Materials:
The basic materials used in ALRIMAL GRC are:
Cement:
Ordinary Portland and Rapid Hardening Portland Cement. Other cements, including
sulphate resisting and HAC, are also used.
Aggregates:
Fine sand with an aggregate / cement ratio from 0.25 upwards and a particle size in the
range 150 microns to 1 mm are typically used. Other fine inorganic aggregates can be
used.
Water:
Clean and fresh water should be used. The water / cement ratio is low by concrete
standards.
Admixtures:
Water reducing, plasticising and workability aids may be used. Accelerators may be used
to speed demoulding.
ALRIMAL AR Glass Fibre
Supplied as continuous roving, which is cut during the making of GRC in to strands 12 to
38 mm long, or as precut chopped strands 12 to 25 mm long. The typical level of fibre
used in GRC is between 4 and 6% of total weight.
Manufacture:
ALRIMAL GRC can be produced by several differing techniques, ranging from very
manual processes to sophisticated mechanized systems. All systems first require the
production of a slurry of the matrix components. The fibre is then added either at a
further mixing stage before casting or by simultaneous spraying on or in to moulds.
Compaction and cure of the product then takes place followed by demoulding. The time
required for production depends on the type of component and manufacturing method
and can range from days to minutes. ALRIMAL GRC products are normally manufactured
in a factory to a high standard of finish and to close tolerances. After cure they can be
machined by normal masonry techniques if required.
Fixing and assembly:
ALRIMAL GRC components can be fixed using conventional precast concrete techniques
and can be assembled by normal techniques such as screwing, bolting, bonding etc.
Surface finish and textures:
The surface of components made in ALRIMAL GRC is as varied as the range of surfaces
against which it can be formed. High quality flat surfaces can be obtained and almost all
textures can be achieved, depending on the type of mould used. Deep and re-entrant
shapes can be produced using flexible mould techniques. Aggregate facings, as found in
the precast concrete industry are often used. By means of coloured cements or pigments,
through colour can be achieved though often surfaces are painted.
Mechanical properties:
The properties of GRC are influenced by factors such as fibre content and distribution,
type of matrix and method of manufacture. The properties can therefore be tailored to
meet the design requirements of particular components. This data sheet gives typical
characteristics for two types of GRC: sprayed material containing 5% fibre and premix
material containing 4% (Table 1). In the case of the sprayed material the alignment of
the fibres is random in two dimensions (a laminar construction) whilst in the case of a
premix the orientation of the fibre is three-dimensional.
The values are at 28 days – the normal test period for concretes.
The stress / strain characteristics of a typical 5% sprayed composite are illustrated in the
graph opposite. The fibre significantly improves both the tensile and bending strengths
and increases the strain to failure resulting in a strong and tough material.
Typical design stresses used for ALRIMAL GRC are shown in Table 2.
Creep and stress rupture:
GRC is capable of bearing loads over prolonged periods of time. In common with other
cementitious materials, GRC components show an initial elastic deformation under load
which is followed by a further slow creep deformation. These creep strains are small and
the creep strain rate decreases with time. No stress rupture has been observed in tests
carried out at twice the recommended working design stress in period up to 8 years.
Fatigue:
At the normal working stress levels, fatigue lives greater than 10 million cycles are
obtained, both in bending and tension.
Physical properties:
The thermal, fire, moisture and acoustic properties are indicated in Table 3. Most of
these are controlled by the matrix components, i.e. the hydraulic cement and aggregate.
Chemical resistance:
The chemical resistance of ALRIMAL GRC is broadly similar to that of concrete and the
effect of the chemical on the cement is the predominant factor. Because ALRIMAL GRC
has high density and a high cement content, it generally has a higher chemical resistance
than concrete. Further improvements can be obtained by using specially formulated
cements such as sulphate resisting, super sulphate and alumina cements. Alkaline
solutions do not present a problem to ALRIMAL GRC.
Electrical:
The electrical properties of ALRIMAL GRC are largely determined by the amount of
moisture present in the material in the prevailing conditions of use. In common with that
of other cementitious materials, the electrical resistivity of GRC can be increased by
surface coatings or impregnation.
Table 1- Mechanical Properties of ALRIMAL GRC:
The mechanical properties of GRC can vary from their initial levels with time depending
upon the particular material and the working environment. In dry conditions there is little
change but in moist or wet conditions there is some loss of tensile and impact strength.
The long term strength levels are well known and are taken in to consideration in the
design levels used.
Table 2 – Typical design stresses used for ALRIMAL GRC:
These design values may be varied in certain product areas e.g. formwork. Limit state
methods are also used.
Table 3 – Physical properties of ALRIMAL GRC:
General Information for GRC
Glassfibre Reinforced Concrete (GRC*) is one of the most versatile building materials
available to architects and engineers. Developed in the twentieth century, GRC has been
making a significant contribution to the economics, technology, and aesthetics of modern
construction worldwide for over 30 years.
GRC is not a single material but a family of high-performance cement-based composites
reinforced with special alkali resistant glass fibres, which can be engineered to suit a
wide range of applications.
GRC products can be formed into sections as thin as 6 mm (1/4 inch) so their weight is
much less than traditional precast concrete products. The design and manufacture of GRC
products is covered by international standards, which have been developed in Europe,
America, Asia and Australasia. GRC is manufactured in over 100 countries.
Technical Appendix
GRC is a family of materials that can be defined by the addition rate of alkali resistant
glass fibre. At one end of the spectrum low dosages of dispersible fibres are used to
control plastic shrinkage cracks in normal concretes (PCR). At the other end, integral
fibres are used at high dosage levels to reinforce cement-rich mortars (GRC). Fibre
contents for different GRC product types (kg of fibre per m3 of concrete)
Most GRC products are manufactured by one of two processes – Vibration Casting
and Spraying.
The vibration cast form is normally referred to as “Premix GRC”. Premix GRC is
produced in a two stage process. A mixture of cement, sand, water and chemical
admixtures is first prepared in a high speed mixer. Fibres are added in the second stage
with a slower speed. The Premix GRC is then poured into moulds and compacted by
vibration.
Sprayed GRC is sometimes called “Hand Spray GRC” or “Machine Spray GRC”
depending on the method of manufacture. A mixture of cement, sand, water and
chemical admixtures is prepared in a high shear slurry mixer. This is then placed in
a machine that conveys the slurry to a special spray gun where the fibres are added
at the nozzle as the GRC material is sprayed onto a mould.
A third production process called “Spray Premix” is also often used for the manufacture
of smaller elements and the application of renders.
GRC and the Environment
The main constituents of GRC are based on the naturally occurring 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
alkaline. It is normal for factories to have settlement tanks so that solids do not enter the
drainage system.
The reduced weight of GRC compared to steel reinforced concrete products does provide
environmental benefits. An assessment carried out as part of a UK government
ETR/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