Cost-effective, durable and sustainable product a popular choice for precast

As the demand for more sustainable construction grows and shortages of raw materials like steel continue to be a challenge, fiber-reinforced concrete has become an increasingly popular choice for both poured-in-place and precast applications.

Fiber reinforcement makes concrete significantly more resistant to cracking from plastic and drying shrinkage, leading to a highly durable end product with a longer lifespan. Some fiber types can even greatly reduce construction time and, in some applications, completely eliminate the need for conventional steel.

Although fiber has long been employed to improve the durability of precast concrete, it hasn’t been implemented as a complete substitute for conventional reinforcement. However, the creation of ASTM C1765 — Standard Specification for Steel Fiber-Reinforced Concrete Culvert, Storm Drain and Sewer Pipe in 2013 along with various DOT and other municipal standards have set the foundation for incorporating different types of fiber as a prospective choice for precast reinforcement in the future.

Why Use Fibers in Precast Concrete?

Regarded as a cost-effective, durable and even sustainable alternative to conventional steel like rebar and wire mesh, fiber-reinforced precast concrete boasts a variety of benefits such as:

  • Preventing plastic and drying shrinkage cracks
  • Increasing production speed and reducing labor costs
  • Eliminating the need for storage and installation of standard steel reinforcement in precast elements
  • Lowering material storage and transportation costs
  • Reducing precast element breakage.

Plus, unlike steel reinforcement, fiber is mixed throughout the entire precast element, which eliminates concrete cover requirements. In certain cases, this will even allow for a reduction in element thickness and weight.

Types of Fiber Reinforcement

The prevalent types of fiber currently utilized in the precast industry are steel, polypropylene and fiberglass. Steel fibers, primarily produced from carbon or stainless steel, play a crucial role in the prevention of cracking in concrete products, with varying geometries designed by manufacturers. Commonly applied in floor slab construction, steel fibers are increasingly being used in other precast structures such as concrete tanks.

Polypropylene fibers, belonging to the synthetic fiber category, offer similar benefits to precast products as steel alternatives. With characteristics akin to steel, polypropylene fibers are employed to prevent cracks and enhance durability, finding applications in precast structures such as septic tanks and burial vaults. These fibers are further divided into two categories of micro and macrofibers. Synthetic fiber concrete reinforcement resists rust, ensuring performance throughout the concrete’s lifespan.

Glass fibers are primarily used for architectural purposes. When added to a concrete mix, glass fibers enable the production of thin decorative elements and cladding systems with minimal weight, thereby reducing loading and providing excellent thermal properties.

Despite these advantages, some challenges remain in the widespread adoption of fiber reinforcement. Variability in the sizes and shapes of fibers complicates the selection process for specific projects, especially without comprehensive standards. Ensuring uniform fiber distribution in precast structures also requires further attention for consistency across different mixes.

How Synthetic Fibers Compare to Steel Reinforcement

Although synthetic microfibers provide superior resistance to plastic shrinkage cracking over welded wire reinforcement, they aren’t resistant to expanding crack width openings caused by drying shrinkage, structural load or other forms of stress. However, these fibers can — and should — be regularly specified in any concrete type to improve cracking resistance, spall protection, freeze-thaw durability and concrete homogeneity during placement. Microfibers are available in ½-inch to ¾-inch (12 – 19 mm) lengths, with dosage rates varying from 0.5 – 1.5 lbs/yd3 (0.3 – 0.9 kg/m3) depending on the fiber product and application type.

Synthetic macrofibers not only provide resistance to plastic shrinkage but also enhance concrete’s durability, toughness and limited structural capacity when properly designed. Dosed at amounts equivalent to conventional reinforcement, these fibers are distributed three-dimensionally throughout the concrete section. Synthetic macrofibers can be likened to the use of steel fibers but are usually easier to place and finish due to their lighter weight, non-corrosive nature and high pumpability. Macrofibers are typically 1 ½ inches to 2 inches (38 – 50 mm) in length, with dosage rates varying from 3.0 – 15 lbs/ yd3 (1.8 – 9.0 kg/m3) depending on the fiber product and application type.

With all of this considered, can synthetic fibers compete “head-to-head” with steel? Absolutely! Macrofibers offer durability and residual strength capacity equivalent to steel. Assuming an adequate fiber design is performed, synthetic macrofibers generally require five to 10 times less weight of material compared to steel, streamlining on-site handling and storage. They are also non-magnetic and non-corrosive, making them ideal for exterior precast where aesthetics and safety are a concern. Since concrete containing macrofibers is mixed, the fiber also becomes somewhat pliable and not nearly as abrasive to pumping lines and equipment.

Design Considerations for Fiber-Reinforced Precast Concrete

Fiber-reinforced precast elements are typically designed by a licensed professional. However, when a design requires a certain wall thickness to resist all flexural and bending conditions applied with conventional steel, the dosage rate of fiber can be calculated directly from the specified steel reinforcement that acts as an equal or better reinforcing option under the same conditions.
The use of fiber-reinforced concrete in this type of system can be and has been successfully implemented to achieve a more durable and economical structure. It should be noted that this analysis is valid only for single-layer reinforcement where distributed steel is evenly spaced. This procedure is also very well suited to precast elements where the steel has been designed to act as temperature and shrinkage reinforcement.

For thin wall precast, poured-in-place and shotcrete applications where specified steel is distributed and has already been designed, the use of synthetic macrofiber reinforcement can be used and calculated to provide an equivalent moment capacity in bending as the originally specified steel.
This analysis also considers the location of the steel and assumes that the existing reinforcement has been correctly placed in the concrete as designed. It is also assumed that all other concrete placement and finishing practices have been properly applied.

Standardized test methods, such as ASTM C1609 — Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete, measure the post-crack performance capacity of fiber-reinforced concrete. This performance value is commonly referred to as “residual strength” or toughness. Once the required flexural residual strength has been established, the appropriate dosage of fiber can be calculated by converting the residual strength to a moment capacity provided for by the steel reinforcement, as determined in accordance with ACI 544.4R — Guide to Design with Fiber-Reinforced Concrete. Some fiber manufacturers provide mobile applications that provide easy conversions of single-layer conventional reinforcement to appropriate dosage rates of fiber reinforcement.

Concrete Mix Design for Precast Applications

Adding fiber often reduces the slump of a concrete mix, which measures the workability of fresh concrete during placement. This is likened to adding more ingredients to the mix and, therefore, requires more fluid to maintain an apparent slump — hence the appearance of workability loss. Microfibers, used at typical dosages, generally only decrease slump slightly and don’t require significant changes to maintain placement characteristics. Yet, synthetic macrofibers and steel fibers can affect the workability of concrete in a more significant way, depending on fiber type and dosage.

To improve the workability of fiber-reinforced concrete, the ACI 544.4R standard offers recommendations and guidance to potentially modify the mix design. Additionally, the use of chemical admixtures like superplasticizers or water reducers increase the workability of concrete without adding water. It is recommended that trial batches are performed to ensure mixture workability.

Factors such as fiber material type, architecture, dimension and dosage may affect the surface finish of fiber-reinforced precast. Stiff or rigid fibers typically have a greater tendency to protrude up through the element than flexible fibers. If necessary, a torch can burn off synthetic fibers on a concrete surface — but should not be used until all desired hardened concrete properties are achieved.

Proper external vibration is another key factor to consider in the production of fiber-reinforced precast concrete. The general recommendation is to use the same consolidation techniques and approximate timing as with conventional concrete.

Compared to more established options like rebar or welded wire, fiber reinforcement represents a less mature industry, but the versatility of fiber ensures its place in modern precast operations. Ongoing research and innovation continue to uncover exciting possibilities in this evolving field.

Michael Mahoney is a licensed professional engineer and director of marketing and technology for fiber-reinforced concrete at Euclid Chemical. He is a fellow and member of the American Concrete Institute and has served on various committees for the National Precast Concrete Association and American Society for Testing and Materials. To learn more, visit