How Micro Synthetic Fibers Work in Concrete

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

Micro synthetic fibers significantly improve the early-age performance of concrete, particularly by reducing plastic shrinkage cracking and enhancing surface durability. Although they are non-structural fibers, their role during the first few hours after placement is critical. Understanding how micro synthetic fibers work in concrete requires examining the mechanism of crack formation and how fibers interrupt these failures at a microscopic level.

How Micro Synthetic Fibers Work in Concrete — The Mechanism of Crack Formation in Fresh Concrete

Fresh concrete undergoes rapid changes during the first few hours. As water evaporates from the surface, internal tensile stresses develop faster than the concrete can resist them.

Plastic Shrinkage — What Happens in the First 2–6 Hours?

Plastic shrinkage cracking occurs when evaporation rate > bleeding rate.As the surface dries, moisture evaporates, creating:

• Capillary tension

• Negative pore pressure

• Surface settlement

Without reinforcement, micro-cracks begin to form in random directions.

Early-Age Tensile Stress — Why Concrete Fails So Easily

Concrete in its plastic state has almost zero tensile capacity.Any combination of these factors accelerates cracking:

• Wind exposure

• High temperature

• Low humidity

• Rapid finishing operations

• Large slab areas

This is why early-age cracking is one of the most common problems in industrial floors, slabs-on-grade, screeds, and precast elements.

How Micro Synthetic Fibers Work in Concrete — Interrupting Crack Formation

Micro synthetic fibers modify the behavior of fresh concrete through physical interaction, not chemical reaction. They create a 3D reinforcement network capable of resisting capillary pressure and early tensile forces.

Fiber Bridging Effect — Stopping Cracks Before They Form

As soon as micro-cracks begin to open, the fibers:

• Resist crack widening

• Distribute stresses across the matrix

• Prevent crack coalescence

• Interrupt crack propagation

This mechanism is called the bridging effect.

Micro fibers do not eliminate shrinkage, but they significantly reduce the size and visibility of cracks.

Improving Early Tensile Resistance

During the first 30–120 minutes, fibers:

• Provide internal support to the plastic matrix

• Slow down settlement of aggregates

• Increase cohesion

• Reduce bleeding channels

The result is concrete that behaves more uniformly, especially in hot or windy weather.

How Micro Synthetic Fibers Work in Concrete — Performance Improvements Demonstrated by Tests

Laboratory tests consistently show that micro polypropylene fibers improve early-age performance.

ASTM C157 Shrinkage Tests — Reduced Length Change

PP micro fibers reduce early-age shrinkage strain by:

• Interrupting water movement

• Limiting capillary channels

• Stabilizing the fresh matrix

Mixes containing 0.6–1.2 kg/m³ micro fibers show 10–40% reduction in shrinkage deformation.

Bleeding Reduction and Surface Integrity

Fibers reduce the size and continuity of bleeding channels, resulting in:

• Fewer plastic settlement cracks

• Lower water permeability

• Improved abrasion resistance

• Better surface finishability

This makes micro synthetic fibers ideal for industrial flooring, toppings, overlays, and decorative finishes.

How Micro Synthetic Fibers Work in Concrete — Key Factors Affecting Fiber Performance

The performance of a micro synthetic fiber depends on its physical properties and dispersion behavior.

Fiber Length and Diameter

Typical dimensions:

• Diameter: 10–40 microns

• Length: 6 mm, 12 mm, 19 mm

Shorter fibers disperse more evenly, while longer fibers offer better bridging.

Dispersion Quality in the Mix

Good dispersion ensures:

• Consistent crack control

• Uniform mechanical behavior

• Reduced clumping (“balling”)

• Optimal efficiency at low dosages

Self-dispersing PP fibers offer superior uniformity in ready-mix applications.

Material Type (PP / PP Fiber Mesh / PET)

The performance of micro synthetic fibers in concrete depends heavily on the material type. Although all three belong to non-structural synthetic fibers, they behave differently in terms of modulus, dispersion, and crack control efficiency.

Comparison of Micro Synthetic Fiber Materials

Material Type	Characteristics	Typical Use Cases	Notes
PP (Polypropylene) Micro Fiber	Hydrophobic, chemically inert, low density, excellent dispersion	Industrial floors, slabs, precast, shotcrete	The most widely used micro synthetic fiber for plastic shrinkage crack control
PP Fiber Mesh (Fibrillated Micro Mesh Fiber)	Net-like structure, higher surface area, improves bonding & shrinkage resistance	Mortar, plaster, overlays, screeds	Provides better mechanical interlock; ideal for improving toughness in thin-section applications
PET (Polyester) Micro Fiber	Higher elastic modulus, good bonding, better heat resistance	Mortar, plaster, architectural finishes	Offers improved tensile stiffness; often preferred where surface durability is critical

Material choice influences tensile modulus, melting point, and shrinkage control performance.

Learn more about HPM® PP polypropylene microfiber 

See all micro synthetic fiber types

Compare with macro synthetic fibers