GENERALNow-a-days properties of concrete by addition of steel fibres.

GENERALNow-a-days concrete is widely used as a construction material. The versatility and mould ability of this material, its high compressive strength and the discovery of the reinforcing and pre-stressing techniques which enhances its low tensile strength have contributed largely to its wide spread use.Cement Concrete is that where cement is used as a binding material. It is one of the simple but actually complex materials. All though, rapid advances have been made during the last few decades in the use of concrete, many of its complex properties are yet to be identified to employ this material with advantage and economy. There is a saying that broken stone, sand, cement and water makes good concrete. But, the same proportions of this material may yield to bad concrete. The quality of the concrete depends more on the man on the job than the constituent materials. However, the improved methods of inspection and testing are resulting in control of qualities of concrete with in more and better defined limits.Researchers have been trying to modify the properties of conventional concrete to suit the individual needs. Earlier notion of using high amounts of cement for concrete has now changed on favour of increased use of high amounts of mineral ad-mixtures and super plasticizers with reduced amounts of cement and water in the concrete mixtures. Though the use of admixtures and additives are being frowned upon by some technologists, there are many of the contrary, who highly reconstructed and faster the use and development of admixture as it imparts many desirable characteristics and effects economy in concrete construction. Attempts were also made to improve the flexural strength and dynamic properties of concrete by addition of steel fibres. The steel fibre reinforced concrete is gaining rapid importance in the field of concrete technology.Today, more than ever before, the civil engineer is required to give thought and time to the problems of concrete making and its utilization with economy. ADVANCES IN CONCRETENormal concrete made with Portland cement and conventional natural stone aggregates undergo several disadvantages. Attempts to over-come these deficiencies have resulted in the development of special concretes. High strength concretes have been produced industrially by using super plasticizers or water reducing admixtures, pozzolonas etc.POZZOLONIC MATERIALS    Pozzolonic materials are finely divided siliceous and aluminous materials which have little or no cementitious value. In the presence of moisture at ordinary temperature, pozzolonic materials chemically react with calcium hydroxide liberated during hydration, to form compounds possessing cementitious properties. The engineering benefits to be derived from the use of pozzolonas in concrete includes enhancement of ultimate strength and impermeability due to pore refinement, improved resistance to thermal cracking because of lower heat of hydration, a better durability to chemical attacks such as acid, sulphate water and alkali-aggregate expansion.Types of Pozzolonas    Pozzolonic materials can be divided into two types:Natural pozzolonas Natural pozzolonas such as clay and shale, diatomaceous earth etc., are processed by crushing, grinding and size preparation, including thermal activation if necessary. The natural pozzolonas have lost their popularity in view of the availability of more active pozzolonas available as industrial by products.     Artificial pozzolonasArtificial pozzolonas can be produced deliberately, for instance by thermal activation of kaolin-clays to obtain metakaolin which is highly reactive, or can be obtained as waste or by-products from high-temperature process such as fly ashes from coal fired electricity production, silica fume from silicon smelting, and burned organic matter residues rich in silica such as rice husk ash.