NJDOT A-5 – Scratch Hardness Test for Coarse Aggregate Particles
Scope. This test method is used to determine the percent of soft particles contained in coarse aggregates on the basis of scratch hardness. This test method
is suitable for use on No. 67, No. 57, or equivalent sized coarse aggregate.
Scratch hardness apparatus consisting of a brass rod, 1/16 inch in diameter, with a rounded point, mounted in a device so that a load of 2 ± 0.1 pounds is applied to the specimen tested. Ensure that the brass rod is of suitable hardness so that when filed to a sharp point, it will scratch a copper penny (US Lincoln design), but fail to scratch a nickel (US Jefferson design).
Sieve sizes 1 inch, 3/4 inch, 1/2 inch, and 3/8 inch conforming to AASHTO M 92.
Dry the coarse aggregate sample and split into portions for the various tests to be performed. Ensure that the portion of the sample for scratch hardness testing is sized so that it will yield not less than the amounts of the sizes specified in Table A-5-1 that are available in amounts of 10 percent or more.
Table A-5-1 – Minimum Size of Sample to be Tested
Sieve Size (Inches)
Sample Mass (grams)
3/8 to 1/2
1/2 to 3/4
3/4 to 1
Perform a gradation test on 1 portion according to AASHTO T 27 to determine the percent retained on the 3/4, 1/2, and 3/8 inch sieves and the percent passing the 3/8-inch sieve.
Separate the test sample into fractionated sizes as specified in Table A-5-1. Discard material passing the 3/8 inch sieve or retained on the 1 inch sieve. Also, discard fractions on sieves that represent less than 10 percent of the overall sample.
If the sample contains less than 10 percent of any of the sizes specified in Table A-5-1, do not test that size. For the purpose of calculating test results, consider the fractionated size to contain the same percentage of soft particles as the average of the next larger and the next smaller size. If one of these sizes is absent, consider the fractionated size to contain the same percentage of soft particles as the next larger or next smaller size, whichever is present.
Take a representative sample meeting the sample size requirements in Table A-5-1 of each of the fractionated sizes to be tested.
Test each particle of aggregate in a fractionated size using the following Steps:
Place the piece of coarse aggregate under the scratch hardness apparatus forcing the piece of aggregate to touch the sharpened, brass rod.
Push up on the 2-pound weighted brass rod with the piece of aggregate and pull the aggregate forward. Ensure that the brass rod remains in firm contact with the piece of aggregate at least from the center to the edge of the aggregate on one face.
Examine the particle to determine if it was scratched. Consider the particle to be soft if a groove was made in it without deposition of metal from the brass rod or if separate particles are detached from the rock mass. Consider the particle to be hard if a metal streak was made on it by the brass rod and no groove was made. If a particle contains more than one type of rock and is determined to be partly hard and partly soft, consider the particle soft if the soft part is more than 30 percent of the particle.
Assign the particles to groups of soft or hard particles.
Separately weigh the hard and soft particles groups for the fractionated size to the nearest 0.1 percent.
Repeat Steps 6 and 7 for each of the fractionated sizes.