Conventional concrete made from Portland cement, cement aggregate, fine aggregate and water is extensively used as an important construction material all over the world and will continue to be in wide use in times to come. As a result of continued research recent development material and construction technologies, it has been possible to introduce new concept of the formulations of concrete. The concrete developed based on new concept has been termed as new concrete. New concrete can be broadly divided in the following categories.
- Polymer Concrete
We know that cement concrete is a porous material. The presence of pores reduces the strength and durability of concrete and it exhibits a tendency to deteriorate when exposed to action of serve chemicals. As a result of continued research it has been established that addition of polymer in concrete brings about marked improvement in its compressive strength, fatigue, resistance, toughness and durability. Polymer concrete is observed to be highly impermeable and resistant to attack by acids, alkalies and other chemicals.
Polymer concrete can be classified in following three categories:
Polymer Impregnated Concrete (PIC)
It is a hardened Portland cement concrete that has been impregnated with a monomer (low viscosity liquid organic material) and subsequently polymerised in situ. In this case the cement concrete is cast and cured in the conventional manner. After the concrete product gets hardened and dried, air from its voids is removed under partial vaccum and a low viscosity monomer (viny chloride, styrene, chlorostyrene etc.) is diffused through the pores of the concrete. The concrete product is then finally subjected to polymerisation by radiation or by heat treatment thereby converting the monomer filled in the voids into solid plastic. Polymer impregnated concrete products on account of their high resistance to wear and tear, impact etc., are used as kerb stones, pre-cast slabs for bridge decks, roads, in marine structures, food processing building etc.
Polymer Cement concrete (PCC)
This type of concrete is provided by incorporating an emulsion of a polymer or a monomer in ordinary Portland cement concrete. The ingredient comprising of cement, aggregates and monomer are mixed with water and the monomer in the concrete mix is polymerised after placing concrete in position. The resultant concrete has improved work-ability, strength, adhesion, chemical resistance, impact resistance, abrasion resistance besides having increased impermeability and reduced absorption. Polymer cement concrete can be used with advantage in marine works.
Polymer concrete (PC)
In this type of concrete polymer/monomer is used to act as binder in place of cement. The monomer and the aggregate are mixed together and the monomer is polymerised after placement of concrete in position. It is however, necessary to per heat the coarse and the fine aggregate while mixing monomer. Polymer concrete serves as a very good dielectric material. It can be used with advantage for the construction of irrigation works.
- Super Plasticized Concrete
This type of concrete in which highly effective plasticisers known as super plasticisers are incorporated as admixture to produce following concrete which is more of less self compacting type. Addition pf super plasticisers increases the workabiity of concrete, permits use of less quantity of water, thereby producing high strength concrete without increasing cement. Super plasticized concrete can be adopted in the following three possible situation.
- When it is required to produce concrete of required strength with reduced quantity of cement. We know that to achieve concrete of desired strength, it is necessary to maintain, certain fixed value of the water cement ratio. In case the cement content in the concrete is reduced, it would be necessary to have corresponding reduction in the quantity of mixing water to maintain the water cement ratio. Reduction of water can render to concrete mix comparatively dry, difficult to be placed in position and hence unworkable. Addition of super plasticisers in suitable doze, compensates for the reduction of mixing water and makes it possible to maintaining the water cement ratio as well as workability of the concrete with reduced quantity of cement.
- When it is required to produce high strength concrete having a very low water cement ratio. The increase in strength of concrete is generally proportional to the reduction in water cement ratio. It has been established that by adding super plasticiser it is possible to achieve water reduction up to 30% and water cement ratio as low as 0.2. Thus to achieve high strength concrete the water content of the mix is reduced, maintaining the cement content same and the admixtures of super plasticiser compensates the reduced workability due to use of less water without producing any determental effect either in the plastic or in the hardened concrete.
- When it is required to produce concrete having good flow-ability without any danger of bleeding, segregation or reduction in strength. By admixture of super-plasticiser it is possible to produce flowing concrete which is self compacting and self leveling type. In situations where concrete is to be placed in heavily reinforced structural members, super-plasticisers can be added to the concrete (without effecting any reduction in the cement or water content) with the sole aim of increasing the slump. In this process it is possible to achieve increase in slump from 75 to 200 mm. without producing adverse effect on the strength of the concrete.
- Fibre Reinforced Concrete (FRC)
Because of this low tensile strength and impact resistance, plain cement concrete is considered to be a brittle material. It has now been established that by addition of small diameter, short length, randomly distributed fibres it is possible to bring about marked improvements in the tensile strength and impact resistance properties of concrete. The fibre could be of steel, glass of asbestos. Concrete with fibre is termed as Fibre Reinforced Concrete.
Out of various possible types of fibre reinforced concrete the following two types are mostly being recommended.
- Steel Fibre Reinforced Concrete: This type of concrete is formed by adding steel fibres in the ingredients of concrete. Steel fibres are normally produced by cutting 10 to 60 mm., length of low carbon steel wires 0.25 mm to 0.75 mm., in diameter. Besides round fibres use of flat sheet steel fibres is also common. Flat sheet steel fibre are produced by shearing 0.15 mm to 0.40 mm thick steel plates in widths ranging from 0.25 mm to 0.90 mm and length 10 mm to 60 mm. Steel fibre have a tendency to cluster together which creates difficulties in ensuring their uniform random distribution in the concrete. This difficulty in overcome using fibre-bundles (Fibre loosely assembled in the from of bundles with the help of water soluble glue). The steel fibre in the fibre-bundles separate out during the process of mixing concrete and get distributed in a random fashion in the concrete mix. By addition of 2 to 3% of fibres (by volume) it is possible to achieve two to three times increase in the flexural strength of concrete and substantial increase in explosion resistance, crack resistance and other properties of the concrete. Steel fibre reinforced concrete is considered suitable for the construction of pavements, bridge deck pressure vessels, tunnels, lining etc.
- Glass Fibre Reinforced Concrete: It is observed that strength of glass fibre increases as its diameter is reduced. It is also seen that although small diameter glass fibre are reasonably strong in tension, they are very brittle and cannot be used in long lengths. Moreover glass fibres get corroded due to the effect of alkalie present in Portland cement. Thus to utilise glass fibres are micro-reinforcement they are suitably treated (coating with epoxy resign compounds etc.) to protect them from alkali attack. It is seen that addition of 10% glass fibre (by volume) brings almost two folds increase in tensile strength and substantial increase in impact resistance of concrete. The use of glass fibre reinforced concrete in the manufacture of precast products like spun pipes, wall cladding etc.
- Sulphur Impregnated Concrete
This type of concrete consists of a mixture of sulphur, coarse aggregate and fine aggregate (There is no cement or water). The concrete is formed by adding molten sulphur to the pre-heated aggregates in a mixer. The hot concrete mix is a poured in moulds to from pre-cast units. Sulphur impregnated concrete develops high compressive strength at an early age. It needs no curing and the mould can be stripped soon after the concrete mix get solidified. The main advantages of this type of concrete is that it can be re-used and re-moulded without any wastage. Sulphur impregnated concrete has good chemical durability and high strength. This concrete is more durable in acidic environments but in unstable in alkaline conditions. Due to its brittleness and corrosive action of sulphur on reinforcement, this type of concrete is not suitable for structural work. Thus concrete is used in industrial plant and for lining canals and tunnels.
- Roller Compacted Concrete
This type of concrete can be defined as a dry concrete materials which has been consolidated by external vibrations using vibratory rollers. It differs normal concrete basically in its consistency. The roller compacted concrete should be dry enough to support the load of vibratory rollers and at the same time it should be wet enough to permit proper consolidation by vibrations. This type of concrete is used with advantage in mass concrete work. It can be laid in thin layers of about 25 cm depth in continuous operation. This results in substantial reduction in the heat of hydration and hence the need to perform special concrete cooling operation in mass concrete work can be avoided. The use of roller compacted concrete permits faster construction and saving in labor cost.