Special Concretes

Special Concretes

Concrete is most vital material in modern construction.

In addition to normal concrete, other varieties in use are, high strength and high-performance concrete, self-compacting, lightweight, high density, fiber reinforced, polymer, colored concrete, etc.

The making of concrete is an art as well as a science.

Special Concretes

Special types of concrete are those with out-of-the-ordinary properties or those produced by unusual techniques. Concrete is by definition a composite material consisting essentially of a binding medium and aggregate particle, and it can take many forms.

These concretes do have advantages as well as disadvantages.

Types of special concrete

  1. High Volume Fly Ash Concrete.
  2. Silica fume concrete.
  3. GGBS, Slag based concrete.
  4. Ternary blend concrete.
  5. Lightweight concrete.
  6. Polymer concrete.
  7. Self-Compacting Concrete.
  8. Coloured Concrete.
  9. Fiber-reinforced Concrete.
  10. Pervious Concrete.
  11. Water-proof Concrete.
  12. Temperature Controlled Concrete.

High Volume Fly Ash Concrete

It is used to replace a portion of the Portland cement used in the mix.

According to IS: 456 – 2000 replacement of OPC by Fly-ash up to 35% as binding material is permitted.

HVFAC is a concrete where excess of 35%of fly-ash is used as replacement.

Use of fly ash is because of many factors such as-

  1. Abundance of fly ash i.e. 110million tons of fly ash is produced in India every year.
  2. Fly ashes from major TPP are of very high quality i.e. quality of fly ash.
  3. Economic factor i.e. Cost of fly ash within 200 km from a TPP is as low as 10% to 20% of the cost of cement.
  4. Environmental factors i.e. reduction in CO2 emission.

Silica fume concrete

Very fine non-crystalline silica produced in electric arc furnaces as a by-product.

Highly reactive pozzolan used to improve mortar and concrete.

Silica fume in concrete produces two types of effect viz.-

  • Physical effect
  • Chemical effect

The transition zone is a thin layer between the bulk hydrated cement paste and the aggregate particles in concrete. This zone is the weakest component in concrete, and it is also the most permeable area. Silica fume plays a significant role in the transition zone through both its physical and chemical effects.

Ternary blend concrete

Ternary concrete mixtures include three different cementitious materials i.e. combinations of Portland cement, slag cement, and a third cementitious material. The third component is often fly ash, but silica fume is also common.

Other material in combination with Portland and slag cement, such as rice husk ash are not currently in common usage.

Slag cement has been used in ternary mixtures for decades.

Ternary mixtures can be used and have been used in virtually any concrete application-

  1. General construction (residential, commercial, industrial)
  2. Paving
  3. High performance concrete
  4. Precast concrete
  5. Masonry and masonry units
  6. Mass concrete
  7. Shotcrete

Light weight concrete

Structural lightweight concrete is similar to normal weight concrete except that it has a lower density.

Made with lightweight aggregates.

Air-dry density in the range of 1350 to 1850 kg/m3

28-day compressive strength in excess of 17 Mpa.

Structural lightweight concrete is used primarily to reduce the dead-load weight in concrete members, such as floors in high-rise buildings.

Structural Lightweight Aggregates:

 Rotary kiln expanded clays, shales, and slates

 Sintering grate expanded shales and slates

 Pelletized or extruded fly ash

 Expanded slags

Polymer concrete

Polymer concrete is part of group of concretes that use polymers to supplement or replace cement as a binder. The types include polymer-impregnated concrete, polymer concrete, and polymer-Portland cement concrete.

  • In polymer concrete, thermosetting resins are used as the principal polymer component due to their high thermal stability and resistance to a wide variety of chemicals.
  • Polymer concrete is also composed of aggregates that include silica, quartz, granite, limestone, and other high quality material.
  • Polymer concrete may be used for new construction or repairing of old concrete.
  • The low permeability and corrosive resistance of polymer concrete allows it to be used in swimming pools, sewer structure applications, drainage channels, electrolytic cells for base metal recovery, and other structures that contain liquids or corrosive chemicals.
  • It is especially suited to the construction and rehabilitation of manholes due to their ability to withstand toxic and corrosive sewer gases and bacteria commonly found in sewer systems.
  • It can also be used as a replacement for asphalt pavement, for higher durability and higher strength.
  • Polymer concrete has historically not been widely adopted due to the high costs and difficulty associated with traditional manufacturing techniques.

Coloured concrete

Coloured concrete can be produced by using coloured aggregates or by adding colour pigments (ASTM C 979) or both.

If surfaces are to be washed with acid, a delay of approximately two weeks after casting is necessary.

Coloured aggregates may be natural rock such as quartz, marble, and granite, or they may be ceramic materials.

Synthetic pigments generally give more uniform results.

The amount of colour pigments added to a concrete mixture should not be more than 10% of the mass of the cement.

For example, a dose of pigment equal to 1.5% by mass of cement may produce a pleasing pastel colour, but 7% may be needed to produce a deep colour. Use of white Portland cement with a pigment will produce cleaner, brighter colours and is recommended in preference to grey cement, except for black or dark grey colours.

Mass concrete

“A large volume of cast-in-place concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and attendant volume change to minimize cracking.”

Mass concrete includes not only low-cement-content concrete used in dams and other massive structures but also moderate to high cement content concrete in structural members of bridges and buildings.

As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting.

The width and depth of cracks depends upon the temperature differential, physical properties of the concrete, and the reinforcing steel.

Pervious concrete

Pervious (porous or no-fines) concrete contains a narrowly graded coarse aggregate, little to no fine aggregate, and insufficient cement paste to fill voids in the coarse aggregate.

Low water-cement ratio, low-slump concrete resembling popcorn held together by cement paste.

Produces a concrete with a high volume of voids (20% to 35%) and a high permeability that allows water to flow through it easily.

Pervious concrete is used in hydraulic structures as drainage media, and in parking lots, pavements, and airport local groundwater supply by allowing water to penetrate the concrete to the ground below.

Pervious concretes have also been used in tennis courts and greenhouses.

The compressive strength of different mixes can range from 3.5 to 27.5 Mpa.

Drainage rates commonly range from 100 to 900 lit per minute per square meter.

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