Why Not Concrete / by Watershed Materials

Although I’ve been asked this question a hundred times, by engineers, building officials, concrete contractors, and other skeptics, I’m always a bit unprepared for it.

I first started building with rammed earth nearly forty years ago in a quest for low-cost construction solutions and with a certainty that sourcing “free” material from the site had to be economical.  Over the years, as code compliance and client preference compelled us to achieve higher strengths and more uniform wall quality, we found ourselves forced to import quarry products and stabilize with higher cement ratios.  In effect we were pulled by market forces towards a product that was indeed akin to concrete.

Hence my difficulty with the question.  Is rammed earth less expensive than concrete?  Does it represent a reduction in carbon footprint?  Is it stronger or more durable?  Is it less expensive to heat and cool?  Does it improve indoor air quality?  Is it healthier?  Is it more attractive?  Is it better for the planet?

First, rammed earth is not necessarily less expensive than concrete.  Even though the forming systems for the two materials are similar and take more or less the same man-hours to erect, layering and compacting rammed earth into the form takes considerably more labor and equipment than pouring and vibrating concrete.  The only savings possible are a reduction in aggregate and cement costs.  To achieve these savings a mix design must be developed that utilizes a major portion of either on-site or other free mineral soil and a minimum rate of stabilization. 

Rammed earth can represent a much lower carbon footprint than concrete.  If the soil for the walls is close to the project, then transportation fuel consumption will be low.  If careful formulation allows design strengths to be met with 7% or less cement, then a big savings in CO2 will accrue.  The question of design strength is another challenge altogether.  Where some structural engineers are comfortable designing one and two-story rammed earth walls with an f’c of 600 psi, other engineers specify strengths as high as 1500 psi.  A design strength of 600 psi can be attained in an ideal soil blend with 1-1/2 sacks of cement per yard, but it can take up to 3-1/2 sacks to get 1500 psi.  What this shows is that engineering design plays a big role in carbon reduction.

Is rammed earth stronger or more durable than concrete?  The answer is clearly no, but the question should be: is rammed earth strong and durable enough for its intended purpose?  If the soil is selected correctly and the wall built properly, then rammed earth meets all the required performance standards and will provide decades of serviceability with little or no maintenance.  Rammed earth has an inherent beauty that transmits a warmth and natural character very different from concrete. 

Rammed earth walls are typically much thicker than a concrete wall, which makes them much more effective at controlling indoor temperature fluctuations.  An 18” wide wall creates a 12-hour thermal flywheel - outside temperatures take half a day to migrate through the wall to the inside face.  This means a mass wall balances out diurnal temperature swings.  In most climates and with proper exterior shading, this makes for no cooling loads.  Heating loads vary considerably based on orientation and building design.  Heating and cooling efficiency are big contributors to energy consumption and carbon footprint reduction. 

Finally, the questions about whether rammed earth is healthier for the occupants or for the planet.  Many people, especially in Europe, believe that clay in an interior wall works to maintain optimum indoor humidity, which in turn results in improved indoor air quality.  Certainly, natural earth walls are more healthy than materials that may be off-gassing glues, paints, resins, or other chemicals.  There are no factories exposing workers to toxins.  Rammed earth walls create quiet spaces that resonate solidness, which can be perceived of as an increased sense of comfort and well-being.  As for the impact on the planet, rammed earth uses fewer resources, both in construction and over the life-span of the house.