30
Hong Kong Builder
LUI
Some engineers complain that long piles of vibrated concrete are very liable to show cracks. This is the case unless special precautions are taken, but similar cracking occurs with hand-rammed piles of similar length, and although vibration does not appear to reduce the tendency of modern cements to crack it does not increase it.
Against the fact that we can successfully vibrate precast units such as large piles, we must remember that it is easily possible, with a little organisation, to produce a first-class job of such units by hand ramming, and vibration in such cases is little, if any, superior to previous practice.
Suspended Floors.--It seems clear that light or moderately heavy suspended floors may be successfully vibrated if the shuttering supports are flexible. The usual type of strut as in Fig. 6 allows of little or no vertical movement and some modification as in Fig. 7 will be required. This calls for a revision of our present methods of designing shuttering and more expensive falsework. It will require a year or two of experience before we can say exactly how many vibrators of any given type or power will be required per cubic yard of concrete. There still remains the difficulty of vibrating the concrete immediately over the columns and possibly some minor difficulties at construction joints.
Fig. 2, left. Two specimens made to illustrate the im- portance of consolidation. The concrete for the left- hand specimen was thor- oughly packed into the mould; that for the right- hand specimen was placed a trowelful at a time, but without ramming or shak- ing. Fig. 3, right, shows the unrammed specimen after one face had been made good.
Walls and Columns.-Up to the present the writer has seen no method that will successfully cope with vertical work which stands directly on other concrete. There seems to be no practical method of making vibrations pass upwards through the whole mass, and vibrations will not pass horizontally through dry con- crete. When vibration is applied to a vertical surface in contact with dry concrete, only the surface layer, to a depth of an inch or two is affected. If the vibration is continued in the hope of making the effect spread inwards, the surface layer is over-vibrated and the water, cement, and fine sand run up the shutter to the top surface of the concrete. Even with thin walls there is this difficulty with the lowest part of each lift.
The
Internal versus External Vibrators. The use of hand-applied internal vibrators seems a very haphazard process. If the vibrator is withdrawn too quickly it leaves a large hole. If it is left in one place too long it makes a "puddle" of water, cement and fine sand. use of hand-applied internal vibrators for large volumes of in situ concrete does not appeal to the writer as likely External vibrators to succeed under contract conditions. clamped to the shuttering in definite positions and moved in definite rotation present a much more fool- proof system.
Fig. 4, section of a T-beam near the support as designed; and Fig. 5, right, the section as made, allowing for good hand- ramming when placing the con- crete. The figures in circles indicate the crushing (cylinder) strength as designed and as obtained.
(3000)
·10
(3000)
888
As drawn
888
(3000)
(2500)
Air
pockets
(1000)
Making
good
Making good
-9%-
(1500)
As made
Air
pockets