Arguably the main failure of buildings is the damage caused by water and its interactions on the building and local environment. Whether it is rain runoff or moisture in the air, some consider about 75% of buildings failures are due to water (May, 2005). Ingress of water can occur in either vapour or liquid form but only leave the substrate as a vapour. When in liquid form, the water can not only enter by direct means such as rain or direct water contact but also by capillary action (Karoglou et al., 2005). In vapour form molecular transport, solution diffusion and convection are the methods of water ingress (Kunzel, 1995).Therefore it is important to understand the effects water has on the materials making up the building. Where most of the effects occur is at a microscopic level at the boundary of the water and the substrate. To a lesser extent there are effects at a macro level but these are associated with weather extremes and are less common in the UK.
Generally building materials are hygroscopic in that they take up water and subsequently maintain a dynamic equilibrium of water content by absorbing water from the environment or desorb it. According to Hill et al, this behaviour leads to the expansion and contraction of the material which in turn leads to damage through cracking (Hill and Rizvi, 1982).
Condensation is the chief architect of water damage and creates the most problems both internally and externally. However this is mainly a recent phenomenon as it occurs more commonly in newer structures as the humid warm air is trapped in the building and fabric since new building envelopes are much tighter in order to preserve against heat loss (Thompson, 2000).
Stone has been the mainstay of building construction for centuries. The reason for this is because of the material’s physical robustness and its relative abundance. Brick, coming later proved to be even more useful due to the ease of manufacture and logistical benefits, as well as its regular shape and size.
However like all building materials brick or stone is susceptible to weathering albeit at a relatively slow rate due to its durability. There are three types of damage that weathering can cause and all of these are associated with water. They are :
- Physical
- Chemical
- Biological
The two main areas of physical damage of old buildings in the UK is both the built up of crystals and freezing within the pores of the material. Crystals formed from solution caused by the ingress of water dissolving mineral solutes. With evaporation the crystals are formed and as they grow they increase the pressure of the surrounding substrate to potential destruction (E. Winkler, 1972). Correns in 1949, was able to start the base line for crystalline work by postulating that the ability of a crystal to exert pressure in a restricted environment was based on the function of the super saturation ratio (Correns, 1945). More up to date work has been carried out on detecting water deposits in stone substrate to identify potential crystallisation areas. Standard methodology includes capillary rise tests to identify absorption percentages in specimen rocks under laboratory conditions. However this is fairly restrictive as it does not provide the picture of potential crystallisation in situ. Thermal imaging has the potential of mapping out areas of risk of the build-up of crystals (Avdelides, Moropoulou and Theoulakis, 2003).
The pH of the water can be changed by pollutants and this can also change the rates of crystallisation, dissolution and recrystallisation cycles of the soluble salts (Jones and Wakefield, 1999). Furthermore added to these temporal changes, the structure of the crystals themselves are changed which lead to an additional factor in the understanding of building damage due to crystallisation.
Stone with a larger pore size will attract more water and this, dependent on the shape of pores, will hold more water and for longer and will be less susceptible to temperature change. Additionally, not only is the pore size important but also the roughness of the material as condensation will form more readily on the rough material due to increased surfaced area. The relationship between the two is complex (Camuffo, 1995). Camuffo states that pores with a radius of less than a µm, the physical effects of water dominate whereas with the larger pores, the chemistry of the water will also influence the adsorption of the water onto the stone.
Pore size and shape is important. Water condenses first on the inner walls of any pores and stays there for the longest time as it is subject to less evaporative effects as seen below. The relative humidity required for condensation is less for a curved surface ( I.e. less than 100% of the required saturation in the atmosphere) (Camuffo, 1995).
Illustration of internal and external pores in stone showing how water is affected by even the evaporative effects of wind and sun. Adapted from Camuffo (Camuffo, 1995)
Therefore it is arguable that the temperature differences for thermal mapping, will be greater and hence easier to see. Furthermore it is suggested, the large pore stone, is more susceptible to salt crystal decay (Theoulakis and Moropoulou, 1997).
Similarly, when the water freezes the expanding ice increases the pressure and ultimately crack up the stone or brick. The water can expand up to 9% (Lstiburek, 2010) causing substantial damage especially in tight fitting walls. It will push non frozen water through hydrostatic pressure and this liquid water freezes deeper in the material. Not only this, but the differing freezing temperatures will cause different expansion rates.
When drying out again the porosity is the important factor associated with the material (Karoglou et al., 2005). Using kinetic drying mathematical models and adding the porosity values, a forecast of the drying of the material can be achieved.
It has been seen that the physical presence of solutions can damage the substrate due to the presents of crystals. Add to this that the solutions themselves will cause chemical changes as the solutes move into solution they are then able to react with the material which will cause changes to the structure of the stone of brick. This last point assumes that the solutes are internal and are leaching out. However pollutants in the atmosphere will be absorbed by the atmospheric moisture and precipitate onto the material causing potential damage. An example of this is the acid rain of the latter part of the 20th century. This was caused by sulphur dioxide and the nitrous oxides that were produced by the burning of fossil fuels.
Moisture and damp are also vectors for the transportation of biological material. The longer the damp is present, the more biological life will develop. The water and the chemical reactions favour biological life. It is considered that Time of Wetness (TOW) is related in some way to the damage of the stone (Camuffo, 1995) In most cases often fungal spores are the initial source of future damage. The damage can be divided into two. First is the potential damage to health as the damp walls will produce fungi and their spores in huge amounts creating health issues. Second is the physical damage to the substrate by the funguses themselves. One example is Serpula lacrymans. that can spread throughout the masonry in dark damp environments.
Dry rot on a wall (image from diydoctor.org.uk)
Finally, there has be a great deal of work on modelling how various building materials act under both temperature and moisture and what the materials’ properties are. Recently databases have been constructed to allow relevant information to be added to various hygrothermal models. These models are now available on basic computers (Kumaran, 2006).