Gradation & SIze
Test Description

The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The complete test procedure can be found in:

AASHTO T 30, Mechanical Analysis of Extracted Aggregate, is used when performing gradation analysis on aggregates extracted from an HMA mixture.

Summary

The test can be run on either dry or washed aggregate. The washed sieve analysis takes longer but produces a more accurate gradation, particularly the percent passing the No. 200 (0.075 mm) sieve since the washing helps remove these small particles from the larger particles. The dry sieve analysis procedure is often used where rapid results are required.

The basic sieve analysis consists of weighing an aggregate sample and then passing it through a nest of sieves. The nest of sieves is made up of a stack of wire-cloth screens with progressively smaller openings from top to bottom. The material retained on each sieve (Slideshow 1) is weighed and compared to the total sample mass. Particle size distribution is expressed as a percent retained or percent passing by weight on each sieve size. Figure 5 shows the major gradation and size analysis equipment.

Approximate Test Time

The washed procedure takes one to three days from sampling the aggregate to completion depending on the moisture content of the aggregate when it is sampled.

The dry procedure can take about one to two hours from sampling the aggregate to completion.

Basic Procedure

  1. Obtain an aggregate sample of adequate mass from one of the following locations: aggregate stockpiles, bins, dump trucks, conveyor belt, or the roadway.
  2. Mix and reduce the sample (Figure 6) to an amount suitable for testing. This process of reducing a sample size is often referred to as "splitting" the sample (Video 1).
  3. Dry the test sample to a constant mass and determine the sample’s dry mass.
NOTE

Samples may be dried by heating (e.g., a hotplate) provided (AASHTO, 2000b):

  1. Steam is allowed to escape without generating pressures great enough to fracture the aggregate particles.
  2. Temperature is not great enough to cause chemical breakdown of the aggregate.

NOTE

Generally, it is not necessary to dry the coarse aggregate since results are not significantly influenced by its moisture content. Coarse aggregate should be dried if:

  • The NMAS is < 0.5 inches (12.5 mm)
  • It contains appreciable material finer than No. 4 (4.75 mm) sieve.
  • It is highly absorptive.
  1. If using the washed procedure, place the dry sample in a container and cover with water. Agitate the sample to completely separate all particles finer than the No. 200 (0.075 mm) sieve from the coarser aggregate, and to bring the fine material into suspension. Immediately decant the wash water containing the suspended solids over a nest of sieves consisting of a No. 200 (0.075 mm) sieve and an upper sieve with openings in the range of No. 8 (2.36 mm) to No. 16 (1.18 mm).
NOTE
Automated washers (Figure 7 and Video 2) are available, which may aid in the washing process.
  1. If using the washed procedure, repeat step 4 until the wash water is clear.
  1. If using the washed procedure, return the material retained on the nested sieves to the washed sample by flushing with water. Dry the washed sample to a constant mass and allow to cool. Determine mass of the sample after washing.
  2. Select applicable sieves to obtain the information required by the specifications covering the material to be tested. Sieve sizes typically used for Superpave mix design are 1½ in, 1.0 in, ¾ in, ½ in, 3/8 in, No. 4, No. 8, No. 16, No. 30, No. 50, No. 100 and No. 200 (37.5, 25.0, 19.0, 12.5, 9.5, 4.75, 2.36, 1.18, 0.600, 0.300, 0.150 and 0.075 mm) sieves. Assemble the sieves in order of decreasing size of opening from top to bottom and place the nest of sieves over a pan (Video 3).
  3. Pour the sample into the top sieve in the nest (Figure 8).
  4. Sieve the material in a mechanical sieve shaker (Figure 9 and Video 4).
  5. Determine the mass of the material retained on each sieve size (Video 5). Record the cumulative mass retained for each sieve size (the mass retained on a specific sieve size and the mass retained on all sieves with larger openings).
WARNING

Be careful that the sample does not overcharge any individual sieves. Excess material on a particular sieve may prevent may block material that would otherwise pass through the seive.

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