What Are Macro Synthetic Fibers? A Complete Engineering Guide

Suitable for architectural engineers, structural engineers, flooring contractors, precast component manufacturers, and technicians.

Macro synthetic fibers have become a major component of modern fiber-reinforced concrete (FRC). Once considered an alternative to steel fibers or welded wire mesh (WWM), they are now widely used as primary reinforcement in slabs, pavements, tunnel shotcrete, precast elements, and industrial floors.

This guide explains the engineering fundamentals of macro synthetic fibers, how they work inside concrete, and where they deliver the most value.

Definition and Classification of Macro Synthetic Fibers

macro synthetic fibers

Macro synthetic fibers are high-strength polymer fibers—typically polypropylene (PP)—designed to provide structural post-crack reinforcement in concrete. They are larger, stronger, and stiffer than microfibers and are tested according to standards such as:

• ASTM C1116 – Specification for Fiber-Reinforced Concrete

• ASTM D7508 – Specification for Macro Synthetic Fibers

• EN 14889-2 – Polymer fibers for concrete

Macro fibers are included in FRC design codes such as ACI 544, ACI 360 and EFNARC shotcrete guidelines.

Macro vs Micro Synthetic Fibers

	Micro Synthetic Fibers	Macro Synthetic Fibers
Primary Role	Plastic shrinkage control	Structural post-crack reinforcement
Typical Length	3–12 mm	40–60 mm
Diameter	10–50 μm	>300 μm
Effect on Slab Capacity	Minimal	Significant
Can Replace Steel Reinforcement?	No	Yes, in many applications

Microfibers control early-age cracking, whereas macro fibers provide load-carrying capacity after cracking.

What Qualifies as “Macro”?

A fiber is considered macro synthetic if:

• Length: generally > 40 mm

• Diameter: > 0.30 mm

• Aspect Ratio (L/D): typically 40–120

• Residual Strength Contribution: measurable in ASTM C1609 / EN 14651 tests

HTM® Twist and HTM® Emb fibers meet these criteria and are classified as structural-grade macro fibers.

How Macro Synthetic Fibers Work in Concrete

macro synthetic fibers

When concrete cracks, macro fibers restrict crack widening and transfer load across the crack’s faces. This mechanism replaces or supplements conventional steel reinforcement.

Post-Crack Load Transfer

After cracking, macro fibers engage by resisting pull-out, providing:

• residual flexural strength (fR values)

• ductility and toughness

• impact and fatigue resistance

• improved joint spacing performance

The fiber-matrix bond and profile geometry directly influence residual load-carrying capacity.

Bridging Mechanism

The bridging action occurs when fibers:

1. Span across cracks

2. Develop frictional or mechanical anchorage

3. Restrict crack width

4. Maintain load transfer under repeated or dynamic loading

Different fiber designs provide different bridging characteristics:

• Twisted fibers (e.g., HTM® Twist): torsional interlock, high toughness

• Embossed fibers (e.g., HTM® Emb): strong early anchorage, precise crack control

• PP-PE fibers (e.g., HTM® Mono): replace steel fibers, welded wire mesh and conventional reinforcing bars in a wide variety of applications

These mechanisms explain why macro synthetic fibers are effective in flooring, pavements, precast, and shotcrete.

Benefits Compared With Traditional Reinforcement

Corrosion Resistance

Unlike steel mesh or steel fibers, PP macro fibers will not rust, making them ideal for:

• coastal or marine environments

• de-icing salt exposure

• water containment structures

• underground and tunnel shotcrete

Their corrosion-free nature significantly improves long-term durability.

Improved Toughness and Ductility

Macro synthetic fibers increase:

• residual strength (ASTM C1609)

• impact resistance

• abrasion resistance

• fatigue life under heavy wheel loads

In industrial slabs, macro fibers can reduce or eliminate rebar mesh by providing equivalent post-crack performance.

HTM® Twist, for example, is engineered for high energy absorption in tunnel shotcrete and heavy-duty flooring.

Applications of Macro Synthetic Fibers

Macro fibers are suitable for structural and non-structural FRC applications where post-crack performance is required.

Industrial Floors

Applications include:

• warehouses

• distribution centers

• cold storage floors

• logistics facilities

Benefits:

• reduced curling

• extended joint spacing

• better crack-width control

• improved performance under forklift traffic

Tunnel Shotcrete

Used in:

• NATM tunnels

• mining

• slope stabilization

• underground caverns

Advantages:

• high toughness and energy absorption

• improved safety via ductile failure mode

• easier handling than steel fibers

Twisted fibers like HTM® Twist are preferred in shotcrete due to their superior pull-out mechanics.

Precast Elements

Macro fibers reinforce:

• tanks

• pipes

• panels

• vaults

• barriers

Benefits:

• better crack resistance during demolding

• improved transport durability

• no corrosion risk

• simplified production since no steel cage is required in certain elements

Conclusion

Macro synthetic fibers provide structural post-crack performance, improve toughness, and replace steel reinforcement in many applications. With advanced anchorage designs—such as those used in HTM® Twist and HTM® Emb—modern FRC delivers longer service life, greater durability, and improved constructability.