glassfibre reinforced concrete

Glassfibre Reinforced Concrete (sometimes called Glassfibre Reinforced Cement) is a mixture of cement, fine aggregate, water, chemical admixtures and alkali resistant glassfibres.

The GRCA can also give advice on other GRC matrices including specially formulated materials. These new systems improve the long term strength and toughness retention and/or mould turnround of GRC components.

There are a number of different manufacturing processes; the most common are Hand-Spray and Premix, see manufacturing GRC.

Glassfibre Reinforced Concrete (GRC) is a material which today is making a significant contribution to the economics, to the technology and to the aesthetics of the construction industry worldwide.

This environmentally friendly composite, with its low consumption of energy and natural raw materials, is being formed into a great variety of products and has won firm friends amongst designers, architects, engineers and end users for its flexible ability to meet performance, appearance and cost parameters.

Since its introduction in 1969, GRC has matured and today's designer has available to him, depending upon his performance requirements, a range of matrix modifiers such as acrylic polymers, rapid set cements and additives to improve the long term stability of the material. Extensive independent test and performance data are available on all aspects of matrix formulation.

CSN EN 1169 - Precast concrete products - General rules for factory production control of glass-fibre reinforced cement.

TESTS

The following tests shall be carried out on coupons cut from the test boards in accordance with BS EN 1170 Parts 2, 4, and 5. If acrylic polymer is used in the mix, presoaking immediately prior to testing shall not be required for Modulus of Rupture or Limit of Proportionality.

i) Glass Content – BS EN 1170 Part 2

ii) Modulus of Rupture – BS EN 1170 Part 5

(and simplified method in Part 4)

iii) Limit of Proportionality. – BS EN 1170 Part 5 

Test boards shall be produced alongside each day’s production (at least one per day for each production team). The recommended size of these sample boards is 600 x 600 mm. The test boards shall be produced with the same quality, thickness and curing as the actual panels.

Those test boards which are not required for testing should be kept for the duration of the contract, or for a period to be agreed between the manufacturer and engineer. 

The frequency of testing shall be agreed between the architect, engineer and manufacturer.

The recommendation of BS EN 1169 is as follows:

■ Glass content – tested in accordance with BS EN 1170 Part 2. Once per week for each spray team.

(This is in addition to the calibration test referred to in section 3.3).

■ Modulus of Rupture and Limit of Proportionality – tested in accordance with BS EN 1170 Part 5. Should be tested by the manufacturer or by a qualified laboratory as the mix design is being set-up and thereafter at least twice per year, or when the mix design is changed.

■ A simplified bending strength test to determine the Modulus of Rupture (MOR) should be conducted by the manufacturer more frequently  The frequency of testing recommended by BS EN 1169 is for each 10 tonnes of GRC produced, or at least once per week.

■ Water Absorption and Dry Density – tested in accordance with BS EN 1170 Part 6. As the mix design is being set-up, and then for each 10 tonnes of GRC produced, or at least once per week.

Glass fibre Reinforced concrete (GRC) is generally manufactured by either the "spray" process or the "premix" vibration casting process. 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 from "premix" GRC. 

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:

typical formulations

            sprayed GRC            premix GRC

sand*         50kg                    50kg

cement        50kg                   50kg

water      15-17 ltr                17-18 ltr

plasticiser: to manufacturers instructions

polymer: optional optional

fibre+ 4-6% by weight 2-3.5% of weight

*The sand used should have a particle size not exceeding approximately 1mm and should be well graded. Material passing sieve No. 100 should not exceed 10%. The sand should preferably be dry. 

**Dependant on type of fibre and property requirements.

+The water/admixture/polymer content will need to be adjusted according to materials.

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 to set and is covered with polythene sheet to prevent moisture loss. The product is demoulded the next day.

• 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.