Concrete technology deals with the fundamental principles of concrete is known as Concrete proportion, with or without a suitable admixture.
The practical utility of this technology for civil engineers is to enable them to know how to stock properly materials required for concrete and to perform different tests for concrete.
Classification of Concrete
The concretes are classified as follows:
1) Plain Cement Concrete (PCC)
Plain cement concrete consists of cement, sand, and coarse aggregate mixed in suitable proportions in addition to water.
The cement is used as a binding material, sand as fine aggregate and gravel, broken brick or crushed stone as coarse aggregates. PCC is strong in taking compressive loads.
2) Lime concrete:- The lime concrete consists of lime, fine aggregate, and coarse aggregate, mixed in suitable proportions, in addition to water.
The hydraulic lime is used as a binding material, sand, surki, and cinder as fine aggregates and broken bricks, and broken stones as coarse aggregates. Lime concrete is cheaper and has less strength than cement concrete.
3) Reinforced cement concrete (RCC):- Reinforced cement concrete (also called ferroconcrete) is a cement concrete in which reinforcement is embedded.
4) Pre-stressed cement concrete (P.C.C.):- It is a cement concrete in which high compressive stresses are artificially induced before its actual use. This type of concrete can take up high compressive and tensile stresses without the development of cracks.
5) Light-weight concrete:- This type of concrete possesses high insulating properties. Lightweight concrete is used in making precast structural units for partition & wall lining purposes.
Lightweight concrete has a density 1440 to 1840 kg/m’ where regular concrete has a density of about 2400 kg/m. The primary use of this concrete is to reduce the load of the concrete structure.
6) Cellular or aerated concrete:- The cellular or aerated concrete (also called air-entrained concrete) is prepared by mixing aluminum in the cement concrete. It is used for roof slabs and precast units in partitions for heat and sound insulation.
7) Saw dust concrete:- The sawdust concrete can be prepared by mixing Portland cement with sawdust in specified proportions in concrete. Saw dust concrete is used as a heat and sound insulating material.
8) Vacuum concrete:- Vacuum concrete is the concrete from which entrained air and excess water are removed with a vacuum pump, after placing it in position.
Constituents of concrete
cement is a binder, a substance that sets & hardens independently, & can bind other materials together. Commonly used cement for normal construction work is OPC (Ordinary Portland cement).
It is obtained by grading of clinker. Clinker is obtained by burning the mixture of calcareous & argillaceous group material in definite proportions.
Aggregate is an inert material that is chemically inactive. The aggregates may be natural or artificial. Natural aggregates such as crushed rock, sand, and gravel are used for reinforced concrete. Artificial aggregates such as furnace clinker, coke breeze, sawdust, and furnace slag are used for the manufacture of concrete of low density.
Water takes the most important role in concrete and is a less expansive constituent of concrete. pH value of water should range from 6-8. PH value of water is less than 6 is acidic in nature which may lead to corrosion of reinforcement.
Admixtures are the chemical compounds in concrete other than hydraulic cement (OPC), water & mineral additives that are added to the concrete mix immediately before or during the mix to modify one or more properties of concrete in either a fresh or hardened state.
Percentage strength of concrete at various ages
Strength of concrete increases with increase in age. The table below shows the strength of concrete at different ages.
Hydration in concrete
The reaction taking place in water and cement by virtue of which Portland cement becomes a binding agent is called the hydration of cement.
In the reaction time, a considerable amount of heat liberates& this liberation of heat is known as the heat of hydration and varies from 85 to 120 calories/gram. Early heat of hydration is contributed by the hydration of C3S.
The hydrates can be classified as
- Calcium Silicate hydrates.
- Tri calcium aluminates hydrate
- Calcium Silicate hydrates:
2 C3S+6H=>C3S2H3 +3 Ca(OH)2
2 C2S+4H=> C3S2H3 +Ca(OH)2
Where, C = CaO
S = Sio₂
H = H₂O
the product C3H2S3 is calcium silicate hydrate which is a gel structure.
From the above reaction following points may be concluded.
- Tri-calcium silicate needs more amount of water for hydration. This means
- It reacts faster than C2Sand hence provides strength faster than C2S.
- It evolves more heat of hydration.
- Excess quantity of C3Scement results in rapid hardening.
- C3S has more amount of Ca (OH)2 and this compound is not desirable in concrete. Ca(OH)2 is soluble in H2O& makes concrete porous.
- Excess quantity of C2Sin cement results in slow hardening, less amount of heat of hydration, and greater resistance to chemical attack.
- Calcium aluminates hydrates
2CA-21 H=> C4AH13 + C2AH8 => 2C3AH6+9H
From above reaction may conclude following points;
- The reaction of tricalcium aluminate with the eater is very fast & this may lead to a flash set.
- The contribution of C3A in strength is insignificant.
- Due to sulfate attack, the presence of hydrated acuminate is harmful to the durability of concrete.
Concrete mix design
The process of selecting suitable ingredients of concrete &determining their relative amounts with the objective of producing concrete with the required durability, strength, and workability as economically as possible, is known as concrete mix design.
Requirements of concrete mix design
The requirements which form the idea of selection & proportioning of mix ingredients are as follows:
- The minimum compressive strength is required from structural consideration
- The adequate workability isimportant for full compaction with the compactingequipment.
Types of Mixes
- Nominal Mixes:-The mix having fixed cement aggregate ratio which ensure adequate strength is known as nominal mix. Nominal mix is used for M20 or less strength of concrete.
M10 = 1:3:6
1) Standard mixes:-
The nominal mixes of fixed cement-aggregate ratio change broadly in strength and may result in under- or over-rich mixes. These mixes are termed standard mixes.
IS 456-2000 has designated the concrete mixes into a number of grades as M10, MI5, M20, M30, M35, and M40. In this designation, the letter M indicates the mix & number to the specified 28-day cube strength of mix in N/mm2. The mixes of grades M10(1:3:6), M151:2:4), M20(1:1.5:3) and M25(1:1:2).
2) Designed Mixes
In these mixes, the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content is laid down.