What is Shrinkage in Concrete? How to Reduce Shrinkage of Concrete?

Restraint structural members may develop cracks due to concrete shrinkage and structural restrictions caused by other parts of the structure. When the stresses exerted on the structure exceed the concrete’s tensile strength, cracks can form. Notably, cracks may occur at lower stresses than those required for crack initiation. Thus, reducing shrinkage could enhance the strength and durability of concrete.

Shrinkage refers to the reduction in concrete volume due to moisture loss. If this reduction happens before the concrete hardens, it is termed as plastic shrinkage. On the other hand, if it occurs after the concrete hardens, it is called drying shrinkage. Additionally, concrete may experience carbonation and autogenous shrinkage.

By reducing the amount of shrinkage, it is possible to improve the strength and durability of concrete. Restraint structural members are prone to crack development due to concrete shrinkage and structural restrictions imposed by other parts of the structure. Cracks appear when stresses surpass the concrete’s tensile strength. Notably, cracks may propagate at much lower stresses than those needed for crack initiation.

Shrinkage occurs when there is a reduction in concrete volume due to the loss of moisture. Plastic shrinkage refers to the volume reduction that occurs before the concrete hardens, while drying shrinkage happens after hardening. Concrete can also experience carbonation and autogenous shrinkage.

What is Shrinkage in Concrete?

Shrinkage in concrete refers to the reduction in volume of concrete as it dries and hardens. When water is mixed with cement, it hydrates and forms a gel-like substance that fills the voids in the mixture and hardens into a solid.

However, during the hardening process, the gel-like substance shrinks as the water in the mixture evaporates, causing the concrete to contract and pull away from adjacent structures. This can lead to cracks and other types of damage in the concrete. Shrinkage can also occur due to changes in temperature or moisture levels in the environment.

Reasons for Concrete Shrinkage

To effectively solve any problem, it’s important to identify and eliminate the root causes of the issue. In the case of concrete shrinkage, there are certain factors that contribute to this problem. By addressing these factors, it’s possible to minimize or eliminate the occurrence of concrete shrinkage.

Concrete shrinkage occurs as a result of several factors. These factors include the use of low-quality materials, improper mixing and placement of concrete, excessive water content in the concrete mixture, and insufficient curing time. When any of these factors are present, it can lead to the shrinkage of concrete.

To prevent concrete shrinkage, it’s important to take steps to eliminate these factors. This can involve using high-quality materials, ensuring that the concrete is mixed and placed properly, using the appropriate amount of water in the mixture, and allowing sufficient curing time for the concrete. By taking these steps, it’s possible to minimize or prevent the occurrence of concrete shrinkage, leading to a more durable and long-lasting concrete structure.

1. Concrete Quality

The quality of concrete can be influenced by various factors such as the water-to-cement ratio, cement content, heat of hydration, aggregate, and compaction level. The water-to-cement ratio (w/c) is an important factor in determining the strength and durability of the concrete. A lower w/c ratio typically results in higher strength and durability. The cement content also plays a crucial role in the strength of the concrete. Higher cement content typically leads to higher strength, but it can also increase the heat of hydration, which is the amount of heat released during the chemical reaction that occurs when water and cement are mixed.

The heat of hydration can affect the curing process of concrete and can lead to cracking if not managed properly. The type and quality of the aggregate used can also impact the strength and durability of the concrete. Proper selection of aggregate can improve the workability and reduce the risk of cracking.

Compaction level is another important factor in ensuring the quality of concrete. Proper compaction ensures that the concrete is free of voids and has a uniform density. Adequate compaction can improve the strength and durability of the concrete and reduce the risk of cracking.

In summary, the quality of concrete is influenced by several factors, including the water-to-cement ratio, cement content, heat of hydration, aggregate, and compaction level. Proper management and consideration of these factors can result in high-quality, strong, and durable concrete.

2. Excess Water in Concrete

Concrete is a commonly used building material that requires an appropriate amount of water for it to be workable and easily handled during construction. However, the presence of excess water within the concrete can lead to a phenomenon known as concrete shrinkage, which can cause various issues with the structure’s overall durability and stability.

To ensure the proper workability of concrete, an adequate amount of water is necessary. This water is necessary for the concrete to be easily moldable and shaped according to the required specifications during the construction process. However, while water is essential for the handling of concrete, it can also be a significant cause of concrete shrinkage.

Concrete shrinkage occurs due to the presence of free water within the concrete mixture. As the concrete begins to dry and set, the excess water begins to evaporate, leading to the concrete shrinking in size. This shrinkage can cause a range of problems with the structure’s overall strength and durability, potentially leading to cracking, deformation, or other forms of damage.

Therefore, it is crucial to find a balance between the required amount of water necessary for the concrete to be workable and minimizing the presence of excess water that can cause shrinkage. This can be achieved through careful monitoring and management of the water-cement ratio, which can help ensure the proper handling of concrete while minimizing the potential for shrinkage-related issues.

3. Geometry of Structural Member

The amount of shrinkage in concrete can be greatly influenced by geometrical factors such as the surface-area-to-volume ratio and depth of the material. In particular, smaller concrete members tend to experience higher levels of shrinkage compared to larger structural members. This is because the smaller size of these members results in a higher ratio of surface area to volume, which in turn leads to more rapid drying and greater shrinkage.

Another factor that can contribute to concrete shrinkage is the size of the surface area exposed to the environment. Larger concrete surfaces have a wider area that is exposed to the environment, which can increase the likelihood of shrinkage. This is because exposure to air and moisture can accelerate the drying process and cause the concrete to shrink more quickly. As a result, it is important to take into account the size and shape of concrete members when designing structures in order to minimize the risk of shrinkage and ensure the long-term durability and stability of the concrete.

4. Environmental Factors

Concrete shrinkage can be influenced by various environmental factors, including temperature, humidity, and wind velocity. These factors can have a significant impact on the concrete’s volume reduction and overall performance. Temperature, for example, can cause concrete to expand or contract, which can lead to cracking and other forms of damage. High humidity levels can also affect the concrete’s moisture content and cause shrinkage, while strong winds can accelerate the drying process and increase shrinkage rates.

It is important to note that concrete shrinkage can occur naturally as the material dries and cures. However, environmental factors can exacerbate the shrinkage process and result in more significant damage to the concrete structure. Therefore, it is crucial to consider these factors when designing and constructing concrete structures, as well as during maintenance and repair operations.

To minimize the negative effects of environmental factors on concrete shrinkage, several strategies can be implemented. These include using materials that are less susceptible to shrinkage, controlling the temperature and humidity levels during the curing process, and limiting exposure to wind and other weather conditions. By taking these precautions, it is possible to mitigate the impact of environmental factors on concrete shrinkage and ensure the longevity and durability of concrete structures.

How to Reduce Shrinkage of Concrete at Construction Site?

1. Reduce Water Quantity

Reducing the quantity of water used in concrete can have a significant impact on reducing drying shrinkage, which is a common problem in construction. However, it is important to exercise caution when decreasing water content to ensure that the concrete remains workable enough to be placed and compacted properly. Studies have shown that reducing the water-to-cement ratio from 0.45 to 0.35 can reduce concrete shrinkage by as much as 50%.

One of the most common examples of shrinkage caused by drying is surface crazing, which occurs when the surface layer of concrete has a higher water content than the interior concrete. This leads to the formation of shallow, closely spaced, fine cracks on the surface.

To reduce both drying shrinkage and settlement cracking, it is recommended to gradually decrease the water content of the concrete as the wall is constructed from the bottom to the top. This will allow the bleed water from the lower portions of the wall to equalize the water content within the wall. However, it is crucial to exercise careful control and proper consolidation during this process to ensure that the concrete remains structurally sound.

In conclusion, reducing water content in concrete can be an effective strategy to combat drying shrinkage, but it is important to balance this with maintaining the workability of the concrete. Proper procedures, such as gradually reducing water content from the bottom to the top of a wall, can help to minimize shrinkage and cracking, but it is important to exercise caution and control during the construction process to ensure that the concrete remains strong and durable.

2. Add Additives

To decrease concrete shrinkage, the use of additives is a viable strategy. Water reducing admixtures and shrinkage reducing admixtures can both contribute significantly to reducing concrete shrinkage.

Water reducing admixtures work by reducing the water content of the concrete, which in turn reduces shrinkage. Shrinkage reducing admixtures, on the other hand, work by mitigating the internal effects of shrinkage.

Furthermore, the use of shrinkage reducing admixtures can help to compensate for concrete shrinkage by generating expansive crystals. These crystals help to counteract the effects of shrinkage and can lead to stronger, more durable concrete structures in the long run. Overall, the use of appropriate additives is an effective means of reducing concrete shrinkage and improving the quality and longevity of concrete structures.

3. Aggregate

To minimize drying shrinkage in a concrete mix, it is important to use the optimal amount of aggregate. The stiffness of the aggregate also plays a significant role in reducing shrinkage. When the stiffness of the aggregate is higher, it is more effective in reducing the shrinkage of the concrete.

However, the shrinkage rate of concrete varies depending on the type of aggregate used. For instance, concrete that contains sandstone aggregate has a higher shrinkage rate compared to concrete containing granite, basalt, or limestone. In fact, the shrinkage rate of concrete containing sandstone aggregate is about twice as high as that of concrete containing other types of aggregates. This suggests that the type of aggregate used in concrete mix significantly influences the level of shrinkage in the resulting concrete.

4. Ways to Prevent Rapid Drying of Concrete Surfaces

One way to mitigate the impact of environmental factors on concrete is by covering its surface. This can significantly reduce or even eliminate the effects of external conditions until the curing process begins. Additionally, if the concrete batched is exposed to high temperatures, using ice or chilled water can help lower the concrete’s temperature and prevent any potential damage.

On windy days, it may also be beneficial to employ temporary windbreaks to decrease wind velocity, which can negatively affect the concrete’s strength and durability. By taking these precautions, the quality and integrity of the concrete can be preserved, leading to a better end result.

5. Time of Concreting

Concrete shrinkage, particularly plastic shrinkage, can be minimized by selecting the appropriate time for concreting. One way to achieve this is by pouring the concrete at night during summer. This approach avoids exposing the concrete to the high temperatures typically experienced during the daytime, which can lead to excessive shrinkage.

By pouring the concrete at night, the concrete has more time to set and cure properly, reducing the likelihood of shrinkage. This approach is especially important during the summer months when temperatures can be exceptionally high, making it more challenging to control the rate of evaporation and shrinkage.

Proper timing of concreting can have a significant impact on the quality of the final product. By taking steps to minimize shrinkage, such as pouring at the right time, builders and contractors can improve the durability and longevity of their structures.

This, in turn, can lead to lower maintenance costs and a safer built environment. Therefore, it is essential to pay close attention to the timing of concreting, especially during adverse weather conditions such as high temperatures in the summer.

6. Proper Concrete Compaction

To prevent the shrinkage of concrete, it is important to compact it properly during the construction process. This can be achieved by applying appropriate techniques and tools to ensure that the concrete is properly packed and settled. By doing so, the risk of shrinkage is significantly reduced, which can help to prevent damage to the structure over time.

Another important factor to consider when working with concrete is to avoid re-tempering the mixture at the construction site. This can lead to an increase in the water content of the concrete, which in turn can increase the risk of drying shrinkage. As a result, it is recommended to use the concrete mixture as soon as it is prepared, and to avoid adding any additional water or other additives that can affect its composition. By following these guidelines, the concrete can be properly prepared and installed, which can help to ensure its longevity and structural integrity.

7. Proper Curing

Proper curing mechanisms play a crucial role in reducing concrete shrinkage. When concrete is allowed to cure adequately, it can hydrate effectively, which leads to the attainment of the desired strength. This is especially important for reinforced concrete slabs because it allows the concrete to achieve higher tensile capacity before drying out occurs.

One of the primary benefits of adequate curing is the reduction of concrete shrinkage. By minimizing the extent to which concrete shrinks, the structural integrity of the concrete is maintained. Therefore, it is essential to adopt appropriate curing mechanisms that facilitate the hydration process, which is necessary for achieving optimal strength.

In addition to reducing shrinkage, proper curing mechanisms can also improve the overall durability of the concrete. This is because curing helps to prevent cracking and other forms of damage that can result from inadequate hydration. Furthermore, adequate curing can enhance the concrete’s resistance to harsh weather conditions and other environmental factors that can degrade its quality over time.

To achieve the best results, steam curing can be employed as a method of curing concrete. This technique has been proven to be effective in reducing concrete shrinkage. By exposing the concrete to steam, the hydration process is accelerated, which leads to the attainment of optimal strength. Consequently, steam curing can improve the performance and durability of concrete, particularly in reinforced concrete slabs.

8. Adequate Contraction Joints and Steel Detailing

Placing contraction joints properly and using appropriate steel bar detailing are effective ways to mitigate the negative impact of concrete shrinkage and manage the development of cracks. By strategically placing contraction joints, the concrete is able to contract in a controlled manner, reducing the likelihood of uncontrolled cracking. Additionally, carefully detailing steel bars can also help to control the formation of cracks by providing reinforcement in areas where the concrete is under stress. Overall, a combination of proper contraction joint placement and steel bar detailing can significantly improve the durability and longevity of concrete structures.

FAQs on how to reduce concrete shrinkage in construction site