10.1016/j. cemconres.2011.09.014. [12] R. Felicetti, F. Lo Monte, P. Pimienta, A new test method to study the influence of pore pressure on fracture behaviour of concrete during heating, Cem. Concr. Res. 94 (2017) 13–23, https://doi.org/10.1016/j.cemconres.2017.01.002. [13] 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 (3) (2010) 477–487, https://doi.org/10.1016/j.cemconres.2009.10.011. [14] European Committee for Standardization, EN 1992-1-2 – Design of Reinforced and Prestressed Concrete Structures – Part 1–2: General Rules – Structural Fire Design, 2010. [15] European Committee for Standardization, ÖNORM B 1992-1-2 – Design of Reinforced and Prestressed Concrete Structures – Part 1–2: General Rules – Structural Fire Design, 2011. [16] C. Gallé, Effect of drying on cement-based materials pore structure as identified by mercury intrusion porosimetry: a comparative study between oven-, vacuum-, and freeze-drying, Cem. Concr. Res. 31 (10) (2001) 1467– 1477, https://doi.org/10.1016/S0008-8846(01)00594-4. [17] L. Pel, S. Jaspers, F. Pereira, P. Pimienta, H. Carré, Combined NMR moisture, temperature and pressure measurements during heating, MATEC Web Conf. 6 (2013), https://doi.org/10.1051/matecconf/20130603005. [18] R. Jansson, L. Boström, The influence of pressure in the pore system on fire spalling of concrete, Fire Technol. J. 46 (1) (2009) 217, https://doi.org/10.1007/ s10694-009-0093-9. [19] G.H.A. van der Heijden, R.M.W. van Bijnen, L. Pel, H.P. Huinink, Moisture transport in heated concrete, as studied by NMR, and its consequences for fire spalling, Cem. Concr. Res. 37 (6) (2007) 894–901, https://doi.org/10.1016/j. cemconres.2007.03.004. [20] H. Lun, Permeabilität von Beton unter kombinierter thermischer und mechanischer Belastung (Ph.D. Thesis), Material Technology Innsbruck, University of Innsbruck, 2015. [21] L.J. Klinkenberg, The permeability of porous media to liquids and gases, Drilling and Production Practice, 1941. [22] ÖNORM – EN 1936: Natural Stone Test Method – Determination of Real Density and Apparent Density and of Total and Open Porosity, A. Standards, 2007. [23] P. Paulini, A laboratory and on-site test method for air permeability of concrete, Presented at the 2nd International Symposium on Service Life Design for Infrastructure, Delft, 2010. [24] H. Lun, R. Lackner, Permeability of concrete under thermal and compressive stress influence – an experimental study, MATEC Web Conf. 6 (2013), https:// doi.org/10.1051/matecconf/20130603007. [25] International Organization for Standarization: ISO 834-1 – Fire-Resistance Tests – Elements of Building Construction – Part 1: General Requirements, 1999. [26] European Committee for Standardization: ÖNORM 1363-1 – Fire Resistance Test – General Requirements, 2012. [27] European Committee for Standardization: ÖNORM 1363-2 – Fire Resistance Test – Alternative and Additional Procedures, 2000. [28] Austrian Research Association for Roads, Railways and Transport: RVS 09.01.45 – ‘‘Baulicher Brandschutz in Strassentunnel” – ‘‘Fire Safety of Road Tunnels”, 2015. [29] Y. Zhang, M. Zeiml, M. Maier, Y. Yuan, R. Lackner, Fast assessing spalling risk of tunnel linings under RABT fire: from a coupled thermo-hydro-chemomechanical model towards an estimation method, Eng. Struct. 142 (2017) 1– 19, https://doi.org/10.1016/j.engstruct.2017.03.068. [30] K. Mroz, I. Hager, M. Tekieli, ‘‘Effect of cold rim on extent and typo of concrete fire spalling, Presented at the 5th International RILEM Worshop on Concrete Spalling due to Fire Exposure, Boras, Schweden, 2017. [31] M. Ozawa, T. Tanibe, M. Kanematsu, T. Morita, Screening-test analysis of fire spalling behavior with various concrete samples, Presented at the 5th Intrenational RILEM Worshop on Concrete Spalling due to Fire Exposure, Boras, Schweden, 2017.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
On the effect of pore-space properties and water saturation on explosive spalling of fire-loaded concrete Marcus Maier ⇑, Matthias Zeiml, Roman Lackner Material Technology Innsbruck (MTI), University of Innsbruck, Technikerstrasse 13, A-6020 Innsbruck, Austria
h i g h l i g h t s A novel Two-Chamber-Fire-Furnace is presented that allows an improved reproduction of temperature histories. Relevant parameters causing explosive fire-spalling of concrete are identified. Correlations between governing parameters to evaluate the spalling behavior of concrete mixtures are presented. An assessment method to evaluate the spalling risk is introduced.
a r t i c l e
i n f o
Article history: Received 25 April 2019 Received in revised form 30 September 2019 Accepted 3 October 2019
Keywords: Mechanical properties Permeability Transport properties Concrete Fire spalling Assessment method
a b s t r a c t In this paper, the influence of concrete properties such as permeability and environmental conditions (water content) on the spalling behavior of concrete subjected to fire loading is investigated. For this purpose, a special fire-test setup is presented, allowing an improved reproduction of temperature histories observed, e.g., in tunnel fires. Moreover, the developed test setup enables a continuous monitoring of spalling, giving access to the spalling history and the final level of damage of concrete specimens. The obtained results, considering different water/cement-ratios and saturation degrees, are related to the required initial tensile strength determined by means of a numerical assessment tool. Finally, correlations between the identified parameters governing fire-spalling were established and allowed an evaluation of the spalling risk of concrete mixtures. These correlations revealed that – in contrast to requirements given in national and international standards – the combination of water content and permeability may serve as proper key and design parameter, defining the risk of spalling. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Fire accidents in tunnels lead to severe damage to tunnel linings caused by thermomechanical and/or thermohygral processes inducing spalling. In the past, extended research on concrete was carried out to identify the governing parameters causing explosive spalling associated with the aforementioned thermohygral processes (see, e.g., [1–4]). In addition to the mechanical properties of concrete, heating rate, pore volume, moisture content and permeability were identified as main parameters influencing the occurrence and extent of spalling [5–7]. As regards explosive spalling, the increasing water-vapor pressure within the concrete pore system is assumed to be the governing force causing spalling in case the transport properties of concrete are insufficient to evacuate the water-vapor fast enough.
⇑ Corresponding author. E-mail address: [email protected] (M. Maier). https://doi.org/10.1016/j.conbuildmat.2019.117150 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
As regards transport towards the ‘‘cold side”, water-vapor transport results in condensation and hence accumulation of water, forming the so-called moisture clog (see, e.g., [8,9]). This effect and the associated increase in water-vapor pressure was investigated experimentally and numerically by several authors [2,10– 13]. In order to avoid explosive spalling, poly-propylene (PP) fibers were added to the concrete mix, increasing the transport properties of concrete because they provide additional space for the water-vapor in the course of temperature loading. The positive effect of poly-propylene (PP) fibers on the transport properties and fire-protection properties was reported, e.g., in Ref. [4] (see Fig. 1). In case no PP-fiber reinforced concrete is employed, the behavior in case of fire as well as the potential need for fireprotection measures is standardly linked to the water content of concrete, e.g., according to the Eurocode [14], spalling of concrete is not expected for concrete with a water content less than 3 M% [14]. In addition, national appendices to the Eurocode may define different threshold values such as, e.g., the Austrian appendix
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Fig. 1. Effect of transport properties of concrete on occurrence and extent of explosive spalling: (left) ordinary concrete and (right) concrete with PP fibers [4].
giving a threshold value of 2 M% [15]. Actually, the high influence of water saturation and the pore volume (defining the water content for a certain saturation) was also confirmed numerically [1]. This paper is devoted to the experimental verification and quantification of the effect of water saturation and pore volume on the progress and final extent of explosive spalling. In this paper, real-life situations and fire scenarios are considered which resulted in two challenges to deal with: 1. The application of thermal loading in fire tests is of crucial importance, as the temperature history strongly affects the risk and extent of explosive spalling. e.g. fire tests on samples subjected to a moderate heating rate show different spalling results compared to samples subjected to high heating rates. Furthermore, single-chamber furnaces face the challenge to accurately follow the required temperatures within the first minutes of the fire tests where already spalling occurs. To provide remedy to this challenge, a novel Two-Chamber Fire Furnace (TCFF) is presented, providing an improved reproduction of temperature loading. 2. The range of material and environmental properties present in real-life situations needs to be captured, setting conditions on the experimental program covering different types of concrete and different extents of initial saturation prior to fire testing. In addition to the fire tests, temperature-dependent material properties (mechanical and transport properties) were determined experimentally. Based on the obtained experimental results, the following questions shall be answered: 1. How do initial water content and permeability influence spalling? 2. Is it possible to identify key parameters to assess the spalling risk? 3. How to proceed in risk assessment of spalling? This paper is organized as follows: In the following chapter, the employed types of concrete as well as specimen preparation and conditioning are described. The permeability-testing procedure and the developed fire furnace are presented in Section 3. Section 4 contains the obtained experimental results, dealing in detail with the progress and extent of spalling as a function of saturation and transport properties. Section 5 includes the discussion of the obtained results and finally, Section 6 closes with concluding remarks. 2. Materials In order to provide different types of concrete characterized by varying mechanical and permeability properties, normal-weight
concrete with a water/cement-ratio (w/c-ratio) of 0.4, 0.5, and 0.6 was produced. The underlying mix designs are given in Table 1. Compressive strength, Young’s modulus, and tensile strength were measured after 28 days of storage in water on cylindrical specimens with a diameter of 100 mm and a height of 300 mm. The tensile strength was obtained indirectly by conducting cylinder-splitting tests (Brazilian test). The obtained properties for the employed types of concrete are listed in Table 2 (identification of residual permeability parameters and porosity is described in detail in the following section). 2.1. Concrete specimens for fire testing The influence of saturation and transport properties on spalling was investigated on slabs of 500 800 120 mm3 (L W H). All concrete slabs were stored under water for 28 days for curing. It should be noted that this curing condition differs from construction practice where the concrete members will be demolded earlier and are exposed to the environmental conditions. Nevertheless, this curing condition was chosen to ensure fully saturated samples as a starting point for the investigation on the fire spalling behavior. The weight of the fully saturated samples served as initial mass to control the partly saturated stages of the specimens by monitoring its mass loss during drying. Fully saturated samples were therefore fire-tested right after they were removed from the water whereas partly saturated specimens were first dried in a temperature chamber at 60 °C till a desired mass was achieved. During fire testing, the bottom surface of the specimen (500 500 mm2) was exposed to a pre-specified temperature loading (hydrocarbon fire curve, HC1200) for 90 min with a maximum temperature of 1200 °C. In total, six slabs with different initial moisture contents (ranging from saturated to dry) were tested for each w/c-ratio (0.4, 0.5 and 0.6), giving a total of 18 specimens for fire testing. The specimens for fire testing and their saturation degree measured at a depth of 0–2.5 cm are given in Table 3. During fire testing, the temperature distribution within the slab was recorded by two sets of Type K thermocouples positioned at a distance of 5/10/20/30/40/50/70/90 mm from the heated surface.
Table 1 Mix designs of considered types of concrete. Cement: CEM I 42.5R Fly Ash (Hydraulit M) Water w/c-ratio Aggregates: siliceous Agg. size 0–4 mm Agg. size 4–8 mm Agg. size 8–16 mm Agg. size 16–32 mm
kg/m3 kg/m3 kg/m3 – kg/m3 Mass % Mass % Mass % Mass %
290 50 132/165/198 0.4/0.5/0.6 1888 30 30 20 20
3
M. Maier et al. / Construction and Building Materials 231 (2020) 117150 Table 2 Properties of concrete with w/c-ratio = 0.4/0.5/0.6. Water-cement ratio Young’s modulus Compressive strength Tensile strength porosity Permeability at 105 °C
E fc ft n kint b
0.4 40.1 63.7 4.8 10.2 16.9 1.6
GPa MPa MPa – nm2 bar
0.5 35.2 52.2 4.1 11.9 30.8 1.6
0.6 29.5 38.7 2.7 14.1 52.6 1.6
Table 3 Water/cement ratio and saturation degree of specimens prepared for fire testing. w/c = 0.4
w/c = 0.5
w/c = 0.6
Specimen [–]
Saturation [%]
Specimen [–]
Saturation [%]
Specimen [–]
Saturation [%]
WCR04-1 WCR04-2 WCR04-3 WCR04-4 WCR04-5 WCR04-6
58 57 35 42 21 18
WCR05-1 WCR05-2 WCR05-3 WCR05-4 WCR05-5 WCR05-6
87 90 45 52 7 10
WCR06-1 WCR06-2 WCR06-3 WCR06-4 WCR06-5 WCR06-6
90 73 61 75 4 6
These thermocouples consist of two 0.2 mm wires made of NickelChromium and Nickel-Alumel insulated with glass silk. As shown in Fig. 2, the thermocouples are positioned from the top of the mold at the required depth, hence, not affecting the temperature ingress from the heated surface during fire testing. The location of the thermocouples is shown in Fig. 3. The numbers shown refer to the depth from the heated surface (in [cm]). Additionally, a video camera recording the bottom surface was employed, to record spalling events and the damage in the course of the fire test. To investigate the influence of saturation on the spalling behavior, the concrete slabs were dried at 60 °C for various durations. The slabs were stored in a temperature chamber and weighed periodically until the designated mass loss was achieved. The saturation degree of the partly saturated samples was controlled by monitoring the mass loss of the initially fully saturated samples. This was achieved by weighing the samples daily till a designated mass loss was achieved and therefore varying saturation degrees could be obtained. To avoid dehydration of the concrete samples, 60 °C was chosen for drying as temperatures above 60 °C will indicate ettringite dehydration and therefore to a change in the microstructure [16]. To identify the initial saturation degree on the day of fire testing, drill dust was collected from each slab and
Fig. 3. In-plane position of the thermocouples within the concrete slab. The numbers refer to the depth in cm of the measurement points measured from the heated surface.
dried at 105 °C before testing. Concrete structures, especially tunnel linings exhibit different moisture contents e.g. throughout
Fig. 2. Mounting of thermocouples before/during casting.
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the tunnel length as well as throughout the thickness of the concrete member. Inhomogeneous moisture distributions within a concrete member result in different amounts of water vapor during heating. This results in varying vapor pressures within the concrete matrix that has to be released by the permeability and/or the forming cracks to avoid spalling. Due to the build-up vapor pressure, water vapor also moves deeper inside the concrete member to colder regions, condensates and, depending on the available evaporable water, forms the so called ‘‘Moisture Clog”. At this steamtight zone, vapor pressure increases over time till the actual tensile strength of the concrete is met and spalling might occur. To consider this effect the inhomogeneous moisture distribution within the specimens was evaluated within two different depths. The depth of 0.0–2.5 cm and 2.5–5.0 cm was chosen as this was the expected spalling depth. Drill powder samples were taken from both sides of the slab giving a total of 4 samples for each test specimen. Based on the obtained change in density, the saturation degree Sr was determined as
Sr ¼
qs qd nqW
where qs and qd represent the density before and after drying at 105 °C, respectively, n the porosity (see the following section for description of the test method for determination of n) and qw the density of water. Figs. 4 and 5 present the obtained water content and saturation degree for the 18 slabs right before fire testing. 2.2. Concrete specimens for material characterization In addition to fire testing, the employed types of concrete are characterized as regards their (temperature-dependent) mechanical and transport properties (see Table 2). For this purpose, discs with a diameter of 100 mm and a thickness of 30 mm served as
specimens. In addition to the initial properties at room temperature, residual permeability and porosity tests were performed on specimens subjected to heating levels of 80/105/300/600 °C. For this purpose, the specimens were heated with a temperature increase of 2 °C/min in order to avoid cracking due to thermal gradients within the specimens. Each temperature level was kept for 12 h to ensure a uniform temperature distribution before the specimens were naturally cooled down for testing. 3. Methods 3.1. Permeability and porosity As highlighted by many authors, transport properties as well as the water saturation have a significant influence on the spalling behavior of concrete [17–19]. Therefore, in addition to the fire tests, residual air permeability and open porosity tests were performed within the presented research work. Permeability was determined using the experimental setup given in Fig. 6, giving access to the so-called intrinsic residual permeability kint [nm2] and Klinkenberg’s parameter b [bar]. To conduct the residual airpermeability test, the test specimen is placed between two steel pipes and encased with a rubber membrane (Fig. 6). Pressure is applied to the membrane to seal the lateral surface of the specimen and hence to ensure a one-dimensional air flow through the specimen as illustrated in Fig. 6 (right). More details on measurement equipment and calculation of the permeability parameters can be found in Refs. [20,21]. For the open-porosity tests, the air-permeability specimens were reused for testing according to Ref. [22]. Hereby, the open porosity n is obtained by
n¼
ms md 100 ms mh
Fig. 4. Water content by weight loss of WCR 0.4/0.5/0.6 measured right before fire testing (Numbers in Graphs refer to the specimen number).
Fig. 5. Saturation degree for concrete with WCR = 0.4/0.5/0.6 measured right before fire testing (Numbers in Graphs refer to the specimen number).
M. Maier et al. / Construction and Building Materials 231 (2020) 117150
5
Fig. 6. (left) Permeability setup and (right) air flux through the disc-shaped specimen [23,24].
where ms is the partly-saturated mass of the specimen, md the dry mass and mh the mass of the specimen submerged in water. 3.2. Fire-test chamber For the presented experimental program, a novel TwoChamber-Fire-Furnace (TCFF) was developed, consisting of four main parts (see Fig. 7): (i) (ii) (iii) (iv)
oil-burner, combustion chamber, temperature-control unit, and test chamber.
The oil-burner is placed at the front wall of the furnace and feeds the combustion chamber with temperatures up to 1400 °C. This chamber serves as temperature reservoir and is connected to the temperature-control unit. This unit allows an automatic and precise control of the thermal flux into the (adjacent) test chamber (L W H = 1400 500 400 mm3) following any desired time-temperature curve. The temperature-control unit adjusts the burner capacity as well as the thermal flux by taking the mean value of two 3 mm insulated Type K thermocouples placed in the test chamber. The benefit of the proposed two-chamber setup is the separation of the flame (oil-burner) from the test chamber. Accordingly, the thermal flux to and hence the temperature loading within the test chamber does neither depend on the capacity of the
oil-burner nor on the time span of temperature loading. Standardly, single-chamber oil furnaces tend to significantly overdrive moderate heating rates as shown in Fig. 8 (left) or underdrive the prescribed chamber temperature, in case of high heating rates (e.g., hydrocarbon fire curve) are required. As indicated in Fig. 8, the presented TCFF is able to capture both moderate heating rates as specified for the ISO 834 fire curve [25,26] (see Fig. 8 (left)) and high heating rates (hydrocarbon fire curve HC1200 [27], see Fig. 8 (right)) as employed within the experimental program. In fact, the achieved temperature in the TCFF meets the accuracy requirements of RVS 09.01.45 (Austrian Research Association for Roads, Railways and Transport) [28] given by the grey curves (lower and upper limit) in Fig. 8(right). 4. Results 4.1. Concrete properties 4.1.1. Permeability Within this study the residual permeability of the concrete mixtures was evaluated, meaning that the samples were heated up to the designated temperature to cause thermal damage, then cooled down and the residual permeability was measured at room temperature. This procedure was chosen due to the challenges which appear for measuring the so called ‘‘hot permeability” and the lack of available ‘‘hot permeability” instrumentation. It should be noted that the residual permeability might underestimate the permeability measured at the designated temperature due to the fact that the
Fig. 7. Design of Two-Chamber Fire Furnace (TCFF) (dimensions in [cm]).
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150
Fig. 8. Temperature loading provided by TCFF: (left) ISO 834 fire curve and (right) hydrocarbon fire curve HC1200 (SCFF = Single-Chamber-Fire-Furnace; TCFF = Two-ChamberFire-Furnace).
heated specimen expands resulting in expanded cracks and air pathways. Results of the temperature-dependent residual permeability (mean value and standard deviation) are shown in Fig. 9 and given in Table 4. For each data point at least 4 samples were testet. The highest residual permeability was observed for w/ c = 0.6, the lowest for w/c-ratio 0.4. Considering the results at 105 °C (kint = 16.9, 30.8 and 52.7 nm2 for w/c = 0.4, 0.5 and 0.6), a 1.8 and 3.1 times higher permeability for w/c = 0.5 and 0.6 is observed compared to w/c = 0.4. Results of the 80 °C tests show lower residual permeability than at 105 °C except for w/c = 0.6. It should be noted that at 80 °C the specimens still contain free water which hinders the air flow through the specimen. All specimens
tested at 105 °C are considered to be dry with no physicallybound or free water present within the specimen. Temperature loading above 105 °C results in decomposition of the cementpaste matrix associated with micro- and macro-cracking, leading to an increase of the residual permeability. Due to the high extent of decomposition for specimens heated up to 600 °C, no permeability could be measured for w/c = 0.4. 4.1.2. Porosity The open porosity results, obtained according to ÖNORM EN 1936 [22], as a function of temperature loading are listed in Table 5, showing a continuous increase of the open porosity from w/c = 0.4
Fig. 9. Intrinsic residual permeability kint and b as a function of temperature.
Table 4 Intrinsic residual permeability kint [nm2] and b [bar] for concrete with w/c-ratio = 0.4/0.5/0.6 (mean value and standard deviation SD in [%]). 80 °C
105 °C
300 °C
600 °C
kint
b
kint
b
kint
b
kint
b
w/c = 0.4
Mean value SD[%]
6.55 15,1
1.21 40.4
16.9 14,5
1.59 55.3
295.9 12,0
0.75 7.1
– –
– –
w/c = 0.5
Mean value SD[%]
20.2 25.9
1.17 34.0
30.8 44.4
1.59 52.3
316.9 13.6
0.66 26.7
12,083 16.2
2.21 27.9
w/c = 0.6
Mean value SD[%]
54.8 13.4
1.19 42.0
52.7 18.3
1.59 9.5
771.3 23.69
0.82 23.6
19,418 28.9
6.33 11.0
M. Maier et al. / Construction and Building Materials 231 (2020) 117150 Table 5 Open porosity [%]. Temperature level
80 °C
105 °C
300 °C
600 °C
w/c = 0.4 w/c = 0.5 w/c = 0.6
9.6 10.6 13.7
10.2 11.9 14.1
11.6 13.1 16.4
16.5 16.8 19.7
to 0.6. The open porosity defines the fraction of volume that can be occupied by water within an interconnected network of the concrete matrix. These pores and channels are connected to the concrete surface and interact with the surrounding moisture resulting in a change of the saturation of a concrete member. Hence, the open porosity is a parameter to conclude to the maximum amount of water that can be contained within a concrete matrix. Drying and rewetting of concrete samples, as done for the open porosity measurements, results in de-, and rehydration and therefore to a change in the microstructure especially for temperatures above 100 °C. Below 100 °C, ettringite dehydration starts at around 60 °C and the C-S-H dehydration initiates at around 100 °C [16]. To fully remove the evaporable water within the matrix before measuring the open porosity, drying of samples at 105 °C till constant mass is commonly used for concrete specimens. To minimize the effect of de-, and rehydration, specimens were kept in the thermal chamber as short as possible and only specimens dried at 105 °C were further considered within this study. Results of the 80, 300 and 600 °C serve as indicator. The decomposition of the cementpaste matrix at temperatures above 105 °C results in a higher porosity at 300 °C and 600 °C while the presence of free and physically-bound water for 80 °C reduces the porosity. Considering specimens dried at 105 °C, an increasing porosity within the considered w/c-ratios ranging from 10.2% and 11.9% to 14.1% for w/ c = 0.4, 0.5 to 0.6 is obtained. Those results served as input for determination of the saturation degree for the specimens used for fire testing (see Section 2).
7
and 11. These two slabs differ in w/c-ratio, initial residual permeability and saturation degree (see Table 6). The most striking result to emerge from the obtained data is that WCR04-1 exhibited severe spalling with a resulting damaged area of 78% within a short period of time (80 s) whereas WCR06-1 shows a damaged area of only 32% during 120 s of spalling. This is remarkable because the initial saturation degree of WCR04-1 is significant lower (58%) than the saturation degree of WCR0.6-1 with 90%. This behavior highlights the influence of the transport properties, with the intrinsic residual permeability being about 3 times higher for WCR06-1 than for WCR04-1 (see Table 6). Summarizing all fire tests (see Fig. 12), the spalling start was detected between 30 and 139 s with spalling durations from 75 to 120 s. Assessing the contribution of the initial saturation degree on spalling (see Fig. 5) the following trend could be identified: a lower saturation degree generally caused a time lag in the start of spalling (see Fig. 12) whereas the permeability governed the mode of spalling: a low permeability results in explosive spalling of small pieces with a size of a few millimeter, so-called popping of concrete (as shown for w/c = 0.4 in Fig. 10), whereas explosive spalling of bigger pieces was observed for higher permeability (as shown for w/c = 0.6 in Fig. 11), with the size of pieces reaching several centimeters. Indeed, the mode of spalling is found to depend on the saturation degree as well, with a combination of popping and explosive spalling being observed for specimens with a low saturation degree, e.g., for WCR05-3 and WCR05-4. Approximately 18 h after fire testing, the mass of the spalled-off concrete and the damage level were evaluated on the cooled-down slabs. The mass of spalled-off concrete was obtained from the concrete flakes and pieces collected from the bottom of the test chamber after fire testing. The obtained results are given Fig. 13. Additionally the spalling depth at the fire-exposed surface was recorded at given grid points (20 mm distance between adjacent grid points), with the obtained results summarized in Table 7. The maximum spalling depth ranges from 5 to 10 mm with a mean value between 1.6 and 6.5 mm considering all conducted tests.
4.2. Fire tests 5. Discussion In order to highlight the significant influence of transport properties and the saturation degree on the extent of spalling, two concrete slabs (WCR04-1 and WCR06-1) were chosen for a detailed presentation, with their spalling histories given in Figs. 10
In the following, the influence of the identified governing parameters, the water content and the permeability, on spalling is discussed and a method to address the risk of spalling is intro-
Fig. 10. Spalling history of concrete with w/c-ratio = 0.4 (first slab, referred to as WCR04-1) with initial saturation of 58%.
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150
Fig. 11. Spalling history of concrete with w/c-ratio = 0.6 (first slab, referred to as WCR06-1) with initial saturation of 90%.
Table 6 Initial properties and spalling results for WCR04-1 and WCR06-1.
w/c-ratio Saturation degree Intrinsic permeability Spalling start Spalling duration Damage level
WCR04-1
WCR06-1
0.4 58% 16.9 nm2 43 sec 80 sec 78%
0.6 90% 52.6 nm2 44 sec 120 sec 32%
duced. Currently, international standards and national appendices conclude to a spalling criteria of a concrete mixture by limiting the water content to a certain amount e.g. 2 M% (see Refs. [14,15]), to classify if spalling occurs or not. The presented method aims to extend this criteria by involving additional parameters responsible for spalling as well as the water/cement ratio to separate between concrete mixtures to evaluate their spalling risk more accurately. The influence of the water content on spalling is shown in Fig. 14 where the trend of each mixture (w/c = 0.4/0.5/0.6) is indi-
Fig. 12. Spalling start, duration and saturation degree for w/c = 0.4/0.5/0.6.
Fig. 13. Spalled-off mass and damage level for w/c = 0.4/0.5/0.6.
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150 Table 7 Maximum, mean and standard deviation (SD) of spalling depth [mm]. Test Nr.
w/c = 0.4 2
Sr [%] 1 2 3 4 5 6 1 2
58 57 35 42 21 18
w/c = 0.5 Max. depth 9 10 5 8 0 0
Mean 1.6 2.6 2.3 4.4 0.0 0.0
SD 1.4 2.1 1.1 1.8 0.0 0.0
2
Sr [%] 87 90 45 52 7 10
w/c = 0.6 Max. depth No meas 10 5 8 0 0
1
Mean
SD
Sr2 [%]
Max. depth
Mean
SD
– 1.6 3.9 3.6 0.0 0.0
– 1.6 1.2 0.6 0.0 0.0
90 73 61 75 4 6
10 8 No meas1 5 0 0
6.5 5.1 – 2.8 0.0 0.0
1.6 1.3 – 1.1 0.0 0.0
No measurement could be made due to failure of the specimen. Sr. . . saturation degree measured at a depth from 0 to 2.5 cm.
Fig. 14. Influence of water content on damage level for w/c = 0.4/0.5/0.6.
cated by a grey area. Up to a water content of 1.0 M% none of the specimens showed spalling and therefore no damage level was recorded. When considering each mixture separately, damage increased with increasing water content. In addition to the water content, the permeability has a significant influence on spalling. For instance, the specimens with w/c = 0.6 show the highest water content (between 4.6 M% and 5.6 M%) but exhibited damage levels below 34%. Furthermore, the damage level is found to be very sensitive to the initial water content and shows a rapid increase within a small range of M%. The highest sensitivity is observed for w/c = 0.4, where a 1.0 M% increase in water content (from 1.5 to 2.5 M%) results in an increase of 64% of the damage level (from 14 to 78%). A lower sensitivity was observed for w/c = 0.5 where a 2.1 M% change in water content showed an increase of 82% of the damage level. The specimens with a w/c = 0.6 showed the lowest sensitivity resulting in an increase of the damage area of 30% within a 1.0 M% increase in water content. In addition to the water content, the permeability is found to strongly affect the damage level. While permeability and tensile strength are directly linked to the underlying mixture (via the w/ c-ratio) the water content may vary depending on environmental condition. As the underlying mixture affects permeability and tensile strength differently i.e., higher w/c ratio results in an increase of permeability but, on the other hand, in a drop of tensile strength a clear correlation between mixture and damage level is not straightforward. Following the approach presented in [29], the risk of spalling may be assessed by means of the so-called ‘‘required RE
initial tensile strength” f t;0 , being a function of permeability (i.e., concrete mixture) and water content (see Fig. 15). In the work of
RE
Fig. 15. Numerically-determined required initial tensile strength f t;0 for w/c = 0.4/ RE 0.5/0.6 as a function of permeability and water content; f t;0 obtained from [29].
Zhang et al. [29] the authors propose an estimation method for a fast assessment of the fire spalling risk of tunnel linings. The estimation method is developed, based on a well-established coupled thermo-hydro-chemo-mechanical model, considering specific boundary condition of tunnel linings (i.e., plate-like structure subjected to one-side heating). Based on parameter studies which cover a large range of concrete mixes and environmental conditions, empirical functions are developed to estimate the so-called RE
‘‘required initial tensile strength” f t;0 , to quantifying the spalling RE
risk. This required initial tensile strength, f t;0 , describes the minimum numerically obtained tensile strength of the concrete required to withstand spalling during a fire event taking material properties, transport properties as well as saturation of the concrete into account. Therefore, concrete structures with a tensile strength, ft,0 > fRE t,0, have a low risk of fire spalling, structures with a tensile strength, ft,0 < fRE t,0, exhibit a high risk of fire spalling The properties of the underlying concrete mixture were considRE
ered during determination of f t;0 (according to Ref. [29]) but, however, are not included in the presented correlation (water content RE
vs. required strength) in Fig. 15. e.g., f t;0 may be scaled by the actual tensile strength of the mixture f t , as shown in Fig. 16 (left). Alternatively, the water content may be scaled by the permeability of the respective concrete mixture, leading to Fig. 16 (right). RE
By scaling the numerically obtained tensile strength f t;0 [29] by the experimentally obtained tensile strength f t , on the one hand, and the water content by the permeability, on the other hand, the influence of all three parameters (i.e., tensile strength,
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M. Maier et al. / Construction and Building Materials 231 (2020) 117150
RE
RE
Fig. 16. Required tensile strength f t;0 and water content scaled by mixture properties, i.e., initial tensile strength f t (left) and permeability (right), respectively, f t;0 obtained from [29].
testing device was proposed, providing temperature loading within the accuracy demanded by standards. The presented testing procedure was applied to identify the influence of transport properties (triggered by the water/cement-ratio) and the initial saturation degree. The obtained results were compared to the required initial tensile strength obtained from a numerical assessment method recently published in Ref. [29]. Based on the obtained results, the three questions stated in the introduction of this paper are addressed as: 1. How do initial water content and permeability influence spalling?
Fig. 17. Influence of water content scaled by permeability on damage level for w/c = 0.4/0.5/0.6.
permeability, and water content) on spalling becomes apparent. Interestingly, a linear relation for the required tensile strength as a function of the water content/permeability-ratio was encountered (see Fig. 16 (right)). For all investigated concrete mixtures, neither a critical permeability nor a critical water content could be specified exclusively to determine the risk of spalling. On the other hand, the water content/permeability-ratio shows a clear correlation, giving higher required tensile strengths as the ratio increases. The same linear correlation is encountered for the damage level shown in Fig. 14 when scaling the water content with the permeability, (see Fig. 17). As a result, the water content/ permeability-ratio can be identified as the critical parameter for the assessment of the risk of spalling. That means that, for a given water content and permeability, the required tensile strength may be specified in order to avoid spalling. 6. Concluding remarks In this paper, a testing procedure for the assessment of the risk and extent of spalling was presented. This procedure comprises the specification of key material parameter as well as recommendations for fire tests. As regards the latter, a novel two-chamber
As a matter of fact, the presented results highlighted the strong influence of both parameters on spalling. A change in permeability, triggered by the w/c-ratio, also affects the tensile strength of concrete. This change in tensile strength leads to a trend opposite to the beneficial effect of the permeability on spalling. Only for water contents below 1 M% no tested specimen showed spalling. Above 1 M%, a small increase of the water content resulted in a significant increase of damage of concrete, especially for mixtures with low permeability. But also concrete with a high permeability experienced spalling for a water content of 5.6 M%. Accordingly, neither the permeability nor the water content alone could be specified as critical parameter causing spalling. 2. Is it possible to identify key parameters to assess the spalling risk? Observations within the presented study highlighted that no clear correlation between water content and damage level exists but also concrete properties such as permeability and tensile strength need to be taken into account since they clearly affect the damage level. In fact, the water content/permeability-ratio was identified as critical parameter showing a linear relation with the required initial tensile strength which may be used for quantification of the spalling risk of a concrete mixture. Fire exposure of one side of the specimen causes a temperature gradient within the specimen which induces additional stresses to the concrete matrix. These stresses combined with the built-up vapor pressure result in a higher stress level within the matrix and therefore spalling might occur sooner. Due to the selected test setup, where dilatation of the specimen is allowed, the contribution of the stresses due to the thermal gradient is minimized and the vapor pressure governs the stress state within the specimens.
M. Maier et al. / Construction and Building Materials 231 (2020) 117150
3. How to proceed in risk assessment of spalling? In contrast to the critical water content of 3 M% or 2 M% currently used in international standards and national appendices (see e.g. [14,15]), the use of the required initial tensile strength obtained from the water content/permeability-ratio is proposed in this paper. While below a water content of 1.0 M%, no spalling may occur, concrete is certainly at risk of spalling with increasing water content – even for concrete mixtures with high permeability. This can be properly assessed by the water content/permeabilityratio in combination with the required tensile strength according to Ref. [29]. The basis for the combined assessment of the spalling risk employing the water content/permeability-ratio and the required tensile strength [29] is provided by the experimental procedure outlined in this paper, comprising identification of key material properties, such as the mechanical properties (Young’s modulus, compressive and tensile strength), moisture content (measured at different depths), the open porosity, and the permeability as well as the fire-test itself. As regards the latter, a Two-ChamberFire Furnace was designed which allow to accurately follow the time-temperature history required by international standards and recommendations especially at the beginning of the fire test. Special emphasis was laid on the correct representation of the temperature loading, as this affect the risk and extent of spalling. Restraining the thermal dilation during the fire test by, e.g., external pre-stressing of the concrete slabs as performed in [30,31] may be included within the presented fire-test setup and is topic of ongoing work. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support of the Austrian Ministry for Transport, Innovation and Technology (bm. vit) within the KIRAS-project (Austrian security research program) 813794 ‘‘Sicherheit von Hohlraumbauten unter Feuerlast” (‘‘Safety of underground structures under fire loading”). References [1] M. Zeiml, D. Leithner, R. Lackner, H.A. Mang, How do polypropylene fibers improve the spalling behavior of in-situ concrete?, Cem. Concr. Res. 36 (5) (2006) 929–942, https://doi.org/10.1016/j.cemconres.2005.12.018. [2] P. Kalifa, G. Chéné, C. Gallé, High-temperature behaviour of HPC with polypropylene fibres: From spalling to microstructure, Cem. Concr. Res. 31 (10) (2001) 1487–1499, https://doi.org/10.1016/S0008-8846(01)00596-8. [3] J.C. Mindeguia, H. Carré, P. Pimienta, C.L. Borderie, Experimental discussion on the mechanisms behind the fire spalling of concrete, Fire Mater. 39 (7) (2015) 619–635, https://doi.org/10.1002/fam.2254. [4] W. Kusterle et al., Brandbeständigkeit von Faser-, Stahl- und Spannbeton [Fire resistance of fiber-reinforced, reinforced, and prestressed concrete], Bundesministerium für Verkehr, Innovation und Technologie, Vienna, 2004. vol. Tech. Rep. 544. [5] K.D. Hertz, Limits of spalling of fire-exposed concrete, Fire Saf. J. 38 (2) (2003) 103–116, https://doi.org/10.1016/S0379-7112(02)00051-6. [6] 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, Mater. Struct. 34 (2) (2001) 83–91, https://doi.org/10.1007/ BF02481556.
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