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BUILDING STONE - VARIETIES AND QUALITIES OF STONE
CLASSIFICATION OF BUILDING STONES 1. Introduction.- index In order to be able to decide what kind of stone to use under given conditions, a knowledge of the different kinds employed in the various types of construction is essential. It is not necessary for an mason to determine the exact composition of a stone to be used in a structure, but his knowledge should be sufficient to help him or her in selecting or specifying the stone best adapted to the type of structure. The properties of a stone that determine its fitness for construction purposes are durability, strength, hardness, density, and appearance. The quality of a stone is ascertained approximately from a study of its origin and chemical composition and from the results of tests and experiments. 2. Definitions.- index The term rock is commonly defined as a hard mass of mineral matter having, as a rule, no definite external form. In engineering construction the word stone is applied indiscriminately to all classes of hard rocks. 3. Description of Classes.- index According to their geological origin, rocks may be classified as igneous, sedimentary, and metamorphic. The igneous rocks, of which granite and trap rocks are examples, owe their formation to the solidification of molten materials. Sedimentary rocks are formed by the consolidation of particles deposited in any of the three following ways: (a) by the mechanical destruction and subsequent deposition of other rocks, usually by water, as in the case of sandstone or lime stone; (b) by the action of animals and plants, as in the case of coral; (c) by the chemical precipitation of mineral matter from water, as in the case of gypsum. The metamorphic rocks are formed by the transformation, through the influence of heat or chemical action, of either igneous or sedimentary rocks. To this class belong marble, gneiss, and slate. 4. Rocks - index May be classified as stratified and unstratified, depending on their structure. Igneous and metamorphic rocks are unstratified, that is, they are not arranged in any definite form in layers, or strata, but have the constituent parts mingled together. The sedimentary rocks are stratified, or formed in a series of parallel layers, as they are deposited from water. The layers were originally horizontal, but in most cases they are found more or less inclined and curved on account of the action of disturbing forces. Sedimentary rocks are composed of grains bound together by a cementing medium, and their strength and durability depend on the nature of the cementing material. 5. Stones -index May be further classified as silicious, calcareous, and argillaceous, according to the chemical composition of the earths forming their main ingredients. In silicious stones, silica is the principal earthy constituent; in calcareous stones, carbonate of lime is the predominating material; and in argillaceous stones, alumina is the chief component. IGNEOUS ROCKS 6. Granite.- index Granite is an igneous rock, ordinarily composed of feldspar, mica, and silica or quartz. It is formed by the cooling and crystallization of matter below the earth's surface under conditions of heat and pressure which do not obtain in the case of lava ejected on the surface in a molten state. It is found in the eastern part of the United States, in Canada, in many sections of the Rocky Mountains, and, as a rule. Wherever the later rock formations have been worn away by the weather, and the igneous rock has been exposed. 7. Planes of Fracture.- index The structure of granite is quite uniform, but there are often planes of cleavage caused by stresses produced while the molten material was cooling. The plane along which the rock can be split most easily is known as rift; it is often nearly horizontal. Rock can also be split along a plane, known as the grain, which is perpendicular to the rift, but this cleavage is not so easy as that along the rift. Sometimes, the stresses are sufficient to cause fractures, called joints, running parallel to the surface. 8. Qualities of Granite.- index Granite is one of the most valuable stones for construction purposes. Although the quality of granite varies according to the proportions of the constituents and to their method of aggregation, this kind of stone is generally durable, strong, and hard. The hardest and most durable granites contain a greater proportion of quartz and a smaller proportion of feldspar and mica. Feldspar makes granite more susceptible to decomposition by the solution potash contained in it, potash feldspar being less durable than lime or soda feldspar. Mica, being easily decomposed, is an element of weakness in granite. An excess of lime or soda in the mica or feldspar hastens disintegration, as does also an excess of iron. Therefore, stones showing large and dark iron stains should be rejected for outside work. Fine-grained granite weathers better than does granite of coarser grain. Granite has a pearly luster. The color of common granite varies from white through yellow to deep red, and the stone is generally classified as gray and red. Feldspar renders the stone lighter in color. Because of its uniform structure, granite can be quarried in large blocks. The rift, the grain, and the joint planes are advantageous in quarrying, as it is very difficult to cut granite in other places. The uses for which granite is suitable depend on the texture of the stone. Medium-grained stone is best fitted for building construction. Fine-grained stone can be carved and polished, but, on account of its extreme hardness, it cannot be worked readily. Such stone is, therefore, costly when it has to be cut, Coarse-grained granite should be used only for concrete aggregate. 9. Trap Rocks.- index The term trap is generally applied to a large variety of dark-colored, igneous, unstratified rock~ that occur in large tabular masses rising one above another in successive steps like stairs. These rocks consist chiefly of hornblende, lime, feldspar, and augite, with some magnetic and titanic iron. The predominance of one or the other of these minerals gives rise to many distinctive names, as greenstone, olivine, etc. The color varies, being dark gray, dark green, or nearly black, according to the proportions of the different constituents. The texture is usually so fine and close-grained that the character of the structure cannot be determined by the naked eye. Trap rocks are exceedingly dense, hard, and durable. However, they are not much used for structural purposes because of their somber and unattractive appearance, the great cost of working, and the difficulty of securing large blocks on account of the numerous joint planes. As they split and break easily, trap rocks are extensively used for paving blocks, for the aggregate in making concrete, and for the construction of macadamized roads, for which purpose their fine texture especially fits them. They are also used for railroad ballast. 10. Syenite.- index The stone known as syenite consists of feldspar and hornblende, frequently associated with mica and quartz. It is of a granular texture and closely resembles ordinary granite, but is somewhat darker; it is hard and tough, is fairly coarse-grained, an takes a good polish. Owing to its limited occurrence, it is but little used in building construction. SEDIMENTARY ROCKS 11. Sandstone.- index Sandstone consists of fragments of other rocks cemented together. It is a stratified rock and belongs to the later geological periods. Most of the grains are quartz, but often feldspar is also present in sandstone. The cementing material may be silica, oxide of iron, clay, or carbonate of lime. If the cementing material is silica, the rock is very durable, but difficult to work. Iron oxide is a good cementing material and gives the stone a reddish or brownish color. Clay is a satisfactory binder, but it readily absorbs water, which may cause destruction of the stone by freezing. Lime renders the stone particularly liable to disintegration when exposed to an atmosphere containing gases, or when used for foundations in a soil that contains acid. 12. Sandstones - index Are variable in character, some being nearly as valuable as granite and others being practically useless for permanent construction. The best stone is characterized by small grains with a small proportion of cementing material. When broken, it has a bright, clear, sharp fracture. It is usually found in thick beds and shows slight evidences of stratification. When quarried, sandstones are usually saturated with quarry water and are very soft; but on exposure to the air, they dry out and become hard. Water can readily penetrate between the layers of this stone; therefore, in foundations it should be laid on its natural bed, that is, in the same position that it occupied in the quarry, so that the penetration of moisture and possible disintegration by freezing may be prevented as much as possible. The colors of sandstone are white, cream, yellow, dark brown, blue, and red. A fine-grained blue sandstone is known as bluestone. This variety is widely used for trimmings and for stone sidewalks, as it readily splits into slabs. 13. Limestone.- Index All limestones are of sedimentary origin and have for their principal ingredient carbonate of lime. The presence of other minerals gives rise to the division of the limestones into five classes, each of which is designated by the name of the predominating mineral. When clay is present, the stone is called argillaceous limestone; when silica predominates, silicious limestone; when iron is prevalent, ferruginous limestone; when magnesia is present to the extent of 15 per cent, magnesium limestone; and when the carbonate of lime and the carbonate of magnesia are combined in equal proportions, dolomite limestone. Limestones are either granular or compact. 14. Granular limestone - index Consists of grains of carbonate of lime, cemented together by some compound of lime, silica, and alumina. The grains are generally sea shells or fragments of shells and are often mixed with sand. This kind of stone is always porous. It is found in various colors, especially white and yellowish brown. In many cases, it is so soft when first quarried that it can be cut with a knife; it hardens, however, on exposure to the air. The variety of granular limestone called oolitic limestone is composed of egg-shaped grains cemented together. It is one of the most important of the limestone group and is extensively quarried and widely used for building purposes. Each grain is usually of concentric structure, the carbonate of lime enclosing a particle of sand or of some substance of either animal or vegetable origin. 15. Compact limestone - index Consists of carbonate of lime, either pure or mixed with sand or clay. This kind of limestone is generally devoid of crystalline structure, and has a dull, earthy appearance and a dark-blue, gray, black, or mottled color. In some cases, however, it is crystalline and full of organic remains; it is then known as crystalline limestone. The compact limestones are easily worked with the saw and hammer. They resemble light granite in appearance, and are extensively used for building purposes. The variety called shelly limestone, which consists of fossil shells that are cemented together, is sufficiently hard to take a polish; it is much used for interior ornamentation. The condition of the minerals combined with the lime also furnishes a basis for distinguishing names. Thus, the stone is called hornstone when very fine grained silica is present; cherty limestone, when the silica is in the form of rounded masses or nodules; ironstone, when the amount of iron and clay is greater than the amount of lime; rottenstone, when the ironstone is decomposed; and hydraulic limestone, when the rock contains silica and clay in nearly equal proportions. 16. Shale.- index Shale is a typical clay rock that splits readily in lines parallel to the bedding. Sand and lime carbonate are always present in this stone and, with increase of either, the rock grades into shaly sandstone or shaly limestone. Shale is used for light traffic roads and in the manufacture of brick, tile, and other burned clay products, but it is not suitable for concrete aggregate. 17. Conglomerate.- index Stratified rock composed of rounded pebbles of any material, such as limestone, quartz, shale, granite grains, feldspar, etc., cemented together is known as conglomerate. When the pebbles are quartz with silicious binding the rock is strong and hard to quarry or dress. When the interstices between the pebbles are not filled by the binder, the rock is very porous, and may hold great amounts of ground water. This stone is seldom used in building construction. METAMORPHIC ROCKS 18. Marble.- index Metamorphosed limestone gives the masonry material known as marble. It is easily dressed to a smooth surface and polished, and is considered one of the most valuable building materials. It resists frost and moisture well, but like all limestones it does not withstand fire. Marble can be obtained in many colors, some of which are white, gray, red, blue, green, and black. One of the most important characteristics of marble is that it is easy to carve; the finer the grains of the stone, the more suitable it is for this purpose. The fine white-grained varieties that are especially prized for sculpture are called saccharoid marbles. Some of the finest varieties of white American marble are found at Lee, Massachusetts, and in the vicinity of Rutland, Vermont. The dark-blue marble from the Vermont quarries is very durable and has a close grain. A fine black marble is quarried at Glens Falls, New York. Colored marbles, including gray, light and dark pink, buff, chocolate, etc., are found in Tennessee, Georgia, and other states. 19. Slate.- index Slate is a laminated rock of great hardness and density. It splits readily along planes called planes of slaty cleavage. This facility of cleavage is one of the most valuable characteristics of slate, as masses can be split into slabs and plates of small thickness and great area. The most common colors of slat are dark blue, bluish black, purplish gray, bluish gray, and green; occasionally, red and cream-colored slates are also found. Some slates are marked with bands or patches whose color is different from that of the rest of the stone. These marks do not affect the durability of the slate, but they spoil its appearance. Although slate is not strictly a building stone, it is used extensively for covering steps and the roofs of buildings, for wall linings, and for sanitary purposes. Slate is sometimes used to make light traffic macadam, but, although it packs well, it ultimately yields much mud and dust, which are objectionable. 20. Schist.- index Schist has a more crystalline structure than slate, and the crystals are easily seen. It is composed chiefly of minerals that cleave readily, such as hornblende, mica, etc., mixed with a variable amount of granular quartz and feldspar. The presence of the cleavage minerals produces a fine cleavage or foliation, called schistosity. Schist is sometimes used in building construction but it disintegrates very rapidly and is not durable. It should always be set with the planes of schistosity horizontal. 21. Gneiss.- index Gneiss is a coarse-grained, laminated rock, formed by metamorphism of either sedimentary or igneous rock. It is often used as structural material and as concrete aggregate. DISINTEGRATION OF STONE DISINTEGRATING AGENTS 22. Classification of Agents.- index The disintegration or decay of stone is commonly referred to as weathering, and is caused by agents of three kinds; namely, physical or mechanical, chemici1 and organic. The mechanical agents are heat and cold, air in the form of wind, and water in the form of rain and ice. The chemical agents are the various acids present in the atmosphere. The organic agents are vegetable growths that thrive in damp and shady places, and marine insects or boring mollusks, which perforate the stone between the high and low water marks. 23. Heat and Cold.- index An increase in temperature causes expansion in a stone, and a decrease in temperature causes contraction; hence, as a result of ordinary changes in temperature, there is a continual slight movement among the particles of the stone, which may destroy their cohesion, and thus produce a slow and gradual disintegration. 24. Fire.- index All building stones are injured by high temperatures. Sandstones, if somewhat porous, uncrystallized, and free from feldspar, are the most refractory of the common building stones. Gneiss is quite fire resistive when it contains a large proportion of quartz in which the particles are of the nature of sand. Limestones and granitic rocks usually crack when subjected to a high temperature. Stone is subjected to a very severe test when it is heated during a fire and then cooled suddenly by a stream of water from a hose The exterior layer of the stone is cooled much more rapidly than the interior, and in some cases the uneven rate of contraction causes large pieces to break off. 25. Air and Water.- index Air acts mechanically in the form of wind, especially when it carries dust; it erodes the surface and removes small particles, much in the Same way as a sandblast apparatus, thus, exposing new surfaces to be acted on. Rain alone has a slight mechanical effect when simply falling on the stone and washing loose particles away. Rain and wind together, however, act very energetically. Water penetrates into all rocks, no matter how dense or compact they may be, and, when it freezes, it expands and tends to split them. A volume of water occupying 100 cubic inches before freezing would occupy 109 cubic inches after freezing. When this expansion is resisted, the pressure exerted is equal to 150 tons per square foot, which is sufficient to split the strongest rocks. 26. Air and water - index Also act together to produce the following changes in the composition of stones: (1) rusting or oxidation of the iron particles present in the stone; (2) reduction or de oxidation of the oxygen in iron oxide, which is caused by the presence of an organic acid or of continual moisture; (3) absorption of water by an oxide; (4) solution of the constituents that are soluble in water. Absorption occurs only when there is continual moisture, as in bridge piers and abutments. 27. Acids.- index Pure water has but little effect in dissolving the ingredients of stone, but the air contains many acids which, in combination with rain, form powerful solvents of mineral matter. The stones that are most susceptible to this dissolving action are limestone, sandstone, and granite containing feldspar. Carbonic acid, which is contained in the atmosphere to the amount of about 400 parts of acid to 1,000,000 parts of air, has, when combined with water, a corroding action on the carbonates, whether they form the principal constituents of the stone or are only present as cementing materials. This acid transforms the insoluble earthy carbonates of lime and magnesia into bicarbonates, which are soluble in water and can, therefore, be washed away. On granite, carbonic acid acts by eliminating the alkaline constituents in the form of carbonates; a friable or crumbly residue of hydrated silicate of alumina is left, which contains the unaltered particles of quartz and mica. In the case of greenstones the acid acts on the iron present, and also dissolves out the lime, leaving a loose, friable, and bulky stone of a red or brown color. Sandstones containing iron are disintegrated by the solution and washing away of the iron. Nitric acid is frequently present as a constituent of the atmosphere; its destructive action is exerted on the limestones. Sulfuric acid, which results from the combustion of coal, is present in the atmosphere of cities to an extent as great as 250 parts in 1,000,000. It has a marked destructive influence on all stones, and especially on granite. The feldspar is attacked, and the potash, soda, or lime is dissolved out, and in time the stone becomes filled with small holes. 28. Living Agents.- index The disintegration and decay of stone by the inanimate agents are frequently hastened by many forms of life, such as bacteria, mosses, worms, etc., all of which are in a sense destructive agents. Their presence gives rise to small amounts of organic acids which exercise a corrosive influence. CONDITIONS AFFECTING DISINTEGRATION 29. Quarrying.- index Disintegration of stone is hastened or retarded by the methods employed in quarrying, seasoning, finishing, and setting the stone. The excessive use of explosives in quarrying shatters the cohesion of the particles composing the stone and causes cracks and flaws that make the stone more permeable to moisture. Small charges of powder, uniformly distributed over the area to be blasted, have a lesser weakening effect on the stone. Stone cut out by quarrying machinery is preferable to that blasted or wedged out, because the stone is not jarred and cracked by this method and because denser faces are produced which render the stone less permeable to moisture. The position of the stone in the quarry also affects its durability. Stone taken from the exposed faces and the top ledges of the quarry is likely to be less durable than unexposed stone. 30. Seasoning.- index Before a stone is placed in a structure, the interstitial moisture, called quarry water or sap, must be removed by evaporation. This process is termed seasoning, and should be effected by exposing the stone to the drying action of the atmosphere for some months; the stone should be stored under cover for protection against rain. If the stone is not seasoned, the quarry water will be alternately frozen and thawed during a series of years, and the stone will be broken up. 31. Finishing.- index The life of a stone is dependent on the style of finish given to its exposed faces. A smooth or polished surface aids in prolonging the life by facilitating the rapid discharge of rainwater. The methods employed in dressing the stone also affect its life. Minute fissures that render the stone more susceptible to atmospheric influences are produced by impact; hence, stones sawed to the required dimensions are more durable than those hammered and broken to size. 32. Setting.- index The position in which the stone is set in the structure affects its ability to resist disintegration. When stratified stones are placed on edge, and the mortar joints are not properly filled, water enters between the layers and in freezing causes the stone to scale off; therefore, laminated stones should be set with their layers horizontal. The portions of a structure most liable to early decay are those under cornices, belt courses, window sills, etc., on which the rainwater slowly falls or drips. As a protection from this source of decay, the under surface of a projecting stone should have a narrow groove, called a drip, extending its whole length. The water that collects on the upper surface of the projection flows over the upper edge and down the face to the under side, where its further progress is interrupted by the drip; it then falls to the ground. SELECTION OF BUILDING STONES METHODS OF SELECTION 33. Importance of Preliminary Investigation.- index When an important masonry structure is to be built, it is essential to select a stone that is strong and durable. Probably nothing in engineering construction is so neglected as the preliminary inspection of building stone. If it is necessary to employ great quantities of building stone at points where, the stability of the structure depends on the strength of the stone, an inspection of the quarry from which the stone is to be obtained should be made. The engineer should also inspect some building or structure which contains the same material and has been standing for a long time. It is well, however, not to depend wholly on inspection either at the quarry or at a building, but to subject the stone also to laboratory investigation. 34. Inspection of Stone at Quarry.- index Careful inspection at the quarry will frequently reveal much information regarding the durability and uniformity of the stone. Exposed quarry faces will sometimes indicate the weathering properties of the stone, as well as its liability to disintegration caused by moisture and running water containing injurious acids and alkalies. The various grades of stone to be had, and the amount of each grade, can be determined. In first-class work it is imperative that only the best grade of the quarry should be employed, and it is important to find out whether a sufficient quantity of stone of satisfactory texture and color is available to supply the amount of material required for the work. 35. Inspection of Stone in Buildings.- index By inspecting stone that has been in place in a building or structure for a considerable length of time, an excellent idea may be had of its weathering properties. If, after years of exposure in the atmosphere of an industrial city situated in the temperate zone, the building stone shows no disintegration and has retained its original luster and color, except for the soil of dust and smoke stains, it certainly can be considered of the best structural value for building purposes. If a stone from a certain quarry shows poor weathering qualities in a structure, an investigation should be made to determine whether the best grade from the quarry has been used, before the product of the quarry is condemned. 36. Laboratory Investigation.- index Although the quarry and building inspections are of the utmost practical importance, they should, as previously stated, be augmented by laboratory investigation. When the stone to be used is from a new quarry, the characteristics of the product are little known, and this investigation is then necessary. The laboratory investigation of stone usually consists of chemical analysis, microscopic examination, and mechanical tests. The chemical analysis determines both qualitatively and quantitatively the chemical constituents of the stone. In a qualitative analysis, the mineral elements and chemical combinations comprising the stone, together with the impurities and organic matter, are determined. The quantitative analysis shows the proportions of the different elements and chemical combinations. When the chemical composition of a stone is determined in this way, conclusions can usually, though not always, be drawn as to the durability and the weathering properties of the stone. The microscopic examination of building stone is not only less expensive but also more important than the chemical analysis, for by it is revealed the structure of the stone. By the microscope may be observed the size and shape of the particles or crystals composing the stone, their relative closeness, and the character and compactness of the cementing material holding them together. Usually, the mineral constituents of the stone may be determined by microscopic examination, and frequently their proportions and the percentage of impurities contained in the stone may be estimated. In addition, the microscope reveals flaws in the structure, such as cracks, cavities, incipient fractures, and gas bubbles. The mechanical tests of a stone furnish data from which a fair estimate of the durability may be made. The purpose of these tests is to impose on the stone, as nearly as possible, conditions that in the course of a few hours or a few weeks will approximate the effect produced by actual use during a period of years. TESTS OF STONE 37. Specific Gravity.- index The American Society for Testing Materials recommends the following method for determining the apparent specific gravity of stone: The sample should weigh about 1,000 grams (35.3 ounces), and should be composed of pieces that are approximately cubical or spherical in shape and are retained on a screen having circular openings 1.27 centimeters (1/2 inch) in diameter. It should be dried to constant weight at a temperature between 100 and 110 C. (212 and 230 F.), and cooled. The total weight to the nearest ..5 gram should be recorded as weight A.. Next, the sample should be immersed in water for 24 hours, and, as soon as it is removed from the water, the surfaces of the individual pieces should be dried with a towel or blotting paper. The weight in this condition should be recorded as weight B. Then, the sample should be placed in a wire basket having 1/4 inch meshes, and the basket should be suspended in water from the center of a scale pan. The difference between the weight so determined and the weight of the empty basket suspended in water should be recorded as weight C. This is the weight of the saturated sample immersed in water. The weight of the water displaced by the sample of stone is equal to B - C, and the apparent specific gravity S may be calculated by the relation in FIG 1- index
The weight of a cubic foot of stone, called the specific weight of the stone, may be found by multiplying the specific gravity by 62.5, the weight of a cubic foot of water. The average specific weights and specific gravities of various kinds of stone are given in Table 1.
38. Absorption Test.- index The absorption of water per cubic foot of rock may be determined by the following method: An approximately cubical sample weighing between 29 and 31 grams should be dried in a closed oven for 1 hour at a temperature of 110 C. (230 F.) and then cooled for 1 hour in a special glass or porcelain vessel with a tight-fitting cover, called a desiccator. The next operation is to weigh the sample as quickly as possible first in air and then in distilled water having a temperature of 25 C. (77 F.). To facilitate the rapid weighing in water, a sample nearly like the test sample in size and shape should be previously weighed in air and in water, and the loss of weight on immersion found; the approximate weight of the test sample in water can then be computed by subtracting the predetermined loss of weight on immersion from the weight in air, and, at the start, the scales can be set to the calculated weight. The sample should be allowed to remain 48 hours in distilled water. Maintained as nearly as practicable at 25 C. {77° F.), at the termination of which time the water should be brought to exactly this temperature and the sample should be weighed while immersed. The number of pounds of water absorbed per cubic foot of the sample should be calculated by the formula in FIG 2- index
The final value accepted shall be the average of three determinations made on three different samples according to this method. The approximate absorptive capacities of various stones are given in 39. The amount of water - index Absorbed by stone depends largely on its density; a dense stone absorbs less than a porous stone. Stones that have already begun to decompose absorb a much larger quantity of water than do those fresh from the quarry. A low absorption is generally considered to indicate a good quality. Nevertheless, it does not follow that a stone that absorbs a small amount of water will suffer the least through the action of the frost, for the reason that a porous stone will dry more rapidly than a dense stone and will permit the expansive action of freezing water to find relief without forcing apart the particles of which the stone is composed. Hence, a high rate of absorption is more detrimental to a fine-grained stone than to a coarse-grained one. 40. Resistance to Fire.- index The effect of high temperatures on a stone is ascertained by heating samples to a red heat in a special furnace, called it muffle furnace. The ability of the stone to resist the action of both heat and water, as in case of a fire, is then determined by plunging the samples, when slightly cooled, into cold water and noting the cracks or crumbling. 41. Abrasion Test.- index
43. Strength.- index In ordinary engineering structures, stones are generally under compression. Occasionally, they are subjected to transverse, or bending, stresses, as in lintels over openings or in footing slabs. They should never be subjected to direct tension. To determine the crushing strength of a stone, cubical specimens that are accurately dressed to form and dimensions are crushed in a suitably constructed testing machine. The applied load at failure, which is measured on the machine, is divided by the cross-sectional area of the cube to give the unit stress. In making a cross-breaking test, a specimen of the stone whose length is ten times its depth is placed on wedge-shaped supports near its ends. A vertical load applied at the center is gradually increased until failure occurs. The transverse strength, called the modulus of rupture, is computed by the formula in
Table 2 index shows the average resistance to crushing and the average modulus of rupture of the more important stones: 44. Resistance to Acids.- index The smoke and fumes from factories usually contain carbonic, sulfuric, and nitric acids, and in large cities the amount of these acids in the atmosphere may be sufficient to act on building stone. The effect of an acid on a stone may be determined by immersing a sample of stone in water that contains 1 per cent. of the acid. The test should last for several days, and the liquid should be agitated frequently.
45. Color.- index As a rule, the chemical constituents of a stone determine its color. The compounds of iron are the principal coloring substances. The brownish or reddish hues in a stone are due to free oxides of iron, and the bluish or grayish hues are caused by the carbonates or the sulfides. When the stone is free from iron, it is usually white or nearly white. The color of granite, however, is affected by the action of light on the feldspar contained in it; when the feldspar is clear and glassy, and absorbs the light, the rock appears darker than when the feldspar is opaque and reflects the light. The blue and black colors of limestones and marbles are largely caused by the presence of carbonaceous matter. In order to ascertain the permanence of color of a stone, samples are placed in an air-tight vessel in the presence of fumes of nitric, hydrochloric, and other acids for a period of at least 7 weeks. At the end of this time the stones are washed and any change in color is noted. DAMChicago.com
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