ENGINEERS ON Portsmouth's Spinnaker Tower were this week carrying out urgent tests for hidden voids in the concrete.
Initial hammer tests have raised alarm over concrete quality.
Portsmouth City Council has asked the Concrete Society advisory service, consultant Mott MacDonald, and tower designer Scott Wilson to come up with an urgent investigation plan.
Mott MacDonald was expected to deliver recommendations for investigation and testing to the council this week, said assistant city engineer Mike Broomfield.
Spinnaker Tower consists of a 138m tall, heavily reinforced concrete A-frame supporting a 1,100t steel 'sail'.
The A-frame was slipformed by specialist Beirrum, which went into receivership just before completion of the job last year.
Its rough external finish has been smoothed over with a cosmetic mortar render.
Alarm over the quality of concrete in Spinnaker Tower's concrete A-frame was raised earlier this month by main contractor Mowlem.
Workers noticed variations in the 'strike tone' of concrete hit by their equipment during installation of lifts and stairs in the A-frame's hollow legs.
Variations in strike tone - the acoustic pitch of noise generated by concrete when hit - can indicate the presence of voids or inconsistencies in concrete density.
'Honeycombing' can contribute to localised weakness in the concrete, and allow water to penetrate. In the long term this can lead to corrosion of steel reinforcement.
Portsmouth carried out hammer strike tests inside one of the A-frame legs, which suggested 'slight hollowness in selected areas', said engineering project manager David Greenhaigh.
But he stressed that the problem was not thought to be significant. 'There is no issue with the structural integrity of the tower, or with its short or long-term durability.'
The council has followed up with petrographic and crushing strength tests on sample cores taken from the structure.
These should determine whether there are inconsistencies in concrete composition or density.
But results suggest no major irregularities yet, said Broomfield. 'So far we've found nothing to explain the extent of the variations in the hammer test results, ' he admitted.
Broomfield said that Mott MacDonald was expected to recommend resin injection for the next stage of testing.
This will involve creating a regular grid of injection points on the internal and external walls of the A-frame and pumping in a low viscosity nonsetting resin pigmented with dye. The dye will make the resin visible if it reappears at the surface on other points of the structure.
The process will identify areas where voids are present and give some indication of their size.