GRC

Property

Constituent

The typical constituent of GRC mixes:

Glass fibre

Typical glass fibre is a continuous filament, alkali resistant glass fibre with high durability in cement. The fibre composition lies within a critical region of the Na2O CaO ZrO2 SiO2 system.

Type E: standard fibre glass used in the reinforcement of plastics

Type C: is used when mineral acid resistance is desirable

Type A: is a cheap fibre made from scrap sheet or bottle glass

Type S: is expensive, and used for rocket motor cases

Type AR: Alkali resistance

There are two product glass fibre forms used in cement reinforcement:

1. Chopped Strands: consist of continous strand chopped to uniform length and used in premix GRC.
2. Rovings: group of individual parallel strand wound as a bundle, for use in the spray production of GRC or for continous reinforcement.

Typical properties of glass fibre:

Cement

The most widely used cements in GRC manufacture are Ordinary Portland Cement (OPC) and Rapid Hardening Portland Cement (RHPC).

Fillers

Sand

Typical chemical composition for sand acceptable as GRC filler.

Grading for sand:

Crushed aggregate

Many varieties of aggregates used for concrete may be crushed to a suitable grading for use in GRC. Example of such aggregates are marble, limestone, and granite.

Pozzolans

PFA fly ash is a pozzolanic material and is the ash extracted from flue gases of boiler fired by pulverised coal. Blended PFA cements suitable for concrete manufacture are available in many countries. Typically they contain 25-35% PFA.

Silica fume (or micro silica) is a very fine material and is an industrial by-product. It is often supplied in the form as water based slurry. It is usually only used in specialist mix designs and process.

Metakaolin with particular grade has very high reactivity with hydrating cement. It provides GRC with very good long term strength retention.

Water

Water should be clean and free from deleterious matter and should meet relevant standards for water used to make concrete.

Admixture

Standard concrete admixture or those specially formulated for GRC manufacture may be used as appropriate to the particular process and to obtain the required properties of GRC.

Pigments

Pigments may be used to colour GRC. Special care is required to achieve uniformity of colour and the strong colours are not usually achievable without significant variation. Low quality pigments may lose or change colour with time.

Manufacture

Sprayed GRC (Manual/ Mechanised/ Spray-dewatering process)

In the manufacture of GRC by the spray process, simultaneous sprays of cement/sand mortar slurry and chopped glass fbre are deposited from a spray- head onto a suitable mould.

Premixed GRC (Vibration casting, Sprayed premixed, pressing, spray-mix)

All premix processes involve the blending together of the cement, sand, water, admixture and chopped strands of glass fibre in a mixer prior to being formed.

Mechanical Properties

Coefficient of Thermal Expansion

10-20 X 10^-6/ ºC

Chemical Resistance

Sulphate resistance

In the presence of moisture and sulphates a reaction takes place causing degradation of the cement, although GRC is less sensitive than most concrete.

Resistance to sulphate attack is increased by the use of sulphate resisting cement and it is usual practise to use this type of cement for the manufacture of GRC which may be in contact with sulphate solutions.

Acids and Alkalis

Portland cement release calcium hydroxide during hydration and is highly alkaline (pH 12.5). Consequently, alkaline solutions present no particular hazard to GRC.

Marine Environment

Seawater and sea-spray exposure to GRC give mechanical property changes similar to those in fresh water exposure and natural weather at equivalent temperature. Some surface carbonation can occur which may detract from the appearance of the GRC but which is not harmful to its mechanical properties, unlike reinforced concrete where both the salts and carbonation result in increased attack on the reinforcement.

Fire Resistance

Tested to BS 476 for fire resistance, GRC is proven to be non-combustible material.

Thermal Conductivity

0.5-1.0 W/mºC

Water Absorption

12%

Porosity

24%

Abrasion Resistance

0.27-0.30 cm³

Water Vapour Permeability

A typical value of water vapour permeability may be taken to be 0.001 g.m/s MN, corresponding to a water vapour permeance of 0.1 g./ s. MN or approximately 1.2 metric perms for 10mm thick GRC.

Finishes

Exposed (Light/ Medium/ Deep Exposure)

Where some portions of cement and sand are removed from the surface, exposing the coarse aggregate. The extent to which aggregates are exposed or revealed is largely determined by their size.

Smooth

When a glossy, ceramic like finish is desired, plastic liners are normally used. Metal or highly polished wood produced a very smooth surface.

Retarded

Retardation involves the application of a specialised chemical to the face mix surface that delays the cement paste from hardening within a period of time and to a depth depending upon the type or concentration or retarder used.

Sand or abrasive blasting

Sand or abrasive blasting of surface is suitable for exposure of either large or small aggregates. The type and grading of abrasives affect the surface finish and should remain the same throughout the entire project.

Form liners

Form liners may be incorporated in or attached to the surface of a mould to produce the desired pattern, shape, or texture in the surface of the finished GRC units. A form texture can be of considerable influence in assisting as- cast surfaces to appear more uniform. Wood, plastics, steels, and elastomeric materials are most commonly used as form liners.

Acid Etched

Acid etched is most commonly used for light and medium exposure. It may be accomplished by brushing the surface with a long handed stiff bristle brush immersed in the acid solution.

Honed or Polished

The grinding of GRC face mixes produces smooth, exposed aggregate surfaces. Honing and polishing depends on the degree of smoothness of the finish.

Veneer facing

Includes natural stone, ceramic tiles, etc. For some material like natural stone, a bond breaker (such as liquid bond breaker, polyethylene sheets, etc) has to be incorporated.

Applied coating (factory applied or site applied)

Paints may be used for decorative purposes. It shall be the breathing type (permeable to water vapour but impermeable to liquid water). Typically, latex or fluorine paints are suitable for most exterior application.

Sealers should be guaranteed by the supplier or applicator not to stain, soil, darkens, or discolours the finish.

Infill Types

As sandwich construction, materials typically used for infill include phenolic foam, expanded polystyerene, polystyrene bead aggregate concrete, polyisocyanurate foam.