Building Materials AMU (2005)

Chapter II

BUILDING STONES

2.1 Introductions

Building stones are naturally occurring rocks that make up the crust of the earth. Some varieties
of these rocks have been found very suitable for use in building construction. Those types of
stones that possess such properties which make them suitable for building construction are
defined as Building Stones. Thus, they may be used in foundations, in walls, as column, stairs
and as sill, in flooring and even in roofing. Since they are:
(i) Naturally available in abundance,
(ii) Very hard, strong and durable,
(iii) Suitable for shaping
Stones have been used extensively in construction in almost all the countries of the world since
the advent of civilization.

2.2 Classification of stones

Rock from which stones can be extracted for building purposes are classified in the following
three different forms:
i. Geologically (depending up on how the rocks are originally formed)
ii. Physically (depending up on what its structure is)
iii. Chemically (depends up on its chemical composition)

1.2.1 Geological Classification:

Geologists classify rocks in to three categories:

a. Igneous, primary, un-stratified or eruptive rocks.
Inner layers of earth are at such high temperatures that masses of silicates melt. This molten
mass, known as magma, is forced up. Magma solidifies in to rocks when it reaches the
surface of earth and forms basalt and Traps. If however, the magma solidifies before reaching
the surface of the earth it forms solid crystalline rock known as Granite

b. Sedimentary, Aqueous, or Stratified rocks.

The surface of earth is subject to the destructive action of rain, frost, wind, and chemical
actions. These destructive agents break up the surface of earth, which gets further broken up
when carried down by rain and rivers. In the journey of river from mountains to sea the
velocity goes on decreasing as the river moves further. As the velocity of river flow
decreases, debris are deposited which carried by it. The heaver ones being the first to be
deposited followed by smaller and finer ones. Sand and silt get deposited in the voids of
deposited debris. The latter have binding properties in layers. Due to seasonal variations the
materials are deposited in layers. The deposit in layers continues for million of years in which
period the deposited layers subjected to enormous pressure of overlying layers and flowing
water get consolidated forming stratified, sedimentary or aqueous rocks. Sandstones and lime
stones belong to this type of rock.

Lecture Notes (Chapter II)

Building Materials AMU (2005)

Sandstones are solid and generally composite rocks which consist of grains of sand cemented
together by natural cementing agents such as clay, iron oxide, lime carbonate or silica. They
are the result of the consolidation of sand beds, which either contained the cementing
substance or were impregnated with it during the process of solidification. Hence the major
constituent mineral of sandstones is quartz, making as much as 95 percent of the whole rock.
The rest is the cementing material, gravel, broken shells and other minerals. Quartz being
essentially strong, hard and durable, the quality of particular sandstone depends, to a large
degree, on the type of the cementing agent present in the rock. The poorest of the cementing
agents is lime carbonate, which, although strong when dry, disintegrates in water. Clay
cement readily absorbs water and there by weakens the resistance of the stone to freezing.
Perhaps the strongest binder is silica, which normally renders the stone hard and difficult to
work.

Limestone is formed from masses of the remains of sea animals, corals, fossil shells and other
organisms, which have accumulated in the sea bed. Most of these remains consisting largely
of calcium carbonate, limestone is made of inter locking crystals of calcite in which are
embedded shells and other deposits. In most limestone, the lime plus magnesium carbonate
content exceeds 75 percent; however, when the silica content is high the stone might be
classified as sandstone whereas if its clay content is high it becomes shale. In its pure form,
limestone is white, generally however, it is found in brown, reddish-brown or other color
depending on its content of clay, iron oxide or other impurities.

c. Metamorphic Rocks. (Meta: change, morhp: shape)

These are originally either igneous rocks or sedimentary rocks which have undergone some
change in their structure, shape and composition. The change might have been due to an
increase in temperature or pressure or both. Sometimes, the change is due to some
chemically active fluids that act on the pre-existing rocks.
Following are some of the changes to “Metamorphic rocks”.
 Granite (igneous) change to Gneiss
 Sandstone (sedimentary) changes to Quartzite.
 Limestone (sedimentary) changes to Marble
 Shale (sedimentary) changes to Slate
Marble is formed when a limestone crystallizes through the combined effect of intense heat
and pressure. The impurities that are present in the limestone or those that are introduced
during the change give marble a variety of colors and make it one of the most attractive
building materials when polished.

2.2.2 Structural (Physical) Classification.

In these classification stones are grouped into three broad classes depending on the
presence and absence of layered structure:
1. Stratified (layered) rocks in these rocks very easily observed layered structure is the
dominant quality. The layers may be thin (1cm or so) or thick (1m & above) and the
same or different colour and composition. Most of the sedimentary rocks are classified
in this nature. Example: Slate, Sandstone and limestone
2. Unstratified (massive) rock, these types of rocks is free from any stratification and
cannot be easily split in to thin layers. Most igneous rock grouped under this class.
Example: Granite, Basalt and Trap

Lecture Notes (Chapter II)

Building Materials AMU (2005)

3. Foliated: These rocks generally belong to metamorphic group where a layered structure
has been induced under lateral pressure. In a foliated rock, the layers are easily
separable as the cohesion between the adjoining layers are least, sometimes negligible.
Best example of a foliated rock is SLATE (metamorphic). Gneisses and schists also
show foliation.
All sedimentary rocks are essentially stratified where as all igneous rocks are unstratified.
Metamorphic rock may be either stratified or unstratified depending up on the type of rock that
has undergone transformation

2.2.3 Chemical Classification.

On the basis of dominant Chemical composition, following three classes of stones are commonly
recognized:

I. Siliceous rocks - These stones have silica (Si02) as the predominant component. Some
sedimentary and metamorphic rocks like Quartzite may be made up almost entirely (98-100%) of
silica. Naturally, they are considered among the strongest building stones. Some igneous and
metamorphic rocks like Granites and Gneisses respectively have predominantly siliceous
composition. When free from decomposed minerals and micas, these rocks also form excellent
building stones.

ii. Calcareous rocks. In these rocks the dominant component is a carbonate generally of
Calcium and also of Magnesium. Most commonly they belong to sedimentary and
metamorphic groups of geological classification. Best-known examples of calcareous rocks are
Limestone, Dolomites and Marbles.

iii. Argillaceous rocks. They are mostly sedimentary and metamorphic rocks having clay
(Hydrous Alumino-silicate of Ca, Mg, K etc) as the predominant component. The sedimentary
varieties are known as clay stones, siltstones and shales. They are generally soft and disintegrate
easily in the presence of water resulting into muddy slush and hence not used as building stones.
The metamorphic varieties are phyllites and slates. These represent thoroughly baked and
indurate shales. They are quite hard and brittle but being invariably thin-layered are of only
limited use in building construction.

2.3 Criteria for Selection of Building Stones

Three factors generally enter into the consideration of an engineer while deciding the use of
stones in construction.

1. The situation in the building where the stone is used, such as in the foundations, for flooring
and outside pavements, for columns, lintels or for load-bearing walls or partition walls or for
roofing.
2. The availability of varieties of stones within a reasonable distance from construction site so
that he/she is in a position to make a choice in accordance with the situation decided as
above.
3. The cost of the stone, which will involve cost of extraction, cost of transport to the site,
dressing of stone to the required shape and cost for its handling and placement.

Lecture Notes (Chapter II)

Building Materials AMU (2005)

When the engineer is satisfied that he/she can use a building stone in the construction in hand,
he/she will study the various rocks available in the area. For making a final choice, he/she will
generally look for a set of qualities in the stone.

2.3.1 Qualities for selection

Despite the abundance of rocks, relatively few stones satisfy the requirements as building stones.
The important requirements are:
 Strength  Durability  Ease of quarrying
 Hardness & Toughness  Colour and grain  Accessibility
 Workability  Porosity

(a) Strength: Rocks generally possess very high compressive, tensile and bending strengths. But
no fixed values should be assigned to the strength of rocks as they vary considerably within a
wide range. Thus, ultimate compressive strength of igneous rocks may vary between 1400-2100
Kg/cm2 that of sedimentary rocks between 200 to 1400 Kg/cm2 and so on. The actual values
depend on the mineral constitution, texture, structure and degree of deterioration on surface.

(b) Hardness and Toughness.

Hardness: next to strength, the property of hardness of a stone is considered in its selection.
Hardness of a stone may be defined as its capacity to resist scratching or abrasion. It depends on
the mineral composition of the stone. Stones easily scratched with a knife (e.g. limestone)
whereas others will not scratch by it (e.g. sandstones & granites).
Toughness: is a property related to hardness and strength. It is defined as the capacity of a stone
to resist the impact load. It is more important a quality for stones when they are used in road
construction. In building construction, stones used as foundations under heavy machines where
vibrations are a common phenomenon must not only be hard and strong but very tough as well.

(c) Water Absorption. This defines the capacity of stone to absorb water. It is expressed in
percentage terms of dry weight of the stones. It varies between 0.5 to 20 per cent in different
stones. Sandstones and limestone show high absorption values. Marbles and granites have
negligible absorption values. The property becomes very important when the stone has to be
used in a cold humid climate in-exposed condition. Stone is liable to deteriorate quickly in this
case due to frost action.

(d) Appearance. Stones are available in nature in almost all the colors-from milk white to blood
red to pitch black. Naturally, appearance of a stone also becomes an important factor in its
selection. It has it is own aesthetic value. The capacity to take polish is important qualities of
building stones. These depend on their composition. Granites and marbles are excellent in this
regard.

(e) Workability. The capacity of a stone to get dressed at economical costs is called its
workability. Very hard stones are difficult to work and hence costly.
(f) Durability: The life span of a stone for which it can stand in a building defines its durability.
Stones show great variation in this regard. Durability depends on the type of stone and the
environment in which it has been used and also the extent of loading. When used with careful
consideration of these factors, stones withstand the natural forces and the loads imposed on them

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Building Materials AMU (2005)

for hundreds of years. Tests for “Weathering resistance” broadly indicate the durability of a
stone.

2.4 Quarrying (Production of stones)

The art of taking stones of various sizes from natural rocks is known as quarrying.
A quarry is a place where this process (quarrying) is carried out. It is always a “on the surface"
process. The purpose of quarrying is to obtain stones for:
 Masonry
 Ballast in concrete
 Road construction or for any purpose.
A quarry site should be at a place where:
i. Rock of sound quality is available in abundance
ii. Populated areas are at a safe distance
iii. Main road is not very far off
iv. Water is available in good quantity
Depending up on the nature of rocks and the purpose for which stones are needed, quarrying is
done by one of the following methods:

 Excavating
 Wedging
 Heating
 Channeling

Blasting or in other words, quarrying is carried out either without blasting or by blasting. In
quarrying without blasting for example, the wedge method and the channeling method may be
applied. In the first case a steel wedge is hammered in the rock to create cracks into which steel
bars are inserted and the stones blocks separated. In the channeling method, grooves are cut by
power driven machines to separate big blocks of proper dimensions that are lifted through cranes.
Quarrying by blasting requires drilling of blast holes of calculated dimensions and at
predetermined places in the rock. These are charged with the explosives of suitable quality in a
carefully selected manner. The shots so prepared are ignited which explode within the body of
the rock and thus break it into parts and are thrown at distances that depend on the quantity and
quality of the explosives used in the shot. Blasting powder. Dynamite and blasting gelatine are
some of the common explosives used in quarrying.

Lecture Notes (Chapter II)

2.5 Dressing of stones (Stone Surface Finishes)

It implies giving a proper shape to the rough blocks of rocks for use in building
construction. The type and extent of dressing depends on the nature of rock and proposed
use of the stone. Stones used in front portion of buildings may need smoothening and even
polishing. Stones used in walls may need only making their shapes regular. Pitched
dressing, hammer dressing, chisel drafting, rough tooling, punched dressing and boasted
finish are some forms of dressing used in Building Stones.

2.6 Stone Construction

When used as facing, stonework may be divided in to four general categories:

 Paneling
 Veneer
 Trim
 Stone flat work
Veneer may be applied in ashlar pattern or as rubble.

Panelling
Paneling, some times known as dimension stone, consists of slabs of stone cut to dimension
and thickness and drilled and grooved as required, to cut as curtain-walls panels and provide
a finished exterior. They may be supported by bond stones or shelf angles and tied to
backup walls of concrete, brick tile, or blocks, or they may be supported on a steel frame.

Veneer
Stone veneering involves the use of relatively small pieces of various thicknesses, fastened
to the surface of a backup wall, such as block, brick tile, or concrete with some type of
stone anchor. Ashlar requires the use of cut stone and includes broken ashlars, irregular
coursed ashlars and regular coursed ashlars. Support is provided in much the same way as
for stone panels, and galvanized-wire ties provide anchors to the back- up wall
Two styles of rubblework are used: random and coursed. In the first case no attempt is made
to produce either horizontal or vertical course lines. Spaces too small for a regular stone are
filled with spalls, and bond stones must be provided for structural bonding unless ties are
being used for this purpose. In course rubblework, horizontal course lines are maintained,
but no vertical course lines are used.

Stone Trim
Stone trim refers to those pieces of cut stones, which are used for such specific purposes as
base, window stool, stair threads, copings, and similar items that are normally regarded as
finishing.

Stone Flatwork
Stone floors, walks, and patios are made by covering a base of stone, concrete, brick or tile
with flagstones. They may be random flagstones, trimmed flagstones, trimmed rectangular,
or square and rectangular. Random flagstones consist of natural irregularly shaped pieces
laid with out any attempt at pattern. Trimmed flagstones consist of natural random pieces, a
percentage that have one or two edges saw trimmed. Trimmed rectangular flagstones have
four straight sawed edges with right angle corners but are of no specific dimensions. Square
and rectangular flagstones have straight edges, right angle corners and are cut to specific
sizes.

Common building stones

Following rocks are used commonly for building construction where -easily available:
Granites, Dolerites, Syenites, Limestones, Sandstones, Quartzite, Gneisses, Marbles, Slate.

2.7 Deterioration and preservation

Stones, like many other building materials, deteriorate with time. The rate of deterioration
is fast in some environments. Commonly, stone decay is related to frost action, temperature
changes and, chemical corrosion by natural gases and artificial vapours as in industrial
towns. Hence steps are necessary to preserve them from the bad effects of these agents.
This may be achieved at the first place by careful selection of the stone and secondly by its
careful use in construction. The life of a stone can be increased in difficult situations by
applying preservatives on it, which can protect it from the direct attack of the deteriorating
agency. Among the preservatives successfully applied in the case of building stones may be
mentioned: Linseed oil, Paraffin, Coal Tar, Baryta solution and various types of paints.
These preservatives are used selectively depending on the nature of protection required and
the type of stone used.