Glassfibre-reinforced concrete for building restoration and renovation

Glassfibre-reinforced concrete for building restoration and renovation

Gilbert, Graham

Glassfibre-reinforced concrete (GRC) products are thin (10-15mm), with component weight between 20-30kg/m^sup 2^. Their reduced weight is of great value when renovating existing buildings as they do not significantly increase the dead load. This also results in savings in handling, storage, transportation and installation.

GRC is able to accurately reproduce existing architectural details and textures and retain its properties and appearance over many decades. This has led to it being increasingly used to refurbish existing buildings.

GRC has been used as a cladding material for more than 30 years, and the following three examples highlight its use as a terracotta replacement for old buildings.

Shepard Hall, NewYork City

The reconstruction of the exterior of Shepard Hall, the first building constructed specifically for public higher education in New York City, is a multi-phase project that will eventually lead to the replacement of 72,000 units and over 4000 shapes, including many ornate sculptures.

Built in 1907 from a combination of load-bearing stone, glazed terracotta and steel, Shepard Hall is the main campus building of New York City College. By the mid-1980s, the terracotta was suffering significant structural deterioration. Major restoration was required, and the Stein Partnership (now Stein White Nelligan) was commissioned to undertake the work. In his approach to materials specification for the project, partner-in-charge Carl Stein rejected new glazed terracotta and cast stone because of their past poor performance in New York’s polluted environment, preferring instead, after exhaustive testing, the attractive properties of a metakaolin-modified form of GRC.

During the past 12 years, the Stein Partnership has replaced around 23,OM^sup 2^ of terracotta at Shepard Hall. These elements include many non– rectilinear shapes with curving forms in several non-orthographic intersecting planes.

Phase 1 was completed in 1992, and required 12,000 replacement units. The project replaced the upper 20m of the main tower, clerestory window surrounds, coping and finials. In Phase 2, a further 6200 units were used to replace tracery and window surrounds for 12 stained glass windows 1m high, ten highly decorated finials 7m high, and various copings, mouldings and quoins. Phase 3 required about 11,500 replacement units, involving the reconstruction of a 35m-tall tower, six projecting bay windows, each 1 lm high x 6.7m, numerous moulding surrounds, copings and quoins.

The most recent phase has just been granted at a total construction price of around $19m (L12.15m), which includes the pavilions and the lower part of the main tower. Around 8500 pieces will be required. Shapes range from flat panels and window mouldings to full sculptures. There are about 25 different human figures, about half life-size, and a set of finial dogs, about 1.2m high holding shields in their front paws. These pieces will be made and installed over the next 18 months. There will be further phases before the contract is complete in four or five years.

This project clearly shows that GRC can be a valuable tool in complex historic reconstruction work. Indeed, there are many properties that make it a particularly useful material for such applications:

It can replicate and replace a wide variety of materials.

It can be moulded to accurately reproduce decorative and sculptural forms. Since GRC is dimensionally stable during production, original examples can serve directly as models for the production moulds. Multiple copies can be readily fabricated.

It is a thin shell product used to replace massive materials such

as terracotta, precast concrete, natural stone or cast stone. As a consequence, there is an area of free space behind the GRC cladding. Structural repairs can therefore be independently achieved without affecting the decorative cladding.

Unlike many historic restoration materials, GRC is widely available and supply is not limited by environmental constraints.

It has excellent environmental resistance – the finished product will retain its architectural properties when subjected to freeze-thaw attack, ultraviolet exposure, abrasion from wind-borne grit, acid rain, etc.

This testing programme required 4-5 months to complete. It was not until the testing programme was at an advanced stage that some pronounced differences began to appear. These tests were aimed at predicting the long term effects of the New York environment on the appearance and stability of the panels over a predicted life of more than 50 years. The surface of many systems showed signs of discoloration or yellowing plus efflorescence or surface defects, particularly during the acid rain test.

This rigorous testing programme highlighted the exceptional durability and surface retention properties of one system in particular, commercially branded as Cem-FIL Star GRC. Previous independent evaluation was undertaken by CSTB and Bureau Veritas in France and the Center for Advanced Cement-Based Materials, Illinois, USA on the mechanical properties of this system. These tests indicated virtually no change over time. Due to this and the ASTM testing, Cem-FIL Star GFRC was subsequently used for most of the restoration work completed or specified so far.

Classic House, Renfield Street, Glasgow

Reconstructed as a cinema in 1914-15 from a building of 1898, the structure featured a glazed terracotta (faience) facade on a traditional brick-and-steel construction. The building was fire– damaged in 1981. In 1989, it was renovated to provide office and retail accommodation. Part of this work involved removal of the existing faience and replacement with GRC panels. The new facade had to be an exact copy of the original, picking up every detail. For this reason, plus weight and cost considerations, GRC was chosen as the ideal material for recreating the original facade in sufficient clarity and quality.

The facade was carefully removed to provide a 150mm zone in which to fit the new panels and a steel subframe to support the new panels.

Some 300 different panel types (including handed versions) were required, a total of 435 panels being manufactured. Typical mullion/pier panels had face dimensions of 2.45 x 2.35mm with a 100mm zone depth for ribs and a general 15mm skin thickness, all in a white, sealed Portland cement finish. Typical cornice panels had face dimensions of 3.95 X 0.65m with particularly complicated and detailed features, creating an total panel depth between 750 and 1050mm.

To reproduce certain features exactly, mouldings were taken on site from the original facade before removal. Panel weights were generally 200-300kg, with a maximum of 870kg for larger cornice panels. All fixings were stainless steel with generally two support brackets transferring vertical load and lateral wind loads, together with two top restraint fixing for lateral wind load.

Now known as Classic House, the renovated building has become reestablished as one of the landmark buildings of Glasgow City Centre and after more than 10 years in Glasgow’s environment, the cladding is in pristine condition. It is recommended viewing by the Tile and Architectural Ceramics Society.

One North State Street, Chicago

Located in central Chicago, One North State Street was constructed as two connected buildings between 1900 and 1912. In the mid-1980s, a restoration programme was undertaken. From original photographs and portions of the existing, deteriorated facades and column covers, plaster positives and rubber moulds were made for use as formwork for the GRC replacement panels.

In addition to matching the texture and finish of the original terracotta, the new GRC panels had to align with the terracotta above and clear the steel and masonry building structure. For these reasons, the panel cross-section had to be reduced in thickness to 12mm in several locations.

The glazed finishes on the column covers, the cornice and the entablature closely match the existing terracotta above the third floor. Sections of the original terracotta, which have been cleaned, are almost identical in colour to the new GRC cladding panels.

The entire erection process had to be performed at night owing to the heavy pedestrian and vehicular traffic on the adjacent street. The area of each panel was maximised as much as possible. However, the large size, strength and durability of the new cladding panels provide a long-term solution. The panels have already been in position at the heart of Chicago for nearly 15 years without any noticeable deterioration of surface details or aspect.

Conclusions

There is great potential for GRC in high-end architectural applications, both for historic reconstruction and new buildings. At this time, only a small proportion of this potential has been realised and most of this is outside of the UK.

Copyright The Concrete Society Oct 2002

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