Concrete Tip of the Day #7: Understanding and Preventing Cracks in Concrete

Cracking in concrete is one of the most common—and often misunderstood—issues faced on construction sites. While some cracking is inevitable, most can be minimised or controlled through proper planning, mix design, placement, and curing. In this article, we explore the causes of cracking and how to prevent or reduce them effectively.

🧱 Why Does Concrete Crack?

When concrete hardens, it begins to lose the excess water not needed for cement hydration. This drying leads to shrinkage. If the concrete is unrestrained, shrinkage often occurs without cracking. However, most real-world structures involve restraint—from subgrade, reinforcement, formwork, or geometry—leading to internal stress and cracking.

Cracking can result from multiple overlapping causes:

• Shrinkage (drying, plastic, thermal)

• Chemical reactions (e.g., alkali-aggregate)

• Structural movement

• Thermal gradients

• Construction or design errors

  
  

🕒 Timing is Everything: When Cracks Occur

Cracks can be broadly grouped into three categories:

1. Before or During Hardening

Occurs within the first 72 hours:

• Plastic shrinkage (caused by rapid surface evaporation)

• Plastic settlement (restraint from rebar or geometry)

• Thermal contraction

• Formwork movement

• Subgrade movement

✅ Prevention Tips:

• Moist subgrade and formwork before pouring

• Avoid adding water on-site

• Use proper compaction

• Cut contraction joints early (¼ to ⅓ slab thickness)

• Start curing as soon as possible

• Protect from rapid temperature drops (≥12°C overnight)

🌬️ Plastic Shrinkage: The Most Common Cracking Issue

If it’s a perfect day to dry your laundry, it’s a risky day for placing concrete.

Plastic shrinkage happens when the evaporation rate exceeds the bleed rate. This can occur in hot, windy, or low humidity conditions—regardless of season.

Contributing factors include:

• High ambient and concrete temperature

• Wind exposure

• Low relative humidity

• Dark or low-bleed concrete mixes

• Use of retarders or high-range water reducers

• Broomed surfaces (higher evaporation area)

✅ How to Reduce Risk:

• Monitor weather (wind, temp, humidity)

• Use windbreaks, misting, or anti-evaporation films

• Avoid retarders if unnecessary

• Reduce water content and mix bleeding

• Place concrete after roof/walls are up if possible

🔍 Cracks After Hardening

These cracks develop after concrete has stiffened and include:

• Drying shrinkage

• Early-age thermal cracking (e.g., temperature drops overnight)

• Design-related cracking in slabs and walls

• Craze cracking (shallow, aesthetic only)

• Delamination (surface separation due to premature finishing)

Proper curing, correct timing of joint cutting, and smart mix design are key to reducing these risks.

🧩 Controlling Cracks with Joints

Cracks can’t always be prevented—but they can be controlled. Grooves and joints act as planes of weakness, guiding cracks where you want them.

✅ Joint Guidelines:

• Cut joints as soon as concrete is strong enough

• Early-entry saws allow cutting within hours

• Delay beyond 24 hours increases risk of random cracking

• Joints should be ¼ to ⅓ slab depth

🛠 Final Word

Concrete cracking is often the result of multiple interacting factors—not just mix design or workmanship. By understanding the types of cracking and the conditions that cause them, you can take proactive steps to protect your slab from premature failure or costly repairs.

✅ Stay tuned for the next episode of Concrete Tip of the Day!