Qualities of good building stone

Stones

Qualities of good building stones

Followings are the qualities of a good building stone for use:

1. Crushing strength
2. Appearance
3. Durability
4. Facility of dressing
5. Fracture
6. Hardness
7. Percentage wear
8. Resistance of fire
9. Seasoning
10. Specific gravity
11. Texture
12. Toughness index
13. Water absorption
14. Weathering

1. Crushing strength

For a good structural/building stone its crushing strength should not be less than 1000 kg/cm2. Some of the crushing values for different stones are shown below.

Rock	Stone	Crushing Strength in Kg/cm2
Igneous	Basalt	1500 to 1900
	Diorite	900   to 1500
	Granite	800   to 1300
	Syenite	900   to 1500
	Trap	3300 to 3800
Sedimentary	Laterite	20 to 30
	Limestone	550
	Sandstone	650
	Shale	2  to 6
Metamorphic	Gneiss	2200 to 3700
	Slate	750   to 2100

2. Appearance

Appearance of stone is an important property for its facing purpose in construction. Stones should be descent in appearance and passes color uniformly on its surface for a longer time.

3. Durability

For a stone durability is also important as well as its strength. If stone has a greater strength but, not durable for longer period it is of no use at all. So durability is also considered as an important property of a stone for structural use.

Durability is depends upon  various important factors like its chemical composition, texture, location of structure and atmospheric resistance. Followings are important factors which affects durability of stones.

• Heat and cold differences due to change in temperature.
• Alternate wetting and drying due to rain and sunshine.
• Dissolved chemical in the form of gases/liquid in rain.
• Growth of grass, creepers and trees in the stone which makes it cracked.
• High velocity wind.

For  a durable stone its bed should be perfectly horizontal and strong enough to transfer loads coming from upper layers, natural bed of stones should be such that the load will be perpendicular to it.

4. Facility of dressing

Stones can be easily dressed and polished economically.

5. Fracture

Fracture should be sharp, even and clear.

6. Hardness

Coefficient of hardness should be worked out in hardness test, as per result obtained it can be classified as below:

Hardness	Description	Use
<14	Low	No use
14-17	Medium	Use for building works
>17	High	Use for Road and bridge works

7. Percentage wear

Percentage wear can be measure by attrition test. different values stand for its own statement for its use. Percentage wear can be classified as below:

Percentage	Description
<2	Good building stone
=3	Just tolerable
>3	Not satisfactory

8. Resistance of fire

Fire resistance of stones should be such that it can preserve its shape and size during fire attacks on it. Fire resistance of stones are depends upon its chemical composition and alternate heating and cooling. All minerals have its own linear expansion at different degree of heatness. 

Such as quartz expands at temperature <600° C, Limestone resist up to 800° C and then it forms calcium oxide and carbon dioxide. Sandstone with composition of silica can resist fire. Argillaceous stones are poor in strength but having good fire resistance then others. 

9. Seasoning

Stones are not available in the direct usable form that's why we need seasoning operation before its use. Stones available from quarrying operation always contains some moisture in it, this moisture is known as sap. 

Due to moisture presence it can be easily seasoned. Stone dressing and carving done after direct quarrying, and before use of stone it is dried to remove sap from it. Stones are used for structural purposes after seasoning operation.

10. Specific gravity

Specific gravity of building stone should be 2.7 minimum required for structural use.

11. Texture

For building/structural use, stone should be strong and durable enough. As per experts studies the crystalline structure of stone is strong and durable enough.

12. Toughness index

Toughness index of stones can be measured from impact test. different toughness index value can be classified as below:

Index	Toughness
<13	Not tough
13-19	Moderate
>19	High

13. Water absorption

Water absorption is measured by the test and its value should not exceed 0.60% after 24 hours of immersion.

14. Weathering

A building stone in its life period can be attacked by many weathering agencies like rain, wind and temperature etc. It should possess a good weathering resistance towards all this agencies.  

Any one kind of stone is not suitable for all type of weathering and structural purposes. such as there are soft, light and hard stones are available in market, which are of different purposes as shown below:

Type	Use
Soft	Carving
Light	Arches
Hard	High pressure structure

Henceforth, it is necessary to study the purpose of structure to be used and location of use. Except this many other factors such as availability of material, economy, quarry location and transport facility also matters.