mix must be adjusted to the speed at which it can be properly compacted in shallow layers, to ensure that compac- tion takes place from the bottom up- wards, and so allows for the escape of air bubbles.
Compaction by hand rodding should never be used, since it cannot be carried out consistently and system- matically. For in-situ work, immersion vibrators are generally preferred to the usual type of external vibrators which are fixed to the formwork, not only because it is easier to obtain a better finish, but also because the formwork need not be as rigid as is necessary where external vibrators are used.
Care must always be exercised in the use of internal vibrators, not only to ensure uniform and adequate vibra- tion of the mix, but also to ensure that the internal faces of the formwork or lining are not abraded or bruised to such an extent that the surface of the concrete will be blemished.
Release agents
Release agents, which are lubri- cants applied to the face of the form- work, reduce the likelihood of the concrete sticking to the form, and con- sequent damage to the concrete sur- face when the formwork is removed. Research has shown that the type of release agent used has a most marked effect on the appearance of the con- crete surface. Release agents may be classified as one of five general types, namely, neat oils, water-soluble emul- sions, neat oils with a surface activat- ing agent, mould cream emulsions, or paints and impermeable coatings.
It has been found that neat oils tend to encourage the formation of small blow-holes, and that water- soluble emulsions tend to produce on concrete a dark porous skin that has a tendency to form dust. Neat oils to which a small proportion of surface activating agent has been added, and mould cream emulsions, tend to pro- duce concrete of a uniform colour, and appear to reduce the tendency for blow-holes to form.
In practice, the addition of up to 2 per cent of a synthetic surface activat- ing agent is sufficient to provide uni- form dispersion in an oil, or stability in an emulsion. Paints and imperme- able coatings, which are usually ap- plied to the form as a seal or surface impregnation, encourage the occur- rence of blow-holes, but they normally produce a concrete surface of uniform colour. Care must be taken to avoid scabbing when paints and impermeable coatings are used.
Design of formwork
Where good-quality finishes are necessary, consideration must be given to the design of formwork, along the following lines:
(i) The formwork must be strong and rigid enough to withstand without excessive deformation both normal handling, and the pressure exerted by the con- crete. It should be as simple as possible to facilitate erection and stripping.
(ii) Joints should be properly design- ed to prevent leakage between panels and at lift lines, as well as between individual boards or lin- ing sheets.
(iii) Vibrations transmitted from the concrete or applied to the form- work itself should not cause leakage, and the formwork should be made stiff to reduce the amplitude of vibration of the formwork to as small an amount as possible.
The joints between panels and between individual boards require par- ticular attention. Butt joints in un- treated timbers are usually unsatis- factory, because shrinkage, swelling and distortion are inevitable. A tongue-and-groove joint stiffens the panel but, if the joint opens as a result of shrinkage, leakage can occur under pressure.
Shrinkage of timber formwork may be reduced by soaking the boards with water, or by thoroughly impregnating them with oil. However, the use of a strip of foamed plastic is recommend- ed at all joints which may open be- cause of pressure or shrinkage. Linings such as steel, plastic, plywood and hardboard, which are not subject to significant shrinkage, may be butted together and taped on the back with masking tape.
Joints in concrete surfaces
Wherever possible, joints and lift lines should be hidden, or be made a feature, since it is impossible to avoid some discontinuity in alignment and difference in colour at both horizontal and vertical joints. However, attention to these factors in the detailed design of formwork, and optimum techniques for the placement and vibration of concrete, determined if necessary by trials, can do much to reduce attend- ant blemishes.
The position of all construction joints in concrete surfaces should be determined and be clearly indicated on drawings before the work is com- menced. Joints should always bear
some relationship to the general de- sign, by an association with grooves, fillets, and the like. Where a plain butt joint is desired, the use of a steel angle at the top and at the vertical edges of the panel will provide sharp arrises, and will reduce loss of alignment.
Deformation of the formwork can occur while the concrete is being placed, as a result of bending between ties, or compression of timber at tie connections. Therefore it is important to use rigid formwork and connections and, in high lifts, to make provision for tightening the ties as concreting proceeds.
When bonding new and old con- crete, the laitance should first be re- moved from the old surface. Addition- al bond should be provided by lightly roughening the old surface which should then be cleaned and dampened. After a thin layer of fresh motar is spread, the normal concrete mix should be placed. Special care should be taken to ensure complete and thorough compaction at the joint.
It is not advisable to use a wetter mix at the bottom of a lift, as colour variation will occur. However, in heavi- ly reinforced sections, a thin layer of mortar of the same consistency as that used in the concrete itself may be used to avoid honeycombing.
Curing
Concrete is usually stripped as soon as it is strong enough to support its own weight. However, sections cast against rough board or heavily profiled moulds may require a longer period before being stripped, to guard against surface damage. With impermeable linings, early stripping of formwork will contribute to uniformity of colour, by preventing differential cur- ing conditions as a result of warping or settlement of the formwork.
It is probable that economic con- siderations will rule in the choice of stripping times for formwork, but standardization of this procedure is most important, and should be adopt- ed for all surfaces forming one part of a structure. Normal curing methods can be applied after forms are strip- ped. It is essential in hot weather, in particular, that premature drying out of the surface should not occur.
Formwork linings
The lining material imprints its character on concrete placed against it, according to the texture of the surface and the absorbency of the material.3
Board formwork. The primary con- sideration in the selection of timber
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Far East BUILDER, June 1970