Materials of Construction
1.1 STONES
Rocks from which stones are obtained may be classified in the following three ways:
1. Geological classification
2. Physical classification
3. Chemical classification.
1. Geological classification On the process of formation, rocks are classified as igneous rocks,
sedimentary rocks and metamorphic rocks.
(a) Igneous Rocks: These are the rocks formed by cooling of magma. These rocks are strong and
durable. They are further classified as:
Volcanic rocks: Cooling of magma at earth's surface - extremely fine grained and glossy.
Examples: Basalt and trap.
Hypahyssal rocks: Cooling of magma at shallow depth fine grained crystallized structure.
Examples: Quartz, dolerite and gneiss.
Plutonic rocks: Cooling of magma at considerable depth very strong and crystalline structure.
Examples: Granite and dolerite.
(b) Sedime ntary Rocks: Disintegrated rock material is carried by flowing water and deposited
elsewhere. Year after year new layers of materials are deposited and consolidated under pressure,
heat and chemical action. Hence, the rocks so formed are uniform, fine grained and bedded.
Examples: sandstones, mudstones, limestones.
(c) Metamorphic Rocks: These are the rocks formed after igneous rocks and sedimentary rocks
undergo changes due to pressure, heat and chemical actions. These rocks have foliated structure.
For example:
Granite becomes gneiss.
Basalt changes to schist and laterite
Limestone changes to marble
Mudstone becomes slate.
2. Physical classification Based on the structure, rocks may be classified as stratified, unstratified
and foliated rocks.
(a) Stratified Rocks: They have layered structure. Examples: Sandstones, limestones, mud-stones.
(b) Unstratified Rocks: They possess crystalline and compact grains. Examples: Granite, trap,
marble.
(c) Foliated Rocks: They have foliated structure. Along the planes of foliation they split easily.
These planes are not parallel to each other.
Downloaded From: www.EasyEngineering.net
3. Chemical classification
76
(a) Siliceous Rocks: Main constituent is silica. These rocks are hard and durable. Exampfes: Granite
trap, sandstone.
(b) Argillaceous Rocks: Clay is the main constituent. These rocks are brittle. They cannot withstand
shock. Examples: Slate, laterite.
(c) Calcareous Rocks: Calcium carbonate is the main constituent. Examples: Limestone, marble.
1.2 CHARACTERISTICS OF GOOD BUILDING STONES
1. It should possess fine grained structure, uni form and pleasing colour. It should be free from soft
patches, flaws and cracks.
2. The minimum strength of 3.5 N/mm is required for stone to be used for load bearing walls.
3. It should not absorb more than 5% water.
4. The specific gravity should not be less than 2.5.
5. In attrition test, it should not show wear of more than 2%.
6. Toughness index should be more than 10.
7. It should have good resistance to shocks and it should be durable.
Quarrying It is the process of extracting stones from rockbed, located near the earth's surface and
exposed to sun.
Quarrying may be done using hand tools, channeling machine or by blasting
(a) Quarrying using hand tools: In soft rocks hand tools like chisels, hammers, pick axes and
shovels may be used. Heating technique is suitable for getting aggregates. Wedging technique is
suitable for quarrying in thin bedded rocks.
(b) Quarrying using channelling machine: This is suitable for large size quarrying in soft rocks
like marble and limestone. Channels are 50 to 75 mm wide and 2.4 to 3.6 m deep. After
channelling with machine wedges and drills are used to get stones.
(c) Quarrying by blasting: It consists of boring, charging, tamping and firing. The gas produced in
the blast tries to come out by breaking the rock in all directions and succeeds in escaping along
the Line of Least Resistance (LLR).
Dressing of stones The aim of preliminary dressing is to bring the size of stone approximately to the
required size and reduce the transportation cost to great extent. Final dressing is as per the
requirement of the user. The different methods of final dressing are:
(1) Hammer face; (2) Chisel drafted face; (3) Polished face and (4) Tooled finish.
Seasoning of stones The process of removing sap from the pores is known as seasoning. The best
method of seasoning is to allow the stones to dry for a period of 6 to 12 months in a shed.
Preservation of stones The following preservatives are used:
(1) Linseed oil; (2) Solution of alum and soap; (3) Solution of barium hydroxide; (4) Coal tar; (5)
Paint and (6) Paraffin
Tests on stones
1. Smith's test: It is to check whethen'thestone continšninuedely nstbstance. A sample of stone is kept
in a glass of distilled water for 24 hours and stirred well to remove muddy substance.
2. Toughness test: Stone is hit with a hammer and metallic sound is indication of strong stone.
3. Hardness test: Mohr's hardness may be found by scratching the stone with knife.
Laboratory Tests
1. Crushing strength test: 40x 40 x 40 mm cubes are used for testing. Load is applied in a
compression testing machine at the rate 14 N/mm per minute. A stone with crushing strength more
than 100 N/mm is treated as good stone.
2. Water absorption test: For good stone water absorption after 24 hours immersion in water,
should not exceed 0.60.
3. Attrition test: Los Angeles abrasion test is conducted to find the resistance of the stone to surface
wear. Los Angeles value recommended are (a) for bitumen mix: 30%, (b) for base course 50%.
4. Impact test: Impact testing machine consists of a frame with guides. A metal hammer weighing
13.5-15 kg falls froma height of 380 mm. Recommended values for various works are
For wearing coat 30%
For bituminous mechadam» 35%
For water bound mechadam 40%
5. Acid test: Sandstones are checked for the presence of calcium carbonate. Sample of 50-100 gm of
stone is kept in 1% hydrochloric acid for seven days. If calcium chloride is present, edges are
broken and powder is formed.
Common building stones Basalt, granite, sandstones, limestones, marble, quartzite, laterite and
slate. Solid and hollow concrete blocks are known as artificial stones.
Comments
Post a Comment