GLUED FORMWORK FOR CONCRETE

Novel System Provides Re-usable Forms In N.Z. Hydroelectric Project

Reproduced from 'Machinery Lloyd and Electrical Engineering' (Volume 36, No. 26 - 19th December 1964) with kind permission of the publisher - The Certificated Engineer May 1965.

In New Zealand, a large unit of the country's hydro-electric scheme is nearing completion. This is the Benmore project on the Waitaki River which will contain six 90 000 kW generators producing 540 MW of electricity. This project in the South Island will feed into the national grid which, using a cable under Cook Strait, will supply the North Island also.

As in other hydro-electric projects, massive concrete structures are entailing the use of timber forms. Aerolite wood glue played a notable part in the construction of this formwork, all of which was designed and made by the New Zealand Ministry of Works. A novel item of formwork was used for the building of six draught tubes under the turbines for the final discharge of water to the tailrace which was made in such a way that it could be dismantled and used again in each of five subsequent positions. This resulted in a considerable economy over previous methods.

The Project

The ￡31 400 000 Benmore project, which will come into use in 1965, has entailed the damming of the Waitaki River. Although the main dam is of the earth (involving the cartage and compaction of 32 million tons of material), the finished project contains some 45 0000 yd3 of concrete nearly all of which was cast using timber formwork.

In concrete construction, there has recently been a tendency towards larger and higher pours and the formwork thus has to withstand greater pressures. Furthermore, for the sake of economy, there has been a rational trend towards the employment of units which can be used repeatedly and this, too, has necessitated extra strength, quick release, and easy handling features.

It was found that all these characteristics could be facilitated by the use of glue-laminated timber structures incorporating Aerolite adhesive. In particular, such construction enabled a very large economy to be made in the provision of formwork for the draught tubes which take the water from the turbines, turn it through a right angle and discharge it finally to the tailrace. Before describing this part of the scheme in greater detail, it may be well to give a short account of the whole Benmore project.

The dam has a length of 2 700 ft and a structural height of 360 ft, which provides a 305 ft head for driving the turbines. At one end of the dam (represented by the left bank of the river), there is a spillway designed to cope with ten times the average flow I the Waitaki River or 120 000 ft3/sec.

At the other end of the dam (right bank of the river) are the six penstocks feeding the six turbines. Each of these pre-stressed concrete pipes is 424 ft long with an internal diameter of 17! ft. The six draught tubes leading from the turbines were built using formwork which was especially novel in design.

The Use of Timber

A report(1) states that practically all formwork at Benmore involved the use of timber in one form and another. Only lumber that was straight, structurally sound, and well seasoned was used for multi-use forms. Because of its availability, cost, strength to weight ratio, and workability, Radiata pine was the basic timber used, together with smaller quantities of indigenous and other exotic timbers. The annual consumption of timber for form construction averaged about 1? million board feet.

The report emphasises the big advantages obtained by the use of glue-laminated wood. The choice of laminating or using solid timber for forms was determined mainly by the importance of the Component about the whole form or by the need for stability or alignment. With rainfall averaging 15 in or less per year, and with very low humidity, all timbers dried out and tended to distort more than they would have in many other places. For this rea on alone, the lamination of critical members was preferred.

Again, the tendency towards large units of formwork often necessitated the use of long or heavy members, and, as the timber was not normally available in lengths of over 16 ft, the best means of ensuring stability and continuity as by lamination. Climbing shutters for many uses over long periods were typical cases where lamination of studs and walings was the only effective way of ensuring trueness of face. The spillway (10 ft lift) and the powerhouse (7 ? ft lift) standard units were examples of this.

Generally, the higher stresses permitted by laminated members made for lighter framing. Internal ribs in curved forms were laminated in preference to the old method using segments with spreaders at each joint. Lamination provided continuity, required fewer spreaders, and therefore allowed better access.

Other instances of the use of the laminating technique were for curved members such as walings for pier nosings, involving either vertical or horizontal lamination. The forms for the spillway and powerhouse piers and the bifurcation nosings in the powerhouse draught tubes were examples of this. Compound curves were treated similarly, as on the invert upstands and bearers for the second stage draught tube formwork and the screed plates on the intake bellmouth invert.

For laminated work, merchantable grade Radiata pine was used for vertically laminated members and the outer laminates of horizontally laminated members. For the lesser stressed inner laminations of the latter members, boxing grade was regarded as suitable.

Removal of the forms after concreting was achieved by providing steel skid shoe gussets between the box-trestle posts and runners. The skid shoes were tapered to trap grease and had guide plates welded to them both sides of the rails, to maintain fine directional tolerances. Towing lugs were attached to the runners. The forms were towed out in single 15 ft units, or tandem, depending on the length of run available in the tailrace.

After removal of the forms, complete with roof and side panels attached, the roof panels were lifted off the box-trestles. With the roof panels removed access to the box-trestle lifting lugs was available, the 'trestles then being lifted to the next bay with side panels attached.

For the second stage, two different types of mould were used. The lower ones formed the remainder of the barrels either side of the previously poured centre wall. To obtain release, the forms were divided transversely into inner and outer moulds, the line of separating being straight in plan and as close as practicable to the centre wall. Inner and outer moulds were separated by wedges and splices. Some of these moulds were removed on the downside of the draught tube and some on the upside.

For the uppermost section, four equal sheathing panels were supported internally by four independent frames. After the concrete had been poured, these were removed via the top of the tube.

The report says in conclusion that notwithstanding the higher initial manufacturing cost of this multi-use formwork, its performance, even with a few uses as two, proved to be an economical proposition.

The stability required concerning both the framing and the sheathing was made possible through the use of glue lamination techniques and the strategic employment of plywood.

In general, nailing was confined to joints carrying compressive or minor shear loads. No joints with nails in withdrawal were permitted, and the use of nails in end grain was discouraged, but, where these were inevitable, the addition of Aerolite glue helped considerably.

Gussets, cleats, and joints which required rigidity were the main types in which nails were employed, and in some cases, these were used in conjunction with glue or bolts mainly for reasons of durability. Plywood was found to be the best material for gussets, due to its two-way strength. Gussets were made from offcuts.

Downside view of completed framing on all moulds, before sheathing operations.

The surface requirements for the various concrete finishes were in all cases specified, but strength and durability also had to be taken into consideration. For example, sheathing made of tongued and grooved boards might be sufficient for many locations based on a few uses. Its quality, however, deteriorated rapidly on re-use, and in such cases, plywood, either as a full-thickness sheathing or as a lining was found to be an economical alternative. Going further, the utility of plywood could be increased by the application of laminated plastics lining. In general, there were many advantages to using plywood for sheathing. These included high utility, smooth surface, warp, split, and shrink resistance, few joints, and suitability for curved work.

The largest sheets of plywood available were 8 by 4 ft and to overcome the disadvantage of short spans, some of the heavier sheathings were constructed out of two or more thicknesses of plywood and even using composite lamination.

The use of plywood laminations was normally confined to curved work in which the sheets were glued together with Aerolite under pressure. This technique was used for the internal travelling forms for the diversion culvert, for the spillway channel, for the penstock precast units, and the second stage draught tubes.

Where the surface was to be of a high standard, particularly on curved work where there would be insufficient re-uses to justify full plywood lamination, tongued and grooved sheathing was used the line on the contact surface with a- thin veneer. The veneers were of hardboard or plywood, depending on the number of uses.

Plywood linings, 3/10 in or 3/16 in thick, were used on forms having more than one or two uses. These were glued to the back sheathing over the full contact area, as otherwise buckling of the plywood would occur after the first use, the advantage of using the plywood thereby being lost.

Some forms were designed for an exceptionally large number of uses. Also, the surface finish was to be of a continued high standard. The penstock pre-casting forms were examples of such, where 100 uses were expected of each. In such cases, the use of plywood as the contact surface was limited, but a laminated plastic facing made a durable, wear-resistant surface. It was found best to use plywood as the backing medium owing to its smooth, stable surface, and the resulting thin glue line.

To prolong the life of, and protect the surface of multi-use forms that were subject to constant exposure to wet and dry conditions, polyurethane lacquers and epoxy resin coatings were used, both of which were found suitable.

The success of Glue Laminations

The success of glue lamination of formwork components depended entirely on the strength and stability of the glue lines. To obtain the proper bond between the individual laminations, it was necessary to fabricate the components under controlled temperature and humidity conditions. For this reason, glueing was done in an enclosed workshop, containing a laminating press for straight members or sheathing panels, and sufficient working area in which to set up jigs for curved members when required.

Good glue lines resulted from two primary factors smooth, plane surfaces, and adequate, uniform pressure.

The choice of the correct adhesive was an important factor since the members depended to a large extent on the glue for their strength and durability.

An Aerolite formulation was chosen, employing a hardener that was mixed with the glue before application. The report says that this type of adhesive was the easiest to use, most economical, and its durability excellent for the conditions over the period of use.

The glue was applied using a perforated container and spread using a roller or rubber spatula.

Design and Manufacture of Forms for Draught Tubes

Another report(2) refers in particular to the design and manufacture of the set of forms for the draught tubes.

The main forms consisted of central box-trestles supporting separate side and roof sheathing panels, each barrel consisting of two 15 ft units. The limiting factor was weight, which was restricted to 10 tons per unit.

The side panels were attached to the box-trestles through steel parallel linkages which functioned as hinges for attaining simultaneously correct height and width of forms. The panels were held in position against the sidewall nibs by props off the sleepers, and wedges off the box-trestles.

The roof panels were not attached to the box-trestles except by locating pins and sockets, the only movement allowed being vertical, and actuated by screw jacks fixed to the box-trestle stringers.

The box-trestles were carried on rails in each barrel, laid on sleepers.

REFERENCES

1. 'Formwork Designed for Re-use' by D. R. Douglas: Ministry of Works, New Zealand; Benmore Power Project: September 1963.
2. 'Multi-Use Draught Tube Forms', by D. R. Douglas and C. L. Speak; Ministry of Works, New Zealand; Benmore Power Project; September 1963. 
3. https://natlib.govt.nz/records/37118565