Brandywine Shoal Lighthouse stands 7 miles west of Cape May, New Jersey. It is the first reinforced concrete structure of its kind located on a submarine site in the continental United States. The historic structure was recently rehabilitated with several types of concrete to assure its ongoing serviceability in a harsh environment.
DESIGN OF THE LIGHTHOUSE
The lighthouse is made up of a series of decreasing concentric cylinders. The three-level, 26-foot-diameter, cast-in-place concrete dwelling supports a watchroom and an iron lantern and is founded on a precast 35-foot-diameter caisson 18 1/2 feet high. In the original construction, the precast caisson was set on 74 timber bearing piles and anchored by 12 precast concrete piles, then extended 14 feet with cast-in-place concrete.
EVALUATION OF DETERIORATION
Visual inspection, chloride ion testing, and compression tests and petrographic studies of core samples were all part of the evaluation of the lighthouse. Deterioration was determined to have been caused by a combination of reinforcement corrosion and freeze-thaw action on the concrete itself.
The caisson repair required removal of all shotcrete and deteriorated original concrete to expose sound material. Tremie concrete was used to cast a jacket at least 8 inches thick around lower portions of the caisson. Precast architectural concrete was used to replace the first-level brackets and the columns and cornices of the veranda.