Special Concretes

Introduction

Special concretes are the concrete prepared for specific purpose like light weight, high density, fire protection, radiation shielding, etc. concrete is a versatile material processing good compressive strength. But it suffers from many drawbacks like how tensile strength, permeability to liquids, corrosion of reinforcement, susceptibility to chemical attack and low durability.
Modification have been made from time to time to overcome the deficiencies of cement concrete. The recent developments in the material and construction technology have led to significant changes resulting in improved performance, wider and more economical use.
Improvement in durability in terms of increased chemical and freezing resistances.
Improvements in impermeability, thermal insulation, abrasion, skid resistance, etc.

Types of Special Concrete

Light Weight Concrete

Lightweight Concrete can be defined as a type of concrete which includes an expanding agent in it that increases the volume of the mixture while reducing the dead weight.
It is lighter than the conventional concrete with a dry density of 300kg/m² up to 1840 kg/m³.
The main specialties of lightweight concrete are its low density and low thermal conductivity.

High Density Concrete

The high density concrete is used in the construction of radiation shielding i.e. in nuclear power plants. The questions of shielding resolves into protection against X-rays, Gamma rays and neutrons.
High density concrete is also suites for preparing counter balance weights for lifting bridges and ballast blocks for ships, where the high density concrete reduces the volume of concrete required to produce the same dead weight, leading to economy.
High density concrete is also known as ‘heavy weight concrete’. High density concrete is produced by replacing the ordinary aggregate by a material of very much higher specific gravity, usually over 4, compared with the specific gravity of ordinary aggregate of about 2.6.
Another type of natural heavy weight aggregate is iron: magnetite, limonite, hematite and goethite have been used. By using iron are aggregate concrete with densities of between 3000 to 3900 kg/m³ can be made.

Plum Concrete

The original idea of the use of aggregate as an inert filler can be extended to the inclusion of large stones up to 300 mm size in a normal concrete; thus the apparent yield of concrete for a given amount of cement is increased this result concrete is called ‘plum concrete’.
These large stones are called ‘plum’ and used in a large concrete mass.
The volume of plums should not exceed 20 to 30% of the total volume of the finished concrete and they to be well dispersed throughout the mass.
The plums must have no adhering coating. Otherwise discontinuities between the plums and concrete.

Aerated Concrete

Autoclaved Aerated Concrete (AAC) is one of the many building products being touted as green or environmentally friendly.
Produced by introducing air into the concrete.
It is also called cellular concrete having voids between 0.1mm to 1mm size.
Two ways are there to induce the air in concrete- Gas Concrete, Foamed Concrete.
Its quantity is about 0.2% of weight of cement.
Aluminium powder reacts with Ca(OH)2 to liberate hydrogen bubbles.
AAC products include blocks, wall panels, floor and roof panels, cladding panels and lintels.
AAC now accounts for over 40% of all construction in the United Kingdom and more than 60% of construction in Germany.

Fiber Reinforced Concrete (FRC)

In conventional concrete, micro-cracks develop even before loading because of drying shrinkage and other causes of volume change. When the structure is loaded, the micro cracks open up and propagate. The development of such micro-cracks is the main reason of inelastic deformation in concrete.
Fibre reinforced concrete (FRC) can be defined as a composite material consisting of concrete and discontinuous, discrete, uniform dispersed fine fibers. The continuous meshes, woven fabrics and long wires or rods are not considered to be discrete fibers.
The inclusion of fibers in concrete and shotcrete generally improves material properties like ductility, flexural strength, toughness impact resistance and fatigue strength. There is little improvement in compressive strength. The type and amount of improvement is dependent upon the fiber type, size, strength and configuration and amount of fiber.

Polymer Concrete

The partial or surface impregnation improves durability and chemical resistance while total or in-depth impregnation improves structural properties of concrete.
Polymer concrete is a mixture of aggregates with a polymer as a sole binder. There is no other bonding material present, i.e. Portland Cement is not used.
It is manufactured in a manner similar to that of cement concrete. Monomers or Pre-polymers are added to the graded aggregate and the mixture is thoroughly mixed by hand or machine. The thoroughly mixed, steel or aluminium, etc.
Impregnated concrete is produced by impregnating or infiltrating a hardened Portland cement concrete with a monomer and subsequently polymerizing the monomer in situ. It is one of the widely used polymers composite.

Ferro Concrete

It is well known that conventional reinforced concrete members are too heavy, brittle cannot be satisfactorily repaired if damaged, develop cracks and reinforcements are liable to be corroded.
Ferro concrete is a relatively new material consisting of wire meshes and cement mortar. It consists of closely spaced wire meshes which are impregnated with rich cement mortar mix.
This is a growing awareness of the advantages of this technique of construction all over the world.

High Strength Concrete

Pertaining to compressive strength, strength gain with age, specimen size effect, effects of drying, stress-strain curves, static modulus of elasticity, Poisson’s ratio, modulus of rupture, and split cylinder strength.
Using Type I Portland cement, gravel or crushed limestone coarse aggregate, sand from a local deposit, and for some mixes a water-reducing retarding admixture.
Water-cement ratio ranged from 0.70 to 0.32.
Concrete strength of 90-120 MPa.
Has to take care about mix proportioning, shape of aggregates, use of supplementary cementitious materials, silica fume and super plasticizers.

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