Concrete quality
The specification required the con- tractor to prepare trial mixes for each grade of concrete and, with the object that not more than 1 per cent. of concrete would fail to reach the mini- mum specified strength, the average strength required for a trial mix was the specified minimum +2.33 (the
standard deviation) x 700 lb/sq. in.
Three batching plants were set up on the site, each comprising three 21/14 electrically driven, non-tilt mixers and a standby unit of the same capacity. One of the plants was located at the eastern end of the site and was used to cast piles; another placed centrally provided concrete for the precast beams and the third, situated close to the sea wall, supplied concrete for the marine works.
Regular tests were carried out to determine the grading of aggregates and moisture content was determined daily for a stockpile in use. Concrete was sampled at least twice during each pour and tested for slump and com- pacting factor. When casting beams, additional slump tests were made at regular intervals to maintain consis- tency throughout each beam.
At least one set of three 6in. cubes was cast during each pour of an in-situ section. The cubes were later crushed, one after seven days and another at 28 days. The third cube would be crushed only if the 28-day result was not satisfactory.
A minimum of six cubes was made during the casting of each prestressed beam and three of these were left at the beam for crushing before stressing to ascertain that the required strength was reached at transfer.
Construction record
Tenders were invited in April 1966 and the contract was let four months later. Work on the site began in Septem- ber 1966 and the bridge was opened to traffic in October 1968.
The general contractor was Paul Y Construction Co. Ltd.
The total contract cost of the bridge and associated works was about HK$ 13.5m.
The cost of structure per sq. ft. of area (measured between concrete up- stands) was as follows:
Marine
Land
spans
spans
(i)
Piles and pile caps
18.6
12.2
(ii)
Superstructure
39.4
33.8
(iii)
Surfacing, railings, etc.
4.0
4.0
HK$62.0
HK$50.0
Authors
This feature is adapted from the paper Design and Construction of Lai- chikok Bridge', presented last month to the Engineering Society of Hong Kong by Mr. W.A. Maslowicz, BSc, CEng, MICE, senior engineer, Scott Wilson, Kirkpatrick & Partners, and Mr. G. Wilson, BSc, ACGI, CEng, MICE, resident engineer.
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