What is GRC?
GRC is a cement-based composite material reinforced with alkali-resistant fibres. The fibres add flexural, tensile and impact strength. Τhe resulting material allows the production of strong, yet lightweight, products used in architectural, civil engineering and many other applications.
GRC is not a single material but a family of materials with different properties tailored to suit particular applications. The properties of GRC vary with the production method, the amount and type of glass fibre and the composition of the cementitious matrix.
Whereas traditional concrete is classified by its compressive strength (C40 etc.), GRC is classified by reference to its characteristic flexural strength (termed in the industry as Modulus of Rupture or MOR).
Application and uses of GRC
GRC is used worldwide to manufacture a vast range of precast products for the building and civil engineering industries.
GRC can be formed into thin sectioned lightweight elements and provides designers, architects and engineers with substantial advantages when compared to other traditional concrete materials.
- Easily moulded to reproduce shapes, details and textures.
- High strength results in thin sections, which are lightweight and easy to handle.
- Can be coloured with pigments, paints and natural exposed stone facings.
For convenience of specification, GRC is graded into three categories, each grade representing numerically the characteristic MOR value. Marathon Stone ensures the highest quality GRC is manufactured in accordance with the best practices from across the world and particularly. The International Glass Reinforced Concrete Association “Specification for the manufacture, curing and testing of GRC products” [PDF] is a major reference.
The properties of GRC vary according to the production method, mix design, and the type and content of the alkali-resistant glass fibre. Marathon Stone employs three methods of production depending on the characteristics and application of the final product
As the name suggests the glass fibre is mixed into the cementitious slurry and the resulting material is poured or pumped into moulds and compacted using vibration or by using special self compacting or other additives in the mix.
The premix process can be easily automated and is often chosen for standard product manufacture. The moulds used are more complex than those for the spray process as they usually require core moulds as well. The mechanical properties are lower but they are consistent allowing optimum design.
Hand Spray is the most versatile and popular production technique. A special spray gun is used to simultaneously deposit chopped glass fibre and slurry into a mould. This is then compacted by hand using a spring roller. More layers are then sprayed and compacted until the required thickness is achieved. The spray technique allows high glass percentages to be used and this gives high strength mechanical properties. In turn this allows for reduced skin thickness and lighter products.
This is a combination method where the premixed material is sprayed into the mould. It combines the advantages of both methods and is becoming the method of choice for many small architectural items.
Properties of GRC
Premix strengths are generally lower than those of the Traditional Spray method due to lower AR Glass fibre reinforcement contents, higher water/cement ratios, shorter fibre lengths and 3-dimensional fibre orientation.
The Hand spray technique can produce the highest strengths due to a high AR Glass fibre content, low water/cement ratio, long fibre length and planar fibre orientation.
Sprayed premix gives consistent mechanical properties similar or higher than those obtained with vibration cast premix.