Types Of Concrete

Types Of Concrete 

1.Normal Concrete

          The concrete in which common ingredients of aggregate, water, cement are used is known as normal concrete. It is also called normal weight concrete or normal strength concrete.It has a setting time of 30 - 90 minutes depending upon moisture in atmosphere, fineness of cement and other properties of concrete.The development of the strength starts after 7 days the common strength values is 10 MPa to 40 MPa. At about 28 days 75 - 80% and almost at 90 days 95% of the strength is achieved of the total strength is attained.

Properties of Normal Concrete

1. Its slump varies from 1 - 4 inches.

2. Density ranges from 140 pcf to 175 pcf.

3. It is strong in compression and weak in tension.

4. Air content 1 - 2 %.

5. Normal concrete is not durable against severe conditions.

6. Compressive strength : 20 - 40 MPa (3000 - 6000 psi).

7. Flexural strength : 3 - 5 MPa (400 - 700 psi).

8.Tensile strength : 2 - 5 MPa (300 - 700 psi).

9. Modulus of elasticity : 14000 - 41000 MPa (2 - 6 x 10⁶ psi).

2. Light weight concrete (LWC)

         The LWC defined as a type of concrete which includes an expanding agent in that it increases the volume of the mixture while giving additional qualities such as nailbility and lessened the dead weight.The density of normal concrete is of the order of 2200 to 2600. This self weight will make it to some extend an uneconomical structural material.Whereas the Self weight of light weight concrete varies from 300 to 1850 kg/m3. Porous light weight aggregate of low specific gravity is used in this concrete. such as pumice, scoria and most volcanic origin and artificial aggregate such as expanded blast furnace slag, vermiculite and clinker aggregates are used.

Properties of LWC

1. Density of light weight concrete is 240 kg/m³ (15pcf) -1850 kg/m³ (115 pcf).

2. Strength of light weight concrete blocks varies from 7 MPa (1000 psi) - 40 MPa (5800 psi).

3. Some times Air Entrained Admixtures are also added to it giving resistance to freezing and thawing along with strength.

Advantages of LWC

1. It helps reduce the dead load, increase the progress of building and lowers the hauling and handling cost.

2. The weight of building on foundation is an important factor in the design , particularly in case of weak soil and tall structures. The wall and floor are made of light weight concrete it will result in considerable economy.

3. Light weight concrete have low thermal conductivity.

4. Only method for making concrete light by inclusion of air. 

This is achieved by 

a) replacing original mineral aggregate by light weight aggregate

b) By introducing gas or air bubble in mortar

c) By omitting sand fraction from concrete. 

This is called no – fine concrete.

5. Light weight concrete aggregate exhibit high fire resistance.

6. Structural lightweight aggregate’s cellular structure provides internal curing through water entrainment which is especially beneficial for high-performance concrete.

7. Lightweight aggregate has better thermal properties, better fire ratings, reduced shrinkage, excellent freezing and thawing durability, improved contact between aggregate and cement matrix, less micro-cracking as a result of better elastic compatibility, more blast resistant, and has better shock and sound absorption, High-Performance lightweight aggregate concrete also has less cracking.

8. Improved skid resistance and is readily placed by the concrete pumping method 

a) Aerated concrete is made by introducing air or gas into a slurry composed of Portland cement.

b) No fine concrete is made up of only coarse aggregate , cement and water.These type of concrete is used for load bearing cast in situ external walls for building. They are also used for temporary structures because of low initial cost and can be reused as aggregate.

3. High density concrete (HDC)

           High density concrete is a concrete having a density in the range of 6000 to 6400 kg/cu.m. High density concrete is also known as Heavy weight concrete. High density concrete is mainly used for the purpose of radiation shielding, for counterweights and other uses where high density is required. Heavyweight concrete uses heavy natural aggregates such as barites or magnetite or manufactured aggregates such as iron or lead shot. The density achieved will depend on the type of aggregate used. Typically using barites the density will be in the region of 3,500kg/m³, which is 45% greater than that of normal concrete, while with magnetite the density will be 3,900kg/m³, or 60% greater than normal concrete. Very heavy concretes can be achieved with iron or lead shot as aggregate, is 5,900kg/m³ and 8,900kg/m³ respectively. Most of the aggregate specific gravity is more than 3.5.

Advantages of HDC

1.They are mainly used in the construction of radiation shields (medical or nuclear). Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures

2.The ideal property of normal and high density concrete are high modulus of elasticity , low thermal expansion , and creep deformation. Because of high density of concrete there will be tendency for segregation. To avoid this pre placed aggregate method of concreting is adopted.

3.High Modulus of Elasticity, Low thermal Expansion ,Low elasticity and creep deformation are ideal properties.

4.The high density. Concrete is used in construction of radiation shields. They are effective and economic construction material for permanent shielding purpose.

4. Ready-mix Concrete (RMC)

       Ready-mix concrete has cement, aggregates, water   and   other   ingredients,   which are weigh-batched   at a   centrally located   plant. This is   then  delivered   to the   construction site in truck mounted transit mixers and can be used straight away without any further treatment.Ready Mixed Concrete, or RMC as it is popularly called, refers to concrete that is specifically manufactured for delivery to the customer’s construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of Portland cement, water and aggregates comprising sand and gravel or crushed stone. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete. Ready Mixed Concrete is bought and sold by volume – usually expressed in cubic meters. Ready Mixed Concrete is manufactured under computer-controlled operations and transported and placed at site using sophisticated equipment and methods. RMC assures its customers numerous benefits.

Advantages of RMC

1.   A centralized concrete batching plant can serve a wide area.

2.   Better quality concrete is produced

3.   Elimination of storage space for basic materials at site.

4.   Elimination of procurement / hiring of plant and machinery

5.   Wastage of basic materials is avoided

6.   Labor associated with production of concrete is eliminated

7.   Time required is greatly reduced

8.   Noise and dust pollution at site is reduced

        

5. Pozzocrete

          Unfortunately, adding extra water and fine aggregate leads to a weaker concrete. The usual remedies for this are either to increase the cement content, which is costly, or to use chemical admixtures, which can also be costly and may lead to segregation in marginal mixes. There is another and far more effective alternative.

Advantages of including of POZZOCRETE in concrete mixes to be pumped

1.Particle Size. 

Pozzocrete meets IS 3812 Specification with 66% passing the 325 (45-micron) sieve and these fine particles are ideal for void filling.  Just a small deficiency in the mix fines can often prevent successful pumping.

2.Particle Shape. 

Microscopic examination shows most Pozzocrete particles are spherical and act like miniature ball bearings aiding the movement of the concrete by reducing frictional losses in the pump and pining.  Studies have shown that Pozzocrete can be twice as effective as cement in improving workability and, therefore, improve pumping characteristics.

Pozzolanic Activity

        This chemical reaction combines the Pozzocrete particles with the calcium hydroxide liberated through the hydration of cement to form additional cementitious compounds which increase concrete strength.

Sand/Coarse Aggregate Ratio

       In pumped mixes, the inclusion of liberal quantities of coarse aggregate can be very beneficial because it reduces the total aggregate surface area, thereby increasing the effectiveness of the available cementitious paste.  This approach is in keeping with the “minimum voids, minimum area” proportioning method.  As aggregate size increases, so does the optimum quantity of coarse aggregate.  Unfortunately, this process is frequently reversed in pump mixes, and sand would be substituted for coarse aggregate to make pumping easier.  When that happens, there is a need to increase costly cementitious material to compensate for strength loss.  However, if Pozzocrete is utilized, its unique workability and pump ability properties permit a better balance of sand to coarse aggregate resulting in a more economical, pump able concrete.

6. Shotcrete

       Shotcrete is a process where concrete is projected or "shot" under pressure using a feeder or "gun" onto a surface to form structural shapes including walls, floors, and roofs. The surface can be wood, steel, polystyrene, or any other surface that concrete can be projected onto. The surface can be trowelled smooth while the concrete is still wet.

Advantages

1. Shotcrete has high strength.

2. Durability.

3. Low permeability.

4. Excellent bond and limitless shape possibilities.

5. The hardened properties of shotcrete are similar to conventional cast-in-place concrete.

6. The nature of the placement process provides additional benefits.

7. Excellent bond with most substrates and instant or rapid capabilities.

8. Particularly on complex forms or shapes. Addition to building homes, shotcrete can also be used to build pools.

REFERENCES

1.   http://theconstructor.org/concrete/types-of-concrete/966/

2.   http://www.aboutcivil.org/types-of-concrete.html

3.   google wikipedia