Fiber optic sensors catch hidden shear cracks in aging concrete bridges before collapse

Thousands of aging bridges across Europe and North America carry a hidden liability that no inspection camera can see: shear reinforcement ratios far below what modern design codes demand. Many of these structures, built before 1970, show no visible cracking at all, yet their calculated shear capacity falls short of current standards, forcing engineers into an uncomfortable choice between expensive strengthening, traffic restrictions, or outright replacement. A new experimental study from TU Braunschweig suggests a third path, one written in light rather than concrete. Researchers there have demonstrated that hair-thin fiber optic sensors glued to the surface of reinforced concrete beams can detect the dangerous diagonal cracks that precede brittle shear failure at roughly half to three-quarters of the ultimate load, providing a measurable warning of collapse that conventional visual inspection simply cannot deliver.

The research, published in Results in Engineering by Johannes Rathgen and Vincent Oettel of the iBMB Division of Concrete Construction, tackles a problem that has haunted bridge engineers for decades. Shear failure is among the most treacherous failure modes in structural concrete: unlike ductile flexural failure, which announces itself through large deflections and visible sagging, shear failure typically arrives suddenly and without significant warning, governed by the unstable propagation of diagonal cracks inclined toward the load. Design codes handle this uncertainty conservatively, and analyses of extensive experimental databases show that current approaches can substantially overestimate the danger for beams with little or no shear reinforcement. The German team’s calculations, for instance, underestimated the measured failure loads of three low-reinforced beams by factors approaching 1.85, confirming that the true reserve of these structures is often much larger than the codes admit. Monitoring, the authors argue, can unlock that reserve safely by revealing how a structure actually behaves under load.

To test that idea, the researchers cast six reinforced concrete beams spanning a deliberate range of shear reinforcement ratios, from one beam with no stirrups at all to others provided with between 1.3 and 8.1 times the minimum ratio required by the German National Annex to Eurocode 2. All beams used normal-strength concrete with a maximum aggregate size of 16 millimeters, B500B steel bars of 20 millimeters as longitudinal reinforcement, and B500A stirrups of 6 and 8 millimeters. Each beam was loaded to failure in three-point bending, with the load applied at midspan and a shear span of 90 centimeters, deliberately chosen to prevent direct load transfer to the supports. The material properties were carefully characterized on the day of testing: mean cylinder compressive strengths ranged from 36.3 to 56.5 newtons per square millimeter, and mean yield strengths of the reinforcement were determined by tensile testing according to EN ISO 15630-1.

The instrumentation was the heart of the experiment. The team used a LUNA ODiSI 6104 interrogator working on the principle of optical frequency domain reflectometry, a technique that turns a single polyimide-coated optical fiber into thousands of distributed strain measurement points with a gage pitch of just 0.65 millimeters, acquired at 1 hertz. Two contrasting sensor configurations were compared in the shear-critical regions near the supports. In one region, the fiber was bonded in an inclined loop at 45 degrees, aligned with the principal tensile stresses of the uncracked beam so that emerging shear cracks would cross it nearly perpendicularly. In the other region, the fiber ran in three horizontal sections parallel to the beam’s bottom surface, spaced 6 centimeters vertically. The horizontal layout is far simpler to install and requires no prediction of the expected crack pattern, but it sacrifices some sensitivity because inclined cracks intersect it at oblique angles. To keep the fragile fibers alive through large crack openings, the researchers bonded them with a slightly elastic UV-curing resin, allowing controlled debonding instead of catastrophic fiber breakage.

The results were striking. In the beam without shear reinforcement, which failed at 145 kilonewtons, the inclined sensor detected the critical shear crack at approximately 76 percent of the failure load and pinpointed its position to within a centimeter at sensor coordinate 8.2 centimeters. Continued loading produced a pronounced and largely continuous rise in measured strain, an unmistakable early indication of impending failure. In the beams with low shear reinforcement, the picture was even more encouraging: the crack that ultimately governed failure was first flagged at 58 percent of the ultimate load in one beam, 52 percent in another, and 77 percent in a third with an asymmetric stirrup arrangement. The team condensed these observations into a shear-failure warning index, defined as the ratio of the detection load to the failure load, offering a quantitative measure of how much warning a monitoring system can realistically provide before brittle collapse.

Perhaps the most consequential finding concerns what happens after the load is removed. Because bridge inspections are performed under service loads rather than at the extreme levels that may have formed cracks, many shear cracks close up and become invisible to the naked eye. The fiber optic measurements, however, reliably detected and localized cracks even after they had largely closed upon unloading. This means the sensors can supply a structural record that visual inspection cannot, capturing evidence of past cracking events that would otherwise vanish. For the vast population of older bridges that show no visible distress despite calculated shear deficits, this capability could fundamentally change how condition assessments are made, replacing worst-case assumptions with measured structural response.

The study also revealed that detecting cracks is the easy part; classifying them is the real challenge. Flexural cracks form roughly perpendicular to the beam axis, while shear cracks incline at around 45 degrees, but a crack that begins as a benign flexural crack can gradually rotate and develop into the critical flexural-shear crack that destroys the beam. In one test, the top horizontal sensor section registered a crack at 63 percent of the failure load that looked entirely flexural; only when a rapidly propagating shear crack merged with it did the fatal crack emerge, becoming detectable in the lower sections at 77 to 79 percent of the failure load. The researchers therefore propose a conceptual two-step classification procedure: first, estimate crack inclination from the pattern of strain peaks across adjacent, closely spaced sensor sections of a loop-shaped fiber; second, track potentially critical cracks near the supports over successive load levels, watching for continuous strain growth that signals dangerous propagation.

That classification framework, the authors emphasize, remains a concept rather than a validated method, but it points toward something genuinely transformative: an automated, condition-based early warning system for shear-deficient bridges. Combined with predefined warning and action levels, the approach could drive a traffic-light scheme in which green indicates all measured strains remain within expected ranges, yellow triggers additional assessment, and red demands prompt investigation, weight restrictions, or closure. A fixed strain threshold of 1 per mille proved suitable under laboratory conditions for separating crack-induced strain peaks from measurement noise, and its simplicity makes it attractive for automated data evaluation, though the researchers caution that it is a configuration-specific criterion whose robustness under field conditions, varying materials, and environmental exposure still requires validation.

The broader implications reach well beyond the laboratory. Strengthening a deficient bridge with ultra-high performance concrete overlays or carbon fiber reinforced polymer strips demands long planning lead times, temporary closures, and considerable expense, while wholesale replacement is simply infeasible given limited personnel, machinery, and materials. Distributed fiber optic sensing, by contrast, can be installed on existing structures with minimal intervention and no major structural modification, enabling both long-term and large-scale monitoring of the regions where shear failure would begin. The study also found that sensor placement matters more than orientation: multiple horizontal sensor sections stacked near the support, particularly the lowest section in the tension zone, caught cracks earlier than expected, and the horizontal configuration avoids the complex principal-stress trajectory calculations that the inclined layout demands. Notably, no clear relationship emerged between the crack-to-fiber intersection angle and early detection performance, weakening the case for the more elaborate inclined installations.

Laboratory beams are not bridges, and the authors are careful about the limits of their work. Only four of the six tests ended in shear failure, some measurement data were lost to a defective remote module and sensor damage, and the findings apply strictly to the tested configurations and loading conditions. Long-term and variable loading, environmental effects, and full-scale field trials remain necessary before the method can be trusted on live structures. The Braunschweig team is already extending the work to haunched beams, whose tapered support regions alter the stress distribution and crack patterns in ways that complicate sensor placement. Still, the core message stands: light traveling through a fiber thinner than a human hair can reveal the silent, invisible cracks that precede one of structural engineering’s most sudden failure modes, and in doing so may help keep thousands of aging bridges open safely while the slow work of renewal catches up.

Subject of Research: Monitoring shear crack formation in reinforced concrete beams using distributed fiber optic sensors

Article Title: Experimental investigation of fiber optic sensors in shear-critical regions of reinforced concrete beams for monitoring shear behavior

Article References: Experimental investigation of fiber optic sensors in shear-critical regions of reinforced concrete beams for monitoring shear behavior. (n.d.). https://doi.org/10.1016/j.rineng.2026.113052

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113052

Keywords: fiber optic sensors, structural health monitoring, reinforced concrete, shear failure, bridge assessment, distributed fiber optic sensing, crack detection, Eurocode 2, shear reinforcement, concrete beams, early warning system, TU Braunschweig