CONCRETE is made by
mixing:-
CEMENT

  • WATER
  • COARSE AND FINE AGGREGATES
  • ADMIXTURES (if required).The aim is to mix these materials in measured amounts
    to make concrete that is easy to:
  • TRANSPORT
  • PLACE
  • COMPACT
  • FINISH

and which will set, and harden, to give a strong and durable product. The amount
of each material (ie cement, water and aggregates) affects the properties of
hardened concrete.

CEMENT The cement powder, when mixed with water, forms a paste. This paste acts
like glue and holds or bonds the aggregates together. There are six major types
of cement:

  1. Type GP (General Purpose Portland cement)
  2. Type GB (General Purpose Blended Cement)
  3. Type HE (High Early Strength cement)
  4. Type LH (Low Heat cement)
  5. Type SR (Sulfate Resisting cement)
  6. Type SL (Shrinkage Limited cement)

Each type of cement will produce concrete with different properties. The most
common types of cement are Type GP and Type GB. Blended cements contain portland
cement and more than 5% of either fly ash, ground slag, silica fume, or a
combination of these.

STORAGE Cement should be stored off the ground in a well-aired, clean, dry
place. Wrapping the cement bags in plastic sheets gives extra protection, Bulk
cement will normally be stored in silos.

AGGREGATES Aggregates are of two basic types:

  1. COARSE: crushed rock, gravel or screenings.
  2. FINE: fine and coarse sands and crusher fines.

Sand should be concreting sand and not brickies sand or plasterers
sand. 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.

STORAGE Aggregates should be stored where they will stay clean, separated from
other materials and dry. If the aggregates are very wet use less water in the
mix.

WATER Water is mixed with the 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. Many concrete
plants now use recycled water.

Always check bore water before use. Don�t use sea water as it may rust the
steel reinforcement in the concrete.

ADMIXTURES Admixtures are mixed into the concrete to change or alter its
properties, ie the time concrete takes to set and harden, or its workability.

HOW THE PROCESS WORKS 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.