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Sydney Harbour Bridge

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Concrete

The bridge structure sits on four large concrete ‘skewbacks’. Two are on the south shore and two on the north. The excavation for these was directly into the solid sandstone. Each skewback is 40 feet wide (12·192 m), 90 feet long (27·432 m) and up to 30 feet (9·144 m) deep. The upper surface of these, for the main bearings, is at an angle of 45° to take the main bearing casting.

The skewback was formed in concrete, part of it steel reinforced, with the layer directly under the main bearings made from a mix of 1 part Portland cement, ¾ part Nepean sand, and 2¼ parts crushed granite. This concrete sets with a 28-day strength of 430 tons per square foot (41 MPa).

excavation for the south west skewback
John Bradfield, Lawrence Ennis, Ralph Freeman and Kathleen Butler in the excavation for the south west skewback. Photographer unknown, 22 April 1926 Sydney Harbour Bridge Photographic Albums 1923–1933, vol 2 State Records NSW

The concrete was poured in sections, and in an hexagonal pattern.

Problem 1

Can you determine why the concrete was poured in sections and not as a complete block (ignore the practicalities of pouring a single block this size)?

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Problem 2

Why is the strength measurement at 28 days important when specifying concrete?

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Problem 3

Can you determine why metallurgical coke rather than crushed aggregate was used in the deck concrete.

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Coke Concrete

In the process of engineering design, it is often necessary to trade off one property against another in order to get the best result. In the case of the deck structure of the Sydney Harbour Bridge, weight saving was obtained by using a special form of concrete which had a lower, though acceptable, strength than traditional concrete.

Macrograph of the coke breeze concrete
Macrograph of the coke breeze concrete used on the Harbour Bridge deck. (Cement/coke grit mix - light grey; Coke - dark grey)
(Courtesy of Roads & Maritime Services NSW)

The main deck strength comes from steel trough beams producing an effect rather like giant corrugations at right angles to the roadway centre line. The most direct and convenient way of providing a smooth surface was to pour concrete to fill in these corrugations and some distance above them to provide a continuous slab. It was recognised that the full strength of normal, structural concrete such as was used for bridge main girders or building columns would not be needed as the function required was to dissipate the stresses caused by pneumatic-tyred wheels. A resulting permissible strength reduction opened the way for using a lighter material.

The coke concrete that was used replaced the usual aggregates by “coke broken to pass through a ring 1.25 inches in diameter, but caught on a sieve of 0.375 inch mesh” as stated in the specification, and “coke grit screened through a sieve of 0.375 inch mesh, but caught on a sieve of 900 meshes per square inch”. The larger of these particles can readily be seen in photographs of cylindrical cores extracted for information preparatory to the refurbishment of the deck in 2012.

The design figures specified in the Bridge tender document, and reproduced below, show expectation of a reduction in weight of 44 per cent in relation to structural concrete. It was not the practice at that time to conduct strength tests other than on the cement, but tests after 80 years indicate a contemporary strength of about half that of structural concrete and more than sufficient for the purpose.

The design figures specified in the Bridge tender document
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