A lucky baby, protected by its pushchair, suffered only an injured elbow. The chair was wrecked.

The Housing Society, which managed the more than 10-year-old owner-occupier block in Second Street, said such an occurrence was ''very rare'' - because previous falls had been smaller and remained inside the building boundary, said housing manager Ellen Ho Li.

Urgent repairs - which were planned when the lump fell off - were now under way at a cost of $5 million to $6 million, she said. But she did not rule out cracks at other Housing Society blocks of a similar age.

The accident followed the collapse of a balcony from a house in Jordan last October, which injured four people.

Nearly all Hong Kong's buildings are made from concrete reinforced with steel bars, as it is the cheapest way to build high-rises.

Concrete corrosion is a worldwide plague: in the United States, billions of dollars are being spent just to mend bridges, says civil engineering researcher Neil Mickleborough of Hong Kong University of Science and Technology.

But the problem is worse in the territory because poor building work, which allows it to happen in the first place, is exacerbated by chemicals in the air and a slapdash approach to maintenance, he says.

Perfect buildings should last 50 years if properly maintained and 20 years without defects, says Alex Cheung Kwok-wai, managing director of concrete repair company Professional Engineering and a member of the Concrete Repair Association. But in Hong Kong, towers more than 10 years old are cracking up.

Twice a day the Buildings Department receives complaints about concrete falling off or cracking on private buildings. Government buildings come under the Architectural Services Department, public housing under the Housing Department.

Buildings Department chief structural engineer Thomas Tang Shik-tsun says that of 16,700 buildings categorised as needing repair under a survey programme begun in 1989, 3,500 still need inspecting to find out what work is needed.

And despite having increased the number of engineers in the dangerous buildings section threefold to 52 since 1989, by the time the department gets round to looking at all these Category II structures by June 1995, some will have deteriorated to CategoryI, regarded as dangerous and in need of extensive repair or even demolition.

''It's a never-ending cycle,'' Mr Tang said.

''They don't have the resources to inspect all buildings,'' said Mr Mickleborough. ''They are not there to carry out a policing role.'' The Housing Department has a rigorous survey and maintenance programme involving inspections inside every one of its 800,000 flats every 18 months and a thorough external review at least every six years, says senior assistant director Raymond Bates.

But in the private sector, maintenance plans, paid for by building or flat-owners, may be patchy or non-existent, say surveyors.

''If you look at a lot of buildings you can see cracks in all sorts of places. It's such a big problem,'' says surveyor John Kiely of Kiely and Co which specialises in building defects.

Shaun Eddleston, a chartered building surveyor and director of building consultancy Vigers, says cracking will get worse as blocks built to shoddy standards before the early to mid-1980s age.

''It's potentially a much greater problem, perhaps a problem that has yet to surface,'' he said.

''Government and building authorities are very concerned. It's very difficult once the concrete has been put in place to actually stop it.'' Standards of workmanship and supervision on building sites were improved in the 1980s after 26 Housing Department blocks made with substandard concrete had to be demolished.