Strengthening works have recently been completed on Blackglen Overbridge, a 97‑metre‑long, two‑span, post‑tensioned, concrete bridge in south county Dublin. Constructed in 2000, the bridge carries the R113 over the M50 motorway, near Junction 13, and connects key residential and commercial centres in the surrounding area.
Following a series of routine TII Eirspan principal inspections, in addition to a structural investigation and assessment, it was determined that the existing reinforced concrete pier supporting the bridge deck required strengthening. ROD, in close consultation with Transport Infrastructure Ireland (TII), explored various active and passive remediation options, and ultimately, a bespoke, high‑strength, stainless‑steel collar was selected to provide pier strengthening.
The six-tonne collar system provides controlled prestressing to counteract the horizontal tie forces at the top of the pier head. It consists of two 25 mm‑thick half‑collars connected by eight 75 mm MacAlloy bars, stressed to 55% of their ultimate tensile strength (equating to a total force of just under 2,000 tonne).
The use of high-strength stainless steel provided several benefits, including enhanced durability and reduced long‑term maintenance demands. It also eliminated the need for future repainting, a significant advantage given the restricted access to the pier.
The collar was installed in May 2026, and the final prestressing was completed in June 2026. The project underscores not just the value of rigorous inspection and maintenance and early crack detection and monitoring but the important role innovative strengthening measures can play in preventing the deterioration of existing structures.
Commissioned by TII, the project was undertaken as part of the Motorway Maintenance and Renewals Contract (MMaRC), with ROD acting as designer for Jons Civil Engineering Ltd, the main contractor for the works. Thompson of Carlow undertook the fabrication of the collar, and the prestressing was carried out by PHL Hydraulics.
