REQUIREMENTS OF GOOD BUILDING STONES

The following are the quality requirements of good building stones:

1. STRENGTH

Generally most of the building stones have high strength to resist the load coming on it. Therefore it is not of prime concern when it comes to check the quality of stones. But when the stones are to be used in large structures, it becomes necessary to check the compressive strength of stones.

Compressive strength of building stones generally fall within the range of 60 to 200N/mm2.

2. DURABILITY

Building stones should be capable to resist the adverse effects of natural forces like wind, rain and heat. It must be durable and should not deteriorate due to the adverse effects of the above natural forces.

3. HARDNESS

When stones are used in floors, pavements or aprons of bridges, they become subjected to wearing and abrasive forces caused by movement of men or machine over them. So it is required to test hardness of stone.

Hardness of stone is determined by Mohs scale.

4. TOUGHNESS

Toughness of stones means it ability to resist impact forces. Building stones should be tough enough to sustain stresses developed due to vibrations. The vibrations may be due to the machinery mounted over them or due to the loads moving over them. The stone aggregates used in the road constructions should be tough.

5. SPECIFIC GRAVITY

The more the specific gravity of stone, the more heavier and stronger the stone is.

Therefore stones having higher specific gravity values should be used for the construction of dams, retaining walls, docks and harbors. The specific gravity of good building stone is between 2.4 and 2.8.

6. POROSITY AND ABSORPTION

Porosity of building stones depend upon the mineral constituent and structural formation of the parent rock. If stones used in building construction are porous then rain water can easily enter into the pore spaces and cause damage to the stones. Therefore building stone should not be porous.

Water absorption of stone is directly proportional to the porosity of rock. If a stone is more porous then it will absorb more water and cause more damage to stone.

In higher altitudes, the freezing of water in pores takes place and it results into the disintegration of the stone.

Permissible limits of water absorption for some the commonly used building stones are as follow.

7. DRESSING

Giving required shape to the stone is called dressing. It should be easy to dress so that the cost of dressing is reduced. However, the care should be taken so that, this is not be at the cost of the required strength and the durability.

8. APPEARANCE

In case of the stones to be used for face works, where appearance is a primary requirement, its colour and ability to receive polish is an important factor.

Light colored stones are more preferred than dark colored stones as the colour are likely to fade out with time.

9. SEASONING

Good stones should be free from the quarry sap. Lateritic stones should not be used for 6 to 12 months after quarrying. They are allowed to get rid of quarry sap by the action of nature. This process of removing quarry sap is called seasoning.

10. WORKABILITY

Stone should be workable. Stone is said to be workable when the work involved in stone working (such as cutting, dressing & shaping) is economical and easy to conduct.

11. COST

Cost is an important consideration in selecting a building material. Proximity of the quarry to building site brings down the cost of transportation and hence the cost of stones comes down.

12. FIRE RESISTANCE

Stones should be free from calcium carbonate, oxides of iron, and minerals having different coefficients of thermal expansion. Igneous rock show marked disintegration principally because of quartz which disintegrates into small particles at a temperature of about 575°C. Limestone, however, can withstand a little higher temperature; i.e. up to 800°C after which they disintegrate.

Adsorbed water

The term ‘Adsorbed water’ as it applies to the area of reclamation can be defined as ‘ Water in a soil or rock mass, held by physico-chemical forces, having physical properties substantially different from absorbed water or chemically combined water, at the same temperature and pressure’.

How affects the permeability :

Fine particles of clay are surrounded by films of adsorbed water. Forces of adsorption and development of diffuse ion-layer around the clay particles create immobilized hydrodynamic layers of water, thereby reducing the effective pore space available for seepage.