Properties of Rocks and Minerals
In order to select the rock material for the construction the properties are commonly examined
The rocks are aggregates of minerals. If the mineral continents of a rock are hard, free from cleavage and resistant to weathering, it is likely to be strong and durable. The rocks which are rich in weak minerals, such as micas, chlorite, talc, felspars and minerals are not durable.
Fine grained rocks are generally more dense and stronger than coarse grained rocks. It is for this reason that the basalts and dolerites are widely used as road metal.
Many rocks contain structures like stratification, lamination, foliation and cleavage. Such rocks bear greater loads if they are placed in the construction parallel to the planes of weakness. Further, since laminated or banded rocks may scale badly due to weathering, it will not be wise to place them in the civil engineering works with the cleavage or bedding planes vertical.
The porosity of a rock is the ratio of the volume occupied by pores to the total volume of the rock sample. It is generally expressed in percentage of the volume of the sample. If W1 is the weight of the dry rock sample, W2 is the weight of the sample when it is made saturated with water and V is the total volume, the porosity P can be determined as follow
P = W2 – W1 / V x 100
A less porous rock is generally moiré durable and strong and therefore its is preferred for construction purposes.
Permeability is the capacity of a rock to transmit water. It indicates the ease with the water can percolate through the openings of the rocks. The permeability of rocks is particularly important in a number of engineering problems such as those connected with dams, reservoirs, water supply, sanitary engineering and other structures. The permeable rocks are considered harmful because they cause seepage of water which may even lead to the destruction of the structure.
The durability of a stones is of the important Properties of Rocks and Minerals its capacity to retain its original size, strength and appearance throughout a long period. Some rocks which do not resist weathering, decay and loose strength early. The durability of a rock is directly related to its mineral composition and texture.
In cold countries the frost action plays an important role in disintegration rocks, the water which enters into the opening of a rock, freezes during the night. On freezing it induces tensile stresses due to which cracks develop in the rocks. Building stones in big cities are severely affected by carbon dioxide and sulphur dioxide gases which are released into the atmosphere by various industries.
Strength of Rocks
The strength of a rock is main Properties of Rocks and Minerals determined by knowing its crushing strength, shearing strength and resistance to abrasion.
The crushing strength is the resistance offered by a stone to pressure. The resistance offered by the stone to shear stresses which tend to move one part of a specimen with respect to the other is called shearing strength. The resistance of a stone to scratching or rubbing action is called its resistance to abrasion.
The stones which are to be put in the foundation of civil engineering structure, must having high crushing and shearing strength. The stone used for paving and flooring purposes must have high resistance to abrasion.
When rocks are heated to a high temperature and then cooled, they may get damaged. This is particularly the case if they are rich in minerals like calcite and felspars. The rocks in the order of decreasing resistance to damage by fire and sandstone, granites, limestones, gneiss and marble.