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Argyll needs no reinforcement

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Extensive use of ground granulated blastfurnace slag meant that a concrete arch bridge in Argyll needed no reinforcement.

Built in a remote and hostile environment that attacks with a chloride bite, the Inverneill Bridge replacement needs exceptional durability. The nearby sea and frequent deicing salts that this corner of Argyll frequently calls for have already accounted for two earlier bridges that run alongside the sea loch of Loch Fyne.

The original Inverneill Bridge was built in the 1860s and replaced in 1939 to carry the A83 between Lochgilphead to Tarbert over a river that runs into Loch Fyne. The aggressive natural chemicals ate away at the bridge and it was found in 1991 to be unable to carry the new 44t weight assessment. A much-needed road realignment meant the new bridge would stand in a slightly more exposed microclimate.

Client, designer and contractor team of Argyll & Bute Council decided to omit corrosion-prone steel reinforcement, preferring a stone-faced concrete arch design in which no concrete would ever be subjected to significant tension.

The twin 7m span arch structure stretches a total of 30m, and contains 750m 3ofC40 structural concrete, provided by the John Machlachlan Group. A cementitous content of 380kg/m 3was made up of 70% ground granulated blastfurnace slag (GGBFS) supplied by Caledonian Slag Cement. This ensured minimal thermal cracking.

On the northern span, where the largest pour was made, a total temperature differential between the 1.5m thickest portion of the arch with the 400mm crown was recorded at 14infinityC. Maximum temperature generated in the centre of this was 45infinityC.

Long term differential drying shrinkage was averted by the addition of a Grace Construction Products' Eclipse admixture as well as the use of maximum aggregate sizes of 40mm, which also reduces the water/cement ratio. This shrinkage was recorded at 0.003% after 28 days when test cubes had reached a compressive strength of 60.5N/mm 2. The water/cement ratio and therefore the total cementitous content was further reduced by Grace's Adva 115 superplasticiser. In addition to these, Fibrin 23 polypropylene fibres were added at 0.9 kg/m 3to reduce the incidence and size of microcracking.

To enable the solid concrete arch to fully perform as a traditional masonry arch, crack inducers were placed on the shuttering. These ensured the crack formation could be controlled as the formwork was removed. This complete defence against chemical ingress is critical in meeting the bridge's 120 year design life.

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