Concrete is a pervasive material in the built environment, with a presence in buildings and other infrastructure. Maintaining its physical health requires near-constant monitoring. Researchers continue to work on efficient systems to handle that task.
Monitoring the integrity of heavy-use infrastructure, such as bridges and tunnels, is critical to maintaining a resilient built environment. But managing extensive systems in real-time can be a challenge. To address the issue, researchers have been at work on integrated sensors and skins that obtain qualitative information about the physical health of a building or a component. For example, the University of Michigan’s Laboratory for Intelligent Systems and Technologies created thin films with wireless sensors that can be attached to structures, while MIT engineers crafted patches of flexible, titanium oxide–infused plastic for placement atop problem areas.
Last month, researchers from North Carolina State University (N.C. State) and the University of Eastern Finland announced the development of a more thorough and reliable method to detect weak spots in concrete structures. Their “sensing skin” turns a static building element into an active one through the application of a single-layer coating—comprising powdered metal, such as copper, and a liquid paint binder—to new and existing concrete surfaces upon which a series of electrodes have been installed. The coating and electrodes use alterations in conductivity to root out surface-level defects. Unlike earlier solutions that apply films and patches episodically, this coating, which cures in about one hour, covers the entire surface and its analysis provides quantitative imaging of the damage.
The goal is to better spot and address defects before they escalate into significant structural issues, says Mohammad Pour-Ghaz, an assistant professor in N.C. State's department of civil, construction, and environmental engineering.
Here’s how it works: Similar to the human nervous system, the sensing skin has two fundamental components: local nerves and a remote brain. Electrodes—the nerves—send electric pulses through the conductive coating to an integrated measuring tool that transmits the data wirelessly to a computer—the brain—that monitors the system’s voltage. A crack in the concrete ruptures the coating, decreasing conductivity in that zone. Detecting the loss, the computer creates an image of the surface with the location and severity of the cracks shown in bright red (below).
A notched beam undergoes a four-point bending test, which causes its surface to crack.
Credit: Mohammad Pour-Ghaz
The sensing skin was applied to the beam in the previous photo. This computer image shows the beam's crack pattern in red, which indicates low levels of conductivity. Electrical conductivity is indicated by the gray-to-red color gradient.
Credit: Mohammad Pour-Ghaz
Pour-Ghaz’s team developed two algorithms to identify defects. The first detects problem areas and provides basic feedback in less than one second. The second reports the precise location and severity of the damage and takes a few minutes to process. The team recommends that the two programs be run in sequence: If and when the first algorithm signals trouble, the second can jump in to provide more specific information.
The number and distribution of the electrodes depend on the size and shape of the structure. Complex geometries require computational analysis to determine the appropriate quantity and placement, Pour-Ghaz says. If the electrodes are spaced too far apart, the algorithm won't be able to determine the cracks' precise locations. If the spacing is too tight, he says, the electrodes won’t be able to get a clear read due to instrumental noise.
The sensing skin is applied to a polymeric substrate with the crack highlighted in red, left, and a reconstructed image of its electrical conductivity, right.
Credit: Mohammad Pour-Ghaz
The skin is only a surface treatment so the method usually does not detect damage beyond superficial cracks. “If we have internal micro-cracking, the sensing skin will not detect it,” Pour-Ghaz says. “However, if the extent of internal cracking becomes high enough and starts to affect the surface and results in surface cracking, then it can be detected.”
Still, the system is straightforward and versatile enough to be deployed widely for the task of monitoring structural and envelope components in new and existing buildings. Though implementing the system would impose an additional charge not typically included in construction bids, Pour-Ghaz claims it is less expensive than peer technologies. Further testing is required to determine the longevity of the skin and whether contact-based current leaks might cause erroneous readings in cases such as buildings that house high-voltage electrical equipment. However, given the low-level of electrical current employed—within the range of that used in medical imaging—this advantageous technology suggests a future in which coatings might become an active, rather than a passive, surface treatment.
Image courtesy Flickr user bittbox via a Creative Commons license.
Blaine Brownell, AIA, is a regularly featured columnist whose stories appear on this website each week. His views and conclusions are not necessarily those of ARCHITECT magazine nor of the American Institute of Architects.