How Polypropylene Fibers Prevent Plastic Shrinkage Cracks in Concrete

Understanding the Mechanisms, Performance Data, and Best Practices

Plastic shrinkage cracking remains one of the most common and costly early-age defects in concrete. It develops within the first few hours after placement—long before concrete gains any measurable tensile strength—and is often invisible until the slab begins drying and service begins.

In this article, we explain why plastic shrinkage cracks occur, how polypropylene microfibers (PP fibers) mitigate them, recommended dosage levels, and technical findings from real project data.

What Causes Plastic Shrinkage Cracking?

Plastic shrinkage cracks occur during the first 2–6 hours after placement, when concrete is still in the plastic (fresh) state. At this stage, the material has little tensile capacity. Any restraint against shrinkage—such as subgrade friction, reinforcement, or forms—creates tensile stresses that exceed capacity and generate cracks.

Sources of Moisture Loss

Concrete loses moisture faster than it can bleed upward. The primary drivers include:

High evaporation rates caused by

• hot weather

• low humidity

• strong winds

• direct sunlight

Insufficient bleeding rate due to

• high cement content

• fine aggregates

• low water–cement ratio

• use of supplementary cementitious materials

• Large surface area exposure, especially in slabs, pavements, and overlays

When surface evaporation > bleed water rate, negative capillary pressures develop and cause shrinkage. Without reinforcement capable of stabilizing the fresh matrix, cracks form.

Why Slabs and Pavements Are Vulnerable

Slab-on-ground applications are particularly susceptible because:

• Large surface areas accelerate water loss

• Thin sections (75–150 mm) dry faster

• Solar exposure increases surface temperature differential

• Settlement and restraint from subbase friction magnify cracking

This makes slabs, pavements, bridge decks, and industrial floors prime areas where polypropylene microfibers deliver significant performance benefits.

How polypropylene fibers Reduce Plastic Shrinkage

Polypropylene microfibers—typically 6–19 mm monofilament fibers—are highly effective in controlling plastic shrinkage cracks. The mechanism is both physical and mechanical.

Mechanism of Micro-Crack Bridging

PP microfibers distribute uniformly within the concrete matrix, forming a 3D reinforcement network. During the plastic stage:

• Fibers intercept and bridge micro-cracks at the earliest stage

• They help dissipate tensile stresses within the fresh matrix

• They reduce crack width and prevent crack propagation

• Surface shrinkage is more evenly distributed and controlled

• Because the fibers are extremely fine (diameter ≈ 30 microns), they interact with micro-cracks before they become visible.

  
  

Key performance effects:

• Reduces plastic shrinkage crack area by 70–90%

• Improves cohesion and reduces segregation

• Controls initial cracking under hot, dry, or windy conditions

Controlling Settlement Cracking

Settlement cracking occurs when concrete settles around rebar, conduits, or changes in section depth. PP fibers reduce settlement cracking by:

• Increasing cohesion of fresh concrete

• Reducing localized bleeding around embedded items

• Supporting the matrix during initial settlement

This is particularly beneficial in slabs containing dense reinforcement, embedded piping, or precast molds with complex geometry.

Performance Data – What Projects Show

Across international standards, laboratory studies, and field projects, PP microfibers consistently demonstrate quantifiable reductions in plastic shrinkage cracking.

ASTM C1579 Test Results

1. Typical results for PP microfibers at 0.9 kg/m³ show:

2. Crack area reduced by 80–90%

3. Maximum crack width reduced by 60–75%

   
   

Field Performance: Slabs-on-Ground

1. Industrial floor contractors report:

2. Dramatic reduction in random cracking

3. More uniform surface hardness

4. Improved finishing performance due to better cohesion

   
   

Shotcrete & Tunnel Applications

1. Tunnels and mining shotcrete with PP fibers show:

2. Reduced early-age thermal cracking

3. Lower rebound and better adhesion

4. Improved fire spalling resistance

   
   

Precast & Mortar Applications

1. Manufacturers note:

2. Enhanced surface integrity

3. Cleaner edges and fewer voids

4. Lower defect rates

Together, the data confirms that PP microfibers are one of the most reliable and cost-effective methods of reducing plastic shrinkage and early-age cracking.

Recommended Dosage for Shrinkage Control

Concrete Slabs & Pavements

• 0.6–1.2 kg/m³

• Use 1.0–1.2 kg/m³ for hot weather or exterior slabs with high evaporation

  
  

Industrial Floors

• 0.9–1.2 kg/m³ depending on slab thickness and placement conditions

  
  

Mortar & Plaster

• 0.3–0.6 kg/m³

  
  

Shotcrete

• 1.0–1.8 kg/m³ for tunneling and overhead applications

  
  

Finishing Compatibility

PP fibers are designed to remain invisible after finishing, making them suitable for:

• polished concrete

• stamped concrete

• architectural surfaces

PP Fiber vs. No Fiber – Real Comparisons

Without PP Microfibers

High crack area under hot/windy conditions

Uneven surface finish

Settlement cracks around rebar & inserts

Increased permeability and long-term durability issues

Greater risk of delamination in overlays

With PP Microfibers

Crack area reduced up to 90%

Surface more uniform and easier to finish

Better resistance to impact and abrasion

Improved service life and reduced maintenance cost

More predictable performance in adverse weather

Conclusion & Best Practices

Polypropylene microfibers are one of the most effective tools for controlling plastic shrinkage and settlement cracking in concrete. Their ability to form a three-dimensional reinforcement network prevents early-age crack development, improves cohesion, and enhances surface durability.

Best Practices for Maximum Performance

• Choose monofilament PP fibers for plastic shrinkage control

• Use 0.9–1.2 kg/m³ for slabs exposed to wind, sun, or high temperature

• Monitor evaporation rate; follow ACI hot-weather concreting procedures

• Extend mixing time by 30–60 seconds to ensure full dispersion

• Combine with macro fibers if post-crack load-carrying capacity is required

• PP microfibers remain a simple, economical, and highly reliable solution to one of concrete’s most persistent problems.

Learn more about HPM® PP polypropylene microfiber 

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