An Introductory Guide to Macro Synthetic Fibre-Reinforced Concrete

Contents

1. What is synthetic fibre-reinforced concrete?
2. What are the advantages of fibre-reinforced concrete?
3. Typical Applications
4. Steel Fibres vs Synthetic Fibres in Concrete Reinforcement
5. Advantages and Disadvantages
6. Why use Synthetic Fibres instead of Wire Mesh?
7. Tuf-Strand SF

Since ancient times, brittle materials, such as mud bricks, have been reinforced with fibrous materials like straw. Fast-forward to the 20th century, engineers found that since concrete is strong in compression but relatively weak in tension, there was a requirement for a reinforcing mechanism to carry the tensile/flexural stresses caused by ultimate loads or shrinkage/thermal stresses after cracking. 

With technological advancements, synthetic fibres were soon developed to reinforce concrete and replace heavier materials such as steel. From a simple material, concrete has now become a complex solution that can be adapted to specific applications in accordance with the requirements.

Part 1 (A): What is synthetic fibre-reinforced concrete?

A comparison between a wire mesh and synthetic fibres in concrete.Synthetic fibre-reinforced concrete can be defined as a concrete that incorporates macro and micro fibres. The main reasons for using fibres such as the macro-synthetic fibres in concrete is to replace the traditional steel rebar reinforcement and enhance its flexural performance, resist crack formation due to drying shrinkage or temperature movement, increase toughness, durability, water-tightness and overall performance of concrete. 

While traditional steel rebar reinforcement is typically continuous and is specifically positioned in concrete for optimized performance, fibres are discontinuous and are randomly and homogeneously distributed throughout the concrete. For example, steel rebars provide two-dimensional reinforcement and require manual labour for placing and positioning. Macro synthetic fibres provide three-dimensional reinforcement and are introduced into concrete during the mixing stage.

Part 1 (B): What are the advantages of fibre-reinforced concrete?

Fibre Reinforced Concrete (FRC) is a rapidly evolving technology in the construction industry for its technical, economical, and environmental benefits. Here are its top 10 advantages:

1. FRC may be useful where optimal flexural strength and reduced crack width are desired, or when conventional reinforcement is not feasible.
2. It improves the overall toughness of concrete and limits the crack growth.
3. Ductility improvements increase fatigue life and impact resistance, resulting in a longer life cycle of concrete structures that can lead to potential savings in repairs.
4. For industrial projects, macro-synthetic fibres are used to improve concrete’s durability. Made from synthetic materials, these fibres are long and thick in size and may be used as a replacement for steel bar reinforcement or fabric reinforcement.
5. Adding fibres to the concrete will improve its freeze-thaw resistance and help keep the concrete strong and attractive for extended periods.
6. Improve mix cohesion.
7. Increase resistance to plastic shrinkage during curing.
8. In shotcrete applications, fibres reduce rebound and material waste.
9. Controls the crack widths tightly, thus improving durability.
10. Reduces segregation and bleed-water.

Part 1 (C): Typical Applications

• Mining and Tunnelling
• Precast concrete elements
• Slab-on-grade 
• Airport runways
• Highways, roads, and bridge decks
• Metal deck topping

Part 2 (A): Steel Fibres vs Synthetic Fibres in Concrete Reinforcement

As mentioned, fibres are used in concrete to control cracking due to plastic and drying shrinkage. They also reduce the permeability of concrete and thus improve the durability of structures. Below, we look at the two most common types of fibres used in concrete reinforcement:

Steel Fibres

Steel fibre is a metallic reinforcement. A certain amount of steel fibre in concrete can cause qualitative changes in concrete’s physical property. It can greatly increase resistance to cracking, impact, fatigue, bending, tenacity, durability, and others.

Synthetic Fibres

While synthetic fibres also include materials such as Polyester and Nylon, we will only be focusing on Polypropylene fibres which is the most used material for concrete reinforcement.

There are two types of polypropylene fibres categorised as macrofibers and microfibers. Primarily, they differ in the length but more importantly in the function that they perform in the concrete.

1. Macrofibres are also called structural fibres because they can replace the traditional reinforcement in the form of steel bars or wire mesh and transfer loads acting on the structure. Therefore, the time needed to make steel reinforcement, and thus the investment costs, are saved. Their length is usually between 30 and 50 mm.  
2. Microfibers are usually used for plastic shrinkage crack control and typically they do not add any structural capacity to a concrete section. 

 Macro versus Micro synthetic fibres.

Part 2 (B): Advantages and Disadvantages

Part 2 (C): In terms of application:

1. For all flatwork applications as well as shotcrete, synthetic fibres are usually preferred for their lighter weight and ease of handling and no issues with corrosion spots. 
2. For more structural applications such as precast segments, steel fibres are more common because they provide higher levels of residual strength.
3. For shotcrete application typical waste due to rebound for steel fibre is around 25% compared with less than 5% rebound for shotcrete made with macro-synthetic fibres.

Part 3: Why use Synthetic Fibres instead of Wire Mesh?

For micro fibres, they are most beneficial soon after concrete placement by controlling the formation of plastic shrinkage cracks. Wire mesh, on the other hand, does not prevent crack formation, but instead, the mesh holds cracks together after they have formed. 

However, replacing wire mesh or rebar with macro fibres (steel or synthetic) eliminates time and cost associated with purchasing, fabrication, delivering, and installation of reinforcement. In addition, wire mesh must be placed properly to be effective. This is not a concern with fibres, which are disperse evenly throughout the concrete during mixing. And unlike wire mesh, synthetic fibres are non-corrosive and will not rust. So from an economic standpoint, it is also more beneficial to use fibres.

In terms of safety, using fibres eliminates unnecessary personnel on site because ready-mix trucks can simply drive directly onto the subgrade sections and quickly and efficiently place the concrete with only minimal finishing practices needed. This reduces trip hazards while at the same time allow for increased speed of placement.

Part 4: Tuf-Strand SF 

What is Tuf-Strand SF?

Euclid Chemical’s TUF-STRAND SF (TSSF) is a patented polypropylene / polyethylene macro synthetic fibre successfully used to replace steel fibres, welded wire mesh and conventional reinforcing bars in a wide variety of applications. Concrete reinforced with TSSF will have three-dimensional reinforcing with enhanced flexural toughness, impact and abrasion resistance and will also help mitigate the formation of plastic shrinkage cracking in concrete.

Why choose TSSF over traditional steel fibres?

Providing that an adequate fibre design has been performed, the use of TSSF will generally require as much as 5-10 times less weight of material making on-site handling and storage much easier. TSSF is non-magnetic and non-corrosive making it a very attractive option for exterior paving projects where aesthetics and safety may be a concern. As TSSF is mixed, it also becomes somewhat pliable and will not be as abrasive or harmful to pumping lines and equipment. 

Furthermore, in shotcrete applications, adding TSSF can significantly reduce rebound and material waste, permit thicker layers per pass, reduce sagging, and inhibit plastic shrinkage cracking.

Tremco CPG recommends the use of TUF-STRAND SF macro-synthetic fibre for the reinforcement of concrete to replace steel and can provide engineered calculations to demonstrate equivalency with many project references available. Get in touch with us today to find out more.