Which statement describes the centrifuge method step for the 75-μm sieve fraction?

• A. Exclude the ash mass from the calculation.
• B. Ignore the material passing the 75 μm sieve.
• C. Only the mass retained by washing is used to calculate the total 75 μm sieve.
• D. When the extracted aggregate is the centrifuge method (T164), add the mass of any dry material passing the 75-μm sieve by dry sieving, along with the mass of any material (ash), to the mass removed by washing in order to obtain the total passing the 75 μm sieve.

Answer: (D)

In the centrifuge method for the 75-μm fraction, you must account for all material that ends up passing through that sieve, from multiple sources. The correct approach is to add the mass of any dry material that passed the 75-μm sieve by dry sieving, plus the mass of any ash, to the mass removed by washing. This sum gives the total passing the 75-μm sieve, ensuring all fines are included. The other options would undercount fines by leaving out material that passes by dry sieving or by including only the washed portion or excluding ash.