Re-cladding the school

Concrete, Apr 2007 by Foster, Chris, Kingsbury, Colin

In 1946, following the Second World War, a population overspill from London, a lack of skilled manpower and building materials, and the introduction of free secondary school education led to an acute shortage of school buildings in Hertfordshire.

A group of architects and educators were brought together and, using experience gained during the war, combined the skills of designers, users and manufacturers to produce a programme of prefabricated schools to fill the gap. They used many pioneering techniques, including pre-ordering building materials and the use of prefabricated construction.

The first Hills school, named after the steel fabricator Hills (West Bromwich) Ltd, was completed in 1947 and was followed by over 100 such schools in Hertfordshire using the Hills system. The deputy county architect, Stirrat Johnson-Marshall, moved to the Ministry of Education and made this work available to the rest of the country. In total, over 400 Hills buildings were completed before Hills went into receivership in 1962.

The problem

At Bernard's Heath School, as with many of the Hills schools, the concrete cladding had started to fail due to the use of mild steel reinforcement with insufficient cover. Over time, the reinforcement had rusted and caused spalling of the concrete, which affected both structural integrity and the aesthetic appearance. The health and safety concerns due to the spalling led to the decision to replace the panels.

The rest of the Bernard's Heath building was found to be in good condition. No deterioration of the main galvanised structural frame was found and the windows were in a reasonably good condition. By replacing the concrete cladding, it was possible to restore the building to a condition where it will have a very long and robust life.

Design, development and replacement process

A design and development system for the replacement of the deteriorated cladding has been created by consulting civil engineer, C H Kingsbury & Associates.

At Bernard's Heath, Derbyshire-based Evans Concrete, specialist in bespoke precast concrete, was called in to manufacture and install the exposed aggregate concrete panels for the building. The replacement units, including plinths, were designed to modern Standards of cover to steel and included the use of high-yield, deformed stainless steel reinforcement. By thickening the panel slightly and reducing the required cover to reinforcement by using stainless steel, there was sufficient thickness to incorporate cast-in insulation into the panel. This gave the school increased thermal performance of the external leaf of the building. The cavity of the structure was also insulated as the replacement work proceeded.

Overall dimensions in elevation followed the original cladding but the overall thickness was increased by 20mm to give added strength and durability to the panels. The robustness in the replacement panels and the exposed aggregate finish also gave them a high resistance to impact and graffiti damage, as appropriate aggregates that facilitate cleaning were used.

Although the new panels were thicker, part of them consisted of insulation, meaning that they were not any heavier. Therefore, there was no adverse effect upon the steel structure of the school buildings because the overall loading on the structure was not increased.

The system is based on keeping a school functioning during the re-cladding. It was important that no teaching time was lost, so the work was carried out entirely from the outside of the building with no disruption to the internal leaf of the structure.

The project itself consisted of replacing approximately 1300 precast panels and 300 plinth units. All the units were manufactured at Evans Concrete Products' operational headquarters at Ripley in Derbyshire. Deliveries were phased to suit the area of work and timed to avoid busy periods at the school gate. The overall programme for on-site works was approximately 12 weeks. However, this was programmed to coincide with the summer holidays to cause minimal disruption to the teaching day.

Concluding remarks

The benefits of having the work carried out were instantly recognisable. A clean, aesthetically pleasing exterior of the building was provided, with the added benefits of improved thermal properties. The health and safety concerns were eliminated and longevity of the existing structure was extended. The technique has been used successfully on over 50 Hills schools and several Clasp schools.

CHRIS FOSTER, EVANS CONCRETE AND COLIN KINGSBURY, C H KINGSBURY & ASSOCIATES