跌拉压芮近勿請 DANGER
PRESTRESSING WORK
Using master gauge during stressing to calibrate pump gauge
intervals either against a proving ring assembly or by inserting a master gauge to check the jacking pressure shown on the normal pump gauges. When not in use the master gauge was kept by the resident engineer's staff. All proving rings and master gauges were provided with up-to-date calibra- tion certificates issued by a reliable testing laboratory.
Beams of up to 85 ft. in length were grouted successfully from one end, using grout with a water cement ratio of 0.45 and without additive. Grouting pumps which gave a continu- ous flow of grout by screw action were most satisfactory and each pumping installation had facilities for power- mixing, screening and re-circulation of the grout. At the ends of each duct 4 in. dia. entry/exit tubes and valves were fitted, using reducers to suit the anchorage assembly grout hole; valves any smaller in diameter were found to impede grout flow.
After sealing the ends of each duct and immediately prior to grouting, ducts were filled with water, blown through and then filled with water a second time. Grout was then pumped in to displace the water and a super- visor stood by to ensure that grout of the required consistency was flowing prior to closing the outlet valve; pump- ing continued to achieve 100 p.s.i. in the duct before closing the entry valve. Earlier attempts at grouting through wetted ducts after blowing out the water were not successful because high
36
ambient temperatures caused the grout to solidify and block the ducts.
Post-tensioned in-situ frames
In-situ cantilever frames at Canal Road Road flyover were stressed using 12/0.6 in. dia. strand cables, the load being applied in one operation using a Freyssinet jack and anchorage system. On the advice of the manufacturer, to ensure proper seating each dead-end anchor wedge was knocked up with a heavy hammer until approximately half-load was applied; also the male flutes were well greased prior to inser- tion into the female cone to ensure that the wedge was drawn fully home.
Due to the promixity of adjacent occupied buildings, as a safe-guard against cable failure the jack was tied back to the concrete structure. Allow ance was made for jack extension and substantial timber barriers were fixed to stop individual strands. In the event there were no cable failures, but dead- end wedge slip did occur at full load causing the jack to be damaged in such a way as to be unserviceable. A re- placement jack had to be shipped into Hong Kong, fortunately without the delay seriously affecting contract com- pletion, but this instance does serve to illustrate the commonly-encountered problem of specialist equipment being provided in Hong Kong only to order without stocks to provide back-up service.
Column ducts were grouted from the bottom upwards and the grout
Arrangement for calibrating stressing equip. ment against a proving rig
pressure was increased to allow for the weight of the column of grout.
Large pockets left in the columns, giving access to the dead-end ancho- rages, were accessible only from inside the canal. When filling these pockets, an expanding agent was added to the concrete concrete to obviate shrinkage and thereby ensure water-tightness.
Road construction
Trafic diversions
Major traffic diversions were agreed by the consulting engineers with the appropriate authorities at the pre- tender stage and contractors were ins- tructed to allow in their tender for all costs arising from diversions. Each contractor was expected, therefore, to submit his proposals for implementing diversions as they were required, such proposals being examined on site and agreed with the police and other authorities.
Detailed liaison by the resident engineer's staff kept administrative delays to a minimum and procedure which became established as experi- ence was gained ensured that agree- ment in writing, including the circula- tion of sketch drawings to all concern- ed, was achieved in each case without delay to the contractor.
The resident engineer's staff pro- vided co-ordination between the con- tractor and the police for implement- ing each diversion or activity affecting public roads and therafter for ensuring
Far East BUILDER, February 1971
30
20
10