NJDOT A-3 – Petrographic Analysis to Determine Lithological
Composition of Coarse Aggregate
Scope. This test method is used to determine the quantity of the rock types and deleterious material components in coarse aggregates or in the coarse aggregate fraction of a DGA or soil aggregate. Determinations may include the carbonate rock content of crushed gravels and the weathered and decomposed content of crushed stone or soil aggregate.
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.
Oven dry sample to a constant weight. Split the sample to a sample size that will result in a testing sample size as specified in Table A-3-1 after sieving.
Table A-3-1 – Test Sample Size
Sieve Size for Retention
Approximate Test Sample Size
No. 67 or larger
No. 7 or No. 8
DGA or I-5
For aggregate sizes No. 67 and larger, sieve the sample over a No. 4 sieve. For aggregate size No. 7 or No. 8, sieve the sample over a No. 8 sieve. Ensure that the sample size after sieving is as specified in Table A-3-1.
Wash the sample to remove any coating that would make particle examination difficult. If needed, use a mild detergent, such as Calgon to clean the aggregate. After washing, oven-dry the sample to a constant mass at a temperature of 230 ± 9 °F.
Separate the prepared sample into rock types as defined in ASTM C 294. Perform the separation using visual examination with the aid of the binocular microscope, dilute HCl, hammer, and scratching implements.
Examine the sample for deleterious material such as weathered, leached, porous, friable, fault-fractured, altered or otherwise unsound particles that could impair coarse aggregate performance. Use the following guidelines to determine if coarse aggregate particles are deleteriously weathered or decomposed:
Can be broken into several pieces by a light hammer tap.
Show more than superficial oxidation or alteration of feldspars.
Appear porous or are determined by the examiner to be porous.
Show numerous microfractures, cleavage planes or slickensides due to faulting.
Are of abnormal coloration throughout due to chemical or mechanical alteration.
Separate the deleteriously weathered and decomposed fraction into its own group.
Weigh the rock types and deleterious material group to the nearest 0.1 grams and record.
Calculations. Calculate the percentage of rock types and percentage of deleterious material by dividing the individual weight by the weight of the total sample and
multiplying by 100.
Report. Report the percentage by weight of individual rock types, as defined in ASTM C 294, and percentage by weight of