Characteristics of Aggregates: Shape, Size & Specific Gravity

Aggregate Properties You Need to Know, Part 2: Shape, Size & Specific Gravity

Table of Contents:

In another installment of this two-part series on aggregates used in construction materials, we discussed properties related to durability, strength, and abrasion resistance. This time, we will take a look at popular tests and testing equipment that measure other characteristics important in aggregate materials performance; density, shape, and particle size distribution.

Particle Size and Gradation:

Particle size and gradation has more influence on the performance of hardened concrete, asphalt, and base material performance than any other characteristic of aggregates. The size and distribution of particles directly impact properties of stiffness, strength, workability, permeability, stability, skid resistance, and more.

It is no surprise that this is by far the most common and primary test to be performed on an aggregate sample. As with most of these aggregate characterization tests, it is not difficult to perform properly and can be conducted effectively by technicians with minimal training.

Once the proportions of the individual fractions are determined and plotted in graphical form as a gradation curve, the information can be used for more than just a report of grain sizes. The values can qualitatively group the aggregate with classification terms like gap-graded, open-graded, or uniformly-graded to describe particle distribution. This information can be used to adjust the proportions of the fractions to manipulate the qualities of the final mix designs.

ASTM C136 and AASHTO T 27 spell out the requirements of the sieve analysis test for aggregates. ASTM E11 lists specifications and tolerances for the test sieves.

  • Test Sieves are available with a wide variety of opening sizes, as well as different types and sizes of frames.
  • Sieve Shakers are highly recommended for efficient and repeatable processing of aggregate samples.
  • Testing Screens offer a practical solution for the effective sizing of large samples of coarse aggregate.

Specific Gravity (Relative Density):

The ratio of the mass of an aggregate to the mass of a volume of water is equal to the volume of the aggregate particles. Specific gravity is a fundamental property of aggregates that are used in many different calculations and for the proportioning of asphalt and concrete mixtures.

There are many standard test methods to determine specific gravity values of different construction materials, but for our purposes, we will stick to the two test methods for coarse and fine aggregates. Both test methods require care to execute properly, but the procedures are straightforward. The methods also allow calculation of absorption of water, which can indicate aggregate that may not be durable and estimate the amount of asphalt binder that may be absorbed.

Specific Gravity of Coarse Aggregate Test

ASTM C127 and AASHTO T 85 are simply a volume determination by displacement of water. A basic test can be performed with a woven-wire basket and water container, or the complete test can be performed more efficiently using a specific gravity bench for suspended weighing of the specimen from an electronic balance.

The Specific Gravity of Fine Aggregate Test

ASTM C128 and AASHTO T 84) is a comparison of sample masses in oven-dry, saturated surface-dry (SSD), and immersed conditions. This test method offers alternate procedures to determine specific gravity. Either a volumetric flask or a simple pycnometer can be used for the gravimetric procedure. For the volumetric procedure, a special Le Chatelier flask is needed. A conical mold and tamper are also required to confirm that the moisture level of the specimen is in a saturated surface dry condition.

Both of these standard test methods allow the calculation of different types of specific gravity:

  • Apparent Specific Gravity is a measurement of the volume of the aggregate particle, not including the volume of water permeable voids. The mass measurement only includes the aggregate particle. Apparent specific gravity only measures the specific gravity of the solids.
  • Bulk Specific Gravity is a measurement of the volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement only includes the aggregate particle. Since it includes the water-permeable void volume, the bulk specific gravity will be less than apparent specific gravity.
  • Bulk Saturated Surface Dry (SSD) Specific Gravity includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement includes the aggregate particle plus the water within the water permeable voids.

Particle Angularity:

Fine Aggregate Angularity

Measures the void content of an uncompacted specimen of fine aggregate, including sand, to provide an indirect indication of its angularity. A high void content indicates a higher degree of angularity, or surface roughness, of the particles. If the aggregate is too rounded, the shear strength of the matrix is reduced. Asphalt paving mixes used in road construction can result in rutting and shoving of the material in use.

The test method is described in ASTM C1252 and AASHTO T 304. A sample of fine aggregate is placed in the fine aggregate angularity apparatus and flows through a funnel into a calibrated cylindrical measure. Uncompacted void content is calculated using the difference between the volume of the cylinder and the total volume of the fine aggregate and the material bulk specific gravity. The test procedure is uncomplicated, and the equipment is simple and relatively inexpensive.

Coarse Aggregate Angularity

Is similar in function and purpose to fine aggregate angularity, using coarse aggregate angularity apparatus that is proportionally larger to accommodate the coarse aggregate. The test method is specified in AASHTO T 326. As of this writing, ASTM does not list a procedure for this test.

Flat and Elongated Particles

Testing measures dimensional ratios of individual coarse aggregate particles. Particles with significantly greater length compared to their width will tend to fracture across the narrow aspect when loaded and can resist reorientation during compaction of asphalt paving mixtures. The fracturing of the particles also negatively affects the void content, stability, and binder distribution of asphalt. These dimensional characteristics also interfere with the placement and consolidation properties of freshly mixed concrete. In the ASTM D4791 test method, a proportional caliper is used to test and classify a representative sample of about 100 individual aggregate particles from each size fraction.

Flakiness Index

Is looking for some of the same dimensional properties as the flat and elongated test but uses a slotted thickness gauge and a separate length gauge to classify the particles. This test method is based on procedures in British Standard BS EN 932-6 (formerly BS 812) and is preferred by some state departments of transportation over the ASTM flat and elongated method. Individual particles from each size fraction are tried in the thickness and length gauges. Aggregate particles in this test are classified as flaky when their smallest dimension is less than 0.6 of their nominal size.

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 About the Author Ben Backus