What is Plastic Settlement Cracking?

Plastic settlement cracking is a common issue that can occur in concrete structures during the early stages of construction. It occurs when the freshly poured concrete settles and consolidates, causing the water to separate from the mixture and accumulate on the surface.

As the water rises to the surface, it can cause the formation of cracks in the concrete, which can weaken the structure and reduce its durability. These cracks are often shallow and occur near the surface of the concrete, and they can be prevented through careful attention to the construction process.

To prevent plastic settlement cracking, construction workers can take measures such as using the right amount of water in the concrete mixture, using a low-slump mix and vibrating the concrete to eliminate air pockets. Additionally, they can cover the concrete surface with a plastic sheet or apply a curing compound to prevent the water from evaporating too quickly, which can exacerbate the problem.

Causes for Plastic Settlement Cracking

Plastic settlement cracking in construction is caused by a combination of factors, including:

Excess water: When too much water is added to the concrete mixture, it can separate and rise to the surface, causing the concrete to settle and crack.
Insufficient compaction: If the concrete is not properly compacted, it can create voids or air pockets that allow the water to rise and accumulate on the surface, causing cracking.
Improper curing: If the concrete is not cured properly, the water can evaporate too quickly, causing the surface to dry out and shrink, which can lead to cracking.
Concrete mixture issues: The type of cement, aggregate, and other materials used in the concrete mixture can also affect the risk of plastic settlement cracking. If the mixture is not properly proportioned, it can increase the risk of cracking.
Poor construction practices: Improper handling or placement of concrete, as well as inadequate protection from weather conditions during construction, can also contribute to plastic settlement cracking.

When plastic concrete is poured, it may form an arch over the top of each reinforcing bar, which creates tension on the surface. This can cause cracks to appear at consistent intervals, following the line of the highest bars and resulting in parallel cracks. Additionally, shorter cracks may develop at right angles over the bars that run in the opposite direction.

By taking steps to address these issues during the construction process, builders can minimize the risk of plastic settlement cracking and ensure the long-term durability of their concrete structures.

Reducing Plastic Shrinkage Cracks

Plastic shrinkage cracks are caused by the rapid evaporation of moisture from the surface of fresh concrete before it has had a chance to fully cure. These cracks can be unsightly and can weaken the structure of the concrete. Here are some ways to reduce the likelihood of plastic shrinkage cracks in your concrete:

Use shading: One of the most effective ways to reduce the evaporation of moisture from the surface of fresh concrete is to provide shade. This can be accomplished by erecting temporary shading structures or by pouring concrete in the early morning or late afternoon when the sun is less intense.

Control wind exposure: Wind can increase the rate of moisture evaporation from the surface of fresh concrete. Using windbreaks or temporary barriers can help to reduce wind exposure.

Cover the concrete: Covering the concrete with a plastic sheet or wet burlap can help to slow down the rate of moisture evaporation and reduce the likelihood of plastic shrinkage cracks.

Use curing compounds: Curing compounds can be applied to the surface of fresh concrete to help slow down the rate of moisture evaporation and promote more even curing. These compounds can be sprayed on or brushed on, and they can be clear or pigmented.

Add fibres: Adding fibres to the concrete mix can help to improve its overall strength and reduce the likelihood of plastic shrinkage cracks. Fibres can be made of materials like polypropylene, nylon, or steel, and they are typically added to the mix before pouring.

By taking these steps, you can reduce the likelihood of plastic shrinkage cracks and ensure the long-term durability of your concrete structures.

When settlement occurs in concrete, it can create a void beneath any restraining elements such as reinforcement, which can negatively affect the local bond. The depth of the concrete section can be a determining factor in the amount of settlement, as deeper sections tend to settle more. Additionally, changes in section, such as those that occur at beam/slab junctions, can cause different settlement amounts that can result in surface cracks.

In the event that plastic shrinkage cracks occur during the final finishing of concrete, the finisher may be able to remedy them by refinishing. However, it is important to take precautions to prevent further cracking, as outlined above.

What is Drying Shrinkage Cracking?

Drying shrinkage cracks are a common issue in construction that occurs when moisture evaporates from freshly poured concrete or plaster. During the drying process, the material shrinks as the water evaporates, which can cause small cracks to appear on the surface.

These cracks typically occur in areas of the construction where the material is thin or where there is a change in thickness or shape. They can also appear at corners or edges, or where different materials meet. Drying shrinkage cracks are usually less than 0.3 mm in width and can occur up to a depth of about 2 cm from the surface.

While these cracks may not initially affect the structural integrity of the building, they can be unsightly and can lead to water penetration, which can cause further damage over time. To prevent drying shrinkage cracks, builders may use techniques such as reinforcing the material with steel mesh or fibres, adding chemical admixtures to the mix, or controlling the drying rate of the material.

Causes of Drying Shrinkage Cracks

Drying shrinkage cracks in construction are caused by a combination of factors related to the properties of the materials used and the environmental conditions during the curing process. Here are some of the main causes:

Water content: As the moisture content of the construction material decreases during the curing process, it undergoes shrinkage, leading to the formation of cracks.

Humidity and temperature: High humidity and high temperature during the curing process can speed up the drying process, leading to faster shrinkage and a higher risk of cracking.

Type of material: Different materials have different shrinkage properties, and some are more prone to drying shrinkage cracks than others. For example, concrete with a high water-cement ratio is more susceptible to cracking.

Mixing and placement: Improper mixing or placement of the material can lead to uneven distribution of water, resulting in differential drying and cracking.

Curing conditions: Insufficient curing time or inadequate curing conditions can result in premature drying and shrinkage, leading to cracking.

Structural design: Poor structural design or inadequate reinforcement can cause the material to be subjected to more stress than it can handle, leading to cracking.

Overall, proper material selection, mixing, placement, and curing techniques, as well as appropriate structural design, can help prevent drying shrinkage cracks in construction.

Reducing Drying Shrinkage Cracks

There are several ways to reduce the formation of drying shrinkage cracks in construction. Here are some of the most effective methods:

Reduce water content: Using low water-to-cement ratios and avoiding excess water during mixing can help minimize shrinkage and reduce the risk of cracking.

Use additives: Adding pozzolanic materials or other admixtures to the mix can help reduce shrinkage and improve the overall strength and durability of the material.

Control temperature and humidity: Controlling the temperature and humidity during the curing process can slow down the drying process and reduce the risk of cracking. This can be achieved through the use of curing blankets or other methods.

Reinforce the material: Adding steel mesh or fiber reinforcement to the material can help distribute stress more evenly and reduce the risk of cracking.

Control the drying rate: Controlling the rate of drying by covering the material or using a slow-curing mix can help reduce the risk of cracking.

Provide adequate curing time: Proper curing time is essential to allow the material to fully develop strength and reduce the risk of cracking.

Overall, reducing the water content, using additives, controlling temperature and humidity, reinforcing the material, controlling the drying rate, and providing adequate curing time are all effective ways to reduce the formation of drying shrinkage cracks in construction.

The effect of temperature on drying shrinkage cracks

Temperature can have a significant effect on the formation of drying shrinkage cracks in construction materials. Higher temperatures can accelerate the drying process, causing the material to shrink more quickly and increasing the risk of cracking. On the other hand, lower temperatures can slow down the drying process, reducing shrinkage and the likelihood of cracking.

Specifically, high temperatures can cause the following effects:

Accelerated drying: High temperatures can cause water to evaporate more quickly, leading to faster drying and more rapid shrinkage.

Increased thermal expansion: Higher temperatures can cause the material to expand, which can increase internal stresses and lead to cracking when the material subsequently contracts during drying.

Reduced strength: High temperatures can cause the material to cure more quickly, which can result in reduced strength and increased susceptibility to cracking.

On the other hand, low temperatures can cause the following effects:

Slower drying: Low temperatures can slow down the drying process, reducing shrinkage and the risk of cracking.

Reduced thermal expansion: Lower temperatures can reduce thermal expansion, reducing the potential for internal stresses and cracking.

Longer curing time: Low temperatures can slow down the curing process, allowing the material to develop strength more slowly and reducing the risk of cracking.

Overall, it is important to consider the temperature conditions during the curing process and take appropriate measures to control the temperature to prevent or minimise the formation of drying shrinkage cracks in construction.