With 400,000 American construction workers injured each year, and a typical American house takeing at least six months to complete, house building had been the same tiring gritty job for 20,000 years. For this problem, Behrokh Khoshnevis has a solution: A robotic house builder.

In a sunny laboratory at the University of Southern California, a robotically controlled nozzle squeezes a ribbon of concrete onto a wooden plank. Every two minutes and 14 seconds, the nozzle completes a circuit, topping the previous ribbon with a fresh one. Thus a five-foot-long wall rises—a wall built without human intervention.

The wall is humble but portentous. “If you can build a wall, you can build a house,” says Behrokh Khoshnevis, an engineering professor, as he watches the gray mixture squirt out in neat courses from what he calls a contour crafter, a machine about eight feet tall and six feet wide. If all goes as planned, Khoshnevis will use a larger, more advanced version of the device later this year to erect the first robotically constructed house in just one day.

Khoshnevis believes his contour crafter will revolutionize building construction, dragging it into the digital age. Today, despite the advent of tech tools like power saws, mechanized cranes, and pneumatic nailers, construction is essentially the same tiring, gritty job it has been for 20,000 years. Workers still have to cut, grasp, hoist, place, and fasten materials, which is why labor accounts for about half of a building’s cost. The process is dangerous, slow, and wasteful: More than 400,000 American construction workers are injured each year, and a typical American house takes at least six months to complete, generating about four tons of waste.

Khoshnevis, 54, a prolific inventor who emigrated from Iran in 1974 and holds patents in fields ranging from optics to robotics, decided there had to be a better way while troweling plaster cracks in his living room following the 1994 Northridge, California, earthquake. “It occurred to me that a trowel, a very simple tool that has been used for ages, can actually create nice, smooth surfaces. In manufacturing, that is always what you are after. Yet not a single automated manufacturing process used trowels,” he says.

To prove automated troweling could work, he invented a small-scale machine for making objects no more than 20 inches across. Guided by input from a computer-assisted-design program, the little contour crafter has a nozzle that navigates on three axes and squeezes out ceramic clay like toothpaste from a tube. A pair of trowel-like fins smooth the top and side surfaces as the layers stack up, yielding cubes, boxes, bowls, domes, cones, or other shapes specified by the software. The device is similar to rapid prototype machines, which have been used for about 15 years to print out three-dimensional plastic models, but the trowels “allowed a new degree of control on the exterior surfaces,” says Khoshnevis.

It worked well on this small scale, so the engineer said to himself, “Why not use concrete and make a whole house?”

Funded by a National Science Foundation grant and aided by engineering graduate student Dooil Hwang, Khoshnevis designed and built a wall-making machine. “I gave myself three years to build it, but it took only 18 months,” says Hwang. The plucky machine uses two parts standard concrete mix (“You can get it right off the shelf from Home Depot,” says Khoshnevis), two parts sand, and one part water, squeezed out of a steel cylinder by a screw-driven plunger. The mix sets up quickly and easily supports its own weight as succeeding layers are extruded.

A wall alone does not make a house. A contour crafter would also need to insert plumbing pipes, electrical wiring, and ventilation ducts in walls as it builds them. The prototype can’t do that, but Khoshnevis sees that as a trivial problem: “The second hand on your watch was placed robotically on a tiny shaft. Modern robotics can achieve tight tolerances and very high speeds. So having segments of tubing robotically inserted, put atop one another, and welded together as the wall goes up is really a no-brainer.”

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