Building Stone

Building Stone

A rock to be used as rubble for masonry has to satisfy certain requirements. The most important are

Engineering properties

Compressive strength –

The capacity of a rock to stand load . When a rock is crushed under load the failure may takes place in two ways .

a)     The constituent mineral ( mineral composition ) grain themselves may be crushed. b) There may be a differential movement between mineral grains which themselves remain intact individually .( nature of bond between the mineral grains I. e. Texture.

                       The resistance a rock will offer to the crushing of first type will depend on the resistance that constituent mineral  particle will offer. It is directly proportional to hardness and sp. Gravity of mineral .The harder and heavier the mineral in a rock the stronger it is .

Eg. 1  Granite 1600kg/cm2 (quartz, feldspar )  Hard

1)    Marble 600kg/cm2 ( cacite )    Soft

The resistance to the second type of crushing will depend on the strength of the bond between the mineral particle and the freedom they have to move against each other.

a)     In porous rock because of empty spaces grains will have considerable freedom to slide over each other.

b)    Nonporous rocks –Grains are closely packed together without spaces . Therefore little freedom to move each other.

Therefore non porous rocks are stronger than porous rocks . Fine grain rocks are stronger than coarse grain rock . Better consolidated rock stronger it is.

If any planes of weakness in a rock of building stone , bedding planes, plane of schistocity , joints , faults slaty cleavage, etc. Pressure acting along them will bring movement along them where as pressure acting at right angle to them will not bring movement . Poor crushing strength in direction ogf such weak planes and greater strength at right angle to them.

Crushing strength should not be made the criterion for choosing between two competent rocks and there is no point in preferring a rock with a very high crushing strength to a rock with a comparatively lower strength.

Eg. Deccan trap area compact basalt have a strength of up to 2000kg/cm2 while amygdaloidal basalt have strength only up to 600kg/cm2

Eg. The load at the base of a 100m high dam will be of the order of only 30kg/cm2 and will be much less in case of building and other structures. Any rock that is hard enough for being considered suitable for engineering purpose will be competent to stand this load. No consolidated rock will generally have a strength of less than 500kg/cm2 .

Rock which locally available with required strength there is no justification to bring from long distance of higher crushing strength rock .

It is essential not to accept without question the test reports from lab but to checks whether they are consistent or not with the litho logical characters. If they are not the results will have to be  rejected and testing repeated. ( due to faulty testing machines and procedure.)

Such misleading results from testing can be avoided if simple precaution are observed.

Maximum force expressed per unit area which stone can with stand is called compressive strength. Any force beyond the compressive strength will cause failure of the stone.

Co=    p/A        Co=  compressive strength     P=   load of failure 

 A =area of cross section.

The crushing strength of rock depend upon no. of factors such as

a)     Its mode of formation

b)    Its composition

c)     Texture

d)    Structure

e)      Its moisture content

f)      Extent of weathering.