Design voids in concrete without reducing its strength

This is the story of a multidisciplinary collaboration between researchers, laboratories and manufacturers to reduce the environmental footprint of concrete by acting on the design of the structures.

It all started in 2012 at Polyhedral Structures Laboratory at the University of Pennsylvaniadirected by Doctor Massoud Akbarzadeh. He and his team wondered how reduce the impact of concrete in the environmental footprint of buildingsknowing that concrete remains essential for many works. Their answer is in two words: polyhedral or polyhedral structuresthe design of which is however not so simple. They have in fact implemented a new approach, the static 3D graphicsa method of structural analysis and design that represents the balance of forces using geometric tools and allows more complex spatial structures to be analyzed and designed. This approach leads to the optimization of structural forms and provides architects and engineers with a powerful tool for innovative design. There static 3D graphics results in a very sensitive reduction in the quantity of concrete used in a structurethanks to voids created in places where there is no interaction of forces in the structure.

The Diamanti project

As Dr. Akbarzadeh explains: “My team’s expertise is to optimize geometric shapes to transfer loads in the structure. There are applications in the fields of architecture, materials science, mechanical engineering and structural calculation. I was able to bring it into three dimensions. This opened a new horizon for structural design and its applications in architecture“. As a result, the work of Doctor Massoud Akbarzadeh was noticed and he was invited by the organizers of the Biennale of the European Cultural Center in Veniceone of the most important meetings in the world in this area, which will be held from May 10 to November 23, 2025.

Yes, but what to present? The most striking would be to present at scale 1 a work designed according to this methodology. This will be the Diamanti projecta footbridge designed in ten concrete elements printed in 3D and assembled by gluing and post-tensioning.

Here are the main stages of the calculation and design of the 10 Diamanti cells and their assembly by post-tensioning, carried out by the team from the Polyhedral Structures Laboratory at the University of Pennsylvania. © Polyhedral Structures Lab, University of Pennsylvania

Structural analysis and material calibration were carried out by the Advanced Building Construction Lab, City College of New York. © City College of New York

Once the decision was made to carry out a work on a scale of 1, Doctor Akbarzadeh surrounded himself with industrial partners and laboratories capable of carrying out full-scale tests: SIKA, Carsey 3D, Ævia and the Cerib (Centre of Studies and Research of the Concrete Industry).

All partners came together to demonstrate the importance of innovation and collaboration to test and evaluate at scale 1 capabilities or performance as well as technical and environmental obstacles in the field of stamped concrete structural construction in 3D. © PP

Here is Diamanti, the footbridge designed by the Polyhedral Structures Lab at the University of Pennsylvania and 3D printed by Carsey 3D in Coubert in 77, using Sikacrete – 7100 3D, a fiber-reinforced micro-concrete used as a printing ink. printing in the system developed exclusively for Sika 3D printing robots, with the help of the Swiss manufacturer LCA. In Venice, Diamanti will be placed on a wooden structure to clearly show its lightness despite its dimensions: 7.8 tonnes, 9 m long, 1.5 m wide on top, 1.8 m wide below. © Polyhedral Structures Lab, University of Pennsylvania

Sika and 3D printing of concrete

Sika supplied the material SikaCrete (7100 3D, a fiber-reinforced single-component micro-concrete for 3D printing) but also brought technical expertise in 3D printed construction, with modeling, slicing of objects and the evaluation of the feasibility of the project. For Sika, the France is the most advanced country in 3D printing and its first market for this product category.

Carsey 3D, represented by its CEO Alberto Arena, made its 3D printer, the first for industrial use in France, available to manufacture Diamanti. There are only three printing gantry cranes of this type in Europe. Carsey 3D has a printing gantry in Coubert in an air-conditioned enclosure where the temperature is maintained between 18 and 23°C and the relative humidity remains at 80%. An activator is added to the print head which allows the parts to harden in a few seconds. © PP

As Dr. Akbarzadeh states: “If we look at the cross section of the concrete segments, we see that it is made up of only two 3D printed layers, which are printed concrete measuring only a few centimeters wide But again, we rely on curved geometry in three-dimensional space, creating surfaces that add geometric rigidity to the structure. This structure appears bulky from the outside, but, in section, it only has two. diapers”. © Carsey 3D

Ævia, specialized within the Eiffage group in the repair and maintenance of structures, applied the prestressing and offered its scientific and technical support in order to validate the various calculations of Doctor Massoud Akbarzadeh. Cerib carried out various real tests on the finished work, in particular by checking the resistance capacity of the future footbridge.

The results of the CERIB test

Once the nine elements of five different types, produced at Carsey 3D, had been printed, the eight applied post-tension cables were introduced: four at the top and four at the bottom. TS15 cables (15.7 mm² in diameter, significantly more resistant than usual concrete reinforcements) threaded into the cavities made in the printed segments have been sheathed so that they offer the same contact over the entire length and that the compensation is identical at all points of the cables.

The top four cables are straight, but the bottom four cables have a parabolic shape. The ducts were sealed using SikaGrout-217, a shrinkage-compensated sealing and wedging mortar which has very high mechanical resistances in compression and bending. Each end block was filled with SikaGrout-238, to better resist the pressure of the post-tensioning cables. The segments were brought together little by little and lightly pressed together, while the Sikadur-30 structural adhesive took effect. Ævia (a subsidiary of Eiffage) had to adapt its technique to assemble 3D printed concrete sections with cables and its prestressing anchors to prove feasibility. © Cerib

The nine voussoirs were sealed together using epoxy glue on both ends of each voussoir, the assembly was transported to Cerib. The cables threaded into the cavities made in the printed segments have been sheathed in such a way that they offer the same contact over their entire length and that the compensation is identical at all points of the cables. The top four cables are straight, but the bottom four cables have a parabolic shape. The ducts were sealed using SikaGrout-217, a shrinkage-compensated sealing and wedging mortar which has very high mechanical resistances in compression and bending. The designers calculated Diamanti for a load of 500 kg/m²which corresponds to the load required by the standard for footbridges. In its test, the Cerib applied twice as much load. The test was ultimately blocked by the capacity of the Cerib’s actuators. But the test result goes beyond the validation of the calculations: a safety coefficient of more than 2 was determined for the Service Limit State (ELS) compared to the predictive calculations. © Cerib

Gathered at Cerib, the major players in the project witnessed the test. In total, it took two hours to carry out the experiment, proceeding in stages. The loading was controlled in displacement with an imposed speed of 1.0 mm/s for loading at the service limit state (ELS) and 3 mm/s for loading at the ultimate limit state (ELU), in the aim of properly controlling and observing the deformation of the beam. The test began with the application of loading cycles corresponding to the service limit state (SLS), aimed at verifying the flexural rigidity, the state of cracking, and the deflection of the beam. Sensors placed in the center and on the sides of the beam made it possible to monitor deformation measurements in real time. At the end of the application of the loading corresponding to 100% of the SLS, a deformation of approximately 1.5 mm was observed, confirming that the material behaved elastically without any appearance of cracks. When applying a loading corresponding to the ultimate limit state (ULS), a first crack appeared in the concretewithout causing the beam to break or lose its resistant capacity, which was the result expected by the calculations.

And now ?

The quantity of concretecompared to a reinforced concrete walkway of the same characteristics, is reduced by 60% and the quantity of reinforcement is reduced by:

– 81% compared to a system with prestressing,

– and 94% compared to a solution without prestressing.

As Patrice Decroix indicates, the Director of Innovation Sika France“This polyhedral approach allows a significant saving of material, the stakes are therefore high. We had already used this technology on plastic, but never on concrete. The Diamanti project serves as an element of proof for the future, and to be able to imagine applying it to real pedestrian bridges.“. An exclusivity agreement has been signed between Doctor Akbarzadeh, his team and the project partners to use this approach in Europe. We could see at least one real gateway in France in 2025.

Beyond that, this technology can be used for lightweight floors, the creation of beams, etc.

We could see at least one real gateway in France in 2025. © Polyhedral Structures Lab, University of Pennsylvania

Source : batirama.com / Pascal Poggi