After about a day, the mixture formed concrete blocks in the form of any mold that the group used, including two-inch cubes, shoe-sized blocks and armor pieces with struts and cutouts. The two-inch individual cubes were strong enough for a person to stop, although the material is weak compared to most conventional concretes. Blocks the size of a shoebox showed potential to make a real construction.
"The first time we made a great structure using this system, we didn't know if it was going to work, increasing from this little thing to this great brick," said Chelsea Heveran, a former postdoc with the group. Now an engineer at Montana State University and the lead author of the study. "We took it out of the mold and held it; it was a beautiful bright green color and said & # 39; Darpa & # 39; aside." (The mold presented the name of the project financier). "It was the first time we had the scale we were imagining, and that was really exciting."
When the group took small samples to a regular review meeting with Darpa officials, they were impressed, Dr. Srubar said: "Everyone wanted one at their desk."
Stored in relatively dry air at room temperature, the blocks reach their maximum strength over the course of days, and the bacteria gradually disappear. But even after a few weeks, the blocks are still alive; When exposed to high temperatures and humidity again, many of the bacterial cells are reactivated.
The group can take a block, cut it with a diamond-tipped saw, place half in a warm beaker with more raw materials, pour it into a mold and start concrete formation again. Each block could generate three new generations, producing eight descending blocks.
The Department of Defense is interested in utilizing the reproductive capacity of these "L.B.M.s" – living building materials – to aid construction in remote or austere environments. "In the desert, you don't want to have to load a lot of materials," said Dr. Srubar.
The blocks also have the advantage of being made of a variety of common materials. Most concrete requires virgin sand that comes from rivers, lakes and oceans, which is running out of water worldwide, largely due to the huge demand for concrete. The new living material is not so demanding. "We are not pigeonholed in the use of some particular type of sand," said Dr. Srubar. "We could use waste materials such as frosted glass or recycled concrete."