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CLOSE THIS BOOKAsbestos Overview and Handling Recommendations (GTZ, 1996)
Part III. Asbestos substitutes
VIEW THE DOCUMENT1. Technical requirements for Asbestos substitutes
VIEW THE DOCUMENT2 Properties of typical Asbestos fiber substitutes - Overview
3 Fiber substitutes for Asbestos fibers in the building area
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT3.1 Non-textile fibers made of glass wool rock wool and mineral wool as well as ceramic wools
VIEW THE DOCUMENT3.2 Wollastonite
VIEW THE DOCUMENT3.3 Cellulose fibers
VIEW THE DOCUMENT3.4 Polyacrylnitril
VIEW THE DOCUMENT3.5 Polyvinylalcohol (PVA)
VIEW THE DOCUMENT3.6 Polypropylene (PP)
VIEW THE DOCUMENT3. 7 Summary
4 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

Asbestos Overview and Handling Recommendations (GTZ, 1996)

Part III. Asbestos substitutes

1. Technical requirements for Asbestos substitutes

Due to the intense discussions about the environment in recent years, extensive efforts have been made to find substitutes for Asbestos containing materials.

The requirements on the substitutes are given by the physical-chemical properties of the Asbestos product to be replaced, which strongly depend on the technical area of application. The wide breadth of application areas for Asbestos has led to many substitute products. The German Environmental Agency (Umweltbundesamt, UBA) commissioned Battelle-lnstitut e.V. Frankfurt with the compilation of a catalogue of Asbestos substitutes, which was published in 10-volumes in 1985. According to this work, in spite of the unique properties of Asbestos, there are Asbestos-free alternatives for nearly every application. This is because not all of the properties are required for the individual applications, but rather the dominating characteristic for the application must be substituted. The following matrix provides useful information:

Table 16: Technical Properties of Asbestos Containing Materials Depending on their Binding in the Material

Area of Application

Fiber Form

Mechanical Properties

Chemical Properties

Thermal
Stability

Insulation

Specific
Friction
Characterisitic

Specific
Adhesive
Characteristic



Tensile
strength

E-
Modulus

Alkalines

Acids


Thermal

Electrical



Asbestos in inorganic matrix











- Asbestos cement

+

+

+

+







- Fire proof sheets

+

+

+








- Spray compounds

+

+

+








Asbestos in organic matrix











- Friction lining

+

+

+

+

+






- Gasket

+

+

+

+

(+)

(+)





- Plastic reinforcement

+

+

(+)

(+)







Asbestos products with little binding






















- Asbestos textiles

+

+

+

+







- Asbestos cardboard and paper

+

+

+

+







- Filter materials











Legend + of prime importance (+) for special products of importance

Source: Schreiber: Asbest-Ersatzstoffe, in: IACS, Page 13.4

2 Properties of typical Asbestos fiber substitutes - Overview

From the previous discussions it is apparent that Asbestos can be substituted in most application areas by other fibrous materials. These can be natural or synthetic fibers. Primarily those fiber types listed in Table 17 come under consideration.

Table 17: Fiberous Materials for Asbestos Substitution



Wollastonite


inorganic fibers

Attapulgit

Natural Fibers


Sepiolite



virgin wool


organic fibers

cotton



cellulose



flax and hemps



crystalline fibrous materials:


inorganic fibrous

- steel


materials

- carbon fibers



- SiC-Whisker



- polypotassium titanate



amorphous fibrous materials:



- textile glass fibers

Synthetic


A-glass, C-glass, D-glass,E-glass, R-glass,

fibrous


glass, Z-glass

materials


silicic acid fiber



-non-textile glass fibers (insulating fibers)



glass wool,



mineral wool,



ceramic wool



polyester


organic fibrous

polyaramide


materials

polytetrafluoroethylene



viscose



polypropylene



polyacrylnitale

Source: Schreiber: Asbest-Ersatzstoffe, in: IACS, Page 13.4

Applying the Asbestos property matrix shown in Table 16 to the potential Asbestos substitutes results in the property matrix for the substitutes presented in Table 18.

Table 18: Technical Properties of Asbestos Substitutes

Fibrous Materials

Mechanical
Properties

Chemical
Properties

Specific
friction
Characteristic

Thermal
Stability in
C Degrees

Insulation


Tensile
strength

E-Modulus

Alkaline

Acids


350

550

750

1250
and >

Thermal

Electrical

Inorganic amorphous












Fibrous materials












Textile glass fibers

+

(+)



?


+



+


A-Glass





+







C-Glass












D-Glass












E-Glass











+

R-Glass












Z-Glass




+








silicic acid fibers

+

(+)



(+)



+


+


non-textile glass fibers

(+)

(+)



?





+

+

ceramic wool




+





+



glass wool







+





rock wool and mineral












wool





(+)



+




Inorganic crystalline












fibrous materials












Steel

+

+

(+)


+ ?







Carbon fiber (highly resistant, highly modular)

+

+

+

+

+ ?




(+)



Carbon fibers (Pitch Type)

+

+

+

(+)








Carbon felt










(+)


Potassium titanate










(+)

(+)

Silicium carbide-Whisker

+

+

+









Organic fibrous materials












Polyester

+

+


+






+

+

Polyaramide (Arenka)

+

+

+

+

+

+




+


(Kevlar 49)

+


+

+


+




+


Polytetrafluoroethylen












(Teflon)

+


+

+


+




+

+

Polyacrylnitrile

+


(+)

+






+

+

Ployaramide (Nomex)

+


+



+




+

+

Viscose (Reyon)

+




+





+


Polypropylene

+


+

+






+


Legend + suitable (+) conditionally suitable ? possibly suitable
Source: Schreiber: Asbest-Ersatzstoffe, in: IACS

The main result of this section is that particularly for the quantitatively important applications of Asbestos, namely

· fire protection,
· thermal insulation,
· structural elements, construction products (Asbestos cement products),
· friction products,

substitutes are available which enable the replacement of Asbestos. The respective health risks of the substitutes must be specially investigated.

Table 19: Areas of Application for Fibers and Filling Materials as Asbestos Substitutes, their Price Range and Health Effects


Health
&
Safety

Fire
Proof

Thermal
Insulation

Electrical
Insulation

Gasket

Filtration

Friction
linings

Construction
Prod.

Chemical
prod.

Price
Range 1)

Health
Risk2)

Textile glass fibers

+


+

+

+


+


+

·

+

SiO2- fibers

+

+

+

+

+





- -

+

carbon fibers





+


+


+

- -

+

Non-textile glass












fibers

+


+


+

+

+

+

+

+·

-

Ceramic fibers

+

+

+

+






- -

-

Gypsum fibers



+






+

n.d.


Wollastonite

+


+

+



+

+

+

·

+ -

Attapulgit









+

n.d.

- -

Sepiolite









+

n.d.

-

Polyacrylnitrile






+

+

+

+

n.d.

+

ox. PAN

+


+



+

+


+

n.d.

+

Vinylal/Polyvinylalcoh.








+


n.d.

+

Polypropylene




+

+



+


n.d.

+

Polytetrafluoroethylene




+

+

+



nn.d.

+


Aramide

+

+

+


+


+


+

- -

+

Woo

+

+








+

+

Cotton

+



+





+

+

+

Cellulose-fibers





+

+

+

+

+

+

+

Flax and hemp








(+)


+

+

Mica





+


·


+

+

+

Talk





+




+

+

+

Bentonite







+


+

+

+

Diatomite







+


+

+

+

+ substitutes are available (+) restricted possible uses 1) ++substantially cheaper + cheaper · similar prices - more expensive - - much more expensive n.d. no data 2) (Carcinogenic effects were defined based on the critical diameter range): + not critical - critical -- very critical n.d. no data

Source: Schreiber: Asbest-Ersatzstoffe, in: IACS. Page 13.9 - expanded -

Table 19 shows a summary of the application areas of Asbestos substitutes and information on their carcinogenic effects. In Germany, the recent controversial classification of glass fibers with particular characteristics as carcinogenic, according to the TRGS 905 of May 1995, could potentially influence application areas for glass fibers.

For detailed information, the substance catalogue of UBA and the substitute list of the Federal Institute for Occupational Safety and Accident Research (Bundesanstalt fur Arbeitsschutz und Unfallforschung) are referenced.

The current state on Asbestos substitution is presented in the following table. The product groups can generally be divided into two classes:

· Product groups with complete substitution possibilities;
· Product groups with partial substitution.

Table 20: Possibilities for Asbestos Substitution Product Groups Area of Use

Fibrous/filling material

Inorganic synthetic fibers

Inorganic natural fibers

Organic synthetic fibers

Organic natural fibers

Non
-fibrous filling material
- lamina (flakes)
- particles


Health & Safety

Personal thermal resistant clothing

Thermal resistant glass

Flat textile prints

Materials for specific workplaces



Fire proof

Fire proof boards & surfaces

Spraying compounds, insulating plaster

Plastic masses, paints, cement and filters, fire proof mortar

Cardboard, cords, fleeces inorganic foam(pastic) material fire protection cushion

Textiles
- fire extinguishing blankets
- curtains

Protective clothing for fire fighting

Heating insulation

Boards & surfaces

Inorganic spraying compounds

Material for filling joints and cavities

Formed parts and moulding compounds

Textile products


Electrical insulation

Wire and cable

Insulating materials

moulding compounds

Household appliances



Gaskets

Static
- flat gaskets

Dynamic packaging

Cylinder
- head gasket

Heating gas gasket

Compensators


Filtration

Liquid filtration fine and sterile filter media, fil- tering aid media

Gas filtration/ ventilation process air dust collection

Respiratory filter for respirator

Diaphragms, separators



Friction Lining

Disk brake lining

Drum brake lining

Brake soles

Brake lining for industrial use

Clutch lining


Construction elements (Asbestos cement)

Small formatted flat boards

Small formatted corrugated board

Pipes for underground construction
- pressure pipe
- sewer pipe

Pipes for household and property drainage
- ventilation
- waste gas

Landscaping


Chemical products and others

Paint materials and filler

Adhesive materials, gasket compounds, cement

Special products with asphalt or tar matrix

Moulding compounds with plastic-matrix(duro plastic)

Moulding compounds with synthetic material(thermoplastic)


Note: Since Asbestos substitutes (other substitute products) are now available, the use of Asbestos products is therefore no longer necessary.

Source: Schreiber: Asbest-Ersatzstoffe, in: LACS, Page 13.14

3 Fiber substitutes for Asbestos fibers in the building area

In the building area Asbestos primarily is applied in the form of Asbestos cement, so that the most effective substitution is with Asbestos-free fibers (hereafter referred to as substitute fibers - SF- or substitute fiber cement - SFC).

After trials with plant fibers, such as flax or hemp, it was found that such fibers are not very appropriate because of their swelling capability and their low resistance against microbes. Steel fibers, on the other hand, are not appropriate due to their poor dispersing ability in the cement matrix. Trials with polyamides, such as nylon or perlon, also had to be given up after a longer period.

Very promising trials have been performed with polypropylene fibers, which bind extensively with cement. Presently, the following are mainly used as reinforcement for fibrous cement: polyacrylic fibers, such as Dolan 10 fibers, plastic fibers made with polyvinylalcohol or also cellulose fibers. In addition, non-textile glass fibers and wollastonite fibers are used. The spectra of properties of these types of fibers are described in the following sections.

3.1 Non-textile fibers made of glass wool rock wool and mineral wool as well as ceramic wools

Glass fibers produced with diameters in the range of 0.1 pm to 20 pm have become particularly important. Since lengthwise fracturing is not to be expected, these fibers only conditionally lie within the range for penetration to the lungs. Based on their compatibility, the residence time in the organism is relatively short.

Application areas for these mineral fibers are in the loose form in stuffing insulations, mats, felts and sheets, as well as in fibrous filling materials in the areas of fire protection, thermal insulation, noise or vibration insulation, filtration, friction linings and also chemical products and fibrous cement.

Glass fiber concrete (Heidelberger Zementwerke AG) is made of a cement-bound matrix and alkali-resistant, highly firm glass fibers. This material produced in a mixing concrete process has very good properties, especially for facade elements, because of its high bending tensile strength, high impact resistance, corrosion resistance and its good ductility. Its relatively low weight and high fire resistance (Fire Class Al) enable the manufacturing of complicated and multi-functional building parts.

3.2 Wollastonite

The inorganic natural fiber wollastonite is a calcium metasilicate with a chain structure consisting of at least 96.5 % CaO and SiO2. The fiber diameter ranges from 10 pm to 100 pm; under mechanical wear a fracturing into fibrous pieces with diameters as small as 0.1 pm is possible. Critical diameter ranges can be prevented through the use of appropriate production methods, however.

Wollastonite is used as filling material in the areas of fire protection (sheets), thermal and electrical insulation (sheets, fill for moulding material), friction linings, structural elements and other construction products, chemical products and others (paints, glues and moulding material).

3.3 Cellulose fibers

Fibers made of cellulose are used as additives in building products to enable the dispersion of synthetic fibers in the cement matrix.

An example for the application of cellulose fibers which is under development is a so-called fiber cocktail, consisting of cellulose and cellulose fiber acting to disperse polyvinylalcohol or polyacrylnitrile fibers in the cement matrix (Eternit AG). Through special processing procedures, an orientation of the fibers is also achieved, which leads to an improvement in the mechanical properties.

3.4 Polyacrylnitril

Organic synthetic polyacrylnitrile fibers (PAN-fibers) can be manufactured for technical applications with a diameter above 18 pm. They therefore lie outside the range penetrable to the lungs. A lengthwise splitting under mechanical wear is only conditionally possible. PAN-fibers are used for reinforcement in the areas of filtration, friction linings, structural elements and other construction products (as reinforcement up to 2 % in fibrous cement), etc..

3.5 Polyvinylalcohol (PVA)

Organic synthetic polyvinylalcohol fibers (PVA - fibers, PVA fraction at least 85%) can be made water insoluble through combination with aldehydes. The fiber diameters typically lie between 10 pm and 20 pm and are therefore not penetrable to lungs. Data on the lengthwise splitting do not exist, however it may be assumed that this does not occur. PVA- fibers are applied in the building area for reinforcement of fiber cement products (Fraction 2 %).

3.6 Polypropylene (PP)

Organic synthetic polypropylene fibers (PP - fibers) can be manufactured with diameters of 20 pm to 100 pm and are therefore not penetrable to lungs. Lengthwise splitting is not to be expected. The fibers are applied in the areas of gaskets and structural elements and other construction products, particularly fibrous cement. Trials with fiber mats and fiber fleeces showed, however, that due to the arising separation of layers between mats and cement paste, no satisfactory results could be achieved. In Europe only Moplefan (Italy) still manufactures fibrous cement sheets from short-fibered PP fleeces or PP - fiber and cement paste.

3. 7 Summary

The fiber substitute technique has not yet reached the standard of quality of Asbestos fiber reinforcement. Deficiencies, such as frost uplifting and fine fissures from thinning of fibers, occurred particularly with corrugated SFC products.

Aside from several disadvantages which SF have over Asbestos fibers, such as reduced binding and higher price, the new SFC products also have advantages, such as higher elasticity and easier processing.

The mechanical resistance of fibrous cement products is dependent on the mixture of ingredients, which is different for each application area, and on the particular manufacturing process. The maximum temperature resistance lies around 150°C.

As with Asbestos fiber products, SFC products have generally limited resistance against moss formation, fungus collection and mold. This is true to the same extent for Asbestos containing and Asbestos-free products. They are also impaired by acids, vegetable oils and fats, magnesium salt solutions, sulfates, ammonium salts, iron chloride, warm distilled water and hot condensed water. Chlorine, sulfur dioxide and smoke also act destructively over long periods.

SFC products, however, resist alkalis, salts, alcohols, mineral oils (bitumen), and tar. They do not corrode and resist dry gases.

Cement contains free unbound alkali metals, which partly separate during the hardening process, and lead to the formation of hydroxyl ions in aqueous medium. (Cement reacts as an alkaline substance).

These characteristics also apply for housing construction and water pipelines, with the limitation that no adequate SFC pipe products have been found for high pressure requirements.

Aside from SF substitutes, other fiber-free substitutes are usable in the building area, and their application is especially gaining importance in housing construction and water pipeline construction. Their possible applications are therefore discussed in the next two sections.

4 Fiber-free substitutes in construction area

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.

4.2 Fiber-free substitutes in water mains construction

The properties of materials used in water mains construction were already introduced in Section 4.1. Consequently, they are mentioned below only with respect to their application as pipe material.

4.2.1 Metal pipes

· Steel Pipes

Steel pipes have a high strength with great fracture strain and a high notch impact strength. Additionally, a shifting of forces and tensions is possible. The installation of long pipes and the line installation are feasible. Adjustment to the local conditions and heights by cutting and welding in situ as well as simple manufacturing of pipeline fittings and axial force closing joints make this material very popular.

Application occurs primarily where high strength and fracture strains are required, high inner pressures and high pressure surges are likely, and where large pipe sizes occur, for instance at supplier and long-distance pipelines and at special constructions, such as inverted siphons or intersections. Initially, seamless steel pipes were used, and with improvement in welding techniques, welded pipes have been applied increasingly. The seamless and welded pipes are available up to DN 500, above DN 600 only welded pipes.

Disadvantageous are the required corrosion protection measures, such as encasement and lining, since there is a high corrosion probability with insufficient encasement and inadequate protective lining. During transport, installation and bedding, additional measures for protection of the pipe wrapping are needed. For corrosion protection in the interior, cement mortar lining ( DIN 2614) has proven effective. The earlier and still frequently typical bituminous coating has not proven successful. In pipeline fittings the cement mortar lining still presents problems, however. For the outer protection the polyethylene wrapping is used in general. Longer steel pipes are frequently protected cathodically. The electrical conductivity can be either advantageous or disadvantageous, depending on the application area.

· Cast iron pipes

The gray cast iron pipe has been in use for the past 500 years and was previously cast horizontally in two-part sand moulds and later standing in sand moulds. Since 1926 centrifugal casting has become prevalent. Pipeline fittings continue to be cast in sand forms. The important material progress occurred in 1965, when the ductile cast was introduced. The gray cast has a brittle fracture behavior, due to its morphology. However, through special melt additives one acquires the ductile cast, which is superior by its elevated fracture strain coupled with high strength.

The cast iron pipes are offered in 3 classes for water pipes, depending on the nominal widths DN. The corresponding permissible nominal pressure is determined from the nominal width.

The simple manufacturing of the pipe joints is advantageous, as are the bending ability and axial mobility in the joints. Particular measures are required for the uptake of the axial strengths, however. The pipe joints are usually not electrically conductive. Pipeline fittings are available in a wide sortiment.

With the change to ductile cast iron, the corrosion resistance of the gray cast iron has been lost, and hence the pieces must be protected like steel. The cement mortar coating (DIN 2614) is typically used for the inner corrosion protection. Suitable for the outer protection are: PE-encasement (manufacturing plant), cement mortar encasement (manufacturing plant), and PE-foil encasements for joints at the construction site; galvanizing by spraying with a bituminous covering or only bituminous covering may only be applied by proven non-aggressive soils.

4.2.2 Plastic pipes

In water supply mains, primarily polyvinyl chloride (PVC, free of softeners) and polyethylene (LDPE, HDPE) are applied. PVC is used mainly in distribution nets, while HDPE/LDPE-pipes are specially used for house connections and for supply lines of smaller nominal widths, e.g. in rural areas. The properties of both pipe types can be characterized as follows:

· PVC, softener-free

An advantage is the resistance to corrosion, the low weight, the simple manufacturing of the elastically sealed plug socket joints, the axial mobility of the plug socket joints and a very smooth pipe wall.

Difficulties can arise with temperature fluctuations and extreme climate conditions. The sustainable tensions can become reduced depending on temperature and operation time. The impact sensitivity increases at temperatures c 5°C. Particular care must be taken during the bedding of the pipes. PVC pipes are very sensitive to external stress. Generally, there is a very low bending ability in the pipe joints. Furthermore, transition problems sometimes occur, since often the only pipeline fittings available are made of other materials.

· LDPE (Low density PE), HDPE (High density PE)

LDPE/HDPE-pipes are also very corrosion resistant and are often used because of their low weight. They are bendable (particularly LDPE) and are available as endless pipes, which are available on a ring or drum. Relatively long pipes are feasible which do not need bends and can be installed in narrow pipe beds. A very smooth pipe wall and an axial strength joint support the application of these pipes.

As with PVC pipes, the sustainable tensions are reduced in correlation with the temperature and operation time. At low temperatures marked stiffening can arise. The pipes are also very sensitive against local peak tensions (e.g. caused by sharp-edged stones or damage to the surface). Furthermore, the high thermal coefficient of expansion is also disadvantageous. In some cases, only pipe joints made of other materials are available.

4.2.3 Concrete pressure pipes

Concrete pressure pipes are mostly applied for supplying and long-distance pipelines having large nominal widths with low to medium pressures and few built-in components and fittings. The long period of use and the high resistance against inner and outer corrosion are particular advantages of this type of pipe. Furthermore, the static measurement can be determined exactly according to the local loads. The high resistance to denting is also positive.

Negative aspects are the high weight and the brittleness of the pipe material. A good deal of technical knowledge and knowledge of statics are needed, because no axial strength joints are possible, the fittings generally not being pre-tensed, but designed constructively in another fashion or available in steel. Difficulties can also arise through later manufacturing of bonds and through repairs. During the installation great care must be taken, particularly in making the bottom of the ditch and the pipe joints.

Nowadays the following concrete pipe types are primarily used:

· Reinforced Concrete Pressure Pipes

Reinforced concrete pressure pipes are mostly used as pressure pipes, protecting pipes or jacket pipes for holing-through and underground driving as well as sewer pipelines with pressures up to PN 2.5 bar. They are manufactured as jolted, rolled and spun concrete pipes with DN 250-4000 (and larger), lengths of 2.5-Sm and with unstressed ring and axial reinforcement.

· Prestressed Concrete Pipes

The reinforcement is prestressed in the ring direction, for which the winding process and prestressing through core expansion are available. Generally, prestressing of the axial reinforcement is also performed. The diameters lie between DN 400 and 2500, the wall thicknesses are 55-150 mm, the construction length is 5m. The concrete quality of B 55 should be met or exceeded; typically a concrete strength 100 N/mm² is attained.

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