Curly curved concrete

Hundreds of worm screws with pistons are neatly arranged in line in the Stevin laboratory in the faculty of Civil Engineering and Geoscience. The pistons are aligned fifteen centimeters apart, forming what looks like a giant fakir’s bed, With these pin beds Ir. Roel Schipper is producing double curved concrete panels. Schipper was a structural engineer for fifteen years before he started a career at the TU Delft. He believes that the flexible mold he is working on will prove very useful for making buildings with complex geometries. The system is a spin-off of a technique, developed earlier by Dr. Karel Vollers of the Architecture faculty to make freely bended glass panels. The process is simple. Placed above the pistons, that can be adjusted in height using nuts, is a lattice made of thin flexible plywood. On top of that is placed a silicone mold filled with concrete. Once the concrete has hardened a little and reached the right shear strength and viscosity a lattice is lowered on the pistons thereby creating bended concrete panels, the shape of which is controlled by adjusting the height of all the vertical stands.

Normally manufacturers make bended concrete panels by casting concrete in a mold, using a different mold for each geometrically distinct panel. Compared to that, an adjustable mold could save a lot of hard labour and material costs. Yet the technique still needs a lot of finetuning.

“What looks simple on paper is actually very complicated”, says Schipper, who has been receiving a lot of help from master’s students during his research. “First we had to experiment thoroughly to find the right concrete and the right timing for lowering the lattice. The viscosity has to be just right. We spilled a whole lot of concrete on the floor during this process. Finally by trial and error we found the right recipe.” Curved concrete panels, of about a square meter in size, lingering around in the laboratory are the proof of that. Moreover, the position of the actuators is not the only thing governing the shape of the intermediate layer. The elasticity of the formwork material itself is as important. “One has to find the right bending stiffness of the intermediate layer and of the mold. These layers have to be flexible enough to follow the positions of the actuators but not that flexible that they sag in between the actuators.”

With a laser scanner Schipper imaged the geometry of concrete panels to study in detail how the mold draped into the shape set by the actuators. “Unfortunately the mold isn’t listening well enough yet”, he concludes.

“Predicting the behavior numerically is more complex than one might think at first glance”, Schipper adds. “When bending a material in two directions, wrinkles and nods can pop up. This is called buckling. It appears when you are trying to deform a material that actually doesn’t want to deform.”

Schipper has about a year left to apprehend the behavior of the mold and of the concrete in a model before his PhD defense. He also wants to make reinforced bended concrete panels. “For this I won’t be using steel reinforcing bars but patches of glass fiber. I hope that these patches will stay evenly distributed throughout the panel while we bend it.”