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Thermal, moisture and microbiological boundary conditions of slab-on-ground structures in cold climate
ARTICLE IN PRESS
Building and Environment 43 (2008) 736–744 www.elsevier.com/locate/buildenv
Thermal, moisture and microbiological boundary conditions of slab-on-ground structures in cold climate Jukka Rantala, Virpi Leivo Structural Engineering, Tampere University of Technology, P.O. Box 600, FIN-33101 Tampere, Finland Received 17 July 2006; received in revised form 9 January 2007; accepted 17 January 2007
Abstract Coarse-grained fill or drainage layers beneath heated slab-on-ground structures are warm and moist throughout the year. According to the in situ measurements, the relative humidity of the fill layer is high at RH 100%. High relative humidity of the fill layer is not a sign of an un-functional drainage or capillary break layer, but a natural boundary condition for a slab structure adjacent to the moist subsoil. Due to the favourable conditions, microbe growth is very common in fill layers. Fungal or bacterial growth, in general, was detected in 98% of the test specimens taken beneath the ground slabs of heated buildings. Indicator species, either fungal or bacterial, were detected in 79% of the specimens. Yet, no moisture damage related to the ground floors was ever detected or recorded in the test buildings. The high microbe concentration in the fill layer beneath ground slabs is not a sign of moisture damage, but a natural state of the moist and warm fill layer. r 2007 Elsevier Ltd. All rights reserved. Keywords: Slab-on-ground structure; Thermal; Moisture; Microbe; Boundary conditions
1. Introduction Ground slabs and basement walls are a unique part of the building envelope in direct and constant contact with the subsoil stratum or the constructed fill layers. The coarse-grained fill or drainage layer beneath a slab forms an outmost segment of this envelope, preventing free capillary or gravitational water from penetrating the adjoining structural elements. Thermal and moisture conditions in these layers form the boundary conditions of ground slabs as far as the building physics of the structure is concerned. Despite the undeniable necessity to know these boundary conditions, the temperature and moisture content of fill and drainage layers have been almost unknown factors in the design of ground slabs. According to recent studies in Finland, in 25% of all Abbreviations: cfu/g, colony-forming units of microbes per a gram of material; EMC, hygroscopic equilibrium moisture content; hs , thickness of concrete slab (mm); hi , thickness of insulation layer (mm); MVOC, volatile organic compound with microbe origin Corresponding author. Tel.: +358 3 3115 2858; fax: +358 3 3115 2811. E-mail addresses: jukka.v.rantala@tut.fi (J. Rantala), virpi.leivo@tut.fi (V. Leivo). 0360-1323/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2007.01.015
detached, semi-detached or row houses the ground slabs have some degree of moisture damage [1]. Boundary conditions are an essential part of the initial data required for the design of safe and fit structures.
1.1. On thermal conditions Thermal interaction between slab-on-ground structures and the subsoil is a widely studied subject in the literature. Not only the energy efficiency and the heat loss into the subsoil, but also the seasonal frost at the surroundings of buildings with shallow foundations and the problems caused by frost heave, have been the main motivations for the efforts. There are numerous empirical studies [2–6] and several analytical [7–11], semi-analytical [4,12–15] and simplified solutions [11,16–19] in the literature concerning the thermal interaction problem. The average temperature of the fill layer beneath a heated building is relatively high and even throughout the year. This is true especially at the central part of the slab, where the influence of short-term or seasonal fluctuations in the outdoor air temperature are less effective [3].
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
1.2. On moisture conditions Moisture conditions in the coarse-grained fill layers adjacent to ground floor structures are a much less studied problem in the literature. Moisture movement in soil has the greatest importance in many agricultural and geotechnical phenomena, and the mechanisms that transport water and moisture are well known. Yet, the conditions in the layers beneath a seasonally heated building are somewhat of a mystery. These fill layers are in contact with the surrounding subsoil layers and the ground water table. At the same time, the heat flow between a heated building and its surroundings create a temperature gradient inside the layer that induces a water vapour flow towards the cooler zones in the soil. This thermal diffusion tends to decrease the moisture content of the fill layers next to the ground slab. In Finnish climatic conditions, the ground water table is seldom far from the soil surface, and the capillary action in the typical fine-grained subsoils of Finland, till, silt and clay, is significant. In addition, rainfall and the gravitational water seeping downwards in the soil mass increase the moisture content of the subsoil above the ground water table. The coarse-grained materials used as capillary breaking or drainage layers, such as sand, gravel and even crushed stone, are to some extent capillary materials. The volume and level of capillary rise is strongly dependent on the percentage of the finest particles in the material. The smaller the diameter of the pore, the stronger the capillary suction and the higher the capillary rise. Coarse-grained sands and gravels usually have small amounts of fine-grained particles in them, and some volume of capillary rise takes place all the way through the limited thickness of fill layers built beneath the ground slab (Fig. 1). The volume of this capillary rise may be very low at the upper part of the capillary region, but high enough to keep the relative humidity high at the slab–fill interface (Fig. 1) [20]. Hygroscopic equilibrium moisture content (EMC) of coarse-grained soil materials is relatively low. According to laboratory tests [21], the EMC is only w ¼ 0:4; . . . ; 1:0% by weight in sands, gravels or crushed stones. This usually denotes that the moisture content is between 15 and 20 kg=m3 at the hygroscopic region of the fill layer (Fig. 1). The water content of the material in the capillary region may be significantly higher, at several hundreds of kilograms per a cubic meter (Fig. 1). In both cases, hygroscopic or capillary region, the relative humidity (RH) of the pore air gives the same reading, RH ¼ 100%. Thus, RH alone does not convey anything about the volume of free or capillary water in the layer. By determining the water content at the slab–fill interface, one gets a more accurate impression of the moisture conditions in the fill layer. Therefore the moisture content, not the relative humidity, is the parameter measured in the field tests of this study. 1.3. On microbiological conditions In recent years the regulations for indoor air conditions have become tighter, and the new limit regulations allow
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less and less impurities in our indoor air. At the same time, the improved methods of measurement are able to detect lower concentrations of detrimental microbes and MVOC in the air. However, the source of the detected MVOC is not always distinctive. Often the high microbe content of the fill layer beneath the ground slab is the easiest scapegoat. The high concentrations of microbes in these layers are interpreted as a sign of moisture failure. One of the main objectives of this study is to prove or disprove this assumption. The objective of this study is to determine the thermal, moisture and microbiological conditions beneath a ground slab during the lifespan of seasonally heated buildings. The objective was achieved by performing long-term field tests on new buildings in Southern Finland and a series of 49 in situ surveys on already established buildings around the country. 2. Field test arrangements and the implementation 2.1. Long-term temperature measurements The long-term test series included four semi-detached or row houses in Southern Finland [3]. The ground slab structures of the buildings are typical Finnish solutions, which include a block or an element footing filled with compacted gravel (Fig. 2). The in situ cast concrete slab with an underneath thermal insulation layer was built on top of the gravel fill. The temperature of the coarse-grained fill layer was monitored starting from the beginning of the first heating season of the building. At least two measuring points were observed, one near the external wall line of the building and another at the central part of the building, at least 3 m from the nearest wall line. 2.1.1. Temperature of the coarse-grained fill layer beneath a heated building Fig. 3 presents the results of one of the long-term field test surveys. The test building is a row house in Southern Finland. The house was completed during the late autumn and winter, and the first heating season started in February (Fig. 3). The slab includes a hs ¼ 80 mm thick massive concrete slab and a hi ¼ 50 mm insulation layer of expanded polystyrene (EPS) underneath. The building has radiator heating and a basic balanced ventilation system. Fig. 3 presents the measured temperature changes at two measuring locations, one at the central part of the slab, approximately 3.5 m from the nearest external wall line, and the other at the slab edge, 0.5 m from the wall. After the heat was first turned on in February, the fill temperature began to rise rapidly. The increase was over 10 in less than two months. By the end of the summer the fill temperature reached the first year peak at T fill ¼ þ18:5 C. At the central part of the slab, the temperature gradually fell to the minimum of T fill ¼ þ17 C during the following winter. The decrease was only nominal and the thermal conditions beneath the central part of the slab remain favourable for microbe growth throughout the year.
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
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The higher capillary rise level
Capillary region The lower capillary rise level
300
200
100
0
Water content w (kg/m3)
ground water table
Distance from the free water source h (m)
Distance from the free water source h (m)
base surface of the slab Hygroscopic region
0 20 40 60 80 100
Relative humidity of the pore air RH (%)
Fig. 1. A typical water content distribution and the relative humidity in the coarse-grained fill layer beneath a slab-on-ground structure.
_
+ slab insulation
fill
subsoil footing wall Fig. 2. A typical Finnish slab-on-ground structure: a footing wall filled with compacted coarse-grained soil material and an in situ cast concrete slab with or without an underneath insulation layer.
The outcome from the other three test buildings was congruent and indicated similar conclusions [3]. The fill layer beneath the central part of the slab is relatively warm throughout the year, despite the significant decrease in the outdoor temperature (Fig. 3). This conclusion is also valid for the detached and row houses, where the total width of the building is moderate, approximately 10–15 m. The fill temperatures measured in this study correspond well with previous research [2,3,5,6]. Comparison with some analytical and simplified solution methods also suggest, that the heat loss and the thermal interaction between ground slabs and subsoil can be estimated with relative accuracy using the state-of-the art methods [7,8,17,18]. 2.2. Water content measurements and microbe analysis The research included 35 individual buildings in seven different cities in Finland. The random sample included different types of buildings, such as schools, day-care or
health centres, apartment buildings, etc. The construction period of the structures differ from 1910 to 2005. The age distribution of the cases is presented in Fig. 4. Three different basic types of slab structures were detected in these 35 buildings. The most typical structure, type 1, included an in situ cast concrete slab with an underneath insulation layer, most typically an EPS layer (expanded polystyrene) with a varying thickness (Fig. 5). Type 2 was the most common in the older structures and does not include an insulation layer. The third type of structure, type 3, was a so called double-floor structure, where two in situ cast concrete slabs are separated by a thermal insulation layer and/or a water proofing layer (Fig. 5). The distribution of the case structures between these three slab types is presented in Fig. 4. The samples of the fill layer were drawn from +100 mm holes drilled through floor coverings and concrete slabs by a water-cooled diamond drill. The drilling was halted immediately after the concrete slab was first penetrated and the drill hole dried out with powerful vacuum. This way the insulation or moisture barriers beneath the slab were left unbroken and the actual fill layer undisturbed. Moisture barriers and insulation layers were removed manually using disinfected cutters. Two different size samples were drawn from the uncovered fill surface by a disinfected sampler. The smaller sample (10–20 g) was used for microbe analysis and the larger (100–300 g) for water content analysis. The fresh samples were sealed immediately in water and airtight plastic bags. 2.2.1. Water and moisture content of fill layers Water content inside the fill usually varies in different levels of the layer (Fig. 1). This is due to the grain-size distribution and the capillary properties of coarse-grained fill materials. The water content and the relative humidity at the upper part of the layer form the conditions determining the moisture behaviour in the structural layers
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
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Temperature T (°C)
20
10
0
-10 Slab centre
-20
Slab edge Outdoor air
-30 Jan
Mar
May
Jul
Sep
Nov
Jan
Mar
May
Jul
Month Fig. 3. Thermal behaviour of the fill layer beneath a heated building in a cold climate.
a
b
Age distribution of the examined buildings 17 % 17 %
1910-1950 1960-75 1976-90 1991-2005
23 %
43 %
Case buildings distrubuted by the type of the slab-on-ground structure
34 %
Type 1 Type 2 Type 3
52 %
14 %
Fig. 4. Characteristics of the 35 individual case buildings of the research: (a) age distribution; (b) distribution by the type of slab structure.
above. In this research, the water content of fill layers was determined at the fill–slab interface using the weighing– drying–weighing method. This is the standard method to test water content of granular soil materials in laboratory and widely used in geotechnical research. The weighing was performed with a calibrated electronic scale with an accuracy of 0.001 g. The results of 33 individual specimens in 33 different buildings are presented in Fig. 6. The sampling date was in late winter/early spring in February–March, while the ground was still in frost and the ground water table was at its lowest. Thus, the determined water contents can be treated as the annual minimum values. According to the results (Fig. 6) the measured water content of the samples in almost every single case was higher than the EMC of the material in high relative humidity (RH 100%3wo0:5% by weight). Thus, the relative humidity of the fill layer beneath a slab-onground structure is very high throughout the year, including the winter months and the heating season for the buildings. There are only few results in the literature that concern the moisture content of fill layers beneath a
building in operating conditions. However, these comparable studies [5,20] and the hands-on experiment of contractors suggest that the relative humidity at the fill layers is high. 2.2.2. Microbe content of fill layers Altogether, 49 soil samples taken from the 35 case buildings were analysed in the Aerobiology Unit of Turku University, Finland. The samples were cultured in the laboratory for the quantitative analysis where the genus and the species of the microbes were identified. The culture medium was a tryptophan–yeast–glucose–agar mix for the bacteria culture and a malted infusion–agar mix for the fungi culture. The incubation temperature was T ¼ þ25 C. The total number of bacteria and fungi were determined, as well as the number of indicator species in any individual specimen. The incubation time for the determination of the total number of bacteria and fungi was 7 days, 7–14 days for the identification of fungal species and 10–14 days for the identification of Actinomycea bacteria. This method is the standard procedure when the microbe growth in building materials is determined in
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
740
Fig. 5. Three basic types of slab structures detected in the field survey.
7
Water content w (% by weight)
6
5
4
3
2
1
9* 15 16 17 18 19 20 22 23 25 26 27 28 29 30* 31 32 33 34 35 36 37 38 39 40* 41* 42 43 44 45* 46* 47* 48*
0
Fig. 6. Water content of the 33 individual specimens taken from the fill–slab interface beneath the slab-on-ground structures.
Finland. The results are given as the number of detected colony-forming units per a gram of the material (cfu/g). An indicator species is a microbe that does not usually exist in undamaged buildings. The occurrence of these species in a material sample suggests that the material and the building have suffered moisture damage of some kind. The microbes used as indicator species vary in different countries and laboratories, but in this research the species listed in Baarn 1992 [22] and in the WHO 2002 list were monitored.
The fungal growth (cfu/g) detected in the 49 specimens is presented in Fig. 7. In 59% of the specimens some fungal growth was detected, excluding the indicator fungi. The indicator fungi were detected in 55% of all the samples and 20% of them were toxic. The most common indicator fungi were Acremonium (detected in 34% of the samples), Fusarium (14%), Oidiodendron (10%), Aspergillus versicolor (6%), Exophiala (4%), Phialophora (4%), Rhodotorula (2%) and Trichoderma (2%).
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
Fig. 8 presents the analysis results concerning the bacteria contamination of the same specimens. Bacterial growth was very common in the fill layers, as it was detected in 97% of the specimens (Fig. 8), excluding the indicator bacteria (Actinomycea). Some of the concentrations were extremely high, over 10.000,000 cfu/g (Fig. 8). Bacteria were detected in all age groups of the buildings, including the oldest structures. Also the Actinomycea growth was very common, as the colonies were detected in 75% of the samples. In addition, three reference samples of the typical raw material (gravel) of the fill layers were analysed. The specimens were taken from three different sand pits in Southern Finland, at the average depth of 1 m from the
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surface of the already sieved fill material. The average values of the detected microbe content of these specimens are presented in Table 1. 3. Discussion Fill layers beneath a ground floor structure of a heated building are moist and warm throughout the year. This result corresponds well with previous research in this field. These conditions are favourable for microbe growth and therefore the microbe contents, both the fungal and bacterial, detected in the field test samples were high. The occurrence of fungi seems to have a dependency on the age of the building. In the oldest surveyed structures
Fungus cfu/g 1
10
100
1000
10000
2005 - 1990
1980 - 1960
1934 - 1910
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Fig. 7. Detected fungal growth in the 49 specimens.
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Bacteria cfu/g 1
10
100
1 000
10 000
100 000
1 000 000 10 000 000 100 000 000
2005 - 1990
1980 - 1960
1934 - 1910
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
Fig. 8. Detected bacteria growth in the 49 field samples.
Table 1 The average microbe content of the 49 test samples and the three reference samples from the performed quantitative analyses cfu/g
Actinomycea
Other bacteria
Indicator fungus
Other fungal
Average (49 specimens) Maximum Reference tests, average (3 specimens)
61 776 1 484 000 45 158
3 245 879 25 960 000 1 233 439
61 540 248
350 3644 1554
built in the 1930s or earlier, the fungal growth was minimal, practically non-existent. In the structures completed in the 1960s, 1970s and 1980s, the growth was detected occasionally. In the newest buildings, completed in the 1990s or later, fungal growth was very common and
the number of detected colonies high (Fig. 7). The result was surprising, as the general conditions, fill temperature and moisture content, were favourable for microbe growth in all of the surveyed buildings, regardless of the age of the structure. This indicates that initially extensive fungal
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
growth in the fill layer of any new building dies out slowly, as the nutriments required for fungal growth depleted at the relatively sheltered location beneath a slab and the spores left in the layer lose their ability to germinate in the long run. Therefore the samples from the oldest buildings are almost clean or the existing fungal spores did not germinate, and those from the newest are highly contaminated. Some sort of microbe growth was detected in 98% of the fill samples. Indicator species, either fungal or Actinomycea bacterial, were detected in 79% of the samples. Also, the average concentration of the detected microbes was high (Table 1). There is no previous work in the literature concerning the microbe growth in fill layers beneath a heated building. Therefore the assumptions concerning the normal microbe growth levels in these layers presented in this paper are based on the results of this research only. Further research in other climatic conditions and other ground slab types are required before extensive conclusions can be drawn. The water content of the layers did not have any significant influence on the detected microbe contents (Figs. 6–8). The measured water content of the samples was equal or higher than the EMC of the material in very high relative humidity. Therefore favourable moisture conditions for microbe growth were always present in the studied structures. The determined microbe contents of the reference samples taken from the sand pits were almost as high as the average contents of the in situ fill specimens. Since the fill material is already contaminated at the sand pit, the high concentrations of microbes detected at fill layers are natural, as the conditions beneath a ground slab are favourable for microbe growth throughout the year. Thus, microbe growth in the fill layer beneath a heated building must be considered as an existing boundary condition for any new or old ground floor structure. Most of the surveyed buildings did not have any sign of any moisture damage related to the ground slabs, whatsoever. Thus, the detection of microbes in the fill layer is not a sign of moisture damage to the ground slab, and should not be misinterpreted as such. Also, the high relative humidity of the fill layer is not a sign of an un-functional drainage or capillary break layer, but a natural boundary condition for a slab structure adjacent to the moist subsoil. To prevent the detrimental microbes, spores or MVOCs from penetrating the indoor air, the joints between the slabs and the footing walls should be sealed airtight, as well as the joints between the slab and any lead-in that penetrates the coherent concrete slab. 4. Conclusions According to the results of the series of field tests performed in this study, the average conditions, temperature and moisture contents, of the fill layers beneath a ground slab of any heated building are favourable
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for microbe growth. The relative humidity of the fill layer is high at RH 100%, throughout the year. Fill temperature beneath a heated building remain high ðT ¼ þ12; . . . ; þ16 CÞ and even throughout the year, especially at the central part of the slab. High relative humidity of the fill layer is not a sign of an un-functional drainage or capillary break layer, but a natural boundary condition for a slab structure adjacent to the moist subsoil. Due to the favourable conditions, microbe growth is very common in fill layers. Fungal or bacterial growth in general was detected in 98% of the test specimens taken from beneath the ground slabs of heated buildings. The indicator species, either fungal or Actinomycea bacterial, were detected in 79% of the specimens. Also, the average concentration of the detected microbes was high. Detection of microbes in the fill layer is not a sign of moisture damage to the ground slab, and should not be misinterpreted as such. Microbe growth in the fill layer beneath a heated building must be considered as an existing boundary condition for any new or old ground floor structure.
References [1] Leivo V, Rantala J. Typical moisture failures of slab-on-ground structures in Finland. Practice Periodical on Structural Design and Construction 2005;10(2):98–101. [2] Adjali MH, Davies M, Ni Riain C, Littler JG. In situ measurements and numerical simulations of heat transfer beneath a heated ground floor slab. Energy and Buildings 2000;33(1):75–83. [3] Rantala J. On thermal interaction between slab-on-ground structures and subsoil in Finland. Publication 542. Dissertation, Tampere University of Technology; 2005. 130p. [4] Rantala J. A new method to estimate the periodic temperature distribution underneath a slab-on-ground structure. Building and Environment 2005;44(6):832–40. [5] Thomas HR, Rees SW. The thermal performance of ground floor slabs—a full scale in-situ experiment. Building and Environment 1999;34(2):139–64. [6] Zhou Z, Rees SW, Thomas HR. A numerical and experimental investigation of ground heat transfer including edge insulation effects. Building and Environment 2002;37(1):67–78. [7] Hagentoft C-E. Heat loss to the ground from a building, slab on ground and cellar. Department of Building Technology, Report TVBH-1004, Lund Institute of Technology; April 1988. 216p. [8] Hagentoft C-E. Heat losses and temperature in the ground under a building with and without ground water flow—I. Infinite ground water flow rate. Building and Environment 1996;31(1):3–11. [9] Hagentoft C-E. Heat losses and temperature in the ground under a building with and without ground water flow—II. Finite ground water flow rate. Building and Environment 1996;31(1):13–9. [10] Lachenbruch AH. Three-dimensional heat conduction in permafrost beneath heated buildings. Geological Survey Bulletin 1052-B. Washington DC: US Government Printing Office; 1957. [11] Mitalas GP. Calculation of below-grade residential heat loss: low-rise residential building. ASHRAE Transactions 1987;93(1):743–84. [12] Chuangchid P, Krarti M. Foundation heat loss from heated concrete slab-on-grade floors. Building and Environment 2001;36(5):637–55. [13] Krarti M, Claridge DE, Kreider JF. ITPE Technique applications to time-varying two-dimensional ground-coupling problems. International Journal of Heat Mass Transfer 1988;31(9):1899–911.
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[14] Krarti M, Claridge DE, Kreider JF. ITPE Technique applications to time-varying three-dimensional ground-coupling problems. Journal of Heat Transfer, ASME 1990; 112: 849–52. [15] Kusuda T, Bean JW. Simplified methods for determining seasonal heat loss from uninsulated slab-on-grade floors. ASHRAE Transactions 1984;90(1B):611–43. [16] ASHRAE Handbook CD. Fundamentals, American Society of Heating Refrigerating and Air-Conditioning Engineers, Inc. ISBN 1-1883413-90-7; 2001. [17] EN ISO 13370. Thermal performance of buildings—heat transfer via the ground-calculation methods. CEN. 50p; 1998.
[18] Krarti M, Choi S. Simplified methods for foundation heat loss calculation. ASHRAE Transactions 1996;102(1):140–52. [19] Mitalas GP. Calculation of basement heat loss. ASHRAE Transactions 1983;89(1B):420–37. [20] Kunze RJ, Kirkham D. Capillary diffusion and self-diffusion of soil water. Soil Science 1964;97:145–51. [21] Rantala J, Leivo V. Thermal and moisture conditions of coarsegrained fill layer under a slab-on-ground structure in cold climate. Journal of Thermal Envelope and Building Science 2004;28(1):45–60. [22] Samson RA, et al. Health implications of fungi in indoor environments. Air Quality Monographs 1994;2:531–8.
Building and Environment 43 (2008) 736–744 www.elsevier.com/locate/buildenv
Thermal, moisture and microbiological boundary conditions of slab-on-ground structures in cold climate Jukka Rantala, Virpi Leivo Structural Engineering, Tampere University of Technology, P.O. Box 600, FIN-33101 Tampere, Finland Received 17 July 2006; received in revised form 9 January 2007; accepted 17 January 2007
Abstract Coarse-grained fill or drainage layers beneath heated slab-on-ground structures are warm and moist throughout the year. According to the in situ measurements, the relative humidity of the fill layer is high at RH 100%. High relative humidity of the fill layer is not a sign of an un-functional drainage or capillary break layer, but a natural boundary condition for a slab structure adjacent to the moist subsoil. Due to the favourable conditions, microbe growth is very common in fill layers. Fungal or bacterial growth, in general, was detected in 98% of the test specimens taken beneath the ground slabs of heated buildings. Indicator species, either fungal or bacterial, were detected in 79% of the specimens. Yet, no moisture damage related to the ground floors was ever detected or recorded in the test buildings. The high microbe concentration in the fill layer beneath ground slabs is not a sign of moisture damage, but a natural state of the moist and warm fill layer. r 2007 Elsevier Ltd. All rights reserved. Keywords: Slab-on-ground structure; Thermal; Moisture; Microbe; Boundary conditions
1. Introduction Ground slabs and basement walls are a unique part of the building envelope in direct and constant contact with the subsoil stratum or the constructed fill layers. The coarse-grained fill or drainage layer beneath a slab forms an outmost segment of this envelope, preventing free capillary or gravitational water from penetrating the adjoining structural elements. Thermal and moisture conditions in these layers form the boundary conditions of ground slabs as far as the building physics of the structure is concerned. Despite the undeniable necessity to know these boundary conditions, the temperature and moisture content of fill and drainage layers have been almost unknown factors in the design of ground slabs. According to recent studies in Finland, in 25% of all Abbreviations: cfu/g, colony-forming units of microbes per a gram of material; EMC, hygroscopic equilibrium moisture content; hs , thickness of concrete slab (mm); hi , thickness of insulation layer (mm); MVOC, volatile organic compound with microbe origin Corresponding author. Tel.: +358 3 3115 2858; fax: +358 3 3115 2811. E-mail addresses: jukka.v.rantala@tut.fi (J. Rantala), virpi.leivo@tut.fi (V. Leivo). 0360-1323/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2007.01.015
detached, semi-detached or row houses the ground slabs have some degree of moisture damage [1]. Boundary conditions are an essential part of the initial data required for the design of safe and fit structures.
1.1. On thermal conditions Thermal interaction between slab-on-ground structures and the subsoil is a widely studied subject in the literature. Not only the energy efficiency and the heat loss into the subsoil, but also the seasonal frost at the surroundings of buildings with shallow foundations and the problems caused by frost heave, have been the main motivations for the efforts. There are numerous empirical studies [2–6] and several analytical [7–11], semi-analytical [4,12–15] and simplified solutions [11,16–19] in the literature concerning the thermal interaction problem. The average temperature of the fill layer beneath a heated building is relatively high and even throughout the year. This is true especially at the central part of the slab, where the influence of short-term or seasonal fluctuations in the outdoor air temperature are less effective [3].
ARTICLE IN PRESS J. Rantala, V. Leivo / Building and Environment 43 (2008) 736–744
1.2. On moisture conditions Moisture conditions in the coarse-grained fill layers adjacent to ground floor structures are a much less studied problem in the literature. Moisture movement in soil has the greatest importance in many agricultural and geotechnical phenomena, and the mechanisms that transport water and moisture are well known. Yet, the conditions in the layers beneath a seasonally heated building are somewhat of a mystery. These fill layers are in contact with the surrounding subsoil layers and the ground water table. At the same time, the heat flow between a heated building and its surroundings create a temperature gradient inside the layer that induces a water vapour flow towards the cooler zones in the soil. This thermal diffusion tends to decrease the moisture content of the fill layers next to the ground slab. In Finnish climatic conditions, the ground water table is seldom far from the soil surface, and the capillary action in the typical fine-grained subsoils of Finland, till, silt and clay, is significant. In addition, rainfall and the gravitational water seeping downwards in the soil mass increase the moisture content of the subsoil above the ground water table. The coarse-grained materials used as capillary breaking or drainage layers, such as sand, gravel and even crushed stone, are to some extent capillary materials. The volume and level of capillary rise is strongly dependent on the percentage of the finest particles in the material. The smaller the diameter of the pore, the stronger the capillary suction and the higher the capillary rise. Coarse-grained sands and gravels usually have small amounts of fine-grained particles in them, and some volume of capillary rise takes place all the way through the limited thickness of fill layers built beneath the ground slab (Fig. 1). The volume of this capillary rise may be very low at the upper part of the capillary region, but high enough to keep the relative humidity high at the slab–fill interface (Fig. 1) [20]. Hygroscopic equilibrium moisture content (EMC) of coarse-grained soil materials is relatively low. According to laboratory tests [21], the EMC is only w ¼ 0:4; . . . ; 1:0% by weight in sands, gravels or crushed stones. This usually denotes that the moisture content is between 15 and 20 kg=m3 at the hygroscopic region of the fill layer (Fig. 1). The water content of the material in the capillary region may be significantly higher, at several hundreds of kilograms per a cubic meter (Fig. 1). In both cases, hygroscopic or capillary region, the relative humidity (RH) of the pore air gives the same reading, RH ¼ 100%. Thus, RH alone does not convey anything about the volume of free or capillary water in the layer. By determining the water content at the slab–fill interface, one gets a more accurate impression of the moisture conditions in the fill layer. Therefore the moisture content, not the relative humidity, is the parameter measured in the field tests of this study. 1.3. On microbiological conditions In recent years the regulations for indoor air conditions have become tighter, and the new limit regulations allow
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less and less impurities in our indoor air. At the same time, the improved methods of measurement are able to detect lower concentrations of detrimental microbes and MVOC in the air. However, the source of the detected MVOC is not always distinctive. Often the high microbe content of the fill layer beneath the ground slab is the easiest scapegoat. The high concentrations of microbes in these layers are interpreted as a sign of moisture failure. One of the main objectives of this study is to prove or disprove this assumption. The objective of this study is to determine the thermal, moisture and microbiological conditions beneath a ground slab during the lifespan of seasonally heated buildings. The objective was achieved by performing long-term field tests on new buildings in Southern Finland and a series of 49 in situ surveys on already established buildings around the country. 2. Field test arrangements and the implementation 2.1. Long-term temperature measurements The long-term test series included four semi-detached or row houses in Southern Finland [3]. The ground slab structures of the buildings are typical Finnish solutions, which include a block or an element footing filled with compacted gravel (Fig. 2). The in situ cast concrete slab with an underneath thermal insulation layer was built on top of the gravel fill. The temperature of the coarse-grained fill layer was monitored starting from the beginning of the first heating season of the building. At least two measuring points were observed, one near the external wall line of the building and another at the central part of the building, at least 3 m from the nearest wall line. 2.1.1. Temperature of the coarse-grained fill layer beneath a heated building Fig. 3 presents the results of one of the long-term field test surveys. The test building is a row house in Southern Finland. The house was completed during the late autumn and winter, and the first heating season started in February (Fig. 3). The slab includes a hs ¼ 80 mm thick massive concrete slab and a hi ¼ 50 mm insulation layer of expanded polystyrene (EPS) underneath. The building has radiator heating and a basic balanced ventilation system. Fig. 3 presents the measured temperature changes at two measuring locations, one at the central part of the slab, approximately 3.5 m from the nearest external wall line, and the other at the slab edge, 0.5 m from the wall. After the heat was first turned on in February, the fill temperature began to rise rapidly. The increase was over 10 in less than two months. By the end of the summer the fill temperature reached the first year peak at T fill ¼ þ18:5 C. At the central part of the slab, the temperature gradually fell to the minimum of T fill ¼ þ17 C during the following winter. The decrease was only nominal and the thermal conditions beneath the central part of the slab remain favourable for microbe growth throughout the year.
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The higher capillary rise level
Capillary region The lower capillary rise level
300
200
100
0
Water content w (kg/m3)
ground water table
Distance from the free water source h (m)
Distance from the free water source h (m)
base surface of the slab Hygroscopic region
0 20 40 60 80 100
Relative humidity of the pore air RH (%)
Fig. 1. A typical water content distribution and the relative humidity in the coarse-grained fill layer beneath a slab-on-ground structure.
_
+ slab insulation
fill
subsoil footing wall Fig. 2. A typical Finnish slab-on-ground structure: a footing wall filled with compacted coarse-grained soil material and an in situ cast concrete slab with or without an underneath insulation layer.
The outcome from the other three test buildings was congruent and indicated similar conclusions [3]. The fill layer beneath the central part of the slab is relatively warm throughout the year, despite the significant decrease in the outdoor temperature (Fig. 3). This conclusion is also valid for the detached and row houses, where the total width of the building is moderate, approximately 10–15 m. The fill temperatures measured in this study correspond well with previous research [2,3,5,6]. Comparison with some analytical and simplified solution methods also suggest, that the heat loss and the thermal interaction between ground slabs and subsoil can be estimated with relative accuracy using the state-of-the art methods [7,8,17,18]. 2.2. Water content measurements and microbe analysis The research included 35 individual buildings in seven different cities in Finland. The random sample included different types of buildings, such as schools, day-care or
health centres, apartment buildings, etc. The construction period of the structures differ from 1910 to 2005. The age distribution of the cases is presented in Fig. 4. Three different basic types of slab structures were detected in these 35 buildings. The most typical structure, type 1, included an in situ cast concrete slab with an underneath insulation layer, most typically an EPS layer (expanded polystyrene) with a varying thickness (Fig. 5). Type 2 was the most common in the older structures and does not include an insulation layer. The third type of structure, type 3, was a so called double-floor structure, where two in situ cast concrete slabs are separated by a thermal insulation layer and/or a water proofing layer (Fig. 5). The distribution of the case structures between these three slab types is presented in Fig. 4. The samples of the fill layer were drawn from +100 mm holes drilled through floor coverings and concrete slabs by a water-cooled diamond drill. The drilling was halted immediately after the concrete slab was first penetrated and the drill hole dried out with powerful vacuum. This way the insulation or moisture barriers beneath the slab were left unbroken and the actual fill layer undisturbed. Moisture barriers and insulation layers were removed manually using disinfected cutters. Two different size samples were drawn from the uncovered fill surface by a disinfected sampler. The smaller sample (10–20 g) was used for microbe analysis and the larger (100–300 g) for water content analysis. The fresh samples were sealed immediately in water and airtight plastic bags. 2.2.1. Water and moisture content of fill layers Water content inside the fill usually varies in different levels of the layer (Fig. 1). This is due to the grain-size distribution and the capillary properties of coarse-grained fill materials. The water content and the relative humidity at the upper part of the layer form the conditions determining the moisture behaviour in the structural layers
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Temperature T (°C)
20
10
0
-10 Slab centre
-20
Slab edge Outdoor air
-30 Jan
Mar
May
Jul
Sep
Nov
Jan
Mar
May
Jul
Month Fig. 3. Thermal behaviour of the fill layer beneath a heated building in a cold climate.
a
b
Age distribution of the examined buildings 17 % 17 %
1910-1950 1960-75 1976-90 1991-2005
23 %
43 %
Case buildings distrubuted by the type of the slab-on-ground structure
34 %
Type 1 Type 2 Type 3
52 %
14 %
Fig. 4. Characteristics of the 35 individual case buildings of the research: (a) age distribution; (b) distribution by the type of slab structure.
above. In this research, the water content of fill layers was determined at the fill–slab interface using the weighing– drying–weighing method. This is the standard method to test water content of granular soil materials in laboratory and widely used in geotechnical research. The weighing was performed with a calibrated electronic scale with an accuracy of 0.001 g. The results of 33 individual specimens in 33 different buildings are presented in Fig. 6. The sampling date was in late winter/early spring in February–March, while the ground was still in frost and the ground water table was at its lowest. Thus, the determined water contents can be treated as the annual minimum values. According to the results (Fig. 6) the measured water content of the samples in almost every single case was higher than the EMC of the material in high relative humidity (RH 100%3wo0:5% by weight). Thus, the relative humidity of the fill layer beneath a slab-onground structure is very high throughout the year, including the winter months and the heating season for the buildings. There are only few results in the literature that concern the moisture content of fill layers beneath a
building in operating conditions. However, these comparable studies [5,20] and the hands-on experiment of contractors suggest that the relative humidity at the fill layers is high. 2.2.2. Microbe content of fill layers Altogether, 49 soil samples taken from the 35 case buildings were analysed in the Aerobiology Unit of Turku University, Finland. The samples were cultured in the laboratory for the quantitative analysis where the genus and the species of the microbes were identified. The culture medium was a tryptophan–yeast–glucose–agar mix for the bacteria culture and a malted infusion–agar mix for the fungi culture. The incubation temperature was T ¼ þ25 C. The total number of bacteria and fungi were determined, as well as the number of indicator species in any individual specimen. The incubation time for the determination of the total number of bacteria and fungi was 7 days, 7–14 days for the identification of fungal species and 10–14 days for the identification of Actinomycea bacteria. This method is the standard procedure when the microbe growth in building materials is determined in
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Fig. 5. Three basic types of slab structures detected in the field survey.
7
Water content w (% by weight)
6
5
4
3
2
1
9* 15 16 17 18 19 20 22 23 25 26 27 28 29 30* 31 32 33 34 35 36 37 38 39 40* 41* 42 43 44 45* 46* 47* 48*
0
Fig. 6. Water content of the 33 individual specimens taken from the fill–slab interface beneath the slab-on-ground structures.
Finland. The results are given as the number of detected colony-forming units per a gram of the material (cfu/g). An indicator species is a microbe that does not usually exist in undamaged buildings. The occurrence of these species in a material sample suggests that the material and the building have suffered moisture damage of some kind. The microbes used as indicator species vary in different countries and laboratories, but in this research the species listed in Baarn 1992 [22] and in the WHO 2002 list were monitored.
The fungal growth (cfu/g) detected in the 49 specimens is presented in Fig. 7. In 59% of the specimens some fungal growth was detected, excluding the indicator fungi. The indicator fungi were detected in 55% of all the samples and 20% of them were toxic. The most common indicator fungi were Acremonium (detected in 34% of the samples), Fusarium (14%), Oidiodendron (10%), Aspergillus versicolor (6%), Exophiala (4%), Phialophora (4%), Rhodotorula (2%) and Trichoderma (2%).
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Fig. 8 presents the analysis results concerning the bacteria contamination of the same specimens. Bacterial growth was very common in the fill layers, as it was detected in 97% of the specimens (Fig. 8), excluding the indicator bacteria (Actinomycea). Some of the concentrations were extremely high, over 10.000,000 cfu/g (Fig. 8). Bacteria were detected in all age groups of the buildings, including the oldest structures. Also the Actinomycea growth was very common, as the colonies were detected in 75% of the samples. In addition, three reference samples of the typical raw material (gravel) of the fill layers were analysed. The specimens were taken from three different sand pits in Southern Finland, at the average depth of 1 m from the
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surface of the already sieved fill material. The average values of the detected microbe content of these specimens are presented in Table 1. 3. Discussion Fill layers beneath a ground floor structure of a heated building are moist and warm throughout the year. This result corresponds well with previous research in this field. These conditions are favourable for microbe growth and therefore the microbe contents, both the fungal and bacterial, detected in the field test samples were high. The occurrence of fungi seems to have a dependency on the age of the building. In the oldest surveyed structures
Fungus cfu/g 1
10
100
1000
10000
2005 - 1990
1980 - 1960
1934 - 1910
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Fig. 7. Detected fungal growth in the 49 specimens.
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Bacteria cfu/g 1
10
100
1 000
10 000
100 000
1 000 000 10 000 000 100 000 000
2005 - 1990
1980 - 1960
1934 - 1910
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
Fig. 8. Detected bacteria growth in the 49 field samples.
Table 1 The average microbe content of the 49 test samples and the three reference samples from the performed quantitative analyses cfu/g
Actinomycea
Other bacteria
Indicator fungus
Other fungal
Average (49 specimens) Maximum Reference tests, average (3 specimens)
61 776 1 484 000 45 158
3 245 879 25 960 000 1 233 439
61 540 248
350 3644 1554
built in the 1930s or earlier, the fungal growth was minimal, practically non-existent. In the structures completed in the 1960s, 1970s and 1980s, the growth was detected occasionally. In the newest buildings, completed in the 1990s or later, fungal growth was very common and
the number of detected colonies high (Fig. 7). The result was surprising, as the general conditions, fill temperature and moisture content, were favourable for microbe growth in all of the surveyed buildings, regardless of the age of the structure. This indicates that initially extensive fungal
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growth in the fill layer of any new building dies out slowly, as the nutriments required for fungal growth depleted at the relatively sheltered location beneath a slab and the spores left in the layer lose their ability to germinate in the long run. Therefore the samples from the oldest buildings are almost clean or the existing fungal spores did not germinate, and those from the newest are highly contaminated. Some sort of microbe growth was detected in 98% of the fill samples. Indicator species, either fungal or Actinomycea bacterial, were detected in 79% of the samples. Also, the average concentration of the detected microbes was high (Table 1). There is no previous work in the literature concerning the microbe growth in fill layers beneath a heated building. Therefore the assumptions concerning the normal microbe growth levels in these layers presented in this paper are based on the results of this research only. Further research in other climatic conditions and other ground slab types are required before extensive conclusions can be drawn. The water content of the layers did not have any significant influence on the detected microbe contents (Figs. 6–8). The measured water content of the samples was equal or higher than the EMC of the material in very high relative humidity. Therefore favourable moisture conditions for microbe growth were always present in the studied structures. The determined microbe contents of the reference samples taken from the sand pits were almost as high as the average contents of the in situ fill specimens. Since the fill material is already contaminated at the sand pit, the high concentrations of microbes detected at fill layers are natural, as the conditions beneath a ground slab are favourable for microbe growth throughout the year. Thus, microbe growth in the fill layer beneath a heated building must be considered as an existing boundary condition for any new or old ground floor structure. Most of the surveyed buildings did not have any sign of any moisture damage related to the ground slabs, whatsoever. Thus, the detection of microbes in the fill layer is not a sign of moisture damage to the ground slab, and should not be misinterpreted as such. Also, the high relative humidity of the fill layer is not a sign of an un-functional drainage or capillary break layer, but a natural boundary condition for a slab structure adjacent to the moist subsoil. To prevent the detrimental microbes, spores or MVOCs from penetrating the indoor air, the joints between the slabs and the footing walls should be sealed airtight, as well as the joints between the slab and any lead-in that penetrates the coherent concrete slab. 4. Conclusions According to the results of the series of field tests performed in this study, the average conditions, temperature and moisture contents, of the fill layers beneath a ground slab of any heated building are favourable
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for microbe growth. The relative humidity of the fill layer is high at RH 100%, throughout the year. Fill temperature beneath a heated building remain high ðT ¼ þ12; . . . ; þ16 CÞ and even throughout the year, especially at the central part of the slab. High relative humidity of the fill layer is not a sign of an un-functional drainage or capillary break layer, but a natural boundary condition for a slab structure adjacent to the moist subsoil. Due to the favourable conditions, microbe growth is very common in fill layers. Fungal or bacterial growth in general was detected in 98% of the test specimens taken from beneath the ground slabs of heated buildings. The indicator species, either fungal or Actinomycea bacterial, were detected in 79% of the specimens. Also, the average concentration of the detected microbes was high. Detection of microbes in the fill layer is not a sign of moisture damage to the ground slab, and should not be misinterpreted as such. Microbe growth in the fill layer beneath a heated building must be considered as an existing boundary condition for any new or old ground floor structure.
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[14] Krarti M, Claridge DE, Kreider JF. ITPE Technique applications to time-varying three-dimensional ground-coupling problems. Journal of Heat Transfer, ASME 1990; 112: 849–52. [15] Kusuda T, Bean JW. Simplified methods for determining seasonal heat loss from uninsulated slab-on-grade floors. ASHRAE Transactions 1984;90(1B):611–43. [16] ASHRAE Handbook CD. Fundamentals, American Society of Heating Refrigerating and Air-Conditioning Engineers, Inc. ISBN 1-1883413-90-7; 2001. [17] EN ISO 13370. Thermal performance of buildings—heat transfer via the ground-calculation methods. CEN. 50p; 1998.
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