January_1971 — Page 45

in order to improve progress when structural work fell behind pro- gramme.

The flyover geometry was complex with vertical curvature and steep gra- dients combined with varying cross-fall and for this reason column capping beam falsework was erected in con- junction with that for the hollow deck structure to the levels shown on the working drawings and the whole was then checked by eye for any irregulari- ty of line which was then corrected by adjusting wedges below individual props.

Falsework for Canal Road flyover was designed to take account of the high risk factor involved because the ground level area below part of the flyover had to remain open at all times to the public. Although in this ins- tance work was completed without incident the possibility of falsework collapsing is always present no matter what precautions are taken and plac- ing concrete in such localities was always, therefore, a matter for con- cern. There appears to be a strong case for the closure of such areas to the public while overhead work is in pro- gress.

Canal Road flyover approach spans each side of Hennessy Road were en- tirely in-situ because of the curvature of the flyover at this location and can- tilevers beyond the support columns. With both longitudinal gradient and super-elevation for horizontal curva ture, the soffit shuttering was sloping and allowance had to be made for out- of-balance loading.

A major bracing system tied back to the permanent supporting columns was therefore provided at soffit level, with a continuous edge girder to trans-

fer any local unbalanced forces into the main bracing system. The steel props used for the falsework had a rated working load of 3 tons but had been tested by the contractor with a load of 6 tons. Following the erection of the falsework, a crane was used to place a point load of 6 tons at various locations on the soffit as a means of testing the falsework structure.

The use of control cubes to assess concrete strength prior to the removal of false work was allowed but any major concrete member in tension was supported for 18-21 days. The removal of false work and re-propping was not permitted and for early re-use of false- work the contractors' designs had to incorporate means of leaving sufficient props undisturbed when the major part of the falsework was taken down. Reinforcement

With a few exceptions, all rein- forcement was mild steel. Local and other sources were used and to supple- ment manufacturers' test certificates samples were taken on site and sent for further testing at an approved la- boratory. Occasional consignments of smaller size bars were rejected when failing to meet British Standard re- quirements.

There have been few problems in fixing reinforcement except for the usual difficulties of placing at heavily reinforced locations such as the junc- tions of columns and capping beams. For the Canal Road flyover in-situ ap- proach spans, however, special steel support beams had to be provided be- tween the permanent columns where the weight of reinforcement was heavi- er than could be carried by the usual

means.

In-situ concrete

Concrete for the Lai Chi Kok inter- change and for the Canal Road flyover foundations was supplied from off-site batching plants. Otherwise concrete was batched on site using normal con- trol methods; all concrete had to com- ply with probability of failure not ex- ceeding 1%. The results achieved and a summary of mix proportions are shown in Fig. 5. Cube results were ana- lysed monthly, using the grouped fre- quency method (Reference 2).

Mixing plants on site were of sim- ple design, mostly locally produced, but regular checking of batching equipment and supervision at the mix- er produced satisfactory results. Stand- ard methods of sampling and testing materials for concrete were imple- mented at each site.

Crushed rock fines were used ex- tensively in preference to marine sand which might have caused staining on exposed wrought faces. Attempts were made to control the fraction passing a No. 100 B.S. sieve, which varied be- tween 8% and 14%. Such fine material is believed to contribute to surface hair cracking and when this did ap- pear during an earlier contract it was eliminated by stricter control of fines coupled with later trowelling.

Plant requirements and concreting methods were agreed with each con- tractor in detail beforehand, taking account when necessary of delivery problems likely to arise because of traffic congestion around working areas; the longest continuous pour re- quired 18 hours for completion. Deep in-situ beams were concreted in advan- ce of deck slabs to avoid slump crack- ing at the junction between beam and slab.

Location

Class of Concrete

Ave. 28 day strength

Ave. Standard Deviation

Ave. Coefficient of

Variation

Agg./Cement Ratio

SUMMARY OF CONCRETE

For Contracts of: Chatham Road Flyover; Canal Road Flyover; Canal Road Flyover Extension

& Associated; Lai Chi Kok Bridge and Lai Chi Kok Interchange

Canal Road Flyover

"

Canal Road Flyover Extension

Lai Chi Kok Bridge

Lai Chi Kok Inchange

* 3/4"

"

-

6539 5202

445 718

3/4"

** 3⁄41⁄44"

Chatham Road Flyover 6500/4500/ 3750/|6500/ 4500/|3750/ 6500/ 4500/ 3750/6500/ 4500/ 3750/6500/ 4500/ 3750/

" Y" 3⁄4"

Y" 3/" Y"

7739 6248 4740 7611 5862 |52708065 |6038|4601 7587|6072|5042

625 509 428 402 540 460 650 710 466 843 834 745

8.1 8.1 9.0 5.3 9.2 8.3 8.1 11.8 10.1 816 10.9 9.7

1:3.7 1:5.5 1:6.0 1:4.2 1:5.5 1:6.5 1:4.5 |1:6.0|1:7.0 |1:3.7 1:5.51:6.5

43 38 42 43 42 42 40 43 43 35 38 42

9.9 12.6

T

1:4.5 1:5.7

Y"

31.6 31.0

Proportion %

3/8"

24

32 27 22 23

25

25 22 22 35

32

27

-

31.4 31.0

by weight

Fine

32

30

31 35

35

33

35

35

35

30

30

31

-

37.0 38.0

Fig. 5

Far East BUILDER, January 1971

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