Volume 3, Issue 5 June/July 2003

2003
2002
2001

Bricks, Mortar, and... Burlap? by David Pescovitz Printer-friendly version The control specimen was not reinforced with the polymer mesh. One dominant crack is visible. The specimen had low strength and low toughness. An ingeniously simple and inexpensive building-reinforcement system developed at UC Berkeley could dramatically reduce the death toll in major earthquakes like those that recently rocked Turkey and India. Civil Engineering associate professor Claudia Ostertag and her students recently tested a construction technique that prevents the low-cost adobe or brick buildings common in developing nations' rural areas from collapsing after a quake. The magic lies in the cheap and easily-obtainable materials used to reinforce the walls: In India, it's the burlap that coffee bean sacks are made from; in Turkey, the felt-like polymer used to line seat cushions is ideal. "An adobe structure fails when one dominant crack propagates through it," Ostertag says. "So we are applying concepts from fracture mechanics to modify the crack paths in the walls." Professor Claudia Ostertag and one of her students take a close look at a concrete sample about to be compressed to failure in a "split in tension" device. Bart Nagel photo In adobe walls, cracks propagate vertically through each row of bricks. According to Ostertag, engineers have attempted to strengthen every component of the walls, from the bricks to the mortar to the bond between the two. "Nothing ever worked," she says. "So we decided to move in a completely different direction. Strength is important, but not that important. What you really want is ductility, the ability to absorb energy." The goal is to eliminate the single dominating crack, blocking a fracture from propagating all the way up the wall. Even with a multitude of small cracks, "the wall will hold together and the building won't collapse on the occupants," Ostertag says. The best barrier to crack propagation is fiber reinforcement, specifically a mesh fabric. When strips of the material are used to line each row of bricks and sandwich the mortar, a crack may initiate but can't penetrate through the reinforced layer. The impact this extra step has on construction time and labor is minimal. The specimen with the reinforced mortar joints visible. To test the reinforcement technique, Ostertag and her students built a full-scale "specimen" adobe wall. The wall was then subjected to massive force from a hydraulic jack while sensors measured how well the structure held up to the pressure. In an instant, the team had proven their theory. The challenge that remained was finding a suitable material that is cheap and readily available in each developing nation. Fortunately, they didn't have to search long. The reinforced specimen suffered multiple cracks during testing, rather than a single dominant crack. (Blue lines indicate cracks.) These cracks absorb energy and will not cause the wall to collapse. "One student visiting El Salvador asked me what he should look for," Ostertag says. "I told him to visit the local market and see what they carry their produce in. The bags turned out to be a polymer material." After the success of the adobe wall experiment, Ostertag, in collaboration with assistant professor Khalid Mosalam, conducted a similar test in the civil engineering test bay using fired masonry bricks instead of adobe and strips of a high-strength polymer mesh. Again, tiny cracks appeared but the structure did not fail. Will engineers find the secret to safer buildings inside a burlap sack? We want to hear from you... The next step, Ostertag says, is to construct an entire mesh-reinforced building on UC Berkeley's "shake table," the nation's largest earthquake simulator. Located at the university's Richmond Field Station, the twenty-by-twenty foot table is capable of three-degrees of hydraulic motion to accurately replicate seismic activity. The researchers are currently seeking funds to support the shake tests. While Ostertag's efforts are aimed at saving lives in developing nations, the same approach could protect buildings in industrialized nations, she says. "The death count in major earthquakes around the world is unacceptable," Ostertag says. "People don't need to live in unsafe buildings." Claudia Ostertag's Home Page Khalid Mosalam's Home Page UC Berkeley Earthquake Simulator Laboratory "Getting Down and Dirty in the Concrete Lab" by Nancy Bronstein (Forefront, Fall 2002) Lab Notes is published online by the Public Affairs Office of the UC Berkeley College of Engineering. The Lab Notes mission is to illuminate groundbreaking research underway today at the College of Engineering that will dramatically change our lives tomorrow. Editor, Director of Public Affairs: Teresa Moore Writer, Researcher: David Pescovitz Designer: Robyn Altman Subscribe or send comments to the Engineering Public Affairs Office: lab-notes@coe.berkeley.edu. © 2003 UC Regents. Updated 5/30/03.