Improving Traditional Brick
Building techniques vary from region to region in response to climatic conditions and ethnic traditions. Clay walls may be molded by hand or with wooden forms; it may be preformed into bricks and sun-dried. In some regions of Central Asia, builders use turf blocks.
Two significant factors in the collapse of newer buildings have been the dilution of vernacular building methods through an unskillful combination with modern industrial elements and the loss of traditional building skills and knowledge.
The use of heavy ferro-concrete coverings over clay structures increases seismic forces, and causes the collapse of walls that would otherwise have withstood the shock. Inspection of buildings constructed with a mixture of raw and burned brick, or adobe and burned brick, reveals significant cracks caused by deformation as the two materials dry and respond to climate differently.
Modern Studies of Ancient Materials
As part of our attempt to understand and improve upon traditional technologies, and to increase clay's ability to withstand seismic forces, we have experimented with a variety of methods. One successful method is the reinforcement of clay with short fibers, such as the waste from textile and carpet weaving.
In laboratory trials, we dried and pounded clay until it passed through a 1/8-inch (3-millimeter) sieve. Then we mixed one third of the clay with fibers and water before mixing it into the rest of the clay. We formed the clay mix into cubes and prisms, and tested them with a hydraulic press.
The best results were obtained with a one to 1.5 percent thin fiber mix, which had a compressive strength of 580 pounds per square inch (psi) or 4.0 megapascals (Mpa). This was 53 percent stronger than pure clay, which had a strength of 375 psi (2.59 Mpa). Mixes with eight percent fiber had a strength of 406 psi (2.8 Mpa).
After compression, the mixed clay samples retained their form with just a few hair-sized cracks appearing on their surface. After additional pressure, the pure clay samples collapsed, leaving the clay mix cubes and prisms with bigger cracks and distorted shapes.
Strengthening through Fire
A second method for improving the bearing strength of clay walls involves thermoprocessing. According to information passed down through countless generations, when fortress walls were built from clay, longitudinal canals were left in each layer, and fire was kindled in these canals for several days. Seeking to emulate our ancestors' experience, we experimented with small samples and fiber-reinforced clay wall fragments.
Using wooden forms, we formed and dried blocks with dimensions 8 by 8 by 12 inches (200 by 200 by 300 millimeters). After the blocks had dried in the shade but were still workable, we chose samples without cracks and drilled a cylindrical canal through the length of each block using a 3-inch- (80-millimeter-) diameter tin tube with a toothed end. When the blocks had air-dried (to 10-15 percent moisture content), we used a gas burner made of perforated 3/4-inch (20-millimeter) steel pipes to thermoprocess the samples.
Because the purpose of the experiment was to emulate traditional experience, we did not attempt to control temperature and fuel expenditure during thermoprocessing, which continued for four hours. Samples were then cut from the walls and subjected to compression tests, revealing a 25.7 percent increase in compressive strength for heat-treated reinforced clay over heat-treated pure clay.
Besides its compressive strength, another weakness of clay is its vulnerability to freeze-thaw cycles. Thermoprocessing also offers a possibility for improving this. In the lab, we heated clay in a muffle furnace at temperatures between 265 and 750 degrees Fahrenheit (130 and 400 degrees Centigrade) for two to four hours.
At these temperatures, low-melting-point synthetic fibers stick to each other in the clay, binding the clay more tightly, and making it therefore more waterproof. The freeze-thaw tests that we performed on the field-thermoprocessed samples bore out these results.
These experiments suggest some promising possibilities for a revival and renewal of traditional building techniques using a clean, plentiful, and inexpensive material.
Ikrom Khadjiev and Professor Kodir Rosiev conduct their research at Tashkent Architectural Building Institute in Uzbekistan.