State of Art in the Field

Effects on materials

A number of factors contributing to future climate change are likely to have an impact on building deterioration on different time- and length-scales. Trends in atmospheric composition and microclimate parameters will affect the fundamental processes causing damage to building materials [2-4]. Increasing atmospheric concentrations of carbon dioxide and other trace gases alter air chemistry and influence chemical reactions. In general, higher temperatures will influence chemical reactions, as well as reducing the effects of thaw-freezing cycles in many environments, while regions with stable cold weather could experience increased thaw-freezing cycles. Alteration in seasonal and annual rainfall will change the length of time during which surfaces are wet, affecting surface leaching and the moisture balance that influences material decay processes.

Although legislation has greatly reduced concentrations of 'traditional' air pollutants in cities, the changing climate may enhance the effects of some of them. In particular, changes in wetting and drying cycles on building surfaces may alter the deposition of acidic gases onto the surface. Longer sunlight hours can increase the photochemical degradation of polymers used in both modern construction and the restoration of ancient buildings.

Several parameters can influence the degradation of materials exposed to the atmosphere. The degradation process is usually a complex interplay of chemical, physical and biological parameters. For several materials, like calcareous stones, stained glass, metals, and painted surfaces, dose-response functions (DRF) have been derived based on extensive field exposures. The DRF express degradation as a function of climatic parameters (temperature, relative humidity, time of wetness, sunshine radiation, wind speed), gaseous pollutants (SO₂, NO₂, O₃), particulates (including sea salt) and precipitation (amount, pH, chemical composition). For unsheltered areas, material damage is usually discussed in terms of dry and wet deposition of pollutants. Wet deposition includes transport by means of precipitation, and dry deposition transport by any other process. The corrosion attack can thus be expressed by DRF of the general form

K = fdry(T,RH,SO₂, NO₂, O₃, HNO₃, VOC, particles) + fwet(Rain, H+, Cl)

• K is the corrosion attack,
• fdry the dry deposition term, and
• fwet the wet deposition term
.

The changes can in principle be subdivided into the following main categories:

• Temperature increase. The influence of temperature on the deterioration process is complex for several materials. At low temperatures the deterioration rate increases with increasing temperature, due to prolonged time of wetness and faster chemical reactions. In some materials a maximum followed by a decrease in the corrosion rate has been observed at about 9-11°C. The decreasing part is attributed to a faster evaporation of moisture layers after rain or dew periods, and a surface temperature above the ambient temperature due to sun radiation, which results in a decrease in wetness time. Of interest, however, is that this phenomenon does not occur in marine locations where, because of the presence of a surface moisture layer of hygroscopic chlorides, no deterioration rate maximum is observed. Increased temperature also affects the term for wet deposition in the DRF, which is one of the reasons why the rate due to wet deposition is more important for corrosion processes in tropical and subtropical regions than in temperate ones.


• Change in relative humidity. In most materials, a rise in relative humidity causes an increase in deterioration, due to prolonged wetness time, higher deposition rates of pollutants and more favorable conditions for microbiological activities.


• Increase in wind velocity. An increase in wind velocity may affect materials deterioration in several ways. Increased eddies and flows around historic buildings can increase the deposition rates of both gaseous and particulate pollutants, as well as strengthening the effect of driving rain. A very serious effect may be an increased transport of sea salt inland, which can substantially enlarge the areas along sea coasts affected by marine aerosols.


• Change in sun radiation. An increase in sun radiation may accelerate the deterioration of organic materials, such as paint coatings, and materials used for consolidation of stone materials.


• Change in precipitation. Increased precipitation amounts can increase the damage due to wet deposition through the dissolution of surface layers. Changes in chemical composition, especially pH, can affect the deterioration rates of several materials.


• Change in gaseous and particulate pollutants. Changes in levels of SO₂, NO₂, O₃, HNO₃, CO₂, VOC and both primary and secondary particulates can be of crucial importance to the degradation of building materials[4].