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

Growth on masonry, clay and concrete roofing tiles

Biological growth is normally unwanted on buildings, and different kinds of chemical agents are often used to remove the growth. This project shows that the agents used in practice proved to be efficient in combating biological growth.

The project has shown the types of growth commonly found on the materials and the importance of selected material properties and construction conditions. A laboratory method for evaluation of chemical agents and of materials has been developed and tested. The results have been used for a critical review of the possibilities to prevent or combat biological growth.

Better knowledge of materials and types of growth

Background and objective

The project has been carried out in order to obtain greater knowledge of the organisms and their preferred conditions of growth, in order to describe the possibilities for prevention of unwanted growth.

The laboratory method has been developed and tested in order to test agents and methods under controlled conditions and to provide a realistic picture of an agent or a method. This lowers the risk for overdosing or inexpedient use of chemical agents.

The main purpose of the project is to make a survey of biological growth on masonry materials, and clay-tiled and concrete roofs, with a view to determining the most suitable methods for prevention of biological growth.

General interest in the project

The investigation

The project has been carried out by The Danish Technological Institute as a co-operation between three centres: Concrete, Biotechnology, and Masonry.

The Danish Environmental Agency has funded the project, with supplementary funding from The Danish Technological Institute. Several producers of materials and cleaning agents, suppliers of agents, cleaning companies etc. have contributed with information, participated in the steering committee, and helped perform cleaning experiments etc.

The project is based on examples of growth found in Denmark during the progress of the project. No limits for conditions of exposure or age of materials and constructions were set.

Principal conclusions

In the project the importance of selected material properties has been investigated. Roughness porosity near the surface, pH, and nutrients carried from the surroundings have been considered as important for growth. The investigations indicate that the surface conditions of the material are to a high degree decisive for the extent and type of growth.

Important construction conditions have been identified. Choice of a suitable construction may significantly reduce the extent of biological growth. Investigations of the impact of the growth on the materials in general show no sign of damage that may influence the lifetime of the materials. Exceptions are, however, cases where growth may contribute to bad adherence and peeling of surface treatments, or where large moss cushions on roofs may present a risk for water intrusion.

The results of the project have provided better possibilities for evaluation of the necessity for cleaning. In situations without esthetical requirements, which make growth undesirable, cleaning can normally can be omitted.

In connection with evaluation of specific agents or methods to prevent or combat biological growth, the laboratory method described in this project can be used. This is a test using a monoculture of green algae. The potential applications of the test are:

Testing agents for combating or cleaning

Testing agents for prevention of growth

Testing the resistance of materials to the occurrence of growth

This biological test should always be combined with identification of important parameters of the materials, especially in connection with evaluation of prevention agents and evaluation of the natural resistance of materials to biological growths.

The possibilities for prevention of biological growth in the different phases of the life cycle of the materials have been described. The descriptions focus on clay bricks, clay roofing tiles, mortar, and concrete roofing tiles.

Results of the project

Growth

The project has provided a profound knowledge of the types of organisms growing on masonry, clay-tiled and concrete roofs. A colour-coded catalogue has been prepared. This describes the species identified and includes keys for grouping and closer identification of the organisms. 26 species of lichens, 5 species of algae, 8 species of mosses and 3 species of fungi have been identified.

Parameters of materials

The investigations show that pH is important for the type of growth occurring on the materials. On clay bricks or roofing tiles where pH in the surface will fall during a short period there is a tendency for more rapid growth of green algae.

The roughness of the surface is important for how quick and how strong the growth on the materials will be. Roughness is also important for the type of species that grow on the materials.

The porosity of the materials, especially near the surface is considered important for biological growth.

The content of nutrients in the materials is generally considered as less important. In contrast, nutrients supplied by cleaning agents, preceding growth, biological deposits etc. are of major importance.

The humidification properties of the materials – the ability to absorb and release water, and the possibilities for water enclosures in the surface – are considered of great importance. However, the project could not identify a specific parameter of humidity as crucial for the possibilities of growth.

Of special interest are the evaporation tests, which show that water evaporates faster from materials with heavy growth than from materials with less growth.

Cleaning for growth / evaluation of effect and risks

The project has revealed the agents and methods, which at present are used in practice to clean biological growth.

Only one relevant active agent on the market has been approved for combating algae (fatty acid C8 – C18, main fraction decanoic acid). A laboratory test method has been developed in the project. This may, for example, be used to test the efficiency of agents. The method has been tested on this active agent. Furthermore, supplementary cleaning tests have been performed with this agent. These tests showed no satisfactory results on the materials investigated in this project.

During the project hot water cleaning was tested at two different pressures and temperatures. The tests showed that this method could remove the visible growth, but also that the surface of the materials was visibly damaged. This damage enhances the possibilities for new growth, and visible new growth could be detected as soon as 6 months after cleaning.

The investigations show the necessity of revealing the risks of a specific cleaning method before it is used. This should especially be seen in the light that the results of the project show that growth is often only an esthetical problem, and in general will not diminish the lifetime of the materials.

Possibilities for prevention of growth

Raw materials and production processes

The project has shown that not only the amount of growth is decisive for removal of the growth, but also, to a considerable extent, the homogeneity over the entire construction. An important plan of action in order to avoid biological growth is therefore to ensure a process that can minimize the variations of the important material parameters.

Possibilities to avoid or minimise growth by influencing pH, porosity, surface roughness, surface tension, humidification properties, nutrients, and poisons have been described.

In combination with investigations of technological properties of the materials, the test method mentioned earlier may be used for evaluation of the effect of changed parameters of materials.

Prevention by planning and design of constructions

In connection with planning a building and choice of construction it is essential to evaluate the degree to which biological growth can be accepted. Choice of a suitable construction may considerably reduce the amount of biological growth, especially on vertical masonry.

Therefore, as a part of planning a building, an important plan of action to avoid biological growth is to decide the extent to which growth can be accepted. The possibilities to prevent growth by choosing roof eaves, roof slopes, roof ventilation, types of joints, and materials, and avoiding construction defects have been described.

Prevention by construction

Prevention during use and maintenance

The possibilities for prevention of growth by:

Maintenance of joints
Repointing
Cleaning maintenance of surface treatment
Maintenance of surroundings

have been described.

Importance of prevention for the possibilities of reutilization

Only a few of the actions for prevention of growth described in this project are considered vital for the possibilities to reuse the materials after demolition:

Addition of biocides or poisonous substances in the production of the materials.

Use of water repellent surface treatments.

Further information

The results of the project are described in this main report and in the following subreports:

1. Subreport 1: "Investigation of agents and methods". Reveals agents and methods to prevent and combat biological growth.

2. Subreport 2: "Investigations of occurrence". Describes investigations of occurrence of biological growth, its impact on materials and the importance of selected material parameters and construction conditions.

3. Catalogue: "Growth on masonry, clay-tiled and concrete roofs". Descriptions and colour illustrations of identified species, and keys for grouping and identification of organisms. This catalogue has been prepared in cooperation with University of Copenhagen, Institute of Botany (ISBN 87-7944-792-9).

4. Subreport 3: "Development and testing of a laboratory method". Describes the development and testing of a method, which can be used, to test methods and agents to prevent and combat biological growth.

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