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close this bookAsbestos Overview and Handling Recommendations (GTZ, 1996)
close this folderPart III. Asbestos substitutes
close this folder4 Fiber-free substitutes in construction area
View the document4.1 Fiber-free substitutes in housing construction
View the document4.2 Fiber-free substitutes in water mains construction

4.1 Fiber-free substitutes in housing construction

In housing construction the substitution of Asbestos containing products in roofing is of primary interest, since the release of Asbestos fibers from weathering is particularly critical. Therefore, substitutes for these application areas are presented below.

4.1.1 Material made of rock and ciay

· Roof slate

Roof slate is a natural sedimentary rock with fine-grained texture, which can easily be split into thin plates in one plane. Its color is usually blue-gray to black-blue, seldom blackish, reddish, greenish or whitish. In general, the following quality requirements are made:

1. The slate should easily be processed, split, axed, holed, or sawed, without resulting in large splittings.

2. It should largely be flat, have a smooth surface and an even grain size, because then the area of attack is smaller; the break should be thin-leaved.

3. The highest resistance to breakage is advantageous, nevertheless the hardness should not be too great.

4. Hair splits and visible inclusions (nodules, veins) should not be present.

5. Color fastness, i.e. even lightening and no spotty changes.

Chemical analyses are also used for the evaluation of roof slate, since its durability and structural properties depend greatly on the chemical composition. While the large presence of silicium and aluminum-compounds improves the quality of slate, its quality is impaired by the presence of lime, pyrites and carbon in higher quantities. The feldspar in slate belongs to the minerals which relatively quickly break down under weathering influences. With roof slate, therefore, the requirement of very weather-resistant mica layers is of great importance: if these form thick congruent layers, then the damaging substances (carbonic acid, sulfuric acid) from the air and rainwater cannot penetrate, and the slate can be durable even in urban areas in spite of lime and pyrite contents. Weathering effects consist primarily of color changes, moss growth, leafing and complete degradation.

· Roofing Shingles

Roofing shingles are flat, ceramic construction parts for the covering of sloping roof surfaces. They are formed out of clayey compounds, in some cases with additives, and are fired in an oven. Roofing tiles formed out of concrete, plastic, metal or other materials are not roofing shingles. For the manufacturing of high quality wall or roofing shingles, the use of appropriately dosed clay mixtures is a prerequisite. Since the correct clay mixture is seldom found in mining, the desired ratio must be prepared in centrifuges by the addition of quartz sand or its washout.

The shingles can contain components which greatly reduce the quality, if they were not removed or neutralized. Such impurities are in particular: calcium carbonate (mussels, snails), quartz fragments and various salts. These substances are dehydrated during the firing and can then absorb water again when in contact with humid air, thereby potentially leading to fine fissures, swelling and even the destruction of the whole shingle (particularly through frost effects). Overly sandy clay mixtures also promote growth of lichens and algea. The shingle material must have a fine grain size for frost resistance. Nevertheless, the shingles should have a particular porosity enabling active breathing, which to a small extent leads to water permeability. Bending and pressure resistance should be high, but should not negatively influence the weight of the shingle. Additionally, roof shingles should not flake, engobe and glaze must be durable, and the color even.

The quality of roofing shingles can be determined visually. One can judge the tendency for fissures, the water conductance as well as the color, surface density and surface condition. The resonance test provides an indication of the hardness of the shingle, which depends on its density and the length or intensity of the firing. Good shingles have shrill tones, their coloring does not rub off, they have sharp fracture edges, and their inclusions should not be above a grain size of I mm.

· Concrete roofing tile

Concrete roofing tile is a flat or synclinal covering element with or without grooves. The basic components of concrete roofing tile are sand, cement and water. The sand must have adequate hardness and resistance to weathering. Therefore, prepared quartz sands and Portland cement as the binder are primarily used. Depending on the final product, the mixing ratio of cement to sand ranges from about 1:3 to 1:3. 5.

Substances damaging to cement, such as salts, sugar-containing and humic substances, acids, gypsum, and soft rain water, reduce the quality. In particular, salty liquids form bonds with cement components. The resulting swelling pressures generally lead to damage through bursting. Sugar-containing and humic substances are much more dangerous than salts, since even in low concentrations they have a negative impact on the hardening of the binder. Therefore, the purity of the sand used must be assured.

Another problem is posed by cement swelling, which arises from the dissolving effect of soft rain water and in an extreme case leads to destruction of the cement structure. In contact with acidic solutions, crumbling of the concrete occurs. However, to a certain degree cement can be made resistant to damaging substances through treatment with silicones, fluorine compounds or bitumen.

Due to the manufacturing in long-stringed machines, complicated forms and grooves are not possible, the joint seal is limited along the length sides to two-sided double-ribbed grooves, while for the head seal only double supporting ribs are present. The joint seal is improved, though, through a high precision in size and a low plasticity.

Missing head grooves and the simple lengthwise grooves are often problematic in regard to weather resistance. Concrete roofing tiles are not very resistant to heavy rain and snow and are therefore categorized in the lower range for weatherproofing. High expectations on the water-tightness of roofs cannot be met, particularly when the lowest roof slope and lowest height of covering are used.

Water-tightness and frost resistance of concrete roofing tiles themselves are generally assured by the fine sand grains, the manufacturing and the additional steam hardening.

Concrete roofing tiles do have high resistance to breakage and high bearing strength, at least according to the values of the norm examinations. On the other hand, this resistance leads to difficulties in processing. With large formatted concrete roofing tiles, the otherwise advantageous accuracy to size is disadvantageous in the stone plane, since through vibrations of the roof construction in the case of strain there is little flexibility between the coverings, so that particularly the stone edges break off easily.

The weight of the roofing tile itself and the net weight (without safety addition) is relatively high at 40 to 55 kg/m². Therefore, special attention should be paid to the design of the supporting construction

4.1.2 Building metals

· Iron/Steel

Iron is the most widely used metal in the technical field, due to the good availability of the raw materials (iron ore, energy supply) the adjustable profile of characteristics through different alloy additives, the versatile processing possibilities and the excellent mechanical properties (in particular the high tensile strength, shearing resistance and bending resistance.

For roofing, galvanized steel suspension sheets are suitable (Class I 0.2 to 0.6 mm thickness, zinc coating at least 0.022 mm thick), which generally are also coated with synthetic resin, or for special requirements are also enamelled with polyvinyl fluorides or PVC, AMMA or PVF sheets.

Steel suspension sheets can bridge breadths of up to 8 m. Their useful width varies depending on manufacturer and profile type and lies between 610 and 1035 mm. They must be laid out so stiffly, that they do not buckle when walked upon and do not bend more than 1/300 under the highest permissible straint. For this reason, roofing sheets should be at least I mm thick and have an adequate profile. Roof areas should have a continuous decline up to the water drain. Therefore, a minimum slope of 15° is often required.

Corrosion promoted through humid air and oxygen containing water is problematic for roofs made of steel suspension sheets. Rust promoting agents are also acids and most salt solutions; consequently, smoke, sodium chloride, salt water, and binders such as magnesite mortar and gypsum. On the other hand, bases protect iron and can stop or reverse the destruction which has already begun. This fact enables the joint use of iron and cement, since the cement acts as a base. Lime loses its rust-protecting property through carbonation during hardening.

Damage through contact erosion can primarily occur with copper and tin, and to a small extent lead as well. Cast iron and highly alloyed steels do not rust as easily as pure iron types. Stainless steel is the only totally corrosion-free type, however.

There are two possibilities for corrosion protection- one is constructive measures and the other is special surface treatments (galvanic or spray coverings, rust protection coatings).

· Aluminium

Aluminum is rather soft and very easily molded. It is also resistant against water and air, due to the formation of a firmly binding and durable oxide layer. Sheets with a typical thickness of 0.7 mm and 0.8 mm are used for roofing.

Aluminum is attacked by most acids, and particularly by lye, and must be protected against them. Thus, lime and cement or concrete act destructively towards aluminum, particularly during the hardening phase. Aluminum is compatible with gypsum, however. Aluminum reacts sensitively in the presence of electrolytes (water) in contact with other metals (contact corrosion). Zinc, cadmium and rustproof steel are compatible. Appropriate surface treatment through pickling or artificial oxidation can protect against corrosion and chemical attacks.

The importance of aluminum lies in its low weight, the good thermal and electrical conductivity and the adequate alloy possibilities. For the construction industry its weather resistance, good warm and cold forming characteristics and the possibility of numerous types of binding (riveting, welding, soldering, gluing) make the metal interesting.

The lower strength compared to steel can usually be increased through special alloys, or compensated for through special construction methods. In addition, the savings in weight of aluminum constructions compared to those of steel is about 50%.

· Zinc

Zinc is generally only applied in the form of alloys as sheets for roofs, roof gutters and rain collection pipes.

Through its manufacturing, alloyed strip zinc has anisotropic material characteristics. Hence it can be somewhat better trimmed perpendicular to the direction of rolling. The strength of zinc sheet is largest perpendicular to the direction of rolling.

In air, zinc forms a protective oxide layer, which essentially protects the metal from corrosion. However, zinc is sensitive to acids and can be destroyed quickly by strong bases. In contact with more precious metals in an aqueous medium, contact corrosion occurs.

Zinc is valued primarily because of its comparatively low price and its normally high resistance to corrosion in clean air, giving it a lifetime of up to over 50 years in unaggressive atmospheres. There are zinc sheet roofs which have remained functional for almost 100 years. It is easy to handle in use, it can be easily cut, sawed, soldered and bent as well as embossed through heating. Due to its heat expansion, roof sheets and gutter elements must be movably placed.

· Lead

Lead is a light metal which is easily cut, rolled, pressed and embossed. Its heat conductivity is relatively low. The high coefficient of thermal expansion and the brittleness of the material forbid the rigid binding of longer sheets. The metal is so soft, that it only expands in a straight line to a small extent, and buckles up.

In the air, lead becomes covered with an oxide layer, which is barely soluble and therefore largely protects the lead from corrosion. Neither sulfur gases nor diluted carbonic acids are damaging to lead, even diluted hydrochloric acid does not affect lead. The sulfate or carbonate protective layer makes lead also resistant against contact corrosion.

Some organic acids, strong bases and distilled water are harmful to lead. Therefore, it should not come in contact with moist wood. In contact with quicklime or nonhardened cement, lead reacts very sensitively (localized corrosion) and must be protected against this by protective layers or bituminous coatings. Distilled water dissolves lead quickly. Lead is compatible with gypsum.

Lead and its oxides are toxic. Even low amounts are taken up by the body and stored. The result is lead poisoning.

· Copper

Copper is a very soft but relatively ductile material and is a very good thermal and electrical conductor. Alloys, which are widely possible, can reduce these properties and introduce significant changes. Pure copper has a tensile strength of about 200 N /mm² which can be increased by twofold through cold forming and even higher through alloy additives. The ductile material can be easily cold-formed and embossed. Copper and its alloys can primarily be soldered and welded.

Copper's durability in air and water is practically unlimited. It becomes covered with a dark brown, water insoluble layer of copper oxide, which protects it from further corrosion. Through the influence of sulfur dioxide or carbonic acids over time a very hard and weather resistant green protective layer forms of alkaline copper carbonate or copper sulfate. Strong acids (aside from hydrochloric acid) dissolve copper. Some organic acids form copper salts, which generally are toxic, such as verdigris induced by acetic, lactic or tartaric acids.

Damages to copper are limited to a few exceptions. Lime, cement and gypsum do not attack copper. The low damage through seawater is compensated by special alloys.

4.1.3 Building materials made of wood

Wood roofing with wooden shingles is among the oldest roof coverings, whereby only weather resistant types of wood come into consideration (Canadian red cedar, yellow Alaskan cedar, Chilean Alerce, beech and oak; also used are white cedar, pine and spruce, the latter primarily as wall shingles). The main advantages of wood as a building material lie in its good availability, easy processing, high resistance to aging and resistance against environmental impacts.

Without special constructive measures, wood is not a good insulator of heat, sound or vibrations, however.

In the selection of wood, attention should be paid to irregularities, such as crooked growth, irregular annual rings, strongly twisted growth, resin galls, shrinkage cracks, core cracks, internal annular shakes and frost cracks.

The good ability to process, particularly the easy splitting of wood, is due to the lengthwise structure of the plant cells. The strength of wood is strongly dependent on the water content and increases with increasing dehydration. Recently felled wood contains up to 50% wt moistness and should therefore not be statically burdened. Good values are around 10 to 15 %. The loading capacity of wood parallel to the grain is manifold that perpendicular to the grain. Basically, the tensile strength is larger than the pressure resistance.

Wood is very resistant against chemical impacts. It is not affected by diluted acids and bases and is superior to most metals in this aspect. Concentrated acids destroy wood, however. Oxygen has practically no negative impact, and wood is unlimitedly durable under water.

A negative trait is the swelling and shrinking of wood due to moisture changes, which can lead to significant changes in size. Cut pieces of wood often deform during drying. The swelling and shrinkage amount is less in the direction of the trunk axis and can generally be neglected. All wood types are rather susceptible to fire, insects and fungus. The maintenance of the quality of wood through professional installation and protective treatment is as important as the corrosion protection for metals.

The destruction by insects occurs almost exclusively through insect larva. Some of the known fresh wood insects are:/pidae, Cerambycidae, and Siricidae. They do not reproduce in the wood of buildings, however, in contrast to the dry wood pests ( Hylotrupesbajulus, termites etc.).

The outer appearance of the wood decomposing activity of fungus is termed rotting. Growth requirements are abnormal moisture and enough oxygen. Destruction types are corrosive rotting, destruction rotting and mildew rotting (e.g. Merulius domesticus)

As preventive protection measures against fungus, insects and fire, chemical agents can be applied, such as protective oils and salts.

4.1.4 Bituminous roof and sealing sheets

Bituminous roof or sealing sheets are flat coverings consisting of a support soaked and coated with waterproof substances (bitumen, tar). Good supports are crude felt, glass fiber quilt, spun glass fabrics, jute, metal supports (aluminum or copper foils), plastic foils and polyester fiber quilts. The sheets can be sprinkled with stones or stone dust or be coated with plastic foils.

The thermal conductivity (0.16 W/mK) is very low in comparison to other substances. Consequently, bitumen is a thermal insulator. The electric strength is high and the conductivity low.

The effectiveness of the bituminous sealing is based on its waterproofness and its deformability. The latter gives the sealing the capability of adjusting to small movements of the building and its parts without impairing its effectiveness. Even the hardest bitumen qualities must be regarded as fluids, which can be plastically deformed and can therefore give with movements. This also means, however, that bituminous layers yield to sudden impact and thereby lose their binding. Because of this, there are strict guidelines for application, which must be adhered to in all cases: bituminous layers must be free of hollow space and lie on even surfaces and may not be burdened point-wise. Furthermore, bituminous layers may not be used to bridge movement joints.

Another important criterion are the thermoplastic properties of bitumen. These lead to the requirement that the thermal impact may only reach an upper value which is markedly below that of the softening point of the applied bituminous type. Bitumen reacts to even small elevations of temperature. Formation of ripples is the result. Free-lying bituminous layers become coated with particulates often containing aggressive substances, which affect the bitumen in addition to solar radiation and lead to premature aging and embrittlement of the uppermost layer. Net rips and circular rip formations then arise, as well as embrittlement through aggressive gases and rained-out contaminants. For these reasons bituminous sheets may not be left unprotected against weathering and may not be used as a single layer. They attain their sealing effect only through the homogeneous fusion of several layers.

Based on the above remarks, one notes that bituminous sheet roofs require very exact installation and careful maintenance and care ( e.g. renewal of the protective coatings).

Bituminous roof shingles, whose lifetime is longer due to the very laborious installation, and bituminous corrugated sheets for light constructions and subordinate buildings are also used as roofing materials.

4.1.5 Plastic roofs

The multitude of different plastic types can be divided into three large groups, which basically differ according to their deformation properties:

1. Thermoplasts are thermally deformable, through solvents a softening to a lacquer-like condition is possible.

2. Elastomers are elastic like rubber bands, a lasting deformation is no longer possible after manufacturing.

3. Duromers are chemically hardened plastics, which cannot be plastically deformed, the processing must therefore be performed through machining. Duromers are very thermally resistant.

For the flat roof sealing the first two plastic groups are primarily applied. Plastic roofing sheets can be installed in single layers for flat roofs. The sheets should be at least 1.2 mm thick. So-called plastic foils are not suitable for sealing, they serve only as a separating and protective layer.

Among the currently most common plastic materials in the area of roofing, the following product types are found:

· Polyvinyl chloride- PVC - soft

On roofs, primarily soft PVC nb (not bitumen-compatible, Thermoplast) is used in the form of prefabricated sheets (width 1.2 m - 1.5 m). For protection against the lower-lying surface, a separating layer of crude glass fiber quilt or crude felt sheets is installed. Direct contact with bitumen or with particular plastic foams as thermal insulators is to be avoided, due to the danger of softener migration, ie. the migration of solvents used for softening in the manufacturing of PVC sheets. This migration results in shrinkage and embrittlement of the material.

The installation of roofing sheets proceeds loosely in connection with a gravel covering or a plate covering as superimposed load or as mechanical securement. The gluing of the seams occurs through solvent welding or through hot air welding, high frequency welding and heated wedge welding. Connections to other building parts can be attained with contact glues.

Under thermal impact over time, plastic foils regain their original manufactured size. The shrinking process associated with thermal impacts and the shrinkage due to softener migration require a mechanical fixation of the sheets at all roof edges, roof penetrations and connections. However, these effects can also be prevented by including a fabric insert in the foil manufacturing.

In building scalings the bituminous-compatible PVC-soft is currently increasing in use. Due to the lack of solar radiation, the feared softener migration does not occur in this area. Gluing is achieved with normal hot bitumen.

· Polyisobutylene-PIB (Thermoplast)

The processing possibilities are similar to those of PVC. However, PIB is always bitumen-compatible and can therefore be glued in the area with hot bitumen.

· Ethylene, Bitumen- Copolymer- ECB (Thermoplast)

Plastic roofing sheets out of this material, which is made from a combination of polyethylene and bitumen, enable a different application technique from that of the previously described Thermoplasts. Gluing is performed using the conventional methods.

ECB - roofing sheets can be glued in the area with normal gluing bitumen. Consequently, prefabrication of sheets and installation of these materials is not possible. The gluing of seams and joints can be performed with hot air welding. Due to the areal gluing with hot bitumen, the underlying surface is first covered with an additional glass fiber quilt bituminous roofing sheet.

· Chloropolyethylene- CPE (Thermoplast)

This roofing sheet is also made of a modified thermoplastic polyethylene. It can be applied loosely under a heavy surface protection, whereby the seams and joint connections are sealed with solvent welding. PEC has a low strength, which is why fabric-strengthened roofing sheets are often offered.

· Vinyl- Acetate- Ethylene- Copolymer- VAE (Thermoplast)

The seams and joint connections of this bitumen-compatible plastic roofing sheet can be sealed with solvent welding or warm gas welding (hot air welding), whereby an additional protection with a VAE solution is necessary.

· Isobutylene- Isoprene- Rubber- IIR (Elastomer)

These roofing sheet can be installed in the form of prefabricated planes or also as sheets. The material has an excellent expanding and restoring capability, which however makes it tough to deform. This disadvantage has been compensated through the prefabrication of common formed pieces.

The sheets can be glued in the area with a modified bitumen. The sealing in the area of seams and joint overlaps is done according to the principle of cold vulcanization.

· Polychloroprene- CR (Elastomer)

Due to the technical application possibilities for installing roofing sheets of this basic material unglued or with bitumen gluing masses, this material can assume multipurpose tasks. Aside from the possibilities of planes and sheets installation, this material can be used as an elastic interface between bituminous roof sealing and connections of all kinds. A connection with other plastic roofing sheets is not always possible, though. All seams can be sealed with a polychloroprene glue.

· Ethylene- Propylene- Dien- Mixture- EPDM (Elastomer)

The gluing of this material proceeds with hot bitumen glue on all conventional subsurfaces for the flat roof The seam connections are achieved with specially developed gluing tape.

· Chlorosulfonated Polyethylene- CSM (Elastomer)

These roof sealing sheets are suitable for bitumen and plastic gluing in the form of sheets and also as prefabricated planes. In order to guarantee good adhesion, these roofing sheets are generally delivered with a coating of fibers on the bottom side. Similar to PVC sealing sheets, the seams and joints are sealed with solvent welding or hot air welding. Additionally, all connection points can also be sealed with a foil cement.

As a fluid product, chlorosulfonated polyethylene can also be applied layer-wise with a roller, thereby enabling the application onto complicated building parts.

4.1.6 Reed and straw

Reed and straw are among the oldest roof coverings. Reed and straw are placed on roofing batten (typical cross-section 4 x 6 cm), which lie on rafters with an axial distance of about 1 m. Wire nails are used for securement of the raflers. To afix the reed and straw covering, wire is applied with which the roof can either be sewn or bound.

Reed grows everywhere on flat sloping banks of standing or slowly flowing waters and reaches 2-4 m in height. It should be ripe, leaf-free, thin stalked, straight stalked and well cleaned. If professionally treated and processed, it has a lifetime of 40 to 50 years on the northern side and on steep roofs up to 100 years.

Rye and wheat straw must be fully grown, straight, as long as possible and well threshed. Machine threshed is not usable, because the stalks are beaten wide and broken. Good, well treated straw lasts on roofs 25 years and longer.

A disadvantage of reed and straw roofs is the hazard of fire. In order to reduce its flammability, one uses soaking substances to make the reed or straw incombustible to some degree, as well as sprinkler systems in order to protect the roof from flames. Protective measures against lightning need particular attention.

The susceptibility to weather is reduced by a steep roof slope. The steeper the roof, the longer the lifetime of reed and straw roofs, because a long covering by rainwater and snow is prevented, and the pulling effect of wind is reduced. The typical slope of the roofs is 45°, in windy areas such as coastal areas at least 50°.

Reed and straw roofs are inexpensive to purchase, however they require very careful installation. Mistakes such as too thin spots can be taken care of by recovering, but nevertheless leave unsealed spots, since the total binding is destroyed. The rotting and rusting of the binder can then slowly progress.