Bricks – Types, Properties, Preparation, Classification and Tests

0
130

BRICKS

  • Brick is obtained by moulding good clay into a block, which is dried and then burnt.
  • It is the oldest building block to replace stone.
  • Manufacture of brick started with hand moulding, sun drying and burning in clamps.
  • A considerable amount of technological development has taken place with better knowledge about to properties of raw materials, better machineries and improved techniques of moulding drying and burning.
  • Nowadays, bricks are made from specially selected & matured brick-earth consisting chiefly of silica (35-70%) and alumina (10-20%).
  • Too much silica makes the brick brittle & too much alumina makes the brick distort (twist) & crack on burning.
  • It is desirable to add lime, magnesia & iron oxide, which act as colouring agent & flux (fluctuation) to assist fusion during burning of brick earth.
  • Clay when heated to low temperatures, it loses moisture & only physical changes occur.
  • Such half-burnt clay crumbles when placed in water.
  • However, when clay is heated to high temperatures, its constituents fuse, & chemical changes takes place.
  • Size of the bricks: 19 cm × 9 cm × 9 cm.
  • With mortar joints, size of bricks: 20 cm × 10 cm × 10 cm.

Types of Bricks

  • Bricks may be broadly classified as:
    • Building bricks: These bricks are used for construction of walls.
    • Paving bricks: These are vitrified bricks & are used as pavers.
    • Fire bricks: These bricks are specially made to withstand furnace temperature. Silica bricks belong to this category.
    • Special bricks: These bricks are different from the commonly used building bricks with respect to their shape and the purpose for which they are made.
  • Specially shaped bricks
  • Facing bricks
  • Perforated building bricks
  • Burnt clay hollow bricks
  • Sewer bricks
  • Acid resistant bricks
  • Fireclay Bricks
  • Silicon Carbide bricks

 

1) Specially Shaped Bricks:

Bricks of special shapes are manufactured to meet the requirements of different situations.

2) Facing Bricks:

These bricks are used in the outer face of masonry. Once these bricks are provided, plastering is not required. Standard sizes:190 × 90×90 mm or 190×90 ×40 mm.

3) Perforated (Puncture) Building Bricks:

  • These are manufactured with area of perforation of 30-45%. Area of each perforation not > 500 mm2. Perforation should be uniformly distributed over the surface. Size: 190×190×90 mm and 290×90×90 mm.

4) Burnt Clay Hollow Bricks:

    • They are light in weight.
    • They are used for the construction of partition walls.
    • They provide good thermal insulation to buildings.
    • They are manufactured in sizes 190×190×90 mm, 290 × 90 × 90 mm and 290 × 140 × 90 mm.

perforated-bricks-250x250

 

    • Thickness of any shell should not be less than 11 mm and that of any web not less than 8 mm.

5) Sewer Bricks:

  • These are used for construction of sewage lines.
  • They are manufactured from surface clay, fire clay shale or with the combination of these.
  • They are manufactured in sizes 190×90×90 mm & 190 × 90 × 40 mm.
  • Average strength of these bricks should be a minimum of 17.5 N/mm2.
  • Water absorption not > 10%.

6) Acid Resistant Bricks:

    • Used for floorings likely to be subjected to acid attacks, lining of chambers in chemical plants, lining of sewers carrying industrial wastes etc.
    • Made of clay or shale of suitable composition with low lime and iron content, flint or sand and vitrified at high temperature in a ceramic kiln.

7) Fireclay Bricks:

    • Made for lining in devices such as furnaces, ovens and other places which are exposed to high temperatures.
    • Also called as refractory bricks.
    • They are made from special clays called fireclay.
    • These bricks should be laid with fireclay mortar and not with cement mortar, when built as lining for chimneys.
    • They are available as acidic, basic & neutral bricks.

8) Silicon carbide bricks:

    • Made of silicon and carbon.
    • Used in electric resistance furnace.

Preparation of Bricks

  • Following are the steps for preparation of bricks:
    • Preparation of clay
    • Moulding of Bricks
    • Burning of Clay
  • After removal of vegetation, clay deposits are excavated in steps rather than in layers to ensure a better distribution of several constituents which vary in different proportions at different depths.
  • Further processing depends on type of bricks to be made.
    • For ordinary bricks, only mixing by crushing is done.
    • For Ist class facing bricks, clay is allowed to weather by keeping it exposed to open air for a considerable time so that lumps of clay breakdown into smaller particles.
    • For very superior bricks, clay is washed & processed before moulding into bricks.
    • For Clay tiles, processed clay is used.
  • Bricks are moulded in various ways depending on the quality of product to be made.
  • They may be hand-moulded or machine- moulded or pressed by machines or in moulds.

Burning of Clay

  • It is carried out in temporary clamps or in permanent kilns.
  • In clamps, one batch of green bricks is heaped along the firewood, coal, etc. & sealed with clay.
  • It is then fired slowly to high intensity in many days.
  • Modern kilns are permanent structures consisting of many chambers.
  • They are intermittent & continuous kilns.
  • Moulded clay is stacked in the chambers.
  • They are then slowly dried & burned to high temperature & cooled.
  • One cycle of loading, drying, burning, cooling & emptying may take atleast 2 weeks.
  • These processes are carried out intermittently in intermittent kilns & in cyclic order in continuous kilns.
  • Nowadays, kilns have almost replaced the clamps as heat can be controlled better in kilns & bricks produced in kilns are of uniform quality.
  • Also it saves cost of fuel.

Properties of Bricks

  • Colour: Colour should be uniform & bright.
  • Shape: Bricks should have plane faces. They should have sharp & true right angled corners.
  • Size: Bricks should be of standard sizes as prescribed by codes.
  • Texture: They should possess fine, dense & uniform texture. They should not possess fissures, cavities, loose grit & un-burnt lime.
  • Soundness: When struck with hammer or with another brick, it should produce metallic sound.
  • Hardness: Finger scratching should not produce any impression on the brick.
  • Strength: Crushing strength of brick should not be < 3.5 N/mm2. A field test for strength is that when dropped from a height of 0.9 m to 1m on a hard ground, the brick should not break into pieces.
  • Water Absorption: After immersing the brick in water for 24 hours, water absorption should not be > 20% by weight. For class-I works this limit is 15%.
  • Efflorescence: Bricks should not show white patches when soaked in water for 24 hours and then allowed to dry in shade. White patches are due to the presence of sulphate of calcium, magnesium and potassium. They keep the masonry permanently in damp and wet conditions.
  • Thermal Conductivity: Bricks should have low thermal conductivity, so that buildings built with them are cool in summer and warm in winter.
  • Sound Insulation: Heavier bricks are poor insulators of sound while light weight and hollow bricks provide good sound insulation.
  • Fire Resistance: Fire resistance of bricks is usually good. In fact bricks are used to encase steel columns to protect them from fire.

Tests on Bricks

  • The following laboratory tests may be conducted on the bricks to find their suitability:
    • Crushing strength
    • Absorption
    • Shape and size
    • Efflorescence

Crushing Strength

  • Brick specimen is immersed in water for 24 hours.
  • Frog of the brick is filled with 1:3 cement mortar and the specimen is stored in damp jute bag for 24 hours and then immersed in clean water for 24 hours.
  • Specimen is placed in compression testing machine with 6 mm plywood on top and bottom of it to get uniform load on the specimen.
  • Then load is applied axially at a uniform rate of 14 N/mm2.
  • Crushing load is noted. Then the crushing strength is the ratio of crushing load to the area of brick loaded.
  • Average of 5 specimens is taken as the crushing strength.
  • Brick specimens are weighed dry. Then they are immersed in water for a period of 24 hours.
  • Specimen are taken out and wiped with cloth.
  • Weight of each specimen in wet condition is determined.
  • Difference in weight indicates the water absorbed.
  • Then the percentage absorption is the ratio of water absorbed to dry weight multiplied by 100. The averages of 5 specimens are taken.
  • Value should not > 20%.
  • Bricks should be of standard size and edges should be truly rectangular with sharp edges.
  • To check it, 20 bricks are selected at random and they are stacked along the length, along the width and then along the height.
  • For standard bricks of size 190 mm × 90 mm × 90 mm. IS code permits the following limits:
    • Lengthwise: 3680 to 3920mm
    • Widthwise: 1740 to 1860mm
    • Height wise: 1740 to 1860 mm.

Following tests help in ascertaining good quality bricks:

  • Uniformity in size
  • Uniformity in colour
  • Structure
  • Hardness test
  • Sound test
  • Strength test.
  • Uniformity in Size: A good brick should have rectangular plane surface and uniform in size. This check is made in the field by observation.
  • Uniformity in Colour: A good brick will be having uniform colour throughout. This observation may be made before purchasing the brick.
  • Structure: A few bricks may be broken in the field and their cross-section observed. The section should be homogeneous, compact and free from defects such as holes and lumps.

Sound Test: If 2 bricks are struck with each other they should produce clear ringing sound. The sound should not be dull.

  • Hardness Test: For this a simple field test is scratch the brick with nail. If no impression is marked on the surface, the brick is sufficiently hard.
  • Presence of alkalies in brick is not desirable because they form patches of gray powder by absorbing moisture.
  • Hence to determine the presence of alkalies this test is performed as explained below:
  • Place the brick specimen in a glass dish containing water to a depth of 25 mm in a well ventilated room.
  • After all the water is absorbed or evaporated again add water for a depth of 25 mm.
  • After second evaporation observe the bricks for white/grey patches.
  • Observation is reported as „nil‟, „slight‟, „moderate‟, „heavy‟ or serious to mean
  1. Nil: No patches
  2. Slight: 10% of area covered with deposits
  3. Moderate: 10 to 50% area covered with deposit but unaccompanied by flaking of the surface.
  4. Heavy: More than 50% area covered with deposits but unaccompanied by flaking of the surface.
  5. Serious: Heavy deposits of salt accompanied by flaking of the surface.

Classification of Bricks

Bricks can be classified in 3 ways –

  • according to Use
  • according to general physical requirement and
  • according to strength & as in IS classification.

According to Use

  • Bricks are classified as
    • Common bricks
    • Engineering Bricks
    • Facing Bricks
    • Fire Bricks
    • Special Bricks
  • Bricks are classified as Class I, Class II & Class III according to general physical properties. Bricks of different classes differ in their water absorption property. A brick which disintegrates when immersed in water for a long period is not of good quality. This is due to lack of good burning.
Class I Bricks Class II Bricks Class III Bricks
General Requirements Shall have uniform colour, shall be thoroughly burnt, and shall have rectangular faces with parallel sides & sharp straight right angled edges. They shall have a compact & uniform texture. Shall have a uniform colour, may be slightly overburnt, may be slightly distorted & round edges. They shall have a fine compact & uniform texture. May be slightly underburnt or overburnt, may be distorted & have round edges.
Water absorption after 24 hrs immersion in cold water Not > 20% by weight Not > 22% by weight Not > 25%

by weight

Efflorescence Slight Slight Moderate
  • IS 3102-1971 “Classification of burnt clay solid bricks” classifies brick according to their strengths.
Class Designation Compressive Strength Requirements (not less than) Additional Requirements
10 10 N/mm2 Dimensional Tolerance+/- 3%. Surface must be smooth, corners should be sharp, should give a ringing sound when struck.
7.5 7.5N/mm2 Dimensional Tolerance+/- 5%. Permitted to have slight distortion but it should not cause difficulty in laying.
5 5N/mm2 Dimensional Tolerance+/- 8%. Permitted to have slight distortion but it should not cause difficulty in laying.
3,5 3,5N/mm2 Dimensional Tolerance+/- 8%. Permitted to have slight distortion but it should not cause difficulty in laying.

 

 

LEAVE A REPLY

Please enter your comment!
Please enter your name here