CONCRETE AND TYPES

CONCRETE

When we say concrete in the building trade, we actually mean reinforced concrete. Its full name is reinforced cement concrete, or RCC. RCC is concrete that contains steel bars, called reinforcement bars, or rebars. This combination works very well, as concrete is very strong in compression, easy to produce at site, and inexpensive, and steel is very very strong in tension.

To make reinforced concrete, one first makes a mould, called formwork, that will contain the liquid concrete and give it the form and shape we need. Then one looks at the structural engineer’s drawings and places in the steel reinforcement bars, and ties them in place using wire. The tied steel is called a reinforcement cage, because it is shaped like one. Once the steel is in place, one can start to prepare the concrete, by mixing cement, sand, stone chips in a range of sizes, and water in a cement mixer, and pouring in the liquid concrete into the formwork tilll exactly the right level is reached.

The concrete will become hard in a matter of hours, but takes a month to reach its full strength. Therefore it is usually propped up until that period. During this time the concrete must be cured, or supplied with water on its surface, which it needs for the chemical reactions within to proceed properly.
Working out the exact ‘recipe’, or proportions of each ingredient, is a science in itself. It is called concrete mix design. A good mix designer will start with the properties that are desired in the mix, then take many factors into account, and work out a detailed mix design. A site engineer will often order a different type of mix for a different purpose. For example, if he is casting a thin concrete wall in a hard-to-reach area, he will ask for a mix that is more flowable than stiff. This will allow the liquid concrete to flow by gravity into every corner of the formwork. For most construction applications, however, a standard mix is used. Common examples of standard mixes are M20, M30, M40 concrete, where the number refers to the strength of the concrete in n/mm2. Therefore M30 concrete will have a compressive strength of 30 n/mm2. A standard mix may also specify the maximum aggregate size. Aggregates are the stone chips used in concrete. If an engineer specifies M30 / 20 concrete, he wants M30 concrete with a maximum aggregate size of 20mm. He does NOT want concrete with a strength of between 20-30 n/mm2, which is a common misinterpretation in some parts of the world.

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Concrete that is cast in place in its mould is called cast-in-situ concrete. Concrete members that are cast in a concrete factory and then shipped to site are called precast concrete.

TYPES OF CONCRETE

The most common types of concrete are:

High Strength Concrete: the most basic and important property of concrete is its compressive strength. Concrete with a compressive strength of 40Mpa (5,800 psi) is called high strength concrete.

High Performance Concrete: is a new term for some concretes being developed today. It is a fairly broad term that describes concretes that outperform “normal”, everyday concrete in one or more characteristics such as lifespan, lifespan in corrosive environments, permeability, density, ease of placement, or many other parameters.

Lightweight Concrete: is made by using small, lightweight aggregates, such as small balls of styrofoam (thermocole) or by adding foaming agents to the mix of concrete. Lightweight concretes have low structural strength, and are used mostly in non-structural elements. The best is example is aerated autoclaved concrete (AAC) blocks used for making walls. Also called cellular concrete or aerated concrete.

Self Consolidating Concrete, also called Self Compacting Concrete:

Sprayed Concrete or Shotcrete: you can actually spray concrete onto a surface to form a thick, uneven coating. This process is different from other concreting methods in that the concrete is not poured into a form or mould. It is sprayed directly onto a surface, and is used in infrastructure projects and to repair old, cracked concrete surfaces. Shotcreting is also called guniting.

Water-resistant Concrete: normal concretes are permeable to water; that is, they allow water to pass through. Water resistant concretes are engineered to have fine particle cement replacements that do not allow water to pass through. These are very useful for construction below ground, like basements, as well as water retaining structures like water tanks and dams, and of course marine structures like jetties and bridges.

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Micro reinforced concretes: are a new generation of high-tech concretes. They contain small steel, fibreglass or plastic fibres that dramatically alter the properties of concrete.