The fanfare that greeted the opening of the first pedestrian bridge printed in concrete is richly deserved but the indications are that the advent of this technology will not be without pain. Despite having trumpeted plans more than 18 months ago to build the world’s first printed steel footbridge, innovators in the Netherlands have yet to reveal any tangible results. Now a team in Spain has taken the spotlight with its 3D-printed concrete bridge which was officially inaugurated last December in the Castilla-La Mancha Park in Alcobendas, northeast Madrid. The bridge is 12m long and 1.75m wide and is made from eight pre-printed pieces of micro-reinforced concrete.

The client, the local authority of Alcobendas, did not pay for its construction or installation, which was carried out by a multidisciplinary team of architects, mechanical engineers and structural engineers led by contractor Acciona.

The bridge’s design by the Institute of Advanced Architecture of Catalonia was based on principles of organic architecture, which seeks to replicate the shapes found in nature.

The use of computation design combined with 3D printing meant that the eight segments of the bridge could be designed to have optimum structural performance with minimum waste of material. The result has been described by some as a tribute to Gaudi, by others as a giant lattice of white chocolate or even a Lord of the Rings prop.

The history of the making of the new bridge goes back at least nine years – or more, depending on whom you ask. The bridge’s sections were printed by a machine designed and supplied by D-Shape, a company founded by inventor and civil engineer Enrico Dini. Seven years after Dini had a chance encounter with Acciona at a civil engineering trade show in London, he got a call asking whether he would be interested in helping to build a bridge. At the time Dini was a visiting professor at IAAC, where he had a venture capital collaboration in additive manufacturing using not concrete, but soil as the base material. Pylos is an on-going research project into 3D printing as a large-scale construction method using natural, biodegradable, recyclable and locally found materials.

As a consequence of this research work Dini suggested IAAC should be responsible for the design of the Alcobendas bridge. “Behind this project my other inspiration was Gaudi, who made a topology-optimised structure that is the Sagrada Familia, using of course very empirical means, where topology is optimised to get a fully compressed building structure.”

Dini’s role was confined to manufacturing the 3D printer for Acciona. Although he accepted the budget he admits that he actually spent it twice over because of his overwhelming desire to see the bridge printed. He describes the collaboration as fantastic, but not without issues: “The person who was managing the printer is proud now, but at one point she said: ‘Esta maquina me mata’ (‘this machine is killing me’).”

The printer was delivered in September 2015 but it took another six months to make it operational. A main challenge was finding the right mix of materials for the printing process. “3D printing of concrete required some additives because the ratio between water and sand is not normal. So you have to limit the amount of water and swap it with some other fluid in order to get a compressive strength in the order of 25MPa.”

Although Dini cannot say too much about how exactly the 3D printer works, he describes it as being almost fully automated, weighing around 15t and being 2m wide, high and deep.

When in full production, such a 3D printer takes about two minutes to print a 5mm layer, which equates to around 2m per working shift: “In one day you can in principle print one segment, but then you have to take out the unbound material and set the piece free. So it might produce one piece every two days, which would be eight pieces in a couple of weeks.”

The largest challenge for this project, says Dini, was without question the issue of budget. All parties recognised that the project would have to be done at a loss, but the prestige associated with making the first 3D printed bridge was the driver and the reward.

With the process of printing using concrete now demonstrated, Dini claims that 3D printing could be a highly cost-effective method for construction. Aside from the machine itself, the costs would mainly relate to having suitably trained operators to correctly establish the right mix of materials: “It’s a machine so it’s mostly a matter of investment – not because of the printing cost itself. And planning.”

Dini’s future vision is for such printers to be just another piece of equipment in the construction site. He believes that his technology is ‘virtually perfect’ for creating topology-optimised, concrete shells for post-tensioned segments. Given the right level of investment, Dini (shown below with his team, third from the left) says that he could even build a machine capable of printing a whole bridge on site, in one section: “Yes, this is the vision. If somebody gave me 5 or 10 million pounds, it is within my capability.”

While Acciona may be leading the charge in concrete bridge printing, as regards steel bridges there is a different contender. The end could soon be in sight for a new 3D printed steel bridge which is planned to be built across the Oudezijds Achterburgwal Canal in Amsterdam.

As with Alcobendas’ new bridge, this project in Amsterdam is taking its time to come to fruition; Dutch 3D printing specialist MX3D has been working on it for some time (Bd&e issue no 81). This project uses multi-axis robots that are designed to print along the horizontal as well the vertical axis, in theory enabling them to work from each side of the gap to create the bridge structure.

Much of the project’s time has been dedicated to developing a more advanced topology optimisation in Autodesk’s Dreamcatcher software, which has involved creating a custom-made version of the program to work specifically with MX3D’s printing process. The idea here is that the bridge designer specifies the space available for the structure, the forces it will have to carry, the properties of the material which it will be built from and so on, and the software then calculates the most efficient shape to meet those criteria.

The project has been put back a number of times, but speaking with Bd&e at the time of going to press, MX3D said that the design of the bridge was being finalised and that printing was intended to begin in spring.

The bridge will not, however, be printed on site as original visualisations suggested, but instead will be printed in a production facility. No further details are currently available as to how it will be installed or constructed.

As for the next step of 3D bridge building, the jury is still out for this admittedly incipient technology. There is little doubt that the bridge in Alcobendas is a milestone, finally bringing the vision to reality. However both the organisation that designed the bridge and the company that built it are remaining tight-lipped about any more detail of the project so it remains to be seen whether the technology continues to advance, or these small schemes are anything more significant than showcases.