Friday, May 6, 2011

Percent Passing Vs. Percent Retained

Most of the time when you get a gradation test report the test results will be shown as percent passing.  But what does that mean?  Percent passing is the amount of the sample that PASSES the sieve in question.  So if you see the following gradation:

#4     100%
#8     55%
#16   23%

You can determine that 55% of the sample is smaller than the #8 sieve because 55% of the sample passed through the #8 sieve.

Another way that a gradation result can be shown is percent retained.  This report shows you how much material is RETAINED on the sieve in question.  So for the same sample shown above you will see the following gradation results:

#4     0%
#8     45%
#16   77%

This shows that 77% of the sample is larger than the #8 sieve because 77% of the sample was retained on that sieve.  This is a cumulative test result so anything that was retained on the #8 sieve is also included in the amount of material that was retained on the #16.  The percentage refers to ALL of the sample that is larger than that sieve, not just the amount of material between that sieve and the sieve above it.

Which brings me to Individual Percent Retained.  Individual Percent Retained is the amount of material that is retained on an individual sieve, between the sieve above and the sieve in question.  So referring to the same sample as before you would see the following gradation report:

#4     0%
#8     45%
#16   32%

This shows that there is 32% individually retained on the #16 sieve because 32% of the sample is smaller than the #8 sieve but larger than the #16 sieve.

As you can see it is extremely important to know what type of gradation you are looking at because the percentages can be very different.  99% of the time in the construction industry you will see gradations shown as percent passing.  For those of you who are new to gradations this might seem counter intuitive and you might naturally want to think in terms of individual percent retained.  I completely understand because I was the same way for the first month or so but I would highly recommend that you start training yourself to understand and use percent passing.  This is how the industry speaks to each other and if you are constantly converting gradations to individual percent retained you will really struggle to keep up.  Don't get me wrong, both individual percent retained and percent retained are extremely useful in their own ways but usually only for special projects and most of you will probably never have to use them.

Quality Control, in its own way, is a new language to most of the industry so don't feel like you have to understand it all at once.  Make sure to clarify by asking questions and you'll be surprised by how many people may be having the same problems as you! 

1 comment:

  1. This comment has been removed by a blog administrator.