Gradation Requirements

[MCMLXX]

Hello,

Have a quick question regarding the gradation chart for NYSDOT Type 2 Subbase material (Specification 733-04 New York State DOT). The chart is as follows:

Sieve...........%Passing By Weight
4".....................-
3".....................-
2".....................100
1/4"..................25-60
NO. 40................5-40
No. 200...............0-10

Because there is such a large gap between the 2" and the 1/4", would a mix of material with the largest stone size being 1/2" meet this gradation? Technically that size passes 100% at 2" and does not pass thru the 1/4".

For subbase material, I would expect there to be some larger material (2"-1") in the mix. For what I typically see, the gradation is broken down past the 100% with something just beneath at 91% to 95% helping to set a condition where a certain % of a larger stone size in the mix is expected. If the range is intentional, then would it be correct to view the range from 2" to 1/4" a linear line as a method of ensuring a good mix? Maybe it doesn't matter from a structural standpoint and as long as 40% does not pass the 1/4", then it doesn't matter what size it is as long as it passes 2"?

I do not believe this is considered a 'gap' gradation, but could be wrong.

Thanks,

 

[oldestguy]

The mix is darn well graded and my copy of the Navy NAFAC DM-7 calls it the "most favorable composite filter". What goes on above the 1/4" and below 2" seems not critical for its use.

 

[BigH]

This is for a road subbase. Normally road subbase gradations are a bit wider in range than for the base course materials - except here in Malaysia where they actually allow a sand mix to be used. I've included their specs - they have 6 different gradations. Don't worry about the blackened portion - it is just "Type A", etc.