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Costain wins Tsing Ma slipform ruling

COSTAIN has won a crucial battle in its fight to secure compensation worth close to pounds50M for delays on slipforming the main towers of Hong Kong's Tsing Ma bridge.

NCE learnt this week that the UK's leading construction arbitrator John Uff has ruled in favour of the bridge's contracting joint venture AJC, which includes Costain, Kvaerner and Japanese firm Mitsui.

However, a trial in Hong Kong next January will be needed to decide how much of the original pounds48M claim will be allowed. In the meantime the Hong Kong government could appeal the arbitration, which would further delay any compensation payments to the JV.

AJC blamed main designer Mott MacDonald's concrete specification for the delay, alleging that special new clauses on chloride penetration were impossible to meet (NCE 16 October). Slipforming was delayed for months in 1993 while the contractor carried out over 100 trial mixes without success - and work could only begin after the consultant relented on some of the original conditions.

The delay forced AJC to introduce double shifts and slipform both towers concurrently to meet key project deadlines.

Uff was appointed late last year to resolve the dispute. NCE now understands he has agreed with AJC that it was impossible for the contractor to meet the original specification.

It is understood that one key factor influencing Uff's judgment was that Mott MacDonald relaxed its specification when work began on subsequent Hong Kong projects which also required chloride-resistant concrete. Experts agree, however, that the concrete in these and in the Tsing Ma bridge is of very high quality and has a potentially very long life.

At the heart of the dispute is an unusual test for chloride diffusion introduced for what is said to be the first time ever in a major bridge project. Mott MacDonald's stated aim was to specify a concrete that would give the structure a minimum life of 120 years, even in Hong Kong's aggressive marine climate.

In such conditions the biggest threat to reinforced concrete structures is reinforcement corrosion caused by chloride ions penetrating through the cover concrete. Usually specifiers respond to this problem by specifying very low water/cement ratios or high cement contents or the use of such 'pore-blocking' materials as microsilica.

On Tsing Ma the consultant's response was to aim for virtually zero chloride diffusion as measured by the Bulk Diffusion Test.

This involves exposing concrete specimens to salt water at 40degreesC for 28 days, then grinding away the exposed surface in layers to measure the chloride ion penetration at different depths. The test was originally developed as a research tool, and few, if any, major crossings have taken this route to durability.

On Tsing Ma the first mix which eventually 'met the intention of the specification' set too slowly for slipforming, and could be used in piers and anchorages only. Eventually a quicker-setting 'triple blend' mix containing microsilica and blastfurnace slag as well as Portland cement was approved and the towers were successfully slipformed.

Mitsui-Costain had the responsibility for Tsing Ma's towers, anchorages and piers, which in total contained 230,000m3 of concrete. The JV started on site in June 1992, and had to hand over the structure ready for cable spinning by early July 1994. The financial penalties for late handover in the contract were said to be particularly onerous, but in the event Costain-Mitsui's efforts to catch up the delay were successful.

Dave Parker

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