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On form in Portsmouth

Concrete Portsmouth student blocks

An innovative French formwork system is being used by a Welsh-Irish firm to build new student residences at lightning speed in Portsmouth.

Diarmaid Fleming reports.

Students enjoying long lie-ins in Portsmouth will be missing more than their lectures. Anyone emerging in the early afternoon after a hard night's raving would be forgiven for blinking in the daylight: the skyline alongside the city's university is changing daily with the construction of three high-rise blocks of student accommodation at breakneck speed.

The speed of work on the US$28M White Swan project to build 750 student bedrooms and 200 kitchens is due to Tunnelform, an innovative French formwork system which allows work to be carried out in a daily cycle, instead of a range of different or new tasks being carried out over a number of days.

'The site is really like a factory. Everyone here knows their own job, ' says Swansea-based specialist concrete contractor Donal Hayes & Sons contracts manager Jim Hayes. 'You don't have a situation where people are standing around wondering what to do next, or relying on the foreman to tell them what to do.

Because the process is repetitive, everyone knows what's involved and just gets on with it.'

While the process is repetitive on site, Hayes is already familiar with the Tunnel-form system, having used it on 25 sites elsewhere in the country. Devised by French company Outinord and supplied in the UK by Ischebeck Titan, the concept is relatively simple but clever.

Two L-shaped steel forms are used together to form a 'tunnel', around which the walls and floor above are cast. The forms are supported by triangular networks of tubular steel struts, while each assembly sits on a series of jacks which can be raised and lowered, or on a set of wheels which come into play when the jacks are raised. No new timbering or struts are needed for each new pour.

At Portsmouth, the tunnel units are 5m wide and 8m long, with four used in each pour to form a 'module' 10m wide and 17m long. Internal plasterboard partitions divide each module into eight bedrooms, and with two crews working independently on different towers, an impressive daily output of 16 rooms a day is achieved. Other rooms with different partition arrangements form larger flats for students with disabilities, common rooms and kitchen areas.

The forms are lifted into position by crane, and butted against the kicker formed from the previous pour. Levelling is achieved by adjustment of the jacks, which take the entire load on the shutter once concrete is poured.

External vertical shutters provide the remainder of the formwork for the side walls, while the ceiling/floor slab is cast on the horizontal 'roof' of the tunnel forms, which are locked together.

For some on the shift, the process means an early start.

Before the previous day's pour can be stripped, concrete must have reached a compressive strength of 12N/mm 2. Cubes from the pour taken to a nearby lab are crushed at 7am, before the site is given the go-ahead to strike the formwork.

This is achieved by loosening the system-built props, with the panels pushed out of position on their wheels and onto a stripping platform. The horizontal floors can also be separately lowered in the event of any adhesion with the ceiling/floor slab soffit, which is propped until the concrete reaches its full 28-day strength.

A special lifting device enables the tunnel forms to be raised for cleaning and greasing while still on the crane, before being moved into position for the next pour.

'Reinforcement is all prefabricated, with sheets cut to suit offsite and starter bar arrangements prefabricated too, so there is the minimum amount of time required for steel fixing, ' says Hayes.

Slender 160mm thick walls dictate the concrete mix. Main contractor Warings engineering manager Nigel Phillips says that a mix with a high slump and high cohesion is needed so it will flow easily throughout the narrow shutters and around the reinforcement.

'We are using a 140mm slump, C40 mix with Glenium C315 admixture, which acts as an accelerator and plasticiser, ' says Phillips. 'The mix is pretty much similar to that used in precast concrete factories.'

The system is also intended to provide a smooth finish that can be painted on directly. This means compaction must be of a very high order, which caused some initial difficulties.

'We tried to use external vibrators at first, but this didn't give us the finish we needed, ' Phillips says. High frequency electric poker vibrators inside the shutter are being used instead. Some remedial treatment has been needed to the surface to smooth over pinhole bubbles before painting.

All toilet units come as prefabricated 'pods' supplied by Polybeam, and are pre-plumbed.

Services throughout the building run through ducts cast through the concrete floors. Box-outs for doors and windows are fixed to the shutter faces with large 100mm 2magnets, to avoid the need for tedious and time-consuming fixing which would slow the construction cycle.

'They work well and provide plenty of adhesion - you certainly cannot lift them off by hand. They need to be prised or peeled off, ' says Hayes.

Prefabrication of steelwork and the rapid striking and reerection enabled by the system means that the forms are ready for concreting by 2pm. 'Each pour is around 40m 3. Because the site is so confined and in the city centre, we have not been able to pump the concrete, which would be quicker, ' Hayes explains.

'We use a 2m 3skip with the crane and can complete the pour in around three hours for the higher floors, less for the lower floors where the concrete can be placed faster.' Once completed, the site rests until the next cycle begins the following morning after the cube crushing laboratory verifies the strength.

Concreting work began in June on the three buildings - seven, nine and 11 storeys high - and is due for completion later this month. Hayes says that the system's advantages are speed, with less labour required than in traditional construction.

'It's faster and cheaper - we did 12 floors in 13 weeks: using traditional shuttering, it could have taken us up to 26 weeks - twice as long, with around 30% more labour required, ' he adds.

Engineer for client Unite which is building the residences for Portsmouth University is Goury Self and Partners.

According to Ischebeck Titan regional manager Phil Waldron, engineers must decide early on to use the system, as incorporating it is difficult when a structural design is already well advanced.

'The system is suited to highrise modular construction, but early consideration by the engineer is necessary, ' he says.

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