Bricks - Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

BRICKS

Constituents of brick earth

• Alumina (20-30%)
• Silica (50-60%)
• Lime (4-5%)
• Oxide of iron (5-6%)
• Magnesia (about 1%)

Alumina. It imparts plasticity to the brick earth so it can be easily moulded. If present in excess it increases shrinkage and warping during drying and makes bricks very hard after burning.

Silica. It prevents cracking, shrinkage and warping of bricks and thus helps them keep a uniform shape. If present in excess, the cohesion between particles is reduced and the brick becomes brittle.

Lime. A small amount of lime is beneficial as it helps prevent shrinkage. Excess lime causes melting during burning and may change colour. If lime is present as lumps, during burning it converts to quicklime which absorbs moisture and undergoes slaking; this causes internal expansion, cracking and disintegration of bricks. Therefore lime should be finely divided and lumps removed.

Oxide of iron. Iron oxide helps silica and lime to fuse during burning and thus increases the strength of bricks. It also imparts the characteristic reddish-brown colour.

Magnesia. Presence of magnesia reduces shrinkage and gives a yellow tint to the brick.

Harmful ingredients in brick earth

• Excessive lime. Causes bricks to melt or distort on burning and, if present as lumps, leads to cracking and disintegration.
• Iron pyrites. Oxidation of pyrites during burning causes crystallisation and disintegration of bricks.
• Alkalies. Act as fluxes during burning; when excessive they cause bricks to fuse, warp and lead to efflorescence and staining.
• Stones and pebbles. Produce weak and porous bricks because they reduce the effective bonded area.
• Organic (vegetative) matter. Small amounts help firing; however, if not fully burnt, decomposition produces gases that form voids and cracks, reducing strength.

Qualities of a good brick and standard tests

• Bricks should be well burnt, table-moulded and free from cracks.
• Bricks should be of uniform shape and size. Standard modular brick size (actual) is 190 mm × 90 mm × 90 mm and nominal size with mortar is 200 mm × 100 mm × 100 mm. Traditional non-modular brick size is 230 mm × 110 mm × 75 mm.
• Soundness test: bricks should produce a clear metallic ringing sound when struck together.
• Structure test: bricks should be homogeneous and free from voids.
• Water-absorption test: after 24 hours immersion in water, bricks of class up to 12.5 MPa shall not absorb more than 20% of their dry weight, and bricks above class 12.5 shall not absorb more than 15%.
• Compressive strength test: As per IS 3495 (Part 1), the compressive strength is the minimum average strength of five bricks tested, and bricks are designated as 3.5, 5, 7.5, 10, 12.5, 15, 20, 25, 30 and 35 MPa as per IS 1077.
• Drop test (field test): a good brick should not break into pieces when dropped flat on hard ground from a height of about 1 m.
• Hardness test: bricks should be sufficiently hard and should not show impression when scratched with a fingernail.
• Efflorescence test: the brick is placed in a shallow dish with water allowing capillary absorption, dried and examined; efflorescence is graded as Nil, Slight, Moderate, Heavy or Serious as per IS 3495 (Part 3).

Manufacture of bricks

Manufacturing of bricks involves four principal operations:

1. Preparation of clay
2. Moulding
3. Drying
4. Burning

1. Preparation of clay

Preparation of clay is carried out in the following sequence:

• Unsoiling. The top layer (about 200 mm) of soil is rejected because it usually contains most impurities.
• Digging. Clay is dug out and spread on a level field for subsequent cleaning.
• Cleaning. Stones, pebbles, roots, and other impurities are removed.
• Weathering (ripening/mellowing). Cleaned clay is exposed to atmospheric weathering for a few weeks to months to increase workability.
• Blending. Different constituents are mixed in required proportions to get a uniform brick earth.
• Tempering. Water is added and the clay is worked (usually in a pug mill) to obtain the required plasticity for moulding.

Tempering is generally carried out in a pug mill which ensures uniform mixing and removal of air pockets.

2. Moulding

Moulding is the process of giving the desired shape and size to the brick.

• Moulds are made of wood or steel and are usually sized about 8-12% larger than the required finished size to allow for shrinkage during drying and burning.
• A recess called frog (generally 10–20 mm deep with minimum area about 40 cm² in modular bricks) is provided to improve mortar bond and indicate manufacturer details.
• Moulding methods: hand-moulded or machine-moulded.
• Hand-moulded bricks: (i) ground-moulded bricks, (ii) table-moulded bricks. Table-moulded bricks are generally of better quality than ground-moulded ones.
• Machine-moulded bricks: (i) plastic clay moulding, (ii) dry clay moulding.

3. Drying

• Moulded bricks must be dried before burning; direct burning of wet bricks causes cracking and disintegration due to rapid vapourisation of moisture.
• Drying reduces the moisture content of moulded bricks to about 2–4% before burning.
• Drying may be natural (sun drying) or artificial (drying chambers).
• During drying, bricks are usually laid on edges rather than faces to increase air circulation and speed up evaporation.

4. Burning

Burning (firing) imparts strength, hardness and durability to bricks by sintering the particles and making the material denser. Proper burning is essential: over-burnt bricks become brittle and under-burnt bricks remain weak and unable to carry design loads.

• Typical burning temperature for common clay bricks ranges from about 900°C to 1100°C.
• Burning methods: clamp burning and kiln burning.

Clamp burning

A clamp is a temporary stack of bricks and fuel built on the ground and covered with mud to conserve heat. Clamps are generally 4-6 m high and are used when production is small and in dry seasons.

• Construction: a vertical brick and mud supporting wall is built at the lower edge of a trapezoidal stack. Alternate layers of fuel (coal, wood, husk, cow dung) and bricks are laid. The top is covered with mud to retain heat.
• After ignition, the clamp is kept burning for one or two months and requires a similar period for cooling.
• Disadvantages: non-uniform burning (bottom over-burnt and top under-burnt), loss of shape as bricks settle when fuel burns away, high fuel consumption and heat loss, not usable in monsoon.

Kiln burning

Kilns are permanent structures (ovens) for burning bricks. Coal and other local fuels are used. Kilns are of two broad types: intermittent and continuous.

Intermittent kilns

Operations (loading, burning, cooling, unloading) occur in sequence and not simultaneously. Two common types are:

• Up-draught intermittent kilns. Hot gases rise and exit through the top chimney; less efficient than down-draught.
• Down-draught intermittent kilns. Hot gases are directed across the load and down to an exit, utilising heat better and giving more uniform burning.

Continuous kilns

Continuous kilns allow simultaneous operations and higher production. Main types are:

• Bull's trench kiln (BTK):
  A trench excavated below ground in rectangular, circular or oval form, divided into several compartments so that loading, burning, cooling and unloading occur simultaneously. Once started, the firing zone moves progressively from chamber to chamber. These kilns conserve heat due to their semi-underground construction and can produce large numbers of bricks (It is suitable for large-scale continuous production with better fuel efficiency than clamp burning.). A drawback is the open top (no permanent roof) which makes operation difficult during monsoon.
  This Trench is divided generally in 12 chambers so that 2 numbers of cycles of brick burning can take place at the same time for the larger production of the bricks. Or it may happen that one cycle is carried out at one time in all the 12 chambers by using a single process in the 2-3 chambers at the same time.
  
  The structure is under-ground so the heat is conserved to a large extent so it is more efficient.  Once fire is started it constantly travels from one chamber to the other chamber, while other operations like loading, unloading, cooling, burning and preheating taking place simultaneously.
  
  Such kilns are generally constructed to have a manufacturing capacity of about 20,000 bricks per day. The drawback of this kiln is that there is not a permanent roof, so it is not easy to manufacture the bricks in the monsoon seasons.
• Hoffman kiln:
   The main difference between the Bull's trench kiln and the Hoffman kilns are:
  -Hoffman's kiln is an over the ground structure while Bull's Trench Kiln is an underground structure.
  -Hoffman's kiln have a permanent roof while Bull's trench Kiln do not have so it former can be used in 12 months a year to manufacture bricks but later is stopped in the monsoon season.

• Tunnel kilns:
  -These kilns are in the form of the tunnels, which may either be rectangular, circular or oval in plan.
  -It consists of stationary zones in which loading burning & cooling of bricks is carried out simultaneously.
  -Bricks are placed either on trolley or conveyor belts & are passed through different stationary zones in tunnels.
  -Requirement of land in these type of kilns is comparatively more.

Defects in Bricks

1. Over-burnt bricks: These are dark in colour, brittle and distorted due to excessive firing. They may lose shape and strength.
2. Under-burnt bricks: These are light coloured, soft and have low strength due to insufficient burning.
3. Bloating: Swelling of bricks caused by formation of gases during firing.
4. Black core: Dark interior caused by incomplete oxidation of carbonaceous matter during burning.
5. Laminations: Thin layers formed due to improper mixing or faulty moulding.

Brick masonry - terms and elements

• Stretcher. The longer face of the brick.
• Header. The shorter face of the brick.

• Closer. Portion of a brick cut along its length to break vertical joints. Types include: queen closer, king closer, mitered closer.
• Bat. Portion of brick cut along its width. Types include: half bat, three-quarter bat, bevelled bat.
• Quoin. The external corner or angle of a wall (an exterior corner equal to or greater than 90°).

Type of closer :-
(1) Queen closer

(2) King closer

 
(4) Mitered closer

4. Bat: -It is the portion of brick cut along its width.
Type of Bat:
(1) Half bat

(2) Three quarter bat

(3) Bevelled bat


5. Quoin : -It is the exterior angle along the face of the wall i.e. greater than equal to 90º

Common brick bonds

English bond

• Consists of alternate courses of headers and stretchers.
• In English bond, courses are laid alternately as header courses and stretcher courses. It is considered the strongest and most commonly used brick bond.
• A queen closer is provided next to the quoin header in header courses to maintain proper vertical joint staggering.
• Vertical joints in successive courses are staggered by providing a minimum lap of at least one-quarter brick length, though half-brick overlap gives stronger bonding.

Flemish bond

• Each course consists of alternate headers and stretchers placed next to one another.
• To break vertical joint alignment a quoin closer is placed next to the quoin header.
• Flemish bond is more decorative but comparatively weaker than English bond.
• For walls having thickness in odd multiples of half-brick, English bond is often stronger than Flemish bond.

Stretcher bond

• In stretcher bond, bricks are laid as stretchers in each course. It is suitable for half-brick thick walls.
• Half bats are used in alternate courses to break vertical joints.

Header bond

• Header bond consists entirely of headers in each course. It is suitable for thick walls and provides better transverse bonding.
• Three-quarter bats are used where necessary to maintain proper vertical joint staggering.