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 until6 to 8 hours then demoulded.

After demoulding the products are cured under polythene sheets to maintain moist conditions for approximately 7days. Alternatively a polymer curing compound can be used as described for the sprayed process.

The strength of Sprayed GRC is generally higher than Premix GRC.

The table below shows typical values for Sprayed GRC and Premix GRC manufactured with 5% and 3% (by weight of mix) alkali resistant glass fiber.

Property Hand or Machine
Spray GRC
Vibration cast
Premix GRC
Glass fibre Content by Weight of Mix 5% 3%
Ultimate Strength (Modulus of Rupture-MOR) MPa 20-30 10-14
Elastic Limit {Limit of Proportionality – LOP) MPa 7-11 5-8
Ultimate Strength {Ultimate Tensile Strength-UTS) MPa 8-11 4-7
Elastic Limit (Ben Over Point-BOP) Mpa 5-7 4-6
lnterlaminar Shear Strength MPa 3-5 N.A.
In-plane Shear Strength MPa 8-11 4-7
Compressive Strength MPa 50-80 40-60
Impact Strength kj/m2 10-25 10-15
Elastic Modulus GPa 10-20 10-20
Strain to Failure% 0.6-1.2 0.1-0.2
Dry Density Tonne/m3 1.9-2.1 1.8-2.0

GRC and the Environment

The main constituents of GRC are based on the naturally occuring earth oxides that are used in the manufacture of cement and glass fibres. These are not generally regarded as pollutants. Wash water from the manufacturing process contains cement and this is alkline. It is normal for factories to have settlement tanks so that solids do not enter the drainage system.

grc-graph2-01The reduced weight of GRC compared to steel reinforced concrete products does provide environmental benefits. An assessment carried out as part of UK government DETER / Concrete Industry Alliance ‘Partners in Technology’ project compared two precast concrete and GRC products that Fulfil the same function.The results show that GRC has a lower environmental impact.

The main reasons For the reduced environmental impact of GRC compared to traditional precast concrete are:

  • Reduced cement usage per product
  • Reduced transport costs.

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