A few thoughts on “Compressed Earth Blocks” (CEB)

Kurt Rhyner P. (Architect, PhD, Professor)   

Earth is one of my preferred construction materials, and for centuries it has been the best option for millions of people in most countries around the globe. Latin America, Africa, Asia and even Europe once were built largely with clay structures and dozens of different techniques exist to build with raw clay as well as with burnt clay.

In recent decades, clay has been relegated to minor roles, due to many different causes, but mainly because of aggressive marketing of Portland cement and elements made of cement. However, a sizeable part of humanity will not likely have a decent dwelling in this century, unless they build with clay, several scientific analysis point to as much as 50%!

In the seventies of the last century (decade of “appropriate technologies”) the practice of “improving” traditional adobe buildings came into fashion. It seemed as simple as adding one and one…. clay is cheap but has low resistance; cement is more expensive but has high compressive strength. If you mix the two and on top of it apply some pressure, you get a brick that obviously must be better.

The reality is not that simple

Portland cement and clay are both binding materials and they both need aggregates to form a more or less stable matrix. Their chemical and mineralogical structures are not well compatible, the resulting brick might not be as good as it seems. And it is definitely not as low cost as it is supposed to be.

One of the major criticisms of clay buildings is their deficient behavior in earthquakes. However, many serious studies point out that it is most often bad workmanship or bad design that causes collapse in earthquakes, and not the material itself. An interesting Government study after the 2001 earthquakes in El Salvador revealed that the percentage of damage in cement block buildings was higher than in adobe buildings.

Many universities have studied the behavior of adobe structures in earthquakes and invariably they show that the weak point in the wall is the mortar joint between bricks and not the bricks themselves. Walking through an earthquake damaged area you will find that confirmed in reality, most cracks follow the joints and only occasionally break an adobe or a brick.

The conclusion is clear: we have to improve the bonding between the bricks, and not primarily the bricks themselves. Unfortunately, almost all R + D has gone into “improving” the adobes! One of the major lines in this movement is to compress the adobes and add cement to the mix. The result is a brick that definitely has higher compressive strength and will lose those qualities to a lesser degree when exposed to extreme moisture. This higher compressive strength then leads the engineers and builders to make the walls thinner. The smooth surface of the CEB definitely makes the bonding between bricks even weaker than the bonding between handmade rustic adobes. Those two factors (thickness of the wall / bonding between bricks) combine to weaken the structure and lower the earthquake resistance of CEB walls with regard to handmade adobes.

Use cement or not?

The obvious reason to use cement is the fact that the matrix of a clay structure breaks down with moisture, resulting in very low compressive strength. Cement, on the other hand, maintains most of its qualities even under water.

However, as cement and clay do not match, a clay brick with an admixture of cement will not improve as much as generally expected. The very fine clay particles surround the cement grain, creating a coating around the grain that hampers cement hydration to a certain extent. Then the role of cement is limited to improving moisture resistance, mainly because of its hydraulic properties.

Extensive testing in several universities around the globe show that it is mainly the compression that improves the compressive strength of a CEB in relation to handmade “natural” adobes. Tests prove that, clay-sand bricks shaped with more than 60 kg/cm2 (6 Mpa) pressure are actually stronger without admixtures of cement!

If the goal is to create a low cost building material, the admixture of cement is most often a dead-end. Compared to hollow concrete blocks (vibro-compacted sand-gravel-cement mixtures) they often use the same amount of cement (per square meter of wall), and are not necessarily cheaper.

If villagers are given a choice between a CEB and a hollow block they almost always will prefer the latter, be it for social status or for security concerns. CEB only have a chance in the free market if they are much cheaper, which leads us almost always to handmade clay-sand adobes, which people are usually able to produce themselves right on the construction site. Careful workmanship, thick walls and horizontal reinforcement in the foundation and as a base for the roof, make a structure that withstands earthquakes rather well. It is clear that terrible images after the recent earthquake in Iran show fallen adobe buildings. However, at the same time, it is known that many of these buildings are hundreds, even thousands of years of age, and have resisted numerous earthquakes throughout history. What we see less in the new images is the new part of the city, built of cement, that also was destroyed.

Over the last 25 years I have witnessed many NGO- and Government sponsored projects producing CEB’s, using many different types of presses. However, I have not yet found a single producer of CEB’s who is able to sell them on the open market.

You are here: Home Past editions Edition #1, January 2004 A few thoughts on “Compressed Earth Blocks” (CEB)

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