Structural Rammed Earth

April 5, 2021 Jeff Dejong

Something old, something new.

Structural Rammed Earth has been a long time interest. Decades long in fact. Rammed earth construction is older than Jesus. Even though rammed earth construction still continues today in much of the world, some of the earliest examples on the Middle East date from 9,000 BCE and in China 5,000 BCE. There are very few places in the world that do not have a history of rammed earth. In my mind, the traditional rammed earth construction was an important first step to modern concrete.

Traditional Rammed Earth

Rammed earth construction is a building technique that involves compacting layers of earth or soil mixed with a small amount of cement, water, and sometimes other materials such as lime, to create solid, durable walls. The technique has been used for thousands of years and is still used today in many parts of the world. The addition of cement is relatively newer introduction in rammed earth’s history.

The process of rammed earth construction typically involves the following steps:

Site preparation: The building site is leveled and compacted, and a foundation is laid.
Formwork: Wooden or steel forms are installed to contain the rammed earth mixture and create the shape of the walls.
Mixing: The soil or earth is mixed with water and a small amount of cement to create a moist, cohesive mixture.
Laying: The mixture is placed into the forms in thin layers, and then compacted using a tamper or a mechanical rammmer.
Repeating: The process of mixing, laying, and compacting is repeated until the desired height of the walls is achieved.
Finishing: The surface of the rammed earth walls can be finished in a variety of ways, such as plastering, stucco, or painting.

Rammed earth construction is a sustainable and environmentally friendly building method, as it uses locally sourced materials and has a low carbon footprint. Additionally, rammed earth buildings are energy efficient and have excellent thermal mass, which helps to regulate the temperature inside the building.

It's important to note that Rammed earth is not suitable for areas with high water table, heavy rainfall or high humidity. Additionally, it's also important to consult with a professional or an expert in rammed earth construction before proceeding as different soil types and mixes may have different effects on the walls and its longevity.

The size of rammed earth walls can vary depending on the design and purpose of the structure. However, traditionally, rammed earth walls are typically between 18 inches and 36 inches thick. The width of the wall is determined by the height of the wall and the load-bearing capacity of the soil mixture. It is important to consult with a professional or an expert in rammed earth construction to determine the appropriate size and thickness of the walls for a particular structure. It is also important to note that the walls should be thick enough to provide sufficient thermal mass, structural integrity, and fire resistance.

Historic Rammed Earth Recipe

Historic rammed earth recipes vary depending on the region, the time period, and the intended use of the structure. However, traditional rammed earth typically used only soil, water and sometimes lime. Cement was not used in traditional rammed earth as it wasn't widely available until the 19th century.

A common recipe for traditional rammed earth is as follows:

Soil: The soil used in traditional rammed earth construction should be a mixture of clay, silt, and sand. It should be free of organic matter, debris, and large stones. The soil should be screened to remove any large particles and to ensure that the soil is well-graded.
Water: The amount of water added to the soil mixture should be sufficient to create a moist, cohesive mixture that can be easily compacted. Typically, the water content should be between 10% and 15% of the dry weight of the soil.
Lime : Lime was sometimes added to the mixture to improve its workability, and to reduce cracking and shrinking. The amount of lime used can vary, but a common ratio is between 5% and 10% of the dry weight of the soil.

It's important to note that the recipe can vary depending on the type of soil, the local weather conditions, the structure's intended use and the desired strength of the finished walls. Additionally, it's also important to conduct compressive strength test on the soil mixture to ensure that it meets the required standards.

It's also important to consult with a professional or an expert in rammed earth construction before proceeding as different soil types and mixes may have different effects on the walls and its longevity.

The ratio for traditional rammed earth construction is as follows:

Soil: 80-90%
Water: 10-15%
Lime : 5-10%

It's important to note that the ratio can vary depending on the type of soil, the local weather conditions, the structure's intended use, and the desired strength of the finished walls. Also, it's important to consult with a professional or an expert in rammed earth construction before proceeding as different soil types and mixes may have different effects on the walls and its longevity. There should be just enough water in able to make a ‘snowball’ consistency.

Modern Rammed Earth Recipe

A modern rammed earth recipe can include a variety of ingredients, depending on the type of soil, the local weather conditions, the structure's intended use, and the desired strength of the finished walls. However, a common recipe for modern rammed earth is as follows:

Soil: The soil used in modern rammed earth construction should be a mixture of clay, silt, and sand. It should be free of organic matter, debris, and large stones. The soil should be screened to remove any large particles and to ensure that the soil is well-graded.
Water: The amount of water added to the soil mixture should be sufficient to create a moist, cohesive mixture that can be easily compacted. Typically, the water content should be between 10% and 15% of the dry weight of the soil.
Cement: Cement is often added to the mixture to improve its strength and durability. The amount of cement used can vary, but a common ratio is between 3% and 5% of the dry weight of the soil.
Lime: Lime can also be added to the mixture to improve its workability, and to reduce cracking and shrinking. The amount of lime used can vary, but a common ratio is between 5% and 10% of the dry weight of the soil.

A common ratio for a modern rammed earth recipe is as follows:

Soil: 80-90%
Water: 10-15%
Cement : 3-5%
Lime : 5-10%

It's important to note that the ratio can vary depending on the type of soil, the local weather conditions, the structure's intended use, and the desired strength of the finished walls. Also, it's important to conduct compressive strength test on the soil mixture to ensure that it meets the required standards. It's also important to consult with a professional or an expert in rammed earth construction before proceeding as different soil types and mixes may have different effects on the walls and its longevity.

The Evolution of Rammed Earth

Modern rammed earth seemingly has evolved into a concrete hybrid, and many of the advances in concrete seemingly also apply to rammed earth.

The Original Roman Concrete

Roman concrete, also known as opus caementicium, was a type of concrete used by the ancient Romans in the construction of buildings, roads, bridges, and other structures. It is a type of concrete that was made using a mixture of volcanic ash, lime, and water. The Romans discovered that by mixing these materials together, they could create a strong, durable, and long-lasting building material that could be shaped and molded into a variety of forms.

One of the key features of Roman concrete is its ability to harden over time, even underwater. This was achieved by adding a volcanic ash called pozzolan to the concrete mix, which reacted with the lime and water to form a mineral called tobermorite. This mineral is responsible for the strength and durability of Roman concrete, allowing it to withstand the test of time for centuries.

Roman concrete was widely used throughout the Roman Empire for a variety of structures including the Colosseum, the Pantheon, aqueducts and many other monumental structures. Many of these structures still stand today, and the concrete used in their construction is as strong and durable as the day it was poured.

Roman concrete technology was lost for centuries, but its rediscovery has recently sparked renewed interest in this ancient building material. Modern scientists have been studying the properties of Roman concrete, and many believe that it could be used as a model for the development of more sustainable and environmentally friendly concrete.

Roman Concrete Recipes

The ratios used for Roman concrete were different from those used in modern concrete. According to historical records, Roman concrete was made from a mixture of volcanic ash, lime, and water. The ratio of these ingredients varied depending on the specific application and the availability of materials in a certain region.

A common ratio for Roman concrete is:

Volcanic ash: 30-40%
Lime: 30-40%
Water: 30-40%

It's important to note that Roman concrete also included small to medium-sized chunks of volcanic rock called tuff, which provided added strength and stability to the concrete. Additionally, some sources also mention the use of pozzolanic materials, such as crushed bricks or pottery, which helped to improve the durability and strength of the concrete.

It is also important to note that the ratio of ingredients varied depending on the intended use, the local resources, and the availability of materials. Roman engineers and builders had a good understanding of the properties of the materials they were using and adjusted the ratios accordingly.

British Concrete and James Parker

British concrete refers to the type of concrete used in construction in Great Britain. The history of concrete in Great Britain dates back to the Roman period, but it wasn't until the 19th century that concrete began to be widely used as a construction material.

In the early days of British concrete, it was made using a mixture of Portland cement, water, and aggregate materials such as gravel or crushed stone. However, the quality of the concrete was often inconsistent due to variations in the ingredients and the lack of standardization in the manufacturing process.

Modern concrete as we know was essentially invented in the 1850’s (Portland cement (hydraulic lime), aggregate, sand and water-and modest amounts of gypsum), which is an adaptation ‘Roman Concrete’, patented in 1796 by James Parker in England. James Parker’s patent was based on the original Roman recipe 150 BCE (un-clinkered slack lime, aggregate, volcanic ash, alumina and or silica).

The recipe for traditional rammed earth walls used to be 30-40% clay, with the remaining 60-70% being comprised of aggregate and sand. Aggregate design is not discussed here, but is a key part of the structural design. Also, not all clays are equal. There are adverse clays that are plastic (expand in the presence of water). Traditional rammed earth builders were in touch with the local materials and recipes varied by region. Clay is the primary binding agent in rammed earth.

Water ratios are also a consideration. As with modern concrete, traditional rammed earth works better with the correct ration of water-but excessive hydration will weaken each product, cause uneven settling, shrinkage, cracking. Excessive water creates a weaker product. Rammed earth is better on the dryer side and ‘rammed’ into the formwork-hence the name.

Rammed earth has quietly developed quietly alongside of concrete. Over time, modest amount of portland cement were added. Many recipes refer to it a ‘stabilizer’.

We have been experimenting with our own recipe and treating it as a family secret. We believe from trials that it is a premium product. We believe we can achieve between 32-48 mpa (4640-6960 psi). More importantly, it has outstanding aesthetic possibilities.

Stay Tuned.

This information was gained was a collaboration of many sources. My newest understandings are complements of the Sirewall Group.