Plastic shrinkage and settlement cracking in concrete

Summary

In this video, Tyler Lai, a concrete expert, discusses the causes of cracks in concrete before it hardens, focusing on plastic shrinkage and plastic settlement cracking. He explains how cracks can form due to water loss, high temperatures, low humidity, and improper concrete composition, and provides practical solutions like using windbreaks or adjusting mix designs. He also covers settlement cracking, which occurs when concrete settles unevenly over rebar or aggregate. The video emphasizes the importance of proper preparation, weather monitoring, and construction techniques to prevent these common issues.

Takeaways

• 😀 Cracks in concrete can appear before it hardens, and they are mainly caused by tensile stress exceeding the concrete's capacity.
• 😀 Plastic shrinkage cracking occurs when the surface water evaporates faster than it can bleed, causing shrinkage and cracks in the wet concrete.
• 😀 Weather conditions such as high air temperature, low humidity, high wind speed, and direct sun exposure increase the likelihood of plastic shrinkage cracking.
• 😀 Factors like low water content in the mix and high paste content (binder and water) can make plastic shrinkage cracking more likely.
• 😀 Concrete with high amounts of fines or pozzolanic materials (like fly ash, slag, or silica fume) is more prone to plastic shrinkage cracking.
• 😀 To prevent plastic shrinkage cracking, you can use wind breaks, shade the surface, fog the concrete, or pour at night.
• 😀 Plastic fibers in the mix can help retain moisture during the early stages and reduce the likelihood of plastic shrinkage cracking.
• 😀 Plastic settlement cracking occurs when the concrete settles unevenly over aggregates or rebar, leading to cracks above them.
• 😀 Settlement cracking is more likely if the formwork is not rigidly supported or if the concrete is not properly consolidated.
• 😀 When cracks are observed, refinishing the surface with a float can help close them up, but deep cracks may still persist beneath the surface.
• 😀 Proper concrete mix design, careful attention to weather, and good construction practices can help prevent both plastic shrinkage and settlement cracking.

Q & A

What is plastic shrinkage cracking in concrete?

Plastic shrinkage cracking occurs when water evaporates from the surface of fresh concrete before it has set. This causes the surface to shrink while the interior remains unchanged, resulting in cracks that are typically shallow and parallel.

What causes plastic shrinkage cracking?

Plastic shrinkage cracking is caused by water loss at the surface of the concrete before it has set, commonly due to evaporation that exceeds the rate of bleeding. Factors like high temperatures, low humidity, wind, and direct sun exposure can increase evaporation rates.

How does the composition of concrete affect plastic shrinkage cracking?

Concrete with lower water content and higher amounts of binder paste is more prone to plastic shrinkage cracking. Additionally, materials like fly ash, slag, or silica fume can delay set times and absorb water, increasing the likelihood of cracking.

What weather conditions contribute to plastic shrinkage cracking?

Weather conditions that increase the risk of plastic shrinkage cracking include high air temperature, low humidity, high concrete temperature, and high wind speeds. Direct sun exposure can also significantly contribute to the evaporation rate.

What measures can be taken to prevent plastic shrinkage cracking?

Preventive measures include building wind breaks, shading the surface, spraying water to keep it cool, pouring concrete at night, using plastic fibers to aid water movement within the mix, and adjusting concrete mixtures to reduce susceptibility.

What is plastic settlement cracking in concrete?

Plastic settlement cracking occurs when fresh concrete settles unevenly over aggregates or rebar, causing cracks. These cracks are often perpendicular to one another and can follow the pattern of embedded reinforcement or aggregates.

What causes plastic settlement cracking?

Plastic settlement cracking happens when concrete settles over aggregate or rebar, and the latter doesn't move. The difference in movement causes tension in the concrete, resulting in cracks. It can also occur if the formwork is not rigid or stable.

How can plastic settlement cracking be prevented?

To prevent plastic settlement cracking, ensure that formwork is stiff and stable, the subgrade is well-prepared, and proper concrete consolidation is performed. Using low slump concrete and minimizing segregation can also help.

What should be done if plastic settlement cracking is observed after pouring concrete?

If plastic settlement cracking is observed, the surface can be refinished by using a float to smooth and close the cracks. However, care must be taken as the cracks may still exist below the surface, even if they are closed on top.

What is the general rule of thumb for preventing cracking during concrete placement?

A general rule of thumb for reducing cracking risk is to ensure that wind speed is below 10 mph, relative humidity is above 50%, concrete temperature is below 85°F, and air temperature is below 80°F. These conditions are most often found on 20-30 days per year.