Choice of concrete mixture for the construction of the Rotterdam pilot bridge

The replacement of the ‘Botreep bridge’ (a wooden pedestrian bridge) in Hoogvliet Rotterdam, which will take place in the fourth quarter of 2022, is preceded by an entire procedure of mixture testing. In April the first material tests started. 

The main goal of Urbcon is to put good constructive geopolymer concrete on the map and build technical knowledge on the subject. A good side effect is that the CO2 emissions are decreasing when using geopolymer concrete with respect to using concrete based on blast furnace cement. The Netherlands have the obligation to reduce CO2. This was already done by using blast furnace cement, however with ‘Urbcon’ concrete this emission will be even lower. 

Choice of concrete mixture

The City of Rotterdam was able to choose between two mixtures which were developed by TU Delft. The first mixture formed a kind of evolution of a mixture that had already been used before. Kees Blom from the Engineering firm of the City of Rotterdam, in cooperation with TU Delft had been researching and experimenting with geopolymer concrete for some time. A well-known example of this can be found in the Dutch article Rotterdam bouwt samen met TU Delft bank van ‘groen’ beton - Rotterdam Circulair (Rotterdam and TU Delft are building a bench of ‘green’ concrete). 

However, the processability of this mixture was too short. As a result the structure of the material was too hard and too stiff in order to be able to be transported and processed in a standard way. Usually concrete is blubbery and is therefore able to flow through the entire reinforcement. As a contractor needs to be able to work with the material, this mixture was further developed so it remains fluid for a longer period of time and can therefore be used (outside lab conditions).  

Subsequently, TU Delft developed a second mixture, which meets the circular goals even better. One of these goals is to make more efficient use of raw materials, so that less raw materials are necessary. The mixture doesn’t only exist of geopolymer concrete (cement substitutes), but of secondary aggregates as well; aggregates (gravel, sand and small stones to be found in concrete). 

Eventually, this second mixture was chosen. It is technically more challenging, but it has a circular component. The only problem is that one does not know yet how the material will behave. It is possible that creep will occur, just like creep of a wooden shelf. A permanent deformation may occur, significantly more than may be the case with traditional concrete. This may cause a problem for the construction of a bridge (when one doesn’t take creep into account or when it is underestimated). This material is also more porous than regular concrete, which means that pollutants can penetrate more easily and will cause the steel to rust (concrete rot).

Testing the material 

In order to be able to apply the concrete it has to be tested thoroughly first. A testing protocol is drawn up, in order to be able to test the material. If it is behaving according the concrete standards, then one can calculate with it. If it doesn’t behave according the concrete standards, the engineering needs to be adjusted. For example: how stiff is the material, what is the interaction with the reinforcement, is it sensitive to concrete rot et cetera.  

TU Delft started the material tests in April 2021. Three pressure tests were executed according a certain standard. There were two tests concerning porousity and creep. The tests were performed on small testing objects, cubes.

Concessions to bridge design

In 2022 the mixture will eventually be applied in the park bridge in Hoogvliet. In order to make sure that the mixture can be applied, a few mitigating measures will have to be taken in the design of the bridge in order to minimalize the technical disadvantages of the mixture. For example, by making the beam of the bridge just a little bit thicker than would strictly be necessary. By doing this the bridge gets more body and the creep will be a little less. The layer of concrete on the outer reinforcement bar (covering) is made thicker as well. The Netherlands do not have a heavy climate, but nonetheless the covering protects the reinforcement. Additionally it is to be expected that a coating will be applied on the concrete in order to protect it against aggressive substances and the influence of frost-thaw cycles. 

Some concessions will have to be made to the bridge design in order to be able to build the bridge. Fortunately, the architect of the Botreep bridge, Marc Verheijen, is also working for the Engineering firm of the City of Rotterdam, so this makes it easier to determine which adjustments are necessary for the bridge. It is a standard Rotterdam bridge to which middle supports are applied which weren’t there in the original design. With all these adjustments the advanced mixture can be used and by the end of next year there will be a circular pedestrian bridge made of Urbcon concrete in Rotterdam.

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