Aging infrastructure offers a potentially huge market for composite materials. According to a seminal report issued a few years back by the coalition Transportation for America (Washington, DC, US), titled The Fix We’re In For: The State of our Nation’s Busiest Bridges, there were 69,223 structurally deficient highway bridges in the U.S. alone — 11.5% of all US highway bridges — that then required rehabilitation or replacement. Those numbers stimulated development of a number of composites-enabled technologies aimed at mitigating the crisis and extending the useful life of newly constructed bridges.
Life extension, it turns out, is a big need: The early deterioration of concrete due to the corrosion and failure of steel rebar reinforcement has been well documented. In many locales, the useful life of corrosion-prone steel-rebar-reinforced concrete is limited to 25 years, rather than the 75 to 100 years once promised by its advocates. Conventional repairs could cost billions. Therefore, the lifecycle-cost advantages, not to mention the safety benefits, of using composite rebar continue to overcome resistance among change-averse municipalities. That said, once significant activity among composites fabricators looking to replace entire bridge structures has quieted, replaced by a more conservative focus on replacing vulnerable, corrosion-prone concrete bridge decks on steel truss bridges with robust composite replacement decks.
As has been the case in years past, progress is still halting. Faced with limited annual budgets, state and local transportation executives have the choice to replace a certain number of bridges with concrete that could last 30-40 years at best, or half as many using composites that could last up to 100 years. In both cases, their careers will be long finished before anyone will hold them to account, so the easy answer is twice as many low-cost bridges. But projects still make the news, particularly in pedestrian bridges. A rather spectacular example (see photo at left) is SkyPath, a pedestrian-only addition to the side of the 1,020m Auckland Harbour Bridge in Auckland, New Zealand still seems promising. Composites fabricator Core Builders Composites (Warkworth, New Zealand) and the Composites Engineering Team of Auckland-based Gurit (Asia Pacific) Ltd. collaborated on the lightweight composite design for the addition, which will be attached in sections to the existing bridge without exceeding the bridge structure load-bearing limits. SkyPath will be 1.1 km in length, approximately 4m wide and about 5m in exterior height. (Read more about the design in “SkyPath: Scenic bikeway/walkway a winner with composites” under “Editor’s Picks.”) The project, however, despite official approvals, is still stalled, facing a court challenge, the victim of the fact that civil infrastructure projects are, alas, public, and therefore, subject to public controversy.
A recently completed project, unusual in that it handles motor vehicle traffic and that it is a floatingbridge, is the Brookfield Bridge in the Vermont, US, town of the same name. Reportedly the world’s first floating fiber-reinforced-polymer vehicle bridge, it preserves the character of the timber structure it replaces, but undergirds it with a composite buoyancy system designed to last for a century. It has replaced a 1978 version that carried state Route 65 traffic over Sunset Lake. Designed by T.Y. Lin International (San Francisco, CA, US), it is the eighth version of a floating log bridge built in 1820 and an illustration of the ways composites continue to find creative use in bridge building.
To make construction, transport and installation practical, the FRP portion of the bridge structure was built by Kenway Corp. (Augusta, ME) in five separate but identical 15.54m long by 7.01m wide sections called rafts, each formed from two buoyant foam-filled, glass fiber/vinyl ester pontoon structures, assembled back-to-back. Assembled rafts then would be bolted together, end-to-end, using steel splice-plates designed by T.Y. Lin, to form a monolith, undergirding the entire length of the wooden bridge. Pontoons were delivered to the bridge site two at a time, assembled into rafts, then rafts were joined and finally linked to the shore. (Read more in “Composite pontoons undergird update of 1820s-vintage floating bridge” under “Editor’s Picks.”)
In the bridge world, says Scott Reeve, president of Composite Advantage (Dayton, OH, US), the day cannot be won on the classic lifecycle-cost argument alone. Reeve, whose company is among the most successful fabricators of composite bridge decks, particularly for pedestrian bridges, confirms that the “upfront cost” problem still exists. “A composite vehicle deck is about twice the price of a concrete deck. Even accounting for lower installation costs, we are probably 1.8 times the traditional solution. Until we can get that differential down to around 15%, market penetration will remain slow.”
Composites are, however, beginning to compete directly with concrete and demonstrate such value in marine civil infrastructure applications. A case in point is the all-composite dock system for the city of Jacksonville’s (FL, US) Fire & Rescue Department (JFRD) Station 40. Like the new station structures, the proposed dock structures had to be able to survive Category 3 hurricanes — 205-241 kph winds and 2.74-3.66m of sea storm surge. Early bids, based on reinforced concrete, were up to 50% over a budget determined by a federally funded Homeland Security grant. Register Marine (Jacksonville, FL, US) reoriented the program to use pultruded glass/polyester composite pilings, beams and planking to build fixed and floating dock structures and were able to meet all of the technical and grant-funding requirements, including budget and timeline (read more in “Composites upgrade marine infrastructure” under “Editor’s Picks”).