Unearthing the quality of rammed earth through fabric formwork
Since I have been building a number of rammed earth with fabric formwork, I have found that the technology is honestly revealing the innate property of material – soil, showing a great potential to improve aesthetical and functional quality of rammed earth. In this blog, I am going to explain about the construction process of recently completed two fabric formed rammed earth walls and what I have discovered.
The size of the models is 250mm (width) x 600mm (length) x 1000mm (height), which is a small example to show what fabric formed rammed earth walls look like in real life.
The materials for the fabric formwork were all sourced from local materials. The fabric and steel reinforcement bars were common materials that my father have been using for farming, and timber frames were the materials that left after the completed construction of my home.
For the construction of foundations, I dug about 100mm-400mm depth from the natural ground level. The depth of foundation varied according to the soil condition since some part was filled with full of rocks, so no further digging was required. A plastic sheet was laid on the excavation site before it was filled with lime-stabilised rammed earth. 1-2 sheet of wire mesh was embedded during the ramming course to reinforce its tensile strength.
What I considered most carefully for the design of fabric formwork was the installation of steel reinforcement bars. Since fabric bulges by compaction during the ramming course, it is fundamental to plan in advance how to control its bulging with frames –steel bars in this construction, and this consideration influences the design of fabric formwork. To fix steel bars firmly, I made steel bars fit to the holes of wooden frames. Therefore, installed wooden frames served to stretch fabric and fix steel bars. Once steel bars were all installed, they and the fabric were tied with cable ties. Thin cable ties were used to insert between warp and weft threads – it is important here not to interfere the interwoven threads. Otherwise, a small disruption of threads could cause ramification of bigger scale of disruption on fabric.
Another important consideration for building fabric formwork was the fabric joint. The part fabric is cut is the weakest point because weft and waft threads are disconnected. To resolve the issue, I attached wood to both ends of the fabric and these was connected by nailing. The attached wood on the fabric helps to disperse stress given on fabric not making it torn out.
I made two different types of fabric formed rammed earth: fully unstabilised rammed earth and proportionally stabilised rammed earth. For the construction of unstabilised rammed earth, the moisture content of earthen mixtures was higher than typical rammed earth construction, however, it did not cause any problem on the outcome because excessive moisture and air were extracted through the permeable fabric. In fact, I found that slightly higher moisture content helps to create smoother façade of rammed earth than small amount of moisture content as earthen mixtures are moving more freely making them bind together.
For the construction of two models, lime-stabilised earthen mixture (Lime (1): Soil (4): Sand (8), approximately 7% of lime) was compacted up to 250mm height for plinth, and damp proofing course was laid on it. Later, unstabilised earthen mixture was laid and compacted creating 6-8cm depth at each layer. Wire mesh was embedded every 25cm height. For proportionally lime-stabilised rammed earth, about 5-8cm depth of lime-stabilised earth mixture was compacted to the most bulged surface.
For unstabilised rammed earth, the top of the wall was finished with concrete, so a roof timber frame was nailed directly on it to be fixed. On the other hand, the roof timber frame of partially lime stabilised rammed earth was fixed with fully screwed bolts that were embedded in the rammed earth wall during the ramming course.
When the fabric formwork was removed after 7 days, the rammed earth walls exhibited outstanding appearances with delicately undulated forms and haptic textures. If you examine the models carefully, the surface of rammed earth walls is gradually undulated from bottom to top of the wall. The undulation of the surface is a bit intense in the bottom, while it becomes subtle going upwards. This resulted from the difference of weight given on each soil layer – more loads were given on the bottom part of rammed earth walls, so fabric bulged more. It is a basic principle, but the outcome caused by the laws of nature is truly remarkable. In this sense, fabric is an honest material being obedient to nature – it stretches by tension, drapes by gravity, and bulges by pressure. The quality of fabric honestly reveals the intrinsic nature of soil, opening a whole spectrum of the material – sensory and aesthetic experience that couldn’t be discovered from timber formwork. Fabric also opens a door to a new potential of material-driven design. For instance, the undulated form of rammed earth wall can perform not only enhancing aesthetic quality but also potentially drain rainwater effectively.* For fabric formed rammed earth, patterns can be simply created based on the arrangement of frames such as timber, bamboo, or steel bars. They can be arranged schematically in consideration of design and the environmental stress. Therefore, a bespoke shape of rammed earth can be produced by fabric formwork reflecting the local condition and fully taking advantage of the benefits of these materials’ quality.
* As we can learn from the lesson from the vernacular architecture Musgum mud huts, the pattern of the external earth walls can serve multiple functions –to maintain the wall as steps, to drain rainfall, and to symbolise the ethnic group with the distinctive patterns.
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