Experiment R - How (not) to install a complex structure
The completed sculpture is fast becoming popular with the kids in Aarhus – but creating it was no child’s play….
I hope You had a great summer – here in Denmark it was exceptionally warm, dry and sunny, and just right for a relaxing vacation at the beach.
On behalf of all of us here at Hi-Con I would like to welcome you back to our UHPC blog with the final chapter of the advanced casting project, Experiment R. And I should warn you – it’s quite a lengthy tale…..
When we left the project in January we still needed to cast 2 of the elements – and never expected this to take more than 3 months to complete. But if we thought the first 4 casts had been difficult …..
The two final casts was: The largest twisted plate element, and, The largest element of them all, both with almost 2 m casting height.
In both cases the formwork did not stand up to the pressure, and in both cases the main reason was that the 3D cut EPS plates deformed several cm under the pressure despite clamps, tightening straps, heavy support blocks and interlocking conical holes and wedges. This opened gaps between the form pieces, that allowed concrete pressure to build up between the plates and against the outer supporting formwork in places that was not designed for such pressure – and as a result, both forms buckled and ruptured!
The result was firstly that casting had to be interrupted several times to keep the formwork from a total collapse - and ensure the safety of those involved! Secondly, that the elements ended up with a different geometry than intended, and thirdly, that the elements required substantial finishing works before being ready to attempt assembly. A few images of the forms and from the process are shown below.
Open B1 formwork before (top left) and after reinforcement (top right). Lower left shows the formwork splitting and CRC flowing out between the formwork parts (again) – lower right shows the top of the form after casting with the filling funnels used to makes sure the top cast was filled, and two cover plates removed showing that this indeed was successfully accomplished.
C1 formwork during assembly of the reinforcement (top left and right), Final touches to the reinforcement and anchoring inserts (lower left) and the completed reinforced formwork before casting (lower right).
After the elements were finally cast and as well adjusted as possible through diamond cutting, polishing, grinding and extra castings, the architectural school of Aarhus conducted a 3D scan of the B1 element (being the one we had the most doubt about the realized geometry of).
The scans (one example is shown below) indicated that the elements both differed in overall geometry, and that the twist also deviated. Because of this we expected that it would be difficult to execute the assembly, and plans were made to adjust the elements on site, and final finishing work postponed till after the assembly.
3D scan (grey) overlaid with the theoretical model geometry of the element (green). Theoretical because the scale is matched up after the scan.
Elements A2, B1, B2 and C2 at the stockyard waiting to be transported to the installation site.
And so, after a more than 5 years long process, we were ready for installation!!
The sculpture was installed on two pre-prepared concrete foundations with app. 20 chemical anchors drilled into each foundation. Threaded M20 rods set into cast-in sockets in the base of primarily A2 and C1 ensured the force transfer to the foundations. The large number of anchors required was mainly the result of forces from thermal stresses in the elements between the foundations due to the yearly temperature variations.
Finally, at the installation site; foundations ready (top left) first elements arrive on site (top right), unloading the elements (lower left) and preparations for drilling of anchors for C1, the first element (lower right).
The installation of the sculpture ended up being divided into five parts – and consequently took almost two weeks to complete:
1. Installation of C1, B1 and B2
2. Attempted installation of remaining elements, measuring, diamond cutting A1 and C2, diamond drilling of holes for anchors for A2
3. Installation of remaining elements and setting of connecting bolts and anchors
4. JointCasting a new connection between C2 and B2
5. Finishing works and completion of the surrounding landscapes
The first three steps were recorded on a time-lapse camera:
The main obstacle was, that because of the skewing and elongation of the forms of especially C1 and B1, the many connecting bolts did not align with the opposing cast-in sockets, but even worse, the entire sculpture could not fit correctly on the foundations, so that the angle and connection between the elements did not fit – even though we tried several different ways to adjust pitch and rotation of the elements, despite a general tolerance of 20 mm in all connections.
As a result, we needed to cut away a large wedge from element A1 and an entire beam section from element C2, and then find a way to connect these to their opposing element faces in a different way than intended.
To make matters worse, the design of the foundations had been done without taking the fact that the elements were to be connected to them by drilled chemical anchors into account. Therefore, so much reinforcement steel had been placed exactly were all the anchors should be, that diamond core drilling of more than two thirds of all holes was required.
CRC i2® is tough, and so is drilling through steel - Just diamond cutting and drilling took an entire day!
Red line drawn on A1 element indicating the wedge-shaped cut needed for it to align with the adjacent elements prior to cutting (upper left), Cutting in progress and completed of the end beam section of C2 (upper and lower right), hole placement for the connection anchors to the foundation of A2 prior to core drilling (lower left).
Once the elements were adjusted, the assembly could be completed – although not without obstacles:
Firstly, because we had cut away the connecting stringer on element C2, another way of fixing it structurally for load transfer in all directions had to be found – in the end, this ended up being drilling and gluing in reinforcement bars and casting the joint on site, both to make sure the structural connection was there, and to attempt to merge the elements acceptably visually:
The difficult assembly of element C2, the two upper images showing C2 alignment in relation to the adjacent elements, the lower left image shows the gap remaining for tolerance, and the final result after in-situ casting the joint is shown in the lower right image.
The final installation of A2 and A1 went comparatively easier since the bolted connections for the most part could be used after drilling the holes larger and then gluing in threaded rods. Also, the altered geometry from the wedge that was cut away was less visible.
Installation of elements A1 and A2. Even though a lot of drilling and resin was required, it was comparatively easier to assemble than C2.
Following the raw assembly of the elements and the casting of C2 interface, all joints, holes etc. were filled and rendered, and as a finishing touch the entire sculpture was added a layer of semi-transparent paint.
The completed Experiment R installation at the inauguration July 2, 2018.
So, what have we learned from the process of creating Experiment R? Quite a bit, actually:
1. Designing irregular 3D structures obviously requires relying on FEM software, as analytical calculations are hopelessly inadequate. What we did not expect was the amount of time required to assess and dimension connections, when multiple load case scenarios are involved. For similar structures in the future, connections are going to be the major focal point, even more than in this project – see also learning point 5.
2. Reinforcement of complex 3D shapes is quite difficult, but 3D printed templates and scale models of elements and other “navigation” tools helps the people placing the bars and speed up the production process.
3. Formwork can never be too strong and rigid! EPS can only be used in very thin sections, or other materials must be used – the only parts on the precast parts not deforming were the 3D milled MDF parts, even though they shifted relative to each other.
4. With concrete temperatures exceeding 90˚C, the EPS effectively softened/melted and fused with the CRC i2® surface, despite the layers of form wax applied. This needs to be solved for future casts, as removal of the EPS residues took many hours and also resulted in a rather rustic surface texture. Using large blocks of EPS – an insulation material – as formwork, high temperatures cannot be avoided, but better surface coatings must be found to protect the EPS surface from the heat.
5. Pure EPS formwork is not sufficiently stiff to ensure that the geometry is maintained. The larger the elements, and the higher the form pressure (casting depth), the larger the deviations from intended shape. This is especially a problem for the connections, which became painfully obvious at the assembly, where we had huge problems with misalignment. For future projects, EPS will not be used for connecting points and faces.
As an overall conclusion, the process has verified that UHPC in combination with computer aided topological optimization and design can create spectacular and novel shapes and structures with minimized material use, which shows great potential for the future. It has also demonstrated, that production and assembly remain the most difficult part of the process, and that formwork mainly build from EPS – at least currently - does not present a viable solution for larger structures.
This was the final post on Experiment R, and I apologize for the length of it - I just had so much I wanted to say. Nevertheless, I hope you have found it interesting. As always, feel free to share your comments or ask questions at our Wordpress site.
Bendt Kjær Aarup Group Material Development Manager