One of the important steps toward completing the main-span construction is the post-tensioning of the road deck. The process of post-tensioning has been mentioned previously in this update, but not thoroughly explained. As it is a critical part of the construction process, this week’s edition will offer a high-level overview of what it is and how it works.
Post tensioning is a technique for reinforcing concrete. As concrete hardens, it compresses or shrinks, and it loses its capability for being stretched by things like large loads (e.g., trucks) crossing over it or the sun’s warmth causing it to expand. Add steel rebar to enable that stretching by bearing the load or enabling the expansion, and you prevent cracking and other damage to the concrete. Post tensioning has the same sort of effect as rebar, just much stronger.
Here’s how post tensioning works: Ducts or sleeves are placed along the length of each of the 598 16’x30’ concrete panels that make up the new bridge’s road deck. As these panels are laid down side by side, nearly 100 of these ducts will line up, running various lengths of the 2,000‑foot main span; the longest is 740 feet long, the shortest is 230 feet.
Steel cable strands similar to the ones used for the cable stays are threaded through each of the nearly 100 ducts (see the picture above). There are 12 strands per duct. Once all the strands are all installed, they will be pulled tight from both sides and anchored with large caps. After that, the nearly 100 ducts will be filled with grout to keep the strands taut and preserve them from the elements.
Post-tensioning the main span is currently underway. When completed, the two gaps on the main span at the approach structures will be filled, then the road deck will be grinded down in preparation for applying a durable polyester-concrete overlay for the road surface.