About Bricks
Bricks are the only man-made building materials that testify to their use since the early human civilization. With their attractive appearances and superior properties such as high compressive strength and durability, excellent fire and weather resistance, good thermal and sound insulation, bricks are widely used for building, civil engineering work, and landscape design.
Classification of Bricks
Clay bricks can be classified according to their varieties, qualities, and
classes.
Common Bricks
Common burnt clay bricks, which are accepted for use in general brick work with
no special claim for attractive appearances. Walls built with common bricks
require rendering or plastering.
Facing Bricks
Quality burnt clay bricks, which give attractive appearance in their color and
texture. It is used without rendering, plastering, or other surface treatments.
Loadbearing Bricks
Loadbearing bricks, which can be either common or facing bricks, conform to
specified average compressive strength limits depending on their classes as
given in table below.
Class
|
Average Compressive Strength
|
N/mm2
|
P.S.I.
|
1
|
7.0
|
1,000
|
2
|
14.0
|
2,000
|
3
|
20.5
|
3,000
|
4
|
27.5
|
4,000
|
5
|
34.5
|
5,000
|
7
|
48.5
|
7,000
|
10
|
69.0
|
10,000
|
15
|
103.5
|
15,000
|
* Based
on British Standard 3921:1965
|
Engineering Bricks
Engineering bricks are bricks burnt at exceedingly high temperatures. They
possess a dense and strong semi-vitreous body and conform to the defined limits
for strength and water absorption. They are primarily used in civil engineering
works that require high load bearing capacity, good damp-proof, and chemical
resisting characteristics.
Engineering
|
Average Compressive Strength, (No less
than)
N/mm2 U.S.A.
|
Average Water Absorption, %
(No greater than)
|
A
|
69.0 (10,000 psi)
|
4.5
|
B
|
48.5 (7,000 psi)
|
7.0
|
* Based
on British Standard 3921:1965
|
Damp Proof Course
Clay bricks of specified low water absorption used at the base of a wall
(minimum two courses) to resist the upward movement of ground water. Their use
is recommended for free standing wall where otherwise a sheet of DPC material
would create a plane of weakness causing the wall to be vulnerable to lateral
forces.
Properties and Functional
Performances of Brick
Bricks are made from clay by burning it at high temperatures. The action of
heat gives rise to a sintering process that causes the clay particles to fuse
and develops extremely strong ceramic bonds in the burnt clay bodies. Such
bonds are highly stable. As a result, bricks can withstand the severe
weathering actions and are inert to almost all normal chemical attacks.
Strength
Bricks are well-known for their high compressive strength. Their compressive
strength depends on:
1. the raw materials
used,
2. the manufacturing
process, and
3. the shape and size.
Bricks made by a de-aerated extruder and fired to sufficiently high
temperature can easily withstand a compressive pressure exceeding 28 N/mm2 (4,000
psi). They are suitable for almost all structural building applications
Aesthetic appeal
Brick possesses the natural and pleasant colours of burnt clay. Its colour
formation is achieved through a complicate physical chemical reaction during
the firing process. In contrast to colour of stained body, brick colour is
permanent and will not be faded during weathering process. Different clay compositions,
firing temperatures or kiln atmosphere can lead to different colours of the
burnt products. By proper control of these factors, bricks can be made to
exhibit endless variety of natural and attractive colours.
Besides its richness in colour, bricks can be made to various textures. It is
the combination of colour and texture that gives brick such distinctive feature
which is everlasting and meadows with age. In view of the high cost to maintain
the appearance of a building, the unique features of brick become an
unparalleled advantage to housing design.
Porosity
Porosity is an important characteristic of brick. In contrast to other moulded
or pre-cast building materials, the porosity of brick is attributed to its fine
capillaries. By virtue of the capillary effect, the rate of moisture transport
in the brick is ten times faster than in other building materials. Moisture is
released during day-time and re-absorbed during night-time. The ability to
release and re-absorb moisture (a "breathing" process) by capillary
effect is one of the most useful properties of brick that helps to regulate the
temperature and humidity of atmosphere in a house. This distinctive property
makes brick an admirable building material, particularly suitable for houses in
the tropics. On the other hand, all porous materials are susceptible to
chemical attacks and liable to contamination from weathering agents like rain,
running water and polluted air. Porosity of building material is an important
factor to consider in respect its performance and applications.
Experiment results show that bricks with water absorption rate at 8% is 10
times more durable in resisting salt attack than that with water absorption
rate at 20%. Well burnt brick has a normal water absorption rate less than 10%
in contrast to that of concrete block and cement mortar exceeding 15%. This
explains why brick walls require comparatively minimum maintenance in the
course of time.
To mitigate the adverse effects but at the same time retain the advantages
associated with porosity, the rate of water absorption of facing bricks for
masonry brickwork should preferable be maintained around 10%.
A rarely known property of brick is its initial rate of absorption (IRA). It is
in fact the initial rate of absorption that plays a key role in affecting the
strength of bond between bricks and mortar during bricklaying. High value of
IRA tends to remove excessive water from the mortar rapidly and thus hampers
the proper hydration of cement . Experiments show that and an increase of IRA
from 2 kg/m2/min to 4 kg/m2/min reduces the strength of
brickwork by 50%. Generally, bricks with IRA exceeding 2 kg/m2/min
will gives rise to difficulties in laying using common cement mortars. Modern
brick extruder with de-airing action produces denser brick with lower IRA.
Fire Resistance
Brick is inherent with excellent fire resistance. A 100 mm brickwork with 12.5
mm normal plastering will provide a fire-resistance of 2 hours and a 200 mm
non-plastered brickwork will give a maximum rating of 6 hours for non-load
bearing purposes. Brick can support considerable load even when heated to 1000oC
in contrast to concrete wall at only up to 450oC due to loss of
water of hydration.
It is a fact that the non-combustibility of brick helps to promote its use in
building houses against fire. There have been numerous examples in the past
that people chose to use bricks for their houses after a devastating fire that
burned down the whole city. Perhaps the most famous instance is the great
London Fire in 1666, after which the rebuilding was largely done if not
entirely in brick.
Sound Insulation
Brick wall shows good insulation property due to its dense structure. The sound
insulation of brickwork is generally 45 decibels for a 4-1/2 in. thickness and
50 decibels for a 9-in. thickness for the frequency range of 200 to 2,000 Hz.
Thermal Insulation
Brick generally exhibits better thermal insulation property than other building
materials like concrete. Perforation can improve the thermal insulation
property of bricks to some extent. Besides, the mass and moisture of bricks
help to keep the temperature inside the house relatively constant. In other
words, bricks absorb and release heat slowly and thus keep the house cool
during daytime and warm during nighttime.
Energy saving of a brick house is remarkable. A study commissioned by the Brick
Institute of America had demonstrated that a brick house can save energy up to
30% when compared to that built of wood.
A comparison of the thermal conductivities of various materials is given in
table below:-
Typical Thermal Conductivities of Various
Building Materials
|
Material
|
Btu/(sq.ft.-hr-F/in.)
|
W/mK
|
Sand
& gravel aggregate (dry)
|
9.0
|
1.30
|
Cement
Mortar
|
5.0
|
0.70
|
Concrete
(1:4)
|
5.28
|
0.77
|
Concrete
Block (1:5) (four Oval-core)
|
5.2
|
0.75
|
Concrete
Block (1:10) (four Oval-core)
|
6.6
|
0.95
|
Solid
Brick (density:1925kg/m3)
|
5.0
|
0.72
|
Perforated
Brick (25% perforation density:1400kg/m3)
|
4.0
|
0.58
|
Wear resistance
The wear resistance of a substance depends on its particulate bonds. Bricks
shows high wear resistance because of its extremely strong ceramic bonds formed
by the effect of heat at high temperature.
Efflorescence
Efflorescence is a phenomenon that soluble slats dissolved in water are
carried, deposited and gradually accumulated on brick surfaces to form an
unsightly scum. The soluble salts may be originated from the raw material of
bricks. But in most cases, efflorescence is caused by salts from the external
sources such as ground water, contaminated atmosphere, mortar ingredients and
other materials in contacts with the bricks.
Flexibility in Applications
Brick is used for an extremely wide range of applications in an equally
extensive range of building and engineering structures. In particular, it can
be used for load bearing structures which greatly simply the construction
process so as to save materials, time and labour. Besides, brick can be make
into convenient shape and size to facilitate the construction work. It is very
flexible and handy in application almost everywhere.
Durability
Brick is extremely durable and perhaps is the most durable man-made structural
building materials so far. There has been numerous ancient brick-building
standing for centuries as a testimony of the endurance of burnt-clay brick.
