Paratibha Aggarwal, Associate Professor, Babita Saini, Assistant Professor, Sarvesh Tripathi, M.Tech student, Deptt. of Civil Engg., N.I.T. Kurukshetra

Introduction

Concrete has been the leading building material since it was first used and is bound to maintain its significant role in the upcoming future due to its durability, maintenance free service life, adaptability to any shape and size, wide range of structural properties plus cost effectiveness. The concrete is the most important construction material which is manufactured at the site. It is the composite product obtained by mixing cement, water and an inert matrix of sand and gravel or crushed stone. It undergoes a number of operations such as transportation, placing, compaction and curing. The distinguishing property of concrete is the ability to harden under water. The ingredients can be classified into two groups namely active and inactive. The active group consists of cement and water, whereas the inactive group consists of fine and coarse aggregates. The inactive group is sometimes also called inert matrix. Concrete has high compressive strength but its tensile strength is very low. In situations where tensile stresses are developed the concrete is strengthened by using steel bars or short randomly distributed fibres forming a composite material called reinforced cement concrete (RCC) or fibre reinforced concrete. The resistance of concrete to the slipping of reinforcing bars embedded in concrete is called bond strength. The bond strength is provided by adhesion of hardened cement paste and by the friction between concrete and reinforcement. It is also affected by the shrinkage of concrete relative to steel. On an average bond strength is taken approximately as 10% of the compressive strength. The roughness of the steel surface, water, the chemical composition of cement and steel bar diameter are the factors that affect the bond strength of concrete. In pull-out tests on plain bars, the maximum load generally represents the bond strength that can be developed between the concrete and steel. With plain bars the maximum load is not very different from the load at the first visible slip, but in the case of the deformed bar, the maximum load may correspond to a large slip which may not in fact be obtained in practice before other types of failure occur. The load shall be applied to the reinforcing bar at a rate not greater than 2250 kg/mm, or at no-load speed of the testing machine head of not greater than 1.25 mm/min, depending on the type of testing machine used and the means provided for ascertaining or controlling speeds. The maximum load for each type of failure shall be recorded. The new replaces the old and same follows with the buildings. Older buildings require reconstruction for better and higher economic gains and on account of obsolescence on structural or functional grounds and also due to the damages inflicted on them by natural disasters and wars. The rate of demolition showed an upward trend which in turn increased the dumping costs due to unavailability of appropriate sites nearby. Thus efficient use of the demolished concrete would reduce the costs and definitely lead to conservation of the invaluable non-renewable sources of energy and hence must be given due importance. The demolished concrete could be used as aggregate for concrete resulting in large consumption of the material. Recycling is the act of processing the used material for use in creating new product. The usage of natural aggregate is getting more intense with the development in infrastructure area. In order to reduce the usage of natural aggregate, recycled aggregate can be used as the replacement materials. Recycled aggregate are comprised crushed, graded inorganic particles obtained from the materials that have been used in the constructions and demolition debris. These materials are generally from buildings, roads, bridges, and sometimes even from catastrophes, such as wars and earthquakes.

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