Short Answer:

Plastic shrinkage cracking occurs when concrete undergoes shrinkage during the early stages of setting, typically within the first few hours after placing. It happens when the surface of the concrete dries faster than the underlying layers, leading to the formation of cracks. These cracks are usually shallow and sporadic but can affect the appearance and performance of the concrete.

Preventing plastic shrinkage cracking involves controlling moisture loss, improving curing techniques, and using proper admixtures. Measures such as covering the surface with wet burlap, plastic sheeting, or spraying curing compounds can help reduce the evaporation rate and minimize cracking.

Detailed Explanation:

What is Plastic Shrinkage Cracking?

Plastic shrinkage cracking is a phenomenon that occurs in fresh concrete when the surface layer dries and shrinks at a faster rate than the inner layers during the initial hardening phase. This type of cracking happens before the concrete has gained sufficient strength to withstand internal stresses. It is most common in hot, dry, or windy conditions where rapid evaporation of water from the concrete surface leads to shrinkage.

The cracks usually form in a random, map-like pattern and are often shallow (typically no deeper than 25mm). They are most commonly seen in the first few hours after concrete placement, before the concrete has hardened enough to resist shrinkage. If left untreated, these cracks can reduce the concrete’s aesthetic appeal and compromise its structural integrity.

Plastic shrinkage cracks typically occur in paving slabs, floor slabs, foundations, or curbs where large surface areas are exposed to the environment. These cracks can also worsen when improper finishing techniques are used, which may cause the surface to be overly compacted and promote rapid evaporation.

How to Prevent Plastic Shrinkage Cracking?

Prevention of plastic shrinkage cracking involves controlling the factors that lead to rapid moisture loss from the surface of the concrete. Here are several methods to prevent this issue:

1. Control Moisture Loss:

The most effective way to prevent plastic shrinkage cracking is to minimize moisture loss from the surface of the freshly laid concrete. This can be done by:

• Covering the surface with wet burlap or plastic sheets to keep moisture trapped.
• Using wet curing techniques by continuously spraying water on the surface during the initial setting period.

2. Use of Curing Compounds:

Applying a curing compound on the surface of the concrete immediately after finishing helps to form a moisture-retaining layer. This method is particularly effective for large surface areas like pavements or slabs. Membrane-forming curing compounds slow down the evaporation of water from the concrete, preventing premature shrinkage.

3. Reducing the Water-Cement Ratio:

Using a lower water-cement ratio ensures the concrete mix is thicker, thus reducing the potential for water loss. It also helps the concrete to retain moisture longer, which decreases the chances of rapid shrinkage.

4. Adjusting the Timing of Finishing:

Avoid finishing the concrete too early or too late. Early finishing can lead to sealant formation that traps moisture, while finishing too late can lead to excessive moisture evaporation. A careful balance ensures the surface remains workable but not excessively dry.

5. Use of Admixtures:

Certain admixtures can be added to the concrete mix to reduce its susceptibility to plastic shrinkage cracking. Retarders are commonly used to slow down the setting time, allowing more time for the moisture in the concrete to remain.

6. Avoiding High Wind and Temperature:

To prevent evaporation, work in cooler temperatures and avoid windy conditions during the early setting period. When these conditions are inevitable, windbreaks or shade covers can help reduce wind and heat impact.

Conclusion:

Plastic shrinkage cracking is a significant issue in freshly placed concrete, especially when exposed to adverse environmental conditions. By controlling moisture loss, improving curing techniques, using appropriate finishing methods, and taking preventive measures like using retarders or curing compounds, plastic shrinkage cracking can be minimized. Early intervention and understanding the factors that cause shrinkage cracks are key to maintaining the durability and strength of the concrete structure.