In January, Engineering360 examined infrastructure resiliency and sustainability in the article "Can a Rating System Ensure Infrastructure Sustainability?" Here, Engineering360 considers efforts to use recycled and even organic materials in infrastructure projects to help improve their sustainability. A future article will look at how electronics sensors are playing a role in monitoring U.S. infrastructure.

Talk focuses on the aging infrastructure in the U.S., but there is relatively little action to back it up. The American Society of Civil Engineers estimates that U.S. infrastructure needs a $3.6 trillion investment by 2020 to improve overall conditions and performance.

The proposed federal "Grow America Act" would take a step in the direction by allocating $478 billion over the next six years toward repairing decaying roads and bridges, as well as expanding transit systems and maintaining existing ones.

To help the U.S. stretch infrastructure investment dollars in an environmental friendly way, the National Science Foundation (NSF) is investing millions into the development of sustainable technologies and materials for civil infrastructure.

The NSF programs, along with initiatives at many state-level transportation agencies, are working with academic researchers to overcome challenges associated with sustainable infrastructure. These collaborative efforts are yielding alternative materials and new technologies that may contribute to high-performing, longer-lasting infrastructure.

New Uses for Recycled Materials

Infrastructure systems require massive amounts of virgin aggregates like crushed stone, sand and gravel. In 2014, the U.S. produced an estimated 911 million tons of construction sand and gravel, with 69% going toward concrete aggregates, road base and coverings, and road stabilization. Of the 600 million tons of crushed stone generated in 2014, some 82% was allocated for road construction and maintenance.

Not every concrete that is recycled has the exact same chemistry, says Burak Tanyu. To minimize the depletion of the earth's raw materials used in road construction and the energy required to extract and refine these resources (as well as diverting tons of construction and demolition debris from landfills), government organizations are turning to recycled concrete aggregates (RCA). Recycled concrete is typically used as a base course within the pavement structure, helping to convey water that may infiltrate the road structure and weaken it over time.

RCA proponents want the material to be used on more projects, but questions remain. Officials at the Virginia Department of Transportation (VDOT) are working with George Mason University's Volgenau School of Engineering to determine how RCA behaves with another sustainable material; geotextile.

The purpose of the geotextile is to create a filtration system that allows water to pass but retains the finer particles so that they don't accumulate within drainage pipes and hinder their flow capacity. The geotextile eliminates the use of soils, which is the traditional choice for filtration.

Every new road built in the Commonwealth of Virginia must have an independent drainage system containing perforated pipe wrapped around an aggregate, which is then wrapped around a geotextile. The concrete is created from a mixture of water, aggregate, and Portland cement, which is a calcium-rich material that reacts with water to bind together the aggregate pieces.

“One of the concerns is that if you use recycled concrete to create a drainage layer, it is important to evaluate the potential for calcium to be leached from the concrete and precipitate as a mineral called calcite," says Burak Tanyu, assistant professor of geotechnical engineering at Volgenau. In large quantities, the calcite may clog the openings in the geotextile and hinder water flow and drainage performance.

Tanyu's research group is evaluating this phenomenon with three objectives. The first is studying the chemical changes of the RCA, as the material may sit in stockpiles for months or years while being exposed to atmospheric conditions including rain. Next, Tanyu and his team are examining the potential for clogging the geotextile. They are doing this through months-long experiments designed to allow water to saturate the RCA and pass through the geotextile to simulate drainage conditions. The researchers also test water samples to track changes in water chemistry.

The third objective involves evaluating the suitability of an existing quick field test to extract leachate so that the state's transportation department gains a method to evaluate the chemistry of RCA before it is used in drainage applications.

“Not every concrete that is recycled is going to have the exact same chemistry," Tanyu says. Concrete sources could vary from demolished buildings to an old highway.

Rice Husks for Construction

Like RCA, other industrial byproducts may feature chemical and physical properties that make them suitable for roadway applications. The School of Civil and Construction Engineering at Oregon State University explored using rice husk ash—the waste product of rice husks burned for energy that typically heads to the landfill—as a partial replacement for Portland cement in concrete.

"A rice husk is like a honeycomb structure that, when placed into a cementitious material with water, absorbs all that water," says Oregon State professor David Trejo. The more water that is added to a cementitious material, the weaker it gets.

Other researchers had previously tried to grind down the rice husk material to rid it of its honeycomb structure. Although they succeeded, the process wasn't economically viable. Instead, an Oregon State research team reviewed breaking down the rice husk ash with a chemical process, and this demonstrated promising results.

In the case of fly ash, the byproduct of coal burned at electric-generating plants, Trejo says that industry specifications are trying to encourage its use because the ash can partially replace Portland cement and make concrete more durable. However, specifications often indirectly limit its use.

CFRP offers properties that building and bridge owners take advantage of in infrastructure rehabilitation. For example, existing materials almost always limit the amount of chlorides. This element, if present in sufficient quantities, can lead to corrosion of the steel reinforcement in concrete. Standards limit it as a percentage of the overall amount of Portland cement. If rice husk ash or fly ash replaces some of the Portland cement, the amount of allowable chlorides in the concrete must be reduced, and some concrete producers cannot meet these lower chloride limits.

Even so, limited research has been performed to assess how lower levels of chlorides in concrete containing fly ash or rice husk ash perform. This research suggests it may be more resistant to corrosion.

Researchers are now evaluating how fly ash (and other supplementary cementing materials) affect the long-term corrosion performance and service life of reinforced concrete structures. As Trejo says, it is important to build cost-efficient structures, “but there's no value if we build a low-cost structure that only lasts 10 years. You might get a lower initial cost but the short service life ends up costing the users much more."

Advanced Composites Strengthen Capabilities

Roads and bridges aren't made solely of concrete and researchers are demonstrating the potential of advanced composite materials as an alternative to steel in certain infrastructure projects.

At the College of Engineering at Wayne State University in Michigan, Hwai-Chung Wu, associate professor of civil and environmental engineering, has been developing a hybrid composite (HC) reinforcement that replaces traditional steel reinforcements in concrete used in highways, bridges and commercial buildings.

Steel rebar corrosion is a leading source of distress in concrete structural components such as beams and columns. Because HC reinforcement is impervious to corrosion, concrete structures using the advanced composite require less frequent maintenance and repair.

The HC reinforcement—which offers up to twice the strength at approximately one-quarter the weight of steel reinforcements—arrives at the job site in modifiable, ready-to-install preforms. Wu says the HC material costs more than steel, but the savings come from lower handling expenses, expedited construction times and reduced maintenance requirements.

Carbon-fiber Components

Carbon fiber reinforced polymer (CFRP), HC's better-known cousin, offers similar properties that building and bridge owners take advantage of in infrastructure rehabilitation.

CFRP's lightweight, corrosion-resistant and high tensile-strength properties make the material a candidate for strengthening bridge components when a complete replacement is not feasible. CFRP reinforcements also enable quicker, less labor-intensive installation while minimizing traffic disruptions. The composite's most notable limitation is price since CFRP has a relatively short history compared to conventional concrete and steel.

The primary goal of CFRP research is to improve structural behavior such as increasing load-carrying capacity for deteriorated or damaged members, says Jimmy Kim, an associate professor in the Department of Civil Engineering at the University of Colorado-Denver who is responsible for CFRP research. His team is starting to examine the potential use of shape memory alloy for structural repair, offering possibly another retrofit solution that would allow older transportation infrastructure to continue to safely function when complete bridge replacement isn't feasible.