IMPURITIES SUCH AS LOAM ,SILT AND THE ORGANIC MATTER MAY WEAKEN THE BOND WITH CEMENT,SO FINE AND COURSE AGGREGATES SHOULD BE FREE FROM THE IMPURITIES.

Natural aggregates are generally classified as fine and the course aggregate.The size of fine aggregate shall not exceed 4.75 mm gauge . Aggregate of the bigger sizes are called course aggregates.When a favourable gradation is required aggregates are separated by way of the sieving into two or three size groups of sand and several size groups of course aggregate.These are then combined to get a densely packed aggregate.The maximum size of the course aggregate in reinforced concrete is governed by the requirement than it shall easily fit into the forms and between the reinforcing bars .The reinforced concrete work, aggregates having a normal size of 20 mm are generally considered satisfactory.For heavily reinforced concrete members as in the case of ribs of main members , the nominal maximum size of the aggregate should usually be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement which ever is smaller. Crushed rock or gravel in the form of a mixture of several fractions can be used as course aggregate.The fine aggregate ie sand is used as for as possible in two fractions.For preparing the concrete mix the fractions of the aggregates are measured separately. Plums above 160 mm upto any reasonable size may be used in plain concrete work up to a maximum limit of 20 percent by volume of concrete and the plums shall be distributed evenly and shall be not closer than 150 mm from the surface.In ordinarily structural concretes the aggregates occupy 70 to 75 percent of the volume of the hardened mass .The remainder consists of hardened cement paste , uncombined water and air voids .The uncombined water and the air voids do not contribute to the strength of concrete .The more density the aggregate can be compacted ,the better is the strength,weather resistance and economy of the concrete.The gradation of the particle sizes in the aggregates to produce dense concrete is very important.It is also important that the aggregate should have good strength, durability and weather resistance.It should be free from the impurities such as loam ,silt and organic matter which may weaken the bond with cement and have unfavorable chemical reaction with cement .