Plastic Shrinkage Cracking: Understanding the Risk and Prevention

Have you ever noticed that sometimes after pouring fresh concrete into the mold, cracks appear as it starts drying? Well, that is what you call plastic shrinkage cracking. In this article, we will give you its detailed description, the reason behind its occurrence, and most importantly, its preventive methods. Whether you are a construction pro or a DIY enthusiast, knowing the ins and outs of plastic shrinkage cracks is vital to ensure that your concrete projects remain durable.

What is plastic shrinkage?

Plastic shrinkage cracking occurs when fresh concrete sustains a volumetric decline after it is put in a form and is in the plastic state. The shrinkage and cracks are due to quick drying of the plastic concrete surface. They materialize often on horizontal surfaces. However, plastic shrinkage cracking seldom damages the longevity and power of concrete pavements and floors.

 

Why Does Plastic Shrinkage Cracking Occur?

Factors influencing plastic shrinkage cracking are vast and seemingly interconnected. It implies that change in one facet may affect the other ones responsible for cracks. That is why it is important to comprehend its risk factors and decrease its continued growth. 

1. Lower Bleed Rate Or Quantity of Bleed Water

Bleed rate refers to the rate of water rising on the concrete surface and forming a layer. Despite reduced evaporation rates, a mixture with a decreased rate of bleed water is subject to plastic shrinkage. Some of the general factors are, 

• Thinner sections
• Minimized water content
• High cementitious materials content
• Entrained air
• High fines content
• High content temperature

2. Quick Surface Moisture Evaporation

This happens when the rate of surface moisture evaporation surpasses the rate at which the bleed water can replace it. It is the most general reason behind plastic shrinkage. Whenever water retreats beneath the concrete surface, a meniscus usually sets between the cluster and cement fragments. As a result, a tensile force is developed between the plane layers. After the surface starts setting and forms a proper tensile strength, no cracks will appear. But if surface evaporation quickly happens while the concrete is still plastic, cracking will occur as the concrete sets up.

Factors responsible for plastic shrinkage cracking include, 

• Wind speed stronger than 5MPH, 
• Reduced relative humidity

Utilizing synthetic fiber reinforcement in the concrete design mix can assist brittle concrete in resisting tension between the plane layers.

• Delayed Setting

It can surface from many aspects leading to plastic shrinkage which includes: 

• Specific water reducers
• Subgrades and/or cool weather
• Additional cementing materials
• Retarders
• High-level water content, and
• Decreased cement content

Prevention Of Plastic Material Shrinkage Cracking

A contractor must take preventive measures to avoid plastic material shrinkage including minimization in the comparative volume change between the plane and other concrete portions. These strategies involve the utilization of fog nozzles for air saturation over the surface and utilizing plastic sheeting. Let us talk about the techniques and methods to avert this common problem.

1. 1. Use Fog Sprays

During dry and hot periods, utilize fog sprays with extra water. It is applied by exuding a sprayer throughout the air upwind of the concrete surface. It lowers the evaporation rate from the concrete plane and must be continued till curing elements may be applied.

1. 2. Put Vapor Retarders Cautiously

You can relevantly diminish the chances of plastic shrinkage by utilizing carefully placed vapor retarders below the concrete slab positioned on the grade. If the usage is important to adequate curing, you can also use a 3–4-inch coating of trimmable, compactible, and lightly dampened granular fill as a cover, like crusher-run material. 

1. 3. Rapidly Begin Curing 

Initiate curing of the concrete surface as quickly as possible. Spray it with a liquid membrane curing mixture or cover it with moistened burlap. Ensure to keep it consistently wet for at least 3 days to steer clear of plastic shrinkage cracking.

1. 4. Employ Makeshift Wind Breaks

They assist in reducing the wind speed moving throughout the fresh plane. At times, temporary wind breaks offer temperature control with shading to the zone. Besides averting plastic shrinkage, they also help avoid unfavorable situations in the early evening or late afternoon. During hot weather, you must set up the breaks in the early morning to gain more control over the concrete plane temperatures. 

1. 5. Moisten The Subgrade

Ensure to moisten the subgrade in case concrete is to be put on the dry absorptive subgrade during dry and hot weather. However, you must not moisten it to the point of freestanding water before placement. Dampen the reinforcement and formwork as well to repair the shrinkage in plastic moulding.

1. 6. Promptly Set and Finish Concrete

Proper employees, supplies, and equipment are necessary to set up and finish the concrete surface timely. In case of delays, you must utilize moisture-keeping coverings on the plane between the steps to ensure appropriate plastic shrinkage prevention. 

1. 7. Utilize Shrinkage Reducing Admixture (SRA)
   Currently, SRAs are conveniently accessible from famous admixture players. It helps reduce the impact of cracks from both drying and self-desiccation. Its operating mechanism is developed by evaluating the Helmholtz free energy. For reducing plastic shrinkage cracking, SRAs amplify the energy fraction utilized in forming interfacial by diminishing the capillary action of the liquid-vapor interface. 

In case cracks must occur throughout the final finishing, the finisher might be capable of closing them through refinishing. Nonetheless, you must take the above-mentioned precautions to avoid further plastic shrinkage in concrete. Before placement, it is essential to think ahead of time about situations conducive to such cracks and diminish their possibility.  

Conclusion

Cracks might form aesthetic issues, specifically for architectural concrete. Also, plastic shrinkage might create strength problems as per the crack depth and width, cracking intensity, and exposure conditions of the concrete. In case elements and water enter the concrete by cracking, the probability of concrete damage and reinforcement corrosion from thawing and freezing can increase. So, the most effective method to eschew durability and aesthetic issues associated with such cracks is to comprehend the susceptibility of concrete mix to cracks. 

You must schedule flat work after the erection of walls. These cracks generally surface in the initial few hours after the concrete setting and usually before completing the finishing work. Though it typically occurs in concrete flatwork, the cracking may also take place on horizontal, exposed surfaces like foundations, tops of walls, and beams. Moreover, knowing the reasons behind plastic shrinkage, supervising job site conditions, and executing precautionary steps can minimize or even prevent unfortunate circumstances.