Post 464 –by Gautam Shah
Stone craft consists of many distinct trades, like quarrying, handling-transporting, sizing, cutting, dressing, finishing and masonry work. The first stone craft was using the stone to make tools for working with materials. A right choice of stone quality and appropriate size-shape were important then, and continue to be so today.
Stone is used for many different purposes.
- as industrial raw materials for minerals,
- as a constituent material in various composites,
- in buildings, for masonry, flooring and applique work,
- as an art and craft material.
Strength of a stone is checked for following types of stresses:
● Compressive stresses, tend to decrease the volume of the material, causing breaks with a shattering effect.
● Tensile stresses, produce cracks and fissures, and torsion (or twisting). Generally, fine-grained rocks are stronger than coarse grained. Rocks with interlocking between the crystals are stronger than rocks with poor interlocking. Stratified rocks have poor strength along the plane or strata. Stratified rocks as a rule have lower strength than igneous and non-stratified homogeneous rocks.
● Shear stresses, which move one part of a stone with respect to another, under certain conditions, inducing a permanent change of shape. These are best avoided by appropriate angle of extraction and cut, by careful orientation during coursing a masonry.
● Torsional stresses are important for structures of stones such as piers. Heat induced stresses were once critical for structures like fire places and hearths, but optional materials have obviated that as the criteria of design.
The general requirements for stones used in Buildings can be summarized as follows:
■ Sound, uniform rock material.
■ Presence of rifts to facilitate workability by hand tools.
■ Porosity advantageous for cementing, provided it does not decrease the resistance to weathering.
■ Inherent chemical stability to prevent fluorescence.
■ High strength (as required in certain cases).
■ Low specific gravity (necessary for easier handling and in light weight structures).
■ High abrasion resistance (an important factor for flooring, steps).
Masonry walls of stones require specific methods of construction such as:
1 Heaviest and thickest of pieces should be used for lower courses.
2 Small pieces of stones should not be used on outer face.
3 Best flat face with a smallest area should form the wall face.
4 Each stone must rest on a flat surface, if required flat face should be achieved preferably by dressing of the stone, by bedding material or mortar, or by use of splinters and wedges.
5 Wedges should be placed with their wider face on the inside and narrower face on the outside.
6 All loose particles, cleavages, layers should be removed before using a stone.
7 Joints must be staggered.
8 In case of very thick walls, if more than two stones form a width, several full width stone should be employed for keying.
9 For all walls especially random masonry, the corners should be made of long rectangular stones of even thickness (preferably dressed).
10 Stratified stone materials should be used for compressive loads to occur across the section or strata.
11 For tension bearing areas stratified and sedimentary stone material should be avoided.