The structural design of earthen structures with robotic shotearth fabrication

Abstract

In the near future, pressure on sustainability requirements in building structures will increase further as governments around the world are forced to tackle climate change issues. The construction industry is one of the most resource intensive and wasteful sectors in the world and is estimated to keep growing at a fast pace due to growing populations. Earth building materials and construction systems are perhaps one of the most attractive options available. The advantages of raw earth are that it is considered to be widely available, recyclable, can be sourced at low costs, can exhibit high strength and can provide an excellent indoor climate for occupants. The interest in building with earth is experiencing a renaissance but advancements in Earth Building Technology is currently at its infancy. Most improvement in earth buildings has been related to fabrication methods, where current developments and opportunities in additive manufacturing has potential to create a new form of earth architecture. From a mechanical perspective, earth materials are strong in compression and weak tension, making compression-only structures like arches, vaults and domes especially interesting.
The purpose of this thesis is to determine how feasible it is to combine earth material with a robotic sprayed earth fabrication method to build wall and arch structures. Emphasis has been put in understanding the material for robotic fabrication. Therefore, an in-depth literature study of earth materials and their properties was conducted to understand how they gain strength. In AM, modelling the structural-up of a printed structure requires time-dependent material properties where early-age behaviour is especially critical. As a result, time-dependent compressive strength and Young’s modulus of potential shotearth with and without cement stabilisation was predicted. Equations from the material predictions were input into the design tool developed by Witteveen+Bos to explore geometrical shapes, assess printing time and dimensions that can be achieved.
The results of this work show that earth building materials is essentially impossible to predict without testing as soil composition has a big influence on mechanical strength. Prediction of raw earth properties was based on understanding of soil consistencies and water content. Prediction of cement stabilised earth properties was based on existing data from 3D extruded and sprayed concrete experiments. 3D printed structures, extruded and sprayed, can be compared to ancient masonry structures in their structural design and inspiration from these structures sees different layering techniques adopted to minimise formwork and maximise structural efficiency and strength. Through a geometrical non-linear FE analysis with the time-dependent properties defined and varying robotic printing speeds between 1000 -6000 mm/min, a free standing wall structure of one meter width can reach heights between 0.1 and 0.62 m. For the overhanging structures, a barrel vault and a half dome, considering no formwork, an 8.5 -10.5 degrees arch or tilt can be achieved, compared to the vertical free wall. It is found that for lower printing speeds, more structure can be printed before failure. For the half dome almost the full geometry can be achieved. The unstabilised and cement stabilised upper bound material models resulted in comparable geometric dimensions for all structures analysed. Results of this study also compared the barrel vault and half dome using the roman (radial) and nubian (inclined) layering techniques.