Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water. This can be easily moulded to desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension. The tensile property is introduced in concrete by inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete and Ferro cement.
In this article proportioning, mixing, curing, properties, tests and uses of plain concrete is dealt in detail. The other improved versions of concrete are explained and their special properties and uses are pointed out.
Major ingredients of concrete are:
1. Binding material (like cement, lime, polymer) 2. Fine aggregate (sand) 3. Coarse aggregates (crushed stone, jelly) 4. Water.
A small quantity of admixtures like air entraining agents, water proofing agents, workability agents etc. may also be added to impart special properties to the plain concrete mixture. Depending upon the proportion of ingredient, strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by mix design procedure. In the absence of mix design the ingredients are proportioned as 1:1:2, 1:1-1/2:3, 1:2:4, 1:3:6 and 1:4:8, which is the ratio of weights of cement to sand to coarse aggregate.
In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various works are shown in following table.
Functions of Various Ingredients in Concrete
Cement is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.
Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin. They should be clean, sharp, angular and hard. They give mass to the concrete and prevent shrinkage of cement. Fine aggregate consists of river sand. It prevents shrinkage of cement. When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete, since it fills the voids. Denser the concrete higher is its strength.
Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete.
Figure 1 shows the variation of strength of concrete with water cement ratio. To achieve required workability and at the same time good strength a water cement ratio of 0.4 to 0.45 is used, in case of machine mixing and water cement ratio of 0.5 to 0.6 is used for hand mixing.
Uses of Concrete
- As bed concrete below column footings, wall footings, on wall at supports to beams
- As sill concrete
- Over the parapet walls as coping concrete
- For flagging the area around buildings
- For pavements
- For making building blocks.
However major use of concrete is as a major ingredient of reinforced and prestressed concrete. Many structural elements like footings, columns, beams, chejjas, lintels, roofs are made with R.C.C. Cement concrete is used for making storage structures like water tanks, bins, silos, bunkers etc. Bridges, dams, retaining walls are R.C.C. structures in which concrete is the major ingredient.
Reinforced Cement Concrete (R.C.C.)
Concrete is good in resisting compression but is very weak in resisting tension. Hence reinforcement is provided in the concrete wherever tensile stress is expected. The best reinforcement is steel, since tensile strength of steel is quite high and the bond between steel and concrete is good. As the elastic modulus of steel is high, for the same extension the force resisted by steel is high compared to concrete.
However in tensile zone, hair cracks in concrete are unavoidable. Reinforcements are usually in the form of mild steel or ribbed steel bars of 6 mm to 32 mm diameter.
A cage of reinforcements is prepared as per the design requirements, kept in a form work and then green concrete is poured. After the concrete hardens, the form work is removed. The composite material of steel and concrete now called R.C.C. acts as a structural member and can resist tensile as well as compressive stresses very well.
Properties of R.C.C./Requirement of Good R.C.C.
- It should be capable of resisting expected tensile, compressive, bending and shear forces.
- It should not show excessive deflection and spoil serviceability requirement.
- There should be proper cover to the reinforcement, so that the corrosion is prevented. 4. The hair cracks developed should be within the permissible limit. 5. It is a good fire resistant material.
- When it is fresh, it can be moulded to any desired shape and size.
- Durability is very good.
- R.C.C. structure can be designed to take any load.
Uses of R.C.C.
- It is a widely used building material. Some of its important uses are listed below: R.C.C. is used as a structural element, the common structural elements in a building where R.C.C. is used are: (a) Footings (b) Columns (c) Beams and lintels (d) Chejjas, roofs and slabs. (e) Stairs.
- R.C.C. is used for the construction of storage structures like (a) Water tanks (b) Dams (c) Bins (d) Silos and bunkers.
- It is used for the construction of big structures like (a) Bridges (b) Retaining walls (c) Docks and harbours (d) Under water structures.
- It is used for pre-casting (a) Railway sleepers (b) Electric poles
- R.C.C. is used for constructing tall structures like (a) Multi-storey buildings (b) Chimneys (c) Towers.
- It is used for paving (a) Roads (b) Airports.
- R.C.C. is used in building atomic plants to prevent danger of radiation. For this purpose R.C.C. walls built are 1.5 m to 2.0 m thick.
Reinforced Brick Concrete (RBC)
It is the combination of reinforcement, brick and concrete. It is well known fact that concrete is very weak in tension. Hence in the slabs, lintels and beams the concrete in the portion below the neutral axis do not participate in resisting the load. It acts as a filler material only. Hence to achieve economy the concrete in tensile zone may be replaced by bricks or tiles. Dense cement mortar is used to embed the reinforcement. The reinforcement may be steel bars, expanded mesh etc.
Prestressed Concrete (PSC)
Strength of concrete in tension is very low and hence it is ignored in R.C.C. design. Concrete in tension is acting as a cover to steel and helping to keep steel at desired distance. Thus in R.C.C. lot of concrete is not properly utilized.
Prestressing the concrete is one of the method of utilizing entire concrete. The principle of prestressed concrete is to introduce calculated compressive stresses in the zones wherever tensile stresses are expected in the concrete structural elements. When such structural element is used stresses developed due to loading has to first nullify these compressive stresses before introducing tensile stress in concrete.
Thus in prestressed concrete entire concrete is utilized to resist the load. Another important advantage of PSC is hair cracks are avoided in the concrete and hence durability is high. The fatigue strength of PSC is also more.
The deflections of PSC beam is much less and hence can be used for longer spans also. PSC is commonly used in the construction of bridges, large column free slabs and roofs. PSC sleepers and electric piles are commonly used. The material used in PSC is high tensile steel and high strength steel. The tensioning of wires may be by pretensioning or by post tensioning.
Pretensioning consists in stretching the wires before concreting and then releasing the wires. In case of post tensioning, the ducts are made in concrete elements. After concrete of hardens, prestressing wires are passed through ducts. After stretching wires, they are anchored to concrete elements by special anchors.
Fibre-Reinforced Concrete (FRC)
Plain concrete possesses deficiencies like low tensile strength, limited ductility and low resistance to cracking. The cracks develop even before loading. After loading micro cracks widen and propagate, exposing concrete to atmospheric actions. If closely spaced and uniformly dispersed fibres are provided while mixing concrete, cracks are arrested and static and dynamic properties are improved.
Fibre reinforced concrete can be defined as a composite material of concrete or mortar with discontinuous and uniformly distributed fibres. Commonly used fibres are of steel, nylon, asbestos, coir, glass, carbon and polypropylene. The length to lateral dimension of fibres range from 30 to 150. The diameter of fibres vary from 0.25 to 0.75 mm. Fibre reinforced concrete is having better tensile strength, ductility and resistance to cracking.
Uses of FRC
- For wearing coat of air fields, roads and refractory linings.
- For manufacturing precast products like pipes, stairs, wall panels, manhole covers and boats.
- Glass fibre reinforced concrete is used for manufacturing doors and window frames, park benches, bus shelters etc.
- Carbon FRC is suitable for structures like cladding and shells.
- Asbestos FRC sheets are commonly used as roofing materials.
It is a light weight concrete produced by introducing large voids in the concrete or mortar. Its density varies from 3 kN/m3 to 8 kN/m3 whereas plain concrete density is 24 kN/m3. It is also known as aerated, foamed or gas concrete.
Properties of Cellular Concrete
It has the following properties:
- It has low weight.
- It has good fire resistance.
- It has good thermal insulation property.
- Thermal expansion is negligible.
- Freezing and thawing problems are absent.
- Sound absorption is good.
- It has less tendency to spall.
Uses of Cellular Concrete
- It is used for the construction of partition walls.
- It is used for partitions for heat insulation purposes.
- It is used for the construction of hollow filled floors.
The term ferro-cement implies the combination of ferrous product with cement. Generally this combination is in the form of steel wires meshes embedded in a portland cement mortar. Wire mesh is usually of 0.8 to 1.00 m diameter steel wires at 5 mm to 50 mm spacing and the cement mortar is of cement sand ratio of 1:2 or 1:3. 6 mm diameter bars are also used at large spacing, preferably in the corners. Sand may be replaced by baby jelly.
The water cement ratio used is between 0.4 to 0.45. Ferro-cement reinforcement is assembled into its final desired shape and plastered directly. There is no need for form work. Minimum two layers of reinforcing steel meshes are required.
According to American Concrete Institute “Ferro cement is a thin walled reinforced concrete construction where usually a hydraulic cement is reinforced with layers of continuous and relatively small diameter mesh. The mesh used may be metallic or any other suitable material.” Ferro-cement is fast emerging as an alternate material for timber.
The history of ferro-cement goes back to 1843 (even before RCC). Joseph Louis Lambet constructed several rowing boats, plant plots and garden seats using ferro-cement. In early 1940’s noted Italian engineer and architect Pier Luigi Nervi carried out scientific tests on ferro-cement and used it to replace wood wherever possible. He built small tonnage vessels, the largest being 165 tons motor sailor.
Nervi also pioneered the architectural use of ferro-cement in buildings. Ferro-cement can be given the finish of teak wood, rose wood etc. and even for making tables, chairs and benches it can be used.
Properties of Ferro-Cement
- Its strength per unit mass is high.
- It has the capacity to resist shock laod.
- It can be given attractive finish like that of teak and rose wood.
- Ferro cement elements can be constructed without using form work.
- It is impervious.
Uses of Ferro-Cement
It can be used for making:
- Partition walls
- Window frames, chejjas and drops
- Shelf of cupboards
- Door and window shutters
- Domestic water tanks
- Precast roof elements
- Reapers and raffers required for supporting roof tiles.
- Pipes, silos, furniture, manhole covers, boats.