How do PP fibers reduce plastic shrinkage cracks?

PP microfibers form a 3D network that bridges microcracks and reduces capillary pressure, lowering plastic shrinkage crack formation by up to 90%.

Can PP fibers prevent settlement cracking?

Yes. PP fibers increase cohesion, reduce bleeding channels, and minimize rebar shadow settlement cracks.

What is the recommended dosage?

0.6–1.2 kg/m³ for general crack control, and up to 1.5 kg/m³ for hot weather or industrial flooring.

Settlement Cracking vs Plastic Shrinkage: How PP Fibers Address Both

Jason
4 days ago
3 min read

Concrete at early age is vulnerable to two major types of cracking: plastic shrinkage and plastic settlement. While they occur for different reasons, both reduce durability and long-term performance. PP fibers（polypropylene fibers for concrete） are among the most effective materials to mitigate both mechanisms.

Polypropylene (PP) microfibers are one of the most effective and widely adopted solutions for mitigating both forms of cracking. This article explains the differences between settlement and plastic shrinkage cracking, how PP fibers work, dosage recommendations, ASTM standards, and best-use scenarios.

Difference between settlement cracking and plastic shrinkage in concrete

Difference Between Settlement & Shrinkage

What Is Plastic Shrinkage?

Plastic shrinkage cracking is caused when evaporation exceeds the bleeding rate, producing rapid moisture loss at the concrete surface. As capillary pressure increases, the concrete surface enters tension before it has developed enough tensile strength, leading to random cracks.

Typical characteristics include:

Irregular, random-oriented cracks
Shallow depth (usually 2–10 mm)
Occurs within 0–4 hours
More common in hot, windy, or low-humidity conditions

This is especially common in concrete slabs, pavements, industrial floors, and shotcrete applications.

What Is Plastic Settlement?

Plastic settlement occurs when solid particles settle downward, but the presence of reinforcement, forms, or geometric changes restrains the settlement. This generates internal stresses and causes cracking directly above these restraints.

Typical features:

Cracks following steel reinforcement patterns (rebar shadow effect)
Cracks over deck corrugations, embedded items, or section transitions
Occurs within 0–2 hours after placement

Key Differences Between the Two

Type	Trigger	Timing	Appearance	Location
Plastic Shrinkage	Rapid evaporation	0–4 hours	Random surface cracks	Surface layer
Settlement Cracking	Particle settlement + restraint	0–2 hours	Cracks over reinforcement or geometry	Surface or near-surface

Micro polypropylene fibers reducing plastic shrinkage cracks

PP Fiber Mechanisms for Each Crack Type

How PP Fibers Reduce Plastic Shrinkage Cracking

PP microfibers disperse throughout the concrete to create a three-dimensional micro-reinforcement network. This network limits crack initiation and crack propagation during early-age shrinkage.

Key mechanisms include:

Microcrack bridging: Fibers bridge early microcracks before they widen
Reduced free strain: Fibers limit deformation from moisture loss
Lower capillary pressure: Slower moisture transport reduces tension

Performance Data

ASTM C1579 (“Plate Test”) has repeatedly demonstrated that adding 0.9 kg/m³ of PP microfiber can reduce plastic shrinkage cracking by 70–90%.

How PP Fibers Reduce Settlement Cracking

Settlement cracking occurs internally when solids descend but reinforcement restrains that movement. PP fibers help by:

Increasing mix cohesion
Reducing bleeding channels
Enhancing particle suspension
Reducing rebar-induced settlement differentials

This makes PP fibers especially effective in slumpier mixes, precast elements, and deck slabs.

Concrete settlement cracking above rebar

Dosage & Timing Considerations

Recommended Dosage Rates

0.6–1.0 kg/m³ — general crack control
1.0–1.5 kg/m³ — hot weather, high evaporation, or windy conditions
≥ 1.2 kg/m³ — industrial floors, pavements, and large surface areas

Timing

Since both settlement and shrinkage cracking occur within the first 1–4 hours, PP fibers must be incorporated before placement and fully dispersed in the mix.

Best Use Cases: Slabs, Mortars, and Plaster

Concrete Slabs & Industrial Floors

Slabs have high surface area and are the most susceptible to rapid moisture loss. PP fibers significantly reduce surface cracking and improve long-term abrasion resistance.

Mortar & Plaster

PP microfibers reduce surface checking, increase cohesion, and minimize sagging or slumping.

Precast Elements

Settlement cracking is especially common in precast forms. PP fibers increase green strength and uniformity during vibration, transport, and demolding.