Searching for Hidden Damage to Pre-stressing in the Faroe Islands

The Streymin Bridge is located on the Faroe Islands; it is a striking connection between two islands across the Streymin River, which flows into the Atlantic Ocean. During a routine inspection, concrete damage with corroded reinforcing bars was detected in the bridge girders. Although this damage was not directly related to the prestressing, its combination with the bridge’s age prompted the operator to commission a further investigation. This was especially true given that experiences such as the collapsed bridge in Dresden once again highlight just how critical the condition of prestressing can be. Experts from Nebest were flown in to take a closer look at the prestressing.

The Streymin Bridge dates from the 1970s, has nine spans, and is approximately 220 meters long. The bridge deck consists of 54 concrete girders, each with internal prestressing. “In a prestressed structure, a significant portion of the load-bearing capacity lies deep within the concrete,” explains Remco van Osch, a specialist in non-destructive concrete testing at Nebest. “So you can’t easily tell its condition just by looking at the outside—not even if one or more strands were on the verge of breaking. And if something goes wrong, the consequences are severe. Recent incidents in Dresden and Genoa have certainly proven that.”

Ultrasound Examination

In his investigation of the prestressing, Nebest combined various techniques. “We first used radar to map the prestressing channels,” Van Osch explains. “We then used ultrasonic pulse-echo measurements to examine the prestressing channels for voids in the injection grout. The ultrasonic pulse-echo measurements were evaluated using advanced phase analysis. This allows us to determine whether the channels are actually filled properly. Poorly filled prestressing channels, in fact, pose an increased risk of corrosion to the high-strength prestressing steel.”

Viewing Windows

Nebest conducted a representative sample survey on approximately 15 percent of all beams. “Where the measurements warranted it, small-scale destructive testing followed,” explains Gert Wilgenburg, a concrete testing consultant at Nebest. “In total, we created about 44 inspection windows.” Van Osch: “It’s a bit like going to the dentist. First, you take an X-ray and then determine where you need to look further. Not every girder needs to be completely opened up.” Through the inspection windows, the team examined, among other things, the condition of the prestressing steel, any fractures, and voids in the ducts. After inspection, the openings were repaired and protected.

Management Choice

The combination of visual inspection, radar, ultrasonic testing, and inspection ports provided crucial information. “We submitted our findings to the bridge manager so that the structural engineer could use that information to recalculate the bridge’s structural integrity,” says Wilgenburg. “From there, the step-by-step plan for repairs will be determined.” Nebest conducted the measurements in June 2026 and submitted the report to the bridge operator at the end of August.

Preventive Screening

What has been studied in the Faroe Islands is also relevant closer to home. “Many prestressed bridges in the Netherlands are now fifty to eighty years old. Structures from the 1960s, 1970s, and 1980s, in particular, deserve attention. During that period, prestressing was still in its early stages of development, and experience was gained with, among other things, the grouting of prestressing ducts,” Van Osch explains from experience, and he advises: “Don’t wait until damage becomes visible on the exterior. By then, it may already be too late. Just as you take periodic X-rays at the dentist to detect problems early, we should also conduct preventive, in-depth inspections of prestressed bridges.”

In the Netherlands, work is now underway on a guideline designed to provide bridge managers and engineering firms with more guidance when inspecting prestressed bridges. Meanwhile, the Streymin Bridge illustrates why this development is necessary: sometimes the most important part of a bridge is precisely what you cannot see.