STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-4

2.6 Grading of aggregate 

One of the most important factors for producing workable concrete is good gradation of aggregates. Good grading implies that a sample of aggregate contains all standard fractions of aggregate in required proportion such that the sample contains minimum voids. A sample of the well graded aggregate containing minimum voids will require minimum paste to fill up the voids in the aggregates. Minimum paste will mean less quantity of cement and less quantity of water, which will further mean increased economy, higher strength, lower shrinkage and greater durability. 

2.7 Tests of aggregate  Since concrete is an assemblage of individual pieces of aggregate bound together by cementing material, its properties are based primarily on the quality of the cement paste [1]. This strength is dependant also on the bond between the cement paste and the aggregate. If either the strength of the paste or the bond between the paste and aggregate is low, a concrete of poor quality will be obtained irrespective of the strength of the aggregate. But when cement paste of good quality is provided and its bond with the aggregate is satisfactory, then the mechanical properties of the aggregate will influence the strength of concrete. For this reason assessment of strength of the aggregate is made by using the following tests:

* Impact Value Testing Machine

Aggregate crushing value machine

1. Aggregate crushing value test: Aggregate crushing value gives a relative measure of the resistance of an aggregate sample to crushing under gradually applied compressive load. Generally, this test is made on single-sized aggregate passing 12.5 mm and retained on 10 mm sieve. The material crushed to finer than 2.36 mm is separated and expressed as a percentage of the original weight taken on mould. This percentage is referred to as aggregate crushing value. 

2. Aggregate impact value test: With respect to concrete aggregates, toughness is usually considered the resistance of the material to failure by impact. The most successful method of test is the one in which a sample of standard aggregate kept in a mould is subjected to fifteen blows of a metal hammer of weight 14 kg falling from a height of 38 cm. The ratio of the weight of the fines (finer than 2.36 mm) formed to the weight of the total sample taken is expressed as a percentage. This is known as aggregate impact value.