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TAR MACADAM PLANTS FOR ROAD SURFACING.

3rd July 1923, Page 35
3rd July 1923
Page 35
Page 36
Page 35, 3rd July 1923 — TAR MACADAM PLANTS FOR ROAD SURFACING.
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The Making of Hot Bituminous Macadam in Portable and Stationary Plants for Road Construction.

THE USE of bituminous macadam and asphalt as a road carpeting has grown to an enormous extent in recent years, whilst tar macadam has become equally popular with road en

• gineers. because of its ability to carry heavy traffic.

John L. McAdam introduced his system of constructing a road surface of broken stone, bound together by sand and water, about 1815, and although this was a big development upon the road surfaces of that day, the macadam or waterbound road is now being super. soded because it is quite unsuitable for carrying heavy traffic, and is unplea

santly prone to the production of dust.

Tar macadam consists of .broken stone, slag, clinker, or other suitable road ma' tertal, which has been dried and heated to a certain tknperature, an:el mixed with a hot, binder, which is wholly or principally . tar, although bitumen, pitch, ail, or proprietary binders may be added. Tar macadam can be laid either hot or cold and, having this advantage, it can be made at the quarries and •des-patched to long distances and, if necessary, stored for fairly . long -periods.. Bituminous macadam is made in much the same way, but the material .must be kept in the drying and heating

ovens for a longer period, and it should be laid and rolled hot, or, at any rate, before it has so far cooled as to lose its plastic property. Once this plastic property . has gone the binding of bitumen is much less effective; hence it is necessary for a bituminous macadam to be made within a quick-delivery, radius of the point where it is to be laid. • • Asphalt, properly defined, consists of a natural or refined bitumen, homogeneously mixed, principally with sand cd very fine grade. Sometimes it is mixed with ;stone or aggregate up to in. in size: It. is thus• only suitable for top-surfacing or carpeting.

. Modern road surfac;es are . generally laid in, at least, two separate coats, the 'bottom coat comprising stone of from 2 ins, to 1 ins,, whilst the top coat should comprise much finer material.

The materials that are generally used in making tar macadam and bituminous macadam are granite, basalt, limestone, basaltic whinstone, quartzite, sandstone, blast slag(hot or cold), steelwork slag, clinker, gravel, or Kentish rag, their weights per cub; ft. of crushed material ranging from 50 lb. to 110 lb.

We have lately been watching at their work the Ransomes batch-type combined drying and mixing plants, and have been extremely interested to note the efficiency of the plant at every phase of the operations . We have seen this

• .plant both ba its portable foridand also at .a stationary plant Each type includes an elevator skip, one ot more drying drums with a 'furnace below and a flue and fan, a mixer, ,driving pulley and gear, and the underframe, which forms the operating platform. In addition, there are weighing machines and measuring troughs, whilst storage tanks, pumps, arid dust-collectors are some. times included in the equipment. The sequence of the operations is as follows. The aggregate which is to be employed is fed by chute, or shovelled, c4.5

or tipped into a skip which measUres and weighs a definite batch. This skip is then hoisted by power, strikes an automatic knock-out gear and discharges its contents into a receiving hopper at the top. of the apparatus. From the hopper the aggregate passes into the drying chamber or drum, which isrotated over the furnace, the hot gases passing through the drum, and the sand and other material being continually lifted by baffle plates and dropped through the heated gas. Taking a small batch of, say, 400 lb., the -material is heated for about 10 minutes (the temperature being roughly 400 degrees F.). When the moisture has been evaporated and the aggregate heated to the required temperature, it is taken by a chain elevator to another hopper, tipped out, and, in souse cases, where a definite knowledge is required of the quantity of material used, is again ,weighed (because there is a distinct loss of weight by the removal of the moisture). It is -then tipped into a mixing drum. Bitumen melted in boilers is brought in buckets and tipped into a measuring trough (this is sometimes hung upon a spring-balance in order that accuracy in weight may be secured), and thence into the mixing drums, where it is thoroughly incorporated with the aggregate. The whole hatch is then discharged into a receptacle, from which it is nsually tipped into lorries for conveyance to the scene of operations. Here it is tipped on to a stage from which it is taken by barrows to the gang laying the surface, and, after the requisite thickness has been laid, the steam-roller or motorroller passes over it once or twice. . This plant is made by the Ransomes Machinery Co. (1920), Ltd., 14-16, Grosvenor Gardens, London; S.W. 1, in various sizes from a batch capacity of 6 cub. ft. to 27 cub. ft., with all hourly output of from 3 tons to 24 tons, but the greatest output for a portable plant is 12 tons per hour, in this case using aggregate measuring up. to 2k ins. As a rule, a batch of material can be turned out every 21 minutes. Ransomes plant is employed by many of the county councils, county boroughs, urban and rural district councils, quarry owners and contractors. •

Tags

Organisations: US Federal Reserve
People: John L. McAdam
Locations: London

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