No_2_1960 — Page 48

Pressure

kit

Piling

bored piles

special foundations

underpinning

site investigation

rock drilling and coring

THE PRESSURE PILING CO (HONGKONG) LTD

1201 UNION HOUSE · HONGKONG · 38021

46

rise to interesting uses of rubber tube to form ducts in the concrete.

The walls of the dam are often cellular in character, built of a number of self-contained chambers. Seepage makes it necessary to drain. these internal chambers and conduct the seepage water to the outside. The water is conducted from a central drain and discharged at low level by means of a network of ducts formed in the thickness of the con crete dam walls.

The mass concrete construction of a dam is often 30 or 40 ft. thick and during the construction of such a huge mass of concrete, difficulties may arise due to the heating of the concrete. It is im. portant that this heating should be kept within limits if the ultimate strength is not to be impaired due to honeycombing or other faults arising from bad curing. The set- ing concrete may be cooled by form. ing ducts at intervals through which cold water or air is passed. This is a most effective way of liberating the troublesome internal heat.

The formation of construction joints is a frequent problem in con- crete design. During the erection of a concrete building it is custom- ary first to position the metal rein- forcement bars and then place con- crete so as to have part of the metal projecting into the next section of concrete, which will be done later. By this means

a proper bond is subsequently attained between the two sections of work, The reinforce- ment is usually in two vertical planes just inside the front and back faces of the concrete wall.

The temporary provision of a vertical stop-end to the concrete is

an

elaborate operation requiring careful cutting around the steelwork, usually with considerable wastage of the material when taken down for

re-use.

Inflated rubber tubing inserted be- tween the two vertical skins of steel reinforcement is an excellent means of forming an edge to the concrete. The tube maintains intimate contact with the metal and the slight undulations created by the pressure of the metal against the tube, and the half-round channel left in the concrete by the tube, form a valu- able key between this and sub- sequently placed concrete.

or

The tubes are quickly demounted and assembled in the next position without any waste of material time. This system is equally applic able to the construction of expan- sion joints in concrete.

A further use in the construction of in situ concrete is the insertion of rubber tubes in the foundation concrete of engine beds, stanchion bases, and wherever holding-down bolts are required. The concrete is placed around the inflated tube and when it has set, the tube is with- drawn, leaving a cavity for the in- sertion of the bolt.

Two methods are used: (1) where a rope or wire is wound round the rubber tubing to form a key in the wall of the resultant duct to receive the bolt, or (2) where the bolt is first positioned, complete with ກ bottom plate, through a cylindrical former around which small diameter tubing is closely wrapped. After casting, the former and tubing are removed, leaving the bolt already in position.

Figs. 5 and 6 illustrate precast concrete products which were made with several ducts in order to reduce their weight. This was necessitated by the capacity of the crane on the site making it imperative to cut down the weight from 2 tons 9 cwt. to 2 tons 2 cwt.

The construction of moulding tables for the casting of often

concrete rubber

owes something to duct formers. The sides of the moulds may themselves be of con- crete through which a continuous duct conducts hot air. This has a helpful effect on the curing of the green cement in the mould.

Although it is believed that this and the preceding article cover all the important uses of rubber duct formers the author is aware that there may be other applications devised by many who have handled the material and have seen its trem- endous possibilities. He would be glad to hear from any readers who are aware of any novel applications.

The author is indebted to the Ductube Company, Ltd., of London for their help in preparing this ar- ticle.

(This is the second part of a two-part article issued by the Natural Rubber Development Board).

THE HONG KONG & FAR EAST BUILDER

VOLUME 15, NUMBER 2