Collapsing building © Daniel FriedmanAsbestos Cement Products
Asbestos cement roofing, siding & millboard composition, history, production, uses, identification & safety

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Cement asbestos materials: this article series describes asbestos cement products & materials and the history of their production and use.

This article includes text & data Adapted from Rosato (1959 out of print) on the production and use of asbestos-containing materials, adding photographs, inspection, repair and maintenance advice, and updates on asbestos hazards involved in the installation, repair, or demolition of asbestos cement products such as roofing and siding or asbestos millboard that remain in place on buildings.

Our page top photograph shows badly broken cement asbestos roofing shingles on a building in upstate New York.

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The History, Production Methods, & Contents / Properties of Cement Asbestos Products©

Cement asbestos roof shingles (C) Daniel Friedman

Asbestos is safe and legal to remain in homes or public buildings as long as the asbestos materials are in good condition and the asbestos can not be released into the air. - US EPA

[Click to enlarge any image]

Asbestos Cements and cementious products including some cement pipes contained asbestos fibers as a strengthening material. Asbestos cement roof ing is described just below, and cement wallboard and cement siding containing asbestos are described later in this document.

W.H. Ransom provides a succinct description of asbestos cement as it was used in roofing, siding, millboard and other products that we have annotated just slightly.

Asbestos cement is made from asbestos fibers [usually white chrysotile asbestos], ordinary Portland cement, and water [and possibly silica flour]. The cement hydrates and sets around and between the asbestos fibres which act as reinforcement.

The fibers are based on the mineral chrysolite and have an average length of around 5 mm. The relative proportions of asbestos fibre and cement used vary with the particular product to be made: for roof sheets and slates the proportion of asbestos is around 10% by weight of the dry materials. [19]

As we cite in detail in this adaptation from his original text, Rosato provided extensive historic detail about asbestos cement products and puts the asbestos content by weight as ranging from 10% to 75%.[20] Typically chrysotile asbestos is the most-common form of asbestos found in asbestos-cement products.

Ransom points out that asbestos-cement materials harden with age and that the material also loses its impact strength over time through atmospheric carbonation. Thus asbestos cement roofing in particular but perhaps also siding become more brittle and more vulnerable to impact damage. We add that such roofing may also shed asbestos in roof runoff and debris.

Cracks in asbestos cement products used outdoors may also occur due to the stresses occurring as different faces (exposed and covered) are exposed to very different degrees of atmospheric carbonation and thus differential shrinkage.

However in our own experience, damage to cement asbestos roofing and siding are most pronounced where the materials were exposed to mechanical damage from ice, impact, foot traffic, and carless contractors such as renovators or cable-TV or telephone wire instsallers.

The following text wasadapted from scanned text 8/30/2013 - History of Use of Asbestos. Edited, expanded, and illustrated with © protected materials to clarify titles; tables & illustrations from the original text are supplemented with additional product photographs, references, citations

D.V. Rosato

The following text is Adapted from Rosato (1959) p. 62-66 [1] © 2013

Types of Asbestos Cement Products

Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and ConradThe following text is adapted & expanded from Rosato p. 62-66. [1]


Asbestos-cement products are primarily composed of Portland cement reinforced with asbestos fibers.

The proportion of cement to fiber varies over a range of approximately 10 to 75 per cent, by weight; it depends upon the physical characteristics desired in the finished product. Percentage of asbestos also can depend upon the type of manufacturing process to be used and cost of the finished product.

When compared to cement, the cost of asbestos is considered high.

The strong asbestos fibers behave similarly to the steel bars used in reinforcing concrete. With the use of asbestos fiber in cement, approximately 70 to 80 per cent of the weight of nonreinforced concrete is eliminated when roofing, siding, pipe and other similar products are manufactured.

Photo at above-left shows transite (cement asbestos) air ducts buried in a building slab. Sewer gas leaks from a failed cast iron sewer line entered this duct system. Click to enlarge any image at InspectAPedia.

Generally, most asbestos-cement products are made under pressures varying from 100 to 10,000 psi. The specific pressure generally depends upon the type of product or process being used. This particular industry uses the greatest quantity of asbestos. Chrysotile fiber is used principally. Some crocidolite is used; amosite is rarely used because of its low strength.

The fibers used involve group 4 of the Canadian classification.

This grade of chrysotile asbestos as well as groups 5 and 6 are generally classified as medium length fiber; they are used extensively in the manufacture of such asbestos- cement products as pipes, flat or corrugated sheets, roofing or siding shingles, clapboard, hand molded products, and wall boards.

The general characteristics of asbestos-cement products are given in Table 3.1 (below).

One basic requirement for the asbestos is that it must be fully fiberized prior to mixing it with cement in order to obtain the best properties in the final product. It is also important to remove or minimize such impurities as dust and grit.

In the United States, the standard grade of Portland cement is a low-heat cement or an aluminous cement. Products are manufactured by processes generally classified as the molding process, the dry process, the wet press process and the wet mechanical process.

Asbestos-cement mixtures do not lend themselves readily to extrusion processes. There has been time spent experimentally in attempting to develop suitable processes but commercially there is little manufacturing by this method.

Ludwig Hatschek, an Austrian, invented the asbestos-cement shingle in 1900 which has now developed into a major commercial product. The original wet process which was developed produced a variety of flat sheets. Later, a dry process was developed which initially provided for competitive development with regard to wet versus dry process. The wet mechanical process is the principal process used today.

Asbestos-cement products lend themselves to rapid construction; therefore, they have been particularly useful for lightweight housing and for industrial buildings. In the last few years, approximately one-half of the asbestos used in the United States was used for asbestos-cement sheets and pipes as well as for floor tiles. It is reported that in 1950 approximately one billion sq ft of asbestos-cement products for building had been produced.


Group Use Grade or Type Process of Manufacture Modulus of Rupture, PSI Tensile Strength PSI Compression Strength PSI Water Absorption %
Shingles Roofing or Siding Plain, veneered & textured surface Wet machine, semi-dry machine 2,000 - 5,000 PSI 500 - 1,500 PSI 4,000 - 12,000 PSI 12 - 30
Clapboards Siding Plain & textured surface Wet machine, wet-press 2,000 - 5,000 500 - 2,000 4,000 - 16,000 12 - 30
Flat Sheets Walls, sheathing, industrial & residential construction, industrial equipment, insulation Plain, impregnated, integrally waterproofed Wet machie, wet process 500 ! to 5,000 150 ! to 5,000 1200 ! to 30,000 0.31 to 140 !
Corrugated sheets Roofing, siding, ornamenal Plain gray with different sized corrugations Wet machine, wet press 2,000 to 6,000 500 to 3,000 4,000 to 20,000 2.0 !! to 25
Pipe Water works, drainage, sewage, industrial processing 3-36-in. diameter up to 13 ft. long Wet machine 2,600 to 6,000 1,500 to 6,000 6,000 to 15,000 2 !! to 25
Molded Products Industrial purposes Plain, impregnated MOlding 500 to 5,000 150 ! to 2,500 1,000 to 15,000 0.3 !! to 25 !

* Encyclopedia of Chemical Technology, Vol. 2, New York, The Interscience Encyclopedia, Inc.

! These values refer to low-density products, which are usually used as thermal insulation.

!! These values refer to impregnated and waterproofed products.

Decorative asbestos-cement building products are available and conform with modern structural and color design.

See the asbestos-cement sheeted Arched Ceiling in New York's Grand Central Station shown in Figure 3.1 below. In the case of chemical plants as well as other plants or buildings, there exists the prime requirement that roofing be resistant to corroding fumes.

This requirement also exists in chemical transporting pipes. Durable and fireproof roofing is obtained by the use of asbestos-cement corrugated boards which can be installed as quickly as sheet iron or aluminum roofing. The asbestos roof does not need paint, will not rot; it is close to being permanent.

How to visually distinguish drywall or plasterboard from cement asbestos millboard

Asbestos millboard fragment (C) Daniel Friedman Drywall layers (C) Daniel Friedman

Above we illustrate a fragment of cement asbestos millboard that is about 1/8" thick and his hard - cementious, and at above right in our second photograph we take a look at the edges of two layers of drywall or plasterboard in an older home - showing that that material, principally made of gypsum, is comparatively soft, paper-faced, and easily cut with a knife.

On that wall the original covering was a soft wood-product fiberboard - pointed to by the tip of my pen. Wood fiberboard or paper based insulating board are not asbestos products. See SHEATHING, FIBERBOARD.

Asbestos mural base at Grand Central Terminal in New York City (C) D Friedman (Rosato)

As we stated earlier, asbestos cement board is a cementious product distinguished by eye from friable and dangerous soft asbestos panels used as building fireproofing tremolite asbestos ceililng panels [image].

More about these tremolite panels can be read

Figure 3.1 (Courtesy Johns-Manville Corp.)

The great arched ceiling in Grand Central Terminal is surfaced with asbestos-cement sheets on which a mural of the heavens is painted.

More photos are in our




Research on the Asbestos Content & Hazard in Asbestos Cement or Fibre-Cement Products


Continue reading at CEMENT ASBESTOS PRODUCT MANUFACTURE - a continuation of the original Rosato text as adapted and expanded, and where we discuss processes used to make various asbestos and cement based products, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see Companies Producing Asbestos-Cement Products described at ASBESTOS PRODUCING COMPANIES a most-complete list of companies that produced products containing asbestos


Or see these

Articles on Asbestos-Cement Products

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