Logistics of concrete placement in 1923

From Engineering Heritage New South Wales


    At the time of construction of the Sydney Harbour Bridge and it approaches the concrete supply industry had not developed. There were no permanent batching plants supplying concrete to order and no transit trucks to deliver mixed concrete to sites. For structures well above ground level such as the Harbour Bridge abutment towers cranes could used to lift hoppers, but in other cases hoist towers carried the concrete to great heights to allow further distribution by gravity. For ground level works the common method of delivery was by gravity using long and sophisticated chutes, sometimes from an elevated batching plant. The design of a concrete structure thereby required considerable extra design and construction to establish a delivery path for the concrete.

Concreting the skewback for the harbour bridge at Dawes Point. The huge block was poured in hexagonal strips. The forms would tend to be forced upwards by the pressure of the wet concrete. To avoid this the shutters were weighed down with old railway wheels and axles. Since this is a Dorman Long and Co site the chute is not the PWD preferred Insley type. 6 July 1926

   Delivery of concrete for mined railway tunnel roofs was by rail-mounted skips, with the final placement done by manual shovelling. Very late in the period of construction concrete began to be delivered by concrete guns, especially to remote tunnel sites. These machines were properly ‘guns’ rather than pumps as they required a relatively large chamber to be filled with concrete and sealed air-tight. Compressed air then introduced to force the contents of the chamber through pipes in a single pulse. The chamber was then opened and refilled.

   On the City Railway at least one fatal accident resulted from an attempt to clear a blockage without disconnecting the firing mechanism.

   Cement was always supplied in bags, rather than in bulk, and the bags weighed one hundredweight, (112lbs or 50kg). In preparation for a big pour huge stockpiles of bagged cement were accumulated at the mixer and covered with tarpaulins pending the start of work.

   Much of the concrete used, especially for mass concrete applications such as foundations, was specified as ‘sandstone’ concrete – that is, concrete with crushed sandstone as the aggregate. More intensively loaded applications used crushed hard igneous rock to produce ‘bluestone’ concrete. The Dorman Long and Co sections of the Harbour Bridge – the abutment towers, skewbacks and approach span piers, used crushed granite as the aggregate. This was a by-product of the quarry at Moruya which was producing cut stones for the abutment towers and pylons.

   In late 1923 we have a snapshot of the batching equipment available given in Bradfield’s DSc(Eng) thesis. It comprised one 16 cubic foot (about 0.5 cubic metre) steam driven “Foote” mixer and three 10 cubic foot (about 0.3 cubic metre) electrically driven “Armstrong-Holland” mixers. Three 10 cubic foot electrically driven “Foote” mixers were on order at that date.


Preparing for the concreting the arched roof of St James. There are three sets of chutes supported by cables and tackle. Note the stockpile of bagged cement in the park. 11 November 1927
Concreting tunnels under Macquarie Street from the ‘batching plant’ in front of Sydney Hospital. 21 July 1924


Concreting the foundations of the Elizabeth Street retaining walls. The Foote steam powered concrete mixer is in the centre of the picture.24 April 1922
Concreting the roof of Museum station. Concrete flows by chutes supported by cables and a strut off the excavation batter from mixers above the excavation. 507259 10 October 1923
This photo at Museum would seem to show chutes being dismantled of re-arranged on the far side of the excavation while those on the nearer side are still in use. It seems to have been common for workers to ‘ride’ the chutes to adjust them. 507276 4 January 1924

   In the 1920s large construction projects did not have long reach cranes covering the whole site, so the delivery of mixed concrete to a wide site such as Museum or St James station involved long runs of chute supported by cables and tackle from the higher ground above the excavation. Elaborate articulations were built into the chutes to allow redirection to the place of use.

   Bradfield discusses these in his thesis and states that the design

   giving the most satisfactory results being the imported “Insley” Chute. This chute is of a deep parabolic section, about 12 inches wide, by 13 inches deep [300 x 350mm], and is obtained in lengths of 10, 20 and 30 feet [3, 6 and 9m], provided with suitable hoppers, swivel heads and plates.

   In locations where tunnels were below roads or parks, 6-inch (150mm) holes were bored from the surface, the batching plant established in the park or the middle of the street above and the concrete delivered directly to the space behind the formwork deep below. Men worked in the space spreading and packing the concrete. This was eminently possible as most concrete was not reinforced, or only reinforced very lightly. On the Harbour Bridge the piers of the approach spans are of plain concrete only, and the more than a metre thick walls of the abutment towers have horizontal reinforcement only – two ⅞ inch (22mm) bars, one near each face, at 18-inch (450mm) centres! (Freeman ICE Journal 1934)

Within a tunnel a skip is hauled up a ramp by a compressed air winch to a staging from where its payload will be shoveled behind the forms. 9 December 1929
An engineer checks the formwork before the next ‘pour’. The tunnel is mined through the soft ground using the curved rails supporting laths. From these are hung ‘T’ irons which support timber walings as formwork. The centre walings are omitted to allow shoveling either side. 23 June 1931
Both sides of the arch have been filled. Note that there is no reinforcing. June 1931
The centre section of the arch will now be filled one bay at a time as the forming timbers are placed in short lengths. 23 June 1931

   Just before work began on the City Railway and the Bridge there had been another immense concrete project in Sydney – the Glebe Island wheat silos, and there the placement of concrete by gravity had been taken to its logical extreme. To serve a huge slip-form, concrete was hoisted to a great height in timber towers and then allowed to flow to all corners of the many cylindrical silos by a switchyard high above the form.

The Glebe Island silos about 1919. The mass of timber in the centre of the photo is the slip form for the nest of silos. Timber towers soar overhead as hoists with concrete chutes supported by a crane arm.
The hoist towers are amazingly tall. At there base are materials hoppers and the batching plant. About 1919
The elaborate route for concrete from mixer to place of use. It is first hoisted up the tower and then tipped into the chute to reach the centre of the nest of silos where a distribution hub can direct it to any corner of the work as required. About 1919
The silos were slip formed. The mechanism for the process is not illustrated in the photo set but progress was quite rapid. About 1919

 

Concreting the roof of St James station as a single pour across all four platforms with three chute systems. 11 November 1924
Concreting a double track tunnel at St James using two chute systems. Note the worker on the chute at the swivel joint 26 August 1924
A concrete pump as used in the City Railway tunnels. 517723 20 January 1930
A concrete pump as used in the City Railway tunnels. The supply of concrete would appear to be from a hole in the roof and delivery is via a pipe to the right. 12 January 1930
Concreting the tunnel at North Sydney. A hoist is loaded directly from the mixer and the material raised to the top of the formwork. From there it is probably distributed by wheel-barrow. 3 March 1925
The concrete and reinforcement used in the Harbour Bridge. This is the parapet of the footway near the southern end, cut to allow access to a modern lift. Although this wall is about 1.5 metres tall and 300 – 400mm thick it contains just five small steel bars – marked by the grey squares of sealant applied to protect the steel, now exposed by the saw cut. Bill Phippen photo 2021
The corresponding cut at the northern end of the footway also has only five bars, though they are more camouflaged in this view. The concrete mix is different. This may be accidental or may reflect a different source for the material north and south of the harbour. As these works are on the approaches, rather than the bridge itself they are part of NSW Public Works Department effort, and the aggregate is not crushed granite. Bill Phippen photo 2021
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