February_1971 — Page 41

Compacting and filling at the east abutment of Lai Chi Kok Interchange

Completed Lai Chi Kok flyover interchange

to achieve a good appearance in line and level for both straight and curved sections of rail.

Traffic direction signs were detail- ed and constructed to meet the re- quirements of the Traffic Engineering Division. "Scotchlite", which is ex- pected to have a seven year life was used, and the structure was therefore detailed to be as maintenance free as possible. The tubular supports were mild steel with full paint protection and the backing plates and stiffeners were aluminium with chromium-plated nuts and bolts and suitable means of isolating steel from aluminium.

Asphalt surfacing

Two grades of dense hot-rolled asphalt surfacing were used, incor- porating 11⁄2 in. aggregate with 4.8%

Far East BUILDER, February 1971

bitumen content for the base course and 4 in. aggregate with 6% bitumen content for the wearing course (Figs. 8 & 9). Flyover concrete decks were finished with 2 in. thickness of wear ing course asphalt; elsewhere roads were surfaced with 4 in. of asphalt in two layers of base course and wearing

course.

Asphalt was obtained from various quarries in Kowloon and Hong Kong; these were proposed initially by the contractors and subsequent acceptance was dependent upon satisfactory trial mixes. Mix design was based on the grading found satisfactory for the asphalt surfacing of the Kai Tak Air- port runway (Reference 3).

Asphalt was spread either by machine or by hand; satisfactory re- sults were achieved with the latter

method providing that strict control during laying was available and that continuous timber edge profiles were provided at longitudinal joints be- tween kerbs. Asphalt was delivered to site with a mean temperature of 240°F and compaction began when this had fallen to within the range 180°-200°F. The actual laying temperature was determined by the behaviour of the asphalt under compaction and it was found advantageous to adopt a break- down rolling technique using a 2-3 ton smooth wheel roller initially and fol- lowed by a 10-12 ton smooth wheel roller for final compaction.

A particular problem was compact- ing asphalt on flyover ramps with gradients as steep as 1 in 10; for these it was necessary to compact only in the up-ramp direction with the drive wheels leading, the roller then return- ing down the ramp clear of the area being compacted.

At Lai Chi Kok interchange and Canal Road flyover, mild steel round bars were fixed at intervals transver- sely across steep ramps to provide positive keying action and thereby pre- vent inter-surface slip between asphalt surfacing and the concrete deck slab.

Some instances of relative move- ment between wearing course and base course asphalt occurred at locations. where vehicles impose considerable transverse force at road level. At such locations the 11⁄2 in. thick wearing course was taken up and, after scarifi- cation of the base course and re-laying of the wearing course, no further movement was recorded.

A problem being investigated at the time of writing is the excessive wear in patches of the surfacing at some loca- tions. This is believed to be caused by the action of heavy vehicle wheels coupled with the washing action of heavy rain after damage of the bitu- men binder by diesel fuel spillage or deposits from exhausts.

References

1) Design and Construction of Lai Chi Kok Bridge by W. A. Maslowicz and G. Wilson: Far East Builder, April 1969.

2) Concrete Technology and Prac- tice by W. H. Taylor, Hal-Stead Press, 13.2.2. Grouped Frequency Method.

3) Asphalt Work For The New Hong Kong Airport by F. J. Parker and J. K. Holt: Paper No. 10 of the 1960 I.C.E. Conference on Civil En- gineering Problems Overseas.

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