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.