Lost Forms in Box Girder Construction: Disposable Molds for Complex Geometry

In box girder bridge construction, a lost form (stay-in-place formwork) is a temporary mold left permanently embedded in the concrete. Unlike traditional plywood/steel forms that are stripped, lost forms are sacrificial—made of expanded polystyrene (EPS), corrugated plastic, fiber-reinforced polymer, or thin-gauge steel—and remain as part of the finished structure.

Why Use Them?

• Complex geometry: Pre-molded to intricate bottom-slab profiles (cells, ducts, variable depth) impossible with flat panels.
• Speed: No stripping, cleaning, re-oiling. Multiple cells cast simultaneously.
• Quality: Factory-molded surfaces yield smooth, consistent soffits; no form-release residue.
• Composite action: Thin steel/plastic forms can act as tensile reinforcement (if designed).
• Access elimination: No need for workers inside cells post-pour—safety gain.

Trade-offs

• Cost: Higher material cost vs. reusable forms; justified by labor savings.
• Inspection: Interior surfaces hidden; requires pre-pour QA.
• Fire: EPS requires intumescent coating if fire rating needed.
• Moisture: Trapped condensation risk; venting details critical.

Applications

Common in segmental box girders (balanced cantilever), precast trough girders, and U-girders. EPS "void formers" also create post-tensioning ducts. The technique exemplifies prefabrication-driven bridge engineering.