What each material actually is
GRG is glass reinforced gypsum: a plaster-based material cast in moulds for internal use. It gives a fine plaster surface and takes complex internal geometry well.
GRP is glass reinforced plastic: a resin laminate, lightweight and dimensionally stable, suitable internally and externally, and capable of curvature that would be uneconomic in other materials.
GRC is glass reinforced concrete: a cementitious material used externally where the element needs the appearance and durability of cast stone at a fraction of solid concrete weight.
Where each belongs
If the element is internal and needs a plaster finish, it is GRG. If it is external and cementitious in character, it is GRC. If it is lightweight, curved, or has to be repeated many times with tight dimensional control, GRP is usually the answer.
- Internal domes, cornices, coffers and columns: GRG
- External panels, screens, copings and surrounds: GRC
- Lightweight canopies, sculpted forms and repeated features: GRP
- Anything exposed to weather: never GRG
The specification mistakes that cost money
The most expensive mistake is specifying GRG externally, usually because a drawing note travelled from an internal detail to an elevation. The second is leaving the fixing design undefined, so the tender returns are not comparable.
The third is not stating who resolves the interface with structure and services. Where that is silent, every tenderer prices it differently and the cheapest number is usually the one that excluded it.
What to put in the tender
- The material, stated explicitly, per element
- Internal or external position and exposure
- Finish reference and how it will be approved
- Fixing and support responsibility
- Interfaces with structure, services and adjacent trades
- Whether installation is included
