Researchers at Purdue University have developed in situ piezoelectric concrete sensors that can monitor in real time the strength of concrete as it cures. This development has the potential to significantly accelerate construction schedules and reduce spending on infrastructure projects.
The sensors work by converting electrical energy into mechanical energy before sending a mechanical wave into the concrete and measuring its propagation and speed using a resonator. When concrete is initially placed, it has little resistance to such waves, explains Luna Lu, Ph.D., the American Concrete Pavement Association professor of civil engineering at Purdue. However, as concrete cures, its resistance to the propagation of the waves grows. Using mathematical and physical models, the researchers established the relationship between the wave impedance and the concrete strength.
These sensors have the potential to replace the two most used methods of judging concrete’s strength: cylinder testing and maturity curves, both of which test a specific concrete mix in the laboratory. Because the sensors test concrete on-site rather than in a lab, they can work on any mix and under any temperature or humidity conditions.
In cylinder testing, concrete is placed in a cylinder in a lab, then tested to destruction to establish the mix’s strength. But specimens in the lab may have lower break values than in the field, leading mix designers to compensate by adding roughly 10%-15% more cement into the mix than might otherwise be necessary, Lu says. Because the new Purdue sensors work on-site, such guesswork is eliminated. Less cement can be used, cutting costs and reducing the carbon footprint of the project.
Maturity curves, the other commonly used method, measure the temperature of concrete as it cures as a proxy for strength. “The temperature is a direct representation of the hydration process,” Lu explains. But in the field, many factors can affect that expected curing time. If water, an accelerator, or a decelerator is added, for example, those maturity curves are rendered inaccurate. Extreme temperatures can also impact the curing rate. By working on-site and in real time, the new sensors avoid these issues.
The sensors were first developed in 2017 for the Indiana Department of Transportation, which wanted to accelerate its roadway construction times. The sensors were deployed in three highways in Indiana in 2019. It was estimated at the time that the process could save millions of dollars annually. (In March 2021, this project was named an ASCE Gamechanger, an innovation that helps solve the problems identified in ASCE’s 2021 Report Card for America’s Infrastructure.)
The sensors are now being tested in a building as well. In late 2020, 12 sensors were placed on the third floor of the five-story Engineering and Polytechnic Gateway Complex project at Purdue, six in columns and six in decks. The building is under construction by F.A. Wilhelm Construction Co. Inc., with an expected completion date in fall 2022.
Currently, Lu and her team are developing a way for the sensors to automatically and wirelessly generate, store, and transmit data. Her team is also working on more robust packaging. “As you know, the construction industry is not the most gentle or elegant,” she says. The team is also working with six states and the Federal Highway Administration to complete additional sensor tests so that the method can be accepted for use nationwide, she says.
The sensors are being developed as part of Purdue’s Center for Intelligent Infrastructure, which focuses on research in sustainability and resiliency, smart sensing technology, autonomy and adaptivity, and artificial intelligence applications. It is led by Lu, who is the center’s director, and associate director Shaoshuai Mou, Ph.D., an assistant professor in the university’s school of aeronautics and astronautics.
“The vision we have for the future of infrastructure is that it really has to be sustainable,” Lu says. “The next generation of infrastructure needs to be more adaptive. It needs to be more resilient, and it needs to have autonomous features.”
This article first appeared in the May/June 2021 issue of Civil Engineering as “Real-Time Concrete Sensors Could Redefine Construction Schedules.”