Technology Advancements in Synthetic Fibre Reinforced Concrete

Transcription

1 Technology Advancements in Synthetic Fibre Reinforced Concrete Craig Wright - Civil and Industrial Manager, Elasto Plastic Concrete Australia Pty, Ltd. Abstract The benefits of synthetic fibre include lower cost, improved durability, and increased efficiency. These benefits are driving demand for synthetic fibre on a global scale. As with all new technologies, synthetic fibre has begun to move from major projects into general concreting works and by residential builders for jobs such as housing slabs and external concrete works. While the use of synthetic fibres is increasing, it has become clear that the industry s understanding of fibre technology is still limited. Unlike overseas, engineers in Australia are still coming to terms with the fundamentals of fibre reinforced concrete design. Using synthetic fibre reinforced concrete design standards from the UK s Concrete Society, Eurocode and a selection of marquee global projects, this presentation aims to inform the conference delegates on synthetic fibre technology as a whole, incorporating key design principles, manufacturing processes and best practices for the end use of synthetic fibre reinforcement. Key Words: Fibre, Concrete, Fibre Reinforcing, Synthetic Fibre, Lower cost reinforcing. polymers it was not until the late 1990 s that Introduction The use of fibres to provide crack control in viable synthetic alternatives became available. concrete is by no means a recent discovery. In fact straw and horse hair, were used to reinforce cementitious materials. In the You will notice throughout this paper much of the information has been referenced from overseas publications. However there is 1970 s the use of steel fibre reinforced significant research and testing being concrete (SFRC) became recognised and accepted by designers and contractors around conducted government in Australia authorities with and within private the world for use in areas that were once the domain of traditional welded wire mesh organisations including precast factories, tunnelling contractors and research engineers. reinforcement. As early as the 1960 s polymer fibres were being experimented with as a substitute for steel fibres in concrete, and with the advent of modern manufacturing techniques and the development of better The aim of this paper is to provide information relating to: The most commonly available synthetic fibres.

2 Definitions and standards for macro synthetic fibres. Testing methods of fibre reinforced concrete. How fibres enhance the concrete s properties. Durability of macro synthetic fibre reinforced concrete. How designers can determine the difference between fibres. Application of fibre reinforced concrete. Fibre Types and Classifications As identified previously, steel fibres have been widely used for around 40 years. This paper however is focussing on the development of synthetic fibres, more so the recent developments that have taken place since the late 1990 s with structural synthetic fibres. Synthetic fibres are manufactured from a variety of polymers, and vary in appearance and colour. They range in length from 6mm 65mm. There are two distinct types of synthetic fibres commonly used, Micro fibres and Structural (Macro) synthetic fibres. Macro synthetic fibres will appears as crimped, embossed, flat ribbon, or twisted strands. Because of this vast gap between suppliers and their products, many countries have developed guidelines to assist designers. Under BS EN (2006) Polymer fibres Definition, Specifications and Conformity, covers fibre manufactured from: polymeric materials such as polyolefin e.g. polypropylene, or polyethylene, polyester, nylon, pva, polyacrylic, aramids and blends of them The same code divides them into two classes according to their physical form: Class 1 Micro Fibres Class 1a < 0.3mm in diameter, Monofilament. Class 1b < 0.3mm in diameter, Fibrillated. Class 2 Macro Fibres > 0.3mm in diameter. CSTR34 Concrete industrial ground floors, states At normal dose rates of 1kg 2kg per M3 short synthetic micro fibres do not enhance the ductility of concrete and hence slabs containing such fibres should be designed as though they were plain concrete..structural (macro) fibres will give suitable values of Re3 This paper will focus on the latter of the two, which are commonly used as an alternative to temperature and shrinkage crack control mesh and steel fibres.

3 Fig 1. Class1b fibrillated fibres. Fig 2. Class 2 (Macro Synthetic Fibres). Standards and Conformity Concrete Society Technical Reference 65 (CSTR65) Guidance on the use of Macro synthetic-fibre-reinforced concrete, was published in The guide at the time points out that much of the published material on macro synthetic fibres had been largely a result of fibre manufacturers own testing and that the performance versus the widely used steel fibres was largely anecdotal. However, as there can be significant cost savings available to contractors that switch to synthetic fibres many have conducted their own rigorous testing. There also been a number of leading independent studies conducted and the evidence has identified the significant advancements some manufacturers have made. For example, in the article, (Mechanical properties of a large scale synthetic fibre reinforced concrete ground slab. Amir L. Alani, Derrick Beckett, Construction and Building Materials edition )..the use of synthetic fibres (BarChip Shogun) at a dose rate of 7kg per cubic metre compares favourably with hooked end steel fibres at 40kg per cubic metre... A testing program was undertaken to compare post crack performance of macro synthetic fibre reinforced round concrete panels and steel mesh in round concrete panels measuring 1200mm diameter x 150mm thick. The results showed that 3kg of 42mm macro synthetic fibre will easily exceed the flexural post crack performance of SL72 steel mesh when placed 30mm from the top surface of a concrete slab Dr E.S. Bernard TSE Report Other topics covered by CSTR65 which provide designers with a background on the properties of synthetic FRC concrete include, design approaches, quality control and applications. Some fibre manufacturers products have been granted CE Marks, for those of you looking to investigate option for the first time this may take some of the initial guess work out of which fibres should be considered. In order to obtain a CE certificate manufacturers

4 fibres must comply with a number of modules. - Fibres are also evaluated on their physical characteristics and the effect their fibre has on the workability of the concrete. So designers have a starting point that is already verified the validity of fibre manufacturers claims to their criteria. British Tunnelling Society guidelines stipulate that macro synthetic fibres must have a tensile strength greater than 500Mpa. products. Specifying FRC and understanding the performance of different fibres. What are the properties of FRC? Ductility (Toughness). Residual Flexural Strength (Post Crack Control). Crack Control. Shear Resistance. Impact Resistance. Many myths surround the use of fibres and how they work and their performance. Not all fibres are the same! And there are numerous examples of where one brand of fibre is substituted for another brand at exactly the same dose rate. A closer examination of this risk later. Fibres are dosed in concrete at rates of between % by volume, or from 2.5kg -10kg per cubic metre. Fibres are used in concrete to provide temperature and shrinkage crack control and in some instances load bearing capacity. Dependent on the fibre type and dose rate FRC is said to have ductility or sometimes referred to as toughness. The Ductility can be measured by ASTM C beam tests. The load deflection curves (see fig 5) generated show the Re3 value, as a measure of the ductility, which is the average load applied as the beam deflects to 3mm expressed as a ratio of the load to first crack, also, referred to as the Equivalent flexural strength. Some fibre types and dose rates will provide acceptable equivalent flexural strength ratio values. Most important to note is The dosage of fibres should be sufficient to give an equivalent strength of a least 0.3 (30% Re3) otherwise the concrete should be treated as plain.

5 Fig 3. Load deflection curve. Testing of FRC. There are a number of accepted methods for testing FRC. Fig 5. Round Determinant Panel (RDP). Euronorm EN EFNARC Panel Test. ASTM C1550 Round Determinate Panels (RDP). ASTM 1609 Beam test. - - Fig 6. ASTM 1609 beam test Fig 4. EFNARC panel It is not uncommon to come across fibre specifications that will read concrete shall include between 3kg and 6kg of discreet graded monofilament high performance polypropylene fibres as per the manufacturers recommendation. A specification like this leaves it open to

6 interpretation, and potentially not what the designer had in mind. Comparing this to how fibre specification may read for shotcrete linings in tunnels.. fibre reinforced shotcrete will be required to achieve toughness as measured by energy absorbed in causing central deflections of 5mm and 40mm must be equal to or exceed 100 Joules and 400 Joules respectively at 28 days.. A comparison of fibre dose rates So what are the risks when one fibre is substituted for another fibre at the same dose rate? The following table shows results from cast 150mm x 150mm C-1609 beams on the three most commonly used macro synthetic fibres by municipal councils in Australia. According to a specification that states concrete shall contain between 3kg and 6kg of discretely grade macro synthetic fibre Fibre Kg Re3% Mpa A- Macro/Micro Blend 4.6kg B- Macro strand 3.8kg C- Macro fibre Embossed 2.5kg Fig 7. Cast 150mm x 150mm C-1609 Beam Tests Only product A and B conform. However, according to BS EN product A does not achieve a Re3 of 30% even though it is the highest dose rate the designer should still consider the concrete as unreinforced. The equivalent flexural strength ratio (Re3) is dependent on fibre type, bond with the concrete, modulus of elasticity and dose rate and concrete strength, not just the dose rate. This should be determined by testing as outline previously or requesting from the fibre supplier test certificates from a NATA certified facility. Durability of FRC Technologies in Structural Engineering (TSE) conducted exposure testing of steel and synthetic fibre reinforced panels to determine the ability of fibre reinforced shotcrete (FRS) to maintain residual load capacity once cracks have occurred. The panels were retrieved for retesting at 7. 14, 24 months. The durability of FRS reinforced with macro synthetic fibres was found to be excellent in both inland and coastal environments. This high level of durability was independent of crack width. In contrast, corrosion and loss of performance of FRS reinforced with steel fibres was substantial, even after only 7 months exposure. Crack widths were found to influence the degree of deterioration suffered by the steel FRS. Cracks greater than 0.1mm width led to significant rates of deterioration Fibre type 12 mth exposure % Difference 28 Days Macro Synthetic Hook-End Steel Fig 8. C-1550 panel tests

7 A copy of the full report Durability of cracked fibre reinforced shotcrete by Dr E.S. Bernard is available on request. FRC panel exposure Applications of FRC Globally FRC is gaining in popularity with contractors for many reasons that I have outlines already, e.g. performance and durability. However as the technology improves and more products enter the market the cost saving realised by contractors is driving the designers to consider synthetic fibres over steel fibres and traditional reinforcing methods. In Europe alone BarChip synthetic fibres have been employed for temporary and permanent linings in nearly 50 tunnels. FRC Precast tunnel lining segments. Across South America and parts of Europe and Japan almost 1 million square metres per annum of commercial and industrial flooring is constructed using macro synthetic fibres in place of steel mesh. Industrial Floor Chile Helsinki Metro. Some of North America s largest precast companies use BarChip fibres in lieu of wwf in their operations.

8 References Technical Reference 34, Third Edition. Concrete Industrial Ground Floors. Concrete Society Technical Reference 65, First Edition, Guidance on the use of Macro synthetic fibre reinforced concrete. Concrete Society Information Bulletin 39, Fibre Reinforced Concrete. CCANZ. Belfast International Airport. Transcity JV the contractors for the Legacy Way tunnel in Brisbane are using Macro synthetic fibres for crack control in the 36 cast in-situ cross passages. Durability of cracked fibre reinforced shotcrete. Dr E.S Bernard, TSE Pty, Ltd. Mechanical Properties of large scale synthetic fibre reinforced ground slabs. Amir M. Alani, Derrick Beckett, Dept. of Civil Engineering, University of Greenwich, UK Conclusion The pressure designer s face to continually keep abreast of advances in technology is on-going. This paper has hopefully provided you with a brief explanation of how just one of the many products that you may be asked to review and consider by contractors and outside consultants can be incorporated into your current and future projects. That there are standards in place that should give you confidence that providing the supplier is conforming to these guidelines you can be certain that macro synthetic fibres can replace steel in many of your assets, and most likely the life of that asset will be longer, also the capital cost of building that asset can be reduced, and most likely will be delivered sooner than it would have been if you had chosen to use traditional steel reinforcing.