WHAT IS FIBER-REINFORCED CONCRETE AND WHY IS IT USEFUL?
by TRP Ready Mix on July 11, 2019
Ready Mixed Concrete Suppliers Discuss the Benefits of Using Fiber-Reinforced Concrete
We’ve heard of concrete being reinforced by materials before, but what’s the purpose of this? Is it something we’ve recently developed, or has it been around for a long time?
Simply put, this process is what strengthens concrete, ensuring the structure’s integrity and security – basically what keeps skyscrapers from toppling over.
To learn more about fiber-reinforced concrete, here’s a look at its purpose, types, uses, and its many benefits.
Read More: The Problem with Steel-Reinforced Concrete
WHAT IS FIBER-REINFORCED CONCRETE?
Fiber-reinforced concrete (FRC) is concrete that has fibrous materials mixed in to increase the concrete’s durability and structural integrity. FRC has small, short, and discreet fibers that are randomly oriented yet uniformly distributed throughout the concrete.
The fibers can be circular or flat, and often make up one to three percent of the concrete mix’s total volume.
Common fibers used in reinforced concrete include steel, glass, synthetic, and natural fibers.
WHY IS IT USED?
On its own, concrete lacks tensile strength and is prone to cracking. But fiber-reinforced concrete can improve tensile strength and control cracking in concrete structures that is often caused by plastic shrinkage and drying shrinkage.
Fibers in concrete can also reduce the permeability of concrete, which limits the amount of water that bleeds out, further reducing shrinkage cracking during curing.
Some types of fibers are also used to make concrete more abrasion-, impact-, and shatter-resistant.
Fiber-reinforced concrete is often used for:
- Ground-level applications, such as sidewalks and building floors
- Basement foundations
- Building pillars
- Support beams
- Burial vaults
- Roof shingles and tiles
- Shotcrete applications—such as pools, basins, agricultural waterways, and rock walls
- Drainage pipes
- Septic tanks
- Sewer systems
- Precast and prefabricated shapes—such as composite decks and thin cement sheets and panels
- Vaults and safes
TYPES OF FIBRES USED IN CONCRETE
Here are some of the most common fibers used in reinforced concrete:
Steel is one of the most commonly used materials for fiber-reinforced concrete. Round steel fibers are made by cutting round wire in short lengths. And flat, rectangular steel fibers are created by silting steel sheets.
Steel fibers add strength to concrete mixes by distributing localized stresses. Steel fiber reinforcement also reduces the amount of structural steel needed, such as rebar and mesh. It can also reduce freeze-thaw damage and cracking caused by plastic shrinkage while increasing impact resistance.
Polypropylene fiber reinforced (PRF) concrete uses the cheap and abundantly available polypropylene polymer. Polypropylene fibers are resistant to most chemicals and have a high melting point of 165˚C. So it can withstand a working temperature of 100˚C for short periods.
Since these fibers are hydrophobic, they can also be easily mixed and evenly distributed in the concrete without clumping together.
Glass fiber reinforced concrete (GFRC) is another common type. It is most often used in the production of thin-sheet concrete products.
Asbestos mineral fibers are naturally available and inexpensive. Asbestos fibers are thermal- and chemical-resistant, so they are suitable for sheet product pipes, tiles, and corrugated roofing elements. But since there are health risks associated with asbestos, concrete suppliers use safer materials these days.
Carbon fibers have high elasticity and flexural strength, with their strength being comparable or even superior to steel fibers. But carbon fibers are more vulnerable to damage than even glass fibers, so they must be treated with a resin coating.
Natural fibers, such as vegetable fibers, are cheaper than other types of fibers. However, a large volume of natural fibers is often required to achieve control over cracking. And natural fibers tend to be more difficult to mix and evenly distribute throughout the concrete. So a superplasticizer might be needed to avoid problems with mixing and ensure the fibers have a uniform distribution.
Plastic fibers are relatively new in the world of concrete reinforcement. But these fibers do present an opportunity for recycling the abundance of plastic in the world for a more eco-friendly reinforced concrete.
Nylon fibers share many of the same characteristics of polypropylene fibers. Nylon fibers are also stronger than welded wire fabric when used in concrete.
FACTORS THAT AFFECT PROPERTIES OF FIBER-REINFORCED CONCRETE
When using fiber-reinforced concrete, many factors will affect its performance and workability, such as:
Relative Fiber Matrix Stiffness
To ensure the concrete has efficient stress transfer, the modulus of elasticity of the concrete matrix must be lower than that of the fiber.
For example, nylon and polypropylene fibers have a low modulus of elasticity. So while they are unlikely to improve overall strength, they will help absorb large amounts of stress energy, making the concrete tougher and more resistant to stress.
On the other hand, steel, glass, and carbon fibers create stronger yet stiffer concrete.
Volume of Fibers
The amount (volume) of fibers used in the concrete will affect the concrete’s strength and toughness. The tensile strength and toughness will increase as the volume of fibers increases.
Aspect Ratio of the Fiber
As the aspect ratio of the fiber increases, so will the strength and toughness of the concrete, but only up until a certain point—an aspect ratio of 75. If the total volume of fibers exceeds this maximum aspect ratio, then the fibers can actually reduce the concrete’s durability instead of strengthening it.
While fibers are randomly oriented in reinforced concrete, their orientation can affect the strength of the concrete. Fibers that are aligned parallel to the load provide more tensile strength and durability than those perpendicular or randomly distributed.
Steel fibers reduce the workability of concrete and make it difficult to compact concrete. The length and diameter of the steel fibers will also affect the workability and ease of compaction.
Also, the non-uniform distribution of fibers in the concrete will reduce workability and compaction. To improve workability and compaction, concrete suppliers may increase the water/cement ratio or use water-reducing admixtures.
When mixing fiber-reinforced concrete, the maximum size of the coarse aggregates should be 10 mm to avoid reducing the strength of the concrete. Friction-reducing admixtures and admixtures that increase the cohesion of the mix can help improve the mix.
Special considerations of the above factors must be made when mixing fiber-reinforced concrete to avoid:
- Fibers balling together;
- The segregation of fibers; and,
- A non-uniform distribution of fibers.
Adding fibers to concrete before the water is added can help ensure that the fibers are evenly dispersed throughout the concrete mix.
ADVANTAGES OF FIBER-REINFORCED CONCRETE
All fibers reduce the concrete’s need for steel reinforcements. And since fiber reinforcement tends to be less expensive than steel rebar (and less likely to corrode), it makes concrete more cost-effective.
Fibers can also improve the concrete’s:
- Tensile strength
- Durability—by controlling and reducing crack widths
- Freeze-thaw resistance
- Abrasion- and impact-resistance
- Resistance to plastic shrinkage while curing
- Resistance to cracking
- Shrinkage at an early age
- Fire resistance
With so many options to choose from and all these benefits, consider using fiber-reinforced concrete for your next project. You can even combine fiber types to enjoy a superior concrete with the combined benefits of each fiber.