How Micro Synthetic Fibers Reduce Plastic Shrinkage Cracks

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

Plastic shrinkage cracks remain one of the most common early-age defects in concrete, especially in slabs-on-ground, pavements, overlays, and industrial floors. Micro synthetic fibers—particularly micro polypropylene fibers—have proven to be one of the most reliable methods to prevent early-age cracking and enhance durability. This article explains how micro fibers work, what happens inside concrete during the first few hours, and how recommended dosages influence performance.

What Happens in the First 6 Hours of Concrete Setting

Plastic shrinkage cracks form before concrete gains measurable tensile strength. The first 6 hours are the most critical, especially under hot, dry, or windy conditions.

Bleeding

After placing concrete, water begins to rise to the surface—a natural process known as bleeding. If the rate of evaporation is higher than the bleeding rate, negative pore pressure forms, pulling the surface apart.

When evaporation exceeds 1.0 kg/m²/hour, the risk of plastic shrinkage cracking increases dramatically according to ACI guidelines.

Surface Tension

As bleed water evaporates, capillary forces build within the concrete matrix. These internal tensile stresses exceed the very low early-age tensile strength of concrete, resulting in large, irregular cracks across the slab surface.

Micro synthetic fibers directly address this mechanism by distributing tensile resistance throughout the matrix.

Micro Synthetic Fibers as Early-Age Tensile Reinforcement

Micro polypropylene fibers act as a three-dimensional micro reinforcement system, significantly increasing early-age tensile capacity and reducing cracking.

Fiber Bridging

Well-dispersed micro fibers create thousands of micro-bridges within the concrete. These bridges limit the development of micro-cracks by providing:

• Early-age tensile resistance

• Control of crack initiation

• Higher resistance against capillary stress collapse

This mechanism makes micro fibers uniquely effective compared to mesh or steel fibers in early-age performance.

Preventing Crack Propagation

Even if micro-cracks start to form, the fibers distribute stresses across a larger area. Rather than allowing cracks to widen, the fibers restrain them at the micro level—preventing visible cracking.

This is why micro polypropylene fibers are essential for:

• Industrial floors

• Pavements

• Precast concrete

• Mortar & plaster

• Shotcrete

• Repair overlays

Micro fibers are non-structural reinforcement but are unmatched in plastic shrinkage control.

Scientific Evidence & ASTM Test Results

Micro synthetic fibers have been extensively validated in both research and field performance.

ASTM C157 – Length Change of Hardened Concrete

Studies show micro fibers reduce drying shrinkage by improving moisture distribution and limiting microcrack formation. Concrete with micro fibers experiences 10–30% lower shrinkage strain compared to control mixes.

ASTM C1579 – Plastic Shrinkage Cracking Test

ASTM C1579 is specifically designed to measure plastic shrinkage performance.

Results from typical micro PP fibers show:

• >80% crack reduction at 0.6 kg/m³

• Up to 95% crack reduction at 1.0 kg/m³

• Significant increase in early tensile strain capacity

• Reduced crack width even under accelerated evaporation conditions

These results confirm micro fibers as a reliable alternative to additional curing measures in challenging environments.

Recommended Dosages for Crack Reduction

0.6 kg/m³ – Standard Crack Control

Best for:

• Residential slabs

• Precast elements

• Trowel-finished floors

• Mortars & screeds

Provides strong plastic shrinkage mitigation while maintaining excellent workability and surface finish.

1.0 kg/m³ – Maximum Crack Reduction

Used for:

• Industrial floors under harsh evaporation

• Pavements in hot/windy climates

• Shotcrete tunnel linings

• High-performance overlays

Provides near-complete elimination of plastic shrinkage cracking.

Learn more about HPM® PP polypropylene microfiber 

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