A framework for large-scale structural applications of 3D printed concrete
Wolfs, Rob
In this work, a framework for large-scale structural applications of 3D printed con-crete is presented, which serves as an important step to develop a regulatory basis for ap-proval in the Netherlands. The steps in this framework, consisting of a design phase, testing phase and manufacturing phase, towards a final output were presented and discussed theo-retically. The framework was then applied to the case of a 29 m 3D printed bridge, funded by Rijkswaterstaat Major Projects and Maintenance, constructed in the Netherlands. The full application of the framework illustrates that despite the absence of standards, it is possible to safely apply 3D printed structures in practice. With the gradual increase of testing data ex-pected to become available over the coming years, the extent of the application of the framework can be reduced step-by-step.
 I want to present today a project of a 3D printed concrete bicycle and pedestrian bridge. And the framework or the process, if you like, that we followed to make this bridge a reality. This is what it looks like. It was opened in the summer this year in the city of Nijmegen in the Netherlands. And because the printed concrete itself is fulfilling the main load-bearing function, we had to solve some challenges and prove in some ways that this was indeed feasible. So it's actually in use right now. Now, the history of our 3D printed bridges goes back a couple of years when we made this small and simple printed bridge in this small city of Gehmert. But the principle that we applied there is actually similar to what I will show you today. As you can see, we have multiple elements that are printed separately and they are then post-tensioned together into one, again, a compression-only structure. So we remain under compression in this presentation as well, which were all printed individually in our lab and then put on a truck assembled on the building site into a full structure. So based on these experiences and the ambitions that we wanted to apply in this project, we came up with a framework for such projects. And this might be a bit overwhelming, but it's not that different from traditional design to manufacturing processes. We just added a few steps. So on the left, it's a starting point, is our design process, which takes place in a complete parametric environment where we can come up with different design iterations. But because of this absence of design rules, there is no Eurocode 3D concrete printing, we have to enhance this parametric environment with some information. This comes from the two data blocks, which are the orange blocks on the bottom and the top of this slide. And on the one hand, this is structural information. So what is the mechanical properties of printed concrete on a small scale, but also the information that we got from structural testing on that previous bridge that I showed. And all that information is put into this design environment. At the same time, we also have to check if we can print these objects, and what are the limits in terms of manufacturing of this technology, so that we make sure that what we design can also be made. Now, if we then inform this design environment with this information, we can come up with some design that we want to move forward with. And then we've added another step in between. This is a sort of validation step where we create a part of the bridge on a full scale and validate if indeed it can be manufactured and then it acts structurally as we expected based on all this information coming from different sources. Now, if all of that fulfills its requirements, then we can move to the manufacturing of the final structure. And then finally, before this opens, we also test the final object to make sure that what we tested in the lab scale also holds in practice. So these are all the steps that we had to go through, and I will explain a bit more in detail how they were solved. This was the starting point. The conceptual design of the bridge developed by Michiel van der Kleij, which in his words, inspired by nature, we have a double curved bridge of about 30 meters with tapered columns sprouting out of the deck into the water. And you can clearly see this was a next step in terms of size and in terms of design freedom compared to that previous simple pre-stress bridge that I showed you. Now, this conceptual design was then put into that parametric environment and we came up with this geometry that we wanted to move forward with. So we have a bridge of about 30 meters, but in fact, from a structural perspective, it is five smaller bridges, each spanning from a support to a pair of columns from one way to the other. And each of these five parts is a small pre-stress bridge on its own. So actually we have a system of five statically determined bridge girders, which is relatively simple to analyze and to design. Now, each of these parts is then further subdivided, as you can see, in multiple elements, and this is because of manufacturing constraints and logistics. So we wanted to print again in an off-site condition, so we limited the size and the weight of these elements and had to print them in multiple parts which were then assembled together, which I will show in a minute. Now, the cross-section of these elements is not solid, but it has what we call a bottle-shaped infill pattern, which I will show you some pictures later, to reduce the amount of material and to allow these pre-stress cables to run through. What is also new in this project compared to that previous bridge is that we now have columns and we need to also print them somehow. But because of their steep curvature, we could not print these elements in one go with the column attached to them. So we decided to make a horizontal cut and print the columns individually in a vertical position and printed all the elements rotated 90 degrees separately. Now, from a structural perspective, these columns are not that exciting because they are printed and then filled with traditional concrete. So we only had to make sure that we could print them, but from a structural perspective, we didn't have to do any additional tests. We could consider them as normal columns. But the formwork, the printed concrete acts as a lost formwork, so we have consistent overall appearance of the structure. The material properties to design this bridge, and specifically to define the dimensions and so on, was based on an extensive experimental program in our group where we tested the bond strength between printed layers in different orientations based on different process parameters, so different time intervals, different orientations and so on. And that was used to define thickness, for instance, of the layers, but also define the pre-stress level to make sure that in the direction of the span, the structure was completely under compression. Now then let's go to some printing and structural testing. Well, the column, that was the first one we looked for, and this is one of the many different column designs. And, well, you don't need a structural model like we did to realize that this one might be very susceptible to failure during printing, due to its very short contour length and its very steep cantilevering angles. So, to be able to make this, we already had some experience with applying a temporary support material during printing to support these cantilevering angles. And that's something that we did, but what turned out is that if you have some parts of this column actually cantilevering to the outside, well, then this support material doesn't provide any support, of course. And this is what exactly happened, and this column did fail after all during printing. The nice thing is that because of this whole digital workflow that we had set up, I remember at that time I was working in Paris, and my colleague was doing the printing of this column. He gave me a phone call and we changed a few things. We generated a new script, and immediately in the same afternoon, we were able to print these columns successfully. So, because we had this design logic set up, we could limit the overhang angles in the column design so that the support material could actually fulfill its function over the full object height. Now then to the bridge girder segments. So, these are rotated 90 degrees, as you can see. And because of the very long contour length, there wasn't much of a problem in terms of print stability, apart from some segments which were very steeply curved to follow that double curved geometry. So, we did locally again, as you can see, apply some of the support material, but not everywhere. It's quite labor intensive, it's still manual work. And this picture also nicely shows that bottle shaped infill that I mentioned before. So, we have these alternating gaps to reduce the overall amount of material, but still have sufficient material connecting the deck and the bottom part of the bridge for the shear forces to flow through. And these openings are then the openings where the pre-stress cables are being put into as well. So, it's an unbounded pre-stress system that theoretically could be taken apart. Now, after testing the printability, so can we manufacture these type of geometries? We had to lift them out of our print bed. Now, I think that for this project, this might have been one of the most difficult parts, not the printing, but how to pick them up and how to handle them. All of these different objects and the stability was a bit complicated, but well, we figured it out. And then we repeated that process for one part of the bridge. So, six of these elements were printed that together provided one statically determined system that we could start to assemble. Well, that was the next step. And similar to our previous presenter, we had to come up with some counter curved formwork to be able to assemble these elements. And what you can also see on this slide is the anchor blocks. So, that's the end point on both sides of the bridge, which is made out of conventional cast concrete. So, the reason for that is that this is where the pre-stress is introduced. There are very strong stress concentrations there. So, this is made out of solid concrete with a reinforcement mesh inside. But the geometry, the outer geometry is printed. So, again, we follow the same geometry. On the outside, we have a consistent appearance, but the anchor blocks themselves are solid for the pre-stress to be evenly distributed into the concrete elements. Now, for this project, similar to the previous one, the elements are still all glued together. So, it's a web join, the permanent connection. But we are now in the follow-up project looking at how we can make these dry connections so that we can also fully demount the bridge and potentially reuse them. So, after all of these steps, well, then we had our first bridge of about six meters, or part of the actual bridge, I should say. And we went for the structural testing. We did this in two configurations. So, it's a four-point bending test. So, the bridge is supported on both ends. And there's a line load running from one way to the other. And we can move these two positions. So, we tested the bridge in two configurations. One where there is a dominant bending moment in the middle. That would be the critical loading situation. And one, and that is the photo is in right now, is where the loads are relatively close to the supports and the shear stresses would be the dominant loading case. So, we could test in both loading configurations if the bridge would fulfill its requirements. These are the load displacement graphs of these two tests. So, the bending test and the shear test. First, we did the bending test. And we step wisely loaded and unloaded the bridge until it reached its ultimate limit state value. And as you can see by the complete linear shape of this graph, there wasn't any damage observed at all in this test. So, after we had reached the target design value, we unloaded the bridge again and we changed the setup. So, using the same samples or the same bridge elements, we then applied the shear test. Again, step wisely loading, unloading until we reached our design load, which is over here. And then still no damage at all, no cracking or no loss of stiffness was observed. And because we were interested to learn how such a structure would fail, we decided to increase the load further and see what the potential failure pattern would be. But after we had reached about three times the ultimate limit state value, we decided to stop the test because we were reaching the limit of what we could practically do, and maybe also a bit of safety. So, we stopped the test, we unloaded it. And you can see there is a bit of nonlinearity, so there might have been some damage, but nothing that we could clearly observe. And because it was that much higher than the design value, we could conclude that these tests were successful and that the structural principle, at least as how it was designed, would hold. So then we had completed the mock-up phase, we demonstrated that we could print all parts of the bridge, we demonstrated that structurally the whole concept of sound and we could move on to the full production of all the bridge elements. The one that was tested was not reused, so we reprinted all of the elements. This was done by the industrial partners involved in the project, all in their off-site printing factory in Eindhoven. They were all put on a truck and brought to the building site in Nijmegen, where they were then assembled in the same principle, so gluing them together into the full bridge. Now to do this, they made this nice little assembly yard just near by the final location of the bridge, where all the elements would be placed in the right orientation, right position, glued and pre-stressed. And then for each of the five parts, before it would be placed in its final location, we would again test and steps these elements to their ultimate limit state. So we really wanted to make sure that what we tested in a lab environment was also exactly what was happening in practice, so that each of these elements was indeed safe, and only then were they put into their final location and assembled into the full bridge, as you can see here. Now in the end, there was also a printed railing, which is not from a structural perspective, not that challenging, placed on top and of course a deck to make sure that when you bike over it, you don't get a flat tire because of all the ribs. So this is how it looked like when it was opened. The project learned us that following these steps, this framework of testing on a material scale all the way to a mock-up level, all the way to what we call a diagnostic test of the actual in-use object, can be used to design structural load-bearing applications of 3D printed concrete. At the same time, I think from an economic point of view and also from a material point of view, this is not something that we want to keep doing forever. We want to make our objects twice before we can actually use them. So I think that these data blocks that I showed you will have to grow over the course of the project and feed more and more information into that early design stage, which means we can reduce the amount of tests we have to do in these later stages. I think that's an important challenge that we as a community have to address somehow. In terms of next steps, we're looking at improving this parametric environment with more different types of analysis, more degrees of freedom. The connections which I already mentioned are now still wet joints, and we want to develop them towards dry joints so we can remove them and reuse potentially some of the bridge parts. And finally, we're also looking at how to integrate structural health monitoring, so censoring. So instead of having to do these destructive tests, we can in a non-destructive way measure the performance of these elements, and not just at one moment in time, but also during the live use, which will also help to gain more understanding of these structural elements. So also this project, of course, was a very nice collaboration. We have Rijkswaterstad and our university together with the municipality of Nijmegen, Michiel van der Kleij designed the bridge and Weber-Beemings and Baum together printed and assembled the bridge, and with de Venenbosch, the structural engineer and cement engineering, in the person of Dieter Rieck-Venendal doing all the parametric modeling work. So that's it. Thanks a lot for your attention. Thank you, Rob, for a wonderful presentation. Maybe picking up on something that Philip has presented also briefly, I'll mention briefly, he said the whole bridge was designed and built with this compass framework. It's kind of a design to fabrication pipeline, where they also integrate structural calculations, and it's extended towards fabrication called compass fan. And I was always very impressed by your simulation of the wet printed concrete. Are you developing a similar pipeline, or is it in use, or are you intending to publish it somehow as kind of an open source workflow or plugin or something? Okay. I think the short answer is to the last question. Yes, that's something we are working on and trying to also enable others to use. We have the parametric environment where the geometry is being designed. What we are trying to do now is to embed different types of modules, where we can do, for instance, structural analysis in its final state, but also make the link to those structural models in the fresh material state. And then in one integrated workflow also generate, in our case the G code, the printing script. These are different models programmed all in Grasshopper and Rhino, so in that sense it's, I think, very easily accessible. The first part, which is the slicer part, is already now on GitHub open access, so that's something that we are doing. And I think as we develop these models, they will become more and more available. Yes. It's not in the stage of the Compass framework yet. Cool. Thank you.
 I want to present today a project of a 3D printed concrete bicycle and pedestrian bridge.  And the framework or the process, if you like, that we followed to make this bridge a reality.  This is what it looks like. It was opened in the summer this year in the city of Nijmegen in the Netherlands.  And because the printed concrete itself is fulfilling the main load-bearing function,  we had to solve some challenges and prove in some ways that this was indeed feasible.  So it's actually in use right now.  Now, the history of our 3D printed bridges goes back a couple of years  when we made this small and simple printed bridge in this small city of Gehmert.  But the principle that we applied there is actually similar to what I will show you today.  As you can see, we have multiple elements that are printed separately  and they are then post-tensioned together into one, again, a compression-only structure.  So we remain under compression in this presentation as well,  which were all printed individually in our lab and then put on a truck assembled on the building site into a full structure.  So based on these experiences and the ambitions that we wanted to apply in this project,  we came up with a framework for such projects.  And this might be a bit overwhelming, but it's not that different from traditional design to manufacturing processes.  We just added a few steps.  So on the left, it's a starting point, is our design process,  which takes place in a complete parametric environment where we can come up with different design iterations.  But because of this absence of design rules, there is no Eurocode 3D concrete printing,  we have to enhance this parametric environment with some information.  This comes from the two data blocks, which are the orange blocks on the bottom and the top of this slide.  And on the one hand, this is structural information.  So what is the mechanical properties of printed concrete on a small scale,  but also the information that we got from structural testing on that previous bridge that I showed.  And all that information is put into this design environment.  At the same time, we also have to check if we can print these objects,  and what are the limits in terms of manufacturing of this technology,  so that we make sure that what we design can also be made.  Now, if we then inform this design environment with this information,  we can come up with some design that we want to move forward with.  And then we've added another step in between.  This is a sort of validation step where we create a part of the bridge on a full scale  and validate if indeed it can be manufactured and then it acts structurally  as we expected based on all this information coming from different sources.  Now, if all of that fulfills its requirements, then we can move to the manufacturing of the final structure.  And then finally, before this opens, we also test the final object  to make sure that what we tested in the lab scale also holds in practice.  So these are all the steps that we had to go through, and I will explain a bit more in detail how they were solved.  This was the starting point.  The conceptual design of the bridge developed by Michiel van der Kleij,  which in his words, inspired by nature, we have a double curved bridge of about 30 meters  with tapered columns sprouting out of the deck into the water.  And you can clearly see this was a next step in terms of size and in terms of design freedom  compared to that previous simple pre-stress bridge that I showed you.  Now, this conceptual design was then put into that parametric environment  and we came up with this geometry that we wanted to move forward with.  So we have a bridge of about 30 meters, but in fact, from a structural perspective,  it is five smaller bridges,  each spanning from a support to a pair of columns from one way to the other.  And each of these five parts is a small pre-stress bridge on its own.  So actually we have a system of five statically determined bridge girders,  which is relatively simple to analyze and to design.  Now, each of these parts is then further subdivided, as you can see, in multiple elements,  and this is because of manufacturing constraints and logistics.  So we wanted to print again in an off-site condition,  so we limited the size and the weight of these elements  and had to print them in multiple parts which were then assembled together,  which I will show in a minute.  Now, the cross-section of these elements is not solid,  but it has what we call a bottle-shaped infill pattern,  which I will show you some pictures later,  to reduce the amount of material and to allow these pre-stress cables to run through.  What is also new in this project compared to that previous bridge  is that we now have columns and we need to also print them somehow.  But because of their steep curvature,  we could not print these elements in one go with the column attached to them.  So we decided to make a horizontal cut  and print the columns individually in a vertical position  and printed all the elements rotated 90 degrees separately.  Now, from a structural perspective, these columns are not that exciting  because they are printed and then filled with traditional concrete.  So we only had to make sure that we could print them,  but from a structural perspective, we didn't have to do any additional tests.  We could consider them as normal columns.  But the formwork, the printed concrete acts as a lost formwork,  so we have consistent overall appearance of the structure.  The material properties to design this bridge,  and specifically to define the dimensions and so on,  was based on an extensive experimental program in our group  where we tested the bond strength between printed layers  in different orientations based on different process parameters,  so different time intervals, different orientations and so on.  And that was used to define thickness, for instance, of the layers,  but also define the pre-stress level to make sure that in the direction of the span,  the structure was completely under compression.  Now then let's go to some printing and structural testing.  Well, the column, that was the first one we looked for,  and this is one of the many different column designs.  And, well, you don't need a structural model like we did  to realize that this one might be very susceptible to failure during printing,  due to its very short contour length and its very steep cantilevering angles.  So, to be able to make this, we already had some experience  with applying a temporary support material during printing  to support these cantilevering angles.  And that's something that we did,  but what turned out is that if you have some parts of this column  actually cantilevering to the outside,  well, then this support material doesn't provide any support, of course.  And this is what exactly happened,  and this column did fail after all during printing.  The nice thing is that because of this whole digital workflow that we had set up,  I remember at that time I was working in Paris,  and my colleague was doing the printing of this column.  He gave me a phone call and we changed a few things.  We generated a new script, and immediately in the same afternoon,  we were able to print these columns successfully.  So, because we had this design logic set up,  we could limit the overhang angles in the column design  so that the support material could actually fulfill its function  over the full object height.  Now then to the bridge girder segments.  So, these are rotated 90 degrees, as you can see.  And because of the very long contour length,  there wasn't much of a problem in terms of print stability,  apart from some segments which were very steeply curved  to follow that double curved geometry.  So, we did locally again, as you can see,  apply some of the support material, but not everywhere.  It's quite labor intensive, it's still manual work.  And this picture also nicely shows that bottle shaped infill that I mentioned before.  So, we have these alternating gaps to reduce the overall amount of material,  but still have sufficient material connecting the deck and the bottom part of the bridge  for the shear forces to flow through.  And these openings are then the openings where the pre-stress cables are being put into as well.  So, it's an unbounded pre-stress system that theoretically could be taken apart.  Now, after testing the printability,  so can we manufacture these type of geometries?  We had to lift them out of our print bed.  Now, I think that for this project, this might have been one of the most difficult parts,  not the printing, but how to pick them up and how to handle them.  All of these different objects and the stability was a bit complicated,  but well, we figured it out.  And then we repeated that process for one part of the bridge.  So, six of these elements were printed that together provided one statically determined system  that we could start to assemble.  Well, that was the next step.  And similar to our previous presenter, we had to come up with some counter curved formwork  to be able to assemble these elements.  And what you can also see on this slide is the anchor blocks.  So, that's the end point on both sides of the bridge,  which is made out of conventional cast concrete.  So, the reason for that is that this is where the pre-stress is introduced.  There are very strong stress concentrations there.  So, this is made out of solid concrete with a reinforcement mesh inside.  But the geometry, the outer geometry is printed.  So, again, we follow the same geometry.  On the outside, we have a consistent appearance,  but the anchor blocks themselves are solid for the pre-stress to be evenly distributed into the concrete elements.  Now, for this project, similar to the previous one, the elements are still all glued together.  So, it's a web join, the permanent connection.  But we are now in the follow-up project looking at how we can make these dry connections  so that we can also fully demount the bridge and potentially reuse them.  So, after all of these steps, well, then we had our first bridge of about six meters,  or part of the actual bridge, I should say.  And we went for the structural testing.  We did this in two configurations.  So, it's a four-point bending test.  So, the bridge is supported on both ends.  And there's a line load running from one way to the other.  And we can move these two positions.  So, we tested the bridge in two configurations.  One where there is a dominant bending moment in the middle.  That would be the critical loading situation.  And one, and that is the photo is in right now,  is where the loads are relatively close to the supports  and the shear stresses would be the dominant loading case.  So, we could test in both loading configurations if the bridge would fulfill its requirements.  These are the load displacement graphs of these two tests.  So, the bending test and the shear test.  First, we did the bending test.  And we step wisely loaded and unloaded the bridge until it reached its ultimate limit state value.  And as you can see by the complete linear shape of this graph,  there wasn't any damage observed at all in this test.  So, after we had reached the target design value,  we unloaded the bridge again and we changed the setup.  So, using the same samples or the same bridge elements,  we then applied the shear test.  Again, step wisely loading, unloading until we reached our design load, which is over here.  And then still no damage at all, no cracking or no loss of stiffness was observed.  And because we were interested to learn how such a structure would fail,  we decided to increase the load further and see what the potential failure pattern would be.  But after we had reached about three times the ultimate limit state value,  we decided to stop the test because we were reaching the limit of what we could practically do,  and maybe also a bit of safety.  So, we stopped the test, we unloaded it.  And you can see there is a bit of nonlinearity, so there might have been some damage,  but nothing that we could clearly observe.  And because it was that much higher than the design value,  we could conclude that these tests were successful and that the structural principle,  at least as how it was designed, would hold.  So then we had completed the mock-up phase,  we demonstrated that we could print all parts of the bridge,  we demonstrated that structurally the whole concept of sound  and we could move on to the full production of all the bridge elements.  The one that was tested was not reused, so we reprinted all of the elements.  This was done by the industrial partners involved in the project,  all in their off-site printing factory in Eindhoven.  They were all put on a truck and brought to the building site in Nijmegen,  where they were then assembled in the same principle,  so gluing them together into the full bridge.  Now to do this, they made this nice little assembly yard just near by the final location of the bridge,  where all the elements would be placed in the right orientation, right position,  glued and pre-stressed.  And then for each of the five parts, before it would be placed in its final location,  we would again test and steps these elements to their ultimate limit state.  So we really wanted to make sure that what we tested in a lab environment  was also exactly what was happening in practice,  so that each of these elements was indeed safe,  and only then were they put into their final location  and assembled into the full bridge, as you can see here.  Now in the end, there was also a printed railing,  which is not from a structural perspective, not that challenging,  placed on top and of course a deck to make sure that when you bike over it,  you don't get a flat tire because of all the ribs.  So this is how it looked like when it was opened.  The project learned us that following these steps,  this framework of testing on a material scale all the way to a mock-up level,  all the way to what we call a diagnostic test of the actual in-use object,  can be used to design structural load-bearing applications of 3D printed concrete.  At the same time, I think from an economic point of view  and also from a material point of view,  this is not something that we want to keep doing forever.  We want to make our objects twice before we can actually use them.  So I think that these data blocks that I showed you  will have to grow over the course of the project  and feed more and more information into that early design stage,  which means we can reduce the amount of tests we have to do in these later stages.  I think that's an important challenge that we as a community have to address somehow.  In terms of next steps,  we're looking at improving this parametric environment  with more different types of analysis, more degrees of freedom.  The connections which I already mentioned are now still wet joints,  and we want to develop them towards dry joints  so we can remove them and reuse potentially some of the bridge parts.  And finally, we're also looking at how to integrate structural health monitoring,  so censoring.  So instead of having to do these destructive tests,  we can in a non-destructive way measure the performance of these elements,  and not just at one moment in time, but also during the live use,  which will also help to gain more understanding of these structural elements.  So also this project, of course, was a very nice collaboration.  We have Rijkswaterstad and our university together with the municipality of Nijmegen,  Michiel van der Kleij designed the bridge and Weber-Beemings and Baum together printed and assembled the bridge,  and with de Venenbosch, the structural engineer and cement engineering,  in the person of Dieter Rieck-Venendal doing all the parametric modeling work.  So that's it. Thanks a lot for your attention.  Thank you, Rob, for a wonderful presentation.  Maybe picking up on something that Philip has presented also briefly,  I'll mention briefly, he said the whole bridge was designed and built with this compass framework.  It's kind of a design to fabrication pipeline, where they also integrate structural calculations,  and it's extended towards fabrication called compass fan.  And I was always very impressed by your simulation of the wet printed concrete.  Are you developing a similar pipeline, or is it in use,  or are you intending to publish it somehow as kind of an open source workflow or plugin or something?  Okay. I think the short answer is to the last question.  Yes, that's something we are working on and trying to also enable others to use.  We have the parametric environment where the geometry is being designed.  What we are trying to do now is to embed different types of modules,  where we can do, for instance, structural analysis in its final state,  but also make the link to those structural models in the fresh material state.  And then in one integrated workflow also generate, in our case the G code, the printing script.  These are different models programmed all in Grasshopper and Rhino,  so in that sense it's, I think, very easily accessible.  The first part, which is the slicer part, is already now on GitHub open access,  so that's something that we are doing.  And I think as we develop these models, they will become more and more available.  Yes. It's not in the stage of the Compass framework yet.  Cool. Thank you.
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I want to present today a project of a 3D printed concrete bicycle and pedestrian bridge. And the framework or the process, if you like, that we followed to make this bridge a reality. This is what it looks like. It was opened in the summer this year in the city of Nijmegen in the Netherlands. And because the printed concrete itself is fulfilling the main load-bearing function, we had to solve some challenges and prove in some ways that this was indeed feasible. So it's actually in use right now. Now, the history of our 3D printed bridges goes back a couple of years when we made this small and simple printed bridge in this small city of Gehmert. But the principle that we applied there is actually similar to what I will show you today. As you can see, we have multiple elements that are printed separately and they are then post-tensioned together into one, again, a compression-only structure. So we remain under compression in this presentation as well, which were all printed individually in our lab and then put on a truck assembled on the building site into a full structure. So based on these experiences and the ambitions that we wanted to apply in this project, we came up with a framework for such projects. And this might be a bit overwhelming, but it's not that different from traditional design to manufacturing processes. We just added a few steps. So on the left, it's a starting point, is our design process, which takes place in a complete parametric environment where we can come up with different design iterations. But because of this absence of design rules, there is no Eurocode 3D concrete printing, we have to enhance this parametric environment with some information. This comes from the two data blocks, which are the orange blocks on the bottom and the top of this slide. And on the one hand, this is structural information. So what is the mechanical properties of printed concrete on a small scale, but also the information that we got from structural testing on that previous bridge that I showed. And all that information is put into this design environment. At the same time, we also have to check if we can print these objects, and what are the limits in terms of manufacturing of this technology, so that we make sure that what we design can also be made. Now, if we then inform this design environment with this information, we can come up with some design that we want to move forward with. And then we've added another step in between. This is a sort of validation step where we create a part of the bridge on a full scale and validate if indeed it can be manufactured and then it acts structurally as we expected based on all this information coming from different sources. Now, if all of that fulfills its requirements, then we can move to the manufacturing of the final structure. And then finally, before this opens, we also test the final object to make sure that what we tested in the lab scale also holds in practice. So these are all the steps that we had to go through, and I will explain a bit more in detail how they were solved. This was the starting point. The conceptual design of the bridge developed by Michiel van der Kleij, which in his words, inspired by nature, we have a double curved bridge of about 30 meters with tapered columns sprouting out of the deck into the water. And you can clearly see this was a next step in terms of size and in terms of design freedom compared to that previous simple pre-stress bridge that I showed you. Now, this conceptual design was then put into that parametric environment and we came up with this geometry that we wanted to move forward with. So we have a bridge of about 30 meters, but in fact, from a structural perspective, it is five smaller bridges, each spanning from a support to a pair of columns from one way to the other. And each of these five parts is a small pre-stress bridge on its own. So actually we have a system of five statically determined bridge girders, which is relatively simple to analyze and to design. Now, each of these parts is then further subdivided, as you can see, in multiple elements, and this is because of manufacturing constraints and logistics. So we wanted to print again in an off-site condition, so we limited the size and the weight of these elements and had to print them in multiple parts which were then assembled together, which I will show in a minute. Now, the cross-section of these elements is not solid, but it has what we call a bottle-shaped infill pattern, which I will show you some pictures later, to reduce the amount of material and to allow these pre-stress cables to run through. What is also new in this project compared to that previous bridge is that we now have columns and we need to also print them somehow. But because of their steep curvature, we could not print these elements in one go with the column attached to them. So we decided to make a horizontal cut and print the columns individually in a vertical position and printed all the elements rotated 90 degrees separately. Now, from a structural perspective, these columns are not that exciting because they are printed and then filled with traditional concrete. So we only had to make sure that we could print them, but from a structural perspective, we didn't have to do any additional tests. We could consider them as normal columns. But the formwork, the printed concrete acts as a lost formwork, so we have consistent overall appearance of the structure. The material properties to design this bridge, and specifically to define the dimensions and so on, was based on an extensive experimental program in our group where we tested the bond strength between printed layers in different orientations based on different process parameters, so different time intervals, different orientations and so on. And that was used to define thickness, for instance, of the layers, but also define the pre-stress level to make sure that in the direction of the span, the structure was completely under compression. Now then let's go to some printing and structural testing. Well, the column, that was the first one we looked for, and this is one of the many different column designs. And, well, you don't need a structural model like we did to realize that this one might be very susceptible to failure during printing, due to its very short contour length and its very steep cantilevering angles. So, to be able to make this, we already had some experience with applying a temporary support material during printing to support these cantilevering angles. And that's something that we did, but what turned out is that if you have some parts of this column actually cantilevering to the outside, well, then this support material doesn't provide any support, of course. And this is what exactly happened, and this column did fail after all during printing. The nice thing is that because of this whole digital workflow that we had set up, I remember at that time I was working in Paris, and my colleague was doing the printing of this column. He gave me a phone call and we changed a few things. We generated a new script, and immediately in the same afternoon, we were able to print these columns successfully. So, because we had this design logic set up, we could limit the overhang angles in the column design so that the support material could actually fulfill its function over the full object height. Now then to the bridge girder segments. So, these are rotated 90 degrees, as you can see. And because of the very long contour length, there wasn't much of a problem in terms of print stability, apart from some segments which were very steeply curved to follow that double curved geometry. So, we did locally again, as you can see, apply some of the support material, but not everywhere. It's quite labor intensive, it's still manual work. And this picture also nicely shows that bottle shaped infill that I mentioned before. So, we have these alternating gaps to reduce the overall amount of material, but still have sufficient material connecting the deck and the bottom part of the bridge for the shear forces to flow through. And these openings are then the openings where the pre-stress cables are being put into as well. So, it's an unbounded pre-stress system that theoretically could be taken apart. Now, after testing the printability, so can we manufacture these type of geometries? We had to lift them out of our print bed. Now, I think that for this project, this might have been one of the most difficult parts, not the printing, but how to pick them up and how to handle them. All of these different objects and the stability was a bit complicated, but well, we figured it out. And then we repeated that process for one part of the bridge. So, six of these elements were printed that together provided one statically determined system that we could start to assemble. Well, that was the next step. And similar to our previous presenter, we had to come up with some counter curved formwork to be able to assemble these elements. And what you can also see on this slide is the anchor blocks. So, that's the end point on both sides of the bridge, which is made out of conventional cast concrete. So, the reason for that is that this is where the pre-stress is introduced. There are very strong stress concentrations there. So, this is made out of solid concrete with a reinforcement mesh inside. But the geometry, the outer geometry is printed. So, again, we follow the same geometry. On the outside, we have a consistent appearance, but the anchor blocks themselves are solid for the pre-stress to be evenly distributed into the concrete elements. Now, for this project, similar to the previous one, the elements are still all glued together. So, it's a web join, the permanent connection. But we are now in the follow-up project looking at how we can make these dry connections so that we can also fully demount the bridge and potentially reuse them. So, after all of these steps, well, then we had our first bridge of about six meters, or part of the actual bridge, I should say. And we went for the structural testing. We did this in two configurations. So, it's a four-point bending test. So, the bridge is supported on both ends. And there's a line load running from one way to the other. And we can move these two positions. So, we tested the bridge in two configurations. One where there is a dominant bending moment in the middle. That would be the critical loading situation. And one, and that is the photo is in right now, is where the loads are relatively close to the supports and the shear stresses would be the dominant loading case. So, we could test in both loading configurations if the bridge would fulfill its requirements. These are the load displacement graphs of these two tests. So, the bending test and the shear test. First, we did the bending test. And we step wisely loaded and unloaded the bridge until it reached its ultimate limit state value. And as you can see by the complete linear shape of this graph, there wasn't any damage observed at all in this test. So, after we had reached the target design value, we unloaded the bridge again and we changed the setup. So, using the same samples or the same bridge elements, we then applied the shear test. Again, step wisely loading, unloading until we reached our design load, which is over here. And then still no damage at all, no cracking or no loss of stiffness was observed. And because we were interested to learn how such a structure would fail, we decided to increase the load further and see what the potential failure pattern would be. But after we had reached about three times the ultimate limit state value, we decided to stop the test because we were reaching the limit of what we could practically do, and maybe also a bit of safety. So, we stopped the test, we unloaded it. And you can see there is a bit of nonlinearity, so there might have been some damage, but nothing that we could clearly observe. And because it was that much higher than the design value, we could conclude that these tests were successful and that the structural principle, at least as how it was designed, would hold. So then we had completed the mock-up phase, we demonstrated that we could print all parts of the bridge, we demonstrated that structurally the whole concept of sound and we could move on to the full production of all the bridge elements. The one that was tested was not reused, so we reprinted all of the elements. This was done by the industrial partners involved in the project, all in their off-site printing factory in Eindhoven. They were all put on a truck and brought to the building site in Nijmegen, where they were then assembled in the same principle, so gluing them together into the full bridge. Now to do this, they made this nice little assembly yard just near by the final location of the bridge, where all the elements would be placed in the right orientation, right position, glued and pre-stressed. And then for each of the five parts, before it would be placed in its final location, we would again test and steps these elements to their ultimate limit state. So we really wanted to make sure that what we tested in a lab environment was also exactly what was happening in practice, so that each of these elements was indeed safe, and only then were they put into their final location and assembled into the full bridge, as you can see here. Now in the end, there was also a printed railing, which is not from a structural perspective, not that challenging, placed on top and of course a deck to make sure that when you bike over it, you don't get a flat tire because of all the ribs. So this is how it looked like when it was opened. The project learned us that following these steps, this framework of testing on a material scale all the way to a mock-up level, all the way to what we call a diagnostic test of the actual in-use object, can be used to design structural load-bearing applications of 3D printed concrete. At the same time, I think from an economic point of view and also from a material point of view, this is not something that we want to keep doing forever. We want to make our objects twice before we can actually use them. So I think that these data blocks that I showed you will have to grow over the course of the project and feed more and more information into that early design stage, which means we can reduce the amount of tests we have to do in these later stages. I think that's an important challenge that we as a community have to address somehow. In terms of next steps, we're looking at improving this parametric environment with more different types of analysis, more degrees of freedom. The connections which I already mentioned are now still wet joints, and we want to develop them towards dry joints so we can remove them and reuse potentially some of the bridge parts. And finally, we're also looking at how to integrate structural health monitoring, so censoring. So instead of having to do these destructive tests, we can in a non-destructive way measure the performance of these elements, and not just at one moment in time, but also during the live use, which will also help to gain more understanding of these structural elements. So also this project, of course, was a very nice collaboration. We have Rijkswaterstad and our university together with the municipality of Nijmegen, Michiel van der Kleij designed the bridge and Weber-Beemings and Baum together printed and assembled the bridge, and with de Venenbosch, the structural engineer and cement engineering, in the person of Dieter Rieck-Venendal doing all the parametric modeling work. So that's it. Thanks a lot for your attention. Thank you, Rob, for a wonderful presentation. Maybe picking up on something that Philip has presented also briefly, I'll mention briefly, he said the whole bridge was designed and built with this compass framework. It's kind of a design to fabrication pipeline, where they also integrate structural calculations, and it's extended towards fabrication called compass fan. And I was always very impressed by your simulation of the wet printed concrete. Are you developing a similar pipeline, or is it in use, or are you intending to publish it somehow as kind of an open source workflow or plugin or something? Okay. I think the short answer is to the last question. Yes, that's something we are working on and trying to also enable others to use. We have the parametric environment where the geometry is being designed. What we are trying to do now is to embed different types of modules, where we can do, for instance, structural analysis in its final state, but also make the link to those structural models in the fresh material state. And then in one integrated workflow also generate, in our case the G code, the printing script. These are different models programmed all in Grasshopper and Rhino, so in that sense it's, I think, very easily accessible. The first part, which is the slicer part, is already now on GitHub open access, so that's something that we are doing. And I think as we develop these models, they will become more and more available. Yes. It's not in the stage of the Compass framework yet. Cool. Thank you.
Netherlands. And because the printed concrete compression in this presentation as well, which absence of design rules, there is no Eurocode 3D to the manufacturing of the final structure. And conceptual design of the bridge developed by showed you. Now, this conceptual design was then appearance of the structure. The material and immediately in the same afternoon, we were its function over the full object height. Now bridge elements, we then applied the shear test. happening in practice, so that each of these steps, this framework of testing on a material in-use object, can be used to design structural load-bearing applications of 3D printed concrete. an important challenge that we as a community work. So that's it. Thanks a lot for your of a design to fabrication pipeline, where they develop these models, they will become more and