Lifting and carrying, digging and pounding, these women - and two million other male and female construction workers like them across India - do it eight hours a day, seven days a week, whenever they can find the work.
They are migrants with no homes other than the tents and huts put up on the sites where they are working. They get no benefits, no paid days off. Their children, who run and play in the scaffolding and rubble, almost never go to school.
They are the hard hats of India, the people who build the country not with bulldozers and backhoes, but with their own wiry hands and strong backs.
In this land where machines are very expensive but labor is very cheap, even the tall modern skyscrapers of New Delhi are frequently built without the help of cement mixers or elevators or cranes. It is far cheaper to hire an army of men, women and children, and have them do the mixing and carrying and moving at the equivalent of about $1.20 a day.
"Yes, India is a great modern nation, but it is an ancient feudal land, too," said Govinda Mukhoty, a New Delhi lawyer whose group, Peoples Union for Democratic Rights, has brought numerous lawsuits on behalf of the construction workers in recent years.
"For these people, little has changed from the time (in the 17th century) that 16,000 workers spent 22 years building the Taj Mahal for Shah Jahan," he said. "Then, they got food and no pay. Now they get a little pay but no food.
"Everywhere the laborers are seen, but still they are invisible."
Probably most unnoticed by local eyes are the women workers, true beasts of burden who carry bricks and gravel and sand on their heads and, not allowed any skill training, can aspire to no higher.
They can be a striking sight even as they do their lowly work: Rajasthani women in their bright red skirts and silver nose pins, tattooed Biharis loaded with wrist and ankle bangles that jingle as they walk. They wear whatever wealth the family owns.
Ten years ago, a social welfare group in New Delhi did a survey of construction workers and concluded there were about 40,000 of them in the city. About 20 percent, they found, were women, and not one was in anything but an unskilled, low-paying job. And because many of the women had to periodically stop work to nurse and care for children, they were regularly paid less than men doing similar work.
Especially now that most of the urban construction workers are migrants - whole families who leave their villages because of famines, or because they have no land, or because they need cash to pay dowries or old debts - they are a group without roots or power.
Construction has been booming in many parts of India, but the workers have no union and very little leverage. A strike would be unthinkable, since there are always too many landless and desperate Indians who will work for whatever is offered.
Still, some public-spirited Indians - especially in cities such as New Delhi, Bangalore and Bombay, where a substantial Indian middle class has emerged - have recently begun to set up programs to provide minimal services or, like lawyer Govinda Mukhoty, to agitate for their rights.
Mukhoty got involved with the construction workers in 1982, when a massive building program was under way in New Delhi in preparation for the Asian Games. Huge colonies of an estimated 125,000 workers were living in squalor then along roadsides and building sites, and some of Mukhoty's associates began to investigate the situation.
"What we found was that there were laws on the books requiring contractors to do many things, but none of them were being enforced," Mukhoty said. "But most appalling, we found that these people - who were working for the government of India - were being paid less than the government-approved minimum wage."
Mukhoty brought a class-action suit on behalf of the workers - one of the first in India - and the Supreme Court ultimately ruled in his favor. By the time the court ruled, however, the construction was long since completed and the workers were gone.
"We never really helped them much," he said, "but maybe our work will help others in the future."
Mukhoty now is suing over working conditions at a huge hydroelectric plant in Kashmir.
Another pioneer in helping the construction workers was a New Delhi woman named Meera Mahadevan, who had been a follower of Mahatma Gandhi. Both she and her husband had been involved in planning the 100th anniversary of Gandhi's birth in 1969, a celebration that involved the construction of buildings, statues and parks in New Delhi.
Once the work began, Mahadevan couldn't help but notice the harsh conditions under which people labored, and the way in which young children were often left unattended. In the spirit of Gandhi, she decided to do something to help them.
What was needed most, she concluded, was a day-care center for the children, something that she called a creche. A woman of great energy, she translated that original idea into a program that today runs centers at 24 construction sites in New Delhi, and as many in Bombay and other cities. It is now funded by the government of India and international relief groups such as Oxfam and Save the Children.
The centers are spartan - usually several small dirt-floor rooms in the workers' quarters, or a completed room in the building under construction - but they at least provide a place where infants and children can sleep and play with supervision during the long workday.
Recently, at one of the centers in New Delhi - an oasis of shade and cleanliness in the baked jumble of the construction site - children danced and laughed with a trained teacher rather than playing in the rubble outside.
One of the mothers came in to nurse her infant, who was asleep in a makeshift cloth cradle hung from a wooden sawhorse.
The woman, Sita Ishar, had left her village in Bihar five years ago with her husband and their children. They left during a famine and, since they have no land anyway, they see no reason to go back.
The woman said she was very grateful to be working at a site with a creche, since most sites where she has worked did not. But soon, she knows, she and her family will have to be moving on, because the work here is almost done.
"This is a hard life for us, and no life for the children," she said through an interpreter. "But what is the choice?"