PHYSICAL PROPERTIES OF ROCKS 

Crushing Strength :

 It is also termed as compressive strength of a stone. It may be defined as maximum force expressed per unit area which a stone can withstand. Any force beyond the compression strength will cause a failure of the stone. Mathematically, compressive strength is expressed by simpler method as follows:

Co =P/A

Where Co is the Compressive strength, P= Load at failure, A = Area of cross section of stone under P.

The determination of compressive strength of a building stone involves making standard test specimens (which are either cubes of 5cm side or cylinders of length: diameter ratio of 2 or 2.5). These specimens are then loaded gradually one at a time after placing on the base plate of a universal testing machine, till the first crack appears in the specimen. Any further loading will crush the specimen. The compressive strength determined in this way using the above relationship is called “unconfined or universal compressive strength”.

The crushing strength of a rock depends on a number of factors, such as its

i. Mode of formation

ii. Composition

iii. Texture and structure

iv. Moisture content and

v .extent of weathering it has already suffered

Igneous rocks are crystalline rocks. They are compact and characterized by interlocking in texture and uniform in structure. These rocks possess very high crushing strengths compared to sedimentary and metamorphic rocks.

In the sedimentary and metamorphic rocks, the presence of planes of weakness along bedding planes, foliation and schistosity and cleavage, greatly affects the compressive strength, both in direction and magnitude. The sand stone may show a very low crushing strength when loaded parallel to bedding planes than when loaded perpendicular to the same structure. Except for sandstone, quartzite and most other sedimentary and metamorphic rocks are composed of clays, calcareous and hydrated silicate minerals which are inherently weak is strength.

Shear Strength:

Shear strength is the resistance offered by a stone to shear stresses, which tends to move one part of a specimen with respect to the other. It is obtained by using the relationship.

S = 2P/A

Where P = load at failure;

 A = area of cross section of the specimen.

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 shear strength of most common building stones ranges from 70 to 140 kg/cm². In laboratory testing, a bar shaped specimen is held with grip and is supported at ends below, is loaded from above. Rupture occurs when the shear strength is exceeded.

Tensile Strength:

Tensile strength of a rock is related to its ability to withstand breakage. It may be determined directly or indirectly. The tensile (pulling) strength that has to be applied to a material to break it. It is measured as a force per unit area. The direct method would require elaborate means to avoid bending while applying tensile forces by gripping the specimens at the ends. Since tensile stresses are seldom required accurately, an indirect method is commonly applied.

The indirect method is called the Brazilian test. It consists of loading a test cylinder diametrically in such a way that the applied loads would develop tensile rupturing along the diametrical plane of the specimen.

Loads are gradually increased till the cylinder fractures. The load P, at rupture being thus known. Transverse strength Ts is calculated by using the formula

Ts = 2P/mDL

D = diameter of the specimen; L = length of the specimen

Porosity:

The shape, size and nature of packing of the grains of a rock give rise to the property of porosity or development of pore spaces within a rock. Numerically it is expressed as the ratio between the total volume of pore spaces and the total volume of the rock sample. Porosity is commonly given in percentage terms. Presence of interlocking crystals, angular grains of various sizes and abundant cementing materials are responsible for low porosity of stones. Conversely the rock will be highly porous it composed of spherical

 or rounded grains, (sandstone) or if the cementing material is distributed unevenly or is of poor character. Porosity is an important engineering property of rocks. It accounts for the fluid absorption value of the stones in most cases and also that a higher porosity signifies a lesser density which generally means a lesser compressive strength. Porosity values for a few common building stones.

Granite-0.1 to 0.5%,

Basalt- 0.1 to 1%, Sandstone- 5 to 25%,

Limestone- 5 to 20%,

Marble- 0.5 to 2%,

Quartzite- 0.1 to 0.5%,

Absorption Value:

It defines the capacity of a stone to absorb moisture when immersed in water for 72 hours or till it gets full saturation. It is generally expressed in percentage terms of original dry weight of the mass. . It may be obtained from the relationship

Absorption value = Ws – Wo/Wo * 100

Where Ws = weight at saturation; W0 = dry weight of the sample used.

Density: It is defined as weight per unit volume of a substance. But in the case of rock it is not only the solid mineral matter which wholly accounts for the total volume of a given specimen. A part of the rock may comprise of pores or open spaces, which may be empty, partly filled or wholly filled with water. Accordingly, three types of density may be distinguished in rocks. They are

A) Dry density: It is the weight per unit volume of an absolutely dried rock specimen, it includes the volume of the pore spaces present in the rock.

b) Bulk density: It is the weight per unit volume of a rock sample with natural moisture content where pores are only partially filled with water.

c) Saturated density: It is the density of the saturated rocks or weight per unit volume of a rock in which all the pores are completely filled with water.

Abrasive Resistance: It is more a qualitative than a quantitative property. It may be broadly defined are the resistance which a stone offers to rubbing action of one kind or another. Determination of this is of considerable significance when stones are intended for use in situations where rubbing by natural or artificial causes is involved as a routine. Example a) stones used in paving along roads, b) Facing stones in buildings of arid region where strong sand laden winds are blown. These type of situations demand stones that have not only high abrasive resistance but also of essentially uniform, composition. So that the wear is as uniform as possible.

Frost and fire resistance: Many building stones show quick disintegration of building stones or rocks when used in situations involving frost formation (excessive cold) or heating. Frost causes disintegration by expansion of water on breezing within the rock pores. In the case of fire, the unequal expansion in different mineral components and also at different depths from surface inwards may cause disintegration. This effect becomes more pronounced when the rock is first heated and then suddenly cooled by water by water. Heavy stones including granites crumble to pieces under such a treatment. It is easy to understand that rocks which are found porous and weak in strength are easily deteriorated in cold humid climates by frost action. Limestone and sandstones fall in this category. They show very poor frost resistance.

Fire resistance is especially determined when the stone is intended for use around stoves, heating places and in the wall of furnaces. Only compact and massive sandstones and quartzites suite reasonably well in fire and heating places.