This is a very synonymous trade with construction works, especially the construction of buildings. Most of the modern construction has its structural frame fabricated out of concrete. There are two main types of concrete:
· Mass Concrete: This is usually a 'weak' form of concrete. This means that it does not perform any load bearing duties. It is used in ground floor slabs, platforms or simple bases for light works.
· Structural (reinforced) concrete: This is used to carry loads and is therefore a common part of buildings and other structures like retaining walls. It is usually added strength properties by the use of steel (reinforcement). The specification of concrete is usually dependent on the amount of strength it is supposed to carry.
Concrete is made from a mixture of the following main parts;
· Ballast (Course aggregate): This forms the bulk of concrete parts. It is made from crushed stone and the size of the aggregates determines (partly) the strength and workability of concrete.
· Sand (fine aggregate): This is the second largest part of concrete. In concrete, it fills the gaps in between the course aggregates.
· Cement (binder): This binds together the other parts of concrete and hardens in a chemical process to form a strong mass, shaped to the desired form and size.
The three parts form the main type of concrete, which is usually given as ratio as in the following manner; (1:2:4 - One part Cement, Two parts Sand and Four parts Ballast). The three are mixed with clean water in a specified quantity. Cement reacts with water to form its strength.
Other constituents of concrete include;
· Reinforcement: Usually in the form of steel bars, but also in the form of fibrous material. Gives concrete enhanced strength properties.
· Additives: Chemical compounds added in concrete to give it enhanced characteristics like more flowability and water resistance.
Alongside this, concrete also needs moulds, commonly known as formwork, to give it support as well as shape it in the desired form, for example circular shape in columns. The conventional formwork was made of timber and poles, although this is quickly changing to other materials, mainly steel and aluminium.
Concrete works constitute the following works, which are either carried out by hand, by machine or a combination of both.
· Formwork construction: Depending on the material to be used, this can be purely a carpentry affair or a simple steel assembly. It also includes works like leveling and waterproofing. It also involves removal of formwork after a specified period, for example seven days, a process known as striking.
· Mixing: As earlier discussed. It is either done on site or mixed offsite and delivered in premix trucks.
· Steel Fixing: Where reinforced concrete to be set up, steel bars are usually fabricated and laid into the formwork before concrete is put. This involves works like bending, cutting and binding. This process is usually guided by an engineer's drawing. This is mostly a manual affair.
· Placing: This refers to the act of putting concrete into the formwork. It is in most cases done above the ground level and as such involves either hoisting or dropping. Hoisting is either done manually or mechanically using hoists or concrete pumps.
· Compacting: This is done to make sure that concrete forms a compact mass, thus enhancing its strength. In small scale projects, it is done by hand, but in large projects, it is either done using poker vibrators (hand held or otherwise) or by use of additives to achieve self-compacting concrete.
· Curing refers to the task of allowing proper chemical bonding of cement by hydration (watering). It is usually done for a specified period of time, for example thirty days.