What causes plastic shrinkage in concrete?

Plastic shrinkage occurs when surface evaporation exceeds the bleeding rate, causing internal tension in fresh concrete before setting.

Why can't steel reinforcement prevent plastic shrinkage cracks?

Steel mesh sits too low in the slab and only becomes effective after concrete hardens, while plastic shrinkage occurs before initial set.

How do micro synthetic fibers reduce early-age cracks?

Micro fibers create distributed reinforcement that reduces capillary tension, controls micro-cracks, and improves surface durability.

Why Concrete Cracks: Understanding Plastic Shrinkage

Jason
1 day ago
3 min read

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

Diagram showing early-age plastic shrinkage cracks in concrete slabs

Concrete cracking at early age remains one of the most common—and most misunderstood—problems in construction. Even when mix design, curing, and finishing are well controlled, many slabs, pavements, and industrial floors still develop early-age surface cracks. The primary cause is plastic shrinkage, a rapid moisture-loss phenomenon that occurs within the first few hours after placement.

Micro synthetic fibers—especially micro polypropylene fibers—are among the most effective solutions for preventing plastic shrinkage cracking. They form a distributed reinforcement network that stabilizes the fresh concrete matrix before initial set, reducing crack width and frequency. Before understanding how fibers work, it is essential to understand what plastic shrinkage is and why traditional reinforcement cannot stop it.

What Is Plastic Shrinkage?

Plastic shrinkage refers to cracking that appears before concrete hardens, typically 30 minutes–6 hours after placement. These cracks form when the evaporation rate of water at the surface exceeds the bleeding rate of the concrete.

When water evaporates faster than it rises to the surface, capillary tension builds inside the matrix, causing the still-plastic concrete to pull apart. This leads to characteristic short, irregular, spider-web patterns across slabs, pavements, and large exposed surfaces.

Evaporation Rate vs Bleeding Rate

Evaporation rate is driven by weather conditions:

High wind speed
Low relative humidity
High air temperature
High concrete temperature

When evaporation exceeds 0.5 kg/m²/hr, the risk of plastic shrinkage cracks increases dramatically. If evaporation reaches 1.0 kg/m²/hr, cracking is almost guaranteed unless protective measures are taken.

Bleeding rate depends on mix design:

Low water-cement ratio
High fines / cementitious materials (slag, silica fume)
Smaller aggregates
SCC and high-performance mixes

These modern mix designs reduce bleeding, making concrete much more vulnerable.

Hot Weather Consideration

In hot weather (≥ 30–35°C), the surface dries extremely fast. Even with curing compounds, cracking may still occur because:

Bleed water evaporates instantly
Wind accelerates surface moisture loss
Low humidity increases vapor pressure

This is why hot-weather concreting requires wind breaks, fogging, curing blankets, or micro synthetic fibers.

Graphic comparing concrete evaporation rate and bleeding rate under weather conditions

Main Causes of Early-Age Cracks

Plastic shrinkage cracking is a weather-driven process, and three key environmental factors accelerate moisture loss.

Wind

Wind increases evaporation exponentially. Even a moderate 10–15 km/h wind can double the evaporation rate.

High Temperature

High air or concrete temperature accelerates:

Water evaporation
Cement hydration
Set time

This combination creates high shrinkage stress before the concrete gains tensile strength.

Low Humidity

Low relative humidity (< 40%) causes rapid capillary water loss. Large exposed slabs dry unevenly, producing differential shrinkage and surface tension cracks.

Micro polypropylene synthetic fibers used to prevent plastic shrinkage cracks

Why Traditional Reinforcement Cannot Prevent Plastic Shrinkage Cracks

Steel reinforcement, mesh, and rebar are essential for structural load capacity—but they cannot prevent plastic shrinkage cracking, because early-age cracks develop before concrete hardens.

Two technical reasons explain this.

Mesh Location Issue

Wire mesh is typically placed:

In the lower third of the slab (due to chairs or foot traffic displacement)

Plastic shrinkage cracks occur at the surface, so mesh is positioned far below the cracking plane. It cannot provide restraint where the cracks form.

Timing Mismatch

Plastic shrinkage cracks occur:

Before initial set
Within the first 2–6 hours

Traditional reinforcement becomes effective only after concrete hardens and bonds to steel. Thus, reinforcement is inactive when plastic shrinkage occurs.

Comparison of concrete surfaces with and without micro synthetic fiber reinforcement

How Micro Fibers Prevent Early-Age Cracking

Micro synthetic fibers—especially micro polypropylene fibers—are highly effective in mitigating plastic shrinkage because they work in the fresh concrete phase, when traditional steel cannot.