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Rascor's white-tank system uses a patented crack inducer, consisting of a rigid plastic element fixed into the formwork cage every six metres.

Something like a waterstop, it sits parallel with the crack. In the midpoint of the inducer there is a large rectangular channel with a porous foam on either side.

Martin Smith of Rascor explains: 'Once the concrete has hardened and is old enough for most shrinkage to have taken place, an operator drills into the joint using his skill to find the channel. '

A steel tube packer is inserted and a nut tightened to expand a pressure collar against the concrete hole. Into this is pumped an acrylic resin, mixed from two separate components as it is injected.

It has the same viscosity as water and rapidly fills the whole channel joint and then the crack itself through the foam. In about two minutes it 'sets' into a gel which is hydrofractured by a second burst of the liquid mixture.

'That allows a pressurised injection which seeks out any untreated gaps, ' explains Smith.

The point of all this is to catch all the cracks by forcing them to take place in a fixed position where the resin can quickly be applied.

A little repair work is needed for the occasional random crack that inevitably occurs, though these are infrequent.

Rascor offers a 10-year guarantee for tunnel watertightness and acts as both supplier for the materials and consultant for designing the concrete mixes to achieve high density and low shrinkage.

The company's head office team also calculates reinforcement and concrete placing criteria.

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