28
0" high elliptical arches, 5'-8" thick by 6'-0" base width were formed in each pier. The piers each measuring 49-8" long by 5'-8" wide by 12-3" high, were cast in two lifts using 1:6 ready mixed concrete supplied by Pioneer Ready Mix Concrete Ltd.
All piers and the abutments were completed in April 1966.
Precast Prestressed I-beams
A total of 15 main beams and 9 side beams were used for the bridge. The 106-10" long main beams were 60 in. in height with a 7" web. Top and bottom flanges were 28" and 30" in width respectively. Side beams were 7" higher with an 18" wide top flange. Twelve cables, each formed with 12 No. 7 m.m. diameter high tensile plain wires, were adopted in the design of the main beams and ten cables in the side beams. The Freys- sinet system of posttensioning was adopted for prestressing.
were
Several trial concrete mixes with different water/cement ratios carried out. A 1:4.5 concrete mix with a 34" maximum size of aggregate was finally adopted for the beams.
Flexible
ducts,
metal sheath 1-21-32" diameter with the wire ten- sioning cables inside and fitted with Freyssinet cone anchorages, were fix- ed in position by means of mild steel master stirrups. The ducts were carefully checked before concreting to ensure that they were suspended in the designed positions to within the allowable vertical and horizontal tolerance of %". External vibrators were attached to the bottom flange of the steel formwork at 3'-6" centres and used together with two poker vibrators. Concreting of the beam was carried out in a continuous operation from one end.
One of the two 12-wire E-type hydraulic Freyssinet jacks used in post-tensioning the cables of the main beams
across the bridge spans.
After curing for seven days and before transfer. sliding packing wedges were fitted to each end sec- tion of the beam soffit to enable the weight of the beam as it hogged un- der the stressing force to be trans- mitted onto the packing. In this way any slight movement between the beam and its support was taken up by the moving wedge, thus avoiding ex- cess shearing stresses being developed in the tapered end section of the beam.
Shuttering was removed from the beam after three days and curing was continued by covering the beam with hessian and keeping this continuously wet for a further four days.
Beam handling and Trans- portation
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Stressing was not applied to the Care was taken to ensure that beam until the crushing strength of beams were cast in a sequence to
test cubes taken had achieved facilitate subsequent selection and to minimum strength of 5,300 lb. per reduce movement after tensioning to sq. in. The post-tensioning operation a minimum. Due to the restricted was carried out in the designed se- width of the casting yards on the ap- quence, tensioning each cable from proach embankments it was essential both ends simultaneously using two that the beams which were cast with hydraulic 12-wire Freyssinet jacks. skew ends did not require turning up- After tensioning the sheath ducts their length before launching were thoroughly flushed with fresh
Layout of the contractor's main beam casting yard
on
water and dried with compressed air and filled with cement grout pumped in at a maximum pressure of 56 lb. per sq. in.
Camber recorded for most of the beams immediately after tensioning
was
1% and this increased to a maximum of 2% after six months.
Load testing was carried out on one of the beams selected arbitrarily to test beam deflection and stress in the concrete. Eighteen dial gauges were set in pairs under the beam soffit with 36 electric micro-strain gauges plac- ed at predetermined positions on the beam and connected to static strain
meter.
The testing load was applied at mid-span with increments of 10 tons up to a maximum of 40 tons and equal to one and a quarter times the sum of the dead and imposed loads designed. The test load was maintain- ed for 24 hours and readings from dial gauges and the strain meter were recorded during each increment of load and at 24 hours after removal of load. Deflection recovery 24 hours after removal of the load was 97.7% and was considered very satis- factory. Stresses derived from the measured strain results closely agreed with theoretical values calculated.
Far East Architect & Builder February, 1967
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