Tests on bricks

 To know the quality of Bricks, various tests are performed. They are

1. Compressive  strength  test 
2. Water Absorption  test 
3. Efflorescence test 
4. Hardness  test 
5. Size, Shape  and Colour   test 
6. Soundness  test 
7. Structure test 

1. Compressive strength test

              This  test  is  done  to know  the  compressive  strength of  brick.  It  is also called crushing strength of  brick. Generally  5  specimens  of  bricks  are  taken to laboratory for  testing and tested  one  by  one.  In this  test  a  brick specimen is  put  on crushing  machine  and applied  pressure  till it breaks.  The  ultimate  pressure  at which  brick  is  crushed  is  taken  into account.  All  five brick  specimens  are  tested  one  by  one  and  average  result  is  taken  as  brick’s compressive/crushing  strength. 

2. Water absorption test

          In this  test  bricks  are  weighed in dry  condition and let  them immersed in fresh  water  for  24  hours. After  24 hours  of  immersion those  are  taken  out  from water  and wipe  out  with cloth. Then brick is  weighed in wet  condition. The  difference between weights  is  the  water  absorbed by  brick.  The  percentage  of  water  absorption is  then calculated.The less  water  absorbed  by  brick  the  greater  its  quality.  Good quality  brickdoesn’t absorb more  than 20%  water  of  its  own weight. 

3. Efflorescence test

          The  presence  of  alkalies  in  bricks  is  harmful  and  they  form  a  grey  or white  layer  on brick surface  by  absorbing  moisture. To find out  the  presence  of  alkalis  in bricks  this  test  is  performed.  In this  test  a  brick  is  immersed in fresh  water  for  24 hours  and then it’s  taken out  from  water  and allowed  to dry  in shade.If  the  whitish layer  is  not  visible  on surface  it  proofs  that  absence  of  alkalis  in brick.  If  the  whitish layer  visible  about  10%  of brick  surface then  the presence of  alkalis  is  in  acceptable  range.  If  that  is  about  50%  of surface then  it  is  moderate.  If  the  alkalies’  presence is  over  50%  then  the brick  is  severely affected  by  alkalies. 

4. Hardness test 

            In this  test  a  scratch is  made  on brick surface  with a  hard thing.  If  that  doesn’t left  any  impression on brick then that  is  good quality  brick. 

5. Size,  shape  and  colour  test

                In this  test  randomly  collected 20 bricks  are  staked along lengthwise,  width wise  and height  wise  and then those  are  measured to know  the  variation of sizes  as  per  standard.  Bricks  are  closely  viewed  to  check  if  its  edges  are  sharp  and  straight and uniform  in shape.  A  good quality  brick  should have  bright  and uniform  colour throughout. 

6. Soundness test

               In this  test  two bricks  are  held by  both hands  and  struck  with one  another.  If the  bricks  give  clear  metallic  ringing  sound  and don’t  break  then those  are  good quality bricks. 

7. Structure  test

In this  test  a  brick is  broken or  a  broken  brick  is  collected  and  closely observed.  If  there  are  any  flows,  cracks  or  holes  present  on that  broken face  then that  isn’t good quality  brick. 

Constituents of good brick earth

   In order to get  a  good quality  brick, the  brick earth should contain the  following  constituents. 

1. Silica 
2. Alumina 
3. Lime 
4. Iron oxide 
5. Magnesia 

1. Silica

• Brick  earth should contain about  50 to 60 %  of  silica. 
• It  is  responsible  for  preventing  cracking, shrinking and warping  of  raw  bricks. 
• It  also  affects  the  durability  of  bricks. 
• If  present  in  excess,  then  it  destroys  the  cohesion  between  particles  and  the brick becomes  brittle.

2. Alumina 

• Good brick earth should contain about  20%  to 30%  of  alumina. 
• It is  responsible  for  plasticity  characteristic  of  earth,  which  is  important in  moulding operation. 
• If  present  in excess, then the  raw  brick shrink and  warp during  drying. 

3. Lime

• The percentage of  lime  should be  in the  range  of  5%  to 10%  in a  good brick earth. 
• It  prevents  shrinkage  of  bricks  on drying. 
• It  causes  silica  in clay  to melt  on burning  and thus  helps  to bind it. 
• Excess  of  lime  causes  the  brick to melt  and brick looses  its  shape. 

4. Iron  oxide 

• A good brick earth should contain about  5%  to 7%  of  iron oxide. 
• It  gives  red colour  to the  bricks. 
• It improves  impermeability  and  durability. 
• It  gives  strength and hardness. 
• If  present  in excess, then the  colour  of  brick becomes  dark blue  or  blakish. If  the  quantity  of  iron oxide  is  comparatively  less,  the  brick becomes  yellowish colour 

5. Magnesia

• Good brick earth should contain less  a  small  quantity  of  magnesia  about 1%
• Magnesium in  brick  earth  imparts  yellow  tint to  the  brick. 
• It  is  responsible  for  reducing  shrinkage 
• Excess  of  magnesia leads  to  the decay  of  bricks. 

Bricks

 Brick is the most commonly used construction material. 

• Standard size : 190mm x 90 mm x 90mm
• Nominal size  : 200mm x 100 mm x 100 mm

On the basis of manufacturing and preparations,

1. First class bricks
2. Second class bricks
3. Third class bricks
4. Fourth class bricks

1. First class bricks

• Table moulded and burnt in kilns
• Standard shape, edges are sharp, square, smooth and straight
• Used for superior work of permanent nature

2. Second class bricks

• Ground moulded and burnt in kilns
• Surface is rough and shape is irregular 
• Edges are not sharp and uniform
• Used at places where brick work is to be provided with a coat of plaster

3. Third class bricks

• Ground moulded and burnt in clamps
• Rough surface with irregular and distorted edge
• Used for unimportant and temporary structures

4. Fourth class bricks

• Over burnt bricks
• irregular and dark colour
• Used as aggregate for concrete in foundations, floors, roads etc 

Timber: Structure of Trees

 From  the visibility  aspect, the structure of a tree can be divided into two categories 

1. Macro structure 

2. Micro structure 

1. Macro structure:  

         The structure of wood visible to the naked eye or at a small magnification is called macro structure. Figure shows the macro structure of exogenous tree.

Image source: http://4.bp.blogspot.com

(i) Pith:  

     The innermost central portion or core of the tree is called pith  or medulla 

(ii) Heart wood: 

    The inner annual rings surrounding the pith is known as heart wood. It imparts rigidity to tree

(iii). Sap wood:  

      The cuter annual rings between heart wood and cambium  layer is known as sap wood 

(iv) Cambium layer:  

      Thin layer of sap between sap wood and inner bark is known as cambium  layer 

(v) Inner bark:  

        The inner skin or layer covering the cambium  layer is known as inner bark  

(vi) Outer Bark:  

       The outer skin or  cover of the tree is known as outer bark 

(vii) Medullary rays:  

       The thin radial fibres extending from  pith to  cambium  layer are known as medullary rays

2. Micro strructure:  

      The structure of  wood apparent only at great magnifications is called micro structure under micro scope, it becomes evident that the wood consists of living and lead cells of various sizes and shapes. 

Timber: Classification of Trees

 Depending upon their mode of growth trees may be divided in the following two categories 

(i) Endogeneous trees:

 These trees grow inwards and fibrous mass is seen in their longitudinal sections. Timber from these trees has very limited engineering applications Ex: bamboo, cane , palm  etc 

(ii) Exogeneous trees:  

These increases in bulk by growing outwards and used for engineering purposes. Exogeneous trees are further sub divided into two groups   

      a) Conifers or evergreen trees:

These trees having pointed, needle like or scale like leaves and yield soft wood

        b) Deciduous trees: 

The trees having flat broad leaves and leaves of those trees fall in autumn and new ones appear in spring season. Timber for engineering purpose is mostly derived from  deciduous trees. These trees yield hard wood. 

Eg.. ash, beach, oak, sal, teak, shishum and wallnut 

Bulking of sand

        The presence of moisture in sand increases the volume  of sand. This is  due to fact that  moisture causes film  of water around the sand particles  which result in  the increase of volume  of sand. For a  moisture content of 5 to 8  percent, the increase in  volume may  be about 5 to 8 percent, depending upon the grading of sand. The finer the  material, the  more will be the increase in volume for a given moisture content. This  phenomenon is known as bulking of sand.  

         When  moisture content  is  increased by  adding  more water, sand particles pack near each other and  the amount of bulking of sand is decreased. Thus the dry  sand and the sand completely  flooded with  water have  practically  the same  volume.

Grading of sand

      According to the state of  grains, sand is classified as fine, coarse and gravelly 

     Sand passing through a screen with clear opening of 1.5875mm  is known as fine sand. It  is generally used for masonry works. 

     Sand passing through a screen with clear openings of 7.62mm  is  known as gravely sand. It is generally  used for plastering. 

     Sand passing through a screen with clear opening of 3.175mm  is  known as coarse sand. It is generally  used for masonary  work