Shrinkage is a natural characteristic of concrete that occurs as a result of various factors leading to the loss of moisture at different stages. This phenomenon can be described as the alteration in volume that is observed in concrete. Throughout its lifecycle, concrete undergoes changes in moisture content, and as a consequence, it experiences shrinkage. This inherent property manifests as a reduction in size and volume, which can occur due to a variety of reasons. These causes can include environmental conditions, hydration processes, or the presence of certain materials within the concrete mixture. Ultimately, the occurrence of shrinkage in concrete is a result of the moisture loss that takes place at different stages, leading to noticeable volume changes.
Types of Shrinkage in Concrete
Shrinkage is a phenomenon that occurs in various materials, and it can be classified into different types depending on the specific circumstances. In the case of shrinkage in construction materials such as concrete, there are several classifications that are commonly used.
One type of shrinkage is known as plastic shrinkage. Plastic shrinkage occurs in concrete during its early stages of curing, typically within the first few hours after placement. It happens when the water content in the concrete evaporates at a faster rate than it can be replaced by bleeding or hydration. This rapid loss of water causes the concrete to shrink, leading to potential cracking and surface defects.
Drying shrinkage is another type of shrinkage that occurs in concrete. It happens over a longer period as the concrete continues to cure and moisture gradually evaporates from the material. Drying shrinkage can be influenced by factors such as the mix design, ambient humidity, and temperature. As the moisture content decreases, the concrete undergoes volume reduction, which can result in cracking and deformation if not properly controlled.
Autogenous shrinkage is a unique type of shrinkage that is not influenced by external factors like moisture loss or temperature changes. It occurs as a result of the chemical reactions happening within the concrete itself during hydration. Autogenous shrinkage happens when the hydration process consumes more water than what is available, leading to internal volume reduction. Although it is generally less significant than other types of shrinkage, it can still contribute to the overall deformation of the concrete.
Lastly, carbonation shrinkage refers to the shrinkage that occurs due to carbon dioxide (CO2) exposure. When carbon dioxide penetrates the concrete and reacts with the calcium hydroxide present in the cement paste, it forms calcium carbonate. This reaction leads to a reduction in the volume of the concrete, resulting in carbonation shrinkage. The extent of carbonation shrinkage depends on various factors, including the carbon dioxide concentration, concrete porosity, and exposure conditions.
In summary, shrinkage in construction materials, particularly concrete, can be classified into different types based on the specific mechanisms involved. Plastic shrinkage occurs during the early stages of curing, while drying shrinkage happens over a longer period as moisture evaporates. Autogenous shrinkage is a result of the internal chemical reactions within the concrete, and carbonation shrinkage is caused by carbon dioxide exposure. Understanding and managing these different types of shrinkage are crucial for ensuring the durability and integrity of concrete structures.
Plastic Shrinkage in Concrete
During the concrete casting process, it is common to observe the escape of water necessary for the strength development of the concrete. This water evaporates from the surface of the structure, leading to the formation of cracks. Another factor contributing to shrinkage cracks, specifically of the plastic shrinkage type, is the absorption of water from the concrete by the aggregates. As a result, the presence of aggregate particles or reinforcement can hinder the settling process, potentially causing cracks to form on the surface or internally around the aggregates.
In scenarios such as floors and pavements, where a large surface area is exposed to the drying effects of the sun and wind, rapid drying occurs, resulting in plastic shrinkage. Furthermore, in mix designs with a high water-cement ratio, there is a possibility of excessive water pathways leading to bleeding. This excess water, accumulating on the surface of the slabs, dries up and collapses when exposed to dry weather conditions, ultimately creating cracks.
Prevention of Plastic Shrinkage
To prevent plastic shrinkage in concrete, various remedies can be applied. One effective measure is to cover the surface with polyethylene sheeting, which acts as a barrier to prevent the escape of water. By inhibiting water evaporation, the occurrence of plastic shrinkage can be minimized.
Another preventive measure involves ensuring proper vibration of the concrete. Through appropriate vibration techniques during the pouring and placement process, the concrete is compacted uniformly. This compaction helps to reduce the risk of plastic shrinkage.
Additionally, the use of aluminum powder can aid in mitigating plastic shrinkage in concrete structures. The inclusion of aluminum powder in the concrete mix modifies its properties, improving its ability to resist shrinkage.
Lastly, the utilization of expansive cement can contribute to controlling plastic shrinkage. Expansive cement has the unique characteristic of expanding during the curing process, counteracting the potential for shrinkage. This type of cement can be employed strategically to minimize the occurrence of plastic shrinkage in concrete structures.
By implementing these remedies, such as covering the surface, proper vibration, incorporating aluminum powder, and utilizing expansive cement, the detrimental effects of plastic shrinkage in concrete can be effectively reduced.
Drying Shrinkage in Concrete
Drying shrinkage in concrete is a phenomenon characterized by the loss of water absorbed on the surface of the calcium silicate hydrate (C-S-H) gel and the decrease in hydrostatic tension within the small pores. Conversely, swelling represents the opposite effect to shrinkage. The primary cause of shrinkage is the deformation of the paste, although the stiffness of the aggregates can also influence it. This process occurs after the concrete has set and is referred to as drying shrinkage. Typically, most drying shrinkage takes place during the initial months of the concrete structure’s lifespan.
Drying shrinkage occurs as water is withdrawn from the concrete, which is stored within the unsaturated air voids. This loss of water results in shrinkage. However, it is possible to recover a portion of this shrinkage by immersing the concrete in water for a specified period, which is known as moisture movement. The calculation of moisture movement can be done using Schorer’s Formula, where the shrinkage strain (Es) is determined by multiplying 0.00125 with the difference between 0.90 and the relative humidity (h) as a fraction.
The rate at which drying shrinkage occurs decreases over time. Within two weeks, approximately 14 to 34% of the shrinkage is observed, while within three months, the range increases to 40 to 70%. Within the span of one year, nearly 80% of the shrinkage would have taken place. These figures highlight the significant extent of drying shrinkage that can occur within the early stages of a concrete structure’s lifespan.
Factors Affecting Drying Shrinkage
Drying shrinkage in concrete is influenced by several factors. The first factor is the selection of materials. It is crucial to choose high-quality ingredients for the concrete mix, adhering to the specified standards of the region. The quality and specifications of the ingredients greatly impact the potential for drying shrinkage.
The water-cement ratio is another significant factor. A higher water-cement ratio in the concrete mix increases the chances of drying shrinkage. As the water-cement ratio increases, the strength and stiffness of the paste decrease, leading to greater shrinkage.
Environmental conditions also play a role in drying shrinkage. The relative humidity of the site affects the concrete structure’s shrinkage. Increased humidity in the environment results in reduced shrinkage.
The cement content in the mix influences the rate of shrinkage. Higher cement content correlates with increased shrinkage.
The type and size of aggregates used in the mix also affect drying shrinkage. Increasing the maximum size of aggregates decreases shrinkage, while the aggregate grading and shape have minimal influence. Aggregates with a rough surface tend to resist shrinkage.
The type of cement used is another factor. Different types of cement exhibit varying shrinkage characteristics. Rapid hardening cement, for example, leads to more shrinkage compared to ordinary Portland cement due to its faster hardening properties. Shrinkage compensating cement can be used to reduce or eliminate shrinkage cracks.
The addition of admixtures can impact shrinkage. Calcium chloride, when used as an admixture, increases shrinkage. However, replacing it with lime can decrease the rate of shrinkage.
The size and shape of the concrete specimens also play a role. The surface-to-volume ratio influences the rate and magnitude of shrinkage. Increasing the surface-to-volume ratio decreases the rate of shrinkage.
Lastly, certain factors like the method of steam curing have minimal effect on shrinkage, except when conducted at high pressures.
Autogenous Shrinkage in Concrete
Volume changes can occur in concrete structures even after they have been set. These changes can manifest as either shrinkage or swelling. In the presence of water, ongoing hydration can lead to expansion of the concrete structure. However, in the absence of moisture, the concrete can experience swelling. This particular type of shrinkage is a consequence of water being drawn out from the capillary pores within the concrete. This water withdrawal is a result of the hydration process, which requires water for the hydration of cement. The phenomenon of water being extracted from the capillary pores to facilitate cement hydration is known as self-desiccation. Autogenous shrinkage, or autogenous volume change, refers to this shrinkage that occurs within the internal regions of the concrete element. The magnitude of autogenous shrinkage typically falls within the range of 100 x 10-6.
Factors Affecting Autogenous Shrinkage
The rate of autogenous shrinkage, which is associated with the hydration process, is influenced by temperature. As the temperature rises, the volume change caused by autogenous shrinkage becomes more pronounced.
The amount of cement content in a concrete mix also affects autogenous shrinkage. Regardless of the water content in the mix, higher cement content leads to increased hydration. Consequently, the volume change resulting from shrinkage is amplified.
Moreover, the composition of the cement used plays a role in autogenous shrinkage. Certain types of cement with a high concentration of tricalcium aluminate (C3A) and tetra calcium alumino ferrite (C4AF) promote greater autogenous shrinkage by facilitating the formation of hydration products.
The inclusion of mineral admixtures in concrete can further contribute to autogenous shrinkage. For instance, mineral admixtures like fly ash provide a larger surface area for increased reactions and the formation of high-quality hydration products. This, in turn, necessitates more water from the capillary pores, resulting in an increased volume change in the concrete due to shrinkage.
Carbonation Shrinkage in Concrete
Concrete casts are prone to undergo a reaction with atmospheric gases such as carbon dioxide, particularly when moisture is present. This reaction leads to the formation of carbonates within the concrete. A significant byproduct of the hydration process in concrete is calcium hydroxide, which is often abundant within the material. When calcium hydroxide interacts with atmospheric carbon dioxide, it undergoes a reaction and converts into calcium carbonates. Consequently, the surface of the concrete becomes carbonated or acidic in nature. This phenomenon is known as carbonation and is characterized by shrinkage that becomes noticeable on the surface. Carbonation shrinkage tends to manifest in areas with moderate humidity conditions. These effects typically become evident over time during the operational lifespan of the concrete structure.
The carbonation process in cement leads to the decomposition of certain compounds, causing the formation of carbonates. These carbonates have the beneficial effect of filling up the pores within the cement, resulting in a decrease in permeability. As the permeability decreases, the strength of the cement increases. However, when shrinkage is constrained either partially or completely, whether from internal or external factors, cracks can develop. These cracks occur due to the tensile stresses that arise as a result of the restraints on the cement’s shrinkage. To address this issue, appropriate joints can be incorporated into the structure during its casting to accommodate contraction and expansion movements. This type of shrinkage also facilitates the close grouping of steel reinforcements, thereby enhancing the bond between the steel and the cement.