The research demonstrates how combining two advanced sensing techniques, Distributed Fiber Optic Sensing (DFOS) and Digital Image Correlation (DIC), can improve the way engineers monitor concrete structures under load.
The team from Rzeszów University of Technology tested three precast high-strength concrete beams prestressed with CFRP bars.
They used DFOS, a contact method that senses strain inside the concrete, alongside DIC, a non-contact optical technique that tracks surface deformation and crack propagation.
While both methods have been used individually and in combination in earlier research, this study aimed to assess how effectively they work together during full-scale structural loading, specifically when beams are pushed to failure in a three-point bending test.
The DIC system was calibrated using virtual sensor points aligned with traditional linear variable differential transformers (LVDTs) and piston displacement readings.
This calibration was critical for ensuring accuracy once LVDT sensors were removed at high loads.
Strain Detection: Where Each Sensor Works Best
The two systems performed differently depending on the loading stage. DFOS excelled at detecting small strains early in the test. However, once cracks formed – particularly in the tension zone – DFOS readings became unreliable at those locations.
In contrast, DIC performed better with large strains and remained effective through failure.
Importantly, DFOS strain measurements in the compression zone remained valid throughout testing. And while DFOS could detect crack formation, its reliability dropped once crack widths exceeded approximately 0.5 mm.
DIC, capturing full-field surface data, enabled detailed tracking of strain patterns and displacements. This was especially important once LVDTs were removed to prevent damage as the beams approached failure.
Identifying Failure Modes
Only the DIC sensor was able to reliably capture the failure modes of the beams in their final loading stages. The beams ultimately failed due to rupture of the CFRP bars, followed by concrete crushing at the top (compression) zone.
These findings were visualized through high-resolution strain distribution maps generated by the DIC system, which clearly showed the number, position, and shape of both vertical and horizontal cracks.
Notably, the tested beams carried about 35 % more load than their nominal design capacity.
The authors suggest this may be due to higher-than-specified material strengths, particularly in the CFRP bars.
Complementary Strengths for Smarter Monitoring
The researchers concluded that DFOS and DIC, when used together, provide a complementary toolkit for assessing structural behavior under load.
DFOS offers precise strain monitoring in early, low-load stages, while DIC enables accurate assessment of larger deformations, crack widths, and ultimate failure modes.
They also cautioned that while these findings are promising, they cannot yet be generalized quantitatively beyond the specific test conditions.
However, they noted that similar methods have already been used to monitor a real bridge, with results from that project to be published soon.
Journal Reference
Wiater A. et al. (2025). Comparison of Distributed Fiber Optic Sensing and Digital Image Correlation Measurement Techniques for Evaluation of Flexural Behavior of CFRP-Prestressed Concrete Beams. Sensors 25(23):7357. DOI: 10.3390/s25237357