No_7_November_and_December__1950 — Page 4

WATER PRESSURE. 200. P

Fig.

8-O' INVER SAM

Fig. 10.

CONCRETE BAGUNA

Fig. 12.

-

NETH CONGTETE

JS® CONCRETE KUKORI

- A LONG

SERUGHE A MA

SEE DETAL

S" DE PENT HORTRE

PECTION

·

Fig. 11.→→

SECTION THRO| TROUGH

SMEETING

STRETCHED TO THE MACTAN 29

2 GARVES BASE PET OF STEEL

BOURGE - PUR JODI PRESUŠE

SERPONG KAMLAY

CURVED WELL, AFTER,

ONE ON ENCH Lar on Songs BNG

up the hardening of the concrete by the combined treatment with vibration, heat and pressure. The compressive stress induced in prestressed concrete pipes is usually chosen to be between 2000 and 2500 lb. per sq. in. This stress should be reduced to nil under test pressure which is generally double of the service pressure. The prestressed concrete pipe is about 16% lighter than the equivalent cast iron pipe, and about twice the weight of the equivalent steel pipe proper not taking into account the usual lining and external covering of steel pipes.

The method of production usually employed in U.S.A. is that of wire wrapping by mechanical means. Such pipes are produced by the Lewistown Pipe Co. and Lock Joint Pipe Co. A high tensile wire is wound under controlled tension on a prefabricated concrete core pipe which is preferably spun; and a layer of cement mortar is applied to the steel spiral as a finish either by cement gun or by brushes.

rather

The design of prestressed tunnels, is in principle, exactly the same as that of prestressed pipes. The methods of con- struction are different due to the fact that the prestressing has to be done on the site, while for pipes the methods are adapted to factory production: and further due to the fact that space during the construction of tunnels is restricted. One of the various construction methods used is shown in Fig. 12. The lining consisted of a plain concrete backing and an inner lining of accurately moulded concrete voussoirs. Each ring of voussoirs was surrounded by one parallel-wire cable placed in an annular box of steel sheeting to allow free movement; the ends of all wires were anchored in the concrete backing. In each box, between the cable and the bottom, there was an inflatable copper ring which was used as a jacking device and remained incorporated in the structure. By pressing a quick-hardening resin, such as Brauthite, into the inflatable ring the cable resting on it was expanded radially: the steel cable was stretched and the concrete lining compressed to the designed strains.

In the design and in the construction of prestressed storage tanks the U.S.A. have certainly progressed far beyond the experience of Europe; and most of the existing prestressed tanks have been built there. The tanks are used for the storage of water, Diesel oil, or petrol. They have been erected for 20 to 35 ft. head, the diameters varying from 50 to 200 ft., and storage capacities varying from 200,000 to 4 million gallons.

Fig. 13. →

The main disadvantage of large diameter tanks built in normal reinforced concrete is that owing to the dense rein- forcement the vertical walls must be cast in horizontal layers, thus creating working joints which frequently open under the influence of shrinkage. Prestressed concrete tanks using stretched hooped steel are usually concreted in circular segments of the full height and 20 to 30 ft. wide without horizontal working joints. The radial vertical working joints Pre- cannot open, as they are under constant compression. stresses are sometimes also applied in vertical direction to counteract the effect of shrinkage and temperature differen- tials.

The first proposal for a prestressed concrete tank was put forward by Hewett, 1923. The latest development is the "merry-go-round" equipment, Fig. 13, designed by the Pre- load Co. which moves around the wall and pays out the continuous peripheral wire. The wire is pulled from the coil through a resistance mechanism which imparts the specified tension to the wrapped wire. The machine is hung from a supporting carriage mounted on the rim of the tank and moves at an average speed of 2 m.p.h. The wire bands are protected by a permanent cover of pneumatically applied mortar of approx. 3.4 in. thickness which also serves to bond the steel bands and the concrete wall together. Where more than one layer of wire bands is required, each layer is shot with mortar before the succeeding layer of wire is applied.

To be continued)

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