Adobe wallsGernot Minke: Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) GmbH. 7.1 General
Blocks of earth produced manually by throwing wet earth into a formwork are called adobes, or mud bricks or sometimes sun-dried earth blocks. When moist earth is compacted in a manual or powered press, the compressed elements so formed are called soil blocks. Blocks produced by an extrusion process in a brick plant, are called green bricks in their unburnt state. Larger blocks compacted in a formwork by ramming are called rammed earth blocks.
There are many different shapes known all over the world. Fig. 7-1 shows some samples.
Different manually operated presses are known too. Fig. 7.3 shows one of the first, the widespread CINVA-Ram, and Fig. 7-4 the CETA-Ram, which can produce 3 smaller blocks at the same time. Blocks produced by these presses have the advantage of a more exact shape with sharp corners than handmade adobes. Their disadvantages are that they usually need the addition of 4-8% cement to the soil in order to obtain sufficient stability and that the production output is only half by comparison with adobes.
7.2 Internal reinforcement
The "Instituto nacional de investigación y normalización de la vivienda (ININVI), Peru, developed a system of adobe walls which are stabilized by vertical bamboo rods that fit into holes of 5 cm diameter, formed by grooves at the side of square adobes and halved ones, see Fig. 7-5. Corner buttresses and intermediate buttresses stabilize the wall, see Fig. 7-6. In Fig. 7-8 it can be seen that the horizontal elements of the roof trusses rest on and are fixed to the buttresses. It is important to mention that if the length of a wall is 12 times larger than its thickness, it should have an intermediate buttress, see Fig. 7-6 and 7-9. Interior walls must also have a buttress when they meet the exterior walls. Fig. 7-7 shows a simple design with shorter walls and a separate kitchen built in Salvador (Equipo Maiz, 2001).
Horizontal bamboo rods, as shown in Fig. 7-5, normally do not strengthen the structure. They weaken the walls as they disturb the transfer of shear forces. This is due to the fact, that in practice there is not sufficient bonding between the rods and the adobe, as they are not always covered by 2 cm of mortar and as the quality of the mortar is too poor to take the shear forces.
The buttresses at the corners can be substituted by columns of reinforced concrete, see Fig. 7-10. In the case of the solution shown on the left, it is necessary to place horizontal steel bars at least every 50 cm which grip into the joints with their angles. This detail was forgotten in all publications which illustrate this solution. Without this bond, the column will separate from the wall when strong horizontal shock occurs in the earthquake.
7.3 Interlocking blocks
Walls without mortar can be built with interlocking blocks. The blocks have holes for vertical reinforcement elements from steel rods or bamboo canes fixed by pouring cement sludge into the holes. The blocks are pressed in special molds and normally stabilized with cement, see Fig. 7-11. If they have enough vertical reinforcement elements, at least at corners and intersections, and if these are well fixed to the plinth and the ring beam, these walls are supposed to be earthquake-resistant due to their flexibility. The system was developed at the Asian Institute of Technology, Bangkok. Figs. 7-12 and 7-13 shows the first demonstration building, built in 1984 in Thailand. In this case the holes were filled with a mixture of cement and sand in the ratio 1:3.
Fig. 7-14 shows a similar system, developed by the University of the Andes, Mérida, Venezuela. The blocks have grooves and tongues which interlock. Horizontal ring beams of reinforced concrete are placed at a height of 1.20 m and on top of the wall.
If stacked without mortar these walls do not show any high resistance to horizontal forces as the interlocking effect is only given by a height of some millimeters.
Therefore the author developed an improved interlocking system, with blocks showing tongues and grooves 40 mm high, in horizontal as well as in vertical joints, see Fig. 7-15. If these blocks are displaced or lifted up by seismic shocks, they always fall back into their normal position. The holes act as gripholes for easy 0 handling, but can also be used to install vertical reinforcement elements. But these additional reinforcement elements are not necessary if the wall corners are formed by concrete columns, which interlock with the block as shown in Fig. 7-15, and these columns are interconnected by a ring beam.
7.4 Concrete skeleton walls with adobe infill
Normal masonry walls are not very stable against seismic shocks. Therefore, nowadays the masonry walls are often framed by concrete which forms a skeleton structure with adobe (or brick) infill, see Fig. 7-2 and 7-3. The vertical concrete columns should have 4 steel bars of at least 14 mm diameter. It is important that the concrete and the adobes interlock, as shown in Fig. 7-16. For low-cost housing projects this solution is normally too expensive. Moreover it shows hardly any flexibility.