Using rammed earth
The Napa guest house gave us a chance to experiment with rammed earth instead of concrete. I’m going to take you back to the spring of this year (2010) when the project was just getting started. The use of the rammed earth turned out to be a great success. Ian Neilson of Grassi and Associates worked closely with our firm to bring our designs to life. Ian had just been finishing up another project where he used rammed earth, but in less complex fashion. We were trying to evoke a feeling of rock of the earth being cut through to reveal the columns and walls of the structure.
Many projects that use rammed earth are authentic to the method of rammed earth which is typically done in consistent ‘lifts’ which yet, can look a bit like unfinished concrete which doesn’t really play up the significance of the material being utilized in contrast with concrete. As the methods are refined, the trend is toward techniques that evoke natural stratification. So we chose a slightly more complex layering and coloring.
We know that it’s always a trade off between rammed earth and concrete. The increased cost gives you a much prettier result and an average reduction in energy use and carbon emissions.
We turned to David Easton of Rammed Earth Works to provide the expertise and placement of the rammed earth in the forms. For this project the cost hovered around 110 per sq. ft surface area due mostly to the extra labor for the slightly more refined forming and David’s crew for the earth ramming. We had 3 different colors in lifts, set in a stratified or sedimentary pattern we had designed for these structural elements.
Unfortunately the structural engineer was not comfortable with anything less than 1500 psi strength for any of the rammed earth walls or columns. This led to a high cement ratio so it may have been a bit less green, but our overall score for the house was very high regardless (kudos to Eric Rogers at our firm). There were several steel columns in the rammed earth walls.
Whereas reinforcing bar (rebar) is used directly in the rammed earth (just like concrete) large steel is treated differently due to the significant difference in expansion and contraction with temperature fluctuation. Ian had his crew epoxy seal the beams then wrap them in foam board to provide for sufficient slip allowance. I was surpised to find that no release agents were used but rather just the smooth face MDO plywood for forming was used. This prevented the rammed earth from being stained or having inconsistent results. At first glance, it may not seem significant, but the arrangement of steel, rebar and foam board in the form is important so I show a picture of the form just prior to filling.
Let’s say that I want to work in the look, to complement the main rammed earth walls, but I need concrete for a thinner wall, or just can’t afford it in every spot or the placement of the wall is less significant, are there ways to combine our new knowledge with the age old practices with concrete? In response to some concepts that David Easton puts forth: An 18″ wide wall has a 12-hour thermal flywheel effect, that means that the outside temprature takes half a day to migrate through the wall to the inside face. This brings to light the value of such solid masses in green building practices. However, it would be nice to bring some of that beauty to regular concrete and other placements.
As an aside, I also wonder if the substitution of fly ash can achieve a similar reduction in green house gases and reduction in energy use compared to rammed earth. For thin infill walls that we want to do as concrete, is it possible to do some kind of concrete modification to simulate rammed earth or is this just disingenuous rather than authentic to the material? Another possible avenue is thin wall applications over insulation to fit smaller spaces or uses. At this time, rammed earth just can’t be thin, so perhaps onsite methods with modified equipment might be worth exploring. I think both may be possible expansions of the use of rammed earth or similar processes, while bringing the authenticity to the material.
Architects frequently attempt to utilize materials in a more authentic manner, this means that instead of covering up something like a concrete floor or wall, we just polish it and leave it as is and appreciate it for what it is. Done in the right way, it can be really cool. Or perhaps treat it in a way that brings out the better aspects of the material’s appearance and strengths such as throwing in a little colorant into the mix.
This does a couple things, it saves money because you are not putting on additional layers to achieve the same goal, and it should make the assembly more reliable since less complexity is typically better for such purposes. However, some materials just don’t deliver on what many thought would the their strengths. Anybody remember Cor-Ten? It’s a particular metal blend that many in the 70s in the building trades thought would behave well in exterior applications. It’s a lot cheaper than copper and seemed to have the potential to similarly patina over the long term in a visually appealing fashion. One good example comes to mind is where a colleague who used it as a slatted cladding for large gate across driveway.
Our colleague who worked during that era now detests the material because it may have not fallen apart or rusted out, but instead left very visible and unsightly stains on the driveway from all of the oxidized material that sloughed off. Additionally, the clients don’t really want to touch the gate due to the oxidized material. So this brings to light the difficulty in using a material authentically yet not compromise on the actual function. A basic distinction between architects is how they deal with this balance.
I think the rammed earth walls on this project strike a very good balance.
There are newer images of the project on the post named Napa Guest House Update. Images used by request (and permission) of Wagstaff Architects September 10, 2010.