COMPOSITION OF CONCRETE

COMPOSITION 
  The compositions of concrete mainly depends upon the requirement in the construction field like on the type of structure being built ,how the concrete will be mixed and delivered and how it will be placed to form this structure. The main ingredients which are mixed in the concrete mixture are following:-

• Cement
• Aggregates(fine & coarse aggregates)
• Water
• Reinforcement 
• Chemical admixtures,Mineral admixtures and blended cement

These aggregates will be discussed in next blogs............ 

MEASUREMENT UNIT CONVERTER

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HISTORY OF THE CONCRETE

Concrete is a material used in building construction, consisting of a hard, chemically inert particulate       substance, known as an aggregate (usually made               from different types of sand and gravel), that is             bonded together by cement and water.

The Assyrians and Babylonians used clay as the                                                       bonding substance or cement. The Egyptians used               lime and gypsum cement. In 1756, British engineer,           John Smeaton made the first modern concrete          (hydraulic cement) by adding pebbles as a coarse           aggregate and mixing powered brick into the                 cement. In 1824, English inventor, Joseph Aspdin       invented Portland Cement, which has remained                     the dominant cement used in concrete production.               Joseph Aspdin created the first true artificial                  cement by burning ground limestone and clay                    together. The burning process changed the chemical properties of the materials and Joseph Aspdin                created a stronger cement than what using plain           crushed limestone would produce.

The other major part of concrete besides the cement              is the aggregate. Aggregates include sand, crushed          stone, gravel, slag, ashes, burned shale, and burned            clay. Fine aggregate (fine refers to the size of aggregate)          is used in making concrete slabs and smooth surfaces.   Coarse aggregate is used for massive structures or                 sections of cement.

Concrete that includes imbedded metal (usually steel)            is called reinforced concrete or ferroconcrete. Reinforced concrete was invented (1849) by Joseph Monier, who received a patent in 1867. Joseph Monier was a Parisian gardener who made garden pots and tubs of concrete reinforced with an iron mesh. Reinforced concrete           combines the tensile or bendable strength of metal              and the compressional strength of concrete to withstand heavy loads. Joseph Monier exhibited his invention                   at the Paris Exposition of 1867. Besides his pots and tubs, Joseph Monier promoted reinforced concrete for use in railway ties, pipes, floors, arches, and bridges.

BLACK COTTON SOIL

     Vertisols owe their specific properties to the presence of swelling clay minerals,
mainly montmorillonite. As a result of the wetting and drying, massive expansion
and contraction of the clay minerals takes place. Contraction leads to the formation of
the wide and deep cracks. These cracks can be wide enough to make the terrain
treacherous for animals. Surface material accumulates in these cracks during the dry
season and is “swallowed” by the soil in the wet season, creating the ‘self mixing’ or
‘self mulching’ action of the vertisols.
The cracks close after rain when the clay minerals swell. During expansion of the
clay minerals high pressures are developed within these soils, causing a characteristic
soil structure with wedge shaped aggregates in the surface soil and ‘planar’ soil
blocks in the subsoil. The slippage of one soil block over the other leads to the
formation of typical polished surfaces known as “slickensides’ on the blocks’

Concrete

Concrete

                              Concrete is a stone like substance obtained by permitting a carefully proportioned mixture of cement, sand and gravel or other aggregate and water to harden in forms of the shape and of dimensions of the desired structure.

Reinforced cement concrete
                                           Since concrete is a brittle material and is strong in compression. It is weak in tension, so steel is used inside concrete for strengthening and reinforcing the tensile strength of concrete. The steel must have appropriate deformations to provide strong bonds and interlocking of both materials. When completely surrounded by the hardened concrete mass it forms an integral part of the two materials, known as "Reinforced Concrete".

                              

 Advantages of reinforced concrete

• It has relatively high compressive strength
• It has better resistance to fire than steel
• It has long service life with low maintenance cost
• In some types of structures, such as dams, piers and footings, it is most economical structural material
• It can be cast to take the shape required , making it widely used in pre-cast structural components
• It yields rigid members with minimum apparent deflection
• Yield strength of steel is about 15 times the compressive strength of structural concrete and well over 100 times its tensile strength
• By using steel, cross sectional dimesions of structural members can b ereduced e.g in lower floor columns
  
  
  
  
  
  
  Disadvantages of reinforced concrete
  
  
  • It needs mixing, casting and curing, all of which affect the final strength of concrete
  • The cost of the forms used to cast concrete is relatively high
  • It has low compressive strength as compared to steel (the ratio is about 1:10 depending on material) which leads to large sections in columns/beams of multistory buildings Cracks develop in concrete due to shrinkage and the application of live loads
    
    
    
    

    Loads

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    Structural members must be designed to support specific loads. Loads are those forces for which a structure should be proportioned. Loads that act on structure can be divided into three categories.
    1. Dead loads
    2. Live loads
    3. Environmental loads

    Dead Loads:

    Dead loads are those that are constant in magnitude and fixed in location throughout the lifetime of the structure. It includes the weight of the structure and any permanent material placed on the structure, such as roofing, tiles, walls etc. They can be determined with a high degree of accuracy from the dimensions of the elements and the unit weight of the material.

    Live loads:

    Live loads are those that may vary in magnitude and may also change in location. Live loads consists chiefly occupancy loads in buildings and traffic loads in bridges. Live loads at any given time are uncertain, both in magnitude and distribution.

    Environmental loads:

    Consists mainly of snow loads, wind pressure and suction, earthquake loads (i.e inertial forces) caused by earthquake motions. Soil pressure on subsurface portion of structures, loads from possible ponding of rainwater on flat surfaces and forces caused by temperature differences. Like live loads, environmental loads at any given time are uncertain both in magnitude and distribution.