Researchers think aged concrete bridges with subsequent reinforcement
Many aged concrete bridges in Europe and the USA could become a safety risk in the coming years. After all, most of the road and railway bridges were rebuilt within thirty years after the Second World War – and are now at least forty years old. However, reinforced concrete also ages and secures under environmental influences. For example, reinforced concrete bridges in road construction are exposed to massive salt loads and this attacks the reinforcement inside the concrete structures. Many of the bridges are also too weak for the significantly increased volume of traffic and the improved traffic loads – and no longer meet the currently applicable standard.
Reinforcement of the concrete bridges
“That is a big problem. Because over the decades, a huge economic fortune has grown in infrastructure buildings. These structures should last as long as possible. But since the safety required in these structures is often no longer available due to deficits in the steel reinforcement, we need methods to strengthen these structures,” says Professor Juergen Feix from the Work area solid and bridge construction at the University of Innsbruck.
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It is both economically and ecologically advantageous to use the stock as long as possible. The demolition and new construction of bridges are associated with lengthy traffic diversions and if the train has to take detours, then that is a multiple of the CO2-Load, which alone requires the new construction of the bridge.
Concrete anchor bolt as replacement reinforcement
Professor Feix and his team have been researching a repair option for aging concrete bridges for over ten years. They have found a solution to subsequently improve existing structures with deficits in the steel reinforcement in order to establish the load-bearing capacity according to today’s standards. Additional advantage: The replacement reinforcement can be carried out while the road or railway line is in operation.
The innovation consists of the further development of the concrete screw. This can be inserted into the concrete beam like a dowel using a self-tapping thread and anchored at the other end with a type of washer.
For the so-called concrete anchor bolt there is already an approval – and a design mode based on how many screws are installed so that the higher load-bearing capacity is given according to today’s standard for the component, explains Professor Feix. The first applications of this replacement reinforcement show that its costs can be ten times lower than those of demolition and new construction. The team in Innsbruck is now working on an extension of this solution that is even more economical.
Range of existing building fabric
The problem with the inexpensive repair of old concrete bridges is the wide range of existing building fabric. Because the bridges were still without from the 1950s shear reinforcement built. This serves – in combination with the concrete – to increase the load-bearing capacity of bridge beams. “When a load IS TRANSMITTED across a bridge, the concrete beam can take the pressure, but it deflects, creating tension on the underside of the beam. In order to absorb this tension, you need bending reinforcement on the underside of the beam and, at the same time, shear reinforcement that connects the pressure zone on the top of the beam with the tension zone on the underside of the beam,” explains Professor Feix.
This results in a construction of steel reinforcement that is based on half-timbered construction. This means that steel elements running linearly on the top and bottom of the beam are connected to steel elements running diagonally or vertically upwards.
shear reinforcements
The first shear reinforcements were not integrated into the slabs of concrete bridges until the 1960s and 1970s. At first it was shear deflections, where a steel element coming straight from the bottom goes up at a 45 degree angle where it continues straight again. It is also a continuous S-shaped iron. In the 1970s and 1980s, the stirrup reinforcement through. This is a closed rectangular shape that is inserted practically vertically into the concrete beam. “The brackets hang the compression struts, which are coming down, up again and this allows this framework effect to appear in the beam,” says Professor Feix.
Presumably based on the current standard, deficits in the transverse force reinforcement are measured for both types of reinforcement – namely when the shear deflection and stirrups, which connect the compression zone and tension zone, are not available to a sufficient extent. The researchers at the compensate for this deficiency University of Innsbruck through a subsequent vertical replacement reinforcement in the form of concrete anchor bolts.
Most economical solution
In the previous project, the researchers had not yet taken into account the differences in the existing building structure. This means that their calculations were based on a building structure without shear reinforcement. However, assume that having steel reinforcement reduces the need for concrete anchor bolts. Therefore, in the current project, they are investigating how concrete anchor bolts and the existing reinforcement form interact. It needs to be clarified whether these should simply be added together or whether certain deductions should be made. The aim is to achieve the highest possible load-bearing capacity with as few reinforcement elements as possible, so that the system is also economical.
Replica of the building fabric
In the laboratory tests, the two relevant types of existing building fabric are reproduced in terms of reinforcement levels and cross-sectional dimensions. That is, there is a reinforced concrete beam with shear deflection and a concrete beam with stirrup reinforcement. In addition, such beams of different heights are also tested. Because we now know that different bar heights can indicate the effectiveness of the reinforcement. This applies to the normal shear force reinforcement as well as to reinforcement with concrete anchor bolts. This means that as the beam gets higher, the reinforcement works stronger, explains Professor Feix.
Three-point bending tests
In the experiments, the real traffic loads are reproduced true to scale in order to obtain data on the fracture behavior of the reinforced concrete beams. These are tested once with and once without replacement reinforcement. The attempts take the form of alleged Three-point bending tests. Think of a bridge beam between two piers that carries a load in the middle. The traffic load is simulated with a hydraulic press that can generate loads of up to 160 tons. This corresponds, for example, to a locomotive crossing a bridge.
Doubling of the payload
The tests show that reinforced concrete beams with equivalent reinforcement achieve double the load capacity. However, the evaluation of the enormous amounts of data is still ongoing and will only be completed in about two months. Which is why Professor Feix cannot yet comment conclusively on this. At the moment, however, there is much to suggest that the interaction of the existing reinforcement form and concrete anchor bolt is actually easy to add – without making any deductions.
It is not possible to say in general terms to what extent the service life of existing bridge structures can be extended by subsequent reinforcement of the reinforcement, because this depends on the overall condition of the concrete bridges. Some buildings are already severely damaged by environmental influences. But when it comes to the concrete anchor bolts, they have been proven to have a service life of at least 50 years – even under the influence of salt.
The project is managed by the Austrian Federal Railways (ÖBB) and Motorway and Expressway Financing Joint Stock Company (ASFINAG) supported.
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