JULY 2011

WHAT IS GLASS REINFORCED CONCRETE ? AN INTRODUCTORY GUIDE

GRC or Glass Reinforced Concrete is a composite material manufactured by reinforcing a concrete matrix with alkaline resistant glass fibre strands. Products manufactured using the material have significantly higher tensile, flexural and impact resistant strengths when compared to other forms of pre cast concrete. These characteristics allow GRC to be moulded into thin (generally 12-15mm) section lightweight profiles whilst still retaining the tactile and visual appeal of traditional precast concrete. GRC was developed over a thirty year period before being introduced in the late 1960s. Since then it has become widely used throughout the world primarily as an architectural cladding and facade material but also in engineering, utility and landscaping applications. Despite advances in the quality of fibres, cements and other additives such as acrylic polymers the basic manufacturing process remains the same and is used by leading GRC producers throughout Europe, Australasia, China, USA and Russia. GRC offers architects, designers and engineers virtually unlimited expression of form. Products can vary from small architectural ornamentation such window cills to 20m² cladding panels. Unlike most forms of other precast concrete complex multi directional curves and shapes can be created with the rear profile generally mirroring the exposed face. High quality GRC is highly resistant to carbonation and chemicals whilst offering good sound and insulation properties. The material will not corrode and of course offers significant weight savings over other types of pre cast concrete due to the thin sections that can be manufactured. GRC is generally manufactured using one of two recognised processes, premix and spray.

02

WHAT IS GRC?

Large scale cladding panels, Canada Water, London

Projecting horizontal and vertical fins during installation

Door surrounds, private development, Wiltshire

GRC is covered by various British and European Standards and Specifications. Because the material can be used in a variety of applications there are no product specifications. As such component performance (dimensional tolerances etc.) should be agreed between the manufacturer and purchaser. BS EN 1169 1999 Precast concrete products-General rules for factory Production of glass-fibre reinforced cement BS EN 15191 2009 Precast concrete products-Classification of glass-fibre reinforced concrete performance BS EN 1170-1 1998 Measuring consistency of the matrix-”Slump Test” method BS EN 1170-2 1998 Measuring the fibre content in fresh GRC, “Wash out test” BS EN 1170-3 1998 Measuring the fibre content of sprayed GRC BS EN 1170-4 1998 Measuring bending strength-”Simplified bending test” method BS EN 1170-6 1998 Measuring bending strength, “Complete bending test” method BS EN 1170-6 1998 Determination of the absorption of water by immersion and determination of the dry density. BS EN 1170-7 1998 Measurement of extremes of dimensional variations due to moisture content. BS EN 1170-8 2008 Cyclic weathering type test IGRCA Specification for manufacture, curing and testing of GRC Products International Glassfibre Reinforced Concrete Association The GRCA is the world’s only recognised trade association for manufacturers, suppliers , engineers and consultants involved in GRC. With headquarters at the Concrete Society in Camberley membership currently covers over 20 countries. The GRCA has representation on British Standards Committee (Precast Concrete Products) and publishes many of its own documents relating to the design, production and testing of GRC For more information please visit www.grca.co.uk

03

STANDARDS & SPECIFICATIONS

The premix process, as its name suggests, involves blending pre-chopped fibres into a concrete slurry. This is then poured into moulds or formers in much the same manner as all other pre cast or in-situ concrete. These moulds generally have a removable core to allow hollow core items to be produced. An alternative is to suspend polystyrene or similar into the mould allowing the slurry to flow around the core and encapsulate it within the profile of the unit. As would be expected the fibre content of premix is relatively low (usually 2-3%) thus allowing the slurry to remain viscous which is necessary with all types of pre-cast concrete. Fibre lengths are generally less than 25mm with 13mm being the most popular. Water/cement ratio is very similar to that of wet cast concrete at around 0.35-0.40. The strength characteristics of GRC are primarily attributed to the fibre length, content and orientation. As such the premix process is only suitable for smaller items. A relatively recent development is the spray application of premix. Although the mix design is the same the GRC is sprayed into open moulds. This method can offer cost savings where complex closed moulds would otherwise be required. Due to the low fibre content premix is generally only used to manufacture smaller items.

04

PREMIX PROCESS GRC

13mm Chopped AR Fibres as used in Premix process

Premix GRC being pumped into open mould

Premix GRC being pumped into closed mould

For large scale GRC applications the spray process is the production method of choice in most countries. Sprayed GRC contains approximately 5% of AR fibres. In addition fibre lengths are a minimum of 25mm in length (BS EN 15191) and are applied in a multi directional orientation The water cement ratio of spray process is also much lower. As a result both elastic and ultimate bending strength are increased significantly along with an Increase in impact strength. At such high fibre concentrations and low water/cement ratio it is virtually impossible to mix and pour the concrete into moulds. As such the concrete matrix and fibres are sprayed into open moulds, the two being combined at the point of delivery. The concrete is pumped to the spray gun, whilst continuous strands of AR fibre are fed through the gun and chopped to length. A high pressure air feed atomises the concrete and this and the chopped fibres are sprayed into an open mould. In order to achieve the correct fibre content calibration of the concrete flow and fibre feed is critical. Application is by several layers to build the GRC up to the required thickness. The process allows a unique form of pre cast concrete where the rear and exposed faces follow the same shape as the form-work or mould. As such sprayed GRC can be used to create virtually any shape Including complex and multi directional curves. Sprayed GRC is a very skilled process involving specialist equipment, skilled staff and extensive production based quality tests during the manufacturing cycle.

05

SPRAY PROCESS GRC

Spray Process

Continuous roving AR fibres as used in Spray process

Spray Process

Whichever production method has been used for the manufacture of GRC products it is generally important on architectural components, that fibres are not visible on the surface. On low fibre premix with a fibre content of less than 2% it would usual to see only a small amount of fibres on the finished surface even after a light acid or grit blast finish. Dependant on colour this may be considered acceptable. Natural Portland stone for example contains small particles of shell which do not look dissimilar to exposed fibre. As colours become darker any fibres become more pronounced. To prevent this most dark shade GRC and that with a fibre content above 2.5% would generally have a mist or face coat applied. A mist coat is a light coating of the mould before casting or spraying using the concrete slurry without any fibres. The application of a mist coat effectively ensures no fibres are visible and should be specified for all high quality GRC. Where a purchaser requires decorative or exposed aggregates to the surface of a GRC product this can be achieved by applying a face coat. This consists of a layer, again without fibre, but containing decorative sands or aggregates. The face coat can be applying by spray to both horizontal and vertical mould faces providing the aggregate is less than 2/3mm maximum particle size. Where a coarser aggregate is required this would be poured and levelled before the application of the GRC backing. The maximum aggregate size recommended is 6mm and the mix design should be formulated to minimise or eliminate any differential shrinkage. It is very difficult to apply large aggregate face mixes to the vertical sides of the mould. Face coats would normally be finished by acid wash or grit blast to expose the underlying aggregate.

06

MIST AND FACE COATS

Application of face coat

Face coat before pouring or spraying of GRC

The recommended curing process for GRC is seven days at 20°C and 95% relative humidity. This is known as wet curing. This is generally not practical and most high quality GRC is thus dry cured by the addition of acrylic polymers. The addition of such polymers to the GRC prevents early moisture loss by providing a film around the cement and sand particles. Polymer modification offers several benefits in addition to ensuring maximum hydration of the cement. Water ingress is reduced thus allowing

the products to retain their supplied appearance longer

Harder and denser surface with less crazing

Improvement to long term mechanical properties particularly aged flexural strain to failure Reduces or eliminates efflorescence. The benefits of the addition of polymer have been recognised on an international basis for many years. Today most high quality GRC, particularly that made using the Spray process, is polymer modified due to the benefits obtained by its incorporation into the matrix.

07

POLYMER MODIFIED GRC

San Francisco Towers, 155,000m² Polymer Modified GRC

Sheppard Hall New York, 1990s Renovation with Polymer Modified GRC

Heathrow Express Tunnel Linings in Polymer Modified GRC

Photos courtesy GRCA

Useful Information Available from www.grca.co.uk

International from www.bsi.org BS EN 1169 General rules for factory production of glass– fibre reinforced cement BS EN 1170 1:1998 Measuring the consistency of the matrix “Slump test” method BS EN 1170 2:1998 Measuring the fibre content in fresh GRC “Wash out test” BS EN 1170 3:1998 Measuring the fibre content of sprayed GRC BS EN 1170 4:1998 Measuring bending strength “simplified bending test” BS EN 1170 5: 1998 Measuring bending strength “complete bending test method” BS EN 1170 6:1998 Determination of the absorption of water by immersion and the determination of the dry density BS EN 1170 7:1998 Measurement of extremes of dimensional variations due to moisture content

BS 8297:2000 Design and installation of non-www.grconline.com

www.grctesting.com © Glass Reinforced Concrete UK Ltd 2011 Not to be reproduced in whole or part without the express permission of the author. Whilst every care has been taken in the preparation of the document to ensure that any advice, recommendations or information is accurate, no liability or responsibility of any kind is accepted in respect of Glass Reinforced Concrete UK Ltd

Glass Reinforced Concrete UK Ltd, 1-3 Redwood Court, Campbell Way, Dinnington, Sheffield. S25 3NQ T: +44 1909 566911 F: +44 1909 569399 info@grcuk.com

www.grcuk.com

© Glass Reinforced Concrete UK Ltd 2011 Not to be reproduced in whole or part without the express permission of the author. Whilst every care has been taken in the preparation of the document to ensure that any advice, recommendations or information is accurate, no liability or responsibility of any kind is accepted in respect of Glass Reinforced Concrete UK Ltd