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Pel, L (1995) Moisture transport in porous building materials. ["eprint_fieldopt_thesis_type_phd" not defined] thesis, Technische Universiteit Eindhoven.

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Moisture plays a dominant role in various deterioration mechanisms of building materials, e.g., frost damage or salt crystallization. In the case of masonry, this deterioration may reveal itself in various forms, e.g., decolouring of the surface, cracking and chipping, or desintegration of brick and/or mortar. A detailed knowledge of the moisture transport is essential for understanding the durability, which in this respect may be regarded as the resistance of a material or construction against deterioration caused by attacks from its environment. In this thesis the isothermal moisture transport in various porous building materials, i.e., fired-clay brick, sand-lime brick, mortar, gypsum plaster is discussed. The isothermal macroscopic moisture transport in porous materials can be described fairly well by non-linear diffusion equation. In this diffusion model, all mechanisms for moisture transport, i.e., liquid flow and vapour diffusion (with the associated enhancement mechanisms), are combined into a single moisture diffusivity which is dependant on the actual moisture content. In principle, the moisture diffusivity can be determined directly from measured transient moisture profiles. With Nuclear Magnetic Resonance (NMR) techniques, which yield an output signal, that is proprtional to the amount of hydrogen nuclei in a certain selected region of the material, the moisture profiles can be determined non-destructively. Because of the large amount of paramagnetic ions present in many porous building materials, the hydrogen nuclei in these materials exhibit very short transverse relaxation times and a broad resonance linewidth. Therefore specialized NMR equipment was developed, which enables the measurement of the moisture profiles in these types of materials with a one-dimensional resolution of 1 mm and an aabsolute accuracy of a few percent. Usin the NMR equipment the moisture profiles are determined durin water absorption and drying. For both of these processes it is found that, within the experimental accuracy, a single unambiguous relation exists between the moisture diffusivity and the actual moisture content, which is dependent on the type of material. For the porous building materials discussed in this thesis, except for gypsum, it is found that the moisture diffusivity for absorption can be approximated by a single exponential function of the moisture content. It is shown that the limited number of discrete positions at which the moisture profiles are scanned and the one-dimensional resolution of the NMR equipment have only a minor influence on the determination of moisture diffusivity for drying. The error in the calculation of the moisture diffusivity is dominated by experimental noise and inhomogeneities in the porosity of the materials under investigation. In the drying experiments, it is observed that after some time, a receding drying front develops that moves through the material with constant velocity. In this stage of the drying process the moisture transport is dominated by vapour transport, which limits the overall drying rate. A new so-called receding front method is developed that uses the constant velocity of the drying front to model the isothermal moisture transport at low moisture constants. In masonry brick and mortar are bonded, and these will therefore interact. Hence attention is given to the moisture transport across the brick/mortar interface. It is found that during the bonding process the hygric properties of the mortar change significantly compared to mortar hardened independently. Preliminary absorption experiments using brick/mortar samples indicate that there is no perfect hydraulic contact, and suggest a jump in suction across the interface. Due to the limited number of experiments on the latter study no satisfactory explanation can be given yet for the moisture transport phenomena observed near the brick/mortar interface.

Item Type: Thesis (["eprint_fieldopt_thesis_type_phd" not defined])
Uncontrolled Keywords: NMR; Moisture movement; Moisture measurement; Masonry
Subjects: English > Building Envelope
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Depositing User: Anna Samuelsson
Date Deposited: 28 Mar 2008 09:20
Last Modified: 04 Apr 2017 12:49

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