New Jersey

Department of Transportation

Standard Specifications

for Road and Bridge Construction

2007


NJDOT A-2 – Determining Percentage of Mica in Fine Aggregate

  1. Scope. This test method is used to determine the mica content of fine aggregate.

  2. Apparatus. Use the following apparatus:

    1. Binocular microscope (Leica, Zeiss, Wild, or equivalent) with 20× minimum magnification and using a standard 10× high quality lens.
    2. No. 10 and No. 100 sieves conforming to AASHTO M 92.
    3. Mechanical sieve shaker conforming to AASHTO T 27.
    4. Microsplitter or spinning riffler splitter.
    5. Balance conforming to AASHTO M 231 and having a minimum capacity of 100 grams with a precision of 0.1 gram.
    6. Balance for weighing separated mica in stainless steel containers having a minimum capacity of 50 grams with a precision of 0.001 grams and conforming to AASHTO M 231.
    7. Stainless-steel sample container with lid.
    8. Oven for drying sample to a constant weight at a temperature of 230 ± 9 °F.
    9. Stainless-steel spatula.
    10. Size 000 or smaller paintbrush with loose hairs trimmed.
    11. Microscopic picking tray as used by the American Association of Petroleum Geologists or equivalent for placement within microscope working area.
    12. Water in a suitable container for wetting paintbrush.
    13. Various suitable larger brushes for moving and spreading sample on picking tray.

  3. Procedure. Perform the following steps:

    1. Ensure that the examiner has a minimum 4-year degree in geology or a related science and an understanding of mineralogy, petrographic analysis, and optical microscopy.
    2. Take the test sample from a representative sample from a field stockpile. After oven drying to a constant mass at a temperature of 230 ± 9 °F, split the sample into approximately 200 grams weighed to the nearest 1 gram.
    3. Sieve the 200 gram sample according to AASHTO T 27 using a No. 10 sieve and a No. 100 sieve. Retain the material passing the No. 10 sieve and retained on the No. 100 sieve.
    4. Reduce the retained sample to approximately 20 grams. Using a microsplitter or a spinning riffler further split the sample into 20 approximately 1-gram samples of fine aggregate. Choose 2 of the twenty 1-gram samples at random for analysis.
    5. Place the first chosen sample on a microscopic picking tray and quarter using a stainless-steel spatula. With a large brush, sweep 3 of the 4 sample aliquots to the periphery of the picking tray for possible further analysis.
    6. Wet the bristles of the small paintbrush and under 10-20X viewing power, separate the mica minerals from the rest of the fine aggregate
    7. After picking at least 95 percent of the mica from the sample aliquot, carefully brush the separated mica into pre-weighed and labeled stainless-steel sample container with lid. Brush the mica-free portion of the sample aliquot into another pre-weighed and labeled sample container with lid.
    8. If deemed necessary, the examiner may analyze other quarter aliquots of the sample brushed aside in Step 5. Follow Steps 6 and 7 to analyze additional aliquots brushing the mica and the mica-free portions into their respective containers with the first aliquot analyzed.
    9. With the lids off, dry both portions of the sample to constant weight.
    10. Using a scale with a precision of 0.001 grams, weigh both samples with the lids on to prevent loss. Record weights of the mica and the mica-free portions. The sum of these is the “Weight of Sample” and should be approximately 0.25 grams for each aliquot analyzed.
    11. Repeat Steps 5 through 10 for the second randomly chosen 1-gram sample.

  4. Calculations and Report. Calculate the percent of mica for each sample analyzed using the following equation:


    5.   Percent of Mica = 
    Weight of Mica in Grams

    		  x 100
    Weight of Sample  
    		 

  5. Report. Report the final mica content as the average of the results of the 2 samples to the nearest 0.1 percent.

Last Document Correction:
December 14, 2007