Once Arch Henderson's Lerwick office team had decided to consider a concrete infill repair option Grace Construction Products was called in to advise on concrete performance possibilities.
'It was a very demanding brief, ' recalls Grace engineering services manager Bruce Perry.
'The mix would have to be fluid enough to be pumped up 13m from the seabed without segregation yet still achieve high early strength so the shutter could be stripped after 12 hours. And in the longer term it would have to be very durable and abrasion resistant.'
Only a limited range of primary concrete making materials was available. Fine aggregate had to be imported from the mainland. Only one source of coarse aggregate - Vatster granite - offered the potential of high performance.
Portland cement came from one mainland source. Perry says it took 'many evenings doing trial mixes in the readymix depot' to get the plastic properties right.
High fluidity and high early strength were achieved by a combination of high cement content and a non-retarding new generation superplasticiser, Grace's Adva 130. Maximum free water/cement ratio was 0.41. A small addition of polypropylene fibres minimised bleeding and shrinkage cracking. But there was another concern that had to be met.
'As the concrete rose up the shutter it displaced the seawater, but the reinforcement would still be wet, ' Perry points out. 'This means high chloride levels actually on the steel.
'To minimise this corrosion risk we included our DCI S calcium nitrite corrosion inhibitor in the mix - which also helped accelerate early strength gains.'
Although initially designed as a C40 mix, test results so far have shown strengths of around 28N/mm 2at 24 hours and 55N/mm 2at seven days. Perry also pays tribute to the batcherman at Hanson's Brindister readymixed concrete depot, which has supplied all the concrete for the repair project. 'Billie Cromwell's willingness to stay on late and produce one trial mix after another made a major contribution to the success of this project, ' Perry says.