Rammed Earth Vs. Adobe

Of all the building materials at our disposal, what do we have the most of? Actually, let me rephrase that. Regardless of where on the planet you live, what material – ANY material you can think of – is there an unlimited amount of? – Earth right? I don’t think we’ll be running out of “dirt” any time soon. It’s like making a sand castle on the beach…you ain’t gonna run out of sand no matter how big your castle is.

Some of us in the more modern, technologically advanced parts of the world, may associate earthen houses with the poorer, less fortunate populations of say, Africa, parts of Asia, and maybe some islanders out in the Pacific. But as some may know, earth has been an extremely vital building ingredient for thousands of years, in the making of not only mud houses with thatched roofs, but considerably larger, more “elegant” structures.

For example, the adobe brick is one such building block – literally – that’s been in use to construct large and magnificent structures, such as the Great Mosque of Djenne in Mali and the citadel of Bam in Iran. These and other simpler structures as well, are testimony to the versatility of earth-based building materials. Earth is just another very viable source of raw building material.

In contrast, wood must be felled and usually ripped in order for it to be effectively used in construction – a very labor-intensive process and requires at the very least a saw blade and/or an axe. Looking at it through the eyes of someone with only their bare hands, it’s easy to understand the advantages of earth-based construction – although it can be just as labor-intensive.

Thermal Benefits to Earth-based Structures

There is a reason why you see adobe and other earthen structures primarily

in desert or semi-desert regions such as North Africa, some parts of North and South America, as well as the Middle East. Similar to concrete, earthen structures have inferior tensile properties, which requires them to be thick. This thickness provides a massive heat reservoir that absorbs heat all day long and and emits it at night when the temperature drops.

By the time the heat is depleted from the “reservoir”, the sun’s peeking out from behind the eastern horizon, ready to start the cycle again. But too late for the scorching sun, the house is stone-cold from the long, cold night, and will take another 12 or so hours of sunshine to penetrate through to the interior. The incredible thermal mass of earthen structures creates a natural temperature control system.

Rammed Earth

Funny name, impressive results. Rammed earth is what results from the process of pouring a damp mixture of earth, with appropriate amounts of gravel, sand, and clay into formwork to construct walls. Sounds simple enough no? The key in making this method work though, is to seriously compact the mixture into the formwork. Pour about 20

Rammed earth wall - note the layers

centimeters at a time and through “ramming”, reduce the height by 50%.

Although similar to the more popular concrete, rammed earth does not need a “curing time”. Formwork can be removed upon completion of the wall, and additional surface work should be done at this point if desired. All surfaces exposed to rain should be sealed to prevent premature erosion. You should know that ramming the earth by hand is a very exhausting task, so I suggest getting as many hands as possible, and possibly some pneumatic tampers – if that’s an option.

As with concrete, rammed earth gets stronger in tension when reinforcement is used. Theoretically, anything can be used, including wood and bamboo, and even straw. The incorporation of reinforcement in some form is especially recommended in earthquake zones. A simple way to think of rammed earth is a drier form of concrete used exclusively for walls. (See What is Concrete Formwork.)

Adobe Brick

Comparatively speaking, this is more of a household term. Adobe bricks are easy Adobe bricks drying in Sunto make, don’t require any heavy machinery, and provide versatility in design and construction due to its unit size. Adobe bricks are a mixture of sand, clay, water, and some organic reinforcement such as straw or sticks. Constituent ingredients can be found almost anywhere in the world and they do not require much in the way of skill to make.

Once made, the wet mixture is pressed into frames or molds to dry. They can be removed from the molds quickly to dry to free the molds for continued brick-making. In this way, bricks can be made fast and building can progress quickly. Drying bricks in direct sunshine in hot, dry climates can cause cracking. If this is a problem, dry them in the shade.

Knowing how to frame and dry the bricks is great, but you also need to know the correct clay-sand ratio so you can make quality bricks. Conducting a simple soil content test will ensure this.

A Simple Soil Content Test for Adobe Brick-making

Take a transparent jar and place a sample of the soil you have to work with. Put water in the jar to where the soil sample is completely dissolved. Screw the lid on and shake the jar vigorously for about a minute. Now place the jar of muddy water on a flat

surface where it won’t be disturbed for about a day. After a day or so has passed, you are ready to “analyze” the test results.

Larger, heavier particles will naturally settle to the bottom first, with sand next, and silt coming after. Clay and other organic matter will most likely take another day or two to fully settle. This miniature “geologic column” will provide you with the data needed to determine whether or not your soil is suitable for the making of adobe bricks.

The New Mexico US Extension Service recommends a mix of no more than one-third clay, no less than half sand, and never more than one-third silt. Use this formula to ensure your mixture is suitable for the task, and if it is, you’re good to go. Add one-sixth straw to the mix and start framing out your adobe bricks. Mortar is simply the same mix minus the straw.

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