Regarding your report on the Spinnaker Tower (NCE 2 September), voids close to the rebars often occur in heavily reinforced slipformed concrete sections. Friction from the moving forms tends to pull the plastic concrete away from the leading edge of the bars leaving lens shaped air pockets in places. The Ministry of Transportation in Ontario, Canada published some reports on this about 15 years ago.
They tried slipforming barrier walls on bridge decks and had to change to fixed forms due to these voids. The voids in your report may be due to the same mechanism.
Roger Northwood (M), Nanaimo, Canada rpnorthwoo@aol. com