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The University of Hong Kong may use ultrasound or X-ray to detect the internal structure before deciding the rectification plan after holes were found in concrete walls and pillars at a dormitory construction site, a veteran engineer said.
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Speaking on a radio program yesterday, structural engineer Ngai Hok-yan said the formation of honeycomb-like voids was due to issues with concrete vibration.
"A common reason for this condition is insufficient vibration for the concrete to strengthen the structure. This leads to the formation of air bubbles, which causes the voids in the structures."
Commenting on the possible challenges during rectification, Ngai said: "The voids found on the surface of walls and beams could be easily fixed. Yet, we fear that insufficient concrete vibration would lead to more voids, affecting the structure of the building."
To determine the type of rectification work needed, he said equipment such as ultrasound and X-ray should be used to check the internal structure of the walls and beams.
Regarding the exposure of steel bars, he said that improper grouting, insufficient fillings in the floor and insufficient concrete vibration are all possible reasons contributing to the issue. He said it was not ideal to see such a construction issue occur.
"The steel root would not be able to offer fire protection without a sufficient protective layer. Such an issue might also lead to the spalling of concrete," Ngai said.
According to HKU, the construction site is at 142 Pokfulam Road. The project involves the construction of two new buildings to be used as residential halls for students. Two other buildings are also being redeveloped into senior staff quarters.
The Buildings Department inspected the buildings involved in the High West Site Development project on Monday.
Apart from discovering that some reinforced concrete elements on lower ground floors of one of the senior staff quarters building were defective, the department has found no obvious structural danger.
A registered structural engineer for the project explained that some of the concrete defects were spotted from end of April to early June by the technically competent persons (TCPs) he appointed.
The TCPs then completed a non-conformity and rectification report in accordance with the code of practice for site supervision.
The contractor subsequently carried out repairs for the concrete elements with relatively minor defects, and submitted to the structural engineer a remedial proposal on concrete elements with a larger extent of defects.
Rectification work would be carried out upon approval and under supervision of the structural engineer.
