Tips for Concrete Making:
CONCRETE is made by mixing:

  • CEMENT
  • WATER


  • COARSE AND FINE AGGREGATES.
  • ADMIXTURES (if required).
 
Broad objectives to be kept in mind before making concrete
  • Concrete is to be made in such a way that
  • It is workable
  • Has the required compressive strength.
  • It is Durable.
  • The constituents is to be mixed in such a way that it is easy to
  • TRANSPORT
  • PLACE
  • COMPACT
  • FINISH concrete
 
Right place for storing cement
  • Cement can absorb moisture and hence must be stored carefully
  • Cement should be stored off the ground in a well-aired, clean, dry place.
 
Aggregates
Aggregates are of two basic types

  • COARSE: crushed rock, gravel or screenings.Size is more than 4.75 mm


  • FINE: fine and coarse sands and crusher fines.Size is generally less than 4.75 mm .
 
Properties of aggregates should be checked before use in concrete
Aggregates generally occupy 70 -80% of concrete depending on the grade of concrete. Hence the properties of aggregate influence the concrete quality.
Aggregates should be:

  • Strong and Hard: A stronger, harder aggregate will give a stronger final Concrete. Never use a crumble or flakey rock like sandstone.
  • Durable: To stand up to wear and tear and weathering.
  • Chemically inactive: So the aggregates don’t react with the cement.
  • Clean: Dirt or clay sticking to the aggregates will weaken the bond between paste and aggregates.


  • Graded: Aggregates should range in size so that they fit together well.

  • This gives a stronger and denser concrete.
  • Rounded aggregates give a more workable mix. Angular aggregates make concrete harder to place, work and compact, but can make concrete stronger.
 
Water for concrete

In large projects water is normally tested for its suitability.

  • We must ensure that water of the right quality is used since water is mixed with cement powder to form a paste which holds the aggregates together like glue.
  • Water must be clean, fresh and free from any dirt, unwanted chemicals or rubbish that may affect concrete.
  • Check bore water before use.
  • Don’t use sea water as it may rust the steel reinforcement in the concrete.
 
Purpose of chemical admixtures in concrete
  • Admixtures are mixed into the concrete to change or alter its properties, i.e.
    the time concrete takes to set and harden, or its workability or to increase the rate of gain of compressive strength.
 
Process of mixing ingredients
  • Measured amounts of the coarse and fine aggregates are mixed together.
  • A measured amount of cement is added and mixed in. Enough water is added to make the mix workable. All the materials are then mixed together well. The cement powder and water form a paste which bonds the aggregates together like glue.
 
Four main properties of concrete
The four main properties of concrete are:

  • WORKABILITY
  • COHESIVENESS


  • STRENGTH
  • DURABILITY
 
Different states of concrete
Concrete has three different states:

  • PLASTIC
  • SETTING
  • HARDENING
 
Distinguishing between three states of concrete
  • Plastic State : When the concrete is first mixed it is like 'bread dough'.It is soft and can be worked or moulded into different shapes. In this state concrete is called PLASTIC. Concrete is plastic during placing and compaction.
  • The most important properties of plastic concrete are workability and cohesiveness.
  • A worker will sink into plastic concrete.
  • Setting State: Concrete then begins to stiffen. The stiffening of concrete, when it is no longer soft, is called SETTING.Setting takes place after compaction and during finishing. Concrete that is sloppy or wet may be easy to place but will be more difficult to finishing.
  • Concrete that is sloppy or wet may be easy to place but will be more difficult to finish.
    A worker leaves footprints in setting concrete.
  • Hardening State: After concrete has set it begins to gain strength and harden. The properties of hardened concrete are strength and durability.
    Hardened concrete will have no footprints on it if walked on.
 
Workability of Concrete
Workability means how easy it is to:

  • PLACE
  • HANDLE
  • COMPACT
  • FINISH a concrete mix.

Concrete that is stiff or dry may be difficult to Handle, Place, Compact, and Finish and, if not constructed properly, will not be as strong or durable when finally hardened.
 
Major Factor affecting the workability of concrete
Workability is affected by the amount of cement paste, which is the soft or liquid part of the concrete mix. The more paste mixed with the coarse and fine aggregates, the more workable a mix.
 
Aggregate grading
Graded Aggregate means that there is a range of size of aggregates, from large rocks to smallsands. Well-graded aggregates give a more cohesive mix, too much coarse aggregate gives a boney mix.
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