Lisø PhD Dissertation Manuscript - NTNU

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Today, frost resistance of brick, concrete and rendering mortar is tested according to methods given in different international and national standards. There are designated test methods for different countries. However, there exists no classification of climatic regions in Norway for frost durability assessment purposes, simply because there is no easily accessible data which can illustrate a correlation between the test exposure and the outdoor climatic exposure conditions. Determination of frost decay or frost damage risk ideally requires hourly values of air temperature and rainfall. Long-time weather data at most observing stations in Norway do not exist as hourly values. According to NS-EN 771-1:2003 [7] “the freeze/thaw resistance of a clay masonry unit shall be declared by the manufacturer by reference to its applicability to masonry or elements subjected to passive, moderate, and severe exposure.” Annex B of the standard includes examples for masonry or masonry elements subjected to different climatic exposures dependent on the design of the construction, i.e. the risk of frost decay in a given climate. However, the climatic conditions are not specified in the standard. Until a European test method is available the freeze/thaw resistance of clay masonry units is to be evaluated and declared according to provisions valid at the intended place of use. The European Committee for Standardization has arrived at an agreement on the development of a test procedure for the assessment of frost resistance of clay roofing tiles (NS-EN 539-2: 1998 [8]). The tiles shall, as stated in NS-EN 1304:2005 [9], be tested “according to the test method to choose with respect to the geographic zone designated for their use”. However, it is a paradox that the climatic exposure is to be based on geographical zones at a far too coarse national level, rather than regional- or local-level climate exposure data. 3. Moisture in materials Every porous, mineral material exposed to moisture in vapour or liquid form will to some degree absorb moisture. Thus, there is always some amount of moisture in building materials. The moisture could have been added during production, or it could be attributable to contact with moist air in the surroundings, water from precipitation or from the ground. The content of moisture in a material depends on the type of material, the characteristics of the pore structure and the ways in which the moisture is bound in the material. The moisture content in a material at a certain point of time is normally also influenced by previous moisture impact (often referred to as the materials moisture history). Table 1. Moisture transfer mechanisms and their driving forces. State (or phase) Transport mechanism Driving force Water vapour (Water vapour) Diffusion Differences in vapour pressure in air on each side of a material, structure or component. Water vapour Convection Differences in air pressure on each side of a material, structure or component. Liquid Gravitation E.g. leaks from roofs into underlying materials, structures or components. Liquid Wind pressure Wind pressure can force liquid water in to cracks or other openings in the building enclosure. Liquid Capillary suction Adhesion forces between water and the pore surface, allowing water being sucked into the pores. The surface stress of the water, the diameter of the pore and the angle of contact between the water and the pore surface determine the strength of this capillary force. At equilibrium the capillary force equals the pressure of the pore water multiplied with the cross-sectional area of the pore. Page 4 of 13
The main moisture transfer mechanisms in all types of building materials, structures and components are diffusion, convection, gravitation, wind pressure and capillary suction, or a combination of two or more of these mechanisms (see Table 1). The type and moisture state of the material determines which transport mechanism that will be dominant. The moisture storing capacity and transfer properties of a material largely depends on the form, size and distribution of the pore system. The diameter of the pores can vary from several millimetres down to molecular dimension. There are not any straightforward ways of perfectly describing a porous material. The porous system could be interpreted based on the materials structure, as presented in Fig. 2. The maximal amount of water that can be retained in a material is determined by the porosity, defined as the volume of pores in relation to the total volume. Fig. 2. Different types of material textures and pore systems: a) no pores (e.g. metal), b) continuous solid substance with locked pores (e.g. foam glass), c) both solid substance and pore system are continuous (e.g. concrete and wood), d) discontinuous solid substance (e.g. mineral wool and sand) [10]. 4. Frost resistance Frost resistance of porous, mineral materials is not a simple mechanical property. The frost resistance depends on a complex set of material properties and on the climatic impact on the material. Several researchers, e.g. Jacobsen [11] and Fagerlund [12], have thoroughly studied the effects of freezing and thawing on the durability of concrete. Freezing and thawing of masonry has not received the same kind of attention, in spite of the fact that frost damage on brick façades represents a considerable challenge in severe climates. Freezing of a façade is a gradual process, mainly governed by [13]: The rate of heat transfer through the façade. The increase in the concentration of dissolved salts in the still unfrozen pore water (the increase depresses the freezing point). The fact that the freezing point varies with the pore water pressure, i.e. size of the pore. Exposure to saltwater may complicate the freezing process because the salt produces osmotic pressure and causes movement of water toward the top layer of the slab where freezing takes place [14]. Marchand et al. [15] presents a comprehensive review of the topic. The frost resistance of a porous material largely depends on its mechanical properties, e.g. tensile strength, extensibility and creep. However, the determining factors to be considered are the degree of saturation and the properties of the pore system [13, 16]. Dührkop et al. [17] recommended the characterization of frost resistance of brick according to the degree of saturation. If the water content is higher than some critical Page 5 of 13
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Building envelope performance asses
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Lisø, K.R./ Building envelope perf
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BUILDING RESEARCH &INFORMATION (200
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LisÖ et al. on how to cope with an
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LisÖ et al. the exposure to contin
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LisÖ et al. business premises. The
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LisÖ et al. construction industry
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Research in Building Physics, Carme
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4 CLIMATE CHANGE SCENARIOS FOR NORW
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with large amounts of precipitation
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will most likely be worse, even wit
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Graves, H.M. & Phillipson, M.C. 200
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766 Nordvik and Lisø The topic of
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768 Nordvik and Lisø into the futu
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770 Nordvik and Lisø climate chang
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772 Nordvik and Lisø 0 Â Â = p d
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774 Nordvik and Lisø probably to a
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BUILDING RESEARCH &INFORMATION (200
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cost of repairing process-induced b
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change in quality (Mehus et al., 20
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on the impacts of different climati
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different strategies: risk-based, p
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Learning from experience - an analy
Page 87 and 88: An important aspect of the programm
Page 89 and 90: Concrete walls 32 % LECA masonry 10
Page 91 and 92: construction industry, and academic
Page 93 and 94: BUILDING RESEARCH &INFORMATION (JAN
Page 95 and 96: greater variations (Lisø et al., 2
Page 97 and 98: Typical problem areas and recommend
Page 99 and 100: Figure 3b Recommended design of win
Page 101 and 102: Figure 7 Example showing the recomm
Page 103 and 104: Figure 11a Poor solution for a £as
Page 105 and 106: Acknowledgements This paper was wri
Page 107 and 108: Climate adapted design of masonry s
Page 109 and 110: 2. Masonry defects in Norway 2.1. S
Page 111 and 112: Fig. 2. Example illustrations of th
Page 113 and 114: The presented results are in good a
Page 115 and 116: appropriate in a more severe type o
Page 117 and 118: screen in a two-stage weatherproofi
Page 119 and 120: INCREASED SNOW LOADS AND WIND ACTIO
Page 121 and 122: The hurricane (Beaufort number 12)
Page 123 and 124: The extensive revisions of the code
Page 125 and 126: The number of buildings investigate
Page 127 and 128: As is apparent from the values in t
Page 129 and 130: climate change is now dominated by
Page 131: TABLE 3. Summary of findings Struct
Page 134 and 135: Lisø, K.R./ Building envelope perf
Page 136 and 137: temperature of -4°C or less. See F
Page 140 and 141: degree of saturation, freezing will
Page 142 and 143: accelerated frost damage or frost d
Page 144 and 145: A possible objection to the present
Page 146 and 147: [17] Dührkop H., Saretok V., Sneck
Page 149 and 150: Decay potential in wood structures
Page 151 and 152: Wood decaying fungi will only be ac
Page 153 and 154: to promote decay prevails. Two of t
Page 155: Acknowledgements This paper has bee
Page 158 and 159: (a driving rain gauge), however, is
Page 160 and 161: Fig. 1. Map of Norwayshowing the lo
Page 162 and 163: Frequency 25 20 15 10 5 0 highest p
Page 164 and 165: interesting to compare those result
Page 167 and 168: Effects of wind exposure on roof sn
Page 169 and 170: In ISO 4355 ”Bases for design of
Page 171 and 172: variety of roofs and wind exposures
Page 173 and 174: Li and Pomeroy [13] evaluated hourl
Page 175 and 176: Fig. 5. Exposure coefficients for 3
Page 177 and 178: meteorological stations are expecte
Page 179 and 180: Many of the meteorological stations
Page 181 and 182: the ground with return period of 30
Page 183: References [1] Standards Norway. De
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