Guide to fiber-reinforced concrete: advice for design, specifications and application

According to the American Concrete Institute CT-18, fiber-reinforced concrete is a concrete mix reinforced with randomly oriented and dispersed fibers. These fibers can be used in place of traditional reinforcement, such as steel rebar. Defined by their composition, fibrous reinforcement products for concrete fall under several types of classification. A common classification is polymer fiber, which includes polypropylene, polyethylene, polyester, acrylic, and aramid. Other types of fibers include natural, cellulose, glass, and high or low tensile steel fibers.

Fibers can also be classified as macrofibers or microfibers. Macrofibers have in the past been called structural fibers and are intended to support load and used to replace traditional reinforcement in certain non-structural applications and limit the width of cracks. They are also used to minimize or eliminate early or late cracking of concrete.

Microfibers, on the other hand, are generally used to minimize early cracking. They are typically used at low volumes in concrete and are easily specified.

chemical euclidI published an article highlighting the main advantages of using fiber-reinforced concrete, especially following the COVID-19 pandemic. While this sparked interest in several benefits that many contractors were unaware of, it also raised questions about how to properly use fiber reinforced concrete. Let’s explore the key considerations for fiber reinforced concrete, including its design, specifications, application, and how to properly finish the product.

The design of fiber-reinforced concrete

Fiber-reinforced concrete is specified by its strength, type of fiber and content in the concrete mix. Fibers added to concrete will most often not change the behavior of uncracked concrete. However, after the concrete cracks, the fibers can bridge the cracks and support the tensile stresses, providing a bearing capacity called residual or post-crack strength. The level of residual strength of fiber-reinforced concrete will depend on the type of fibrous material used, as well as its size, geometry, adhesion characteristics, dosage and, more importantly, their combined effect in hardening the concrete.

In order to characterize the performance of fiber reinforced concrete, bending tests are usually carried out and the post-crack parameters derived from these tests are used in the design. The most common and preferred bend test in North America is ASTM C1609/C1609M. To perform this test, the complete before and after crack response of a 6 inch x 6 inch x 20 inch fiber reinforced concrete beam is recorded.

For the design of slabs on grade with fiber reinforcement, Chapter 11 of the American Concrete Institute (ACI) Guide 360R provides detailed information and calculations. The load capacity of a slab on grade is a function of its thickness, the modulus of the foundation and the interaction between the slab and the foundation.

Macrofibers can contribute to stress redistribution and improve the flexural strength of the slab after cracking. Therefore, a toughness parameter called residual strength ratio – expressed as R 150 / (T, 150) (or Re3 ) is commonly used.

  • Re3 =fe3 /Fr (Where be3) is the equivalent flexural strength after cracking
  • Fr = the measured flexural strength to cracking

For a given concrete mix with known flexural strength, higher macrofiber dosages will provide higher toughness values.

Fiber-reinforced concrete specifications

Guide to fiber-reinforced concretechemical euclidWhen specifying fiber-reinforced concrete, specifications should be performance based and application dependent. Fibers should be specified based on desired technical performance and approved methodologies, such as those provided in ACI Guide 544.4R if the purpose of fiber reinforcement is to provide post-crack flex and tensile capacity to a concrete section. Once the appropriate calculations have been made for the design, parameters such as residual strength should be used for fiber reinforced concrete specifications.

Additionally, fiber reinforced concrete can be specified for serviceability to meet specific crack widths. This alternative specification approach is primarily used for underground structures and other pre-engineered structures. For this method, a European notched beam test is usually performed to determine the residual strength at given crack widths.

For shotcrete applications, the performance of fiber-reinforced shotcrete is determined by the energy absorption capacity in panel tests per ASTM C1550 or EN 14488-5. In this case, the energy would be classified as the area under the load-displacement curve.

Determination of fiber dosage

The amount of fiber required for a given project and application is determined by the performance requirements specified for the specific fiber-reinforced concrete being used. For plastic shrinkage crack reduction using microfibers, a crack reduction rate is usually specified and the required microfiber determined from ASTM C1579 standards is provided by the fiber manufacturer. Microfibers can also improve the properties of hardened concrete such as abrasion, permeability and weather resistance. For hardened concrete using macrofibers, the fiber dosage is chosen to meet the specified average residual strength, post-crack equivalent flexural strength or energy absorption capabilities, which also take into account factors such as as concrete thickness, strength, temperature and loading requirements.

For concrete slabs on grade, minimum dosage rates are determined by fiber manufacturers based on standardized product testing to meet industry standards. For composite metal decks, the IBC-2015 standard has a specified minimum requirement allowing for a dosage of 4.0 lb/yd3 for synthetic macrofibers, or 25.0 lb/yd3 for steel fibers used for temperature and shrinkage control and wire mesh replacement.

Considerations for mix design and application

Guide to fiber-reinforced concreteAdobe Stock Images | By DanielleAdding fibers to concrete will often reduce concrete slump, which is a measure of the workability of fresh concrete during placement. Adding fibers to a concrete mix is ​​likened to adding more ingredients to the mix and would therefore require more fluid to maintain apparent slump, hence the occurrence of loss of workability. Microfibers, used in typical doses, generally decrease sag only slightly and require no significant changes to maintain placement characteristics. Macrofibers and steel fibers can affect workability more significantly, depending on fiber type and dosage.

To aid in the workability of fibre-reinforced concrete, the ACI 544.3 standard provides recommendations and guidance for possibly modifying the mix design to improve workability.

Additionally, the use of chemical admixtures, such as superplasticizers or water reducers, will help increase the workability of concrete without adding water. It is recommended to run trial batches to ensure the workability of the mix.

Fiber-reinforced concrete finishing

Guide to fiber-reinforced concreteThe fibrous products available for concrete reinforcement from Euclid Chemical are PSI Fiberstrand “bundled” fibers, TUF-STRAND macro-synthetic fibers and PSI steel and blended fibers.chemical euclidThe type of fibrous material, architecture, dimension and dosage can all affect the surface finish of fiber reinforced concrete, as well as the required surface finishing method that is used. Rigid or rigid fibers generally have a greater tendency to protrude through the slab more than flexible fibers.

The use of appropriate external vibrations is a key factor when finishing fiber-reinforced concrete. It is generally recommended that fiber reinforced concrete be finished with the same finishing techniques and approximate schedule as conventional concrete. When casting samples for test purposes, no internal vibration in the molds can be used. Synthetic fibers in concrete can also delay the appearance of bleed water on the surface, which can impact the overall schedule of finishing operations on larger slabs.

There are a variety of broom finishing tips available to help minimize surface appearance, such as brushing in one direction only and using specific broom bristles to help align surface fibers. A torch can also be used, if necessary, to burn off the synthetic fibers to the surface of the concrete, but should not be used until all the desired properties of the hardened concrete have been achieved. When finishing industrial, commercial and warehouse floors where high dosage levels of synthetic macrofibers and steel fibers are used, a laser screed or vibrating screed is generally recommended.

As the uncertainties of the COVID-19 pandemic remain in 2022, fiber reinforced concrete continues to be a financially sound and viable health and safety solution for concrete applications. Using fiber-reinforced concrete can also offer significant cost savings while reducing the labor required on a job site. Fiber reinforced concrete will provide a more resilient, durable and cost effective building solution and will still meet the desired engineering properties compared to conventional reinforced concrete. By following these best practices, you’ll be well positioned to utilize the many benefits of fiber reinforced concrete on your next project, while ensuring the finished project is just as visually appealing.

About the Author

Michael Mahoney is a Professional Engineer and Director of Marketing and Technology – Fiber Reinforced Concrete for Euclid Chemical and a Fellow of the American Concrete Institute.

Resources

ASTM C1609/C1609M, www.astm.org/c1609_c1609m-12.html

Chapter 11 of the American Concrete Institute (ACI) 360R Guide, cecollection2.files.wordpress.com/2020/05/360r-10-guide-to-design-of-slabs-on-ground.pdf

ACI 544.4R Guide, concrete.org/Portals/0/Files/PDF/Previews/544.4R-18_preview.pdf

ASTM C1579 Standards, astm.org/c1579-21.html

IBC-2015 Standard, sdi.org/wp-content/uploads/2017/02/ANSI-SDI-C-2017-Standard.pdf

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