How Micro Synthetic Fibers Improve Concrete: A Technical Overview

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

Micro synthetic fibers are essential additives used to enhance the early-age and long-term performance of concrete. Unlike macro fibers—designed for structural post-crack reinforcement—micro synthetic fibers primarily target the first hours of concrete life, controlling plastic shrinkage, reducing settlement cracking, and improving durability through crack-width reduction.

This engineering-focused article explains how microfibers function, how they interact with cement paste, and how they should be applied in floors, precast elements, screeds, repair mortars, and shotcrete.

Introduction to Micro Synthetic Fibers

micro synthetic fibers

Micro synthetic fibers are very fine polymer fibers used at low dosages to provide early-age crack control and improve durability. They are typically made from polypropylene (PP) or polyester and are designed to remain uniformly dispersed throughout the concrete matrix.

Types of Microfibers

Common microfiber categories include:

1. PP Monofilament Fibers

• Most widely used

• 6–12 mm length

• Excellent for plastic shrinkage control

• Hydrophobic and chemically stable

2. PP Fibrillated (Mesh-Type) Fibers

• Net-like structure

• Improves paste bonding

• Great for mortar and screeds

3. Polyester Microfibers

• Higher melting point

• Good for high-temperature applications

4. Ultra-Fine Microfibers（< 0.1 mm）

• Designed for specialized repair mortars

• Improve density and segregation resistance

Typical Dosages and Mix Compatibility

Typical recommended microfiber dosages:

Application	Dosage
Plastic shrinkage control	0.6–1.0 kg/m³
Durability enhancement	1.0–1.5 kg/m³
High-performance mortars	1.5–2.0 kg/m³

Micro synthetic fibers are compatible with:

• OPC and blended cements

• SCC, pump mixes, shotcrete

• Mortars, repair materials, UHPC blends

They do not significantly affect slump loss or finishing when mixed correctly.

Key Roles Fibers Play in Early-Age Concrete

Plastic Shrinkage Crack Reduction

Microfibers reduce plastic shrinkage cracking by:

• Restraining movement of bleeding water

• Reducing capillary pressure-driven cracks at the surface

• Providing tensile resistance before concrete sets

This is critical for:

• slabs-on-ground

• large pours exposed to wind

• pavements

• topping slabs

Settlement Crack Mitigation

Fibers help reduce settlement cracking around:

• rebar

• embed plates

• conduits

They hold aggregates in suspension and reduce settlement through improved particle stability.

Bleeding and Segregation Control

Microfibers slow down the mobility of water within fresh concrete, resulting in:

• reduced bleeding

• better cohesion

• minimized aggregate segregation

This is especially useful in SCC and pumped concrete.

Fiber Mechanisms Inside the Cement Matrix

How Microfibers Form a 3D Reinforcing Network

Dispersed microfibers form a uniform, three-dimensional network that:

• increases tensile strain capacity

• distributes shrinkage stresses

• delays crack initiation

The network is active from the moment concrete is placed—unlike mesh, which only engages after cracking.

Fiber–Cement Bonding Performance

Bond is primarily mechanical rather than chemical.Polypropylene microfibers create anchorage through:

• surface embossing

• fibrillation

• micro-roughness

This bond improves resistance to microcrack formation.

Impact on Hydration Heat Distribution

Microfibers help dissipate localized thermal gradients by:

• stabilizing paste during hydration

• reducing early thermal cracking tendency

This is especially beneficial in mass concrete and hot-weather concreting.

Mechanical & Durability Benefits

Toughness Improvement

Although microfibers do not provide structural-level toughness, they significantly increase:

• energy absorption before cracking

• strain tolerance

• surface wear resistance

This results in longer concrete life.

Impact Resistance

Micro synthetic fibers reduce brittle chipping and impact damage on:

• precast elements

• repair mortars

• pavements

• shotcrete linings

Permeability Reduction（Indirect）

Fibers reduce permeability not by blocking pores, but by:

• limiting crack width

• preventing interconnected crack networks

• improving surface integrity

This leads to better freeze–thaw durability and reduced chloride ingress.

Practical Engineering Applications

Floors, Precast, Screeds, Repair, Shotcrete

Micro synthetic fibers are used in:

• slabs and pavements

• precast panels and pipes

• industrial screeds

• thin toppings

• repair mortars

• tunnel and mining shotcrete

When to Choose Micro vs. Macro Fibers

Select microfibers for:

• plastic shrinkage control

• surface durability

• crack inhibition in thin sections

Select macro fibers when:

• post-crack structural reinforcement is required

• replacing mesh or steel fibers

• slabs need increased joint spacing

For many projects, micro + macro (hybrid system) provides the best performance.

Summary & Engineering Recommendations

Micro synthetic fibers significantly improve the early-age and long-term durability of concrete. Engineers should:

• use 0.6–1.5 kg/m³ depending on performance needs

• combine with macro fibers when structural toughness is required

• specify fibers early in mix design for best results

• use certified PP microfibers such as HPM® PP for predictable performance