09-10-2012, 04:47 PM
CONCRETE
CONCRETE UNIT 3.ppt (Size: 2.46 MB / Downloads: 79)
Concrete is a mixture of cement (usually Portland cement) and stone aggregate.
When mixed with a small amount of water, the cement hydrates to form a microscopic opaque crystal lattice structure encapsulating and locking the aggregate into its rigid structure. Typical concrete mixes have high resistance to compressive stresses.
If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists compression, bending, and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel resists the tension can be made into almost any shape and size for the construction industry.
USE IN CONSTRUCTION:-
Concrete is reinforced to give it extra strength; without reinforcement, many concrete buildings would collapse.
Reinforced concrete can encompass many types of structures and components, including slabs, walls, beams, columns, foundations, frames and more.
Reinforced concrete can be classified as precast concrete and in-situ concrete.
Concrete has flexibility and is used to design free forms.
Reinforced concrete street light pole. Concrete poles are
less expensive than traditional steel or alumunium poles and
may outlast them.
RESISTANCE TO CORROSION OF STEEL :-
Good quality concrete provides good protective coating around steel reinforcement.
The alkalinity of cement concrete leads to the formation of a thin visible protective oxide film on steel reinforcement.
To achieve good protection against corrosion of steel, sufficient thickness of cover of good quality (dense and impermeable) concrete should be provided.
The concrete cover should not allow ingress of moisture and air which are necessary for corrosion of steel to occur.
In porous concrete, the reinforcement is likely to corrode quickly. On corrosion the volume of steel increase and this causes concrete cover to crack and further increase the process of corrosion by providing easy ingress of moisture and air.
Sands containing salts if used in concrete, absorb moisture from air and cause efflorescence and weaken the concrete cover leading to ingress of moisture to steel reinforcement.
Use of sea water or water containing sulphates and chlorides beyond certain permissible limits in cement concrete causes volumetric instability and unsound porous concrete cover to steel. This results into corrosion of steel. Chlorides accelerates corrosion of steel to some extent.
The inadequacy of reinforcement may also cause cracks in concrete cover, thus allowing the moisture to reach steel for initiating corrosion.
The permeability of concrete is the most important factor which affects the process of corrosion of steel reinforcement due to ingress of water.
All factors which reduces the permeability of concrete are also responsible to improve the resistance of concrete to corrosion of steel.
CRACKS IN STRUCTURE
Materials like bricks , mortar and concrete, which contain considerable amount of water at the time of construction, dry out subsequently and undergo major contraction. Tensile stresses are produced due to restraint to contraction movements. If these stresses exceed the tensile or sheer strength of the material cracks developed.
When placed concrete typically contains more water than is required for hydration of the cement. As the concrete hardens and starts to lose the excess water, shrinkage begins. If the concrete is unrestrained, no cracks will develop. But it is virtually impossible to support a structure of any appreciable size without some restraint.
The cracking phenomenon is complex and depends upon a number of things; rate and amount of drying, drying shrinkage, tensile strength, tensile strain, creep, elasticity, degree of restaint, and other factors.
While most types of cracking do not affect structural stability or durability, all cracks are unsightly and in extreme cases cracking can reduce the use and servicability of the structure. For this reason, cracking should be kept to a minimum.
In the laboratory, drying shrinkage tests are the most easily and most frequently performed tests in relation to shrinkage/cracking problems. However, there is sometimes too much emphasis on the drying shrinkage of hardened concrete as the criterion of susceptibility to cracking.
Drying shrinkage alone is influenced by many factors - water/cement ratio; amount, fineness and composition of cement; mineral composition, stiffness, shape, surface texture, and grading of the aggregate; characteristics and amount of any admixture; size and shape of the concrete mass; and conditions of humidity and temperature.
Preventative measures at the time of placing and curing remain the best means of minimizing cracking.