What is the difference between steel and synthetic fibres in concrete?

Fibres are often used in concrete to provide added structural and durability performance, crack minimisation and even to eliminate the need for reinforcing bars altogether.

There are several types of fibres that are commonly added to the concrete mix to improve certain properties of the hardened concrete, derive constructibility benefits or reduce cost of materials and placement. The most common types being steel fibre, synthetic fibre and carbon fibre and glass fibers.

Whilst steel fibres have been used in concrete for decades, recent advancements in materials technology and inclusion of synthetic fibre reinforcement in building standards has led to the increased use of synthetic fibres in practical applications. These benefits include lower cost, corrosion resistance, environmental advantages and lightweight nature compared with steel.

Composition of steel vs synthetic fibres

Synthetic fibres are commonly made of monofilament and fibrillated polypropylene, monofilament polyester, nylon or a blend of any of these. Synthetic fibres are classed as either macrofibres or microfibres depending on their diameter.

Microfibers have a diameter of less than 0.3 mm whilst macrofibres are 0.3 mm or greater. Steel fibres, usually composed of black steel, generally range from 0.5 to 1.1 mm in diameter and from 15 to 60 mm in length.

Synthetic fibres are much lighter than steel fibres, weighing just 910 kg/m3 compared to 7,850 kg/m3 for steel fibres.

Structural performance

Microfibres typically aren't used for structural applications, i.e. where the addition of fibres contributes to the load-bearing capacity of the concrete element. However, macrofibres can be used to either enhance structural capacity, or completely replace crack control mesh and reinforcement bar altogether.

Steel fibres have a much higher Young's modulus (210,000 MPa) and tensile strength (500-2,000 MPa) than synthetic fibres (3,000-10,000 MPa and 200-600 MPa) as well as a higher traction resistance.

Synthetic fibres have a low Young's modulus (between 3,000 and 10,000 MPa) which means they are effective in preventing cracks during the early stages of concrete curing such as plastic shrinkage cracking, but become less effective over time as the concrete hardens.

Synthetic fibres are more susceptible to creep than steel, particularly at higher temperatures, causing them to decline in effectiveness of preventing concrete cracking over time and in hot climates.

Design codes for the fibre reinforced concrete structures are as follows:

fib Model Code for Concrete Structures 2010

Considered the preeminent authority for FRC design

Adopted and used in Europe

Based on a partial safety factor methodology

ACI committee 544 - Fiber-Reinforced Concrete

544.4R-18: Guide to Design with Fiber-Reinforced Concrete

References and incorporates fib Model Code theory

 

 

Important Factors Affecting Properties of Fiber Reinforced Concrete

Volume of Fibers:

The strength of the composite largely depends on the quantity of fibers used in it. The increase in the volume of fibers, increases approximately linearly, the tensile strength and toughness of the composite. Use of a higher percentage of fiber is likely to cause segregation and harshness of concrete and mortar.

Aspect Ratio of the Fiber:

Another important factor which influences the properties and behavior of the composite is the aspect ratio of the fiber. It has been reported that up to aspect ratio of 75, increase on the aspect ratio increases the ultimate concrete linearly. Beyond 75, relative strength and toughness is reduced. Table-1 shows the effect of aspect ratio on strength and toughness.

Table-1: Effect of the aspect ratio of the fiber on the plain concrete with randomly dispersed fibers

Aspect ratio	Relative strength	Relative toughness
0	1	1
25	1.5	2.0
50	1.6	8.0
75	1.7	10.5
100	1.5	8.5

 

Use Cases

Synthetic microfibers are used as secondary reinforcing for non-structural applications such as plastic shrinkage crack control, abrasion resistance and spalling resistance in slabs, precast and shotcrete.

Synthetic Microfibres are effective for reducing plastic shrinkage cracks in fresh concrete. This is both because of their greater ability to distribute evenly within the paste and therefore sit close to the surface and their ability to slow water evaporation and therefore reduce bleeding.

They are also useful for passive fire protection by minimising concrete spalling in the presence of fire. This is due to their low melting point which opens pores in the concrete, allowing the concrete to release built-up vapours and internal stresses.

Synthetic Macrofibres, on the other hand, can be used for structural applications, either to enhance the structural integrity, replace crack control mesh and replace structural reinforcement bars in some applications. Macrofibres are effective for use where an increase in residual (post-cracking) flexural strength is required.
They can be used where an equivalent reinforcing option to steel fibres, crack control mesh and light gauge reinforcing bars are required in pre-cast concrete, slabs on grade, composite steel decks and shotcrete. They are particularly suited to corrosive environments where the use of steel would inhibit long-term durability.