Purdue team tests concrete strength sensors, no maturity curve required

Engineers at Purdue University in West Lafayette, Ind. have developed slab-embedded sensors that can safely speed up a construction timeline by determining concrete strength directly onsite in real time. Their technology removes the need for extensive offsite testing by allowing contractors to verify concrete maturity onsite.

Purdue University Civil Engineering Professor Luna Lu (left) helps install an acoustic wave-enabled sensor into a slab forming the third floor of Engineering and Polytechnic Gateway Complex. The sensors she and her team have developed can instantly measure concrete strength, speeding up a construction timeline.
PHOTOS: Rebecca McElhoe for Purdue University

“Our sensors could help make better data-driven decisions to determine the construction schedule and improve the quality of concrete,” says the school’s American Concrete Pavement Association Professor of Civil Engineering Luna Lu. Her team is working with F.A. Wilhelm Construction Co. to test and compare the technology with traditional commercial sensors installed in a floor of what will be Purdue’s five-story Engineering and Polytechnic Gateway Complex.

Lu and her research team also are testing the sensors in highways across Indiana as part of an effort to better determine when newly placed concrete is ready for heavy traffic loads. “We’re trying to work with contractors to find out how much saving we can do for them in terms of time, cost and the number of people needed at a site, which reduces risk and improves construction safety,” Lu observes. “That starts with industry collaborations to evaluate how well the sensors work.”

Over the last decade, she adds, general contractors have used traditional sensors to make reliable and accurate estimates of concrete strength and maturity. But before even pouring the concrete, the method requires a monthlong process of testing the concrete mix design in the lab. A line graph is generated to note the strength of the mix design based on specific temperatures over time. This line graph is then used to match up temperature measurements from sensors in the field. Strength values on the graph, called a “maturity curve,” help workers estimate when the concrete is strong enough to continue construction.

If unexpected weather or any other schedule impact calls for changing the approved mix design, the contractor has to generate a maturity curve all over again for the new mix.

The Purdue lab sensors are geared to measure concrete strength directly from the floor deck in real time, eliminating the need for generating a maturity curve beforehand. “These new sensors are more of a ‘plug and play.’ We could make judgment calls on the fly,” affirms F.A. Wilhelm Quality Assurance Manager Ryan Decker.

Like commercial sensors, Lu’s strength-measuring devices remain in the concrete. The Purdue sensors provide a more direct measurement of strength by using electricity to send an acoustic wave through the concrete. How concrete responds to particular wave speeds indicates its strength and stiffness.

“A wave propagating through concrete can tell us a lot of information. We can find out not only how strong the concrete is, but also detailed information about the microstructure,” Lu explains. Twelve sensors have been installed into various sections of the Gateway Complex’s third floor so that the team can best understand how well they work compared with commercial sensors in use on the site. The Purdue lab has validated the technology and is now developing a system that contractors can use to remotely receive the concrete’s strength information.

The sensor technology is detailed in a patent application filed through the Purdue Research Foundation Office of Technology Commercialization. Construction of the Gateway Complex is expected to be completed in fall 2022. The interdisciplinary hub will connect project-based instructional labs, design studios and other collaborative spaces to increase interaction between the Purdue community and Indiana companies.