The need to rehabilitate America’s infrastructure has escalated past the point of urgency. According to the American Society of Civil Engineers, the current backlog of repairs includes 46,154 structurally deficient bridges and nearly 2 million miles of roadways in poor condition—coming with an estimated price tag of $560 billion. Another concern is the impact this failing infrastructure could have on the nation’s economic capacity, as more frequent extreme-weather events exacerbate deterioration problems and further undermine the reliability of transportation systems to move people and goods safely and efficiently across the country.
For bridge and highway owners, it makes good engineering and financial sense to use stronger and more durable construction materials that require less maintenance, extend service life and lower life-cycle costs. In search of advanced solutions, researchers at the Federal Highway Administration (FHWA), academic institutions and industry partners are tapping into the emerging science of nanotechnology. Considered the next frontier in high-performance building materials, nanotechnology is proving especially valuable in the concrete industry.
The Promise of Nanotechnology
Nanotechnology is defined as the understanding, controlling, and restructuring of matter at the nanoscale, with one nanometer equivalent to one billionth of a meter or, to put it in perspective, 100,000 times smaller than the width of a human hair. It is a rapidly growing area of scientific exploration, where manipulating the physical and chemical properties of matter at the nanoscale can have outsized impacts on the behavior of materials at the conventional scale.
Nanomodification (the alteration of one material with a nanomaterial) of concrete could be key to America solving its long-standing challenges of keeping its infrastructure in good health to support economic growth. Some nanomodified concretes are stronger or more durable. Others are better at conducting electricity or have different magnetic properties. Many also enable the use of less cement while achieving higher performance.
Commonly Investigated Nanomaterials for Concrete
The various nanomaterials available allow custom mix designs, modified at the molecular level, which result in amazing properties at full scale. And though the production process may be similar to the current practice, the properties of the resulting concrete will be quite different.
Among the many different types of nanomaterials, nano-silica, nano-clay, carbon nanotubes (CNT), carbon nanofibers (CNF), nano-graphene oxide, and nano-titania offer the greatest potential for developing the next generation of high-performance concrete.
Growing interest in nanomodified concrete is being driven by its exceptional strength and durability improvements to prolong service life, as well as by novel “smart” applications. At the same time, industrywide commitments to employ more sustainable building strategies continue to progress.
Improved strength and durability.Calcium-silicate hydrate (CSH)—the glue that holds concrete together—starts forming at the early stages of cement hydration. Nanomaterials such as nano-silica allow hydration to seed faster and gain early-age strength. They also add pozzolanic activity whereby additional CSH binder is created and particles pack more tightly within voids. The resulting dense concrete is less permeable to water and salt-based deicing chemicals.
Higher stiffness. Modulus of elasticity indicates the stiffness of a material. Concrete with a high modulus of elasticity has a greater resistance to being deformed when a stress is applied. The interface region in cement paste near aggregates—one of the weakest links in concrete—tends to be more porous and flexible. A very small dose of nanomaterial significantly densifies this area. For example, the addition of just 0.1% of well-dispersed CNT by weight of cement can increase the elastic modulus by up to 50%.
Crack prevention. Controlling early formation of cracks mitigates other durability issues, such as corrosion of embedded steel rebar and freeze-thaw damage. Crack resistance is fundamentally a fracture process governed by the size and weakness of defects, such as the interface between large aggregates and the cement paste. By making that interface denser, stronger and stiffer, nanomaterials such as CNT will increase the crack resistance of concrete.
Self-sensing. Concrete is an insulating material with very high electrical resistance. Embedding conductive nanoparticles such as CNF within the concrete establishes a conductive network. When this nanomodified concrete is subjected to deformation or stress, the conductive network is disturbed, producing a measurable change in resistance. This has the potential to turn concrete into a “smart” material that can sense strain, cracking or other damage while maintaining or improving mechanical properties.
Self-cleaning. Photocatalysts, usually nano-titania, activate in concrete when exposed to sunlight. This changes the electric charge, causing a repelling effect between the concrete, dirt and airborne pollutants. In the presence of ultraviolet light, the particles cause organic materials such as soot, grime, oil, mold and algae to decompose, thus preventing these materials from discoloring exposed surfaces.
Although many promising property enhancements have been identified, there has not been a systematic approach to demonstrate code compliance or performance of nanomaterials on large-scale batches of concrete in infrastructure applications. In addition to further research in this area, there are a variety of processing and financial challenges that need to be addressed.
Hydrophobic nanomaterials, such as CNT and CNF, pose particular challenges because they require significant energy for dispersion, which with current technologies could negate other sustainability benefits. The cost of nanomodified concrete relative to other property-enhancement strategies is another consideration, as is the expense of additives necessary to process the nanomaterials for use in concrete.
The environmental, health and safety impacts of nanomaterials are also not fully understood. Progress in this area is essential to establish the regulatory certainty needed before using nanomodified products in the construction industry.
A Promising Future
In the years ahead, the material that will be called concrete will be very different from what we have today. Designing materials at the molecular level using nanotechnology will be at the forefront of innovating next-generation technologies to make large-scale change happen. We see the beginnings of this today, but much work lies ahead to bring these visions to reality.
CTLGroup’s Materials Consulting Group understands the nuances of nanotechnology and our team has played a significant role in researching and furthering the development of this exciting new field. To learn more, visit the websites of the National Nanotechnology Initiative, the FHWA and the American Concrete Institute, or contact the materials consulting team at the CTLGroup.
About the Author:
David Corr, Ph.D., P.E. serves as Principal Engineer & Materials Consulting Group Director at CTLGroup and is one of the nation’s leading experts related to structural performance, material characterization, and material development. Dr. Corr’s knowledge focuses on both traditional and emerging building materials. Specifically, he has studied the durability of concrete, the rheology and fresh-state behavior of concrete, and fracture and cracking in cement-based materials.
Prior to joining CTLGroup, Dr. Corr was Clinical Professor and the Director of Graduate Studies in the Department of Civil & Environmental Engineering at Northwestern University. His most current research focused on nanotechnology of cement-based materials, large-scale additive manufacturing (3D printing), and cross-laminated timber. He is a member of the American Concrete Institute (ACI), the past Chair of the Cements Division of the American Ceramic Society and is a licensed professional engineer in the state of Illinois.