Properties Of Stones
The following properties of the stones should be looked into before
selecting them for engineering
works:
(i) Structure: The structure of the stone may be
stratified (layered) or unstratified. Structured
stones should be easily dressed and suitable for super structure.
Unstratified stones are hard and difficult
to dress. They are preferred for the foundation works.
(ii) Texture: Fine grained stones with
homogeneous distribution look attractive and hence they
are used for carving. Such stones are usually strong and durable.
(iii) Density: Denser stones are stronger. Light
weight stones are weak. Hence stones with specific
gravity less than 2.4 are considered unsuitable for buildings.
(iv) Appearance: A stone with uniform and
attractive colour is durable, if grains are compact.
Marble and granite get very good appearance, when polished. Hence they
are used for face works in
buildings.
(v) Strength: Strength is an important property
to be looked into before selecting stone as building
block. Indian standard code recommends, a minimum crushing strength of
3.5 N/mm2 for any building
block. Due to non-uniformity of the material, usually a factor of safety
of 10 is used to find the permissible stress in a stone. Hence even laterite
can be used safely for a single storey building, because in such structures
expected load can hardly give a stress of 0.15 N/mm2. However in stone masonry
buildings care should be taken to check the stresses when the beams
(Concentrated Loads) are placed on laterite wall.
(vi) Hardness: It is an important property
to be considered when stone is used for flooring and
pavement. Coefficient of hardness is to be found by conducting test on
standard specimen in Dory’s
testing machine. For road works coefficient of hardness should be at
least 17. For building works stones
with coefficient of hardness less than 14 should not be used.
(vii) Percentage wear: It is measured by
attrition test. It is an important property to be considered
in selecting aggregate for road works and railway ballast. A good stone
should not show wear of more
than 2%.
(viii) Porosity and Absorption: All stones
have pores and hence absorb water. The reaction of
water with material of stone cause disintegration. Absorption test is
specified as percentage of water
absorbed by the stone when it is immersed under water for 24 hours. For
a good stone it should be as
small as possible and in no case more than 5.
(ix) Weathering: Rain and wind cause loss of
good appearance of stones. Hence stones with
good weather resistance should be used for face works.
(x) Toughness: The resistance to impact is
called toughness. It is determined by impact test.
Stones with toughness index more than 19 are preferred for road works.
Toughness index 13 to 19 are
considered as medium tough and stones with toughness index less than 13
are poor stones.
(xi) Resistance to Fire: Sand stones resist fire
better. Argillaceous materials, though poor in
strength, are good in resisting fire.
(xii) Ease in Dressing: Cost of dressing
contributes to cost of stone masonry to a great extent.
Dressing is easy in stones with lesser strength. Hence an engineer
should look into sufficient strength
rather than high strength while selecting stones for building works.
(xiii) Seasoning: The stones obtained from
quarry contain moisture in the pores. The strength of
the stone improves if this moisture is removed before using the stone.
The process of removing moisture
from pores is called seasoning. The best way of seasoning is to allow it
to the action of nature for 6 to
12 months. This is very much required in the case of laterite stones.