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Renere teknologi til undgåelse af biologisk vækst på murværk, tegl- og betontage

Summary and conclusions

Background for the project
Purpose
Definition and approach
Conclusion

Background for the project

It is not unusual to see roofs and walls, which are green with algae or patterned by yellow, grey, and black lichens. In the light of the existing knowledge, it is likely that some organisms cause more damage than others, but to achieve more knowledge of how and where very different organisms cause damage, it is necessary to have a more thorough knowledge of which organisms are relevant.

Better knowledge of the organisms and their preferred growing conditions will improve the possibilities to prevent unwanted growth.

Purpose

The purpose of the project phase "Investigations of growths" is to

	identify different types of growth
	identify significant growth parameters
	identify how growth occurs on the materials
	describe the effect of the biological growth on the materials
	identify relevant material characteristics
	identify relevant construction conditions
	survey and evaluate types of damage
	identify potential detergents/methods

The above knowledge will contribute to describing the applied and potential preventives.

A distinction must be made between the possibilities in the following categorised life-cycle phases:

	extraction of raw materials
	production
	construction
	construction/execution
	use/maintenance

This part will not be reported until the final report on conclusion of the project phase: "Proposal for plans of actions".

Definition and approach

In the project the term "biological growth" is used as a common term for

	bacteria
	algae
	fungi
	lichens
	mosses

Higher plants such as facade plantings are not covered by this project. The project only includes organisms that grow in Denmark on the materials below:

· masonry materials including

- bricks

- calcium silicate bricks

- granite/gneiss

- masonry mortar

- rendering mortar

- whitewashed masonry surfaces

- painted masonry surfaces

· clay tiles

· concrete tiles

Special focus is on bricks, clay tiles and concrete tiles. The project distinguishes between the following life cycle phases for masonry and roofs:

	Extraction of raw materials
	Production of building materials
	Design
	Construction/Execution
	Use/maintenance
	Demolition – Recycling/reuse

This project is based on the examples of growth found during the course of the project. Thus the examination differs from a number of the related examinations described in literature. These usually deal with selected materials that have been laid out for exposure in natural conditions or in the laboratory.

The primary advantage of the approach of this project is that the survey/knowledge of the growths, which can be found on relevant materials, becomes more substantial, as no limitations have been made in exposure conditions or age of the materials.

The disadvantages of the approach are that there are many different variables to relate to (age, geography, exposure conditions, etc.).

As the knowledge of the sorts of growth on masonry, clay, and concrete tiles was very modest at the start of the project - no results of similar examinations are available - the project group judged the advantages of the chosen approach to be bigger than the disadvantages.

Conclusion

Introduction

As described earlier, the project is based on the examples of growth found during the course of the project. Setting priorities for examinations and relationships has partly been dictated by the combinations of growth and material we have been able to examine more closely.

During the course of the project it was soon apparent that it was easier to get permission to exchange overgrown roofing tiles with new ones than to take samples of bricks, mortar and rendered surfaces from masonry. Therefore the project has focused on the organisms which are able to grow on roofing tiles.

A large number of the organisms identified on this substrate, however, also occur on masonry, and there is no reason to believe that knowledge gained from examinations of roofing tiles cannot to a large extent be transferred to masonry.

As no fungi have been recorded on roofing tiles, examinations of fungi have only been given a modest part of the project. Fungi have especially been found on painted and rendered surfaces from which it has only been possible to get a few samples for closer examination in this project. Therefore very few conclusions have been made in the following paragraphs with decisive parameters for fungi.

Types of growth

The project has provided thorough knowledge about which organisms grow on masonry, clay, and concrete tiles.

A colour-coded catalogue, which describes the identified species, has been prepared. In addition the catalogue contains keys for identification of the organisms.

The work on identifying the different organisms has revealed an extremely large richness of species on a relatively modest material. The work also showed that the most common species, with the largest coverage, were repeatedly found, which means that, in our opinion, the catalogue will be a useful tool in the future. Such a comprehensive examination of organisms found on Danish materials did not previously exist. The examination is thought to be of European interest.

26 species of lichens, 5 species of algae, 8 species of mosses and 3 species of fungi have been identified. The algae found on masonry, clay and concrete tiles belong to the group of aerial algae i.e. they are spread through air.

There seems to be a tendency that endolithic lichens (lichens that grow down into the brick or tile medium) under normal conditions prefer concrete and mortar and do not thrive to the same extent on red clay bricks or tiles. The theory is however uncertain, as the estimate of whether Physcia caesia - a lichen of common on clay bricks or tiles - is epilithic (lichens that grow on the surface of the brick medium) has been made on the basis of a single, thin section. The theory should be examined more closely in future examinations of overgrown materials.

The increased knowledge of which organisms grow on masonry, clay and concrete tiles, increases the basis for understanding:

	The demolition mechanisms on masonry, clay bricks of tiles and concrete, if any
	The mechanisms spreading the organisms
	The effect/lack of effect of cleaning
	The need for prevention of growth
	How to develop relevant test methods
	How to give a better and more qualified guidance

To the trade, this means that there is now a better possibility to examine a series of factors including:

	whether discoloration is due to chemistry or biology
	whether the cause of growth is the material, construction, execution, or environment
	how growth can be prevented and combated.

Important growth parameters

Generally it is known from literature that three factors must be fulfilled for an organism to establish itself on a material:

	a surface to which the organism can be root itself
	sufficient nourishment for evolution and growth
	sufficient water to supply the physiological functions and in many cases their formation and spreading (cyanobacter (sulphur bacteria), algae, mosses and lichens). The moisture decides the composition of kinetics, biomass, and vegetation.

The content of the nutrient of the medium, moisture conditions, and pH are parameters important to the composition of the flora. Other factors, such as frost, pollution, light, and temperature, as well as the characteristics of the material, and their mutual influence are also important.

Occurrence of growth on materials

As far as possible, a review has been made in the colour-coded catalogue of the material types on which the specific organisms have been identified in this project and of which material types the organisms prefer according to literature.

On concrete roofs and mortar there are generally more lichens than on clay bricks and tiles. Some species of lichens are, however, also quite common on clay bricks and tiles - for example the silver-grey lichen Physcia caesia appears frequently. The diversity of species of lichens on concrete roofs is, however, generally much larger, and often the lichens are the dominant growth on concrete roofs.

On concrete roofs, the orange lichen Xanthoria parietina is often very dominant in the overall visible impression of the growth.

On clay roofs, algae are more frequently the dominant growth, but often with scattered lichen pads. However, in the course of the project clay tiles have been seen completely overgrown by lichens.

Mosses most frequently appear between tiles, as well as on base surfaces, where the roughness is larger and thus provides a better attachment. In addition mosses frequently appear in mortar joints, often originating in a thin crack between brick and joint. On concrete tiles, examples have also been seen where mosses have become attached to the surface of the concrete. This has not been seen on clay tiles.

On masonry algae, lichens and mosses have been observed. The lichen and moss growth often originates in the mortar joints, from where they spread over the bricks.

Fungi have been identified on rendered and/or painted masonry. However, as mentioned earlier, we did not focus on these, and further examination would probably result in identification of many more than the 3 species we have found.

Decisive material characteristics

The following material parameters have been examined:

	porosity
	pH
	density
	water absorption
	rate of evaporation
	nutrients
	surface roughness

The examinations show that pH has importance to the species of growth that appear on the materials. The examination also indicates that pH may have an influence on how quickly green algae grow on the materials. Most of the identified lichen species apparently prefer a pH above approx. 9.

Density and water absorption has no clear influence on growth.

To clay bricks it is a well-known phenomena that the water-repelling ability does not commence until after a certain period after roofing. This change in the tile can also be expected to have an influence on the rate of evaporation of water from the bricks. Therefore, from examinations of new bricks, it is not possible to report the actual rate of evaporation after roofing.

The rate of evaporation of newly produced bricks cannot immediately be used to assess the tendency of the material to become overgrown. Comparative determinations of the rate of evaporation surprisingly showed that bricks with rapid growth have a higher rate of evaporation than bricks with weak growth. The examinations only covered bricks with growth of algae and lichens.

The content of nutrients in the materials is assessed to be of secondary importance to the occurrence of growth. However, the kind of nutrient emitted from detergents, previous growth, and biological deposits from animals and plants etc. are considered important. The materials covered by this project do not generally contain agents which can be expected to be poisonous to biological growth. The examination, however, did not extensively cover paints, glazing and other surface treatments. Therefore it has not been possible to conclude anything about the importance of such nutrients.

The surface roughness is important for how quickly growth appears on the material. Materials with a rough surface will be overgrown at the most. However, in the course of time smooth surfaces can become overgrown, if other conditions are present, but growth will typically be delayed compared to similar materials with a rough surface.

The surface roughness is of importance to the kind of species that may overgrow the material.

Surface roughness will change over the course of time, especially for cement and lime calcareous materials, for example due to effects from acid rain.

For clay bricks and tiles it seems that porosity, expressed as the amount of air, has a certain influence on growth. The microanalyses carried out from thin sections show a higher air content in the material in areas with rapid growth than in areas with weak growth. Where air pores lie very close to the surface penetration into these can be seen.

With regard to concrete, porosity (expressed in content of air pores) does not seem to have an effect on the occurrence of growth. However, where growth occurs penetration into the pores very close to the surface (0.5 mm from this) can be observed.

On painted surfaces growth occurs mainly where the paint has been decomposed/penetrated. In these areas penetration is observed along bricks/tiles close to the surface. It is, however, not possible to decide if the growths burst through the paint or if they penetrate where the paint has already been burst. It is possible that growth may contribute to decomposition of the paint layer, where underlying porosities cause weaknesses in the paint.

Identify decisive construction properties

All construction properties that affect light, warmth, and moisture conditions appear to be significant to the occurrence of biological growth. Furthermore properties that affect the possibility of attachment from the growth seem to be extremely important, especially the design of mortar joints, the proper choice of mortar for the given construction, and any finishing treatments that might affect the surface roughness, for example when crumbling starts.

In the project the following aspects have been subject to closer examination:

	Geographic location and orientation
	Roof eaves
	Roof slope
	Underroof
	Roof ventilation
	Rising soil moisture
	Defects (leaky drainpipes etc.)
	Design of mortar joints
	Quality of mortar

There is general disagreement on the effect of biological growth on the materials and to what extent biological growth can cause significant deterioration of building materials. In paragraph 9 "Types of damage", an account has been given of the results of our own examinations, as well as an exposition of relevant examinations from literature.

In connection with the microscopic investigations of thin sections, a study was also made of how far into the actual material the biological growth can be detected. Only up to 0.5 mm penetration of growth has been recorded. As described earlier a distinction was made between epilithic and endolithic lichens, where endolithic lichens are able to grow into the actual tile material. However, the extension of the endolithic lichens on the tiles chosen for the microscopic investigation of thin sections has been modest. Only at 3 microscopic investigations of thin sections has the microscopic investigation been laid through an endolithic lichen, and in none of these cases was the growth very strong.

In literature (cf. clause/paragraph 9) it is possible to find many articles that conclude that biological growth is the cause of deterioration of materials. With the documentation that is available for different chemical/biological processes there is no doubt that the growth can start a number of different deterioration processes. However, the articles mentioned often deal with examinations of historical monuments of a much greater age than would normally be expected for buildings. Many of the articles deal with growth on sandstone and/or limestone, neither of which has been included in this project.

In connection with adherence of surface treatments (paint, lime, etc.) examples were noticed during the course of the project, where the biological growth is estimated as a contributor to poor adherence and peeling of the surface treatments.

Therefore, for new surface treatment it seems relevant to carry out thorough cleaning of the construction to prevent poor adherence and peeling.

On the basis of microscopic investigations of thin sections of painted concrete tiles it is possible that in some cases growth may contribute to deterioration of paint layers in places where underlying porosity results in weaknesses in the paint. However, it can be assumed that this deterioration is very slow and will hardly be the decisive cause of a need to clean and repaint.

Many people consider a change in the aesthetic impression in the form of local growth/colour changes/discoloration as damage. It is naturally very subjective whether growth is considered as discoloration or an undesirable change of the aesthetic appearance of the construction. Similarly, it is subjective if an aesthetic change can be characterised as actual damage.

In the project, tests have been carried out with hot-water cleaning using 2 different pressures and temperatures. The method showed that it was possible to remove the visible growth, but for all tested combinations of the 2 pressures and 2 temperatures there was visible damage to the surface of the material. Approx. 6 months after cleaning took place, visible growth appeared again on the tiles.

The tests have been reported in detail in the subreport "Development and testing of test methods".

Increasing knowledge of the species of growth has provided greater opportunity to estimate the risks of different cleaning methods:

	By using mechanical cleaning there may be a risk of spreading the organisms through incomplete cleaning, instead of removing them. The surface may appear clean, but if small endospores are deposited throughout the porosities in the material, new growth will soon emerge again.
	For growth of endolithic lichens down into the materials, it may be impossible to remove all organisms by mechanical cleaning without destroying the surface of the materials.

When developing and assessing any cleaning method, there should be careful consideration of the risks the method implies for the material. In subreport No. 1, a short review is given of the risks that each method may imply. This subreport also lists a number of items which should be considered before cleaning is started.

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