Rammed earth construction presents a fascinating intersection of ancient building practices and modern environmental considerations. Here’s a comparison between raw earth and stabilized rammed earth construction:
Raw Rammed Earth:
This technique involves compacting a mixture of silt, clay, and gravel into forms, without the use of modern binders. It has been used for centuries, especially in regions where these materials are readily available. The absence of synthetic additives makes raw rammed earth more environmentally friendly, in terms of embodied energy. And the material is often locally sourced, which further reduces the carbon footprint required to haul it to the build site.
Raw rammed earth has two shortcomings — water resistance and imprecise structural definitions. On one hand, raw rammed earth is more susceptible to weathering due to its lack of water resistance. Historically, this has been mitigated by wide eaves and protective coatings.
On the other hand the structural calculation variable is more of a modern problem, as building codes typically require calculable values for compressibility. Without stabilization, the material’s strength and durability can vary — making it tricky to meet modern code requirements.
Stabilized Rammed Earth:
By adding a binder, rammed earth improves in terms of strength, durability, and water resistance. Historical binders like casein, tar, lime, and straw have also been used, adding to the material’s versatility. But modern construction uses a mixture of 5% concrete (or cementitious additive) to make the material more predictable, easier to work with, and more parallel to a mainstream construction material. This does decrease the embodied carbon advantage slightly.
The use of silica-based admixtures or other modern additives can enhance the material’s resistance to water.
Finally, admixtures can alter the color of the rammed earth, providing varying shades and hues in the layer lifts giving a complex resulting finish.
Practical Challenges:
Rammed earth, whether raw or stabilized, is not a mass-produced product with standardized properties. So it’s difficult to provide the uniform environmental performance data required for Environmental Product Declarations (EPDs), certifications that are becoming a required part of bids and procurement. As a result, rammed earth faces a hurdle in being specified for projects with strict sustainability criteria. In Tim Krahn’s excellent work “Rammed Earth Construction” (2019) Krahn has an insightful observation that this is a difficulty more broadly for efforts in sustainability that are outside the EPD process. I highly recommend Krahn’s work for anyone interested in the topic, his depth of knowledge, experience, and writing skills make this the essential resource on the topic that I’ve run across.
Cost is an obstacle to adoption as well. On the face of it there is nothing particularly costly about the techniques or material. It’s basically dirt. But the construction is often more expensive than conventional methods due to the need for specialized skills and the unfamiliarity of the technique among builders.
Conclusion:
While stabilized rammed earth offers improvements in terms of structural integrity and durability, the question remains whether it simply becomes “packed concrete” with a lower carbon footprint (Krahn, 2019). The balance between traditional, low-impact materials and the need for modern performance standards continues to be a key question for sustainable architecture. Rammed earth is a test case for this puzzle.
Here is a link to some recent examples of rammed earth projects.
And finally, an affiliate link to Krahn’s book:
