Types of cladding

Several systems of GRC cladding and roofing have been developed over the past decade, including:

- single-skin cladding or roofing;

- profiled sheeting;

- composite panels with foamed polyurethane;

- cast or moulded products.

In single-skin form, GRC can be mounted onto a metal stud framing (Fig. 4.5).This framing provides support and stiffness against out-of-plane forces, and permits expansion and contraction of the GRC skins. This approach has been extensively used in Germany and the USA and increasingly in the UK and continental Europe.

The choice of the type of construction (single skin, profiled or sandwich) will be governed by a combination of requirements that need to be satisfied: fire, thermal, acoustic, weight etc.Table 4.3 shows the relationship between various types of wall construction and the performance requirements for spans, fire resistance and weight, which are usually the most critical.

GRC pane!

Grc Cladding Fixing Details

GRC pane!

4.5 Fixing method for a stud frame system.

The total effective depth of large panels can usually be reduced using sandwich construction, as large single-skin panels would require deep stiffening ribs to prevent deflection under load.

In sandwich construction a larger overall thickness than indicated by strength calculations would usually be used to satisfy fire and thermal characteristics and also minimize bowing. This may also be minimized by limiting panel length or width, because the effect is proportional to the square of either dimension; increased panel thickness has a directly beneficial effect in minimizing bowing. Bowing has

Stud frame

Steel bracket

Sandwich Type Construction

Main sleel structure

Stud frame

Steel bracket

Main sleel structure

Bent bar welded to frame not been a problem In single-skin construction with separate insulation applied on site.

If the panel is required to act as a true sandwich, and for the core to have sufficient shear strength, it is important to form a good bond between the GRC and the insulating core in order to prevent delamina-tion of the core from the skin, which may be caused, in turn, through the bowing of the panel.

Sandwich panels have tended to predominate in the UK. For example, the RACS Superstore,Thanet, Kent (architects: Royal Arsenal Co-operative Society Ltd) is clad with what were claimed at the time of

Types of cladding 81

Table 4.3 Relationship between wall construction and performance requirements

Wall construction

Maximum recommended span

Fire resistance

Weight

(1.0 kN/m2 wind pressure)

BS 476 Part 8

(approximate)

Single skin

Metres

Hours

kg/m2

Flat GRC 8 mm thick

None claimed 16 None claimed 24 0.5 40

Flat GRC 8 mm thick

Flat GRC 12 mm thick

Profiled or ribbed GRC + 50 mm glass-fibre insulation + plasterboard

Profiled or ribbed GRC + 50 mm glass-fibre insulation + 100 mm concrete block

Increased depth of profile and stiffening ribs increases the spanning capability of the panel

None claimed 16 None claimed 24 0.5 40

Sandwich construction

Overall panel thickness 70 mm

Overall panel thickness 70 mm

Overall panel thckness 150 mm

10

mm

GRC

50

mm

polystyrene

10

mm

GRC

70

mm

10

mm

GRC

60

mm

polystyrene

10

mm

GRC

70

mm

10

mm

GRC

50

mm

PBAC

30

mm

polystyrene

50

mm

PBAC

10

mm

GRC

150

mm

None claimed 46

Note: PBAC - polystyrene bead aggregate concrete.

construction to be the largest GRC panels manufactured in Europe.These panels weigh just over 4 t and have thermal insulation of 0.9 W/m2oC. In overseas contracts, however; this requirement for thermal insulation and other performance requirements were achieved using single-skin construction, ribbed where necessary, with separate insulation applied on site. For example, the Sports and Leisure Centre in Saudi Arabia (architects/consulting engineers: Slater Hodder/ Cooper McDonald) uses GRC on the fascia panels to the roof and barrel vaults. This move away from sandwich panels is due partly to the greater thermal stresses and hence bowing problems in hot countries and partly to manufacturing difficulties.

Bridging webs used to give stiffness to larger panels can cause ghosting of the surface finish, as in the case reported by Young (I 978) at the UOP Fragrances factory, where, although a special case using a site-applied urethane paint finish, the egg crate construction could be seen on the panel face.Young suggests the most probable explanation is that paint on the insulated skins (130 mm polystyrene) cured at a different rate from that in those areas where webs linked skins and at the edges of window surrounds. However, he also reports (1980) feedback from the Melrose Centre, Milton Keynes, where 77 mm polystyrene core was used with two skins of 6 mm GRC, and no ghosting of stiffening webs occurred. He suggests that this may be because the panels were left as white Portland cement on all surfaces, and because the method of production meant that the insulation was not added until about 2 hours after spraying. The decision to use the non-moulded finish on the outside of the panels may account for the same lack of apparent ghosting on the panels at the Scion Computer Centre, where 50 mm polystyrene core was used.

In order to improve the thermal performance of the panel and, at the same time, improve its fire resistance and stiffness characteristics, multiple sandwich panels of GRC and rigid polystyrene foam have been used (Table 4.2).

Was this article helpful?

0 0

Post a comment