In this article we will discuss about:- 1. Meaning of Building Stones 2. Properties of Building Stones 3. Important Building Stones.
Meaning of Building Stones:
A building stone may be defined as a sound rock that can be safely used in some situation in the construction as a massive dressed or undressed unit. Granites and marbles used in the form of finely dressed blocks or slabs or columns in monumental and costly buildings are good building stones.
Similarly, sandstones and limestones used in forts, retaining walls and boundary walls and also as blocks in stone houses and bungalows are typical building stones. Slates used in many areas as roofing material for ordinary constructions and in pavements also fall in the category of building stones.
Stone masonry is an engineering art that is preserved in many historical buildings in all parts of the world. This skill is still used, though on a lesser scale (because of the advent of concrete) in the construction of common residential houses and palatial buildings in many places. The Taj Mahal at Agra, the Red Fort in Delhi and temples of Jagannathpuri are some of the best known stone marvels of India. Such examples may be compiled from all countries of the world and the number may run into many hundreds.
Properties of Building Stones:
What physical, structural and other properties a rock should possess to be recommended as a building stone will depend upon the type of construction and the situation within that particular construction where the stone is intended to be used. Thus a stone to be used in flooring for a building need not possess all those qualities that are must for a stone to be used in foundations and load bearing walls.
Similarly, stones required for making retaining walls need not have all the properties required for that intended to be used in facing walls.
It is, therefore, considered convenient to discuss various qualities governing the selection of building stones under three sub-headings:
(A) Strength Characteristics,
(B) Geological characters and
(C) General characters.
(A) Strength Characteristics:
Stones, like all other solids, fail when subjected to loads beyond their strength. The failure takes place under compressive, tension and shear forces at different values. It is, however, the unconfined compressive strength, which is taken as the most important index property of stones.
1. Compressive Strength:
It is also sometimes referred as crushing strength of a stone and may be defined as the maximum force expressed per unit area, which a stone can withstand without rupturing. Any force applied beyond the compressive strength will cause a failure or rupture of the stone.
Mathematically, uniaxial compressive strength is expressed by a simple relationship:
Co = P/A
where Co = Compressive strength/unit area;
P = Load at failure,
A = Area of cross-section of stone under P.
Practical determination of unconfined compressive strength of a building stone is carried out in a well-equipped civil engineering laboratory. Representative test specimens of standard shape (either cubes of 5 cm side or cylinders of length-diameter ratio of 2 – 2.5) prepared carefully are loaded gradually, one at a time, on the base plate of a UTM (Universal Testing Machine). The loading is continued till the first crack appears in the test specimen indicating beginning of the failure. Any further loading will crush the specimen.
The load at failure (P) divided by the area of cross-section of the sample gives the unconfined compressive strength of the rock. The value is termed unconfined or uniaxial because the test specimen has no lateral support or restraint and is being compressed only along one axis.
When the compressive strength is tested by a method providing a lateral support, as by keeping the specimen in a special cell (Triaxial cell) filled with a liquid under pressure, the value obtained is called a Confined or Triaxial Compressive Strength. It is, however, the unconfined or uniaxial compressive strength that is generally taken into consideration in civil engineering construction with stones.
The compressive strength of a rock depends on a number of factors such as its mode of formation, its composition, texture and structure, its moisture content and extent of weathering it has already suffered. Igneous rocks being crystalline in character, compact and interlocking in texture and uniform in structure possess very high compressive strengths compared to sedimentary and metamorphic rocks.
In the latter two groups, abundance of planes of weakness such as bedding planes, foliation, schistosity and cleavage greatly affect the compressive strength of the respective rocks in magnitude and direction. Talking about direction, sandstone may show a very low compressive strength when loaded parallel to bedding planes than when the same stone is loaded perpendicular to the same structure. Except for some varieties of massive sandstones, limestones, quartzites, marbles and gneisses, most of the sedimentary and metamorphic rocks are inherently weak in crushing strength.
The strength values as obtained from laboratory testing may also show considerable variation with respect to the same rock depending upon the size of the test specimen, the smoothness of the contact planes of the test specimens, the rate of loading and duration of loading etc. It is, therefore, implied that all the laboratory tests are carried out strictly in accordance with Standard Codes as issued by the specified authority of the country. (The Bureau of Indian Standards Codes should be referred in India in designing construction with stones).
Compressive strength of many types of building stones is generally much higher than the loads they are often supposed to withstand in ordinary type of building construction.
Following values give a general idea of the compressive strength range for some common building stones:
During the last few years, thousands of tests have been carried out on rock strength in different parts of the world.
An attempt has been made by Deere and Miller to classify rocks on the basis of compressive strength range:
2. Transverse Strength:
It is defined as the capacity of a stone to withstand bending loads. Such loads are only rarely involved in situations where stones are commonly used.
But when a stone is intended for use as a beam or as a lintel, its transverse strength is determined as Modulus of Rupture using the following relationship:
R = 3wl/2bd2
where R = Modulus of Rupture;
w = load at which sample breaks,
I = length,
b = width and
d is thickness of the test specimen.
Transverse Strength is determined practically by loading transversely a bar shaped test specimen, generally 20 × 8 × 8 cm and supported at ends from below. The load at which the stone breaks (ruptures) is taken as V from which R can be calculated. It has been found that in stones, transverse strength is generally 1/10th to 1/20th of their compressive strength values. Shear Strength of a stone is also not commonly determined except when the stone is to be used as a column. It has been observed that in important building stones the shear strength lies between 70-140 kg/cm2.
The size, shape and degree of packing of the component crystals or grains in a rock give rise to its porosity. Numerically it is expressed as the ratio between the total volume of the pore spaces and the total volume of the rock sample and is expressed commonly in percentage terms.
Porosity (range) values for few common types of building stones are:
Low porosity is caused by interlocking crystals, angular grains of different sizes and uniformly distributed cementing material in the rock. Conversely, the rock will be highly porous if composed of spherical or rounded grains (e.g. sandstone) or if the cementing material is distributed unevenly or is of poor cementing value.
Absorption Value defines the capacity of a stone to absorb moisture when immersed in water for 72 hours or till saturation. It is generally expressed in percentage terms of original dry weight of the stone samples. Samples of cube shape are made from the stone and dried at 100°C till they attain a constant weight, W.
These samples are immersed in water and are weighed after regular intervals of 8-12 hours, till they attain a constant weight, S, which indicates their saturation with absorbed water.
The absorption value, which indicates the pore volume or porosity, is given by the relationship:
Absorption Value = [(S – W) / W] × 100
where W and S give the dry and saturated weights respectively for the specimens being tested.
Sandstones and limestones may show in some varieties as high absorption values as 10% or more. Selection of such highly porous varieties of stones for use in building construction, especially in moist situations, would be greatly objectionable. Presence of water within the pores not only definitely decreases the strength of the stone but also makes them vulnerable to disintegration due to frost action in cold humid climates.
It is defined as weight per unit volume of a substance, in this case a stone. But in the case of stones, the concept of the unit volume has to be clearly understood. A cubic centimeter of a stone shall not always be made up 100% of solid matter. It may contain 1% to 25% of pore spaces (porosity) which may be empty, partly filled, or wholly filled with water.
Accordingly, three types of density values may be differentiated in the case of rocks as follows:
(i) Dry Density:
It is the weight per unit volume of an absolutely (oven) dried rock. Obviously it includes the volume of the pore spaces present in the rock.
(ii) Bulk Density:
It is density of the rock with natural moisture content. It is assumed that in such cases all the pore spaces are not filled with water.
(iii) Saturated Density:
It is the density of rock fully saturated with water. In most engineering calculations it is the bulk density that is taken into consideration.
Bulk density values in grams/cubic centimeter for some common building stones are:
Granite – 2.7
Basalt – 2.9
Sandstone – 2.6
Limestones – 2.2 – 2.6
Knowledge of density values of stones helps in making an estimate of approximate strength values of the stones and also in calculating the loads implied in stone masonry at different levels.
5. Abrasive Resistance:
It is more a qualitative than quantitative property and may be broadly defined as the resistance, which a stone offers to rubbing action of one kind or another. This quality acquires considerable importance when a stone is intended to be used in a situation where rubbing by natural or artificial causes may become a routine.
Thus stones used in paving along roads, in flooring in buildings, as lining in tunnels or facing stones in building where sand loaded winds blow quite often are all exposed to rubbing action. Hence their resistance to abrasion should better be known before use.
Mineralogical composition of a stone plays a great role in resistance to abrasion. A stone made up of the same mineral will wear uniformly when soft; e.g. limestone and marble. It may not wear at all if made up of hard minerals, e.g. quartzite and sandstone. But another stone, like granite, having an appealing appearance but made up of minerals of different hardness (felspars and quartz) may get abraded unevenly. It may present a bad, pitted appearance after a few years of its use in areas where sandstorms are quite frequent.
Abrasion resistance of a stone is commonly determined in civil engineering laboratory using Dorry’s Abrasion Testing Machine. It consists of a revolving steel disc provided at the top with two metallic funnel-shaped projections placed at diametrically opposite ends. One projection is for holding a properly prepared cylindrical stone sample of dried weight and proper dimensions (2.5 cm height, 2.5 cm dia.)
The other projection is used for feeding standard sand at a standard rate that causes the stone to wear at the base during rotation of the disc and thus losing some weight in the process. After a fixed number of revolutions, (generally 1000), the stone sample is taken out and reweighed. The loss in weight is a measure of abrasive resistance of the stone.
Commonly, it is first calculated in percentage terms of original dry weight of the sample, and then, a coefficient of wear is obtained from the following relationship:
Coefficient of wear = 40 / (% loss in weight)
The safe value for coefficient of wear is 20, when the loss in weight of stone after 1000 revolutions of the disc is only 2%.
6. Frost and Fire Resistance:
Many building stones show quick disintegration when used in situations where either frost forming climate prevails in first case or intensive heating is involved in the second case.
Frost causes disintegration by expansion of water on freezing within the pores of rocks during winters; it’s melting during summers and the process getting repeated year after year. This is called frost action as already explained in weathering of rocks. Porous varieties of limestones and sandstones often show very poor frost resistance.
Fire resistance is especially determined when a stone is intended for use around stoves, heating places and in the walls of kilns or furnaces. Ironically, many otherwise sound building stones like granites and other coarse grained igneous rocks are poor in fire resistance. These may become unsuitable for use in heat intensive situations.
Their poor fire resistance is due to their multi-mineral composition. Different minerals expand at different rates on getting heated. The unequal expansion becomes the cause of internal stresses within the body of the rock leading ultimately to its disintegration.
Similarly, some rocks, though made up primarily of one mineral, may also disintegrate when subjected to heating conditions; limestones, for example, are made up of calcite that decomposes under temperatures beyond 750°C. Only compact and massive sandstones and quartzites will prove durable around heating places.
(B) Geological Characters:
1. Mineralogical Composition:
Rocks as we know are actually made up of smaller units, called minerals. Hence properties of rocks ultimately depend on the composition and nature of packing in the rock. It has been found that rocks made up chiefly of silica, especially quartz (SiO2), especially in the free form, are strongest in many respects. Quartzites, sandstones and granites are examples of quartz-dominated strong rocks.
Carbonate rocks show a very wide variation in their engineering properties and hence each variety has to be tested thoroughly for such properties before use in construction of some importance. Presence of minerals like mica, gypsum, sulphides, glauconite, tremolite, flint, chert and clay, even in small proportions, in otherwise sound rocks, destroy their inherent strength and quality.
2. Texture and Structure:
Texture defines the size, shape and mutual relationship of the mineral constituents of a rock whereas structure determines the development of some typical features on a large scale in the rock mass as a whole.
Rocks may be coarse-grained, medium-grained or fine-grained and also equi-granular or inequigranular in texture. Experience has shown that fine-grained, equi-granular textured rocks are, other things being same, better building stones compared to coarse-grained and inequigranular rocks. In the latter case components of different sizes behave differently under the imposed loads offering complex reaction.
Structurally speaking, such features as stratification, foliation, lineation, cleavage, joints, micro-joints and flow structures have to be given due consideration before selection of a stone for construction. Very often these may not be seen in the small-sized test specimen. The geology of the rock mass as a whole and in field occurrence should be studied for their likely effects on the quality of these stones.
Most of these structures are to be treated as effective planes of weakness and when closely spaced, they make obtaining proper-sized blocks from the rock mass very difficult if not impossible. When some blocks from such rocks have to be used, they should be placed in such a way that the planes of weakness are not parallel to the effective loads.
3. Resistance to Weathering (Durability):
It is essentially a geological character that is determined by the composition and texture of a stone on the one hand and also the environment where it is ultimately used in construction on the other hand. Nothing is everlasting, stones included. But the rate at which a stone deteriorates under the attack of natural processes (i.e. it weathers) varies from rock to rock.
A stone (e.g. granite) may remain fresh and untarnished when used in the interior of a building for hundreds of years but the same stone used on the exterior of the same building may get badly pitted and tarnished within a few decades. Similarly, limestone used in industrial towns may weather badly due to reactions with sulphurous acid vapours polluting the air whereas the same rock used in temples and forts in cleaner climates (e.g. Kashmir valley) has remained fresh for centuries.
An engineer, especially a town planner has to bear in mind the compatibility of a stone proposed to be used in a particular environment. Rocks are sensitive towards the environment in which they are placed. Their durability which means their capacity to withstand the imposed loads without undergoing any deterioration depends on the fact whether or not they have been used in an environment which is in tune with their geological characters.
Durability of a stone can be experimentally determined by subjecting the stone samples to disintegrating action of sodium sulphate. Test specimens, generally of 5 cm side-cubes, are dried perfectly and weighed. They are suspended in 14% solution of sodium sulphate decahydrate for 4 hours at 27 +/-2°C. These are taken out, and oven dried at 100 +/-5°C. This makes one cycle. These samples are then subjected to 30 such cycles and loss of weight determined at the end. Greater the loss in weight, poorer the durability of the stone under test.
(C) General Characters:
It is as important a consideration in the selection of a stone for building consideration as its engineering properties or geological characters. A stone may be quite suitable for use in a building but it may still not be selected because of its high cost with respect to the place of construction.
Cost of a building stone depends on its availability, Grain and Hardway in Granites accessibility and workability. Good quality building stones are not available everywhere. Their transport from a quarry at a particular place to another place of construction might be a very costly affair compared to other materials such as bricks or concrete. Rajasthan is rich in marble of good building qualities. Still these stones, though exported, are not used as a common building stones in our own country or even in Rajasthan. They are quite costly compared to other conventional building materials.
By workability of a stone is understood the ease in effort and the economy with which it can be extracted from its natural place of occurrence and finally given a proper shape (called dressing). Harder and stronger rocks require more effort and cost for their quarrying and dressing.
Quarry engineers often take advantage of natural structures of rocks such as stratification and jointing in sedimentary rocks and rift and grain in igneous rocks to make quarrying easier and economical. However, the overall economy in construction still remains a deciding factor for large scale use of stones in building construction.
It is the property of appeal (i.e. aesthetic) and gets importance only when a stone is to be used in situations exposed to public view. Thus stones for use in foundations and dams or where they have to be given outer plastering may be strong and durable and of any colour. However, in the case of walls in residential and official buildings, dark colours are seldom used. White, red and pink, grey and green shades are the popular shades of stone for residential buildings.
The colour of a rock is a geological character and depends on the mineralogical composition of the rock. Granites are generally light coloured; sandstones occur in a variety of shades from grayish to white, red and purple; limestones and basalts tend towards darker shades. Marbles occur in almost all shades ranging from pure white to pink, red, green and black, depending upon the type of impurity present.
Important Building Stones:
Any type of rock that satisfies the above considerations may be used as a building stone. Some rocks, however, occur abundantly whereas others are much rare. It can be said that these are the common rock types that are commonly used in ordinary building construction.
Following is a brief account of general properties of these stones:
These are the most commonly used building stones of all the igneous rocks. They generally possess all the essential qualities of a good building stone except for fire resistance. Granites show very high crushing strength, low absorption value, least porosity, interlocking textures, variety of appealing colours and capacity to get brilliant polish.
For common granite, general properties may be summarized as follows:
India has got good reserves of granites and granitic rocks. The Archean group of rocks of Peninsular India are comprised chiefly of gneisses and granites that form inexhaustible source of good building stones.
Massive sandstones consisting of closely interlocking angular grains of quartz and free from structural defects find extensive use as building stones in all areas where they occur. Varieties rich in clay and iron are better avoided. Quartzites having uniformly distributed siliceous cement are quite suitable under heavy loads.
India has got immense reserves of good quality sandstones distributed in many states of the country. Most of these rocks are quite suitable for common type of construction. Some of them are in fact of excellent qualities and have been used in many historical buildings in the past as well as during recent times.
The so well-known Vindhyan Sandstones occurring in Madhya Pradesh, Rajasthan, parts of Bihar and Uttar Pradesh are available in abundance in a variety of colours like white, cream, deep red and gray. These stones are available in a large area of the country, for over 35000 sq. km. extending from Bihar to Aravallis. Many superb buildings of Delhi, Agra and Rajasthan are built of these stones.
Dr. Wadia, an eminent Indian Geologist said about these stones:
“No other rock formation of India possesses such an assemblage of characters, rendering it so eminently suitable for building or architectural work.”
The Gondwana formations of India have also yielded very good quality of building stones. The fine-grained sandstones of Cuttack (known as Athgarh sandstones) have been used widely; many famous temples including that of Jagannathpuri are built of them. Similar good deposits occur in Tirupati and Ahemad Nagar.
These sedimentary rocks are also very extensively used as building stones at places of their occurrence but all varieties are not suitable for this purpose. Limestones show great variation in their properties and appearance. Thus in crushing strength, they may be softer than a good brick (less than 200 kg/cm2) or may be as strong as 1000-1500 kg/cm2. A similar variation in other properties may also be observed in different types.
Use of limestones as facing stones, even if quite strong; should be avoided in situations where:
(i) The air is polluted with industrial gases, such as in industrial towns;
(ii) Along sea coasts where sea-water- rich winds can attack them directly.
The reason for first precaution is that sulphuric acid vapours contained in the industrial gases react with calcium carbonate of limestone producing calcium sulphate (gypsum) crystals. This change involves an increase in the volume and results in the disintegration of the surface layer of the rocks. Similarly, salt crystals may be formed from seawater by absorption and evaporation on the rock surface and cause disintegration at the surface.
Limestones occur in many geological formations of the country; in fact some geological formations are entirely made up of limestone and run for many hundred kilometers.
Important deposits of useful limestones are:
(i) The Cuddapah System, outcropping in Andhra, Tamil Nadu, Delhi and Chhattisgarh.
(ii) The Bijawar, Khondalite and Aravallis groups;
(iii) The Vindhyan System of Madhya Pradesh and Rajasthan.
(iv) The Hill Limestones found in many hilly regions in northern Himalayas including those of Kashmir, Jammu and Himachal Pradesh.
These are metamorphic rocks that are used for ordinary structural work as well as for decorative, monumental and architectural designs. Marbles occur in different colours and textures. Their absorption value is generally low. They have sufficient strength to bear building loads and at the same time can be given any desirable shape because of their low hardness. Marble can take brilliant polish.
India has got fairly widespread deposits of marbles of very good quality. Most important source of commercial marbles is the crystalline formation of Rajasthan.
The rock is quarried at a number of places, e.g.:
(i) Makrana in Jodhpur, wherefrom white and pink varieties are quarried.
(ii) Kharva in Ajmer, which is the source of green and yellow varieties.
(iii) Kishengarh and Jaipur are famous for black and dense marbles.
It is another metamorphic rock characterized with a perfect slaty cleavage. It is used only as a paving and roofing stone in ordinary type of construction in hilly terrain where it may be available. In India, slate deposits occur at quite a few places of which Kangra (Himachal Pradesh), Pir Panjal (Jammu and Kashmir) and Rewari (Rajasthan) may be mentioned.