Restraints in structural members, combined with concrete shrinkage, can cause the development of cracks. These restraints are often caused by other parts of the structure. When stresses on the structure exceed the tensile strength of the concrete, cracks can occur. It’s worth noting that cracks may propagate at lower stresses than those needed for crack initiation. Therefore, reducing shrinkage would improve both the durability and strength of concrete.
Shrinkage is the decrease in concrete volume due to moisture loss. If this volume reduction occurs before the concrete hardens, it’s referred to as plastic shrinkage. However, if the volume reduction happens after the concrete hardens, it’s known as drying shrinkage. Carbonation and autogenous shrinkage are also potential causes of concrete shrinkage.
Reasons for Concrete Shrinkage
Concrete shrinkage can be effectively addressed by implementing a strategy that focuses on eliminating the factors that contribute to the problem. In this case, it is crucial to identify the specific factors that are responsible for causing concrete shrinkage. Once these factors have been identified, a plan can be put in place to eliminate them and prevent further shrinkage from occurring. It is important to note that there may be several factors at play, so a thorough analysis is necessary to ensure that all contributing factors are addressed. By implementing this strategy, it is possible to effectively tackle the problem of concrete shrinkage and prevent it from occurring in the future.
1. Excess Water in Concrete
Concrete is a construction material that requires a sufficient amount of water to be workable and easy to handle. However, the presence of free water in the concrete is precisely what leads to concrete shrinkage.
To ensure that concrete can be used effectively, an appropriate amount of water must be added during the mixing process. This water is necessary to hydrate the cement and provide the necessary fluidity for the concrete to be molded and shaped as desired. Without enough water, the concrete will be too dry and difficult to work with.
On the other hand, once the concrete has been poured and begins to set, the presence of free water becomes a problem. As the concrete dries, the water evaporates, causing the concrete to shrink. This shrinkage can result in cracks, which can compromise the strength and durability of the structure.
Therefore, it is important to balance the need for workability and handling with the risk of shrinkage. By carefully controlling the amount of water used during mixing and taking steps to minimize the amount of free water in the concrete after it has been poured, it is possible to create strong, durable concrete structures that are resistant to shrinkage and cracking.
2. Environmental Factors
Concrete shrinkage can be influenced by a variety of environmental factors, including temperature, humidity, and wind velocity. These factors can have a significant impact on the overall shrinkage of the concrete, and may cause it to shrink more or less than expected.
Temperature is one of the most important environmental factors affecting concrete shrinkage. When concrete is exposed to high temperatures, it tends to shrink more quickly than when it is exposed to lower temperatures. This is because high temperatures can cause the water in the concrete to evaporate more quickly, which can lead to greater shrinkage.
Humidity is another factor that can affect concrete shrinkage. When the humidity is low, the concrete will tend to shrink more than when the humidity is high. This is because low humidity can cause the water in the concrete to evaporate more quickly, which can lead to greater shrinkage.
Finally, wind velocity can also have an impact on concrete shrinkage. When there is a high wind velocity, the concrete will tend to shrink more than when there is little or no wind. This is because the wind can increase the rate of evaporation of water from the concrete, which can lead to greater shrinkage.
In summary, environmental factors such as temperature, humidity, and wind velocity can all contribute to the overall shrinkage of concrete. Builders and engineers must take these factors into consideration when planning and designing concrete structures, in order to ensure their long-term stability and durability.
3. Concrete Quality
The quality of concrete is affected by various factors such as water to cement (w/c) ratio, cement content, heat of hydration, aggregate, and compaction level. The w/c ratio is the ratio of the amount of water to the amount of cement in the concrete mix, and it plays a significant role in determining the strength and durability of the concrete. A higher w/c ratio can result in weaker and less durable concrete.
The cement content is also a crucial factor in determining the quality of concrete. A higher cement content can increase the strength of the concrete, but it can also increase the heat of hydration, which can cause cracking and shrinkage in the concrete. The heat of hydration is the heat generated when cement reacts with water, and it can be controlled by using cement with a lower heat of hydration or by using a higher w/c ratio.
The aggregate used in concrete also affects its quality. The size, shape, and texture of the aggregate can impact the workability, strength, and durability of the concrete. The compaction level of the concrete mix is also important, as insufficient compaction can result in voids and weak spots in the finished product.
Overall, a proper balance of these factors is necessary to ensure high-quality concrete. It is essential to carefully consider the w/c ratio, cement content, heat of hydration, aggregate, and compaction level when designing a concrete mix to achieve the desired strength and durability.
4. Geometry of Structural Member
Concrete shrinkage can be greatly influenced by geometrical factors such as the surface-area-to-volume ratio of concrete and the depth of concrete. Concrete members that are smaller in size are more susceptible to higher levels of shrinkage when compared to larger structural members. This is because the larger the concrete surface, the wider the concrete area exposed to the environment, leading to a higher likelihood of shrinkage occurring.
In summary, concrete shrinkage is a complex phenomenon that can be affected by various factors, including the geometry of the concrete members. Engineers and construction professionals should be aware of these factors and take them into account when designing and constructing concrete structures. Failure to do so can result in significant structural problems, which may compromise the safety and durability of the building or infrastructure.
How to Reduce Shrinkage of Concrete at Construction Site?
1. Reduce Water Quantity
The reduction of water quantity in concrete can be a significant factor in reducing drying shrinkage. However, caution must be exercised to ensure that the concrete remains workable enough to be placed and compacted effectively. Studies have demonstrated that by decreasing the water to cement ratio from 0.45 to 0.35, concrete shrinkage can be reduced by 50%.
Small-scale shrinkage due to drying can manifest as surface crazing on walls and slabs. This phenomenon typically occurs when the surface layer of the concrete has a higher water content than the interior. The outcome is a series of shallow, closely spaced, fine cracks.
To mitigate both drying shrinkage and settlement cracking, decreasing the water content of the concrete gradually as the wall is constructed from the bottom to the top can be helpful. By following this process, bleed water from the lower sections of the wall will tend to even out the water content within the wall. However, this technique necessitates careful control and appropriate consolidation.
Fig. 1: Reduce Water Quantity
2. Add Additives
Concrete shrinkage can be reduced through the addition of additives. Two important additives are water reducing admixture and shrinkage reducing admixture. Water reducing admixture works by decreasing the water content in concrete, which in turn reduces concrete shrinkage. On the other hand, shrinkage reducing admixture addresses the internal effects of shrinkage. By using this additive, the shrinkage of concrete is diminished.
Aside from water reducing and shrinkage reducing admixtures, another effective additive is the shrinkage reducing admixture that creates expansive crystals. This type of additive compensates for concrete shrinkage by producing expansive crystals that counteract the shrinkage. By adding this type of admixture, concrete shrinkage can be significantly reduced.
3. Aggregate
To minimize drying shrinkage in concrete, it is important to use the optimal amount of aggregate in the mix. Aggregate stiffness plays a significant role in reducing concrete shrinkage. The higher the stiffness of the aggregate, the more effective it is in reducing the shrinkage of the concrete.
Concrete that contains sandstone aggregate is known to have a higher rate of shrinkage compared to concrete that contains granite, basalt, or limestone. In fact, the shrinkage rate of concrete with sandstone aggregate can be up to twice as high as that of other types of aggregate.
Therefore, it is crucial to use the appropriate type of aggregate when mixing concrete in order to minimize shrinkage. By using aggregates with higher stiffness, such as granite, basalt, or limestone, the amount of drying shrinkage in the concrete can be significantly reduced.
4. Protect Concrete Surface from Rapid Drying
There are a few strategies that can be employed to minimize the impact of environmental factors on concrete. One option is to cover the concrete surface until the curing process begins. By doing so, the concrete will be shielded from external factors such as rain, sun, and wind, which can compromise its integrity.
Another approach is to use ice or chilled water to lower the temperature of the batched concrete. This can be particularly helpful during hot weather when the concrete can become too hot and set too quickly, leading to potential cracking and other issues. By cooling the concrete, it can set more gradually and evenly, resulting in a stronger finished product.
In addition, temporary windbreaks can be used during windy days to decrease the velocity of the wind. This can be important because high winds can cause concrete to dry out too quickly, leading to cracks and other problems. By reducing the wind speed, the concrete can set more evenly and avoid these issues. Overall, these strategies can help to ensure that the finished concrete is strong, durable, and capable of withstanding the environmental factors that it will be exposed to over time.
5. Time of Concreting
Proper timing for concrete pouring can play a significant role in reducing concrete shrinkage, specifically plastic shrinkage. In this regard, it is advisable to pour concrete during the nighttime in summer to avoid exposing the concrete to high temperatures during the daytime. This approach can help prevent the rapid evaporation of moisture from the surface of the concrete, which occurs due to high temperatures, wind, and low humidity. Such conditions can lead to the formation of cracks on the concrete surface, negatively impacting the strength and durability of the structure.
By pouring concrete at night, the ambient temperature is typically lower, and humidity levels are higher, which slows down the rate of moisture evaporation. Additionally, the cooler temperature can provide a more conducive environment for the cement hydration process, allowing the concrete to gain strength and durability more effectively. This approach can also minimize the risk of thermal cracking, which can occur due to sudden changes in temperature. In summary, selecting the appropriate time for concrete pouring can help reduce the risk of plastic shrinkage and other issues that can compromise the integrity of the structure.
6. Proper Concrete Compaction
To avoid shrinkage of concrete, it is important to compact it properly. Additionally, concrete should not be re-tempered at the construction site, as doing so can increase drying shrinkage due to the added water content. Therefore, it is essential to ensure that the concrete is properly mixed and compacted before it is transported to the construction site. Once on site, the concrete should not be tampered with or adjusted in any way that could alter its water content or consistency. By taking these precautions, the risk of shrinkage can be minimized, which in turn can help ensure the long-term durability and stability of the concrete structure.
7. Proper Curing
Proper curing mechanisms play a crucial role in minimizing concrete shrinkage. By allowing concrete to hydrate sufficiently, adequate curing can aid in achieving the desired strength of concrete. In particular, reinforced concrete slabs can significantly benefit from proper curing as it can enhance their tensile capacity before the onset of drying.
Steam curing is a specific method that can effectively reduce concrete shrinkage. This technique involves applying steam to concrete in order to accelerate its curing process. Steam curing can be a useful approach for projects that require quick curing and reduced shrinkage.
8. Adequate Contraction Joints and Steel Detailing
Proper placement of contraction joints can play a significant role in reducing the negative impact of concrete shrinkage and controlling the development of cracks. Along with that, appropriate detailing of steel bars also holds great importance in this regard.
When concrete undergoes shrinkage, it may lead to the development of cracks, which can significantly compromise the structural integrity of the construction. However, by placing contraction joints at the correct locations, the tensile stresses caused by concrete shrinkage can be reduced, leading to a lesser likelihood of cracks appearing.
The detailing of steel bars is also crucial in ensuring that the concrete structure remains intact. Properly detailed steel reinforcement can help to reduce the amount of shrinkage-induced cracking and control the crack widths if any cracks do appear. Additionally, appropriately detailed steel reinforcement can help to redistribute the stresses in the concrete and prevent localized cracking.
Therefore, the combination of properly placed contraction joints and appropriately detailed steel reinforcement can effectively mitigate the detrimental effects of concrete shrinkage, leading to a more durable and long-lasting structure.
FAQs on how to reduce concrete shrinkage in construction site
What is the shrinkage of concrete?
Shrinkage is a phenomenon that occurs in concrete when moisture is lost from its volume. This process is characterized by a reduction in the overall volume of the concrete structure, which can lead to various issues, such as cracking and deformation. The loss of moisture can occur due to a variety of factors, including environmental conditions, curing processes, and mix design. Shrinkage can also have a significant impact on the durability and lifespan of a concrete structure, as it can affect the overall strength and stability of the material over time. Therefore, it is crucial to consider shrinkage when designing and constructing concrete structures to ensure their long-term performance and safety.
What are the types of concrete shrinkage?
Concrete is a widely used construction material that undergoes several types of volume reductions during its lifespan. The type of shrinkage that occurs before the concrete hardens is called plastic shrinkage. This type of shrinkage is caused by the evaporation of water from the surface of the concrete, which leads to a decrease in volume. On the other hand, drying shrinkage occurs when the concrete has already hardened and experiences moisture loss. This results in a reduction in volume, which can cause cracking and other forms of damage to the concrete structure.
Apart from plastic and drying shrinkage, concrete can also experience carbonation and autogenous shrinkage. Carbonation shrinkage occurs when carbon dioxide from the atmosphere reacts with the concrete’s hydrated cement paste. This reaction leads to the formation of carbonates and a decrease in the volume of the concrete. Autogenous shrinkage, on the other hand, occurs when the concrete undergoes self-desiccation, which causes a reduction in volume.
It is important to understand the different types of shrinkage that can occur in concrete to prevent damage and ensure the longevity of concrete structures. Proper curing and maintenance practices can help minimize the effects of shrinkage and extend the life of concrete.
What are the causes of concrete shrinkage?
Concrete shrinkage is influenced by several factors. The first factor is the presence of excess water in the concrete when it dries. If there is an excess of water, the concrete will shrink more as it dries, leading to cracking and other forms of damage.
The second factor that affects concrete shrinkage is the environment. Factors such as temperature, humidity, and wind velocity can impact the rate at which the concrete dries and, consequently, its shrinkage.
Concrete quality is another critical factor that affects shrinkage. The water-cement (w/c) ratio, cement content, heat of hydration, aggregate, and compaction level all play a role in determining the amount of shrinkage that will occur in concrete.
Finally, geometric factors such as the surface area to volume of concrete and depth of concrete can also influence concrete shrinkage. Smaller concrete members are more susceptible to shrinkage compared to larger structural members.
What is shrinkage reducing admixture?
A shrinkage reducing admixture is a substance used in the production of concrete to mitigate the negative effects of shrinkage. This admixture works by introducing expansive crystals into the concrete mixture, which counteract the natural tendency of the material to shrink as it dries and cures. By compensating for this shrinkage, the admixture helps to ensure that the resulting concrete is more stable and less prone to cracking or other forms of damage over time.
The expansive crystals created by the admixture work by exerting pressure on the surrounding concrete, which helps to offset the shrinkage that would otherwise occur. This pressure can be quite significant, and is typically sufficient to counteract the majority of the shrinkage that would otherwise take place during the curing process. As a result, the use of shrinkage reducing admixtures has become increasingly popular in recent years, particularly in large-scale construction projects where the risk of concrete damage is particularly high.
Overall, the use of shrinkage reducing admixtures represents an important advancement in the field of concrete production. By helping to mitigate the negative effects of shrinkage, these admixtures make it possible to produce more durable and reliable concrete structures that are better equipped to withstand the rigors of long-term use. As such, they are likely to continue to be an important part of the construction industry for many years to come.
What are the common types of concrete shrinkage?
Concrete is subject to two common types of shrinkage that can significantly impact its performance: plastic shrinkage and drying shrinkage. These types of shrinkage are known to have a profound effect on the durability and longevity of concrete structures.
Plastic shrinkage occurs during the early stages of concrete curing when the surface is still plastic and has not yet hardened. During this stage, rapid moisture loss can cause the concrete to shrink, resulting in cracks and other types of damage. This type of shrinkage is typically caused by hot and dry weather conditions or wind, which increases the rate of evaporation from the surface of the concrete.
Drying shrinkage occurs later in the curing process when the concrete has hardened and the moisture content has decreased. As the moisture continues to evaporate from the concrete, it causes the concrete to shrink, which can lead to cracking and other forms of structural damage. This type of shrinkage can also be caused by changes in temperature and humidity.
Both types of shrinkage can have a significant impact on the performance and longevity of concrete structures. It is important to take measures to minimize these types of shrinkage during the curing process to ensure that the concrete remains durable and structurally sound over time.