Rammed earth on the rise
Of more than twenty different types of earth construction techniques, rammed earth has been lauded for its durability, sophisticated environmental performance and striking earthen beauty.
Rammed earth construction in South Africa has generally been stigmatised as a substandard and primitive building construction method reserved ‘for the poor’. Yet it is now gaining popularity for community social projects, as well as among wealthier clients. Although a building standard is yet to be formalised in South Africa, the rammed earth industry is established in many African countries, Europe, United States and Australia. This means plenty of expertise and established international building standards already exist and this construction method is now far removed from its primitive roots.
By nature of the material (earth comprising mainly sand, gravel, silt, and clay), the rammed earth construction process is simple while also being labour intensive, which facilitates job creation for disadvantaged communities. Upper market residential and higher profile projects are also revealing the striking beauty and creative possibilities of this building material. As demand for environmentally sustainable alternative building technologies increases, the intrinsic environmental qualities of rammed earth walls – such as high thermal mass, compressive strength and earthen material’s ability to modify humidity naturally – creates healthy living environments. It therefore presents an attractive alternative to carbon-heavy concrete and brick.
Connecting with the earth
A rammed earth wall is made up of a compact mixture of sand, gravel, silt and clay earth. Because of the unique characteristics of each soil in each unique location, the proportions of the mix used for making the wall will differ according to the soil’s clay content. The clay helps to bind the materials. If there is too little clay in the earth available, cement or lime might be used as a stabiliser. The addition of a stabiliser helps to make the wall more water resistant and durable. The ideal percentage of the stabiliser will depend on the specific characteristics of the available soil. Layers of the earth mix (200mm) are then poured into a shuttered frame (usually timber) and compacted either by hand with a tamping tool or with a pneumatic ramming machine. The frame is removed after the compacted mixture has dried and the beautiful rammed earth wall is revealed. The imperfect layers of earth can be arranged to express the natural colours of the mixed earth, and different finishes are available to emphasise the rich sandy textures.
Architect Paul Marais of Simply Sustainable began exploring the process of building with natural materials in 1986 and is currently working on his PhD looking at the feasibility and acceptability of rammed earth construction in Southern Africa, with the aim of using this research to help develop an appropriate South African standard for rammed earth. Marais believes the aesthetic beauty of rammed earth is currently driving its popularity, followed by its thermal properties.
Notably, the majority of the architects and builders who specialise in natural building materials have spent time in rural areas working hands-on with the materials and learning the innovative techniques long practised by those communities. Design cannot be separated from the materials out of which the building is made. Perhaps obvious, yet it is contrary to the practice of many conventional architectural systems.
In parallel to the aesthetic sensibilities of rammed earth, it is also an abundant, cheap and durable material. It is well-suited to the South African context where the climate is relatively hot and dry. Mike Beukes developed his method of stablised rammed earth wall construction called Rammteck in 2011 and has used this technology very successfully on a variety of social community projects across South Africa – notably in the Hoedspruit area in Limpopo province. He says for most projects the sand is sourced from the local area, otherwise it can be found not too far afield, shortening the supply chain. Rammteck uses a 5% cement additive in order to stabilise the wall. The walls, which are usually 300mm thick and very heavy, can reach a hardness of 25 – 30MPa. A double-storey building usually requires minimum wall strength of around 2.5MPa according to international best practice. “With a crew of six to eight people, we can ram a 7m² area of wall per day,” Beukes says. Shutter boards (which can be timber or steel) stay on for 24 hours before they are removed. The wall dries within about two days, during which time it can be patched easily.
Rammteck’s core vision is to go to underprivileged areas and train people to use the technology. Members of the local community volunteer to work for a small stipend, during which time they are trained in basic building construction techniques (including bricklaying, plastering, and tiling, etc.) and receive a certificate at the end. This gives them the tools to become financially self-sufficient in future and some of them have already started their own construction related businesses. Over six years Rammteck has trained 250 people and this number continues to grow. They encourage women to apply for the work so that the whole community is involved.
Beukes thinks one of the reasons for the growing desire to use rammed earth is so that people can go off the grid. The walls insulate water pipes and no additional heating and cooling is required inside the building, and gas and solar are also very easy to install. Rammteck usually builds pitched corrugated iron roofs, which help to protect the walls from moisture while creating the opportunity for rainwater to be harvested. Off-the-grid construction methods ideally suit rural areas where infrastructure is insufficient.
A question of standards
Gaining approval from municipal councils for rammed earth buildings in South Africa can be a challenging task because a local official standard for rammed earth is not yet in place. The construction materials of a proposed building are usually measured against a national standard that specifies basic rules for compliance. When no standard exists, there are alternative methods for gaining council approval but they are more costly and onerous, explains architect and earth building expert Andy Horn of Eco Design Architects.
Having a standard makes it a lot easier to build with a specific technology because it establishes a prescriptive rule that anyone can apply to a project in order to ensure it will be safe to inhabit. Use of the material becomes much more accessible to the general construction market because plans can go through council without questions. There are various earth building standards around the world, adds Horn, which could be used to guide the construction of rammed earth but unfortunately not enough to get through South African bureaucracy. The harmonisation of the standards process known as SADCSTAN (Southern African Development Community Cooperation in standardisation) – in which South Africa has not taken part – has adopted the rammed earth standard of Zimbabwe. There are plans for this to be adopted as an All Africa standard.
The South African Bureau of Standards (SABS) declined to comment on whether rammed earth might achieve an official certification in South Africa in the future. As there are no formal regulations for rammed earth, the National Home Builders Registration Council (NHBRC), which governs the residential market, also does not recognise this building method, explains Beukes. This makes it impossible for clients and builders to attain local bank financing to build with these materials. It is unfortunate that government bodies are resisting the opportunity to lead the way in enabling the industry to easily build with sustainable technology that reduces harmful impacts on the environment.
One of the main concerns preventing the approval of a standard, is that the structural strength and stability of rammed earth construction is presumed to be below par. There are alternative ways to achieve approval for innovative materials if they do not have a standard – one is through Agrément South Africa – but these must undergo stringent testing for structural stability and many products have failed the tests when measured against what Horn suspects are unrealistic structural requirements. When asked whether having a standard would encourage the market to use natural buildings as an alternative to concrete and brick, SABS also declined to comment.
Suspicion remains as to whether a bias for conventional building materials (like concrete and brick) exists, says Horn, because some of the people on the technical committees represent those industries with vested interests. (see earthworks, April/May 2015, Between a Rock and Hard Place, issue 25, pp. 117 – 126). “The addition of cement or other stabilisers to rammed earth produces walls that meet the standard for masonry or concrete walls [such as Rammteck's cement stablised walls], although there is a view that lime stabilisation produces walls less prone to cracking and lime is better at combining with the existing clay in the soil,” Marais adds. Horn argues that walls stabilised with cement are not strictly “rammed earth”, and thus it is preferable to have a unique standard for “normal” rammed earth walls.
Horn says it’s easy to overcome any structural concerns in the construction design of a rammed earth building. Rammed earth is strong in compression but weak in tension, so he advises adding a ring beam at the top of the walls, which can be either timber or concrete that “bond” the tops of the walls all round, makes the building much more stable against earthquakes and cracking from possible lateral forces or from any point loads. You also need to build proper foundations for the rammed earth walls. He says that foundations should be built with a solid masonry material like conventional fired brick, concrete, urbanite, or stone, up to a level of at least 350mm above the natural ground level to protect against splashing. “An earth building should be built with a good pair of boots and a good hat, i.e. a plinth and roof,” he says.
For a house on Monaghan Farm in Johannesburg, completed in 2014, Marais designed earth foundations that were stabilised with 2% cement and 5% polymer. He monitored the performance of these foundations as part of a PhD research project through Cardiff University, Wales. The research paper discusses the use of polymer stablised earth foundations and their benefit for producing lower CO2 emissions (as compared to concrete). It concludes that the foundation methodology has performed satisfactorily to date but further research will optimise the technology and streamline the construction process.
Although the stabilised earth is more durable than conventional rammed earth, Horn advises against using earth as a foundation material altogether.
Scaling the heights
Although South African legislation still has progress to make with regards to fully integrating alternative and natural building materials and techniques into the mainstream building industry, it seems market demand is now pushing that forward, albeit at a slow pace. It’s clear that despite being an ancient material, current methods of rammed earth construction can achieve a level of sophisticated design and structural integrity that matches, if not exceeds, conventional building materials. We need not search further afield than nature to find the answers.
By Mary Anne Constable
See earthworks Issue 31, April-May 2016 for the full feature, which includes the project examples: Nourish Community Library, Limpopo, South Africa (2014 by Rammteck); OR Tambo Narrative and Environmental Centre and Caretaker’s House, Gauteng, South Africa (2012, with consulting by Eco Design Architects); and Monaghan Farm house, Johannesburg, South Africa (2014, by Paul Marais).