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Aggregate Properties and Testing

In the latest edition of the RoadReady Newsletter, we took a look at what makes an aggregate suitable for asphalt paving. But how do we determine if our rocks will perform well before putting them in the roadway? Let’s explore a few of the tests that road builders use to assess the quality and characteristics of their aggregates.

A standard sieve nest used for gradation testing

Size and Gradation

One of the most well-known and common aggregate tests is the sieve test. Used for determining the gradation of an aggregate sample, the sieve test, aptly named, involves passing the aggregate through a number sieves. Each sieve consists of a metal cylinder with a wire screen at the bottom, which contain square holes of a specified width. Therefore, any rock that passes through a given sieve must have maximum dimensions smaller than the diagonal of the square. Sieves are stacked sequentially, with larger sized holes at the top, and smaller sizes at the bottom. The specific sieves to be used depend upon the particular specifications for the aggregate. Material is introduced to the top sieve, and the entire stack is agitated to promote passing of the particles through the screens. After a specified time interval, the mass of the aggregate retained on each of the sieves is weighed. This allows calculation of the proportion of the total sample within each size range.

Aggregate Shape

While gradation affects how dense an aggregate matrix is, the shape of the aggregate also impacts the strength of this matrix and how particles lock together. Depending on the size of aggregate, shape is assessed in different ways. For large aggregate particles, shape is determined by visual inspection. Depending on specifications, aggregate particles are divided into different categories based on the number of fractured faces. Generally, a number of required fractured faces is determined, and particles are designated as either pass or fail. The percentages of particles that pass and fail are determined on a by-mass basis.

Fine aggregate particles are too small and numerous for visual inspection, so a different method must be used. Because particle shape cannot be assessed directly, the void content of uncompacted aggregate is used to estimate this parameter. In general, the more angular an aggregate sample, the more resistant to compaction. Therefore, the presence of angular faces and rough particles will increase the void content of the aggregate. An uncompacted condition is achieved by passing the sample through a funnel and allowing it to fall into a cylinder of known volume. The mass of the aggregate in the sample is then measured, and air void percentage is calculated from the specific gravity of the aggregate.

A rotating drum for toughness testing

Durability and Toughness

The ability of aggregates to withstand freeze-thaw cycles (durability) can be extremely important in asphalt pavements depending on weather conditions. Aggregates can be damaged when trapped water freezes inside of individual particles and expands. Resistance to this phenomenon is gauged using soundness tests, which simulate the formulation of ice crystals through chemical processes. Aggregate samples are placed in a sulfate solution, which causes salt crystals to form in the aggregate pores. Like ice crystals, these crystals create outward pressure on the aggregates, and can lead to cracks and breaks. The aggregate sample is subjected to a number of repetitions of this process, and sieved to assess particle disintegration.

Aggregate toughness is determined through brute force. Aggregates are placed in a rotating drum, along with a number of steel balls. The drum is rotated at a specified speed and duration, allowing the steel balls to crush the aggregates inside to a certain extent. Again, this test is concluded by sieving the remaining material to determine the amount of crushing it has undergone. This test is commonly called the Los Angeles abrasion test.

For additional information on aggregate testing, check out the following links:

Gradation Test

Los Angeles Abrasion Test

Coarse Aggregate Angularity

Fine Aggregate Angularity

Aggregate Testing Bundle-Designed to familiarize technicians and engineers with the origins, procedures, calculations, and results associated with common aggregate tests, this bundle is comprehensive in its coverage in all things related to determining aggregate quality.

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