What is concrete?
The word “concrete” originates from the Latin word “concretus” which means “to grow together”.
Concrete is a composite product produced by mixing cement, fine aggregate, coarse aggregate, water, and admixtures (if needed) in a suitable proportion.
Concrete is the second-most-used substance in the world after water and it is the most widely used building material in the construction industry.
What are the ingredients of concrete?
As stated above, the main ingredients of concrete are,
- Fine aggregates
- Coarse aggregates
Let’s discuss them one by one below.
Cement is a basic ingredient of concrete. It acts as a binder. The binder glues the aggregates to form a robust, stone-like material called concrete. In general, Portland cement is the most commonly used binder in cement concrete.
b) Fine aggregate
Fine aggregates stand for sand particles that are in the size range of 75 microns to 4.75mm.
c) Coarse Aggregate
Coarse aggregates are generally coarse gravel or crushed rocks such as limestone, or granite which are in the size range of 4.75mm to 75mm.
Aggregates are being used as filler materials in concrete to reduce the cost of concrete production.
Aggregates are the important constituent in concrete that occupies 65 to 70% of the volume of concrete. Therefore, it should be chemically inert, strong, hard, and durable.
Admixtures are the optional materials available in powder or liquid form that are added during concrete mixing to enhance the properties and performance of fresh concrete. The most common admixtures are accelerators and retarders.
Classification of concrete
We know that concrete is the most widely used construction material in the construction industry. It is used for the construction of beams, columns, slabs, floors, footings, etc.
For some of the above works like slab concreting, it is been used with the reinforcement bar but for some other works like flooring, it need not to be used with reinforcement bars.
Hence, we classify concrete broadly into two categories namely,
- Plain Cement Concrete
- Reinforced Cement Concrete
a) Plain Cement Concrete (PCC)
PCC is a basic concrete which consists of water, cement, aggregates, and admixtures (sometimes). Plain cement concrete can be cast without the use of any reinforcement bars. Such casted elements are only good in compression. Its tensile strength is only about one-tenth of its compressive strength.
It is generally used in foundation beds, parking slots, etc.
b) Reinforced cement concrete (RCC)
RCC is a composite material which is made up of cement, aggregates, water, admixtures, and reinforcing steel bars. These reinforcement bars are required to withstand the tension produced in the structural member when it is subjected to bending.
In other simple words, RCC is nothing but a PCC with a steel bar in it. Steel bar is selected as a reinforcement to concrete because the coefficient of thermal expansion for both steel and the concrete remains same.
What is the Grade of concrete?
The grade of concrete denotes the strength of the concrete at the end of 28 days. There are several grades of concrete available. The grade of concrete is designated by the letter M followed by a number.
- The letter M refers to the Mix
- The number refers to the characteristic compressive strength of a 150 mm size cube at 28 days, expressed in N/mm2.
According to IS 456: 2000, amendment 4, we can group concrete into 3 major categories based on their grades as listed below.
Characteristic compressive strength of 150 mm cube at 28 days in N/mm2
High Strength Concrete
Mix proportions of concrete
Concrete production involves the mixing of the above-stated ingredients in a suitable proportion. There are two types of mix proportions available. They are,
- Nominal mix proportion
- Design mix proportion
a) Nominal Mix
In the nominal mix, all the ingredients of the concrete are fixed to a certain standard amount. The proportions of ingredients are specified in the ratio of cement: Fine aggregate: Coarse aggregate. The ratios are already fixed based on standard empirical studies.
Since it lacks scientific background, we can use the nominal mix only for smaller and unimportant works.
As per the ‘Indian Standard- IS 456:2000’, a nominal mix may be used for concrete of M20 grade or lower grade such as M5, M7.5, M10, and M15.
The nominal mix can be easily identified by seeing their ratio itself. Generally, the nominal mix follows the 1: n: 2n relation.
Nominal mix proportions
Grade of concrete
Cement : FA : CA
|1: 5: 10|
1: 4: 8
1: 3: 6
1: 2: 4
1: 1.5: 3
b) Design Mix
Design mix can be defined as the process of selecting relative proportions of ingredients of concrete to produce concrete that possesses certain minimum strength and durability.
It is not a fixed proportion like the nominal mix. Here many factors such as the environment of concreting, available pieces of machinery, and quality of ingredients of concrete are taken into consideration.
The design mix is more scientific than the nominal mix and it is widely used for more extensive and important concrete works.
Design mix concrete is preferred over nominal mix concrete, especially where the concrete is above M20 grade.
Standard codes for concrete
There are various standard guidelines available for concrete and its design. Some of the important standards are,
a) Indian Standards
IS 456: 2000 – Code of practice for plain and reinforced concrete
IS 10262: 2009 – Concrete mix proportioning guidelines
b) American standards
ACI 318-95 -Building Code Requirements for Structural Concrete
ACI 319:1989 – Building code requirements for reinforced concrete
C) British Standards
BS 8500 is the British Standard for specifying and producing concrete. It is the complementary British Standard to BS EN 206. BS 8500 is split into two parts. Part 1 covers specifications and gives guidance to the specifier. Part 2 covers the constituent materials in concrete and contains the information required by the concrete producer.
Hope this article gives you a brief outline about the concrete and if you have any queries leave it in the comment section.