A good aggregates should not contain ant deleterious materials which may cause physical and chemical changes in the concrete . An aggregates should have clean , uncoated , properly shaped particles of strong , dense , durable mineral and rock materials . Some of the important characteristics of aggregae are given below which may affect the performance of the concrete .
1. shape of the particles . As per IS : 383 - 1963 shape of the particle of the aggregate may be classified as rounded , regular , angular and flaky .
Rounded aggregates have about 35 percent of voids . It gives minimum ratio of surface area to a given volume of aggregates and hence require minimum water for lubricant . it gives good workability for the specified amount of water and also requires less cement for a particular w/c ratio . But such an aggregates does not have good interlocking effect and hence bound between aggregate and cement plaster is low and hence it does not give high strength concrete .
Irregular aggregates gives higher percentage of voids which may be as much as 37 percent . This aggregates exchibits lesser workability and requires more of cement . Its interlocking effect is better than rounded aggregates , but still not adequate enough for high strength concrete .
Angular aggregates have void percentage up to 40percent or 41 percent . it requires more water for lubrication and exhibits least workability for a specific /c ratio . it has very good interlocking effect and hence most suitable for high strenght concrete .
Flat , elongated or flaky particles in aggregates , are objectionable . besides decrease in workability , flaky particles in the aggregates should not be more than 10 to 15 percent .
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