• Delayed Ettringite Formation Damage. Delayed ettringite is an internal sulfate attack on concrete. The FHWA research was exploring how delayed ettringite forms and causes damage in concrete, in transforming from an amorphous ettringite gel to nanoscale crystals. The research involves the application of synchrotron radiation to study the relationship between ettringite crystal formation and concrete expansion.

Lightweight Aggregates

Lightweight aggregates comprise a ceramic material produced by expanding and vitrifying select shales, clays and slates in a rotary kiln. This pyroprocessing produces an aggregate that is structurally strong, durable, environmentally inert and low in density, according to the Expanded Shale, Clay and Slate Institute. Such aggregates are ideal for use in applications where weight is an issue, for example, as aggregates in mixes for bridge decks.

Synthetic aggregates can be made from recycled materials. For example, in the SYNAG process of the Western Research Institute, coal combustion fly ash is cold-bonded chemically to produce a hardened product that can be crushed and sized for construction applications, with properties specified by ASTM and AASHTO.

Lightweight aggregates refer to a class of building materials that weigh less than 70 pounds per cubic foot, but more than 55 pounds per cubic foot (lightweight aggregates less than 55 pounds per cubic foot are used in insulation, agriculture or horticulture, but are too weak for use in robust applications). They generally exhibit a porous structure, with the weaker, lighter aggregates exhibiting high porosity, and the stronger displaying a finer, more evenly distributed porosity.

Aggregates produced by altering both physical and chemical properties of a parent material may be considered synthetic or artificial aggregates. Some are produced and processed specifically for use as aggregates; others are the byproduct of manufacturing and a final burning process, such as ground granulated blast furnace slag.

While there are naturally occurring lightweight aggregates — such as pumice and other volcanics — these tend to be too light and weak for construction use. Instead, engineers prefer “pyroprocessed” natural materials, that is, those that have been chemically and physically altered by the heat of a rotary kiln.

Pyroprocessed lightweight aggregates include those made from shale, clay and slate, which expand into lightweight aggregates when heated to temperatures in excess of 1000 degrees C (1800 to 2100 degrees F). This synthetic lightweight aggregate, according to ESCSI is a ceramic material produced by expanding and vitrifying select shales, clays, and slates in a rotary kiln, not unlike cement manufacture, but at a lower temperature.

“The process produces a high quality ceramic aggregate that is structurally strong, physically stable, durable, environmentally inert, light in weight, and highly insulative,” ESCSI says. “It is a natural, non-toxic, absorptive aggregate that is dimensionally stable and will not degrade over time.”

Clay is a very fine-grained, moisture-retentive, naturally occurring material composed principally of aluminum silicate compounds, derived from the decay of igneous micas and feldspars, such as found in granite. Sedimentary rock made from clay is called shale, and if it is metamorphosed it becomes slate. Thus pyroprocessing of all three materials – clay, shale and slate – results in essentially the same product, a lightweight aggregate that is ceramic in nature.

Aggregates and Unpaved Roads

Two-thirds of the road network system in the United States – and nearly 90 percent of the roads in the world – are unsurfaced or lightly surfaced low-volume roads. These unpaved roads may be surfaced with crushed stone, gravel or dirt – or even covered with a chip seal – but an aggregate/fines mix is a very popular combination for surfacing.

Piezoceramic- based devices – dubbed “smart” aggregates – someday may monitor dynamic seismic response and perform structural health monitoring for large-scale concrete structures.

If unpaved roads must serve year-around traffic under all weather conditions, then aggregate surfacing is the best solution. The South Dakota Local Technology Assistance Program (LTAP) recommends a minimum of 3 inches of aggregate, which won’t provide much structural strength, but provides the minimum amount of aggregate for blade maintenance purposes that will permit an operator to shape and work the aggregate without getting into the earth subgrade.