Fiberreinforced Concrete Uses Benefits and Key Questions

Concrete serves as the foundation of modern construction, with continuous improvements in its performance being a primary focus in engineering. The emergence of fiber-reinforced concrete (FRC) has brought new vitality to this essential material. However, it also raises numerous questions: What exactly are fibers? What distinguishes macrofibers from microfibers? How should the appropriate fiber type and dosage be selected? This article provides an in-depth exploration of fiber-reinforced concrete, addressing practical application concerns and offering guidance for constructing more robust and durable buildings.

Fibers play a crucial role in concrete, functioning as reinforcement materials rather than mere additives. As defined in ACI CT-18 (Concrete Terminology), fibers bridge cracks, binding concrete together and significantly enhancing its overall performance.

The primary distinction between macrofibers and microfibers lies in their size, typically measured by equivalent diameter or linear density. In the FRC industry, linear density is often expressed in denier, representing the mass (in grams) of 9,000 meters of fiber.

Key differences between microfibers and macrofibers include:

• Synthetic fibers: ASTM D7508/D7508M specifies requirements for macrofibers, microfibers, and hybrid fibers (combinations of both). This standard also uses tensile strength and cut length as additional distinguishing properties.
• Natural microfibers: ASTM D7357 outlines compliance requirements for cellulose fibers used in FRC.
• Steel fibers: Classified as macrofibers, ASTM A820/A820M establishes minimum requirements for steel fibers in FRC applications.

• Microfibers: Primarily control plastic shrinkage cracking. At typical dosages, they do not provide temperature or shrinkage crack control, nor do they enhance other concrete properties. However, fibrillated microfibers used at minimum dosages of 1.5 lb/yd³ (0.9 kg/m³) can replace lightweight welded wire reinforcement (WWR) for temperature and shrinkage crack control.
• Macrofibers: Significantly improve concrete performance, including post-crack residual strength, load transfer across cracks, enhanced durability, flexural toughness, fatigue resistance, impact resistance, and shear capacity.

Various synthetic macrofiber types exist, each with unique advantages. Beyond physical properties like tensile strength, elastic modulus, material composition, or shape, less obvious differences include mixing performance, balling tendency, surface finish quality, and bond characteristics with concrete.

The key to successful synthetic macrofiber use lies in selecting appropriate dosages that meet both performance requirements and workability/ finishing needs. Stronger fibers or those with better bonding characteristics may require less material than weaker alternatives. Manufacturers should provide test data to support dosage recommendations, with trial mixes recommended when uncertainties exist.

Microfibers primarily control plastic shrinkage cracking. Research shows they provide negligible load-bearing capacity in hardened concrete at typical dosages. Higher microfiber dosages can complicate mixing due to increased fiber count and surface area, potentially causing workability issues and significant slump loss.

Compared to shorter fibers of the same type, longer fibers have greater surface area, improving anchorage in hardened concrete and enhancing post-crack performance at equivalent dosages. However, an optimal length exists for each fiber type, depending on specific shape, stiffness characteristics, and concrete compressive strength.

Pre-project trial mixes considering placement methods and finishing requirements help determine ideal fiber lengths, preventing issues like balling or surface fiber protrusion.

Synthetic macrofibers (particularly large, coarse monofilaments) can provide residual strength comparable to steel fibers, depending on dosage. Other factors like anticipated deformation and environmental conditions should also influence fiber selection. For some structural applications requiring steel fibers, synthetic alternatives haven't been validated.

While synthetic macrofibers typically have lower tensile strength and elastic modulus than steel fibers, they provide significantly more fibers to bridge potential cracks. When properly dosed, total stress transfer capacity across cracks should be equivalent. Effective sectional strength also depends on fiber-matrix bond quality—high-strength fibers with insufficient bonding make poor reinforcement candidates. FRC performance reflects composite behavior, not individual fiber properties.

Fiber-reinforced concrete (FRC) incorporates discrete fibers (typically up to 2.5 inches or 64 mm) in hydraulic cement concrete. Fiber-reinforced polymers (FRP) use substantially longer fibers embedded in polymer matrices without cement or aggregates.

Required fiber quantities depend on specified FRC performance criteria. For microfibers controlling plastic shrinkage, crack reduction ratios (CRR) guide dosage determination per ASTM C1579. For macrofibers in hardened concrete, dosages should meet specified residual strength (ASTM C1399), post-crack equivalent flexural strength (ASTM C1609), or energy absorption capacity (ASTM C1550), considering element type/thickness, concrete strength, reinforcement specifications, and loading requirements.

Many suppliers provide design tools (particularly for slabs) to calculate appropriate dosages.

For slabs, minimum dosages are typically established by manufacturers based on standardized product testing to meet requirements or industry standards.

ANSI/SDI C-2017 for composite metal decks specifies minimum macrofiber dosages of 4.0 lb/yd³ (2.4 kg/m³) for synthetic fibers and 25.0 lb/yd³ (14.8 kg/m³) for steel fibers when used for temperature/shrinkage control. Under UL requirements, upper limits are 5.0 lb/yd³ (3.0 kg/m³) for synthetic and 66.0 lb/yd³ (39.2 kg/m³) for steel fibers. Fiber reinforcement currently doesn't replace negative moment steel in composite deck assemblies.

Dosages below manufacturer recommendations or code requirements aren't advised. Engineers should consult fiber manufacturers when uncertain about applications or dosages.

Several sources provide design guidance for macrofiber-reinforced concrete in various structural applications:

• ACI 544.4R-18: Guide to Design with FRC
• ACI 360R-10: Guide to Slab Design
• ACI 322-14: Residential Concrete Code Requirements

Fiber manufacturers can offer additional product-specific guidance.

Macrofiber FRC specifications should be performance-based and application-specific, using approved methods from ACI 544.4R to calculate characteristic parameters such as:

• Average residual strength (ASTM C1399)
• Residual strength or equivalent flexural strength (ASTM C1609)

Sample specification: "Fiber dosage shall provide minimum 200 psi (1.4 MPa) residual strength in 4,000 psi (28 MPa) concrete." Compliance should be verified through manufacturer test data per relevant ASTM standards.

For shotcrete, fiber-reinforced shotcrete (FRS) performance is determined by energy absorption capacity (ASTM C1550 or EN 14488-5). Sample specification: "Fiber dosage shall provide minimum 280 J energy absorption at 7 days in 4,000 psi (28 MPa) shotcrete."

FRC's post-crack performance can be evaluated via ASTM C1609, C1399, or C1550 (energy absorption). Engineers should determine appropriate values based on design intent and required performance levels, referencing ACI 544.4R for guidance.

Certain fibers (e.g., steel) near concrete surfaces in exposed environments may corrode. While such localized corrosion doesn't compromise structural integrity, its aesthetic impact should be evaluated beforehand. Synthetic and natural fibers are non-corrosive and chemically inert, unaffected by environmental conditions.

While fibers typically don't increase concrete's first-crack flexural strength (modulus of rupture, ASTM C78), they enhance slab flexural capacity and fatigue strength. Appropriate fiber-specific toughness values enable thinner concrete sections to support design loads.

ANSI/SDI C-2017 permits steel or synthetic macrofibers (at manufacturer-determined dosages meeting minimum requirements) to replace welded wire reinforcement (WWR) for crack control, not structural stress resistance.

Under UL and ICC-ES evaluations, some microfibers are recognized as WWR alternatives in specific fire-rated floor/ceiling assemblies.

This historical issue primarily occurred with synthetic microfibers when improper finishing techniques were used. Modern monofilament microfibers (typically limited to 1.0–1.5 lb/yd³ or 0.6–0.9 kg/m³) and macrofibers minimize this effect. Proper mixing, placement, and finishing produce excellent surfaces. Surface fiber protrusion doesn't compromise slab integrity—rosebud torches can melt exposed fibers if aesthetic concerns arise.

While some synthetics (e.g., nylon) absorb small amounts of batch water, common polypropylene/polyethylene fibers are hydrophobic. Apparent slump reduction at higher dosages results from fibers acting as cohesive agents rather than water absorption.

Adding water reduces concrete strength. When high fiber content affects workability, chemical admixtures—not extra water—should be used.

Ideal addition points vary by fiber shape, stiffness, and dosage—some perform best as first ingredients, others after all materials are loaded. Manufacturers can provide guidance, with pre-project trials recommended to determine optimal addition timing and mixing duration.

For normal concrete, 4–5 minutes of mixing after all ingredients are added is typically recommended for ready-mix operations.

All fiber types may ball due to insufficient mixing, improper sequencing, or addition to overly dry mixes lacking sufficient fines to coat fibers. Pre-project trials help verify mix compatibility with intended fiber type and dosage.

Slump effects depend on:

• Initial mix slump (greater impact at lower starting slumps)
• Fiber count and dosage (higher dosages increase impact)
• Total fiber surface area (larger area increases impact)

Note that slump cone measurements indicate batch consistency, not necessarily actual workability. While visual slump may appear reduced, actual workability might be less affected. Pre-project trials determine whether workability adjustments are needed.

Microfibers may impact slump more than macrofibers at equivalent length-to-diameter ratios and dosages due to higher fiber counts per pound. Generally:

• Synthetic microfibers (1.0–3.0 lb/yd³ or 0.6–1.8 kg/m³): 1–3 inch (25–75 mm) slump loss
• Synthetic macrofibers (3.0–10.0 lb/yd³ or 1.8–6.0 kg/m³) or steel fibers (15–50 lb/yd³ or 9–29.6 kg/m³): 1–5 inch (25–125 mm) slump loss

Plasticizers (mid- or high-range water reducers) should offset slump loss. Mixture adjustments may be necessary at high dosages to ensure adequate paste content. Avoid excess water to prevent strength reduction and segregation.

Macrofibers generally don't adversely affect air content or compressive strength. Perceived changes often stem from overmixing, added water, temperature variations, or incorrect moisture measurement in aggregates. Air fluctuations may also relate to actual slump changes. Certain historical fiber surface treatments could introduce unwanted air, but this is now rare.

Unit weight changes depend on fiber type and mix design adjustments. Synthetic fibers typically don't alter unit weight if air content remains constant. Higher-density steel fibers may increase unit weight depending on volume fraction and mix adjustments.

In normal-weight concrete, properly proportioned and mixed fibers neither float nor sink due to material specific gravity and mixed concrete viscosity. Fibers actually help suspend larger aggregates and prevent segregation.

Generally compatible, but some fiber processing aids or spin finishes might affect other concrete chemicals. Always verify with fiber manufacturers.