Rammed earth is often hailed as a low-carbon method of building construction, but some experts are beginning to question its sustainability. Amy Peacock reports.
“People are using rammed earth to sell something that is essentially concrete,” said Rowland Keable, chief executive of the charity Earth Building UK and Ireland and founder of Rammed Earth Consulting.
“I’ve seen rammed-earth projects in the UK that make it into glossy architectural magazines and you think, ‘Wow, that’s a great-looking building,'” added Tim Coleridge, head of sustainability and adaptation programs at the Center for Alternatives. Technology (CAT).
“But [we’re] I’m not talking about the cement that made it possible.”
Rammed earth is a construction method in which soil made up of a combination of aggregate, sand, silt, clay and gravel is packed into formwork and compressed, leaving a solid wall when the formwork is removed.
It is sometimes constructed in its raw form, but is often stabilized with a binder such as cement to improve its strength and durability.
The ratio of cement in rammed earth constructions varies, but some experts say it can be comparable to the amount used in concrete, which is widely considered one of the most environmentally damaging building materials.
Cement-stabilized rammed earth comparable to concrete
Concrete is made by mixing aggregate with cement, and it is a highly energy-intensive cement production that generates large amounts of CO2.
“Once you’re using cement at 5 or 10 percent in compacted soil, it’s not that much better environmentally than using concrete block,” Coleridge said.
“It doesn’t seem like a lot, but it’s a similar proportion of cement to a standard concrete block, so the impact of your local, low environmental impact material has gone from very small to almost comparable to conventional, mass-produced concrete block,” he told Dezeen.
Keable echoed those concerns, adding that he believes once cement is added to soil, it should no longer be considered a soil building material.
“Talk to an organic farmer and they’ll say, ‘We’d love to spray it all with stuff that kills all the little bugs that eat our crops, but then we wouldn’t be organic and that’s our USP’,” he said.
“Until there’s a general awareness of this, then I’ll always have a chip on my shoulder.”
James Souter, director of British engineering firm Elliot Wood, has used cement-stabilized rammed earth in some of the projects he has worked on, including Bushey Cemetery in London.
A raw rammed earth wall in the UK would normally need a large overhang to protect it from rain erosion, but at Bushey Cemetery the cement content meant that monolithic walls with a textured, patterned rammed earth surface were achieved.
“It’s an aesthetic decision”
“It’s primarily an aesthetic decision,” Souter said. “The best way to improve the properties of compacted soil is to stabilize it with cement.”
For Souter, concerns about the cement content of rammed earth are negligible compared to the amount of concrete produced in the UK.
“If you make a 0.1 percent improvement in the concrete mix in all the projects that are built with concrete in the country, it will make a bigger difference than if the rammed earth went from something that was comparable to other materials to this magical material. it cost no carbon,” he said.
“The amount of rammed earth actually used in this country is insignificant, so it won’t make any difference whether it’s 3.5 percent cement or six percent cement. The volume is so small it doesn’t even rearrange.” sunbeds on the Titanic.”
In the UK, it is technically possible to build modern rammed earth buildings without stabilizing the material. Coleridge singled out the Visitor Center at Grimshaw Architects’ Eden Project and the WISE Building at CAT in Wales, a seven-metre-tall drum-like unstabilised rammed-earth structure built from soil sourced from a quarry some 50 miles away.
“It’s a wonderful sustainable material,” he said. “At the end of the life of a WISE building, you can rewet the material with a hose and it will hit the pile.”
“You can mix it up and use it for another rammed earth building, or it’s completely harmless, you can just literally put it back into the soil.”
Although ramming soil into solid building elements is an ancient construction method that is now facilitated by modern machinery, it is still laborious compared to other construction methods.
In addition, many countries, including the UK, do not have building standards for rammed earth structures – so cement is often added to appease building authorities and insurance companies.
“The packed dirt and all the earth building techniques are really old, but they stayed on the shelf,” Keable said.
“Regulations have been made for concrete and other materials, so compacted soil has a lot of catching up to do,” he continued. “It’s a bit of the wild west and we actually need to find a checkpoint.
“Lack of understanding” about choked earth
“There are no standard details, so building with rammed earth requires a lot of bespoke work, which often comes with additional challenges around building inspection, insurance, even lending if you have to get a mortgage on the property,” added Arup sustainability expert Sean Lockie.
“Insurance companies and mortgage lenders often have a very narrow view of what they are prepared to insure and lend against.”
Coleridge explained that where there are no standards for compacted earth to refer to, architects often have to use building codes for similar materials, including concrete, instead.
Building codes and standards for building construction would help popularize the material in its unstabilized form and raise awareness of its properties, he argued.
“If there isn’t a local code for earth building, then designers have to defer to something else, like masonry or concrete standards,” Coleridge said. “They might find they’re forced to add five or even 10 percent cement just to satisfy insurance companies or local authorities, and they’re treating it like concrete masonry to sign off.”
“People use cement in the ground because of a lack of understanding or concern about the material’s performance,” Coleridge continued.
“It’s not going to be right for every situation. Having earth building codes allows for confidence building and allows local building departments to understand the properties of the material.”
In the late 1980s, Keable began setting construction standards for rammed earth construction in countries across Africa that were later adopted as national standards. This included guidelines for wall thicknesses, material compaction, formwork and material mix.
“By 2012, 15 countries had a national standard for rammed earth, and by 2016 it covered the entire continent,” he said.
He has published similar guidelines in the UK, which have not yet been adopted as a national standard, but Keable hopes this can be achieved with more support from the rammed earth industry.
“The big problem in the construction industry is that all the materials that we know are problematic are very well funded, and all the ones that are waiting for wider adoption have very little funding,” he said.
“We just don’t have the craftsmanship”
Alternative stabilizers to cement can be mixed with the soil to improve its performance. Lime is a common alternative, and cob construction involves the addition of a fibrous natural material such as straw or horsehair to increase tensile strength.
There are also mass-produced earth building blocks that aim to make the sustainable natural material less labor-intensive.
However rammed earth has not yet emerged as a popular choice among architects and few contractors in the UK have experience of working with it.
“If you compare rammed earth with things like wood, hemp bricks and undergrowth, a lot of these materials are starting to think about how they can be produced and distributed on a large scale using modern construction methods,” Lockie said.
“At Arup, we see this as an emerging material, but it will address a range of issues such as its ability to scale, who is investing in its availability, what insurers think of it, what is the maturity of the contracting entity – there are not too many suppliers who they would know so well,” he continued.
“Routing rammed earth is more difficult than, for example, fair-faced concrete because the experience in the industry is not there,” agreed Souter. “We just don’t have the processing experience in this country.”
Photograph by Lewis Khan unless otherwise noted.
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