Hong Kong Builder
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Design of Vibrators.—It is rather early days to say what is the effective working life of a vibrator. If a 160-watt machine lasts the equivalent of 500 full working days of 10 hours, it consumes 800 units of electricity, which may cost £2. If the machine costs £25, the depreciation is about 10 times the cost of current. From the contractor's standpoint, therefore, stoutness of construction is more important than low current consumption.
Vibrating and Shuttering Design.—Modern shuttering is usually designed for a lateral pressure of 140 lbs. per square foot per foot depth of wet concrete. Modern vibration liquifies the concrete, and the lateral pressure probably never exceeds this figure. There is no need to increase the strength of the shutters for "impact." (In some very early attempts at vibration, flat-ended vibrators were applied to the top surface of the concrete causing heavy lateral pressure and bursting the shutters). A difficulty occurs at the edges of the panels through leakage of grout, and metal cover trips with spun-yarn in the joint may be necessary.
Vibration and the small Contract-Unless care- fully applied and continuously supervised, vibration can be absolutely disastrous. In its present state, it is not therefore suited to a small contract in charge of a junior foreman with only weekly visits from an engineer.
Strength of Vibrated Concrete. Whatever laboratory tests may indicate, test cubes made on the site by standing the moulds on the vibrating shutters are little, if, any, denser and little, if any, stronger (possibly 10 per cent.) than hand-rammed cubes having the same weight of cement per cub. yard. For a good practical open design not overcrowded with steel, vibration in its present state is therefore little, if any, superior to hand ramming.
NEW
in
Fig. 8-160-watt vibrators placing concrete pre-cast piles. Each machine consists of an enclosed electric motor driving two eccentrically loaded flywheels.
Fig. 6. The usual type of rigid support for suspended floors.
to
Fig. 7.-Supports designed permit vibration of
suspended floors.
Cost of Vibration.-If an outfit costing £150 can place 50,000 cubic yards during its effective life, con- suming some 5,000 units of electricity, the total cost is about 4d. per cubic yard. This is possibly less than the cost of extra supervision required, and the slowing down of the mixer due to handling the very dry concrete. It is a little early yet to put forward any definite figures.
Mixed Methods. Vibration has been successfully used as an aid to hand ramming. A little light vibration sparingly applied to the shutters or reinforcement will help where the structure is heavily reinforced. Un- fortunately, at least one structure so built has been described as being "vibrated concrete." This is, of course, very misleading and has given rise to wrong impressions in some quarters.
The Ideal Vibrator.-The ideal machine is one which will deal with a range of concrete from medium wet to very dry (alternatively a fool-proof automatic water control on the mixer) which will pass concrete down narrow clearances and pack it against the under- side of crowded bars; a machine whose application can easily be checked up so that no part of the concrete can be left unvibrated-in short, a machine which will do for item 10 what the batcher plant and machine- driven mixer have done for items 6 and 7.
The writer has for several years advocated the adoption of mechanical methods of consolidation, but so far his general feeling is one of disappointment.
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