Selection of stones

In contemplating the use of stone for various engineering works, the selection of the nature and quality of stone is governed by the purpose in view, cost of stone, its ornamental value and durability Suitability various types of stones for different purposes and situation is briefly discussed below

1. For face work, in general marble, granite and close-grained sand stone are used in the form of thin slabs (veneers) where the structure subjected to adverse weather effects.

2. For pillars, balustrade, pedestals, columns statues and door and window sill and paving stone, granite marble and compact lime stone can be recommend because they can take good polish.

3. For ornamental works such as molding and carvings, fine-grained sand stone, fine grained marble and fine grained granite are used.

4. For bridges, piers, docks, break-waters and other marine structures the stone should be very hard, heavy, strong and durable granite and gneiss are recommended for this purpose

5. For road metal, stones should be hard, tough, resistant to abrasion and durable. Basalt and course-grained granite are generally recommended for this purpose.

6. For railway ballast, the stone should be hard, dense, durable, tough and easily workable sandstone, compact lime stone, trap and quartzite are commonly used

7. In situation like steps, doors sills, pavings etc where there is a regular flow of traffic, stone should be hard, dense, easily workable and durable. Marble, slates and sand stones are commonly use in such places.

8. In fire proof construction, compact sand stone should always be preferred.

Artificial stones: These are also known as cast stones or reconstructed stones. Artificial stones may take up various forms such as

1. Cement concrete: This is the mixture of cement, fine aggregates, coarse aggregates and water. It may be cast in site or pre-cast if steel is used with cement concrete, it is known as reinforced cement concrete.

2. Mosaic tiles: Pre-Cast concrete tiles with marble chips at top surface are known as tiles. They are available in different shades and widely adopted at present.

3. Terrazzo: This is a mixture of marble chips and cement. It is used for bathrooms residential buildings, temples etc.

Advantages of artificial stones:

• Cavities may be kept in artificial stones to convey pipes, electric wires etc.

• Grooves can be kept in artificial stone while it is being cast which are useful for fixing various fittings.

• It can cast in desired shape

• It can be made in a single piece and hence trouble of getting large blocks of stone for lintels, beams etc is avoided.

• It can be made stronger than natural stone

• It is cheap and economical

• It is more durable than natural stone

• Natural bed is absent in artificial stones and hence, the question of taking precautions with respect to the natural bed of stones does not arise.

Aggregates – Grading: Aggregates is derived from igneous, sedimentary and metamorphic rocks or is manufacture from clays, slag etc. The properties of concrete are directly related to those of its constituents and should be hard, strong, durable, and free from clay, loam, vegetables and other such foreign matters. The presence of clay or dirt coating prevents the adhesion of cement on the surface of aggregates and ultimately retards the setting and hardening of cement and reduces the strength, durability and soundness of concrete. Depending upon their size, the aggregates are classified as (i) Fine Aggregative (ii) coarse aggregates.

(i) Fine Aggregates: The material, most of when passes through 4.75mm I.S. sieve size, is termed as fine aggregates. It should not contain more than 1 to 8% of fine particles, which may be obtained from sea, river, lake or pit may be used as fine aggregates but care should be taken all its impurities must be removed

(ii) Coarse Aggregates: The material whose particles are of such size as are retained on 4.75mm, I.S sieve are called coarse aggregates. The size of the coarse aggregates used depends upon the nature of work. The maximum size may be 23mm for mass concrete such as dams etc. and 63mm for plain concrete. Crushed hard stone and gravel is the common materials used as coarse aggregates for structural concretes. Coarse aggregates usually obtained by crashing granite, gneiss, crystalline lime stone and good variety of sandstone etc.

Grading of Aggregates:

Grading of aggregates consists of proportionating the fine and coarse aggregates in such a ratio, so as to get strongest and densest mix with the least amount of cement. Grading the aggregates is so graded as to have minimum voids when mixed with all ingredients, and water should render a concrete mass of easy workability. The grading of aggregates are done by the following methods

(i) By trail – In this method, proportionating of aggregates as to give heaviest weight for same volume, yield the densest concrete

(ii) By finesse modules method (sieve analysis method): in this method, the samples of both coarse and fine aggregates are passed through a set of nine standard sieve and the percentage of sample retained on each of the said sieves is determined. The total of these percentages divided by 100 gives the finesses modulus of sample

(iii) By minimum voids method: This method is based on the fact, that so obtain dense concrete the quantity of cement should also be slightly in excess of voids more that the fine aggregates. In this method the voids in the fine and coarse aggregates are separately found out with the help of graduated cylinder and water. The percentage of voids I aggregate, “X” given by the equation.

X = {(V1 – V2) x 100}/V2

Where v1, volume of water filled

Where v2, volume of aggregates.

(iv) By arbitrary standards: It is a commonly adopted method of propitiating the aggregates in a concrete mix for small works of moderate importance. This method is not recommended for large works or important works in this method, the volume of cement, sand and coarse aggregates are taken in the proportion of 1:n:2n respectively. The quantity of water to be used a varied suit the workability descried.

Ex: 1:1:2 M250 rich mix for columns, beams

1:1:3 – M200 Water retaining structures etc

1:3:6 – M150 slab’s columns roads etc

1:3:6 – M100 – foundations,

1:4:8 – For mass concrete.