The survey of 6,451 construction workers from 9 cities, including Beijing, Shanghai, Wuhan and Zhengzhou, also found that construction workers were older than workers in other industries.

Thirty-two percent of workers in the construction industry were aged 41-50, 27 percent were 31-40 and less than 30 percent were aged 30 and below.

Liu Xiaohong, president of Beijing Yi Zhuan Yi Wa Cultural Development Centre, which aims to improve the living conditions of migrant workers, confirmed that the number of female construction workers is increasing.

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"Along with Beijing Normal University, we conducted a joint study in 2011 into the workforce of Beijing's construction industry. At that time, only 2 percent of construction workers were female, but the number has been growing. Now, the proportion has reached more than 10 percent in Beijing, and it's up to 20 percent in Sichuan province," said Liu.

The construction industry is the pillar of the Chinese economy. It is estimated that the number of construction workers will continue to increase by at least 15 million each year.

Liu explained that most female construction workers are middle-aged and have difficulties finding other jobs.

"For example, the manufacturing industry prefers to hire young female workers. So older women choose to follow their husbands and take jobs on construction sites," she said.

At first, the women would take odd jobs at the construction site, such as in security and cleaning. But now more and more female workers are taking the same jobs, which are often high-intensity, as their male counterparts.

"In fact, the room for promotion for these women is small, and management positions are held by men. There is also gender discrimination. They're not paid the same as men even though they do the same kind of work," Liu said.

"Male workers can earn 170 yuan for 10 to 11 hours of work placing rebar at a construction site, but women can only earn 150 yuan for the same work."

Women should also be treated differently when they are pregnant or after they give birth, but the reality is that they aren't given any special treatment, Liu added.