January_1971 — Page 46

fitted between the standard side shut-

ters.

Concreting approach spans of Canal Road flyover

Edge sections were concreted after the deck beams and slab were com- plete, which allowed a continuous route centre-line to be marked and agreed at deck level; thereafter this was used as a base line for offset pur- poses when fixing edge shuttering. By this means a good edge alignment was achieved by all contractors.

Vertical concrete surfaces were cured by covering with saturated hes- sian or other similar material or alter- natively by membrane curing; in the latter case adequate shade also was re- quired. Horizontal concrete surfaces were best cured by ponding but some form of damp covering was accepted when early use of the completed slab was required for light loading.

Flyover elevations, including col- umns and walls, were rubbed down manually, using a well wetted car- borundum stone, to achieve uniform colour between in-situ and precast work.

Moulds for precast beams

Contractors chose to fabricate steel moulds for precast beams except where small numbers of a special type were required, in which case top grade water-proof plywood, precoated with polyurethane paint, was used. Each mould was designed with sufficient rig- idity to remain grout-tight when the external vibrators clamped to the mould were used during bottom flange concreting. Complications introduced. by varying skew ends were overcome by using individual timber end pieces

For larger beams with wide bottom flanges and many ducts, openings which would not be visible after erec- tion were left in the bottom flange upper surface for the insertion of pok- er vibrators and were sealed before the web was concreted. Casting-bed soffits were faced with steel or plywood; the former required light oiling to prevent rusting while the latter needed regular attention to remedy surface defeats. Each bed was provided with specially designed ends which were capable of designed ends which were capable of spreading the additional loading indu- ced as the beam cambered concentra- ting self-weight at the ends; this pre- vented cracking around the lower an- chorage pockets.

Cable and duct assembly

End-block reinforcement was pre- assembled by all contractors so as to speed up assembly of reinforcement on the casting beds. Links were posi- tioned at intervals along each beam with brackets pre-welded at the cor- rect positions for supporting each ca- ble duct. These links were fabricated against master jigs and sat on spacer blocks designed to give the correct bottom cover.

In this way the ducts were very easily placed in each beam to correct profile and additional brackets welded to other links were provided to form a rigid assembly after detailed checking had been agreed and recorded. For concreting, the bottom spacer blocks

were removed after the reinforcement and duct assembly had been hung in- side the mould. Strand was cut into lengths and colour coded at each end prior to being made up into cables and particular care was taken to ensure that individual strands ran straight for the full length of each cable. Cables were threaded into ducts with care to avoid twisting before the ducts were placed into the beams.

Strand certificates supplied by the manufacturers were backed up with the results of testing strand at Hong Kong University. It was found that the ultimate load results from testing a 3 ft. length of strand, would not be relia- ble unless a grip length of not less than 6 inches at each end of the test piece could be provided. Shorter grip lengths resulted in individual wires failing due to uneven distribution of load.

Placing concrete for precast beams

Various means of handling con- crete immediately prior to placing were adopted, including skips sus- pended from gantries and skips run- ning on wheels along a platform over the beam mould.

All contractors were required to follow the same procedure when placing concrete. Initially the lower flange was filled and well vibrated over sufficient length to allow an interval of time before web concreting. Finally the top flange was filled and compact- ed but as late as practicable after the web. This was to prevent slump crack- ing at the roots between web and top flange, some of which did appear on early trialcasting sections which were not concreted in this way.

End block concrete was always vib- rated up to the end of the beam rather than away from it, in order to obtain the best possible compaction; careless- ness in this respect on one occasion resulted in anchorage failure due to failure of the backing concrete during stressing.

Handling Precast Beams

After stressing and prior to grout- ing, beams were raised from their beds, either by jacking or by the use of gantries, and then placed on pneu- matic-tyred bogies or trolleys on steel track. Beds were laid out to avoid the need for transverse movement, except at the Lai Chi Kok Bridge where a suitable rail-track layout was provided.

Beams were erected using either specially designed lifting towers, which in principle were based on piling rigs,

Far East BUILDER, January 1971