The new Forth Replacement Crossing, a cable-stayed bridge known as the Queensferry Crossing, is Scotland’s biggest infrastructure project for a generation.
It is the longest three tower cable-stayed bridge in the world, and by far the largest to feature cables that cross mid-span. It is also the tallest bridge in the United Kingdom and its innovative design provides extra strength and stiffness, allowing the towers and the deck to be more slender and elegant.
Concrete admixtures from BASF Master Builders Solutions contributed significantly to the construction of this £790 million bridge that opened in 2017. It was designed and built for the Scottish Government by Forth Crossing Bridge Constructors (FCBC), a consortium comprising of Hochtief Solution, American Bridge International, Dragados and Morrison Construction.
The North and South towers start from a concrete plug on top of the estuary bed 14 metres below water-level, while the Centre Tower’s foundations were only around 2 metres below the waterline, starting from a rocky outcrop.
The scale of the steel caissons used on the new bridge is remarkable. The largest is 30 metres high by 30 metres in diameter – approximately the size of an eight-storey building. It weighs a massive 1,200 tonnes, making it one of the largest steel caissons ever sunk down to the seabed anywhere in the world.
To give some sense of the scale of the project, one caisson alone involved more than 2,100 truckmixer deliveries and 250 barge trips. As one barge discharged, another returned to dock to be loaded and a third was in transit back to the caisson fully loaded, while the fourth was in dock filling up, allowing the concrete pour operation to be executed in one continuous cycle. Each complete barge cycle took up to four hours.
The carefully designed mix and logistical preparation by FCBC and Master Builders Solutions supported the delivery of more than 30,000 m3 of underwater concrete to the three caissons. The biggest single pour for the South Tower foundation, resulted in 16,869 m3 of concrete being produced and delivered without any delays or returned concrete and was tackled in a non-stop process, 24 hours a day, over 15 days which, at its peak, involved up to 100 people.
The new cable-stayed bridge has three slender 207 metre high single column towers, which are equivalent to approximately 48 London buses stacked on top of each other and 50 metres (25%) higher than existing Forth Road Bridge.
Each of the hollow, single-columned, steel-reinforced towers was constructed with 54 concrete pours, each 4 metres high, using the slip form method. The formwork shaped the concrete around an octagonal interior where access staircases for maintenance were to be fitted and cable anchor boxes installed. Finally, a 6 metre x 4 metre concrete slab, complete with entry hatch and parapet walls, was fitted across the top of each tower.
FCBC worked extensively with Master Builders Solutions in order to design and deliver mixes that were capable of being specification compliant whilst providing high strength, consistent workability, long retention times and high flow characteristics. Two admixtures played a significant role in achieving all of FCBC’s desired properties and logistical requirements.
Modern polycarboxylic ether (PCE) polymer based superplasticisers permit a previously unachievable level of concrete performance tailoring that allow truly bespoke solutions to be supplied. The specific and unique challenges posed here resulted in the development of MasterGlenium SKY 903. This was specifically engineered to allow high levels of water reduction and very long workability retention, without sacrificing early strength development performance, therefore not having any negative effect on the construction program. Preliminary laboratory work also identified that incorporating a quantity of a modern generation set retarder (MasterSet R 200) gave further confidence and concrete uniformity, enhancing the placing and finishing operations on the site.