Can moving parts be built from concrete?
How durable are textile-reinforced bridges?
Balance after ten years
New construction methods are often viewed critically. One of the most common questions is: “How durable is the system?” Or, to put it more simply: “Will the whole thing still hold up in 100 years?” Of course, fiberglass and carbon-reinforced bridges also have to ask this question. In 2010 one of the first was built in Albstadt - almost ten years later is a good time to take stock.
In Albstadt-Lautlingen on the Swabian Alb, a reinforced concrete pedestrian bridge was showing its age. It had visual defects, such as rust flags, and posed a risk to road traffic. Concrete fragments fell on a federal road that ran under the bridge and endangered traffic.
The result: the bridge had to be dismantled and replaced with a new one. Since the textile industry has a long tradition in this area, the municipality of Albstadt was very open to the topic of textile-reinforced concrete. After a feasibility study showed that it was possible to build the 97 m long bridge using fiberglass concrete, the builders decided to put this into practice. Since building with fiberglass-reinforced concrete had not been tested for long at the time and approval was required in individual cases (ZiE), those responsible showed in two ways that the bridge would withstand the loads: arithmetically and through extensive component tests. In order to be able to correctly assess the current condition of the bridge, it makes sense to take a look at the materials used, the construction details and the component tests.
Textile reinforcement in bridge constructions
The use of textile-reinforced concrete was and is still something special for bridge construction. But this type of construction has one major advantage: the non-metallic reinforcement is not susceptible to corrosion. In the case of the bridge in Lautlingen, this meant that the concrete cover could be reduced to just 1.5 cm. A comparable reinforced concrete structure would have required a cover of at least 5 cm. Furthermore, by combining textile-reinforced concrete with pre-tensioning without a bond, a slimness of 1:35 could be achieved, which is unusual for concrete structures. In contrast to the usual bridge construction, in which a layer of asphalt is applied to the concrete surface to protect the steel reinforcement against corrosion, this was not necessary for the project in Lautlingen. For this reason, a concrete formulation was developed that meets the high requirements for surface quality and workability and falls into concrete strength class C50 / 60. Due to the severe winters with high amounts of snow in Albstadt, large amounts of de-icing salts are used. In addition, the bridge is regularly cleared of snow in winter with a snow clearing vehicle. For the bridge's planners, this meant two things: On the one hand, they had to take into account the mechanical abrasion of the snowplow and, on the other hand, ensure that the concrete complies with exposure class XF4. With regard to abrasion, the planners assumed, on the safe side, that in 80 years the concrete surface would be reduced by one centimeter due to mechanical effects.
Current state of the bridge
In summary, it can be said: During the planning, the engineers took into account both the features that building with textile concrete brings with it, as well as the peculiarities that a climatically demanding area like the Swabian Alb contains. As a result, the bridge is in very good condition today - after almost ten years of use.
Since the reinforcement made of fiberglass grids does not rust, neither rust flags are visible nor flaking like with the old reinforced concrete bridge. The use of de-icing salt has not harmed the bridge to this day. The surface of the walkway is impeccable, as is the stability of the entire structure. Nico Köllnick, who is employed by Breinlinger Ingenieure Hoch- und Tiefbau GmbH and on behalf of the Albstadt city administration, has checked the bridge in Lautlingen according to DIN 1076, comes to the same conclusion. He says: “All bridges are to be inspected regularly once a year for obvious defects and damage. Here you look at the neuralgic areas, such as the column feet, and check whether they show any abnormalities. "When he checked the bridge in Lautlingen, Mr. Köllnick came to the conclusion:" The bridge is in a very good general condition. " The condition that the bridge in Lautlingen is in - although it is exposed to high demands due to climatic conditions - shows that glass fiber reinforced engineering structures can be a perspective. The prerequisites for this are, of course, similarly good and conscientious planning as was done for the bridge in Lautlingen, and careful construction.
While researching this text, Claudia Ahwany spoke to Nico Köllnick. The civil engineer has the VFIB certificate for building inspection according to DIN 1076 and examined the glass fiber reinforced bridge in Albstadt-Lautlingen.
Ahwany: What tasks did you take on in connection with the bridge in Lautlingen?
Köllnick: Our engineering office received an order from the municipality to inspect several bridges in Albstadt and check them for visible damage. The textile concrete pedestrian bridge in the Lautlingen district was also part of it. The test carried out was carried out as an inspection according to DIN 1076 without the use of inspection devices or equipment as an intensive, extended visual inspection. In this visual inspection we also checked the functional parts of the bridge, e.g. B. bearings, joints, transition structures and anchors, controlled.
Ahwany: In which areas are bridges typically prone to damage?
Köllnick: Wherever surface water containing de-icing salt comes into direct contact with steel-reinforced concrete or as a spray mist. This can be at the base of the supports and abutments. It can also be on the surface of the bridge caps. In addition, the drainage areas and moving parts of the bridges are susceptible to damage if they leak over time.
Ahwany: How do they come about?
Köllnick: As a result of deposits and adhesions from grit containing road salt, chlorides can penetrate the uncoated reinforced concrete pavement surfaces and surfaces through open or leaky joints or seals into the unprotected reinforced concrete over the years. In addition to surface corrosion with oxygen reduction through carbonation of the concrete cover, depassivation can also be initiated by chlorides. As a means of transport in concrete, water plays a decisive role. Depending on the moisture content and the porosity of the concrete, chlorides can be transported by capillary action, but also by diffusion. The passive layer is locally destroyed by the chlorides and the steel is corroded. Experience has shown that reinforcement corrosion in the reinforced concrete components increases rapidly as a result of chlorides. As corrosion progresses, cross-section reductions and thus impairment of load-bearing capacities must be expected.
Ahwany: Could they be recognized by the bridge in Lautlingen?
Köllnick: No, there were no signs of reinforcement corrosion on the textile-reinforced concrete bridge in Lautlingen.
Ahwany: Given the age of the bridge, what was its general condition?
Köllnick: The visible general condition of the bridge was very good.
Ahwany: Assuming that the bridge will be exposed to the same stresses as before for the next five years, how do you assess its condition in five years?
Köllnick: After another five years nothing will change in the general condition of the bridge, provided that regular structural maintenance, i. H. Maintenance and care, as with other bridges, is carried out.
Ahwany: In your opinion, what are the advantages and disadvantages of fiberglass-reinforced concrete in bridge construction?
Köllnick: The advantages of fiberglass-reinforced concrete are that only a small amount of concrete is required, which means that the component thickness and the weight of the components can be reduced. Since the reinforcement is corrosion-resistant, the components have a longer service life. The disadvantage is that the cost of building with textile-reinforced concrete is still very high, as there are no regulations or building inspectorate approvals. At the moment, approval and testing is required on a case-by-case basis for each use.
Ahwany: What else should be mentioned in connection with the bridge in Lautlingen?
Köllnick: It is an innovative development for the engineer u. Building construction, which is and will certainly be a good way to ensure the durability of concrete components in the future.
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