US2008718A - Structural material and method of making the same - Google Patents

Description

E. MA JENKINS STRUCTURAL MATERIAL AND METHOD OF MAKING THE SAME Filed July 25, 1932 INVENT R I Edward Jenkins. "Y

.ATEORNEY Patented July 23, 1935 PATENT oFFIcE STRUCTURAL MATERIAL AND METHOD F MAKING THE SAME Edward M. Jenkins, Somerville, N. J., assignor to Johns-Manville Corporation, New York, N. Y., a corporation of New York Application .nay z3, 1932, serial No. 624,295

17 Claims.

to a compressed and densiiled, reenforced cemenwhich titious composition provided with a multiplicity of cells. In a preferred embodiment of the invention the cellular composition is integrally united to a dense non-cellular portion containing the same kinds of materials as the cellular composition.

'Ihere has been made, heretofore, a cellular cementitious product. Thus, granular naphthalene has been mixed with Portland cement, sand and water, the mixture cast and allowed to harden, and the naphthalene removed by melting and volatilization. Attempts to produce thus a product containing a high proportion of voids cause weakness or susceptibility of the product to breakage.

It is an object of the present invention to provide a structural material and a method of making it in preformed units that are light in weight but suiliciently strong to withstand heavy loading, even when supported in the form of large units at positions several feet apart, that are fire-proof and also heat-resistant, that are thervmal insulating, and that may be also sound-absorbing.

Other objects and advantages of the invention will appear as the description thereof progresses.

The invention is illustrated in the drawing in Fig. l shows a side view of a unit containing a compressed and densied, reenforced cementitious composition including particles of a densitylowering iiller material.

Fig. 2 shows a perspective View of an insulating or sound-absorbing material containing av multiplicity oi.' cells resulting from the removal of the filler material from the structure shown in Fig. 1.

Fig. 3 shows aside view of a unit comprising a cellular core of the kind illustrated in Fig. 2 and dense facing layers integrally united to opposite sides of the core.

Fig. 4 shows a perspective view of an embodiment of the invention in which a dense, integrally united facing layer is perforated in a manner adapted to admit incident sound to a cellular sound-absorbing core material of type shown in Fig. 2.

Fig. 5- is aside view of a ,modification of the invention, comprising a layer of dense material and units of cellular material, of the type illustrated in Fig. 2, united to each side of the dense layer. v

In the` various gures like reference characters denote like parts.

Thus, 1 denotes a cellular structure or material,

suitably in the form of a sheet, comprising a com-v pressed and densied mixture 2, of a cementitious and a reenforcing fibrous material. The cementitious material as used originally, is one adapted to be hardened at atmospheric temperature and suitably is a hydraulic cement. Materials that may Vbe used include magnesium oxychloride cement, calcium aluminate cement (sometimes called cement fondu or Lumnite cement), and Portland cement. vThe latter is the cementitious material preferredat this time.

The fibrous material should be incombustible and also heat-resistant, that is, not susceptible to decomposition, by charting or otherwise, when subjected to moderately high temperatures, such as 350 to 400 C. The bers are preferably inorganic and of the type of asbestos. I prefer asbestos ilbers, particularly because they are adapted to become interlocked with the cement, originally forced thereinto under strong compression, and to facilitate the later removal of tlller in iluid form, as will be described later. When used in the structure of the present invention, asbestos shows no harmful action and no corrosion. I have used to advantage chrysotile asbestos from Canadian mines. One grade usedv consisted of short fibers and showed 100 per cent through a 4mesh screen and 57 per cent through an 8-mesh screen, in a standard screening test.

Distributed throughout the cementitious and brous composition in the modication shown in Fig. 1 are discrete particles of a density-lowering illler material 3, that, in the preferredembodiment of the invention, are subsequently removed to give a inished product provided with a multiplicity of cells 4 corresponding to the space in' itially occupied by the particles of filler material. These cells may be distributed more or`less unlformly throughout the structure. They may be individually relatively small, that is, so small as to be effective in thermal insulation. Also, they may intercommunicate, as by means of narrow capillaries, in such manner as to make the structure sound-absorbing. They may correspond in size and shape to the granules of filler material originally used. By using ller material in various shapes, such Ias pellets, crystals, rods, or the like,v corresponding shapes of voids may be obtained.

The density-reducing material is suitably a solid that may be reduced to the form of granules or pellets and that is compression-resistant (not adapted to undergo pronounced change of form or volume under compression). The material should not seriously stain or otherwise injure the nished product. For use in making cellular sound-absorbing compositions, the illler material should be adapted to be removed without weakening the structure in which it is embedded.l Thus,

the filler should be removable by fusion and/or volatilization at a temperature below that which would cause excessive weakening of the rest oi' the structure. Removable iiller materials that may be used are granular naphthalene, paramn, and para-dichlorbenzol. 'Ihese materials melt and also are volatile in steam. If-the densitylowering ller material is to be left in the nished product, in a modification of the invention, the

,iiller should be one that is incombustible and heat-resistant and of low apparent density, such as granular pumice or diatomaceous earth.

The light-weight structure comprising either the density-lowering material or cells, resulting from the removal of the density-lowering material, may be provided with a dense facing layer 5. This facing layer may consist 'of the same kind of materials, but not necessarily in exactly the same proportions toeach other, as found in the lightweight structure. In the modicatlon illustrated in Fig. 5, there may becentrally spaced dense layer 6 of material of the same kind, integrally united on either side to two sheets of the lightweight structure.

l The .facing layer may be provided with perforations 'i adapted to admit incident sound. Thus, there may be provided a multiplicity of perforations of diameter inch, spaced on l/2 inch centers, for example.

The method o'f making the improved products of the present invention involves the use of certain apparatus that is conventional in making sheet material comprising a compressed and densied and then hardened mixture of Portland cement and asbestos 'vbers. In making such a product, Portland cement may be mixed with as- ,bestos fibers, say, in approximately equal proportions by weight, and suicient water to form a freely flowing slurry. This slurry may then be poured into the bed of a hydraulic press provided with a filtering bottom and constituting a mold. The mold may have removable sides. Then, a plate connected suitably to the plunger of the press Yis brought down on top of the slurry. The slurry is shaped into a sheet; excess water yis removed -therefrom and forced through the filtering bottom in the bed of the press; and the composition is strongly compressed, to densify and strengthen it. The resulting densied sheet is then-removed from the press. It may. be so removed by taking away theasides of the mold, raising the die plate to which the sheet normally adheres through suction eect, then breaking the suction at an edge of the sheet, as by stripping over a small area, and allowing the sheet. to be received upon a support. 'I'he sheet is then allowed to set and harden. y

This process may be modied in such manner as yto produce the products of the present invention, as illustrated in the following examples:

t Example I A ture) and suiiicient water toA give a freely flowing slurry. The slurry is then poured into the bed of a hydraulic press with a ltering bottom. The

amount of slurry pouredinto the press should be sucient to give a nished sheet of the desired thickness. Pressure is then applied to the layer of terial in the bed of the press, as, .for eple, by means of a steel die plate. As the pressure is applied, the slurry is shaped and compressed, with the elimination of Water. The pressure is increased until, at the end, it is very high, say approximately 2000 pounds per square inch. At this stage, the sheet is highly compressed and densifed; it may have approximately a third the thickness of the layer of slurry before compression. y

'I'he densified sheet is removed from the press and allowed to stand until the cement hardens,

say for thirty days. The resulting hard sheets may be trimmed to exact size desired.

Example II In making a sound-absorbing element constituting the preferred embodiment of the invention, a sheet is made as described under Example I, except that 186 parts of granular naphthalene are substituted, as the .iiller material, for the pumice and that the resulting densified sheet is subjected to treatment to remove the naphthalene after the cement in the sheet has taken its initial set.

The removal of the ller material may be accomplished by subjecting the sheet'to an elevated temperature to melt and/or volatilize the said material. Thus, the naphthalene may be removed by maintaining the sheet at a temperature somewhat above the melting point of naphthalene, until much of .the naphthalene has iiowed or dripped from the sheet, and -then raising the temperature of the sheet to a point at which the remaining naphthalene volatilizes rapidly.

For example, the naphthalene may be volatilized by heating in a stream of saturated steam at 100 pounds pressure, with attendant accelerated hardening of the cement in the sheet, or in a .stream of superheated steam, flue gas, or the Exampe III There is made'a' mixture of Portland cement and asbestos fibers, suitably in equal proportions by weight, with water to form a freely fiowing slurry. 'I'his slurry is placed as a layer in the bed of a hydraulic press with iiltering bottom,

as described above. The layer is covered by a screen wire or provided with other vacuum breaking means, for a purpose that will appear later. 'I'he layer is then compressed, as by a die plate of the type described above, to a more or less compact sheet, say one so rm that it may be indented with the nger with diiiiculty. Pressing is then discontinued. .The pressure, up to this point, may be of the order of 50 to 100 pounds per square inch. With such moderate compression, there is formed a sheet that is adapted to lter water in subsequentoperations.

The die plate is then removed, as by bing raised, during which operation the screen breaks the vacuum between the sheet and the die plate and causes the sheet to remain in position in the bed of the press. The wire screen or vacuum breaking means is then removed from the surface of the sheet. y

Over this sheet there is then applied a layer of material containing a density-lowering filler material, preferably, of the type ofthe ller def 'scribed under Example II. This layer may be in the form of a freely iiowing slurry containing parts of Portland cement, 25 ofasbestos bers, 186 of naphthalene, and water in amount to give the consistency desired. Since the cementiyacosms tiousmaterial in the original sheet and also in the layer of material applied thereto is the same, the cementitlous material in the layer is miscible with that in the sheet, and becomes Lntegral therewith. On the other hand, the sheet first formed is sufiiciently compact and firm to retain its shape when the layer of additional material is placed over the sheet.

Pressure is then applied, as before, except that the pressure may now be built up to the full limit of, say 2000 pounds per square inch.

When the filtration of water from the composite layer and sheet has practically ceased, the product is removed from the press and allowed to set, that is, stand until the cement therein has taken its initial set, say for a half day or longer. It is then submitted to a treatment such as described in Example II and including steaming, for removing the volatile filler material and hardening the cement in the composition.

' Example IV The procedure of Example III is followed, with the exception of certain modifications which permit the fabrication of a product having a dense facing layer united to each side of a porous core material.

'I'here is formed at moderate compression an initial sheet of asbestos bers, Portland cement and water, and there is pressed onto this sheet a layer of Portlandcement, asbestos fibers, naphthalene and water, all as described in Example III, with the, exception that a wire gauze, disposed next to the die plate, as vacuum-breaking means, is used in pressing the second layer as well as the initial sheet, and the final compression of the two layers is made in such manner as to maintain the composite in condition for filtering water, say by compression at to 100 pounds per square inch. The die plate is removed, and also the wire screen. 'Ihere is then applied over the top of the layer containing naphthalene a slurry like that used in making the initial sheet of the composite, namely, a wet mixture of Portland cement and asbestos fibers, say

in approximately equal proportions. The whole is then compressed at a iinal pressure of, say, 2000 pounds per square inch, to remove lexcess water and give a strongly compressed and densifled composited article.

When the compression is completed, the composited article is allowed to set to develop initial strength, after which it is subjected to treatment to remove the naphthalene, as, for example, as described under Example 1I. Thus, the composited article may be maintained at a temperature above the melting point of naphthalene. This causes a large proportion, frequently about two-thirds of the total naphthalene used, `to flow in liquid form from the article. When the recovery of naphthalene in liquid form becomes very slow, the temperature is raised to volatilize the remaining naphthalene. For this purpose there may be used to advantage a stream of saturated steam, as at 100 pounds pressure or higher. When the naphthalene has been removed, there remains a structural material or unit comprising a cellular portion or core and dense and strong facing material integrally united to opposite faces thereof. Since all parts of the unit contain the same kinds of material, the various parts are adapted to adhere to each other over agwide range of conditions, 1n spite or the rigidity or the core and facing material.

like the first, containing naphthalene.

Example V A composited article, adapted to absorb sound incident upon either of its two faces but not to transmit it, of the type illustrated in Fig. 5, is made by a method somewhat similar to that described under Example IV, except that the outer layers are made with a removable ller material, that is later removed, while the inwardly disposed or core layer is made of material that remains dense and impermeable to air-borne sound in the finished product.

A slurry comprising Portland cement, asbestos fibers, naphthalene and water is moderately compressed in the bed of a press and formed into Va sheet. Over this sheet is then moderately compressed and united a layer of a composition of Portland cement, asbestos fibers, and water. Then there isformed, over this layer, a sheet The whole is then strongly compressed and densifled, and then subjected to treatment as previously described, to cause hardening and also the removal of the naphthalene.

Such a product may be provided with integrally united outer facing layers, say of Portland cement and asbestos fibers. These faces may be imperforate, if sound absorption is not desired. or perforated, if the sound-absorbing properties of the product are to be utilized.

Example VI In this modification of the invention, a dense facing layer, united to the material containing inter-communicating cells, is made permeable to sound. Thus, the dense facing layer may be provided with small, closely spaced perforations l (Fig. 4). Such holes may be formed by drilling or cutting through the facing layer after the composite has been 'set or hardened, or in other suitable manner.

It will be understood that various types of openings adapted to admit sound may be used, as, for example, openings arranged in the form of ornamental patterns or designs.

The products of the present invention have certain features in addition to those that have been mentioned.

The dense facing layers, in a typical product, weigh about 120 pounds per cubic foot and have a modulus of rupture of about 4000 pounds per square inch. They may be sanded to a hard, smooth surface that is susceptible to decoration, as, for example, by paint or lacquer.

The cellular material made, as described, by the use of and subsequent removal of filler of the type of granular naphthalene, is strong and rigid in spite of the presence of cells corresponding to more than half, say to 90 per cent, of the total volume of such material, and of a density that may be as low at 18 to 30 pounds per cubic foot. The compressive strength may be approximately to 500 pounds per square inch. The exact gures as to strength, percentage of cell volume, and density depend in part upon the proportion of removable filler material used initially. The cellular and/or the dense facing material may be colored by the incorporation of pigment into the original compositions before being shaped into sheets.

Itis not necessary to use an adhesive or to leave substantial amounts of organic material in the finished article. Therefore, the finished article may be not only incombustible but also heatresistantup to relatively high temperatures.

.tion and upon the proportions of materials.

Thus, thesize of the particles of the removable ller incorporated initially into the mixture and later removed has a bearing. With a given proportion of naphthalene used, for example, smallness of size of particles favors increased thermal insulating power, but decreased strength of the product. On the other hand, less eective insulation and greater strength are obtained when the naphthalene particles are larger. With these facts in mind, one may choose a graduation of size of naphthalene particles that suits the par-y ticular requirement. In order to avoid making a porous -material of pronounced capillarity for water and weakness, the particles of naphthaiene should be large enough to leave individual cells of substantial size when.the particles are removed from the composition. To make such cells, I have used to advantage particles of naphthalene of which more than half by weight was ner than 10-mesh. The results of a standard screening test of a granular naphthalene that have used, follows: I

Mesh oi Screen Retained em'ngs per l Percent b1! near inch weight On 14 2l On 20 17 On 4.0 37 0n 60 12 On 8O 5 On 100 3 Through 100 5 naphthalene to each 100 parts dry weight of mix- Sound absorp- Percent of Thickness of tion. percent, naphthalene cellular mateat frequency used rial in inches of 500-1000 cycles The strengt?. of theproduct with the dense facing layers is very high and is outstanding in proportion to the overall density of the material;

Also, the thermal insulating properties are very satisfactory, as illustrated quantitatively by the data for a preformed structural unit madeas described in Example IV. The unit included a core of cellular material and two dense facing layers, each approximately 1% inch thick, containing 50 parts of Portland cement to 50 of as= bestes fibers. The core was 0.93 inch thick and contained'Z parts of Portland cement to 25 of asbestos bers, and was made with the use initially o- 65 parts of naphthalene to 100 of total dry mixture in the core layer. The unit weighed approximately 51 pounds per cubic foot, had an overall thermal conductance at atmospheric temperature of approximately 0.5 to 0.6 British g thermal units per square foot of area, per hour,

calculated for a temperature gradient of, 1 F. Also, this product, when supported in units 42 inches wide, on beams '7 feet apart, broke, under concentrated center loading at 1,266 pounds. It will be seen that the material, even when made in large units, can support heavy loads` over long spans. y

When such a unit is subjected to heavy loading, there is slight flexing before the unit breaks.-

This slight exing is accomplished by compression of the facing layer on which the load bears and tension on the opposite facing material. Since compositions consisting of Portland cement and asbestos fibers are stronger under compression than under tension, the facing layer under tension in the unit will usually break be'- fore the other facing layer breaks. To offset this greater susceptibility to breaking under tension, the preformed unit may be made with a layer of facing material that may be two to five times as thick on the side that is to be subjected to tension, when the unit is loaded, as on the other side. However, it is desirable from the standpoint of minimizing warpage of the unit under varying conditions, such as varying humidity of air in contact therewith, to have the oppositely disposed facing layers of the same thickness, as

well as of identical compositions.

The proportions of brous to cementitious material may be varied within limits. v proportion of ber in VVthe core or light-weight material may be 20 to '75 per cent, say 25 to 40 per cent, of the weight of the mixture of fiber and cement. For the facing layers, which are dense and very strong, there has been used a somewhat higher proportion of fibers, say 40 to 75 percent, advantageously about 50 per cent, of the weight of the mixture of ber and cement.

The units may be made of various dimensions or sizes. Thusthe light-weight or cellular material may be made of thickness greater than 1/2 in ch, say up to about 5 inches. The faces may b'e made of thickness greater than 13g inch. The units may be made in very large shapes, say 31/2 x 1l feet on the face. Such units, particularly when faced on both sides, are admirably adapted for use in oors, walls, partitions or roofs of buildings.

While the invention is not limited to any theory or explanation of the results obtained, the pressure ltration of Water through the composi vtion, during the fabrication, as illustrated in Examples I-VI, has a very important bearing. This filtration makes possible the use initially of very Wet and easily shaped slurries and facilitates the integral union of the adjacent layers. Also, the filtration may carry nepartioles of cement from one layer into pores in another, with the substantial elimination of boundary pla-nes between the cement in onerlayer and that in the, next and blending in the zone of contact. Further, the rapid removal of water from the composition by filtration may open small passages from the interior to an exterior surface and thus provide avenues of escape for the ller during itssubsequent removal.

Thus, 'then-` tion of the filtration process as described in the examples may be used. Thus, the products may be fabricated by a process using a wet or millboard type of machine. In this process, there is provided a paper making stock comprising Portland cement and asbestos fibers, for example, suspended in a large volume of water. The cement and fibers are then collected in the form of a felt and the felt transferred continuously to the collecting drum of a millboard or wet ma.-

chine,'in conventional manner. When the felt on the drum 'reaches the desired thickness, it is composited with a similarly formed felt made from a stock containing a removable filler, suchA -to remove the naphthalene.

The various layers may be composited either on the drum of the millboard machine, as by changing the character of the stock furnished to the machine, or after the individual layers are removed from the drum and straightened into slabs.

Also, layers separately formed by filtration in the hydraulic press equipment, as described in Examples I-VI, may be composited. Thus, separately formed and pressed sheets, one or more with and one or more without filler, may be placed together in mating relationship, before the sheets are hardened, and then pressed together strongly, suitably at a pressure substantially higher than used initially in forming the individual sheets. The adhesive or cementing material, used between the separately formed and compressed sheets in making the composite, may be a material suitable for the purpose, as, for example, a Portland cement mortar. I

During the filtration. step of the process of manufacture, the asbestos fibers facilitate the i'll; tration. Compacted, finely ground Portland cement alone does not filter water readily. Mixed with asbestos fibers, as in the facing layers, thecement composition filters water .rapidly and makes possible the removal of excess water at a rate that may openup passages associated with the fibers through which a volatile filler may later be removed.

A similar effect of the asbestos in the compodtions and method described is the provision of wick-action or otherwise facilitating ready removal of the filler of the type of naphthalene by melting or vaporizing, as described above. In spite of the strong compression and densification of the facing layers of the unit comprising, for example, dense layers on both sides of the naphthalene-containing core layer, raising the `com- .posite to a temperature above the melting point of naphthalene causes a rapid flow of naphthalene therethrough. This result is particularly surprising in view of the campactness and. relative freedom from porosity of the surface layer and is to be explained as due in part to the action of the asbestos fibers present. A

In the finished product, the asbestos bers have a strong reenforcing and toughening action which, with the eifect of compression and densification and knitting together of the structure by the pressure filtration, oifsetsin large measure the weakening effect of voids corresponding to particles of.

initially incorporated and then removed filler.

It will be understood that the details that have been given are for the purpose of illustration and not restriction, and that many variations therefrom may be made without departing from the spirit and scope of the invention.

What I claim is:

l. An article of manufacture comprising a strongly compressed and densifled mixture of a cementitious material adapted to be hardened at atmospheric temperature, reenforcing fibers of the type of asbestos, and a density-lowering material that is readily volatile in superheated steam at a temperature below that which causes weakening of the bond in the cementitious material.

2. An article of manufacture provided with cells intercommunicating with each other and with an outer surface of the article by means of fluidescape channels and comprising av compressed, densied, and then hardened mixture of a hydraulic cementitious material and reenforcing fibers, of the type of asbestos, embedded and interlocked therein.

3. A rigid, strong, incombustible structural unit consisting largely of hydraulic cementitious material and reenforcing fibers distributed therethroughout, and adapted to absorb sound incident upon opposite faces and to prevent the transmission of such sound, said unit comprising outer layers of sound-absorbing material integrally united to a centrally disposed sheet of material that is impermeable toair-borne sound.

4. A structural unit consisting largely of a bydraulic cementitious material and reenforcing fibers, containing the same materials throughout and comprising a cellularportion and a dense portion, the said portions being in the condition of having been integrally united one to the other by filtration, compressed and densined, and then hardened.

5. A structural material provided with a multiplicity of cells and comprising a. hardened mixture of asbestos fibers and Portland cement in the proportion of approximately 20 to '75 parts of asbestos to 100 parts of total weight of mixed asbestos and cement, the said cells corresponding' approximately to the volume originally occupied by a removable filler of the type of naphthalene initially incorporated, in the proportion of not less than parts of ller to 100 parts by weight of total dry material, and subsequently removed.

6. A preformed, sound-absorbing structure consisting throughout largely of a compressed anddensified and hardened vmixture of` a hydraulic cementitious material and reenforcing material and comprising a cellular layer provided with intercommunicating cells adapted to absorb sound,V and a dense, perforated facing .layer adapted to admit incident sound, the said cellular andfacing layers being integrally united to each other to give a strong, rigid, washable unit adapted to absorb more than 40 per cent of incident sound of frequency ranging from 500 to 1000 cycles per second.

'1. A strong, light-weight, preformed strucwral unit comprising a cellular core and dense facing layers integrally united to opposite faces of the core, the said core and. facing layers containing each a compressed and hardened mixture of a hydraulic cementitious material and reenforcing fiber and the core being provided with intercommunicating voids corresponding to the space resulting from the initial incorporation and subsequent removal of granular nller of the type of -maphthalene in proportion equal to approxicore, the said core and facing layers containing a hardened mixture of Portland cement and asbestos fibers.

9. Inthe manufacture of a cellular composition, the method which comprises making a mixture of a hydraulic cementitious material, reenforcing fibers of the type of asbestos, a volatile densitylowering material, and water, forming the mixture into a sheet, strongly compressing the sheet to densify and strengthen it, setting and hardening the compressedsheet, and removing the density-lowering agent therefrom by atreatment including volatilization.

10. In the manufacture of a cellular composition, the method which comprises making a mixture of Portland cement, asbestos fibers, granular naphthalene containing more than half by Weight of particles finer than l-mesh, and water, forming the mixture into a sheet, vstrongly compressing the sheet to densify and strengthen it, setting and hardening the compressed sheet, vand removing the naphthalene therefrom.

1l. In the manufacture of a cellular composition, the method which comprises making a mixture of Portland cement, asbestos fibers, granular naphthalene, in proportion equal to at least 55% of the dry weight of the mixture, and water, forming the mixture into a sheet, strongly compressing the sheet to densify and strengthen it, allowing the cement in the compressed sheet to take its initial set, and then -simultaneously hardening the product and removing naphthalene therefrom by subjecting the product to a temperature above the melting point of naphthalene and below the temperature of substantial weakem, ingo! the cement. s

l2. In the manufacture of a cellular composi tion, the method which comprises making a mixture of Portland cement, asbestos bers, granular naphthalene, and water, forming the mixture into a sheet, strongly compressing the sheet to densify.

' product to saturated steam at a temperature substantially above 100 C., whereby melting and volatilization of the naphthalene are produced.

13. In the manufacture of a structural unit comprising a portion of low density and a dense portion, the method which includes shaping and moderately compressing a'wet mixture of a hydraulic cement and reenforcing fibers, to forniA a sheet adapted to filter water, then applying.

over the sheet a layer of a wet mixture of a hydraulic cement, reenforcing fibers, and a densitylowering material, shaping the' layerand compositing it and integrally uniting it to the said sheet by moderate compression, `strongly compressing the resulting composite to remove water therefrom, to density and strengthen it, and to cause blending inthe zone of contact between the said sheet and layer, and then hardening the product. f

'14. In they ymanufacture of a structural unit `used in moderately compressing hardening the united comprising a portion of low density and a dense portion, the method which includes shaping and moderately compressing a wet mixture of a hydraulic cement and reenforcing fibers of the type of asbestos, to form a sheet adapted to lter water, then applying over the sheet a layer of a Wet mixture of a hydraulic cement and reenforcing fibers of the type of asbestos containing an admixed removable density-lowering material, shaping the layer and compositing it and integrally uniting it to the said sheet by mod erate compression, strongly compressing the resulting composite to remove water therefrom and to densify and strengthen it, and then hardening' the product and removing the density-lowering material therefrom, to form intercommunicating voids. A

15. In the manufacture of a structural unit comprising a portion of low density and a dense portion, the method which includes shaping and moderately compressing a wet mixture of a hydraulic cement and reenforcing fibers of the type of asbestos, torform a sheet adapted to filter water, then applying over the sheet a layer of a wet mixture of a hydraulic cement and reenforcing fibers of the type of asbestos 'containing an admixed density-lowering material, shaping the layer and integrally uniting it to the said sheet by moderate compression, forming a second sheet like the first mentioned sheet over the layer containing the density-lowering material, then strongly compressing the composited article-to remove water therefrom to densify and strengthen it, and hardening the strongly compressed product to give a unit containing a core of low density and dense facing layers integrally united to opposite faces'of the core.

16. In the manufacture of a structural unit comprising a portion of low density and a dense portion, the method which includes shaping and moderately compressing a Wet mixture of a hy- 'draulic cement and reenforcing fibers of the type of asbestos, to form a sheet adapted to lter Water, then applying over the sheet a layer of a wet mixture of a hydraulic cement, reenforcing bers of the type of asbestos, and a removable densitylowering material, shaping the layer andintegrally uniting it to the said sheet by moderate moderately compressing a wet mixture of a hydraulic cement and reenforcing bers of thetype of asbestos, to form a sheet adapted to filter water, forming a similar sheet lcontaining particles of filler material in addition to the ingredients stated, placing the two, sheets together face to face, then uniting them and simultaneous'ly filtering water from 'the united sheets by subjection to pressure in excess of that originally' them, and then sheets. EDWARD M. JENmIjlS.

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