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THE GRAVING DOCK

A graving dock had long been wanted, and in 1881 it was settled that it should be formed towards the eastern end of Albert Basin, alongside Commercial Road, the estimated cost being £36,000, but before it was completed it cost £48,000. It was found that a bed of water-tight clay, with gravel beneath it, extends over the site of the dock to the depth of two or three feet below the level of the sill. The graving dock was specified to be 550 feet long at the bottom and 569 at the top ; 48 feet wide at the bottom and 74 feet at the top ; 20 feet deep below high water level, and 5 feet more above it. It was formed of concrete, with granite facings in some parts. The dock was opened in January, 1885, and the event was celebrated with a great dinner and much speaking and congratulation. It has, however, proved a miserable failure, and the grave of a great sum of money. After undergoing a process of reconstruction it has been given up as incurable. It remained in use, but heavy expense will not be incurred with it in future. A small floating dock of iron, in section like the letter U, with one side shortened, was procured soon after the formation of the graving dock, and a large iron floating dock of the same sort has been procured.

THE MANIPULATION OF THE CONCRETE

Concrete is a substance formed of cement mixed with sand and gravel, wetted and allowed to set. Its quality depends upon the goodness of the cement, the quantity of it used in proportion to the sand and gravel, and the degree of wetness of the mass after mixing. The quality of the cement depends upon the proportions of its principal ingredients — clay and carbonate of lime in the form of chalk or limestone — the temperature at which they are calcined, the length of time this process lasts, and the fineness of the grinding of the calcined materials. The quantity of sulphur in the coals used to calcine the clay and the lime also affects the cement. All these things are better understood now than they were in 1881-1884, when the Graving Dock was made; but even yet the formation of concrete work seems to be conducted very much by rule of thumb. If we look at the pavements of the city some of them are cracked, indicating that the concrete had been too wet; some are crumbling, an indication that it had been too dry or that too much sand had been used. If a shower begins to fall one piece of the pavement grows wet instantly, which is a good sign; another remains dry, which shows that it is too porous.  In forming the concrete for the Graving Dock two mistakes seem to have been made. Firstly, the concrete in the outside walls, which had to be deposited through water, was allowed to set to some extent before being put in the place it was to occupy; and, secondly, too much sand was used.

The consequence of these mistakes was that the concrete was soft and porous, and when the water was 20 feet higher outside than the floor of the dock it was forced through the concrete. This brought in a train of evils. All salts of lime being more or less soluble some of the lime of the cement was dissolved out, and it became only a question of time how long it would take to dissolve the whole and make the concrete an incoherent mass. In cement, however flue the grinding may be, some particles remain unslaked when it is wetted, and if they afterwards get water they slake and expand, and thereby disintegrate the mass. There must also be bad results from the passage of sea water containing chlorides and sulphates of sodium and magnesium through concrete containing silicates of lime and alumina and also some sulphate of lime due to calcining the components of the cement with coal containing sulphide of iron. When these various salts come in contact, especially in water under pressure, chemical changes on a greater or less scale are sure to follow with consequent expansion and disintegration. Soluble substances will be produced, and under the pressure of 20 feet of water from the outside these will be forced inwards, leaving the concrete more and more disintegrated.  Inset - water ingress.
 


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Last modified: 01/09/2013