What is a Concrete?
Concrete is a versatile construction material made by mixing cement, fine aggregates (such as sand), coarse aggregates (like gravel, stone chips, or rock), and water in specific proportions. When these components are properly combined, a chemical reaction between the cement and water occurs, forming a plastic mass that gradually hardens over time. This mixture, both in its plastic and hardened states, is referred to as concrete.
The characteristics of concrete depend on the proportions and quantities of its ingredients. However, these properties can also be significantly altered by the type of cement used in the mix. Concrete excels in compression but has limited tensile strength. In its fresh state, it can be poured into various forms or molds to achieve the desired shape and size. Due to its strength and versatility, concrete is widely used in construction projects, including buildings, bridges, dams, roads, and flyovers.
Composition Of Concrete
1. Binding Materials: While lime was once used in concrete, today cement is the primary binding material. When water is added to the mix, a chemical reaction occurs between the cement and water, forming concrete.
2. Aggregates: Aggregates act as filler materials in concrete. They don't undergo any chemical reactions but contribute to the mass and volume of the concrete while helping to reduce shrinkage.
3. Water: Water is essential for completing the chemical reaction with cement, which is crucial for the formation of concrete. It also plays a key role in determining the workability of the mixture.
4. Admixtures: Admixtures are optional materials added to the concrete mix to improve its properties and performance. There are four main types of admixtures: accelerating, retarding, air-entraining, and water-reducing admixtures, each serving specific purposes in concrete production.
Types of Concrete
Plain Cement Concrete (PCC)
Plain Cement Concrete is a basic concrete mix consisting of cement. fine aggregates, coarse aggregates, admixtures (optional), and water. PCC is very strong in compression. But, due to the absence of reinforcements like steel, it is very weak in tension. The general mix design ratio of PCC is 1:2:4 and 1:3:6. It is commonly used in the foundation of a building. After the hard strata have been reached, a layer of sand is laid followed by Plain Cement Concrete. Above the Plain cement concrete, the reinforcement cage of the footing is placed.
Applications:
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Foundations
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Grade slabs
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Concrete blocks
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Verandas
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Open Parking
Fibre Reinforced Concrete (FRC)
Fibre reinforced concrete is a composite material made up of cement, water, aggregates, and flat or rounded fibres. Various types of fibres like steel fibres, polypropylene fibres, glass fibres, asbestos fibres, carbon fibres and organic fibres are widely used to reduce permeability, bleeding and the formation of minor cracks. Fibres are added in the ratio of 0.1% to 3% of the total volume of the concrete. The dimensions of the fibres are represented using the term “aspect ratio” which is the ratio of the length to the diameter of the fibre. The aspect ratio of the fibre is generally 30 to 150. The orientation of the fibres is mostly random but they can be arranged in parallel or perpendicular fashion depending on the design criteria.
Applications:
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Dams, spillways, basins
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Pavements in airports and highways
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Bridge decks, Thin shelled structures
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Foundation, In refractories, Industrial floors, Machine foundation
Precast Concrete
Precast concrete structures are cast, cured, transported to the site, and erected using cranes. The precast structures are manufactured at the sites using moulds with reinforcement cages present inside them. They may or may not be prestressed based on requirements. The dried concrete members are cured in controlled conditions to achieve the desired strength. Special hooks are provided to the members to lift them. The design of precast members takes into account the handling and erection stresses that may arise during the construction process.
Applications:
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Precast slabs, beams, columns, wall panels can be used for conventional buildings
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Bridge decks, Parking, High rise building, Retaining walls, Sound walls, Culverts
Reinforced Cement Concrete (RCC)
Reinforced cement concrete is a composite material made up of cement, fine aggregates, coarse aggregates, admixtures, and steel reinforcements. It is simply Plain cement concrete with the presence of steel in it. All the limitations in Plain cement concrete can be overcome by reinforced cement concrete. Of all metals, steel is the most suitable reinforcement material because of the near same thermal coefficient of expansion. The thermal coefficient of concrete is 14.5 /C and that of steel is 12 /°C. Due to this property of steel, it is more compatible with concrete than other metals.
Applications:
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Slabs, beams, columns & foundations of buildings
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Dams, Pavements, Bridges Water tanks, Retaining walls
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Underwater constructions, Concrete sewers & pipes Canals
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Chimneys, Power plants
Ready Mix Concrete
Ready mix concrete is a factory-made concrete made of cement, aggregates, water, and admixtures and transported to the site. The ready-mix concrete is preferred when there is less space for storing and mixing the construction materials. The plant-made concrete is loaded into special delivery trucks called the transit mixers which have the provisions to constantly rotate and keep the concrete in motion and prevent setting.
Usually, retarders are added to the concrete mix to slow down the setting process to give allowance to the transportation and placing the time of the concrete. Quality check is performed both in the factory and at the site.
The difference in the slump value of both should not differ by more than 25 mm or 1/8th of the specific value whichever is greater.
Applications:
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Normally used for monolithic concreting of the roof slabs and beams
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Runways
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Pavements
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Lining of tunnels
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Dams and hydraulic structures
Prestressed Concrete
Prestressed concrete structures are made up of high-strength concrete and high-strength steel tendons in addition to the normal reinforcements. When the tendons are prestressed, the stress from the tendon is transferred to the concrete. Thus it improves the deflection resistance, load capacity, and overall structural performance of the member. Prestressed concrete structures are commonly used in the construction of prefabricated buildings.
A concrete member can be prestressed in two ways:
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Pre-tensioning
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Post-tensioning
Applications:
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Bridge decks
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Parking
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High rise buildings
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Retaining walls
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Sound walls
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Culverts
Light Weight Concrete
Light weight concrete is a special type of concrete used to reduce the self- weight of the structure. The reduction in self-weight can be achieved by any weight of the structure. The reduction in self-weight can be achieved by any of the following methods
1. Light Weight Aggregate Concrete:
Using light weight aggregates like silica sand, pumice, sawdust, scoria, volcanic cinder blocks, volcanic slag, tuff, crushed stone, and synthetic aggregates like coke breeze, foamed slag, bloated clay, expanded perlite, thermocol beads, broken bricks etc.
2. Aerated Concrete:
The concrete can be made light by increasing the air density inside to concrete from 300 kg/cu.m to 800 kg/cu.m. The air can be introduced by chemical reactions, using foam or chemicals like aluminum powder, hydrogen peroxide, and zinc compounds.
3. No fines Concrete:
In this concrete, the self-weight is reduced by removing the fine aggregates in the concrete. No fines concrete is made up of cement, coarse aggregate, and water. The Aggregate to cement ratio is set between 6:1 and 10:1.
Application:
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In precast elements
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Bridge decks
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Long span structures
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Filling for floor and roof slabs
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Partition walls
High Density Concrete
High-density concrete is a special type of concrete made up of cement, water fine aggregate, coarse aggregate, and high-density aggregate. The density of normal Plain cement concrete is 2400 kg/m3. For high- density concrete, the density ranges from 3360 kg/m3 to 3840 kg/m3. The density of the concrete is increased by increasing the cement content, reducing voids, and using high-density aggregates like barite, magnetite, serpentine, limonite, goethite, etc.,
Applications:
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Power plants
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Coal plants
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Research institutes
High Performance Concrete
High-performance concrete is a special type of concrete made using cement, water, fine aggregate, coarse aggregate, mineral admixtures, and superplasticizers.
The mix design ratio of the high-performance concrete is designed in such a way that it performs well both structurally and in durability criteria. The performance of the concrete can be improved by making the three phases of the concrete – the paste phase, transition phase, and aggregate phase stronger.
It can be done by increasing the cement content, restricting the water- cement ratio to not more than 0.3, using superplasticizers, mineral admixtures, and non-porous aggregates.
Applications:
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Power plants
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Chemical industries
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Coal plant
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Research Institutes
High Strength Concrete
High strength concrete is a special type of concrete that is made up of cement, water, fine aggregate, coarse aggregate, mineral admixtures, and superplasticizers. Mineral admixtures like fly ash, ground granulated blast furnace slaq, silica fume, rice husk ash have a high specific surface area which plays a major role in increasing the strength. High-strength concrete has a compressive strength of at least 70 MPa.
High-strength concrete is used in the precast industry. The concrete has to be strong enough to withstand the huge amount of prestressing that will be transferred through the process.
Self Compacting Concrete (SCC)
Self-compacting concrete is a special type of concrete made up of cement, fine aggregates, coarse aggregates, chemical admixtures to improve the work ability, flow ability, rheology, and mineral admixtures. SCC is also known as zero slump concrete. It has high work ability and does not require any extra compaction. It is used in congested reinforcements where it is quite complicated to achieve full compaction.
The flowability of the concrete can be achieved using viscosity modifying agents like sikaplast, retarders, air- entraining agents, very fine mineral admixtures, and superplasticizers.
Applications:
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In congested reinforcements like beam-column junctions.
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In places of heavy reinforcements.
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Places where compaction is not possible.
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Deep beams
Pumped Concrete
Pumped concrete is a special type of concrete that is suitable for pumping. Admixtures are added to improve the workability, flowability, and pumpability of the concrete.
Applications:
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Tall buildings
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Tunnels
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Underwater construction
Dry Lean Concrete (DLC)
Dry Lean Concrete (DLC) is an important part of modern rigid pavement. It is a plain concrete with a large ratio of aggregate to cement than conventional concrete and generally used as a base/sub base of rigid pavement.
Type: Domestic & Commercial
Pavement Quality Concrete (PQC)
Pavement Quality Concrete (PQC) is normally employed for providing a strong surface of rigid pavements. PQC is also used for construction of runways in airports in order to distribute the heavy impact load from wheel to sub- grade layers.