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Steel VS synthetic fibres

Steel VS synthetic fibres in concrete reinforcement – what’s best?

Early humans are known to have used horsehair in mortar and straw in mudbricks to build their homes. Master builders used concrete with bamboo fibres to build the Roman Pantheon, and for the last few decades, steel fibres have been widely used to reinforce concrete structures all over the world. These fibres provide additional structural and durability performance, minimise the formation of cracks and in some cases might even eliminate the need for reinforcing bars altogether, therefore reducing cost of materials and placement.

As the global requirement for sustainable materials becomes more vital, we’re seeing the use of synthetic fibres offer an ecological alternative to steel. Synthetic fibres offer corrosion resistance, environmental advantages and are lighter in weight making them easier to transport.

Different types of fibres used for fibre reinforced concrete (FRC)

- Asbestos fibres - Asbestos was cheap to produce and provided mechanical, chemical and thermal resistance, though with low impact strength. The manufacture and use of asbestos products was banned Australia in 2003.

- Carbon fibres - Carbon has a very high modulus of elasticity and flexural strength.

- Glass fibre reinforced polymer (GFRP) - GFRP is durable, non-corrosive, sustainable and has a very high strength-to-weight ratio.

- Natural fibre reinforced concrete (NFRC) - is made up of cellulose fibres that have been processed from pine trees or recycled carpet waste.

- Polypropylene fibre reinforced (PFR) - Polypropylene fibres are resistant to forming chemical reactions. These synthetic fibres are a by-product from textile industry making it cheap and widely used.

- Steel fibre reinforced concrete - Hydraulic cement mixed with randomly dispersed steel fibres.

Steel VS synthetic fibre reinforcement – what’s the difference?

Steel fibres are usually made with black steel and range in size from 0.5mm to 1.1mm in diameter and from 15mm to 60 mm in length. Steel fibres are most suited to heavy duty and high fatigue structural elements for long-term crack control, and in high sunlight applications.

Synthetic fibres are generally made of monofilament and fibrillated polypropylene, nylon, monofilament polyester or a blend of any or all these. Synthetic fibres are called ‘microfibres’ or ‘macrofibres’ depending on their diameter.

- Synthetic microfibers have a diameter of less than 0.3mm and are either monofilament or fibrillated. They are not a structural reinforcing fibre and cannot be used to replace any structural steel elements.

- Synthetic macrofibres have a diameter greater than 0.3mm. These can be used as a replacement for crack control mesh or as structural reinforcement in concrete or shotcrete. Macrofibres are used where an increase in residual (post-cracking) flexural strength is required.

There are also blends of micro and macro synthetic fibres that are used to control cracking. Synthetic fibres are much lighter than steel fibres and weigh 910 kg/m3 compared to 7,850 kg/m3 for steel fibres.

Design codes for fibre reinforced concrete structures

- fib Model Code for Concrete Structures 2010

- The most comprehensive, internationally used, code on concrete structures which includes their complete life cycle; conceptual design, dimensioning, construction, conservation and dismantlement.

- ACI committee 544 – Fibre Reinforced Concrete

- 544.4R-18: Guide to design with fibre reinforced concrete.

- References and incorporates fib Model Code theory.

Structural performance of steel VS synthetic fibres

- Young’s modulus - Steel fibres have a much higher Young’s modulus (210,000 MPa) and a higher traction resistance than synthetic fibres (3,000-10,000 MPa). This means that synthetic fibres are effective for plastic shrinkage control (cracking that can occur in the first 24 hours of concrete cure), impact protection, and fire anti-spalling, but become less effective over time as the concrete hardens.

- Tensile strength - Steel fibres have a greater tensile strength (500-2,000 MPa) VS synthetic fibres (200-600 MPa).

- Durability - Exposure to carbon dioxide in the air and chlorides in the water leaves steel predisposed to corrosion. Unlike steel fibres, synthetic fibres are corrosion resistant, so are ideal in coastal environments, immersed conditions, and splash zones. Also, in areas that are regularly exposed to corrosive chemicals or de-icing salts. However synthetic fibres are more susceptible to UV degradation and high temperatures than steel.

- Creep - Creep can occur when strain in the concrete increases over time under constant stress. Steel fibres are less susceptible to creep than synthetic fibres, particularly at higher temperatures.

- Volume of fibres - The strength of the composite depends greatly on the volume of fibres present. Generally, the greater the volume of fibres, the greater the tensile strength and toughness of the composite. However, use of a higher percentage of fibre can cause segregation and harshness of concrete and mortar.

- Aspect ratio of fibres - An increase on the aspect ratio up to 75 is reported to linearly increase the ultimate strength of concrete. After 75, the relative strength and toughness is reduced.

- Impact protection and fire spalling - Polypropylene microfibres are designed to reduce plastic shrinkage in the first 10 hours of concrete pouring by holding back water and slowing down the evaporation process. They tend to stop the spreading of cracks at the aggregate-paste interface by holding the concrete together.

When to use synthetic microfibres fibres in concrete

Synthetic microfibers are used as secondary reinforcing for non-structural applications. Because of their ability to distribute evenly within the paste and to slow water evaporation, they reduce bleeding and are superior for plastic shrinkage crack control in fresh concrete. Synthetic microfibers are also useful for passive fire protection as they minimise concrete spalling in case of fire. That’s because they have a low melting point which opens channels in the concrete pores, allowing built up vapours to escape and release pressure.

When to use synthetic macrofibres fibres in concrete

Synthetic macrofibres can be used for structural applications. They can enhance structural integrity, replace crack control mesh and even replace structural reinforcement bars in some cases. They’re also a good choice for when an increase in residual (post-cracking) flexural strength is required.

Synthetic macrofibres can be used in pre-cast concrete, slabs on grade, composite steel decks and shotcrete as an equivalent reinforcing option to steel fibres, crack control mesh and light gauge reinforcing bars. They are particularly favourable over steel fibres in environments that are susceptible to corrosion, e.g., coastal areas.

Fibre addition and mixing

All fibres have different procedures regarding when and how they must be added to the concrete. Some fibres must be sprinkled in gradually to avoid balling or clumping problems. This is huge consideration, especially when required in rapid mixing systems and high fibre doses for paving applications. In some cases, if the job site is more than a 30-minute drive away, the fibres will need to be added on site.

Cost of steel VS synthetic fibres

Synthetic fibres are usually much cheaper than steel fibres, in direct cost comparisons and also because a far lower density of synthetic fibres than steel are required in the concrete mix.

For example, for container combi slabs the density of synthetic fibres required in concrete is around 4.5 kg/m3 compared with 35 kg/m3 for steel fibres. So, if the synthetic fibres cost $60 /m3 and steel fibres cost $70/m3, there’s a whopping saving of $10/m3.

Sustainability of steel VS synthetic fibres

We’ve discussed the environmental benefits of Glass Fibre Reinforced Polymer (GFRP) rebar over steel here. Steel production is solely responsible for almost 8% of global emissions. Approximately half of that steel is used in the construction industry, with a significant 44% of it being used for reinforcement. In comparison GFRP creates less carbon emissions and uses far less energy consumption.

Many synthetic fibres are often produced as a by-product of the textile industry. Recycling them for use in concrete is an effective way to prevent contribution to landfill.

If you have more questions about steel VS synthetic fibres in concrete reinforcement, click here to access our Customer Service team.

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