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The Uyllander Bridge being moved into place across the Amsterdam-Rhine Canal in the Netherlands. The bridge was constructed using composite formwork. (Picture courtesy of Poly Products.) Composite sandwich structures: the new milestone in bridge building 0034-3617/13 2013 Elsevier Ltd. All rights reserved NOVEMBER/DECEMBER 2013 REINFORCEDplastics 17
Bridges have always been social and economic driving forces. Many have also become landmarks and icons. Prehistoric bridges were made of stone and wood. Brick, mortar, concrete, iron and steel later sneaked into the bridge builder s toolkit. And now it s the turn of polymer composite sandwiches to offer innovative solutions and a plethora of exciting, new possibilities. Freelance journalist Django Mathijsen reports. Current time = 55.3 min (81,1% filled) Filling time [min] 55.3 49.7 44.2 38.7 33.2 27.6 22.1 16.6 11.1 5.5 0.0 Flow simulation of the resin infusion of the Uyllander bridge formwork panels. (Picture courtesy of Poly Products.) The application of composite materials in civil engineering is growing fast, especially in stairs, landings, balusters and banisters that have to stand up to corrosive environments like sewage or the sea. Also they are used for facades with intricate, aesthetic shapes. Fibre reinforced polymer (FRP) sandwich structures are even finding their way into large, load-bearing structures, like bridges and lock gates. That is about to change, if the Dutch have anything to say about it. The engineering firm Ingenieursbureau Amsterdam for example did know about polymer possibilities, when they designed the new Uyllander Bridge across the Amsterdam-Rhine Canal in the Netherlands. They came up with a groundbreaking solution. supported by a giant falsework : a temporary steel structure. Fitting all that wood and falsework is time consuming, resulting in delays, high labour costs and traffic disruptions. It s also a complicated and hazardous job, hanging under a bridge over a canal. Fitting the form- and falsework on a worksite on the bank and then lugging the bridge formwork, falsework and all into place isn t any easier. Not to mention that the form- and falsework would have to be dismantled after the concrete had set; again: hanging under a bridge over a canal. So Ingenieursbureau Amsterdam had an epiphany: why not try composite polymer sandwich panels for the formwork? They should be strong enough to support the wet concrete without requiring any falsework. Also, composite panels are excellent in resisting the elements. And they d give the bridge a nice, smooth finish on the bottom. So there s no need to remove them after the concrete has set. Were composite panels a viable option? Interlaminar stresses The advantages composite sandwiches (comprising a sheet of core material faced by two FRP skin layers) have to offer seem obvious. Compared to steel, wood and concrete they have a higher strength to weight ratio and stand up well against the elements, resulting in a longer life span and lower maintenance costs. They re also shapable practically without limitations. However, composites are still held back in civil engineering because their possibilities aren t widely known yet. A permanent formwork The bridge over the Amsterdam-Rhine Canal was designed as an arch with steel trusses, 3.8 m apart. The spaces between the trusses were to be filled with a steel reinforced concrete road deck. Normally, the concrete for the deck would be poured into a wooden formwork. But wood isn t strong enough on its own to support the wet concrete, so it must be The job of developing and producing the formwork went to Poly Products in Werkendam, the Netherlands. They re specialised in custom-made composites for civil engineering, industry and recreation. There were a number of design challenges, says Dr. Ir. Albert ten Busschen, Poly Products technical director. The panels had to be lightweight 25 kg/m 2 so they could be installed easily. And the lips where the panels sit on the steel trusses 18 REINFORCEDplastics NOVEMBER/DECEMBER 2013
weren t allowed to stick out more than 20 mm above the steel. At the same time each panel had to be a thick sandwich in the middle to make it stiff enough. Because if it were to sag more than 25 mm when the 850 kg/m 2 concrete was poured onto it, it would ruin the aesthetics and cause extra consumption of concrete. It was tricky making the transition from lip to sandwich strong enough. To fill the 3.8 m wide gaps between the trusses, vacuum infused glass fibre reinforced polyester panels with a polyurethane foam core, 11-15 m long, were needed. The biggest problem was the occurrence of interlaminar stresses in the lip. Those are stresses between the layers, Ten Busschen explains. It s comparable to wood: strong in the direction of the fibres, but load it transversely and it can split. Interlaminar stresses decrease the strength levels of the material, so you have to prevent them in the design. But that s not always possible. You can t fully prevent them in the panel lips. If we d have laid the composite panels on the steel trusses just like that, surface irregularities in the steel would have locally caused excessive stresses which could have led to tearing. We prevented that by putting rubber strips onto the steel trusses. That way we spread out the load, and we had a well-defined surface to lay the panel on. Poly Products produced samples of the intended material composition to test for strength and stiffness. Simulations (FEM) were made of the panel lip, calculating the stresses and deflections. And finally, five test panels, scaled down to 1 m x 2 m but equipped with those dreaded lips were tested in a three-point bending test. That s a worst case scenario, putting the load all in one spot. The actual concrete pour causes a distributed pressure on the upper skin, so buckling is less likely. The test results matched their theoretically predicted values so Poly Product could start producing the panels. During production, meticulous records were kept for each of the panels, recording the raw materials, the room temperature and the settings of the vacuum infusion machine, and checking if all the reinforcing layers were installed in the right order. The panels were then transported to the building site and installed into the bridge while it was still on the bank. Once the bridge was put in place, the steel reinforcement was introduced and the concrete poured. The work was finished in March 2013. the composite solution wasn t just faster, safer and more aesthetic, but also cheaper. The composite panels were of course more expensive than a few slabs of wood, but they annihilated so many labour costs that Ten Busschen is adamant that overall the The bridge panels being manufactured in a vacuum infusion process. (Picture courtesy of Poly Products.) NOVEMBER/DECEMBER 2013 REINFORCEDplastics 19
composite solution wasn t just faster, safer and more aesthetic, but also cheaper. More composite bridges We ve made composite bridge edges before that required a special shape, Ten Busschen says. A lifting bridge for example with a shapely side finish that ended in something resembling a giant counterweight. With other materials that would have been very difficult to make. With composites smooth, doubly curved shapes are no problem. We ve also made composite formworks for intricately formed abutments. But never before have we made a permanent formwork, that is: a formwork that is left in place after the concrete has set. We hope it will lead to more applications of this technique. Then why not go one better and construct a bridge, completely made from composites? There have been bridges like that, Ten Busschen says. But especially in very large spans with composites the limiting factor isn t strength but stiffness. Then it can be wiser to use steel trusses for the main construction and composites for the deck. Otherwise you d need enormously thick sandwiches to achieve the required stiffness. Thick sandwiches involve extra design effort, like partitions to make sure the skins are securely attached to the foam. And all that foam isn t free of charge either. An example of such a bridge with steel trusses and a sandwich deck is the new, 140 m long traffic bridge near Lunetten across the A27, one of the busiest motorways in the Netherlands. It features a loadbearing composite deck bonded to its steel frame with a customised adhesive. Sounds like something you d find in a sports car. And indeed, the lightweight construction was rolled into place in one piece lightning fast. It took just two nights to move it up from the adjacent worksite: Saturday and Sunday night, March 3 and 4, 2012. So traffic disruption was minimal. But that wasn t the main reason why this groundbreaking lightweight version was chosen. Assembly of the Uyllander bridge. (Picture courtesy of Poly Products.) A bridge like a sports car The new bridge was needed because the old traffic bridge was going to be recommissioned as a railway bridge. Initially a concrete bridge on seven supports was designed. But that was going to weigh 2650 tons. And there s a plastic sheet at the intended location 6 m below the ground acting as a ground water dam, making a conventional steel or deep pile foundation to support 2650 tons impossible. A lightweight alternative was needed. 20 REINFORCEDplastics NOVEMBER/DECEMBER 2013
The concrete being poured onto the composite panels of the Uyllander bridge. (Picture courtesy of Poly Products.) The solution was a steel truss viaduct on three supports. The architects (irs. Vegter b.i. in Leeuwarden, the Netherlands) already had experience with composites, working on the first European composites lifting bridge (in 2010 in Oosterwolde, the Netherlands). So they proposed a composite road deck, which had the advantage over a steel road deck of being vibration dampening and sound deadening. The cost turned out to be virtually the same. The longer the projected life span needs to be, the more economical a composite solution will be. The longer the projected life span needs to be, the more economical a composite solution will be, Michiel Galema, director finance and market of composites bridge and lock gate builder FiberCore Europe comments as a general rule of thumb. Wood was no option because a life span of 100 years and low maintenance for at least 50 years was required. The final design consisted of just two steel trusses, 6.5 m apart and 5 m high, connected on the bottom by a GRP and polyurethane foam sandwich road deck and on the top by a triangulation frame to combat wind loads. We don t make classical sandwiches but InfraCore Inside road decks, says Galema. This makes them capable of handling heavy traffic classes, where they have to be able to stand up to impacts and repetitive loads. The whole structure comes in at about 400 tons (140 for the deck and 250 for the frame). And still, it can handle all traffic, from pedestrians to the heaviest weight class (60 tons). It has an eigenfrequency of 3 Hertz and adheres to human-induced vibration regulations. The Dutch Ministry of Waterways and Public Works was skeptical, since a composite road deck had never been employed in this way before. But eventually they accepted it. The lightweight construction allowed a lot of prefabrication. The deck elements were made by FiberCore Europe in 14 weeks while construction company Hillebrand made the trusses. On the building site, just next to the place where the bridge was going to be installed, all those prefabbed bits only had to be assembled. The road deck was prefabricated in seven segments of 24.5 m x 6.2 m (the largest size possible for fabrication and transport). For each part, polyurethane foam and sheets of glass fibre matting were stacked and then vacuum infused with resin. The difference with classical sandwiches is that the foam NOVEMBER/DECEMBER 2013 REINFORCEDplastics 21
is alternated with glass fibre to form skins perpendicular to the direction of the traffic to guarantee the constructive integrity even if the foam perishes. And all internal corners are covered with continuous matting to ensure long term high impact resistance. Tests were done to prove the stress levels wouldn t cause fatigue. The top of the sandwich is an epoxy bonded road surface and the bottom a gel-coat (for aesthetic reasons). The only maintenance needed is hosing it down to prevent moss growth and, if necessary, fixing the road surface. In order to prevent stress due to thermal expansion, the fibre composition was chosen so the sandwich s thermal expansion coefficient would match the one of steel. On the building site, the seven segments were connected to form one stiff slab. Movement and gaps between the steel frame and the composite sandwich would be unacceptable because then moisture could come in, causing the steel to corrode. So the composite floor was inserted into U-shapes on the bottom of the side trusses. The space between the U-shapes and the floor was injected with resin. With temperatures plummeting to nearly -15 C this was done in a tent to ensure the injection could take place at the required 10 C. As a precaution pins were added, driven from the top to the bottom through the U-shapes and the sandwich. Once fully assembled the bridge was rolled into place with SPMTs (Self Propelled Modular Transporters). Cranes could have made the installation even quicker, but would have risked damaging the underground water dam sheet. Next to the 4.5 m wide road there s a raised sidewalk, lying like a composite lid on a gutter for cables, pipes and water drainage. And even more innovations Although groundbreaking, the A27 bridge wasn t the first bridge FiberCore have built using DSM resins. They ve already built more than 150 composite bridges in Western Europe, the US and China, including a bridge that floats on water and a movable traffic bridge. The FiberCore Europe bridge across the A27 motorway in the Netherlands features a load-bearing composite deck bonded to a steel frame. (Picture courtesy of FiberCore Europe.) 22 REINFORCEDplastics NOVEMBER/DECEMBER 2013
On March 10, 2012, they achieved another interesting milestone in the Dutch Eendragspolder : the first bridge using bio-composites. It features DSM s Synolite 7500-N-1, a high strength structural resin which is 50% based on renewable raw materials (corn). With easy vacuum infusion and great resistance to rainy climates it was especially developed for bridges and is currently being evaluated for other applications in building, construction and infrastructure. Nonwovens specialist Lantor in Veenendaal is working on a flexible nonwoven for de-icing purposes, which can be easily laminated with both polyester and epoxy. We add a carbon like conductive additive to the nonwoven, says Reinier Jansen, Lantor s product manager composites group. Once it s laminated it retains electrical resistance, creating heat when you put electricity through it. How did they come up with that? We already work with conductive additives for the electrical cable industry. The different layers in high voltage cables are separated by non-conductive, semi-conductive or fully conductive nonwovens. The innovation is to use those nonwovens for heat generation. And to make them suitable for composites, in which they re impregnated with rather aggressive epoxy and polyester, Jansen adds. We can work both at low voltages below 100 V and high (230 V). And we can reach about 2 kw/m 2. It should be available for small-scale applications like epoxy pipes (for which it was originally conceived) around now. That s to cure the epoxy adhesives that will connect the pipes, at above 100 C temperature. Next year a larger scale version will be available which Lantor is developing for Utec, a Dutch company making composite side elements to embellish bridges. It s to be used for de-icing those so snow build-up doesn t all fall down in one chunk. It might also be suitable for composite road decks, to prevent (black) ice. How to spread the word All these examples illustrate the exciting new possibilities composites are bringing to bridge building. Unfortunately, their application is still hampered by the fact that many people in civil engineering aren t aware of them yet. That s certainly true in the rest of Europe and the world, Galema comments. But in the Netherlands composite bridges are fully accepted now. I don t think there are many countries that have more than ten composite bridges. In the Netherlands we have 200... and counting. So why is the rest of Europe lagging behind in the acceptation of composites bridges? According to Ten Busschen, who is also the chairman of the VKCN (the Dutch Association of Plastic Composites) the current European Standards have a lot to do with that: Fibre reinforced polymers have sort of missed the boat with those standards. Composites have many design codes in specific areas, like aviation, wind propellers, tanks and pipes. But in a general sense, especially in civil engineering, there is no European Standard, whereas wood, concrete, steel, aluminium and even brick and mortar do have them. So the Dutch composites industry, the VKCN and engineering firms like Royal Haskoning formed an initiative to get an official European Standard going. In the Netherlands, companies processing polymers for civil engineering currently use an industry recommendation, written in 2004: the CUR Recommendation 96 Fibre-reinforced polymers in load-bearing structures in buildings and civil engineering works. The idea was to use that as a basis for a European Standard. So the recommendation was revised and updated, put into a European standard form and then translated into English. The Dutch standardisation institute NEN are now introducing it into the European Committee for Standardisation. This CUR Recommendation has procedures for quality control so things like raw materials, resin mix, layer setup and processing conditions are as specified in the design. And it regulates the material properties and safety factors to use. It also offers the option of using project specific material properties instead of standard properties. This means that if you perform tests on the actual material composites and constructions going into your project (as was done in the bridges covered in this article), you re allowed smaller safety margins, leading to lighter and cheaper designs. The standard safety margins were already updated in the CUR revision to prevent unnecessarily high safety margins, Ten Busschen adds. Those safety margins had to be high initially, when composites were still new and exotic. Now that composites have proven themselves in so many projects and products, their safety margins can come down. People in England, Germany and Italy are actively working on a European standard as well, Busschen says. But I think the revised CUR Recommendation is the most comprehensive and concrete document. Once we have that European Standard, composites will have a much more equal position with other building materials. Further information Ingenieursbureau Amsterdam; www.iba.amsterdam.nl Poly Products; www.polyproducts.nl FiberCore Europe; www.fibercore-europe.com DSM; www.dsm.com Lantor; www.lantor.nl VKCN; www.vkcn.nl NOVEMBER/DECEMBER 2013 REINFORCEDplastics 23