By Mark Crawford
When most Americans drive over a bridge, they aren’t wondering if it is going to collapse. But if some travelers do, you can’t blame them – about 11% of all U.S. highway bridges are structurally deficient and need to be replaced.
According to the American Society of Civil Engineers’ 2013 “Report Card for America’s Infrastructure,” the average age of the nation’s more than 600,000 bridges is 42 years. One in nine of these bridges is classified as structurally deficient. The Federal Highway Administration indicates that, in order to repair or replace all deficient bridges by 2028, the U.S. needs to invest $20.5 billion annually in bridge construction, a 70% increase from current spending.
Steel reinforcement is a major reason bridges can deteriorate at such a young age. While using steel allows for faster installation of longer spans and slender beam bridges, it can rust and corrode, weakening the structure. Another option is masonry arch bridges, which are well known for their strength, durability, longevity and minimal maintenance. These factors, combined with their good looks, make them highly popular.
Thanks to a unique new system developed by Northern Ireland-based precast manufacturer Macrete, designers and engineers are turning to precast concrete instead to minimize costs and construction time while building bridges which will last several hundred years.
An innovative solution
The new arch bridge system was created by the School of Civil Engineering at Queen’s University in Belfast, Ireland, in partnership with Macrete. Adrian Long, a civil engineering professor at Queen’s University’s Center for Built Environment Research, invented the system, which contains no steel reinforcement or mortar joints and is ideal for short-to-medium-span arch bridges. Called “FlexiArch,” bridges built with the system require no centering and can be assembled in a day. These advantages make FlexiArch cost-competitive, especially when taking longer service life and reduced long-term maintenance into account.
How it works
FlexiArch bridges use the same basic construction principles as traditional stone masonry bridges. Because the main forces are compressive, no reinforcing steel is required. Voussoirs are manufactured using precast concrete for easy transport and rapid assembly, with angles calculated to produce the correct curvature of the arch when completed. They are then laid side by side and a layer of polymeric reinforcement is placed on top. Finally, a 1 1/2-inch layer of screed is overlain, assuring the voussoirs are interconnected.
FlexiArch units can be cast in convenient widths to suit design requirements, site restrictions and available lifting capacity. The units are transported to the project site flat, then lifted and placed on precast footings. When lifted at the designated anchor points, gravity forces the wedge-shaped gaps to close into the desired arched shape. All of the self-weight is then transferred from tension in the polymeric reinforcement to compression in the voussoirs, functioning the same way as a conventional masonry arch.
Each unit can be placed on site in as little as 10 to 15 minutes. “The degree of taper of the voussoirs controls the geometry of the arch,” Long said. “Flatter arches require less taper and vice versa.”
An integral part of the design is the polymeric reinforcement. The primary function of the reinforcement is to provide enough tensile strength for the FlexiArch units to be lifted safely from the flat casting bay to the flatbed truck, then from the truck into position on the precast sill beams at the bridge site.
“With safe working conditions being of primary importance, we accurately simulate the boundary conditions through carefully designed tests to ascertain the strength of the polymeric reinforcement,” Long said. “Using these results and taking account of creep effects, an appropriate load factor can be applied to eliminate any risk of failure during lifting.”
Only a few arch bridges have been constructed in Europe since the early 1900s thanks to the advent and popularity of reinforced concrete beam and slab systems. Since FlexiArch began business operations in 2007, more than 50 FlexiArch bridges have been constructed in the U.K. and Ireland. Span lengths range from 13 to 52 feet with plans for bridges as long as nearly 100 feet.
FlexiArch can also be used to strengthen existing bridges. For example, Tameside’s 78-year-old Jubilee Bridge, which spans National Cycle Route 66 in Manchester, had been weakened by extensive reinforcement corrosion and spalling. The bridge could not be replaced because it was part of a key transportation corridor and would have disrupted services. Concrete was sprayed on the deck soffit in 1974, but was not expected to be a long-term solution.
In December 2012, the main contractor installed 15-foot-long FlexiArch units. The 24-foot span units were manufactured off site and shipped to the bridge location. A crane individually lifted the spans, which were placed on lightly greased, laterally extended sill beams along each abutment. They were then pushed horizontally in pairs beneath the bridge using two hydraulic jacks.
“When all 14 units had been located, spandrel walls were constructed and the gap between the FlexiArch unit and the original deck soffit was filled with foamed concrete,” Long said. “The result was an aesthetically-pleasing bridge with a design life of 120 years.”
The greatest initial challenge for Long was convincing precast manufacturers that FlexiArch was a viable bridge system. It was also difficult to get the first bridges built because clients and their consultants were reluctant to be the first to use the new concept. Discussions are in progress with companies in Australasia, Sweden and the U.S., and several U.S. precast companies are exploring sub-licensing agreements.
Lengths could be even longer for pedestrian bridges – the FlexiArch units could be transported in two lengths for interconnection prior to installation. The system could also be adapted for skew arch bridges over railway lines, where speed of construction is paramount.
“For angles of skew of around 30 degrees,” Long said, “slightly modified voussoirs could be beneficially deployed. Also, using FlexiArch in conjunction with stress ribbon decks would result in very graceful pedestrian bridges over motorways.”
After contractors, designers and clients have been involved in the installation of a FlexiArch bridge they become strong advocates of the system.
“When this experience is combined with competitive cost, aesthetics, sustainability and durability,” he said, “the potential for widespread use in the construction industry is immense.”
Mark Crawford is a Madison, Wis.-based freelance writer who specializes in science, technology and manufacturing.