Pre-Mixed Concrete
What is Concrete?
Concrete is composed of aggregates (crushed raw materials of rock and sand) bonded together with cementitious binders (cement) and water. The cement and water form a paste that glues the aggregates together when it hardens. Concrete, in its freshly mixed state, is a plastic, workable mixture that can be formed into almost any desired shape. It starts to slowly stiffen when mixed, but remains plastic and workable for several hours. This is a long enough period to allow it to be placed and finished. After the initial set, it continues to gain strength for months and sometimes years if moisture continues to be present.
Concrete production is the process of combining water, aggregates, cement and additives. These different ingredients are mixed at our Campbellfield batching plant. A batching plant consists of large storage silos for the various cement components, and an area for the bulk ingredients like aggregates, additives and water.
The batching plant also features mechanisms for the addition of various additives, machinery to accurately weigh, move and mix all of those ingredients to load the mixed concrete, into a concrete agitator (truck). Concrete is always time-sensitive, once the ingredients are mixed, customers must put the concrete in place before it hardens.
Being one of the most versatile building materials, concrete is used in many forms of construction in Australia. Concrete can be used in residential driveways, house foundations, walls, as well as in major infrastructure, including for the foundations of high rise buildings, lift shafts and major roads. Filling pre-made moulds with concrete is referred to as ‘pre-cast’.
Cement
Cement and Concrete are not the same thing. Cement is a dry powdered chemical that, when mixed with water, slowly reacts to form a hard, solid compound.
Aggregates & Sand
Concrete is a mix of sand (fine aggregate) with particle sizes smaller than about 5mm, which can be natural or manufactured and gravel or crushed stone (coarse aggregate) with particle sizes of 7mm, 14mm and 20mm. Fine and coarse aggregates of a raw natural quality from quarries is added to the cement-water paste.
Water
Water is used for two main purposes at a concrete plant - to mix with the dry products to create concrete, and to clean, or ‘wash out’ the concrete equipment. The objectives for water used in the concrete washout process are to collect and retain all washout water and solids in holding pits, preventing the harsh materials from reaching soil and migrating to the surface of ground waters.
Concrete should be workable; it should not be so stiff or so wet that segregation occurs; nor should it bleed excessively. Bleeding is the movement of water to the surface of freshly placed concrete. Excessive bleeding of water to the surface increases the water-cement ratio at the surface. A weak layer of poor durability may result, particularly if finishing operations take place while the excess water is present.
The cementing property of the paste results from a chemical reaction between the cement and water, called hydration. It is a reaction that requires time and favourable conditions of temperature and moisture. Curing is the provision of favorable temperature and moisture conditions over a period of time long enough to allow the hydration process to approach completion.
With proper curing, hydration takes place very rapidly at first, and then decreases slowly for a long time. This allows the concrete to develop its strength and durability. Concrete needs continued moisture to develop its full potential. It should not dry out too quickly.
Most concrete is designed with a certain cement content and enough water to make the mass workable. Providing work-ability is adequate, reducing the mixing water content makes the batch stronger and the addition of water makes the batch weaker.
Strength of Concrete
The compressive strength of concrete is measured by how much force is required to crush it, this is important in the design of structures. In pavements and other slabs on the ground, the design is usually based on flexural strength (how much force the concrete can withstand in bending before it breaks). The principal factors affecting strength are the water-cement ratio and the extent to which hydration has progressed.
