As engineers evaluate the remains of the recently collapsed Champlain Towers South condominiums in Surfside, Fla., the many potential shortcomings include flaws in the building’s design, construction, maintenance, and inspection. The most unsettling possible culprit, however, is the steel reinforcing in the concrete—a problem that would have remained largely invisible until the collapse. Recent reports suggest that compared to the building’s architectural drawings from 1979, an insufficient quantity of rebar was used in the project. But the more likely source of the tower’s demise was the extensive deterioration of the steel reinforcing caused by water penetration. No conclusive determination has been made, and a thorough investigation may take months to complete. Nevertheless, the sudden collapse, without much warning, has brought increased scrutiny to structures made of reinforced concrete.
As author Robert Courland explains in Concrete Planet, modern concrete is an inherently flawed building material. The extreme reactivity of iron makes steel (which is about 90% iron) a vulnerable reinforcing ingredient. No matter how well the steel is encapsulated by the surrounding concrete, either water, chemicals, or air will find their way in through cracks—eventually causing what is called “concrete cancer.” When the steel rebar corrodes, it expands and further fragments the concrete—significantly reducing its structural capacity. In time, sufficient damage can lead to catastrophic failure.
Although visual inspections can reveal signs of trouble, evidence like spalling and exposed rebar often suggest a long-established problem. Core samples can be drilled before such symptoms present themselves, but invasive techniques have obvious drawbacks and can create new vulnerabilities. Non-invasive forensic methods, on the other hand, offer the best opportunity to catch concrete cancer before it is too late.
Established non-destructive testing methods for reinforced concrete include electromagnetic, elastic wave, and deflection-based techniques. Electromagnetic techniques direct electromagnetic (radar) impulses into a test material and assess how it responds. Examples include ground penetrating radar (GPR), magnetic pulse-induction, and infrared thermography (IRT). Other, less typical methods include elastic wave-based techniques that apply physical force to a material, revealing the behaviors of internal vibrations, as well as deflection-based approaches that measure structural deflections based on the application of external loads. Such techniques are often used in paving applications to predict material performance under wheel loads.
While non-destructive testing methods have been employed for over three decades, they have undergone significant advances in recent years. GPR is one of the most promising techniques, involving lightweight and portable testing instruments requiring no special safety measures. This electromagnetic method uses radar pulses to distinguish between highly conductive steel reinforcing and less conductive concrete. GPR can reveal information about the size and location of rebars and the location of anchors and joints. Notably, the method can indicate steel corrosion as well as concrete defects such as cracks or voids. Consider the Conquest 100, a GPS device from Mississauga, Ontario-based Sensors & Software. Used primarily to detect the location and size of rebar and cabling in concrete, it includes a scanner and display. A quick swipe of the scanner reveals hyperbolic waves on the monitor that indicate subsurface elements. The system creates section and plan views to provide a detailed picture of embedded objects.
Another approach uses electrical currents to detect corrosion in embedded steel reinforcing. Giatec’s iCOR system relies on connectionless electrical pulse response analysis (CEPRA) technology, delivering pulses of narrow DC/AC currents via electrodes and evaluating the voltage between them. Corroded steel exhibits a different voltage response than non-corroded steel, enabling the device to flag any problem areas. Notably, iCOR also detects the rate of corrosion, thus highlighting areas of priority concern.
IRT is another promising tool for detecting structural flaws in reinforced concrete. The method captures the invisible infrared radiation emitted by different materials and displays them as thermographs. The Japan-based West Nippon Expressway Engineering Shikoku Company Limited partnered with thermal imaging manufacturer Teledyne FLIR to develop IrBAS, an IRT technique to identify flaws in concrete structures. “The IrBAS is capable of photographing and diagnosing a wide area at one time by thermography, which helps dramatically reduce inspection time and effort,” says engineer Kazuaki Hashimoto in a FLIR press release. The camera employs indium antimonide-type thermography, revealing minute differences in material temperature variation. As a result, IrBAS shows subsurface damage and distinguishes between different types of defects such as water leaks, spalling, and chemical contamination.
As the world’s many reinforced-concrete structures continue to age and degrade, reliable methods to diagnose internal problems will be increasingly essential. Electromagnetic techniques based on GPR, CEPRA, IRT, and other similar technologies offer many advantages, including portability, rapid reporting, and non-destructive analysis of existing structures. For new structures, the widespread development and deployment of alternative concrete reinforcing materials—such as carbon fiber, basalt fiber, or stainless, galvanized, or epoxy-coated steel—promise to extend the life of those buildings. Meanwhile, building owners and design teams will increasingly use smart concrete technology that can detect and report early structural deterioration, regardless of the type of reinforcement. Although the verdict of what brought the Champlain Towers down is still unknown, what is certain is that we need to improve our forensic capabilities when it comes to structural concrete.
