ENGINEERS have found over 200 small voids in the central concrete core for Spinnaker Tower, a new landmark structure in Portsmouth on England's south coast.
Each seems to be a 'lens' shaped air pocket between 0.2mm wide and 1mm thick, usually about 50mm to 90mm beneath the surface of the concrete, close to the first reinforcement bars.
Some are as big as 800mm by 300mm and most are beneath the inside surface of the 76m high hollow hexagonal box section legs.
The legs form part of the 143m high 'A' shaped tower.
'We think from analysis of the oxidation in the voids that they have never been exposed to the surface' said Mott MacDonald structural engineer David Greenhaigh, who is on secondment to Portsmouth City Council as project supervisor.
'A consensus of opinion by Mott, checking engineer Scott Wilson, contractor Mowlem's materials laboratory and the Concrete Society is that they formed during the plastic phase of the concrete, while it was still hardening.'
No voids have been found on the outside surface where the sounding survey, using tapping to identify hollow areas, is three quarters complete.
The survey follows discovery of hollow sounding patches by main contractor Mowlem in June this year.
The cause of the voids, which typically are found in patches of around 300mm by 300mm, is unknown.
But their location means that structural stress and cracking of the structure is not a likely cause of the problem, said Greenhaigh.
'We do not believe they are a major threat to the structural strength of the tower or its short or long term durability.'
Petrographic examination of cored samples has so far failed to reveal a cause of the voids.
'The Concrete Society is searching world literature on this but so far has not found a similar problem.' said Greenhaigh.
'It is thought that perhaps high temperatures from last summer's hot weather and the containment of heat of hydration might have affected slipforming, ' he said.
'We will continue coring to try and understand the problem.
But further samples will use a resin injection process first'.
Meanwhile remedial work has begun. Most of the 130 patches in the stairwell core of one of the two shafts have been broken out and replaced with 'a proprietary repair product'.