Build a doghouse with earth_Part3: ramming course and the completion of fabric formed rammed earth
Updated: Dec 23, 2020
Before compacting rammed earth, the rammers with rounded heads were prepared. I cannot stress enough the importance of the use of curved rammer heads when compressing rammed earth with fabric formwork. As the fabric is bulging during the ramming course, rounded rammer heads help to compact properly the edge of the rammed earth.
About 15 cm of the bottom layer was made of lime-stabilised rammed earth with the ratio of 1 (lime): 2 (soil) : 3 (sand). To be honest, I was not really successfully at controlling the quantity of water in the earthen mixture. The soil was very sandy, opposite to clayey Scottish soil, so I ended up adding a lot of water relying on a ball-dropping test. When I compressed hard the earthen mixture with a rammer, the surface was bouncy due to the high moisture contents, but it increased workability. It was, actually, easy to compact the edge since the mixture was somehow fluid.
Rammed earth is normally built with a low amount of water compared to other earthen construction methods such as adobe and cob. High amount of water will be problematic for traditional rammed earth since they are trapped between timber formwork, eventually undermining its density and durability. However, I think the high-water contents may not be very problematic for fabric-formed rammed earth since excessive water and air are extracted through the permeable fabric increasing its density. I guess that the moisture content that is normally used between typical rammed earth and poured earth can be reasonably used for fabric-formed rammed earth to enhance workability not compromising its durability and density.
I added wire mesh between the bottom layer of the rammed earth to enhance shear and tensile strength. It is a same reason that concrete is reinforced with steel. In the next day, I laid a damp proofing coursing layer. Since the shape of rammed earth was not uniform, I tailored and overlapped the material to fully cover the top surface. Initially, I intended to cut the overused layer after removing fabric formwork, but interestingly, it was merged quite nicely with completed rammed earth, so I left it as it is.
On the top of a damp proofing course, I compacted unstabilised earthen mixture and finished the top layer with lime-stabilised mixture. Even after the ramming course was completed, the fabric was still wet for a couple of days helping rammed earth’s curing process.
I dismantled fabric formwork after 10 days since the completion of ramming course. Removing it was very straightforward except the bottom layer of the inside. Because the corner of the fabric was bulged hugely due to the absence of vertical timber scaffolding, it was very challenging to remove the horizontal timber frames which were stuck into bulged rammed earth. After removing all timber frames, I uncovered the fabric carefully cutting the sewn edge.
The condition of the used fabric was pretty good. I thought it could be reused for another fabric formwork or other purpose such as a weed barrier. Although there were some small holes caused during the ramming course, it did not cause any problem to produce the outstanding outcome of the rammed earth.
When the fabric was uncovered, the rammed earth imprinted with interesting fabric effects revealed. Not only the woven pattern of geotextile but also natural wrinkle was embedded in the surface of the rammed earth.
The rammed earth displayed a true colour variation given from the earthen mixture from light brown (unstabilised earthen mixture) to light beige (lime-stabilised mixture). The form of the rammed earth exhibited undulated curvatures without any sharp edges except to the top part that was controlled by wooden frames. (I think this will be a flood-resilient feature of fabric-formed rammed earth because curved edges are getting less impact from debris following from floods compared to sharp edges. This flood resilient feature will be demonstrated at my next research project.)
The geometrical outcome of the inside of the rammed earth wall is more interesting. Because the lateral earth pressure was stronger inwards, the level of the undulations of the interior façade was bigger than the exterior, expressing a dynamic fabric-formed surface.
When I completed the construction of Phase 1 on 19th October 2020, the temperature of South Korea started to drop quickly then so it was difficult to start Phase 2 construction. I thought initially that I could finish the fabric-formed rammed earth doghouse within a month, but the production process of the fabric formwork took much longer time than I had expected. I will resume Phase 2 in the spring of next year 2021.