Fiber-Reinforced Concrete: Steel, Synthetic, Glass and Basalt Fibers Compared

Fiber-reinforced concrete is concrete with short discrete fibers of steel, synthetic polymer, AR glass or natural cellulose mixed into the batch, classified as Types I to IV in ASTM C1116/C1116M-23, so that the cracked section keeps carrying load. Which fiber, how much of it, and which standard proves it? Those three choices decide whether a fiber dose controls early cracks only or replaces steel mesh in a slab.

Fiber-reinforced concrete is also written fibre reinforced concrete or FRC, the fibers are called concrete fibers, and buyers search for fibre for cement when they mean the same batch ingredient. Fibers are one of the four families of concrete additives, with chemical admixtures, supplementary cementitious materials and integral pigments.

The split that matters first is size: microfibers have an equivalent diameter below 0.30 mm (0.012 in) and handle plastic cracking at about 0.1 % by volume, while macrofibers and steel fibers at 0.3 % by volume or more improve toughness and crack width (ACI FAQ 733). Doses are stated per cubic meter of concrete: 20 to 40 kg/m³ (33.7 to 67.4 lb/yd³) of steel and 4 to 8 kg/m³ (6.7 to 13.5 lb/yd³) of macro-synthetic fiber in standard concrete, and 0.6 to 0.9 kg/m³ (1.0 to 1.5 lb/yd³) of polypropylene microfiber against plastic shrinkage.

Performance is proved by residual flexural strength, not peak strength. An EN 14889 structural declaration names the dose that gives at least 1.5 MPa (218 psi) at a crack mouth opening of 0.5 mm and 1.0 MPa (145 psi) at 3.5 mm, and ACI 318-19(22) gives shear credit only to deformed steel fibers at 59.3 kg/m³ (100 lb/yd³) or more. EN 1992-1-2:2023 recommends 2.0 kg/m³ (3.4 lb/yd³) of fine polypropylene fiber where fck reaches 70 MPa (10,150 psi) and no specific spalling assessment is made.

Each value is traced to its source: standards and codes to their issuing body, product values to the data sheet named with them, prices to the dated source beside them (methodology and fact-checking).

What Is Fiber-Reinforced Concrete?#

Fiber-reinforced concrete is concrete that contains uniformly dispersed short fibers, which bridge cracks once the matrix cracks. ACI FAQ 733 gives two dose levels: about 0.1 % by volume of synthetic fiber reduces plastic shrinkage cracking, and 0.3 % or more of synthetic or metal fiber improves toughness and crack width.

ASTM C1116/C1116M-23 sorts fiber-reinforced concrete into four types by fiber material: Type I steel, Type II glass (AR), Type III synthetic and Type IV natural (cellulose). The same specification requires the concrete to be delivered uniformly mixed and free of fiber balls, made to ASTM C94/C94M or the volumetric-batching specification as modified, and it excludes thin-section spray-up GFRC.

Fibers as concrete additives, not admixtures#

Fibers are not admixtures in the ACI sense. ACI CT-25 defines a chemical admixture as "a liquid, or dispersible powder, used as an ingredient in a cementitious mixture to improve its economy and/or properties in the plastic and/or hardened state", and a solid fiber is neither. "Concrete additive" is a trade umbrella term, not an ACI, ASTM or EN defined term, and it groups four material families that a producer doses into the batch.

The four families of concrete additives are listed below.

• Chemical admixtures, covered under concrete admixtures
• Mineral additions, covered under supplementary cementitious materials
• Fibers of steel, polymer, glass, basalt or plant origin
• Integral color, covered under concrete pigments and integral color

Microfibers and macrofibers: the 0.30 mm split#

Fibers split by equivalent diameter: microfibers are below 0.30 mm (0.012 in) and macrofibers 0.30 mm or more, never by length or denier. The Fiber Reinforced Concrete Association (FRCA) and EN 14889-2:2006 draw the line at the same diameter.

EN 14889-2:2006 turns the split into three polymer classes: Class Ia for micro monofilament, Class Ib for micro fibrillated fiber, both below 0.30 mm, and Class II for macro fiber. The classes map onto the job. Microfibers act in the first hours, against plastic and settlement cracking, while macro-synthetic and steel fibers act after the hardened concrete cracks, carrying a residual load across the crack.

How Do Fibers Work in Concrete?#

Fibers do little before concrete cracks; once a crack opens, fibers that cross it are pulled out of the paste and keep transferring stress, so the cracked section still carries a residual load. That residual load, not a higher cracking strength, is the property a fiber dose buys.

Crack bridging and pull-out after the matrix cracks#

Deformed steel fibers, hooked-end, crimped or flat-end, bridge cracks and pull out of the paste rather than rupture, and their anchorage gives the cracked section post-crack flexural and tensile capacity. At 20 to 40 kg/m³ (33.7 to 67.4 lb/yd³) they add toughness, not peak strength (Sika Fiber Reinforced Concrete Handbook).

Macro-synthetic fibers work by frictional pull-out. The Concrete Society notes that they carry relatively more at large crack openings, while steel is stronger at small openings, which matters when a serviceability crack width governs. Polypropylene microfibers work earlier: they raise the early-age tensile strain capacity of the fresh paste and reduce settlement and bleeding, with little or no post-crack capacity. Which fiber a slab needs therefore depends on the crack width it must control, and the full trade-off is set out under steel vs synthetic fibers.

Fiber geometry: aspect ratio, length and deformation#

An aspect ratio, length divided by equivalent diameter, of 40 to 80 performs well, and the risk of fiber balling rises above about 100 (Sika Fiber Reinforced Concrete Handbook).

Deformation sets the anchorage. Steel fibers are hooked-end, crimped or flat-end; macro-synthetic fibers are embossed, undulated or twisted to grip the paste. ACI 318-19(22) recognizes only deformed steel fibers with a length-to-diameter ratio of 50 to 100 for shear credit, a narrower band than the general mixing guidance. Steel fiber balls of 100 to 150 mm (3.9 to 5.9 in) form from a high aspect ratio, fast addition or an empty drum, and a uniformly mixed batch free of fiber balls is an ASTM C1116/C1116M-23 requirement.

Types of Fibers for Concrete by Material and Standard Class#

The fiber families used in concrete are steel, polypropylene microfibers, macro-synthetic polyolefin, AR glass, basalt, PVA and natural cellulose. ASTM C1116/C1116M-23 sorts the concrete they make into 4 types: Type I steel, Type II glass, Type III synthetic and Type IV natural.

Steel fibers are cold-drawn wire, cut sheet, melt-extract or mill-cut, and they carry post-crack load. Polypropylene microfibers fight early-age cracking and, in fire, explosive spalling. Macro-synthetic polyolefin fibers carry post-crack load without corroding. AR glass contains at least 16 % ZrO2 by mass of glass, determined by XRF (EAD 260002-00-0301), and reinforces thin GFRC sections; E-glass is not alkali resistant. Basalt is a melt-spun rock filament specified by performance. PVA fibers enable strain-hardening composites: in a study (Li 2007), ECC-M45 with 2 % by volume PVA fiber reached a tensile strain of up to 5 %. Natural fibers such as cellulose, sisal, jute and coir serve non-structural crack control.

The table classifies each fiber family by material, size class and standard.

EN 14889 covers steel and polymer fibers only. AR glass reaches the European market through EN 15422:2008, which specifies glassfibers for mortars and concretes, or through EAD 260002-00-0301 (March 2016, OJEU 2017) and a European Technical Assessment. Natural fibers carry a known limitation: the durability of unprotected plant fibers in alkaline paste (ACI PRC-544.1R-96).

Basalt and carbon outside the product standards#

Basalt fits no ASTM C1116 type and sits outside EN 14889-1 and -2:2006, so it reaches CE marking only through an EAD and ETA and is specified by EN 14651 or ASTM C1609 performance.

Evidence on basalt durability comes from studies, not from a product standard. In a study by Garcia-Mayo and co-authors (ECCM21, 2024), basalt fiber kept its tensile strength after 72 h and 168 h in 1 M NaOH at room temperature but weakened after 30 minutes of boiling. Carbon fiber appears in concrete literature for its stiffness: the Sika Fiber Reinforced Concrete Handbook lists literature values of 300 GPa (43.5 million psi) modulus, 4,000 to 4,500 MPa (580,200 to 652,700 psi) tensile strength and a density of 1.7 g/cm³. No C1116 type, EN 14889 class or dose range covers it.

What Do Fibers Change in Fresh and Hardened Concrete?#

Fibers lower slump in fresh concrete and add post-crack residual strength in hardened concrete. Steel fibers at 20 to 40 kg/m³ (33.7 to 67.4 lb/yd³) raise toughness rather than peak flexural or compressive strength (Sika Fiber Reinforced Concrete Handbook), so the stiffness and strength of the fiber itself explain how each family behaves.

The table compares the modulus, tensile strength and density of the main fiber materials.

Workability with fibers in the batch#

Fibers reduce workability, so slump is restored with a water reducer or high-range water reducer and fibers are added at a set point, never by adding water (ACI FAQ 733).

The slump loss grows with dose and with aspect ratio, so a long, thin steel fiber at a high dose needs more admixture than a short microfiber at 0.6 kg/m³. The dosage and types of plasticizers are covered under water-reducing admixtures.

Residual flexural strength instead of peak strength#

Residual flexural strength, the stress a cracked beam still carries at a set crack opening, is the property fibers are specified by. EN 14651:2005+A1:2007 tests a notched beam of 150 x 150 x 550 mm (5.9 x 5.9 x 21.7 in) on a 500 mm (19.7 in) span and reports residual strengths fR1 to fR4 at crack mouth opening displacements (CMOD) of 0.5, 1.5, 2.5 and 3.5 mm. EN 14889 uses the same beam for its structural declaration: the declared dose must give at least 1.5 MPa (218 psi) at a CMOD of 0.5 mm (0.020 in) and 1.0 MPa (145 psi) at 3.5 mm (0.138 in) in the EN 14845-1 reference concrete.

ASTM C1609/C1609M-24 is the US beam: an unnotched third-point test that reports residual strengths at deflections of L/600 and L/150. FRCA Fibers in Practice 8 relates the two worlds: tensile residual strength at the ultimate limit state is about 0.37 times the flexural residual strength. The same publication gives an example specification of a minimum residual strength of 1.4 MPa (200 psi) in 28 MPa (4,000 psi) concrete.

Early-age cracking and fire spalling#

About 0.1 % by volume of synthetic microfiber reduces plastic shrinkage cracking (ACI FAQ 733), and ASTM C1579-21 proves the effect on restrained panels. That test compares fiber and control panels and requires the evaporation rate to produce an average crack width of at least 0.5 mm (0.020 in) in the controls. The causes of plastic cracking and the mix measures against it are covered under plastic shrinkage cracking.

Fire is the second early-age-type job for microfibers. EN 1992-1-2:2023 (Table 10.1, clause 10) requires a specific spalling assessment or polypropylene fibers when fck is 70 MPa (10,150 psi) or more, or when silica fume is 6 % or more by weight of cement. Its recommended kpp is 2.0 kg/m³ (3.4 lb/yd³) of monofilament polypropylene fiber of 50 µm (0.002 in) or less, unless the National Annex differs. In fire, the monofilaments melt and leave channels that relieve vapor pressure in the dense paste.

Fiber Dosage by Type: Ranges and Basis#

Fiber dosage is stated in kg per m³ of concrete or as a percentage by volume, from below 1.2 kg/m³ (2.0 lb/yd³) for microfibers to 9 to 60 kg/m³ (15 to 100 lb/yd³) for steel (FRCA Fibers in Practice 9). Every range below keeps its basis, because a volume fraction and a mass per cubic meter give very different numbers for steel and polymer.

GFRC is the exception to both bases: its AR glass content is stated by mass of total GRC, and EAD 260002-00-0301 assesses bending strength at 3 % (premix) and 5 % (spray-up), which are assessment contents, not design doses. The design dose of any structural fiber comes from residual-strength testing of the project mix, never from the EN 14889 declared dose, which proves a fiber in a reference concrete only. The mix as a whole is then proportioned by the methods of concrete mix design.

The table lists dose ranges by fiber with their basis and value type.

1 kg/m³ = 1.6856 lb/yd³. The project dose is set by residual-strength testing.

Converting a volume fraction to kg/m³#

A volume fraction converts to kg/m³ by multiplying it by the fiber density in kg/m³. The worked example below, calculated from the ACI FAQ 733 volume levels and the handbook densities of 7.8 and 0.91 g/cm³, uses both fibers.

• Steel at 0.3 % by volume: 0.003 × 7,800 kg/m³ = 23.4 kg/m³ (39.4 lb/yd³), inside the 20 to 40 kg/m³ handbook range.
• Polypropylene at 0.1 % by volume: 0.001 × 910 kg/m³ = 0.91 kg/m³ (1.53 lb/yd³), just above the 0.6 to 0.9 kg/m³ handbook range and below the 1.2 kg/m³ FRCA ceiling.

The same volume fraction thus gives about 26 times more mass of steel than of polypropylene, which is why a dose written as a bare percentage cannot be batched.

Batching fibers at the plant and in the truck#

Fibers are added at batching, at the plant or into the truck mixer (FRCA Fibers in Practice 9), in packaging matched to the fiber. Steel fibers come loose or glued in 20 kg (44 lb) sacks or in bulk and go in by belt or blower. Polypropylene microfibers come in degradable paper bags that are added whole, and macro-synthetic fibers in bundles or pucks wrapped in water-soluble film.

Balling is the batching defect to prevent. Steel fiber balls of 100 to 150 mm (3.9 to 5.9 in) form when the aspect ratio is high, when fibers are added too fast or when they go into an empty drum, so the causes point to the controls: a moderate aspect ratio, steady addition and a drum that is never empty. Macro-synthetic pucks are light and cause less pump wear than steel (Sika Fiber Reinforced Concrete Handbook). The acceptance target stays the ASTM C1116/C1116M-23 condition: uniformly mixed and free of fiber balls.

Fibers sticking out of the finished surface#

Microfibers can show on power-troweled surfaces, and exposed steel fiber ends rust, an aesthetic effect rather than a structural one. Surface fibers depend on the fiber and the finish together, so a specification for an exposed floor or wall states the fiber type, the dose and the finishing method in the same document. Where rust spots from steel ends are unacceptable, a synthetic fiber is the specification decision, made before the order.

Selecting Fibers by Application and Climate#

Fiber choice follows the job: microfibers for early-age cracking, steel or macro-synthetic fibers where cracked concrete must carry load, and AR glass for thin GFRC sections of 10 to 20 mm (0.39 to 0.79 in). UHPC adds a fourth rule: PCI-UHPC (2021) defines it with at least 2 % by volume of high-strength, high-aspect-ratio fibers.

The table matches applications to fiber roles and the governing figures.

Structural credit in ACI 318 and Eurocode 2#

Two code families give fibers structural credit: ACI 318-19(22) lets deformed steel fibers replace minimum shear reinforcement in some beams, and EN 1992-1-1:2023 Annex L designs steel fiber concrete by residual-strength class. ACI 318-19(22) Table 9.6.3.1, with 26.4.1.5.1 and 26.4.2.2(i), sets the conditions: steel fibers to ASTM A820 with a length-to-diameter ratio of 50 to 100, at least 59.3 kg/m³ (100 lb/yd³), in normalweight beams with h up to 600 mm (24 in) and f'c up to 41 MPa (6,000 psi). Macro-synthetic fibers get no such credit. ACI CODE-318-25, published in 2025, is the current ACI building code; its fiber clauses are not reproduced here because their text was not read.

EN 1992-1-1:2023 Annex L classifies steel fiber concrete by a strength class SC from 1.0 to 8.0 (fR,1k in MPa) with a ductility letter a to e. Its minimum classes are 1b for foundations on ground, 2c on piles and 4c for tunnel segments without bars (Hormigón y Acero, 2023). Neither code lets fibers replace the bars of girders, columns, suspended decks or cantilevers.

The table summarizes the code rules for fibers with edition and clause.

Fiber choice for hot, marine and fire exposure#

Exposure narrows the fiber choice: microfibers for hot-weather placement, synthetic fibers for marine and electrified rail work, and polypropylene monofilament for fire. In hot-weather work, the early risk the fiber answers is plastic shrinkage cracking, where about 0.1 % by volume of synthetic fiber applies (ACI FAQ 733).

Marine and coastal work favors synthetic reinforcement because macro-synthetic fibers do not corrode or conduct, which also suits tram and light rail slabs (Concrete Society). For fire, the EN 1992-1-2:2023 trigger at fck 70 MPa (10,150 psi) or 6 % silica fume by weight of cement turns a high-strength mix into a fiber mix by rule, with 2.0 kg/m³ (3.4 lb/yd³) of monofilament polypropylene as the recommended value.

Which Standards Govern Fibers and Fiber-Reinforced Concrete?#

Fibers are governed at three levels: the fiber itself, the fiber-reinforced concrete, and the test that proves residual strength. The fiber level is ASTM A820, D7508, C1666 and D7357 in the US, EN 14889-1 and -2 in Europe and IS 16481 in India; the concrete level is ASTM C1116/C1116M-23 and EN 206-1:2026.

The table sets out the standards landscape by region with edition and status.

Design guidance sits outside these product standards. The Sika Fiber Reinforced Concrete Handbook summarizes that no single harmonized global FRC design standard exists, and designers draw on fib Model Code 2010 section 5.6, EN 1992-1-1:2023 Annex L for steel fibers, RILEM TC 162-TDF, ACI PRC-544.4-18, Concrete Society TR34 4th edition (2013) and ITAtech report 7 (2016).

Product standards in the US, Europe and India#

The US splits fiber standards by material, Europe by steel and polymer, and India covers synthetic microfibers. ASTM A820/A820M-22 covers steel fibers in Types I cold-drawn wire, II cut sheet, III melt-extracted, IV mill cut and V modified cold-drawn wire. ASTM D7508/D7508M-20 (last updated 2020-09-16) covers polyolefin chopped strands for concrete, with conformance on denier, finish content, tensile strength and cut length. ASTM C1666/C1666M-08(2023) covers AR glass fiber for GFRC and FRC and specifies a minimum zirconia content, and ASTM D7357-07(2026) covers cellulose fibers for fiber-reinforced concrete.

In Europe, EN 14889-1:2006 covers steel fibers in Groups I to V (cold-drawn wire, cut sheet, melt extract, shaved cold-drawn wire, milled from blocks) and was confirmed at CEN review on 2022-06-16; EN 14889-2:2006 covers polymer fibers. Attestation follows Delegated Regulation (EU) 2026/1310 (OJ 2026-08-28, in force 2026-09-17), Annex III: fibers for structural concrete AVS 1 (row 180), for non-structural use AVS 3 (row 181), while Regulation (EU) 2024/3110 applies from 2026-01-08. In India, IS 16481:2022 (first revision, published 2022-06-30) covers synthetic micro fibers for cement-based matrices, the ISI mark is compulsory under Quality Control Order S.O. 1627(E) of 2021-04-15, and IS 16481:2016 was withdrawn on 2023-09-05. IS 15607 is the BIS biodiesel (B100) specification, not a fiber-reinforced concrete code.

Residual-strength test methods#

Three tests prove fiber performance: the ASTM C1609/C1609M-24 third-point beam, the EN 14651:2005+A1:2007 notched beam and, for sprayed linings, the ASTM C1550-26 round panel. C1609 uses beams of 100 x 100 x 350 mm (3.9 x 3.9 x 13.8 in) on a 300 mm (11.8 in) span or 150 x 150 x 500 mm (5.9 x 5.9 x 19.7 in) on 450 mm (17.7 in). C1550-26 loads an 800 mm (31.5 in) panel, 75 mm (2.95 in) thick, at its center and records energy to 5 mm (0.20 in) and 40 mm (1.57 in) deflection.

ASTM C1399/C1399M-10(2015), the average residual strength test, was withdrawn in 2024 with no replacement, and ASTM C1018-97 was withdrawn in 2006, yet product data sheets still quote C1399 values. Sprayed concrete adds EFNARC energy classes E500, E700 and E1000: 500, 700 and 1,000 J at 25 mm (0.98 in) deflection on the EN 14488-5 panel. The Sika handbook gives an indicative E700 dose of 25 to 35 kg/m³ (42.1 to 59.0 lb/yd³) of steel or 5 to 6 kg/m³ (8.4 to 10.1 lb/yd³) of macro-synthetic fiber.

ACI 544 reports and other design guidance#

ACI Committee 544 reports and guides cover fiber-reinforced concrete from materials to design, from PRC-544.1R-96 to PRC-544.11-22. The seven documents below carry their reapproval status where the ACI store shows one.

• ACI PRC-544.1R-96, Report on Fiber Reinforced Concrete (reapproved 2023)
• ACI PRC-544.3-08, Guide for Specifying, Proportioning, and Production of FRC (reapproved 2023)
• ACI PRC-544.4-18, Guide to Design with FRC
• ACI PRC-544.5-10, Report on the Physical Properties and Durability of FRC (reapproved 2023)
• ACI PRC-544.7-16, Report on Design and Construction of Fiber-Reinforced Precast Concrete Tunnel Segments
• ACI PRC-544.8-16, Report on Indirect Method to Obtain Stress-Strain Response of FRC
• ACI PRC-544.11-22, covering shrinkage, creep and transport

Complete List of Fiber Types for Concrete#

Each fiber type for concrete has its own page: steel, polypropylene microfibers, macro-synthetic, AR glass (GFRC), basalt, PVA and natural fibers. The table gives each with its ASTM C1116 type and role.

Fiber quotes compare only on equal terms: the C1116 type or EN 14889 group or class claimed, the declared dose and its declaration of performance, the residual-strength report on C1609 or EN 14651 and the safety data sheet. Quotes depend on fiber material and coating, the dose per m³ set by residual-strength design, minimum order and freight. For steel, one dated public record is available: hooked-end steel fiber marketplace listings, FOB China, October 2026, ran from US$550 to 1,000 per tonne, a listing range and not a market index.

When Is Fiber-Reinforced Concrete the Wrong Choice?#

Fiber-reinforced concrete is the wrong choice where fibers would replace the structural bars of girders, columns, suspended decks or cantilevers (FRCA Fibers in Practice 9). Microfibers are also wrong wherever post-crack capacity is needed, because they have little or none. Macro fibers do replace welded wire reinforcement or temperature steel in slabs-on-ground by equivalent residual strength (ACI PRC-544.4-18; FRCA Fibers in Practice 8), and the full comparison is under fiber vs wire mesh and rebar in slabs.

Fiber-reinforced polymer bars and externally bonded FRP strengthening are construction products and are not covered here. Fibers used in repair mortars, screeds and renders belong to construction-chemical products and are outside this page.

What are the downsides of fiber-reinforced concrete?#

Fiber-reinforced concrete has four downsides: slump loss, balling, fibers at the surface and no replacement of required bars. Slump loss grows with dose and aspect ratio and is answered with a water reducer, never water (ACI FAQ 733). Balling starts in the drum when fibers go in too fast. Microfibers can show on power-troweled surfaces and steel ends rust. Bars that a structural code requires stay in the design (NYC Building Code 2022 1905.6.6).

Can you add fiber to concrete?#

Yes: fibers are added at batching, at the plant or into the truck, and slump is restored with a water reducer, never with added water. ASTM C1116/C1116M-23 then asks for concrete delivered uniformly mixed and free of fiber balls, so the order states the fiber, the dose with its basis and the target slump together.

Which safety data applies to concrete fibers?#

Steel fibers are articles with no CLP classification as supplied (ECHA C&L steel entry: not classified), and their hazard is mechanical, from sharp ends. For polymer, basalt, natural and AR glass fibers, see the product safety data sheet. For fibrous glass dust, the NIOSH Pocket Guide lists a NIOSH REL of TWA 3 fibers/cm³ and 5 mg/m³ total, and an OSHA PEL of TWA 15 mg/m³ total and 5 mg/m³ respirable.