Pel, L.
1995
Ph. D. Thesis, Eindhoven University of Technology, Netherland
| 1 INTRODUCTION
In the past decades durability of materials, service life of building constructions, and life-cycle costs have been important topics in building research. The durability of a material and/or construction may be regarded as its resistance against deterioration caused by attacks from its environment. This deterioration may exhibit itself in various forms, e.g., in the case of masonry by decolouring of the surface (visual damage), cracking, chipping, or desintegration of brick and/or mortar (visual and mechanical damage).
Based on a probabilistic approach, the durability may be predicted [BEK92]. If sufficient data are available, such a method will predict the decrease of condition with time (e.g., [CUR93]). Based on such predictions, service life and life-cycle costs can be determined and management decisions can be made with respect to maintenance, restoration or renovation. However, these predictions are based on existing materials and constructions. Therefore no prediction can be made for new types of materials and constructions or different environmental conditions (e.g., other climate, contaminated rain, and pollution).
Looking at the various deterioration mechanisms, however, it is clear that moisture plays a dominant role. It enters by, e.g., driving (contaminated) rain, condensation, run off from roof and facade and/or capillary rise of ground water. In addition water may enter by failure of water services and flooding. Water will transport contaminants such as soluble salts. If wet, the material can become susceptible to freezing damage. It may also act as a substrate for the growth of bacteria, fungi, or algae with possible physical and chemical damages, but also possible health risks. A large fraction of the absorbed water will leave a material again because of drying. While a material is drying salt crystallisation may occur at the surface, causing defacing (efflorescence), or just under the surface, where it may cause structural damages, e.g., delamination, surface chipping, or desintegration.
Often it is tried to determine durability by means of accelerated aging tests in the laboratory. In these tests cyclic conditions are applied in which a specific degradation mechanism is dominant, e.g., freeze-thaw test. However, it is not always possible to relate the accelerated test to natural deterioration as found in outdoor-exposure; a material that performs well in the accelerated aging test may perform poorly in practice and vice versa. Often the exact degradation processes are not clear or not well understood. As a consequence, there are different aging tests which aim to test durability in relation to the same degradation mechanism, e.g., freeze-thaw tests: NEN 2872, DIN 52252, ASTM C67. Another problem is that the combination of two durable materials, e.g., brick and mortar combined to masonry, may perform poorly. This is caused by the interaction between these materials, i.e., moisture will migrate from brick to mortar and vice versa. In figure 1.1 a schematic diagram of possible approaches to durability research is given.
Figure 1.1: Schematic diagram of possible approaches to durability research of masonry From the discussion above it may be clear that a good knowledge of the various degradation processes associated with durability cannot be obtained when the moisture transport is not sufficiently understood. Such a knowledge is needed to develop better accelerated tests. Apart from this, it can give information for prevention and restoration. More insight into moisture transport may also give rise to adjusted technology and building design. Moreover, it can give information for the justification of probabilistic models. The scope of this thesis is to study the moisture transport in porous building materials, as a first step towards understanding of the associated deterioration mechanisms. This study concentrates on isothermal moisture transport. Since in the Netherlands more than 90% of the exterior walls of dwellings consists of unplastered masonry, special attention is given to moisture transport in bricks, mortar, and their combination in masonry.
The moisture transport in saturated porous media was first described by Darcy [DAR56] in 1856. More or less complete studies concerning moisture transport in non-saturated porous media started after 1950. Among the classical studies are those of Philip and de Vries [PHI57, VRI58], Luikov [LUI66], and Berger and Pei [BER73]. Later Whitaker [WHI77] and Bear [BEA90] provided a more fundamental basis for the equations governing moisture and heat transport in porous media on basis of volume-averaging techniques. The objective of chapter 2 is to give a brief survey of the theory of isothermal moisture transport in porous media. The most relevant assumptions and limitations will be given, as well as the resulting diffusion model. In this model all mechanisms for transport are combined into a single moisture diffusivity, which is dependent on the actual moisture content.
For the present study, it was decided to determine the moisture diffusivity directly from moisture profiles as measured during various transport processes, i.e., drying and absorption. It has been shown that Nuclear Magnetic Resonance (NMR) and scanning neutron radiography offer powerful techniques to measure these moisture profiles in a non-destructive way [GUM79, GRO89]. Of these techniques NMR offers the best sensitivity, since it selectively probes the hydrogen atoms. However, complications occur if the materials under investigation contain large amounts of paramagnetic ions, as is the case for most building materials. Therefore an NMR apparatus was designed that was especifically suited to study the moisture transport in these types of material. In addition measurements were performed using neutron scanning radiography, in order to verify results found by NMR. Both instruments will be discussed in chapter 3.
Chapter 4 discusses the applications of, in particular, the NMR instrument for measuring the moisture profiles during water absorption and drying of various building materials, i.e., soft mud machine moulded fired-clay brick, mortar, sand-lime brick, and gypsum plaster. Based on the measured profiles, the moisture diffusivity of the materials under investigation is determined as a function of the moisture content, both for water absorption and drying. A new method is presented to determine the moisture diffusivity for drying at low moisture contents, where the moisture transport is dominated by vapour transport.
In masonry brick and mortar are bonded, and these will therefore interact. The results of preliminary studies of the moisture transport across the brick/mortar interface are presented in chapter 5.
Finally, chapter 6 includes the main conclusions and suggestions for future research. |
