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.
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.
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
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.
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
0
1
1
25
1.5
2.0
50
1.6
8.0
75
1.7
10.5
100
1.5
8.5
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.
According to Aurecon, over 90% of all fibre reinforced overlays in the USA were constructed with structural synthetic fibres.
Steel fibres are most suited to structural elements in heavy duty and high fatigue applications for long-term crack control, and in high sunlight applications.
Synthetic fibres are corrosion resistant unlike steel fibres but are susceptible to UV degradation. Steel, on the other hand, is susceptible to corrosion but is stable in the presence of UV.
The melting point of synthetic fibres is much lower than steel so steel maintains greater resistance to high temperatures such as in the event of a fire.
Synthetic fibres are substantially cheaper than steel fibres, both as like-for-like quantities and also because a much lower density of synthetic fibres is required in the concrete mix compared with steel.
AS an example, for container combi slabs the density required in concrete for synthetic fibres is around 4.5 kg/m3 compared with 35 kg/m3 for steel fibres. This means that Synthetic fibres cost approximately $60 /m3 whilst steel costs approximately $70/m3. And by using synthetic fibres approximately $10/m3 would be saved.
Synthetic fibres are often produced as a byproduct of the textile industry. This means their reuse in concrete is an effective way to upcycle and prevent contribution to landfill.
Steel fibres are generally produced new, as recycled fibres present challenges in quality control and consequently impact concrete performance. Steel is one of the world’s largest producers of carbon with around half of all steel produced consumed by the construction industry. 44% of construction steel is used for reinforcement equating to approximately 1.5% of global carbon emissions attributable to steel reinforcement.
Steel fibers have a high modulus and a high resistance in traction, which means they are not effective against cracks, while the synthetic fibers have a low Young modulus ranging between 3 and 5GPa and are reactive to potential cracks at an early stage. Conversely, synthetic fibers lose significance as the concrete becomes more mature. Steel fibers do not creep at the levels of strain in concrete, while it is not so with synthetic fibers.
Durability of fiber reinforced concrete involves two aspects; the material and the structure. The first aspect concerns corrosion of the fibers, and there is no durability issue in the fiber in the concrete when it comes to synthetic fibers. Corrosion of the fibers may take place in case of steel fibers.
Steel fibers or synthetic fibers
Polypropylene fibers are synthetic fibers obtained as a by-product from textile industry. Polypropylene fibers are characterized by low specific gravity and low cost. Its use enables reliable and effective utilization of intrinsic tensile and flexural strength of the material along with significant reduction of plastic shrinkage cracking and minimizing of thermal cracking.
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Used as secondary reinforcement, polypropylene fibers help reduce shrinkage and control cracking. To use these fibers, concrete mix design does not have to be altered, and no special equipment or slump modifications are required, even for pumping or shotcreting.
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Chemistry. What the fiber is made of, its shape, even its color, can lend itself to a variety of pain sources. Whether it’s steel, polypropylene, polyester, nylon, cellulose, or even natural, fiber makeup determines whether it has issues with corrosion, absorption, safety in use, adding air to the concrete. Each brings its own level of potential pain.
Addition time and process. Different fibers have different protocols with regard to their optimum point addition to the concrete, and many fibers require specific time span requirements for that addition process. A fiber that must be sprinkled in or added in a “chicken feed” manner causes pain for the concrete producer. This is particularly critical for the high-speed, rapid mixing systems for paving applications, especially at high fiber dosages.
For instance, some plants require 40 pounds to 50 pounds of synthetic fiber to be added within a 20- to 30- second loading cycle, which is often impossible for a “trickle-in” fiber without causing balling or clumping problems.
Mixing time. Different than addition time, mixing time for a particular fiber to achieve uniform distribution varies widely by fiber type. Again, this is particularly critical in the short-cycle paving mixers, many of which have a mixing period of 60 seconds or less. That might not be nearly long enough for some fibers.
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Impact protection and fire spalling
“Micro fiber concrete with polypropylene fibers are mainly used to reduce plastic shrinkage in fresh concrete. During the hardening process of concrete, dissipation of heat of hydration of concrete coupled with evaporation of water induces tensile stresses. Beyond a threshold limit of these stresses, micro cracks start developing in the concrete. Micro fiber concrete with polypropylene fibers reduces effective the early shrinkage behavior in the first 10 hours of pouring. The reason is that these types of fibers are able to hold back some water and slow down the evaporation process. They also are able to pick up some limited tensile stresses especially in the early age.
These types of fibers work better to reduce plastic shrinkage cracks and are often added in addition to the reinforcement of concrete.
“Like any secondary reinforcement, the fibers tend to stop cracks from propagating by holding the concrete together so cracks cannot spread wider or grow longer. However, since polypropylene fibers are distributed throughout the concrete, they are effective close to where cracks start at the aggregate-paste interface”.
Durability of fiber reinforced concrete involves two aspects; the material and the structure. The first aspect concerns corrosion of the fibers, and there is no durability issue in the fiber in the concrete when it comes to synthetic fibers. Corrosion of the fibers may take place in case of steel fibers.
Synmix® is a synthetic fibre for concrete, specifically for use where serviceability requirements for deflections, rotations, crack widths and creep are more relaxed than would normally be the case e.g. ground support in mines.
Mixing: proportions, time, etc. During the mixing operation, the movement of aggregate shears these bundles into smaller bundles and individual fibers. If the job site is more than a 30-minute drive, the fibers should be added at the site. (concreteconstruction.net)
Much less density of synthetic fibres needed than steel
Further Sources
https://www.researchgate.net/post/what_is_the_difference_between_steel_glass_and_polypropylene_fibers
http://onlinepubs.trb.org/Onlinepubs/trr/1989/1226/1226-011.pdf
https://www.researchgate.net/publication/304169533_Effect_of_Steel_Fibers_Polypropylene_Fibers_and_or_Nanosilica_on_Mechanical_Properties_of_Self-Consolidating_Concrete
https://bosfa.com/wp-content/uploads/2015/07/Steel-vs-synthetic-fibre-reinforced-concrete.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7767471/
https://bosfa.com/wp-content/uploads/2015/07/Steel-vs-synthetic-fibre-reinforced-concrete.pdf
https://fibermesh.com/wp-content/uploads/2017/04/FIB_ER1603-UndFibReinCon.pdf
Featured image credit/source: longbeachconcreteservice.com
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