Measuring the Size and Shape of Aggregates

Aggregates encompass the bulk of the materials used in highway construction, consequently the performance of the concrete or asphalt pavement relies heavily on the performance of the aggregate materials. Careful selection of these materials, with respect to stringent standards, ensures that the pavements will perform as designed, and not suffer from premature deterioration.

As an example, aggregates can be rounded or angular and/or elongated.  Highly elongated aggregates are not desirable for asphalt pavements because they can break when they are compacted by construction equipment or by repeated loading by large trucks

3/8” aggregate samples. Left: Rounded river gravel, Right: Angular crushed dolomite

Aggregates must pass a "flat and elongated" test whereby each piece of gravel is passed thought the left caliper (in its longest dimension) and then if possible through the right caliper (in its shortest dimension).  If it passes through it fails the test (It is too elongated) and depending on the specification if say 10% of the pieces are too elongated, the entire sample is unsuitable for use in asphalt pavements.

This test is clearly labor intensive, time consuming, subjective, and results are typically not statistically representative enough to base costly technical decisions on.

Proportional caliper, used to test for flat and elongation

The alternative is to use image analyzing computers that can efficiently to make objective test measurements.  Aggregate pieces are paraded in front of a camera by a translucent rotating disk.  Backlit images of the aggregate are taken from on top and at the size, simultaneously at 60 frames per second.

The WipShape size and shape analyzer

Each frame image is digitized, and where aggregate pieces are identified, the the three major dimensions are measured.  From that the nominal size of the piece is determined as well as the aspect ratio.

Top and size profile images of an aggregate piece.  Red areas indicate the profile of the aggregate piece, green line the dimensions

of the piece, and blue lines represent the position of the triggering scans used to detect the presence of the piece in each image.

The measurements for each piece of aggregate are then tabulated and reported.  This provides a fast, efficient, convenient way to characterize the shape of the aggregate pieces that is objective, and efficient enough so that enough measurements can be taken to provide statistically significant conclusions.

Final report showing the distribution of size and aspect ratio as well

as the average angularity (curve radius) of the set of 281 pieces.