Note for Building Materials and Building Construction - BMBC by Karthik M P
- Building Materials and Building Construction - BMBC
- Note
- Anna University -
- Civil Engineering
- B.Tech
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1. STONES STONE AS BUILDING MATERIAL Stone has been defined as the natural, hard substance formed from minerals and earth material which are present in rocks. Rock may be defined as the portion of the earth’s crust having no definite shape and structure. Almost all rocks have a definite chemical composition and are made up of minerals and organic matter. Some of the rock-forming minerals are quartz, felspar, mica, dolomite, etc. The various types of rocks from which building stones are usually derived are granite, basalt, trap, marble, slate, sandstone and limestone. Use of stone in building construction is traditional in the places where it is produced, although even there its high cost imposes limitations on its use. The conditions which govern the selection of stone for structural purposes are cost, fashion, ornamental value and durability. Stone has been used in the construction of most of the important structures since prehistoric age. Most of the forts world over, the Taj Mahal of India, the famous pyramids of Egypt and the Great Wall of China are but a few examples. Stone has also been extensively used in almost all the elements of building structures, as load carrying units as well as for enhancing the beauty and elegance of the structure. As building material stone has gradually lost importance with the advent of cement and steel. Secondly, the strength of the structural elements built with stones cannot be rationally analysed. Other major factors in overshadowing its use are the difficulties in its transportation and dressing which consume a lot of time resulting in slow pace of construction. ROCKS Being aggregations of minerals, the properties of rocks are dependent upon the character of these constituents, identified by their physical properties such as hardness, cleavage, streak, colour, lustre, specific gravity and shape of crystals. CLASSIFICATION OF ROCKS The rocks may be classified on the basis of their geological formation, physical characteristics and chemical composition as shown in Fig. 1.1. Fig. 1.1 Classification of Rocks 1
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QUARRYING OF STONES The only operation involved in the production of natural stone is the quarrying process. The open part of the natural rock from which useful stone is obtained is known as quarry. While selecting a quarry site, the points to be borne in mind are availability of sufficient quantity of the stone of desired quality, proper transportation facilities, cheap local labour, problems associated with drainage of rain water, location of important and permanent structures in the vicinity and site for dumping refuse. TOOLS Some of the quarrying tools shown in Fig. 1.2 are wedge, pin, hammer, dipper or scraping spoon, tamping bar, priming needle, jumper, borer, claying iron, crow bar. Fig. 1.2 Quarrying Tools METHODS OF QUARRYING Rocks suitable for the manufacture of stone materials are called useful minerals and the operations involved in obtaining minerals are called mining. In the process of mining, voids formed are called excavations, and the mined deposits are the quarries. The purpose of quarrying is to obtain stones for various engineering purposes. Knowledge of various quarrying methods is essential but does not make one very much more competent to choose or specify a stone for building work. Depending upon the nature and surface of rocks and the purpose for which stones are needed, quarrying is done by excavating, wedging, heating or blasting. SEASONING OF STONES A freshly cut stone carries some natural moisture known as quarry sap making it soft and workable. The quarry sap is a mineral solution and reacts chemically with the mineral constituents when the stone is exposed to atmosphere after quarrying. The stone becomes harder and compact. The process takes about 6 to 12 months for complete seasoning. When the quarry sap evaporates, it leaves a crystalline film on the faces of the stone and makes them weather resistant. The dressing before seasoning 2
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improves the weather resistance. As such, the dressing, carving and moulding, etc. should be done as early after quarrying as possible. DRESSING OF STONES A quarried stone has rough surfaces, which are dressed to obtain a definite and regular shape. Dressing of stones is done immediately after quarrying and before seasoning to achieve less weight for transportation. Dressing of stone provides pleasing appearance, proper bedding with good mortar joints, special shapes for arches, copings, pillars, etc. The various types of dressed stones are shown in Fig. 1.3. Fig. 1.3 Dressed Stone Surface USES OF STONES Use of stone as building material depends upon the nature of the work, type of the structural element in which it is to be used and its quality, availability and transportation cost. For structural purpose, granite, gneiss, trap, sandstone, limestone, marble, quartzite and slate are most useful. On the basis of the method of manufacture, items and materials from natural stones are classified as Sawn— obtained either from massive rocks by stone-cutting and stone-splitting machines (large stones) or from semi-product blocks by appropriate working (facing slabs, windows sill slabs, etc.); Split obtained by splitting and finishing blocks (curb stones, paving blocks, etc.); Roughly split— manufactured by oriented splitting of blocks (bedded stone); Fractured—produced by blasting rocks and separating finer sizes (quarry stone); Crushed—produced by crushing and screening (crushed stone, artificial sand) and; Ground—obtained by grinding rocks (ground mineral powder, stone powder). • Structure: Foundations, wells, columns, lintels, arches, roofs, floors, damp proof course, etc., • Face Works: Massive appearance to the structure • Paving Stones: Cover the floor of various buildings and also adopted for paving roads, foot paths, etc., • Basic Materials: Converted to form basic material for cement concrete, morum of roads, calcareous cements, artificial stones and hollow blocks, etc., • Miscellaneous: Ballast for railways, flux in furnace, blocks in bridges, piers, abutments, retaining walls etc., 3
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Characteristics of Good Building stone A good building stone should have the following qualities. I. Structure: The structure of the stone may be stratified (layered) or un-stratified. Structured stones should be easily dressed and suitable for super structure. Un-stratified stones are hard and difficult to dress. They are preferred for the foundation works. II. Texture: Fine grained stones with homogeneous distribution look attractive and hence they are used for carving. Such stones are usually strong and durable. III. Density: Denser stones are stronger. Light weight stones are weak. Hence stones with specific gravity less than 2.4 are considered unsuitable for buildings. IV. Appearance: A stone with uniform and attractive colour is durable, if grains are compact. Marble and granite get very good appearance, when polished. Hence they are used for face works. V. Strength: • Strength is an important property to be looked into before selecting stone as building block. Indian standard code recommends, a minimum crushing strength of 3.5 N/mm2 for any building block. • Due to non-uniformity of the material, usually a factor of safety of 10 is used to find the permissible stress in a stone. • Hence even laterite can be used safely for a single storey building, because in such structures expected load can hardly give a stress of 0.15 N/mm2. • Hardness: • It is an important property to be considered when stone is used for flooring and pavement. • Coefficient of hardness is to be found by conducting test on standard specimen in Dory’s testing machine. • For road works coefficient of hardness should be at least 17. • For building works stones with coefficient of hardness less than 14 should not be used. I. Percentage wear: It is measured by attrition test. It is an important property to be considered in selecting aggregate for road works and railway ballast. A good stone should not show wear of more than 2%. VI. Porosity and Absorption: All stones have pores and hence absorb water. The reaction of water with material of stone causes disintegration. Absorption test is specified as percentage of water absorbed by the stone when it is immersed under water for 24 hours. For a good stone it should be as small as possible and in no case more than 5. VII. Weathering: Rain and wind cause loss of good appearance of stones. Hence stones with good weather resistance should be used for face works. VIII. Toughness: The resistance to impact is called toughness. It is determined by impact test. Stones with toughness index more than 19 are preferred for road works. Toughness indexes 13 to 19 are considered as medium tough and stones with toughness index less than 13 are poor stones. IX. Resistance to Fire: Sand stones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire. 4
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