For much of the 20th century, asbestos cement sheets provided builders with a rugged solution for simple roofing and siding projects. While these products were cheap to manufacture and buy, their true cost emerged years later in the form of deadly asbestos-related illnesses. Fibrous cement sheets are more durable than drywall and easier to work with than concrete, and they have numerous applications in construction. Today, the fibers in cement sheets are usually made of cellulose, a substance found in plants, but for the first eight decades after fibrous cement sheets were invented, they were primarily made with asbestos. Asbestos is a naturally occurring mineral that can be fluffed into a wooly consistency and mixed into building materials such as cement. Adding asbestos makes cement more durable, weatherproof and heat resistant, and because asbestos cement sheets are fireproof, builders initially considered them a much safer material than wood.
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Manufacture of asbestos-cement products: this article describes the processes used to manufacture asbestos cement sheet, siding, roofing, pipe, and molded products.
Dry, wet, and wet mechanical processes that were used to produce cemetn asbestos products are explained and illustrated. Our page top photograph shows badly broken cement asbestos roofing shingles on a building in upstate New York. The great arched ceiling in Grand Central Terminal is surfaced with asbestos-cement sheets on which a mural of the heavens is painted. Factors influencing asbestos-cement products include manufacturing process, composition, forming pressure, method of cure, and environmental conditions.
Generally, the long asbestos fibers or higher percentages of fibers will produce higher strength properties. By using higher curing pressures or forming pressures, increased density can be achieved which in turn produces higher strength properties. The higher density products are less porous and sometimes give the effect of improving chemical resistant properties. In the production of asbestos-cement products, as well as asbestos millboard and paper, the density of the opened fiber and its ability to remain in suspension are important factors.
These properties can be related to the buoyancy test method. The buoyancy of a fiber can be determined from a slurry of fiber in water using a definiteweight of fiber in a constant or definite volume of water. After it has been thoroughly mixed, the slurry is allowed to settle; the rate of settling for a given time is read in centimeters for a given time. The air permeability method is another method of examining the degree to which the fiber has opened.
With any manufacturing process, the asbestos fiber is required to be in proper form for best use. Usually, the fiber is opened as much as possible without excessively crushing or damaging it in order to secure the most efficient results.
The asbestos should be fluffed or separated so that it can be distributed uniformly through the product and not appear as hard clumps. Many devices are used for this processing; for some grades, it is adequately accomplished at the mines during the milling and grading operations.
Hammer mills, edge mills, disintegrators, and similar equipment are used for this purpose. In some operations, the opening process is continued after the fiber, cement, and water have been mixed, by using pulp beaters, which aid also in securing uniform distribution of the fibers throughout the mixture.
Fillers and pigments may be added to reduce the raw material cost and to provide coloring. These are preferably inorganic; in order to hold true color they need to be stable against the highly alkaline nature of the Portland cement. This process resembles the method used for forming asbestos plastic molded products.
It is used for the manufacture of shapes and forms which would not be otherwise possible to manufacture. Manufacture by this process is limited to specialties; it represents only a very small part of the total output of asbestos-cement products. In this process, the raw materials are mixed dry, and spread in an even layer on an endless canvas belt. The thickness of the dry stock sheet is greater than the final thickness desired. It is sprinkled with water and compressed between pressure rollers to make it dense.
The amount of water used at this point is critical, because insufficient water can lead to breakage when the product is removed from the belt; and excessive water can result in the material sticking to the roller. The lower of these rollers is generally smooth; the upper roller may be textured or contoured to produce a pattern on the finished product.
Cutters can be fitted on the rollers and simultaneously, the sheet can be trimmed to its final size and shape. The product is then removed from the belt, either mechanically or by hand, and stacked on pallets or frames for setting and curing. Usually, the color is applied as a veneer on the exposed surface. This may be done as a dry addition before or after wetting the spread layer, or it can be sprayed on after the first pressure rolling operation. This dry process is used principally for shingles, but it may be also used for other sheet products.
It is not suitable for the manufacture of pipe or for any product where the sheet must be deformed after final pressing. It has several advantages and disadvantages, relative to the other processes. The product is essentially equal in strength in all directions isotropic. There are no such material losses through filtering as occur with the wet processes.
The machine does not require continuous operation. The product tends to have a lower unit strength than the wet process products; only relatively thin products can be made if the operation is to be practical. Unless the dry stock is spread evenly with uniform control, there are apt to be low density spots or nonuniform thickness.
The following excerpt is not from Rosato. Amy Lamb Woods in her article "Keeping a Lid on It" Asbestos-Cement Building Materials" gives this helpful historical comment on the production of sheet asbestos products:. It was not until that the invention by an Austrian engineer, Ludwid Hatschek, made possible the manufacture of pre -formed asbestos-cement products.
The Hatschek machine, a wet transfer roller, was used to produce the initial asbestos-cement sheets, while two other manufacturing processes included the Mazza process for pipes, and the Magnani semi-dry process for corrugated sheets.
Figure adapted from both Wilden and Woods at above left illustrates the 12 key parts and stages of a simplified Hatschek asbestos sheet goods production machine.
The roller at 7. After being formed, most products were steam cured to achieve the optimum microstructure for strength and durability. It is these asbestos-cement building products that hav e many desirable material characteristics, such as being lightweight, impermeable to water, durable, tough, resistant to rot, termites, soiling, corrosion, warping, and fire, and easy to clean and maintain.
Asbestos-cement also possesses low thermal conduc tivity and is therefore a good electrical insulator. These highly desirable material characteristics, apparent in the new found material, sparked growth in manufacture of a plethora of forms and styles to suit different needs. With the refinement of the asbestos and cement mixture, as well as the forming and curing procedures, the market soon developed major commercial products of synthetic roof an d wall shingles, corrugated wall and roof panels, flat millboard, and decorative wall and ceiling moldings.
Additional manufactured products included water pipes, simulated ceramic bathroom tiles, facings of acoustical materials, electrical switchboard panels, laboratory tabletops, electrical conduits, and even smaller diameter pipes were used for purlins and trusses in wartime construction to conserve steel and lumber.
The principle manufactured products used in building construction were, in order of th eir production volume: siding shingles, flat sheets, roofing shingles, and corrugated sheets. These asbestos-cement products lent themselves to rapid construction techniques and, therefore, were particularly useful for lightweight housing and industrial buildings.
In this process, the raw materials are usually blended dry, using any of several common types of mechanical mixer, and transferred in weighed batches to wet mixing tanks located higher than the press. Each weighed batch is sufficient for just one sheet, and its quantity depends upon the size, thickness, and density desired for the product to be made.
Weighing may be either manual or automatic; the number of wet mixing tanks required depends upon the mixing time relative to the pressing cycle. Sufficient water is added to the batch to form a slurry that will flow readily; after it has been properly mixed, it is passed by gravity to the mold chamber of the press. The consistency of the slurry, particularly for large sheets, frequently requires a quick hoeing in the mold, to aid in the distribution across the chamber.
Here, it is compressed to a dense, hard, and homogeneous sheet. The excess water is filtered out under the pressures supplied, and a dwell time at these pressures is required to permit escape of all of the excess water. The formed sheets are usually placed on steel plates and allowed to set and harden for a period of 24 to 48 hr.
They are then cured in accord with the standard practice, prior to final trimming and shipment. This process permits some variations in the form and shape of the manufactured products; e. Their strength tends to be equal in all directions; they are not likely to have soft or low-density areas. The operation can be shut down without loss of raw materials; however, some material losses occur through filtering.
The wet mechanical process can mass produce varied shapes by the addition of mechanical operations to the basic process. The machinery used is similar to a paper making machine. The mixing of the asbestos and cement slurry is highly important in the wet mechanical process, because the slurry must be distributed to form a thin film across the entire width of the machine.
In some processes, the opened fibers are combined with the cement and mixed with water in a conventional pulp beater Hollander and fitted with knives in order to eliminate clusters of fibers.
The relatively heavy paste mixture in the pulp beater is delivered to a stirring vat where it is thinned, by the addition of water, usually recirculated to this mixer from the filtration cycle at the wet machine.
It is important that agitation be constant and effective in order to prevent separation and settling of the solids in the slurry. Other mixing systems are employed which have replaced the cumbersome Hollander mixer. Some of the modern procedures start with dry mixing, followed by a wet mixing just before the stock is fed to the machine. By this method, excessive hydration of the cement and the asbestos is avoided, faster filtration rates can be maintained, and a higher production rate obtained.
These dry mixers have the added advantage that asbestos and cement, if mixed in the dry state, adhere one to the other. The dry cement crystals become entwined in the asbestos fibers and after a short time this mixture is so well mixed that it would be difficult to separate most of the cement from the asbestos fibers.
Another advantage is that parts made by this process have a higher specific gravity approximately 2. The slurry flows to cylinder vats which are fitted with paddle agitators to prevent settling of the stock. There are usually three to four of these vats on each machine.
Each vat contains a horizontal cylinder covered with a fine wire mesh screen, and fitted with seals at its ends so that flow can occur out of the vat and only through the wire, thus leaving a thin, even film of stock deposited on its surface while removing the major part of the water. The thickness of the film depends upon the slurry level in the vat, its consistency, and the speed of rotation of the cylinder.
The machine is usually operated so that the film deposited is approximately. The continuous woven felt travels above these cylinders, with its hairy side toward them; it is kept in firm uniform contact with them by heavy rubber covered rolls. At the contact surface, the film of stock is transferred to the felt and some water is removed. Each cylinder adds its film thickness to the web of stock on the felt. Additional water is drawn from the web by means of a vacuum box placed under the felt as it travels toward the cast iron or steel accumula-tor roll.
There may be other vacuum boxes between the vats, to seat the stock web firmly on the felt. As the web of stock passes under the accumulator roll, it is transferred to the roll in a continuous operation, until the sheet is built up to the desired thickness. Then the sheet is knifed along a groove in the roll and peeled from it to a conveyor table which draws the sheet clear of the machine.
See Figure 3. During this operation, the machine continues to run, and a new sheet begins to form immediately on the accumulator roll. The water removed during this operation is returned to the mixing area of the machine.
This water contains a small quantity of asbestos and cement from the stock that has passed through the screens and the felts. It is first retained for a period of time in settling tanks to clarify it. In addi-tion, the water very rapidly becomes saturated with soluble materials from the Portland cement.
SME Bolero Ozon. Jessica Elzea Kogel , Nikhil C. Trivedi , James M. Barker , Stanley T. Industrial Minerals and Rocks builds on the strengths of the earlier editions but adds significant new content—ensuring the continued relevance of this classic text.
Asbestos Cement Sheets
Filed July 16, , Scr. N , 17 Claims. A method of improving the filtration rate of a waterlaid web formed from a water 1 slurry of Portland cement and asbestos fibers, by addition to the slurry, just prior to the web formation, of a small percentage of harsh mineral glass fibers. This invention relates generally to cement-asbestos products, and to an improvement in the manufacture of sheet products containing hydrated cured Portland cement as a cementing and bonding material, and asbestos fiber as a structural reinforcing material. These two materials are integrated by the manufacturing process into a monolithic, dense, strong, rigid, hard, durable structure as the final product.
Asbestos Is Being Pushed In Asia As A Product For The Poor
Asbestos is a fibrous mineral that was widely used in the manufacture of building materials and other products. In Australia the use of asbestos was phased out in the manufacture of building products through the s and completely banned in The most common product used in residential properties is asbestos cement. Asbestos cement products pose little risk to health when they are in good condition and undisturbed.SEE VIDEO BY TOPIC: CNC Router Projects- Start to Finish: Working with Concrete & Cement Products
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Asbestos Cement (Transite)
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website. Manufacture of asbestos-cement products: this article describes the processes used to manufacture asbestos cement sheet, siding, roofing, pipe, and molded products. Dry, wet, and wet mechanical processes that were used to produce cemetn asbestos products are explained and illustrated. Our page top photograph shows badly broken cement asbestos roofing shingles on a building in upstate New York. The great arched ceiling in Grand Central Terminal is surfaced with asbestos-cement sheets on which a mural of the heavens is painted. Factors influencing asbestos-cement products include manufacturing process, composition, forming pressure, method of cure, and environmental conditions. Generally, the long asbestos fibers or higher percentages of fibers will produce higher strength properties.
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Free for one month and pay only if you like it. Is the imposition of excise duty on asbestos fibre introduced by the Finance Act of w. All these petitions were heard together and the main arguments was addressed in C. The counsel for the parties agreed that the decision in this writ petition will govern the decision in all other cases, namely The petitioner No. The main constituent for the production of asbestos sheets and pipes is asbestos fibre. Before the supply to asbestos was dependent completely on imports, thereafter the petitioner took on lease certain mines in the State of Bihar under the Mines and Minerals Regulation and Development Act , hereinafter to be called the Act. Section 9 of the Act provides for payment of royalties in respect of mining lease at the rate specified in Second Schedule to the Excise Act. Second Schedule specifies the mineral and entry 30 relates to asbestos and reads as follows AS, A.
Asbestos Products and Companies List
Asbestos is the generic term for a group of naturally occurring fibrous minerals with high tensile strength, flexibility, and resistance to thermal, chemical and electrical conditions. Asbestos fibers are of high-tensile strength, flexible, heat and chemical resistance, and good frictional properties. Cement is the most essential raw material in any kind of construction activity. Ceramics also known as fire clay is an inorganic, non-metallic solid article, which is produced by the art or technique of heat and subsequent cooling. Limestone is a sedimentary rock, mainly composed of calcium carbonate CaCO3.
Their industry, they said at the conference in the Indian capital, saves lives and brings roofs, walls and pipes to some of the world's poorest people. The industry's wonder product, though, is one whose very name evokes the opposite: asbestos. A largely outlawed scourge to the developed world, it is still going strong in the developing one, and killing tens of thousands of people each year. The International Labor Organization, World Health Organization, the wider medical community and more than 50 countries say the mineral should be banned.
Now operating in its th year, the company continues to challenge convention within the construction industry and leads the way in delivering innovative building products and solutions. In every country in which we operate, our goal is to remain at the forefront of the fibre cement industry, capitalising on our global leadership in products, manufacturing and technology for new home construction, renovation and commercial applications. Innovation is vital to maintaining our leading position in the fibre cement industry. We invest heavily in research and development, with a special emphasis on product quality and production efficiency to develop durable and low maintenance products that can stand the test of time.
Tara Strand Senior Content Writer. Jennifer Lucarelli Legal Advisor and Contributor.
About small-scale asbestos mining and milling facilities and 33 large - scale asbestos manufacturing plants, 17 asbestos-cement product manufacturing plants and 16 other than asbestos-cement product plants are situated in India. The present study reveals the exposure of commercial asbestos chrysotile in the occupational as well as ambient air environment of the asbestos-cement AC sheets industry using membrane filter method of Bureau of Indian Standards BIS. The fibre concentrations in 15 samples collected in the occupational environment at ingredient feeding site, sheet-producing site, fibre godown were 0.