facture of construction equipment was valued at £28.000.000.
By the mid-50s this figure had reached nearly £73,000.000, and last year production was worth about £113.- 000.000. Exports, too, have climbed from £14.000.000 in steadily 1950 to £61.000.000 last year.
A transistorised meter that mea sures accurately the depth at which reinforcement lies below the surface
of hardened concrete, and indicates its position and it axis. is being produced by a U.K. firm.
The depth is indicated on a large moving-coil meter calibrated in two scales- one giving readings from () to 1 inches, and the other from 1 to 3 inches.
The power source of the meter is a small nine-volt battery of the type
used in transistor radios. Current consumption is 17-22 milliamps, giv- ing a battery life of several hundred hours in intermittent use. A battery charge check is incorporated.
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The meter. which is housed in polished wood cabinet 83 x 73 x 53 inches and weighs 64 pounds. can also be used for determining the position and depth of electric wiring. in the walls of buildings.
STEEL SHUTTERING COMES TO STAY
TIMBER when used for formwork
is the most expendable item in any works costing. It has come to be looked upon as a "building material." used and disposed of on the building under construction in exactly the same way as the reinforcement. cement, sand.and aggregates.
When it is realised that the cost of the formwork is perhaps 25 per cent of the cost of the concrete con- struction in a structure, the expense of the waste of timber in the building will be appreciated. In the erection of timber shuttering, the carpenters take raw materials and erect within the area of the structure under con- struction. complete a
temporary framework of timber. wasting a con- siderable amount in cutting in the process.
When this shuttering is taken down. it has to be torn apart and unless some sections have been pre- fabricated (at additional expense), its reuse on the same or another site requires further cutting to waste. Often the timber twists and warps under stress of heat and contact with wet concrete, so that it can be used little more than six or eight times.
In the United Kingdom. timber shuttering began to give way to steel formwork in the middle thirties. When war broke out in 1939. normal timber supplies from the Baltic were cut off and engineers quickly produc- ed highly developed systems of steel formwork for reinforced concrete construction. Today, there are steel formwork systems available which are the result of the experience of twenty-five years of service to the construction industry. and in most countries of the world where building is in reinforced concrete they are in
use.
The basic objects in view in the design of steel formwork are:
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Complete prefabrication of ad- justable components with loose or expandable parts. Speed and ease of erection on site by unskilled labour.
• Known characteristics of each component enabling complete calculation of loadings, to ensure use of minimum material to achieve the desired result in the time available.
High degree of durability enabl ing hundreds of uses over many years.
The normal system of steel form. work for concrete frame buildings comprises adjustable steel props under the beams. adjustable steel floor centres between the beam sides and panels above. The loading on the props varies unbraced between 1 ton for the largest size fully extended to 16 feet. to 2 tons for the smallest extended to 10 feet. When suitably braced. the permissible load is 3 tons and 6 tons respectively.
These props normally carry the weight of the beams and in addition. are capable of carrying. on an average. 40 square feet of shuttering with a 9 inch slab above. Con- sequently. to provide a medium whereby the fullest advantage is taken of the maximum possible loading of the props, telescopic steel floor centres have been designed.
These floor centres span unsup- ported up to 28 feet between the beam formwork. Upon the floor cen- tres spaced at either 2 feet or 4 feet centres are placed steel panels, either of a pressed metal or angle welded to plate type.
The saving in labour has always been a foremost concern with the
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4
designers and it is estimated that in normal construction. unskilled labour can erect on an average of 60 square feet of slab formwork per men hour. This is the figure normally realised in the United Kingdom and Western Europe.
A further saving is the speeding up of the cycle of operations on the This is job with less materials. achieved by stripping the main form- work and repropping a short time after the slab has been poured. Con- crete gathers sufficient strength with- in three days to be able to dispense with overall support, provided it can be shored at a few points. It is now accepted practice in many countries where steel forms have been used for a number of years. to remove the formwork under slabs in three days with props under individual panels. left in position in suitable places, thus not disturbing the slab.
This is possible with steel form- work where the dismantling is carried out piece by piece and not the com- plete breaking up of the form as with timber. This method of stripping is recommended by the American Con- crete Institute and by the Cement & Concrete Association in the United Kingdom. It is therefore possible with extra props to reuse steel form- work every third day after pouring.
A further factor to bear in mind is that with the floor centres being free spanning between beams, it is possible to erect the slab formwork for the next floor over the one re- cently stripped as no weight will be imposed on this slab.
There is no doubt that steel shut- tering is repidly replacing timber on construction sites and, once contrac- tors use steel, they do not go back to the wasteful and expensive timber methods.
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