Building Stones (role of geology while selecting building stones)
Role of geology while selecting building stones:
Geology has an important role in the works of civil engineering. The rock products that are used in the construction of buildings, dams, bridges, roads, cofferdams, sea walls, canals, barrages, and other infrastructures are included in the category of ”building stones”. The building stones used for this purpose include (i) building stones in the form of masonry blocks, (ii) stones in the forms of rubble in the form of irregular fragments, (iii) crushed stones which are used to make concrete and bedding of the roads, (iv) limestone, which is used to make lime and cement.
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Properties of Building Stones:
In order to select the rock material for construction, the properties that are commonly examined are : (i) composition of mineral, (ii) texture, (iii) structure of the stone, (iv) porosity of the stone, (v) permeability of the stone, (vi) durability of the building stones, (vii) strength of the building stone, (viii) heat resistance.
(1). Mineral Composition:
The rocks are aggregates of minerals. If the mineral constituents 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, tale, felspars, and clay minerals are not durable.
(2). Texture of the building stones:
Fine-grained rocks are generally denser and stronger than coarse-grained rocks. It is for this reason that the basalts and dolerites are widely used as road metals.
(3). Structure of the rocks:
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 plane 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 world with the cleavage or bedding lanes vertical.
(4). Porosity of the rocks:
The porosity of the rock is the ratio of the volume occupied by pores to the total volume of the rock sample. It is generally expressed in the percentage of the volume of the sample. If W1 is the weight of a dry rock sample, W2 is the weight of the sample when it is made saturated with water, and V is its total volume, and porosity P can be determined as follows:
A less porous rock is generally more durable and strong and therefore, it is preferred for construction purposes.
(5). Strenght of the Rocks:
The strength of a rock is determined by knowing its resistance to crushing, shearing, and abrasion. The crushing strength is the resistance offered by a stone to pressure. The resistance offered by a stone to shear stresses, which tend to move one part of a specimen with respect to the other, is called ”shearing strength”. The stones which are to be put in the foundation of civil engineering structures must have high crushing and shearing strengths. For making road metal the rocks having high crushing strength are selected.
The resistance of a stone to scratching or rubbing action is called its ”resistance to abrasion”. The stone used for paving and flooring purposes must have high resistance to abrasion.
(6). Permeability of the Stones:
The capacity of a rock to transmit or absorb water is known as the ”permeability of the rock”. It is an indicator of water percolation through the pores and openings of the rock. The permeability of the 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 permeability of rocks are considered harmful because they cause seepage of water, which may even lead to the destruction of the structure.
(7). Durability of the rocks:
The capacity of a stone to retain its original size, strength and appearance for a longer period of time is known as ”the durability of the stone”. Some rocks that do not resist weathering, they start decaying and loosing strength early, while those which are resistant to weathering and abrasion, their size, shape, and appearance last for a longer period of time. Thus the durability of the rocks is directly dependent on the composition and texture of minerals.
In cold countries, the fros action plays an important part in distinguishing rocks. The water which enters into the openings of a rock freezes due to extremely low temperature especially at night. On freezing it induces tensile stresses thereby producing cracks in the rocks.
Building stones in big cities are severely affected by carbon dioxide and sulfur dioxide gases, which are released into the atmosphere by various industries and vehicles. The calcite present in the limestones and marbles may be transformed into sulfate and scaling may result.
During continuous rain, the pores in a stone may be filled with a saturated salt solution. then the crystallization of salt occurs, which causes the disintegration of stones.
(8). Heat resistance of the stones:
When rocks are heated up to a high temperature and then cooled down, they may get damaged. This is particularly the case if they are rich in minerals like calcite (calcium mineral) and felspars. The rocks in the order of decreasing resistance to damage by fire are sandstones, granites, limestones, gneiss, and marble.