Shrinkage-reducing concrete admixture is an effective solution to reduce the volume change of concrete due to drying shrinkage, which occurs when the moisture in the concrete evaporates during the hardening process. This type of admixture promotes expansion of the concrete at about the same volume that normal drying shrinkage is contracting it. As a result, the net change in length of the hardened concrete is minimized, preventing shrinkage cracks.
The admixture used in shrinkage-reducing concrete typically contains polyoxyalkylene alkyl ether or similar compounds. When added to concrete during the batching stage, it can significantly reduce both the early and long-term drying shrinkage, achieving reductions of 30 to 50%. This is because the admixture treats the ’cause’ of drying shrinkage within the capillaries and pores of the cement paste, addressing the issue at its source.
It’s important to note that shrinkage reducing admixtures should not be confused with shrinkage compensating materials, which are added at above 5% on cement. Shrinkage compensating materials work by creating an expansive reaction within the cement paste to treat the ‘effects’ of drying shrinkage.
In summary, shrinkage-reducing concrete admixture is an effective solution to prevent shrinkage cracks in concrete. By treating the cause of drying shrinkage, this type of admixture significantly reduces both early and long-term drying shrinkage. However, it’s important to differentiate shrinkage-reducing admixtures from shrinkage compensating materials, as they work differently to address the issue of drying shrinkage.
Materials
Shrinkage reducing admixtures are a type of admixture used in construction that is distinct from most other admixtures in its composition. While most admixtures are water-based solutions, shrinkage reducing admixtures are primarily made up of ethylene glycol derivatives, which are organic liquids. These admixtures are typically completely active, and are soluble in water. They also have a distinct odor, and a specific gravity that is less than 1.00.
Due to their unique composition, shrinkage reducing admixtures offer specific benefits in construction applications. One of their primary uses is in reducing the amount of shrinkage that occurs as concrete cures, which can help to prevent cracking and other forms of damage. Because these admixtures are water-soluble, they are also easy to incorporate into the concrete mixture during the mixing process.
Despite their usefulness, it is important to handle shrinkage reducing admixtures with care due to their organic composition. Additionally, the distinct odor of these admixtures can make them unpleasant to work with. Nonetheless, the benefits of these admixtures make them a valuable tool in many construction applications.
Mechanism of Shrinkage Reduction
Shrinkage reducing admixtures have a unique mechanism of operation. Once concrete is placed, compacted, finished, and cured, excess water begins to evaporate from the surface. This evaporation process creates an air/water interface or “meniscus” within the capillaries or pores of the cement paste present in the concrete.
This meniscus formation has an adverse effect on the concrete’s stability, leading to shrinkage cracks. Shrinkage reducing admixtures work to counteract this process by minimizing the meniscus formation within the concrete. They achieve this by reducing the surface tension of the pore solution, which reduces the magnitude of the capillary tension.
This reduced capillary tension, in turn, decreases the meniscus formation and lowers the amount of water that gets absorbed by the concrete. The reduced water absorption results in less drying shrinkage, leading to fewer shrinkage cracks in the concrete. As a result, shrinkage reducing admixtures are critical in enhancing the durability of concrete structures.
The high surface tension of water creates stress on the internal walls of capillaries or pores where a meniscus forms. This stress pulls inward and can cause the capillary or pore to close up, resulting in a reduction of its volume. This, in turn, causes the cement paste around the aggregates to shrink, leading to an overall decrease in volume.
To mitigate this shrinkage, shrinkage-reducing admixtures are used. These admixtures work by interfering with the surface chemistry of the air-water interface within the capillary or pore, which reduces the surface tension effects. As a result, the shrinkage of the cement paste is reduced when water evaporates from within the concrete. This is because the admixtures hinder the formation of the inward pulling force that causes the capillary or pore to close up, thereby preventing the reduction of its volume and ultimately reducing the shrinkage of the cement paste around the aggregates.
Dosage
The usage rates of a certain substance can differ depending on the design of the batch and the amount of water present. Typically, the rates range from 8 to 25%. To choose the appropriate dosage, several factors need to be considered, including the desired level of shrinkage, the degree of set retardation that can be tolerated, and the curing conditions.
The amount of substance needed can vary depending on the batch design and water content. The rates generally fall within the range of 8 to 25%. The selection of the appropriate dosage relies on several factors such as the desired level of shrinkage, the acceptable degree of set retardation, and curing conditions.
To achieve the desired results, the amount of substance used should be appropriate for the batch design and the water content. The usage rates typically range from 8 to 25%. The selection of the suitable dosage depends on various factors, including the desired level of shrinkage, the degree of set retardation that is acceptable, and the curing conditions.
Effects on Fresh Concrete
Adding 2% of a certain admixture to a concrete mixture can increase its slump value without the need for any further adjustment. However, it’s worth noting that this admixture may have a slight retarding effect on the rate of hydration, which could extend the concrete’s setting time by an hour.
Moreover, this admixture can also affect the air content of the concrete mixture. If a reducing admixture and an air-entraining admixture are added together, a greater quantity of the air-entraining admixture will be needed to achieve the desired air content in the concrete.
Therefore, when using this admixture, it’s important to keep in mind these potential effects on the concrete’s properties and adjust the amount of other admixtures accordingly to achieve the desired outcome.
Effects on Hardened Concrete
Shrinkage is a common issue that affects the strength and durability of concrete. It is caused by a variety of factors including ambient conditions, mixture design, and materials used. The reduction in shrinkage is critical to ensure that the concrete maintains its structural integrity over time. Typically, a reduction of 30 to 50% is desired to minimize the effects of shrinkage.
However, the use of shrinkage reducing admixtures can lead to a reduction in the concrete’s compressive strength. In fact, studies have shown that blending 2% of these admixtures can result in a 15% reduction in strength at 28 days. To offset this effect, it is recommended to reduce the water to cement ratio or add high range water reducing admixture to maintain the desired strength of the concrete.
Thermal cracking is another issue that can arise in concrete due to temperature fluctuations. It can cause curling of concrete slabs, which can lead to serious structural problems over time. To address this issue, it is important to use materials that can withstand changes in temperature without cracking. Additionally, reducing the amount of shrinkage in the concrete can also help to minimize the risk of thermal cracking.
Overall, reducing shrinkage is crucial to ensuring the longevity and stability of concrete structures. By taking into account the factors that contribute to shrinkage and implementing strategies to minimize its effects, it is possible to create strong and durable concrete that can withstand a variety of environmental conditions.
Advantages
Shrinkage reducing concrete admixtures have the ability to minimize cracks in concrete structures. They work by decreasing joint contraction and curling, which can result in fewer cracks in structures that are susceptible to leakage. These admixtures are particularly useful in prestressing applications because they reduce the loss of prestress.
Another advantage of using shrinkage reducing admixtures is that they can offset the effects of high-shrinkage aggregates. However, in cases where the cost of using these admixtures is high, it may be more cost-effective to use good quality aggregate instead.
Shrinkage reducing admixtures are especially useful in situations where the formation of shrinkage cracks could lead to durability issues or where a large number of shrinkage joints are not desirable for economic or technical reasons. Additionally, when new concrete is used to strengthen or repair existing structures, shrinkage reducing admixtures can help to reduce the risk of cracking in what can be a highly restrained environment.
In summary, shrinkage reducing concrete admixtures offer a range of benefits when used in concrete structures. They can reduce cracking by minimizing joint contraction and curling, offset the effects of high-shrinkage aggregates, and are particularly useful in situations where the formation of shrinkage cracks could lead to durability problems or where a large number of shrinkage joints are not desirable. When new concrete is used to strengthen or repair existing structures, shrinkage reducing admixtures can help to reduce the risk of cracking in a highly restrained environment.
Applications
Architectural precast is a construction technique that involves creating building components, such as walls and facades, off-site using molds and then transporting and installing them on the construction site. This method allows for greater precision and consistency in the finished product, as well as reduced construction time.
Pneumatically applied concrete is a method of applying concrete using compressed air to force it through a hose and nozzle onto a surface. This technique is commonly used for applications where conventional concrete pouring or placing is not feasible or efficient, such as on steep slopes or in tight spaces.
Water-retaining structures are structures designed to hold or contain water, such as dams, reservoirs, and tanks. These structures are typically constructed using materials that are impermeable to water, such as concrete or asphalt, and are subject to rigorous safety and engineering standards to ensure their structural integrity.
Horizontal slab applications refer to the use of concrete slabs for flooring, roofing, and parking decks. These slabs are typically reinforced with steel bars or mesh to provide added strength and durability. They are commonly used in a variety of construction projects, from residential homes to large commercial buildings, due to their cost-effectiveness and ease of installation.