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What is the significance of specifying the amount of material passing the 300µm and 150 µm (ASTM No. 50 and 100) sieves?

This is one of the most important characteristics of the fine aggregate grading. Inadequate amounts of materials in these size ranges can cause excessive bleeding, difficulties in pumping concrete, and difficulties in obtaining smooth troweled surfaces. The requirements for the same are tabulated below:

1	10 to 30% passing the 300 μm (No. 50) sieve and 2 to 10% to pass the 150 μm (No. 100) sieve	Generally allowed by most specifications
2	lower limits of above range (given in 1 )	adequate for easy placing conditions or for mechanically finished concrete
3	at least 15% passing the 300 μm (No. 50) sieve and 3% passing the 150 μm (No. 100) sieve	Generally recommended for hand-finished concrete floors or where a smooth texture is needed
4	15 to 30% passing the 300 μm (No. 50) sieve, and 5 to 10% passing the 150-μm (No. 100) sieve	Recommended when concrete is to be pumped through lines less than 150 mm (6 in.) in diameter