Author Interview with Tim Krahn, Rammed Earth Building Expert

by: EJ on 01/31/2019
Posted in: Author Events

Tim Krahn, author of Essential Rammed Earth Construction: The Complete Step-by-Step Guide, is our guest today.  Rammed earth construction is a relatively new construction technique in the modern world but has been used for millennium by ancient cultures.  Tim in bringing this ancient building technique into the world of modern architecture.

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1) In your experience, how likely is it that a builder will find suitable material to build a raw earth mix on their property? If you use a stabilized earth mix, does that make it easier to use local materials?

I have only worked with one client (out of several dozen) who was able to use site materials. That said, it's not just a matter of the material being present, but the overall site and master plan. You may not want to cut a large scar into a hill or create a depression where the most suitable sub-soils for rammed earth exist on your site.However, all of the projects that I have worked on were built with aggregate mixes from quarries within 25 km of site, most much closer than that.

Using a stabilized mix does help increase the range of suitable soils, yes.

2) What are the main stabilizers for rammed earth, and their brief pros and cons?

Aside from clay and its ability to create large matric suction forces, the most common chemical stabilizers for rammed earth are Portland cement, hydrated Lime, calcined clay, ground blast furnace slag, and fly ash - not necessarily in that order.

Portland cement is the most commonly available and generates the most compressive strength per units added. It is almost indispensable for freeze-thaw resistance in walls that will be exposed to the kind of wet-dry/freeze-thaw cycles that are common in most of Canada and the northern US. However, at the same time it reduces permeability, decreases ductility, can cause significant shrinkage and has the highest embodied energy and carbon footprint of the binders listed here.

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Hydrated Lime is also relatively common - masonry supply centers will stock it regularly. It doesn't 'cure' the same way that Portland cement does - it's a much slower set (carbonation versus hydration), and it won't reach the same compressive strengths per units added. It will allow you to take advantage of fine limestone screenings in your soil mix, however, and it can be enhanced by combining it with calcined clay, fly ash or ground blast furnace slag. The main disadvantage of Lime is a relatively high embodied energy and carbon footprint - less than Portland cement, but still up there.

Calcined Clay, Fly Ash, Ground Blast Furnace Slag - these are all considered by-products, so that can mitigate their embodied energy and carbon footprints under certain accounting schemes. It doesn't change that they are produced in high temperature environments that cost a lot of energy and emit significant carbon - but if those costs are offset by the primary production (expanded glass aggregate, coal fired energy and steel production, respectively) there is a certain amount of efficiency gained.

In all cases, these stabilizers need a certain amount of Portland cement or Lime in order to work as a chemical binder. They can significantly reduce the amount of primary stabilizer in a rammed earth mix, and generally increase ductility while reducing shrinkage. These stabilizers have a range of effects on permeability; testing is being done, but mostly on concrete mixes, and mostly with fly ash and ground blast furnace slag.

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3) The accuracy of the field tests seems like they may be dependent on the experience of the builder. Do you have any tips for first time builders that might help increase accuracy of certain tests like the ball drop or slap tests?

Repeatability is key. Have the same person in your team perform the tests every time, and do as many as you can. Practice makes perfect (or helps you get close anyway). There is no silver bullet that will make a person an expert other than experience.

4) Are there any contexts or places where rammed earth isn't a suitable option? For example, is it suitable for high seismic areas such as the Pacific coast?

Unreinforced, raw rammed earth is not likely to be a good fit in high seismic (not to mention very wet) areas such as the Pacific coast. But there is nothing stopping a person from building with reinforced stabilized rammed earth in high seismic areas, or even areas with high precipitation and freeze-thaw cycles. 

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Stabilized rammed earth buildings have been constructed on the Pacific coast in Canada since the 1990s and the US since the 1970s. They have proven very durable, with many in northern California withstanding several large seismic events brilliantly.

5) How does the embodied carbon of rammed earth compare with say conventional stick framed walls over the lifetime of the building?

Using the 1000 ft2 example home that Chris Magwood proposes in Making Better Buildings: A Comparative Guide to Sustainable Construction for Homeowners and Contractors, we can attempt an apples to apples comparison.

If we assume both wall systems will be insulated equally and have the same doors and windows installed, along with any electrical and plumbing fixtures, we can eliminate these materials from our comparison.

For our purposes, we'll use a conventional stick framed wall with 2x6 studs at 16" o/c covered with 1/2" plywood sheathing. For a reasonable durability comparison, we'll clad the exterior with brick. For the interior wall finish I'll assume drywall (gypsum wall board), and we'll need some kind of air barrier, which I'll assume as 6 mil polyethylene. Paint on the interior is another source of embodied carbon, and I'll assume three coats of latex for that.

Conventional wall: 

Component

kg CO2e

 

Studs 257
Sheathing 550
Brick 2530
Polyethylene sheet 38
Gypsum Wall Board 284
Latex Paint 83
Total 3742

For the rammed earth wall, we'll assume an 8" interior wythe and a 6" exterior wythe with 10M rebar in both wythes at 48" o/c. The main variation is the amount of stabilizer used in the mix - we'll look at 8%, 5% and raw rammed earth (without rebar).

Rammed Earth wall, 8% stabilization content

Component

kg CO2e

 

Rammed earth mix 6854
Rebar 163
Total 7017

Rammed Earth wall, 5% stabilization content

Component

kg CO2e

 

Rammed earth mix 4978
Rebar 163
Total 5141

Rammed Earth wall, no stabilizer

Component

kg CO2e

 

Rammed earth mix 1960
Total 1960

* all data from Making Better Buildings by Chris Magwood and the I.C.E. database, volume 2.

 

It's immediately apparent that the bulk of the embodied carbon in rammed earth construction comes from the stabilizer. However, a great deal of durability comes from that same set of ingredients. The raw rammed earth building would require some kind of plaster finish in order to prove durable, and that would have to be re-applied or otherwise maintained during the lifespan of the walls. But if an earthen plaster is used, it wouldn't be a significant addition of embodied carbon, since we are already considering 14" total thickness in these walls.

Operational carbon - this is a very difficult thing to quantify, since we don't know the HVAC system or the lifestyle choices of our occupants. That said, we do know that the rammed earth house will have significantly more thermal mass, and should not require any cooling energy. How much the heating load is reduced is difficult to say with any accuracy - but I think it's reasonable to assume that the rammed earth home will use at least 30% less energy per year. How many years it will take to 'catch up' to the wood framed house depends on the total amount of energy used.

How many years each house will last is another question. Properly built stabilized rammed earth will definitely last longer than a stud framed wall. As soon as the wood framed wall is replaced, the rammed earth wall produces less carbon, even if we assume the highest percentage of stabilizer. If the rammed earth wall lasts for more than two generations of wood framed walls, it starts to look very favorable.

Question from book contest participant:

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6) Living in Ontario, I wonder how much (if any) Portland cement I have to add to my mix to achieve a stable rammed earth construction? How can I run test batches that will allow me to determine not only compressive strength but also to test the effects of weathering?

It is my experience, along with that of many rammed earth and compressed earth block builders here in Canada, that at least 5% binder (by weight) is necessary to achieve a durable exterior wall. That binder need not be completely made up of Portland cement - you can use hydrated lime, calcined clay, ground blast furnace slag or fly ash (or a combination of them) as effective binders. I cover binders briefly in the answer to another question in this contest, and more completely in the book.

Test samples can be created to test for weathering and durability, as well as compressive strength. Weathering/durability testing can be done effectively with water and freezing outdoor temperatures (for freeze-thaw), a wire brush (for erosion) and repetition.

It is not a simple matter to conduct a compressive strength test on your own. The most common tests that laboratories carry out is a crush test on a 6" diameter x 12" tall cylinder. They are not terribly expensive - between $35 and $50 per crush. Details of how to make samples and a testing plan are covered in the book.

Find out more about building with rammed earth in Tim's new book, Essential Rammed Earth Construction: The Complete Step-by-Step Guide

 

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