Equilibrium of Structures

A structure is considered to be in equilibrium if, initially at rest, it remains at rest when subjected to a system of forces and couples. If a structure is in equilibrium, then all its members and parts are also in equilibrium. In order for a structure to be in equilibrium, all the forces and couples (including support reactions) acting on it must balance each other and there must neither be a resultant force nor a resultant couple acting on the structure. For a space (three-dimensional) structure subjected to three-dimensional systems of forces and couples (Fig. 1), the conditions of zero resultant force and zero resultant couple can be expressed in a Cartesian coordinate system (xyz) as

     Σ Fx = 0, Σ Fy = 0, Σ Fz = 0

     Σ Mx = 0, Σ My = 0, Σ Mz = 0

These six equations are called the equations of equilibrium of space structures and are the necessary and sufficient conditions for equilibrium. The first three equations ensure that there is no resultant force acting on the structure and the last three equations express the fact that there is no resultant couple acting on the structure.

Fig. 1 Space Structure Subjected to Three-Dimensional Systems of Forces and Couples

Fig. 2 Plane Structure Subjected to a Coplanar System of Forces and Couples

For a plane structure lying in the 𝑥y plane and subjected to a coplanar system of forces and couples (Fig. 2), the necessary and sufficient conditions for equilibrium can be expressed as

      Σ Fx = 0, Σ Fy = 0, Σ Mz = 0

These three equations are referred to as the equations of equilibrium of plane structures. The first two of the three equilibrium equations express that the algebraic sums of the 𝑥 components and y components of all the forces are zero, thereby indicating that the resultant force acting on the structure is zero. The third equation indicates that the algebraic sum of the moments of all the forces about any point in the plane of the structure and the moments of any couples acting on the structure is zero, thereby indicating that the resultant couple acting on the structure is zero. All the equilibrium equations must be satisfied simultaneously for the structure to be in equilibrium.

It should be realized that if a structure (e.g., an aerospace vehicle) initially in motion is subjected to forces that satisfy the equilibrium equations, it will maintain its motion with a constant velocity, since the forces cannot accelerate it. Such structures may also be considered to be in equilibrium. However, the term equilibrium is commonly used to refer to the state of rest of structures and is used in this context herein.

Concurrent Force Systems

When a structure is in equilibrium under the action of a concurrent force system; that is, the lines of action of all the forces intersect at a single point - the moment equilibrium equations are automatically satisfied and only the force equilibrium equations need to be considered. Therefore, for a space structure subjected to a concurrent three-dimensional force system, the equations of equilibrium are

      Σ Fx = 0, Σ Fy = 0, Σ Fz = 0

Similarly, for a plane structure subjected to a concurrent coplanar force system, the equilibrium equations can be expressed as

      Σ Fx = 0, Σ Fy = 0

• If a structure is in equilibrium under the action of only two forces, the forces must be equal, opposite and collinear.
• If a structure is in equilibrium under the action of only three forces, the forces must be either concurrent or parallel.

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Analytical Models

An analytical model is a simplified representation or an ideal of a real structure for the purpose of analysis. The objective of the model is to simplify the analysis of a complicated structure. The analytical model represents the behavioural characteristics of the structure of interest to the analyst, while discarding much of the detail about the members, connections and so on, that is expected to have little effect on the desired characteristics. Establishment of the analytical model is one of the most important steps of the analysis process; it requires experience and knowledge of design practices in addition to a thorough understanding of the behaviour of structures. Remember that the structural response predicted from the analysis of the model is valid only to the extent that the model represents the actual structure. Development of the analytical model generally involves consideration of the following factors.

Plane and Space Structure

If all the members of a structure as well as the applied loads lie in a single plane, the structure is called a plane structure. The analysis of plane or two-dimensional structures is considerably simpler than the analysis of space or three-dimensional structures. Many actual three-dimensional structures can be subdivided into plane structures for analysis.

As an example, consider the framing system of a bridge shown in Fig. 1. The main members of the system designed to support vertical loads are shown by solid lines whereas the secondary bracing members which is necessary to resist lateral wind loads and to provide stability are represented by dashed lines. The deck of the bridge rests on beams called stringers; these beams are supported by floor beams, which in turn are connected at their ends to the joints on the bottom panels of the two longitudinal trusses. Thus the weight of the traffic deck, stringers and floor beams is transmitted by the floor beams to the supporting trusses at their joints; the trusses, in turn transmit the load to the foundation. Because this applied loading acts on each truss in its own plane the trusses can be treated as plane structures.

Fig. 1 Framing System of Bridge

As another example, the framing system of a multi-story building is shown in Fig. 2. At each story, the floor slab rests on floor beams which transfer any load applied to the floor, the weight of the slab and their own weight to the girders of the supporting rigid frames. This applied loading acts on each frame in its own plane, so each frame can be analyzed as a plane structure. The loads thus transferred to each frame are further transmitted from the girders to the columns and then finally to the foundation.

Fig. 2 Framing System of Multi-storeyed Building

Although a great majority of actual three-dimensional structural systems can be subdivided into plane structures for the purpose of analysis, some structures such as latticed domes, aerospace structures and transmission towers cannot be subdivided into planar components due to their shape, arrangement of members or applied loading. Such structures, called space structures are analyzed as three-dimensional bodies subjected to three-dimensional force systems.

Line Diagram

The analytical model of the two or three-dimensional body selected for analysis is represented by a line diagram. On this diagram, each member of the structure is represented by a line coinciding with its centroidal axis. The dimensions of the members and the size of the connections are not shown on the diagram. The line diagrams of the bridge truss of Fig. 1 and the rigid frame of Fig. 2 are shown in Fig. 4 and 5 respectively.

Fig. 3 Line Diagram of Bridge and Connection

Fig. 4 Line Diagram of Multi-storeyed Building

Connections

Two types of connections are commonly used to join members of structures: rigid connections and flexible or hinged connections. A rigid connection or joint prevents relative translations and rotations of the member ends connected to it; that is, all member ends connected to a rigid joint have the same translation and rotation. In other words, the original angles between the members intersecting at a rigid joint are maintained after the structure has deformed under the action of loads. Such joints are capable of transmitting forces as well as moments between the connected members. Rigid joints are usually represented by points at the intersections of members on the line diagram of the structure, as shown in Fig. 3.

A hinged connection or joint prevents only relative translations of member ends connected to it; that is, all member ends connected to a hinged joint have the same translation but may have different rotations. Such joints are thus capable of transmitting forces but not moments between the connected members. Hinged joints are usually depicted by small circles at the intersections of members on the line diagram of the structure.

The perfectly rigid connections and the perfectly flexible frictionless hinges used in the analysis are merely idealizations of the actual connections, which are seldom perfectly rigid or perfectly flexible. However, actual bolted or welded connections are purposely designed to behave like the idealized cases. For example, the connections of trusses are designed with the centroidal axes of the members concurrent at a point to avoid eccentricities that may cause bending of members. For such cases, the analysis based on the idealized connections and supports generally yields satisfactory results.

Supports

Supports for plane structures are commonly idealized as either fixed supports- which do not allow any movement; hinged supports - which can prevent translation but permit rotation; roller or link - supports which can prevent translation in only one direction.

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Tests on Bricks

I) Field Tests on Bricks

The field test on bricks gives an idea about its basic quality based on its shape, size and colour at first observation without any big appliances. They are very common and easiest way to check the quality of brick. Field tests of brick are very helpful on the site. Some very common tests of brick that is followed to find if brick is good at first observation are as follows.

1) Shape and Size

The clay bricks should have a uniform rectangular plane surface, as per standard size and sharp straight edges. BIS recommends the standard size of brick is 190 x 90 x 90 mm and constructional size is 200 x 100 x 100 mm.

2) Visual Inspection

In this test, bricks are closely inspected for its shape. The bricks of good quality should be uniform in shape and should have truly rectangular shape with sharp edges.

3) Hardness

The clay bricks should be sufficiently hard when scratched by a finger-nail and no impression should be left on the brick surface.

4) Colour

The clay bricks should have a uniform deep red colour throughout. It indicates the uniformity of chemical composition and the quality of burning of the bricks.

5) Texture and Compactness

The surfaces should not be so smooth to cause skidding of mortar. The clay brick should have a pre-compact, homogeneous and uniform texture. A broken surface should be free form cracks, holes grits or lumps of lime.

6) Soundness

When two clay bricks are stuck together, a metallic ringing sound should come.

7) Structure

A brick is broken and its structure is examined. It should be homogeneous, compact and free from any defects such as holes, lumps etc.

8) Basic Strength

When dropped flat on the hard ground from a height of about one meter, clay bricks should not break.

II) Laboratory Tests on Bricks

Laboratory tests on brick determine the mechanical properties of brick and give a scientific approach to ensure the quality of bricks. It is essential while purchasing the brick and examine the properties for the quality of construction. Followings brick tests are performed in the laboratory to determine the quality of brick.

1) Water Absorption of Brick

The brick is porous by nature and Porosity is the ability to release and absorb moisture. Therefore, it tends to absorb the water or moisture. It’s an important and useful property of brick. But if brick absorbs more water than the recommended result, then it affects the strength of brick as well as durability of the structure and will damage plaster and paint over walls. Dry bricks are put in an oven at a temperature of 105° to 115°C till these attain constant mass. The weight (W1) of the bricks is recorded after cooling them to room temperature. The bricks are then immersed in water at a temperature of 27° ± 2°C for 24 hours. The specimens are then taken out of water and wiped with a damp cloth. Three minutes later it is weighed again and recorded as W2.

Water absorption test is performed to know the percentage of water absorption of bricks. Water absorption of bricks should not more than 20% by its dry weight. If brick fails in the water absorption test, possible reasons are like manufacturing error, insufficient burning, error in clay composition etc. If brick fails in water absorption as well as efflorescence than never use those bricks because it will cause in permanent problems and it will be very difficult to solve them. Water bath, weight balance and oven are required for performing this test.

2. Compressive Strength of Brick

The compressive strength of the brick is the most essential property of the bricks because in the construction, bricks are widely used in masonry and it also plays a significant role as a load bearing component. When bricks are used in any structure, the bottom-most layer of the brick will be subjected to the highest compressive stress. Therefore, it is essential to know that any particular brick will be able to withstand that load or not. This test is performed to know the strength of bricks because it affects the overall structure in the way of quality, durability and serviceability.

For testing bricks for compressive strength from a sample the two bed faces of bricks are ground to provide smooth, even and parallel faces. The bricks are then immersed in water at room temperature for 24 hours. These are then taken out of water and surplus water on the surfaces is wiped off with cotton or a moist cloth. The frog of the brick is flushed level with cement mortar and the brick is stored under damp jute bags for 24 hours followed by its immersion in water at room temperature for three days. The specimen is placed in the compression testing machine with flat faces horizontal and mortar filled face being upwards. Load is applied at a uniform rate of 14 N/m² per minute till failure. The maximum load at failure divided by the average area of bed face gives the compressive strength.

Recommended Result of Compressive Strength Test of Brick

Test result recommendations are as follows.

• For first class bricks, it should not less than 10 N/mm² (102 kg/cm²).
• For second class bricks, it should not less than 7 N/mm² (71 kg/cm²).
• For third class bricks, it should not less than 3.5 N/mm² (36 kg/cm²).

In India, the northern and the eastern region produce bricks having good compressive strength than the western region because the western region has black cotton soil, while the soil is good in Gangetic region. If the test result is not as per recommendation, there are many reasons behind it such as the clay composition, degree of burning like over burning or insufficient burning, error in the testing appliance or testing procedure etc. If bricks fail in strength as well as water absorption test, then do not use it. If bricks are irregular in some minor shape/size than it can be corrected with mortar.

3) Efflorescence Test

This test should be conducted in a well-ventilated room. The brick is placed vertically in a dish 30 x 20 cm approximately in size with 2.5 cm immersed in distilled water. The whole water is allowed to be absorbed by the brick and evaporated through it. After the bricks appear dry, a similar quantity of water is placed in the dish and the water is allowed to evaporate as before. The brick is to be examined after the second evaporation and reported as follows.

• Nil : When there is no perceptible deposit of salt
• Slight : When not more than 10% of the area of brick is covered with salt
• Moderate : When there is heavy deposit covering 50% of the area of the brick but unaccompanied by powdering or flaking of the surface.
• Heavy : When there is heavy deposit covering more than 50% of the area of the brick accompanied by powdering or flaking of the surface.
• Serious : When there is heavy deposit of salts accompanied by powdering and/or flaking of the surface and this deposition tends to increase in the repeated wetting of the specimen.

Bricks for general construction should not have more than slight to moderate efflorescence.

4) Dimension Tolerance

Twenty bricks are selected at random to check measurement of length, width and height. These dimensions are to be measured in one or two lots of ten each as shown in Fig. 1. Variation in dimensions is allowed only within narrow limits, ±3% for class one and ±8% for other classes.

Fig. 1

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Classification of Structures

The most important decision made by a structural engineer in implementing an engineering project is the selection of the type of structure to be used for supporting or transmitting loads. Commonly used structures can be classified into five basic categories, depending on the type of primary stresses that may develop in their members under major design loads. It should be realized that any two or more of the basic structural types described in the following may be combined in a single structure, such as a building or a bridge, to meet the structure’s functional requirements. The different type of structures is given below.

1) Tension Structures

The members of tension structures are subjected to pure tension under the action of external loads. Because the tensile stress is distributed uniformly over the cross-sectional areas of members, the material of such a structure is utilized in the most efficient manner. Tension structures composed of flexible steel cables are frequently employed to support bridges and long- span roofs. Because of their flexibility, cables have negligible bending stiffness and can develop only tension. Thus, under external loads, a cable adopts a shape that enables it to support the load by tensile forces alone. In other words, the shape of a cable changes as the loads acting on it change. As an example, the shapes that a single cable may assume under two different loading conditions are shown in Fig. 1.

Fig. 1

Fig. 2 shows the cable structure of the suspension bridge. In a suspension bridge, the roadway is suspended from two main cables by means of vertical hangers. The main cables pass over a pair of towers and are anchored into solid rock or a concrete foundation at their ends. Because suspension bridges and other cable structures lack stiffness in lateral directions, they are susceptible to wind-induced oscillations. Bracing or stiffening systems are therefore provided to reduce such oscillations. Besides cable structures, other examples of tension structures include vertical rods used as hangers (for example, to support balconies or tanks) and membrane structures such as tents and roofs of large-span domes.

Fig. 2 Suspension Bridge

2) Compression Structures

Compression structures develop mainly compressive stresses under the action of external loads. Two common examples of such structures are columns and arches (Fig. 3). Columns are straight members subjected to axially compressive loads, as shown in Fig. 3. When a straight member is subjected to lateral loads and/or moments in addition to axial loads, it is called a beam-column.

Fig. 3 Column

An arch is a curved structure, with a shape similar to that of an inverted cable, as shown in Fig. 4. Such structures are frequently used to support bridges and long-span roofs. Arches develop mainly compressive stresses when subjected to loads and are usually designed so that they will develop only compression under a major design loading. However, because arches are rigid and cannot change their shapes as can cable, other loading conditions usually produce secondary bending and shear stresses in these structures, which should be considered in their designs. Because compression structures are susceptible to buckling or instability, the possibility of such a failure should be considered in their designs; if necessary, adequate bracing must be provided to avoid such failures.

Fig. 4 Arch

3) Trusses

Trusses are composed of straight members connected at their ends by hinged connections to form a stable configuration (Fig. 5). When the loads are applied to a truss only at the joints, its members either elongate or shorten. Thus, the members of an ideal truss are always either in uniform tension or in uniform compression. Real trusses are usually constructed by connecting members to gusset plates by bolted or welded connections. Although the rigid joints thus formed cause some bending in the members of a truss when it is loaded, in most cases such secondary bending stresses are small and the assumption of hinged joints yields satisfactory designs.

Because of their light weight and high strength, trusses are the most commonly used types of structures. Such structures are used in a variety of applications, ranging from supporting roofs of buildings to serving as support structures in space stations and sports arenas.

Fig. 5 Plane Truss

4) Shear Structures

Shear structures, such as reinforced concrete shear walls (Fig. 6), are used in multistory buildings to reduce lateral movements due to wind loads and earthquake excitations. Shear structures develop mainly in-plane shear, with relatively small bending stresses under the action of external loads.

Fig. 6 Shear Wall

5) Bending Structures

Bending structures develop mainly bending stresses under the action of external loads. In some structures, the shear stresses associated with the changes in bending moments may also be significant and should be considered in their designs. Some of the most commonly used structures such as beams, rigid frames, slabs and plates, can be classified as bending structures. A beam is a straight member that is loaded perpendicular to its longitudinal a𝑥is. The bending (normal) stress varies linearly over the depth of a beam from the maximum compressive stress at the fiber farthest from the neutral axis on the concave side of the bent beam to the maximum tensile stress at the outermost fiber on the convex side.

For example, in the case of a horizontal beam subjected to a vertically downward load, as shown in Fig. 7, the bending stress varies from the maximum compressive stress at the top edge to the maximum tensile stress at the bottom edge of the beam. To utilize the material of a beam cross section most efficiently under this varying stress distribution, the cross sections of beams are often I-shaped, with most of the material in the top and bottom flanges. The I-shaped cross sections are most effective in resisting bending moments.

Fig. 7 Beam

Rigid frames are composed of straight members connected together either by rigid (moment-resisting) connections or by hinged connections to form stable configurations. Unlike trusses, which are subjected only to joint loads, the external loads on frames may be applied on the members as well as on the joints. The members of a rigid frame are subjected to bending moment, shear and axial compression or tension under the action of external loads. The design of horizontal members or beams of rectangular frames is often governed by bending and shear stresses only, since the axial forces in such members are usually small.

Frames, like trusses, are among the most commonly used types of structures. Structural steel and reinforced concrete frames are commonly used in multistory buildings, bridges and industrial plants. Frames are also used as supporting structures in airplanes, ships, aerospace vehicles and other aerospace and mechanical applications. The generic term framed structure is frequently used to refer to any structure composed of straight members, including a truss.

Fig. 8 Rigid Frame

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Manufacturing of Bricks

The following operations are involved in the manufacturing of brick.

I) Preparation of Clay

The preparation of clay involves following operations.

a) Unsoiling

The soil used for making building bricks should be processed so as to be free of gravel, coarse sand (practical size more than 2 mm), lime and kankar particles, organic matter, etc. About 20 cm of the top layer of the earth, normally containing stones, pebbles, gravel, roots, etc. is removed after clearing the trees and vegetation.

b) Digging

Clay dug out from ground is spread on level ground about 60 to 120 cm heaps. After removing the top layer of the earth, proportions of additives such as fly ash, sandy loam, rice husk ash, stone dust etc. should be spread over the plane ground surface on volume basis. The soil mass is then manually excavated, puddled, watered and left over for weathering and subsequent processing. The digging operation should be done before rains.

c) Cleaning

Stones, pebbles, vegetable matter etc. should be removed from soil.

d) Weathering

Clay is exposed to atmosphere from few weeks to full season. Stones, gravels, pebbles, roots, etc. are removed from the dug earth and the soil is heaped on level ground in layers of 60 -120 cm. The soil is left in heaps and exposed to weather for at least one month in cases where such weathering is considered necessary for the soil. This is done to develop homogeneity in the mass of soil, particularly if they are from different sources and also to eliminate the impurities which get oxidized. Soluble salts in the clay would also be eroded by rain to some extent, which otherwise could have caused scumming at the time of burning of the bricks in the kiln. The soil should be turned over at least twice and it should be ensured that the entire soil is wet throughout the period of weathering. In order to keep it wet, water may be sprayed as often as necessary. The plasticity and strength of the clay are improved by exposing the clay to weather.

e) Blending

Clay is made loose and any ingredient to be added to it is spread out at top and turning it up and down in vertical direction. The earth is mixed with sandy earth and calcareous earth in suitable proportions to modify the composition of soil. Moderate amount of water is mixed so as to obtain the right consistency for moulding. The mass is then mixed uniformly with spades. Addition of water to the soil at the dumps is necessary for the easy mixing and workability, but the addition of water should be controlled in such a way that it may not create a problem in moulding and drying. Excessive moisture content may affect the size and shape of the finished brick.

f) Tempering

Clay is brought to a proper degree of hardness, then water is added to clay and whole mass is kneaded or pressed under the feet of men or cattle for large scale. Tempering is usually done in pug mill. Tempering consists of kneading the earth with feet so as to make the mass stiff and plastic (plasticity means the property which wet clay has of being permanently deformed without cracking). It should preferably be carried out by storing the soil in a cool place in layers of about 30 cm thickness for not less than 36 hours. This will ensure homogeneity in the mass of clay for subsequent processing.

Pug mill consists of a conical iron tube as shown in Fig. 1. The mill is sunk 60 cm into the earth. A vertical shaft, with a number of horizontal arms fitted with knives, is provided at the centre of the tube. This central shaft is rotated with the help of bullocks yoked at the end of long arms. Steam, diesel or electric power may be used for this purpose. Blended earth along with required water, is fed into the pug mill from the top. The knives cut through the clay and break all the clods or lump clays when the shaft rotates. The thoroughly pugged clay is then taken out from opening provided in the side near the bottom. The yield from a pug mill is about 1500 bricks.

Fig. 1 Pug Mill

II) Moulding

Clay, which is prepared form pug mill is sent for the next operation of moulding. It is a process of giving a required shape to the brick from the prepared brick earth. Moulding may be carried out by hand or by machines. The process of moulding of bricks may be the soft-mud (hand moulding), the stiff-mud (machine moulding) or the dry-press process (moulding using maximum 10 per cent water and forming bricks at higher pressures). Fire-brick is made by the soft mud process. Roofing, floor and wall tiles are made by dry-press method. The stiff-mud process is used for making all the structural clay products.

Fig. 2 Details of Mould

1) Hand Moulding

Moulds are rectangular boxes of wood or steel, which are open at top and bottom. Steel moulds are more durable and used for manufacturing bricks on large scale. Bricks prepared by hand moulding are of two types.

a) Ground Moulding

In this process, the ground is levelled and sand is sprinkled on it. The moulded bricks are left on the ground for drying. Such bricks do not have frog and the lower brick surface becomes too rough. To overcome these defects, moulding blocks or boards are used at the base of the mould. The process consists of shaping in hands a lump of well pugged earth, slightly more than that of the brick volume. It is then rolled into the sand and with a jerk it is dashed into the mould. The moulder then gives blows with his fists and presses the earth properly in the corners of the mould with his thumb. The surplus clay on the top surface is removed with a sharp edge metal plate called strike or with a thin wire stretched over the mould. After this the mould is given a gentle slope and is lifted leaving the brick on the ground to dry.

This method is adopted when a large and level land is available. To prevent the moulded brick from sticking to the side of the mould, sand is sprinkled on the inner sides of the mould, or the mould may be dipped in water every time before moulding is done. The bricks so produced are respectively called sand moulded and slop moulded bricks, the former being better since they provide sufficient rough surface necessary for achieving a good bond between bricks and mortar.

Fig. 3 Type of Strikes

b) Table Moulding

The bricks are moulded on stock boards nailed on the moulding table. Process of moulding these bricks is just similar to ground bricks on a table of size about 2m x 1m. Stock boards have the projection for forming the frog. The process of filling clay in the mould is the same as explained above. After this, a thin board called pallet is placed over the mould. The mould containing the brick is then smartly lifted off the stock board and inverted so that the moulded clay along with the mould rests on the pallet. The mould is then removed as explained before and the brick is carried to the drying site.

Fig. 4 Table Moulding

Fig. 6 Stock Board

2) Machine Moulding

This method proves to be economical when bricks in huge quantity are to be manufactured at the same spot. It is also helpful for moulding hard and string clay. These machines are broadly classified in two categories.

a) Plastic Clay Machines

This machine containing rectangular opening of size equal to length and width of a brick. Pugged clay is placed in the machine and as it comes out through the opening, it is cut into strips by wires fixed in frames, so these bricks are called wire cut bricks. This is a quick and economical process.

b) Dry Clay Machines

In these machines, strong clay is first converted into powder form and then water is added to form a stiff plastic paste. Such paste is placed in mould and pressed by machine to form hard and well-shaped bricks. These bricks have well behaviour than ordinary hand moulded bricks. They carry distinct frogs and exhibit uniform texture. These are burnt carefully as they are likely to crack.

III) Drying

Green bricks contain about 7–30% moisture depending upon the method of manufacture. The object of drying is to remove the moisture to control the shrinkage and save fuel and time during burning. The drying shrinkage is dependent upon pore spaces within the clay and the mixing water. The addition of sand or ground burnt clay reduces shrinkage, increases porosity and facilities drying. The moisture content is brought down to about 3 percent under exposed conditions within three to four days. Thus, the strength of the green bricks is increased and the bricks can be handled safely.

Clay products can be dried in open air driers or in artificial driers. The artificial driers are of two types, the hot floor drier and the tunnel drier. In the former, heat is applied by a furnance placed at one end of the drier or by exhaust steam from the engine used to furnish power and is used for fire bricks, clay pipes and terracotta. Tunnel driers are heated by fuels underneath, by steam pipes or by hot air from cooling kilns. They are more economical than floor driers. In artificial driers, temperature rarely exceeds 120°C. The time varies from one to three days. In developing countries, bricks are normally dried in natural open air driers. They are stacked on raised ground and are protected from bad weather and direct sunlight. A gap of about 1.0 m is left in the adjacent layers of the stacks so as to allow free movement for the workers. The drying of brick is by the following means.

i) Natural Drying – usually about 3 to 10 days to bricks to become dry under sunlight.

ii) Artificial Drying – drying by tunnels usually 120⁰C about 1 to 3 days.

Fig. 7 Method of Drying Bricks

IV) Burning

This is very important operation in the manufacturing of bricks to impart hardness, strength and makes them dense and durable. The burning of clay may be divided into three main stages.

a) Dehydration (400 – 650ºC)

This is also known as water smoking stage. During dehydration,

• The water which has been retained in the pores of the clay after drying is driven off and the clay loses its plasticity
• Some of the carbonaceous matter is burnt
• A portion of sulphur is distilled from pyrites
• Hydrous minerals like ferric hydroxide are dehydrated
• The carbonate minerals are more or less decarbonated

Too rapid heating causes cracking or bursting of the bricks. On the other hand, if alkali is contained in the clay or sulphur is present in large amount in the coal, too slow heating of clay produces a scum on the surface of the bricks.

b) Oxidation Period (650 – 900ºC)

During the oxidation period, remainder of carbon is eliminated and the ferrous iron is oxidized to the ferric form. The removal of sulphur is completed only after the carbon has been eliminated. Sulphur on account of its affinity for oxygen, also holds back the oxidation of iron. Consequently, in order to avoid black or spongy cores, oxidation must proceed at such a rate which will allow these changes to occur before the heat becomes sufficient to soften the clay and close its pore. Sand is often added to the raw clay to produce a more open structure and thus provide escape of gases generated in burning.

c) Vitrification

To convert the mass into glass like substance, the temperature ranges from 900 – 1100°C for low melting clay and 1000 – 1250°C for high melting clay. Great care is required in cooling the bricks below the cherry red heat in order to avoid checking and cracking. Vitrification period may further be divided into

i) Incipient Vitrification

It is the stage at which the clay has softened sufficiently to cause adherence but not enough to close the pores or cause loss of space—on cooling the material cannot be scratched by the knife.

ii) Complete Vitrification

The stage at which more or less well-marked by maximum shrinkage.

iii) Viscous Vitrification

This stage produced by a further increase in temperature which results in a soft molten mass, a gradual loss in shape, and a glassy structure after cooling. Generally, clay products are vitrified to the point of viscosity. However, paving bricks are burnt to the stage of complete vitrification to achieve maximum hardness as well as toughness.

Burning of bricks is done either in clamps or in kilns. Clamps are temporary structures and they are adopted to manufacture bricks on small scale. Kilns are permanent structures and they are adopted to manufacture bricks on a large scale.

a) Burning in Clamp ox Pazawah

A typical clamp is shown in Fig. 8. The bricks and fuel are placed in alternate layers. The amount of fuel is reduced successively in the top layers. Each brick tier consists of 4–5 layers of bricks. Some space is left between bricks for free circulation of hot gasses. The total height of clamp in alternate layers of brick is about 3 to 4 m. After 30 per cent loading of the clamp, the fuel in the lowest layer is fired and the remaining loading of bricks and fuel is carried out hurriedly. The top and sides of the clamp are plastered with mud. Then a coat of cow dung is given, which prevents the escape of heat. The production of bricks is 2–3 lacs and the process is completed in six months. This process yields about 60 per cent first class bricks.

Fig. 8 Clamp

Advantages

• The bricks produced are tough and strong because burning and cooling are gradual
• Burning in clamps proves to be cheap and economical
• No skilled labour and supervision are required for the construction of clamps
• There is considerable saving of clamps fuel

Disadvantages

• Bricks are not of required shape
• It is very slow process
• It is not possible to regulate fire in a clamp
• Quality of brick is not uniform

b) Kiln Burning

The kiln used for burning bricks may be underground, e.g. Bull’s trench kiln or overground, e.g. Hoffman’s kiln. These may be rectangular, circular or oval in shape. When the process of burning bricks is continuous, the kiln is known as continuous kiln, e.g. Bull’s trench and Hoffman’s kilns. On the other hand, if the process of burning bricks is discontinuous, the kiln is known as intermittent kiln. The different types of kiln are explained below.

i) Intermittent kiln

The example of this type of an over ground and rectangular kiln. After loading the kiln, it is fired, cooled and unloaded and then the next loading is done. Since the walls and sides get cooled during reloading and are to be heated again during next firing, there is wastage of fuel. Bricks manufactured by intermittent up drought kilns are better than those prepared by clamps. But, bricks burnt by this process is not uniform. Intermittent kiln is of two types.

Fig. 9 Intermittent Kiln

a) Intermittent Up-Draught Kiln

This is in the form of rectangular with thick outside walls. Wide doors are provided at each end for loading and unloading of kilns. A temporary roof may be installed to protect from rain and it is removed after kiln is fired. Flues are provided to carry flames or hot gases through the body of kiln. The stages are given below.

1. Raw bricks are laid in row of thickness equal to 2 to 3 bricks and height 6 to 8 bricks with 2 bricks spacing between rows
2. Fuels are filled with brush wood which takes up a fire easily
3. Loading of kiln with raw bricks with top course is finished with flat bricks and other courses are formed by placing bricks on edges
4. Each door is built up with dry bricks and are covered with mud or clay
5. The kiln is then fired for a period of 48 to 60 hours draught rises in the upward direction from bottom of kiln and brings about the burning of bricks.
6. Kiln is allowed to cool down and bricks are then taken out
7. Same procedure is repeated for the next burning

b) Intermittent Down-Draught Kiln

These kilns are rectangular or circular in shape. They are provided with permanent walls and closed tight roof. Floor of the kiln has opening which are connected to a common chimney stack through flues. Working is same as up-draught kiln. But it is so arranged in this kiln that hot gases are carried through vertical flues upto the level of roof and they are then released. These hot gases move downward by the chimney draught and in doing so, they burn the bricks.

ii) Continuous Kiln

These kilns are continuous in operations. This means that loading, firing, cooling and unloading are carried out simultaneously in these kilns. The examples of continuous kiln are Hoffman’s kiln and Bull’s trench kiln. In a continuous kiln, bricks are stacked in various chambers wherein the bricks undergo different treatments at the same time. When the bricks in one of the chambers is fired, the bricks in the next set of chambers are dried and preheated while bricks in the other set of chambers are loaded and in the last are cooled. There are three types of continuous kilns.

a) Bull’s Trench Kiln

This kiln may be of rectangular, circular or oval shape in the plan as shown in Fig 10. It is constructed in a trench excavated in ground either fully underground or partially projecting above ground openings is provided in the outer walls to act as flue holes. Dampers are in the form of iron plates and they are used to divide the kilns in suitable sections and it is the most widely used kiln in India.

The bricks are arranged in such a way that flues are formed. Fuel is placed in flues and it is ignited through flue holes after covering top surface with earth and ashes to prevent the escape of heat. Usually, two movable iron chimneys are employed to form draught. These chimneys are placed in advance of section being fired. Hence, hot gases leaving the chimney warm up the bricks in next section. Each section requires about one day to burn. The tentative arrangement for different sections as shown in Fig. 10 may be as follows

   Section 1 – Loading

   Section 2 – Emptying

   Section 3 – Unloading

   Section 4 – Cooling

   Section 5 – Burning

   Section 6 – Heating

Fig 10 Bull’s Trench Kiln

b) Hoffman’s Kiln

This kiln is constructed over ground and hence, it is sometimes known as flame kiln. Its shape is circular to plan and it is divided into a number of compartments or chambers. A permanent roof is provided; the kiln can even function during rainy season. Fig.11 shows plan and section of Hoffman’s kiln with 12 chambers

   Chamber 1 - Loading

   Chamber 2 to 5 – Drying and Pre-Heating

   Chambers 6 and 7 - Burning

   Chambers 8 to 11 - Cooling

   Chamber 12 – Unloading

The initial cost in stalling this kiln is high. The advantages of Hoffman’s kiln are given below.

• Good quality of bricks are produced
• It is possible to regulate heat inside the chambers through fuel holes
• Supply of bricks is continuous and regular
• There is considerable saving in fuel due to pre heating of raw bricks by flue gases

Fig.11 Hoffman’s Kiln

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Classification of Bricks

I) Classification of Bricks Based on Building Process

a) Unburnt Bricks

These are half burnt bricks and the colour is yellow. The strength is low. They are used as surki in lime terracing. They are used as soiling under RCC footing or basement. Such bricks should not be exposed to rainwater.

b) Burnt Bricks

Burnt bricks are made by burning them in the kiln. First class, second class, third class and fourth class bricks are burnt bricks.

c) Over Burnt or Jhama Bricks

It is often known as the vitrified brick as it is fired at high temperature and for a longer period of time than conventional bricks. As a result, the shape is distorted. The absorption capacity is high. The strength is higher or equivalent to first class bricks. It is used as lime concrete for the foundation. It is also used as coarse aggregate in the concrete of slab and beam which will not come in contact with water.

II) Classification of Bricks Based on Quality

Burnt bricks are classified into the following four categories.

a) First Class Brick

These bricks are table moulded and of standard shape. The colour of these bricks is uniform yellow or red. It is well burnt, regular texture, uniform shape. The absorption capacity is less than 10%, crushing strength is 280 kg/cm² (mean) where it is 245 kg/cm² (minimum). It doesn’t have efflorescence. It emits a metallic sound when struck by another similar brick or struck by a hammer. It is hard enough to resist any fingernail expression on the brick surface if one tries to do with a thumbnail. It is free from pebbles, gravels or organic matters. The comply all the qualities of good bricks and used for superior work of permanent nature. The thickness of mortar joints doesn’t exceed 10mm. The uses are given below. In a building of long durability, say 100 years For building exposes to a corrosive environment For making coarse aggregates of concrete Recommended for exposed face work in masonry structures, flooring and reinforced brick work

b) Second Class Brick

These bricks are ground moulded and they are burnt in kilns. The size is standard, colour is uniform yellow or red. It is well burnt, slightly over burnt is acceptable. It has a regular shape; efflorescence is not appreciable. The absorption capacity is more than 10% but less than 15%. Crushing strength is 175kg/cm²(mean) where the minimum is 154 kg/cm². It emits a metallic sound when struck by another similar brick or struck by a hammer. It is hard enough to resist any fingernail expression on the brick surface if one tries to do with a thumbnail. It is used for the construction of one-storied buildings, temporary shed when intended durability is not more than 15 years. These bricks are commonly used at places where brick work is to be provided with a coat of plaster. The thickness of mortar joint is 12 mm.

c) Third Class Brick

These bricks are ground moulded and they burnt in clamps. These bricks are not hard and they have rough surfaces with irregular and distorted edges. These bricks give dull sound when struck together. The shape and size are not regular. The colour is soft and light red coloured. It is under burnt, slightly over burnt is acceptable. It has extensive efflorescence. The texture is non-uniform. The absorption capacity is more than 15% but less than 20%. The crushing strength is 140kg/cm²(mean) where the minimum crushing strength is 105kg/cm². It emits a dull or blunt sound when struck by another similar brick or struck by a hammer. It leaves fingernail expression when one tries to do with the thumbnail. They are used for unimportant and temporary structures and at places where rainfall is not heavy.

d) Fourth Class Bricks

These are over burnt bricks with irregular shape and dark colour. These bricks are used as aggregate for concrete in foundation, floors, roads etc. because of the fact that the over burnt bricks have compacted structure and hence, they are sometimes found stronger than even first class bricks.

III) Classification of Bricks Based on Manufacturing Method

a) Extruded Brick

It is created by forcing clay and water into a steel die, with a very regular shape and size, then cutting the resulting column into shorter units with wires before firing. It is used in constructions with limited budgets. It has three or four holes constituting up to 25% volume of the brick.

b) Moulded Brick

It is shaped in moulds by hand rather being in the machine.

c) Dry pressed Brick

It is the traditional types of bricks which are made by compressing clay into moulds. It has a deep frog in one bedding surface and shallow frog in another.

IV) Classification Based on Strength

The Bureau of Indian Standards (BIS) has classified the bricks on the basis of compressive strength and is as given in Table 1.

Table 1 Classification of Bricks based on Compressive Strength (IS: 1077) 

Class	Average compressive strength not less than (N/mm2)
35	35.0
30	30.0
25	25.0
20	20.0
17.5	17.5
15	15.0
12.5	12.5
10	10.0
7.5	7.5
5	5.0
3.5	3.5

The burnt clay bricks having compressive strength more than 40.0 N/mm² are known as heavy duty bricks and are used for heavy duty structure such as bridge, foundation for industrial building, multi-storey building, etc. The water absorption of these bricks is limited to 5 per cent. Each class of bricks as specified above is further divided into subclasses A and B based on tolerance and shape. Subclass-A bricks should have smooth rectangular face with sharp corner and uniform colour. Subclass - B bricks may have slightly distorted and round edges.

V) Classification of Bricks Based on Raw Materials

a) Burnt Clay Brick

It is obtained by pressing the clay in moulds and fried and dried in kilns. It is the most used bricks. It requires plastering when used in construction works.

b) Fly Ash Clay Brick

It is manufactured when fly ash and clay are moulded in 1000 degree Celsius. It contains a high volume of calcium oxide in fly ash. That is why usually described as self-cementing. It usually expands when coming into contact with moisture. It is less porous than clay bricks. It proved a smooth surface so it doesn’t need plastering.

c) Concrete Brick

It is made of concrete and is the least used bricks. It has low compression strength and is of low quality. These bricks are used above and below the damp proof course. These bricks can be used for facades, fences and internal brickworks because of their sound reductions and heat resistance qualities. It is also called mortar brick. It can be of different colours if the pigment is added during manufacturing. It should not be used below ground.

d) Sand-Lime Brick

Sand, fly ash and lime are mixed and moulded under pressure. During wet mixing, a chemical reaction takes place to bond the mixtures. Then they are placed in the moulds. The colour is greyish as it offers something of an aesthetic view. It offers a smoother finish and uniform appearance than the clay bricks. As a result, it also doesn’t require plastering. It is used as a load bearing member as it is immensely strong.

e) Fire Brick

It is also known as refractory bricks. It is manufactured from a specially designed earth. After burning, it can withstand very high temperature without affecting its shape, size and strength. It is used for the lining of chimney and furnaces where the usual temperature is expected to be very high.

VI) Classification of Bricks Based on Using Location

a) Facing Brick

Facing Bricks are made primarily with a view to have good appearance, either of colour or texture or both. The face material of any building is known as facing brick. Facings bricks are standard in size, are stronger than other bricks and also have better durability. The colour is red or brown shades to provide a more aesthetic look to the building. There are many types of facing bricks which use different techniques and technology. Facing bricks should be weather resistant as they are most generally used on the exterior wall of buildings.

b) Backing Brick

These types of brick don’t have any special features. They are just used behind the facing bricks to provide support.

VII) Classification of Bricks Based on Weather-resisting Capability

a) Severe Weather Grade

These types of bricks are used in the countries which are covered in snow most of the time of year. These bricks are resistant to any kind of freeze-thaw actions.

b) Moderate Weather Grade

These types of bricks are used in tropical countries. They can withstand any high temperature.

c) No Weather Grade

These bricks do not have any weather resisting capabilities and used on the inside walls.

VIII) Classification of Bricks Based on Their Use

a) Common Bricks

These bricks are the most common bricks used. They don’t have any special features or requirements. They have low resistance, low quality, low compressive strength. They are usually used on the interior walls.

b) Engineering Bricks

These bricks are known for many reasons. They have high compressive strength and low absorption capacity. They are very strong and dense. They have good load bearing capacity, damp proof, and chemical resistance properties. They have a uniform red colour. They are classified as Class A, class B, class C. Class A is the strongest but Class B is most used. They are used for mainly civil engineering works like sewers, manholes, ground works, retaining walls, damp proof courses etc.

IX) Classification of Bricks Based on Shape

a) Bullnose Brick

These bricks are moulded into round angles. They are used for rounded quoin.

b) Airbricks

These bricks contain holes to circulate air. They are used on suspended floors and cavity walls.

c) Channel Bricks

They are moulded into the shape of a gutter or channel. They are used in drains.

d) Coping Bricks

They can be half round, chamfered, saddleback, angled varied according to the thickness of the wall.

e) Cow Nose Bricks

Bricks having double bullnose known as cow nose bricks.

f) Coping Bricks

These bricks are used to cap the tops of parapets or freestanding walls.

g) Brick Veneers

These bricks are thin and used for cladding.

h) Curved Sector Bricks

These are curved in shape. They are used in arcs, pavements, etc.

i) Hollow Bricks

These bricks are around one-third of the weight of the normal bricks. They are also called cellular or cavity bricks. Their thickness is from 20-25mm. These bricks pave the way to quicker construction as they can be laid quickly compared to the normal bricks. They are used in partitioning.

j) Paving Bricks

These bricks contain a good amount of iron. Iron vitrifies bricks at low temperature. They are used in garden park floors, pavements. These bricks withstand the abrasive action of traffic thus making the floor less slippery.

k) Perforated Bricks

These bricks contain cylindrical holes. They are very light in weight. Their preparation method is also easy. They consume less clay than the other bricks. They can be of different shapes like round, square, rectangular. They are used in the construction of the panels for lightweight, structures and multi-storeyed frame structures.

l) Purpose Made Bricks

These bricks are made for specific purposes. Engineering bricks are made for civil engineering constructions such as sewers, manholes, retaining walls. Fire bricks are made for chimneys and fireworks. Ornamental bricks are made to use for cornices and corbels. Arch bricks are used in arches.

X) Classification of Bricks Based on the Basis of Manufacture

a) Hand-made

These bricks are hand moulded.

b) Machine-made

Depending upon mechanical arrangement, bricks are known as

Wire Cut Bricks - bricks cut from clay extruded in a column and cut off into brick sizes by wires

Pressed Bricks - when bricks are manufactured from stiff plastic or semi-dry clay and pressed into moulds

Moulded Bricks - when bricks are moulded by machines imitating hand mixing.

XI) Classification of Bricks Based on the Basis of Burning

a) Pale Bricks

These are under burnt bricks obtained from outer portion of the kiln.

b) Body Bricks

These are well burnt bricks occupying central portion of the kiln.

c) Arch Bricks

These are over burnt also known as clinker bricks obtained from inner portion of the kiln.

XII) Classification of Bricks Based on the Basis of Type

a) Solid

Small holes not exceeding 25 per cent of the volume of the brick are permitted; alternatively, frogs not exceeding 20 per cent of the total volume are permitted.

b) Perforated

Small holes may exceed 25 per cent of the total volume of the brick.

c) Hollow

The total of holes, which need not be small, may exceed 25 per cent of the volume of the brick.

d) Cellular

Holes closed at one end exceed 20 per cent of the volume.

XIII) Classification of Bricks Based on the Basis of Finish

a) Sand-faced Brick

It has textured surface manufactured by sprinkling sand on the inner surfaces of the mould.

b) Rustic Brick

It has mechanically textured finish, varying in pattern.

Different Forms of Bricks

Some of the common type of bricks, depending upon the places of use, are shown in Fig.1. Round ended and bull nosed bricks are used to construct open drains. For door and window jambs, cant brick, also called splay brick are most suitable. The double cant brick is used for octagonal pillars. Cornice brick is used from architectural point of view. A compass brick which is tapering in both directions along its length is used to construct furnaces. Perforated brick is a well burned brick, but is not sound proof. The hollow bricks are about l/3rd of the weight of normal bricks and are sound and heat proof, but are not suitable where concentrated loads are expected. Top most bricks course of parapets is made with coping bricks and it drain off the water from the parapets. When the brick is cut along the length, it is called queen closer and when cut at one end by half header and half stretcher, it is known as king closer.

Fig. 1 Forms of Bricks

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