Grit blasting instead of scabbling
The task
Constructing concrete structures.
The problem
A construction company was building one of the stations for the London Underground Jubilee Line extension. The main structure of the station is concrete which was being cast in situ in stages. The meeting surfaces of each section have to be prepared before the adjacent section is cast to ensure an effective bond. In some parts of the structure, this was done using a special expanded metal material which forms the bond itself. However, this material is not suitable for use on thin sections of concrete which meant the joints in these sections had to be prepared in some other way. They could have been prepared using impulsive scabbling tools such as needle guns, but workers would have been exposed to vibration magnitudes of about 18 m/s2 for up to 2 hours per shift. These tools also produce high noise levels.
The solution
The surfaces were prepared by grit blasting. A sub-contract gang was able to blast about 300 m 2 of the surfaces per day per worker. Before the work began, screens were erected around the area to be blasted to prevent dust from blowing around the site. The grit blasting method compares favourably with the use of impulsive scabblers which may prepare as little as 8 m 2 of the surface per day per worker.
The result
- The operators are not exposed to any vibration with grit blasting.
- Grit blasting is much faster than scabbling but may increase exposure to dust and noise which will require further assessment and control measures.
Case courtesy of Tarmac Bachy Joint Venture