Getting Geotechnical: Lightening the Environmental Load of Aggregate Use

Globally, approximately 75 percent of aggregates are used for cement, roads and railway ballast, while about 25 percent supports drainage, void fills, pipe insulation and protection, hard surfaces, water filtration, and sewage-treatment processes. Aggregates are an important component across large swathes of infrastructure development and maintenance.

However, the environmental cost of quarrying aggregates can be high and irreversible. As such, in addition to prioritizing structural resilience to achieve more sustainable construction, it’s important to consider optimizing extracted material to reduce the environmental impact of quarrying.

To that end, expanded shale, clay and slate (ESCS) lightweight aggregate can be a viable solution to more sustainable infrastructure development. While the material initially has a high ecological footprint, due to being fired in a rotary kiln, its qualities can offset this cost to support environmentally conscious construction—even contributing points toward LEED certification.

Environmental Cost of Quarrying

Aggregate extraction can occur through quarrying, marine and river dredging as well as aggregate recycling. Quarries, especially open-pit mining, can destroy carbon sinks and reduce biodiversity, which can disrupt food chains and have a broader effect on the environment. Blasting and heavy truck traffic can cause air pollution. Dredging can destroy habitats and negatively impact the water quality within the bodies of water being dredged. Both processes can increase the risk of erosion and pollution caused by runoff.

Traditional aggregates represent a one-to-one ratio of materials taken to materials used—a producer would need to take 1 cubic yard of material to provide 1 cubic yard of aggregate to a customer. Conversely, ESCS expands two to three times its original size during firing. As a result, it takes less raw material to deliver similar amounts of aggregate. It also should be noted that the extraction of raw materials for ESCS has a particularly low environmental impact compared to gravel and sand materials.

This material is also chemically inert, durable and insoluble in acid and water; it’s readily recyclable when and if needed.

Transportation Costs

After aggregates are extracted, crushed and readied for use, they must be transported to the jobsite. Depending on the size and distance from the aggregate producer, this can represent a sizeable amount of embodied energy and carbon based on the fuel used and emissions created from trucks, barges and railcars.

Traditional quarried materials typically have a dry and loose bulk density of 95 to 135 pounds per cubic foot. Lightweight aggregates typically weigh between 35 to 55 pounds per cubic foot. As such, more of it can be carried per truckload, which reduces fuel consumption and associated emissions. For example, depending on the material, a standard 20-ton dump truck can carry up to 16 cubic yards of traditional geotechnical fill. By comparison, that same truck can carry up to 32 cubic yards of lightweight aggregate, reducing the required loads by 50 to 68 percent.

Further, lightweight aggregate often requires fewer compaction efforts, which reduces labor costs and the embodied energy and emissions represented by onsite equipment.

 

 

Sustainable Construction on the Jobsite

Lighter aggregates in geotechnical fills also can reduce material requirements for foundation and structural systems, resulting in a lowered environmental footprint when a full project is considered. This is especially true when the aggregates have a high internal friction angle.

In addition, aggregate choice can reduce the ecological impact of infrastructure use. In a peer-reviewed article published in the Journal of Cleaner Production, researchers found that ESCS-amended soils used for clear zones alongside roads infiltrated 220 percent more water than sand-amended soil under compaction and improved the soil capacity to remove pollutants, including heavy metals and E. coli. This indicates ESCS lightweight aggregates could potentially be used to treat pollution originating from road infrastructure.

Considering the impact of quarrying and transportation as well as an aggregate’s ability to contribute to more sustainable construction can help engineers plan more ecologically conscious infrastructure projects.

 

Getting Geotechnical Jack Moore