No_1_February_and_March__1939 — Page 23

Hong Kong Builder

29

23

VIBRATING CONCRETE

By G. P. Manning, M.Eng., A.M.Inst.C.E.

2

3

Cement Supply

Sand Supply

Shingle

Supply

4 Water Supply

5

Unloading and Stacking

Gauging

7

Test Cube

Mixing

Stage

Transporting

K.U.

ARCHITECTURE

Placing

10

Consolidating

"

Curing

(Finished Concrete

Fig. 1.-Eleven steps in making concrete.

The main object of vibration is to consolidate and pack the concrete, driving out air bubbles, and to achieve this with fairly dry concrete.

And

Is modern concrete in need of such a process? if vibration is a success, will this result in a marked and striking advance or merely a minor improvement?

Fig. 1 shows the eleven items that make up the finished concrete and these, it will be seen, are mostly in the form of a series or chain. The usual weakness of modern concrete literature is that the "expert" confines his attention to one link only, ignoring the other ten. This leads to distorted, contradictory and erroneous views. Little reliance can be placed on test-cube strength as showing the quality of the finished concrete for there are four "blind" items (Nos. 8, 9, 10, and 11) between the two.

In this country item 4 is usually drawn from a drinking water supply and may be left out of the argument. With this possible exception all the other items are capable of improvement, but as the strength of a chain is only the strength of its weakest link, improvements in the strong links will have little effect on the final result. For example, a modern concrete mixer makes an excellent job of item 7. Modern mixers are not perfect and can be and will be improved, yet this will cause little or no improvement in the finished concrete. Undoubtedly the weakest (and least under- stood) link is item 10, and any appreciable improvement in this item would be a big advance. Vibration is therefore a major issue and not merely a question of detail.

Fig. 2 shows two specimens made by the writer some years ago to illustrate the importance of consolidation. They are both made of a rich, easily workable concrete. The concrete in the specimen on the left has been care- fully and thoroughly packed in the mould. The concrete in the right-hand specimen has been carefully placed a trowelful at a time, but no ramming or shaking has been done. The reader may say that he has never seen any concrete similar to the right-hand specimen. This is probably true as all unconsolidated patches are "made good" as soon as they are exposed. Fig 3 shows the right-hand specimen in Fig. 2 after one face has been "made good."

Test-cubes are readily consolidated, but then they are not reinforced and a great deal of time is spent on making them. If a man spends five minutes consolidat- ing a 6-in. cube, this is at the rate of 18 hours per cu. yd.--eight or ten times the amount spent on the concrete in the structure. Yet the amount of physical energy required is small and it is clear that a perfect con- solidating machine would pack all the concrete in the structure to the same degree of perfection as the concrete in the test-cube at a power cost of a few pence per cu. yard.

The Problem.-Fig. 4 shows a section of a T-beam near the support as conceived by the designer. Practical engineers may say at once that the design is bad and the steel much too crowded, but academic experts and beginners may retort (and usually do retort) that the section is strictly in accordance with the D.S.I.R. Code, Fig. 5 shows the section as built assuming first-class steel-bending and steel-fixing (with a variation in the shape of the bars of only 14 in.) and good hand- ramming for placing the concrete. The figures in circles indicate the crushing (cylinder) strength of the concrete. It is true that we have no exact means of measuring the crushing strength of the concrete in Fig 5, and must rely on the appearance of the concrete and the difficulty (or otherwise) of cutting it alway for our estimate of its soundness; but there can be no doubt that the figures in Fig. 5 show the general state of affairs.

If there were no steel whatever in the beam we could easily make a very sound job of the concreting by hand ramming, even with a fairly dry mix and this could be done with a small expenditure of energy. The problem is therefore to find some fool-proof machine that will supply this small amount of energy exactly where required.

Vibrating Pre-Cast Units.-Provided the moulds are supported on flexible timber sleepers we can place the concrete in pre-cast units, such as large pre-cast piles, very successfully with mechanical vibrators. For example, an outfit consisting of six 160-watt vibrators placed concrete up to 10 cu. yds. per hour in 16 in. by 16-in. piles, the machines being spaced at 6-ft. centres along the piles. Thus, each 160-watt machine vibrated 6-ft. length of pile, or 10.7 cu. ft., equal to 15 watts per cu. ft. With all the machines running the outfit con- sumed only 960 watts or less than one unit per hour.