Roads & Bridges Case Alert: Hot-dipped galvanized rebar serves Bermuda bridge for seven decades

The Longbird Bridge in Bermuda, a swinging asymmetrical steel span, was built across Castle Harbor by the US Navy in 1953 to facilitate movements to St. David’s Island near the Kindley Naval Air Station (now the Bermuda L. F. Wade International Airport). Due to the high chloride environment, hot-dip galvanized steel reinforcement was used in the concrete approach and bridge deck.

Over the nearly seven decades the Longbird Bridge stood, several inspections and corings were conducted to evaluate the galvanized rebar. In 1995, when the bridge was 42 years old, the HDG rebar coating thickness was in excess of the requirement for newly galvanized rebar even though the chloride levels were 3-9 lbs/yd3, well beyond the threshold limit of 1.1 lbs/yd3 for black rebar. The performance of the Longbird Bridge led Bermuda’s Ministry of Works and Engineering (MW&E) to exclusively specify hot-dip galvanized reinforcement for more than 50 years.

In 2008, the Longbird Bridge was closed due to damage from hurricanes and subsequently bypassed by temporary twin galvanized steel bridges. In 2020, nearly 70 years after its construction, Bermuda’s MW&E began demolition of the Longbird Bridge to make way for a new, larger replacement structure. Samples of the rebar were collected for analysis by Zinc Connections, LLC. 

Based on the analysis and published report, the galvanized coating on the rebar samples was in exceptionally good condition, except for one small bare area. The zinc coating thickness measurements on each bar ranged from 3.8 to 11.6 mils. When averaging all samples together, the zinc coating thickness was equal to 6.0 mils which exceeds the minimum coating thickness for new rebar in ASTM A767 Class 1 (150µm or 5.9 mils).