CHANGSHA, CHINA -- In early December, Liu Zhangning was tending her cabbage patch when she saw a tall yellow construction crane in the distance. At night, the work lights made it seem like day.
Fifteen days later, a 30-story hotel towered over her village on the outskirts of the city like a glass and steel obelisk.
"I couldn't really believe it," Liu said. "They built that thing in under a month."
A time-lapse video of the project in Changsha, which shows the prefabricated building being assembled on site, has racked up more than 5 million views on YouTube and left Western architects speechless.
"I've never seen a project go up this fast," said Ryan Smith, an expert on prefabricated architecture at the University of Utah.
In other countries, the most advanced prefab construction methods can reduce building times by one-third to one-half, Smith said. The builders of the Changsha hotel did better, knocking one-half to two-thirds off the normal schedule.
"It's unfathomable," Smith said.
The warp-speed construction is a startling illustration of the building boom in China, where an exodus from the countryside to the cities has swelled the urban population by almost 400 million since 1990.
Skylines are peppered with cranes. Smog-choked streets echo with the pounding of jackhammers. Residential high-rises sprout like weeds in the plains between major cities, creating an endless sprawl along the country's east coast.
The breakneck pace of construction reflects a societal urge to catch up as fast as possible to the developed world after decades of scarcity under Mao Tse-tung, said Zhang Li, a Beijing architect.
The focus on fast construction took root during the economic reforms of the early 1980s, Zhang said. Prefabrication methods, well established elsewhere but just catching on in China, have magnified it.
Raising a 30-story tower in two weeks is possible because most of the work is done in a factory and the foundation has been laid ahead of time. China's abundance of workers also helps.
But a job done quickly is not always a job done well. Zhang said that in their race to the finish line, many Chinese construction companies skimp on the meticulous reviews and inspections that make projects in the West drag on for years.
"Incredible speed also means incredible risk," he said. "But only time will tell how serious the risk is."
The Chinese company behind the Changsha hotel, Broad Sustainable Building, says it cuts no corners on safety. To the contrary, it says, its methods will make China's construction boom safer, cheaper and more environmentally friendly.
In promotional literature, Broad boasts that its technology is "the most profound innovation in human history" and that construction on a third of the world's new buildings will be done this way "in the near future."
The hotel, called T-30, looms over dilapidated concrete homes interspersed with piles of garbage and rows of cabbages and leeks. Dogs and chickens run through muddy alleyways.
In mid-January, a month after the building's announced completion, its interior was a hive of activity. Many of the 500 rooms were finished, with made beds and white sofas. In others, wires protruded from unfinished walls. Paint-splattered workers hauled wooden planks past a grand piano in the pristine marble lobby.
The hotel, which will accommodate visiting clients of Broad Sustainable Building and house some of its employees, is about 400 yards from the cavernous white factory where its components were manufactured. The headquarters of the parent company, Broad Group, is a 90-minute drive away.
"This is the tallest building in this county, and it's also the fastest-built," said Rong Shengli, one of the building's planners, looking over the rural sprawl from a helicopter pad on the hotel's roof. "Next, we're going to build a 50-story building. Then a 100-story one, then a 150-story one. And they're all going to go up fast."
The time-lapse video provides a glimpse of how the hotel was made. Workers in blue jumpsuits are seen assembling "main boards," the building blocks of Broad's structures - 13-by-50-foot slabs containing ventilation shafts, water pipes, electric wiring and lighting fixtures sandwiched between ready-made floors and ceilings.
A counter at the bottom of the screen ticks off the hours as the boards are loaded onto a truck and delivered to the construction site. A crane then stacks them up like blocks. Workers bolt in pylons and piece together staircases; the glass-and-steel exterior rolls up onto the frame like a gleaming carpet.
At 360 hours, the ticker stops.
Building this way costs 20 percent to 30 percent less than traditional methods, said Jiang Yan, a senior vice president at Broad.
It's also safer, said Zhang Yue, chief executive of Broad Group, because factories are typically less risky environments than construction sites.
"The faster, the safer," Zhang said. "It's like crossing the road. If you slowly walk back and forth in the middle of the road, that's not safe."
The China Academy of Building Research has declared Broad's structures earthquake-resistant up to magnitude 9. (The largest recorded quake of the 20th century, which hit Chile in 1960, measured 9.5.) The company says the strength of its buildings comes from their lightweight steel structures and diagonal bracing.
Zhang said he got the idea to manufacture prefabricated buildings after a massive earthquake in Sichuan province in 2008 in which the collapse of poorly constructed buildings killed tens of thousands of people, many of them schoolchildren.
Zhang said it took about 200 of the company's 900 employees to put up the hotel. They are paid $500 to $800 a month, above average for China. Although some company executives acknowledged that many workers put in well over 40 hours a week, Zhang said they do not work later than 10 p.m.
Unlike most Chinese tycoons, Zhang cultivates a reputation as an environmentalist. The company touts its sparing use of concrete to cut down on waste. Its buildings have low-energy lighting, water-saving toilets and elevators that generate electricity on the way down.
Broad Sustainable Building has completed only a handful of projects. Its first was the 15-story New Ark Hotel, which the company built in about six days near Broad Group's headquarters in 2010. Soon afterward, it built a six-story building at the 2010 Shanghai World Expo in less than 24 hours.
Its first international project was a two-story building erected at the United Nations Climate Change Conference in Cancun, Mexico, in 2010. Mexican President Felipe Calderon called it a "revolution of the world's architectural and housing industry."
The company says it is negotiating technology-transfer deals with firms in Brazil, Saudi Arabia, Mexico and India and hopes to establish partnerships in the United States.
Experts say that may not be easy. Broad's buildings may not conform to U.S. fire codes. Labor laws could prevent employees from working the long hours required to construct a building with such speed.
Amy Lelyveld, a professor of architecture at Yale University, described prefabricated buildings in the U.S. as "kind of at the jewelry-making end of architecture" - an expensive niche.
Lelyveld expressed concern about the adequacy of construction oversight on a complicated, hurriedly constructed building like the Changsha hotel.
"I wonder why it was so fast," she said, adding that "if it was slower there might be more opportunities to inspect the work."
Zhang, however, said Broad could adapt to labor and fire safety laws in other countries, and that employees' workdays would drop to eight hours as the company's technology improves. "We will use international standards," he said.
Zhou Weidong, a vice president at Broad Sustainable Building, said the company was developing as quickly as its home country. Looking out the window of a company Buick, he noted that the squat concrete homes, convenience stores and auto repair shops lining the newly paved road between the headquarters of Broad Sustainable Building and central Changsha were at most a year old.
"Three years later, if you come back here, this will be a city," he said. "That's China. It changes overnight."