Manufacturing GRC / GFRC

GLASSFIBRE REINFORCED CONCRETE (GRC) /
GLASS FIBER REINFORCED CONCRETE (GFRC)

GRC on Shephard Hall project video

Glassfibre Reinforced Concrete (GRC), also known as Glass Fiber Reinforced Concrete (GFRC) is generally manufactured by either the "sprayed" process or the "premix" process. Premix GRC can either be vibration compacted, or manufactured using a self compacting GRC mix. The method chosen is normally dictated by factors such as strength requirements, size of mould, architects specification etc.

As a general rule, larger items, such as building cladding panels, are normally "sprayed" whereas small items are manufactured using a "premix" GRC method.

Sprayed GRC is generally stronger than premix vibration cast GRC. The reasons for this are firstly that with sprayed GRC it is possible to achieve a fibre content of 5% - 6% whereas premix GRC is limited to around 3% - 3.5%. Secondly, sprayed GRC has a lower water content than premix GRC.

Typical mix designs for sprayed and premix GRC materials are as follows:

GRC typical formulations

 

Sprayed GRC

  • The water and admixture (and polymer if used) are placed in a "high shear mixer" and the sand/cement are slowly added until a smooth creamy slurry is achieved. The consistency of the slurry can be checked using a simple slump test kit. Mixing time is about 1 - 2 minutes.
  • When ready the mix is transferred to a "pump/spray unit". The pump conveys the slurry at a regulated rate of flow to the spray gun. At the spray gun fibre, in the form of a roving, is chopped to a length of approximately 32mm and added to the slurry. The two materials are projected onto the mould surface using an air supply from a compressor.
  • The GRC material is sprayed and built up in thin layers until the required thickness is achieved - normally 10 - 15mm. Simple hand rollers are used to compact the material between layers.
  • The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.
  • After demoulding the units are covered with polythene and allowed to cure for approximately 7 days. Alternatively, if a polymer curing compound is used in the mix the units can be exposed to the atmosphere immediately although it is advisable to keep them protected from direct sunlight or severe external conditions for a day or two. Reference should be made to the Polymer Supplier's instructions.

Premix GRC

  • The sand and cement are mixed dry and then the water/admixture and polymer (if used) are added. Generally a two speed slurry/fibre blender mixer is used. With this type of mixer, the fast speed is designed to create a smooth creamy slurry. This takes about 1 - 2 minutes. The mixer is then switched to slow speed and fibre in the form of chopped strand (length approximately 13mm) is added slowly. The fibre is blended into the mix for approximately 1 minute.
  • Once the mix is ready, it is poured into moulds which are vibrated using a vibrating table.
  • The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.
  • After demoulding the products are cured under polythene sheets to maintain moist conditionsfor approximately 7days. Alternatively a polymer curing compound can be used as described for the sprayed process.

For more information about manufacturing GRC please see our Specification of GRC