- COARSE AND FINE AGGREGATES
- ADMIXTURES (if required).
The aim is to mix these materials in measured amounts to make concrete that is easy to:
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:
- Type GP (General Purpose Portland cement)
- Type GB (General Purpose Blended Cement)
- Type HE (High Early Strength cement)
- Type LH (Low Heat cement)
- Type SR (Sulfate Resisting cement)
- 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:
- COARSE: crushed rock, gravel or screenings.
- 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 dont 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. Dont 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.