Water-reducing and set-retarding admixtures are chemicals that can be added to cement to modify its properties. These admixtures are designed to slow down the initial reaction between cement and water by reducing the rate of water penetration into the cement and slowing down the growth of the hydration products. This results in the concrete remaining workable for a longer period than it would otherwise, providing more time for placement and finishing.
The length of time during which concrete remains workable is influenced by several factors, including its temperature, consistence class, and water-cement ratio, as well as the amount of retarder used. The use of these admixtures allows for the placement of large volumes of concrete over extended periods, reducing the need for forming, placing, and joining separate units.
However, the use of set-retarding admixtures requires careful proportioning to minimize bleeding due to the longer period during which the concrete remains fresh. Bleeding is the separation of water from the cement mixture, which can cause a weakened surface layer and negatively affect the final strength and durability of the concrete. Thus, proper proportioning is crucial to ensure the desired quality of the final product.
In summary, the addition of water-reducing and set-retarding admixtures to cement can have significant benefits for the construction industry. It provides more time for placement and finishing, allowing for the construction of larger projects with reduced costs and labor. However, careful proportioning is required to minimize bleeding and ensure the quality of the final product.
Effects on Fresh Concrete
Concrete is a versatile building material that is widely used in construction. To improve its performance, several characteristics need to be addressed, such as reducing water requirements, decreasing the rate of slump loss, extending working time, improving finishing characteristics, and delaying setting time.
One of the main concerns in concrete production is the amount of water required, as it affects the strength and durability of the final product. However, it is possible to decrease the water required by at least 5% for the same slump concrete, which can lead to significant improvements in the quality of the material.
Another issue is the rate of slump loss, which can impact the workability and consistency of the concrete. Reducing the rate of slump loss can help to maintain the desired level of workability and allow for a longer working time, which is particularly beneficial in situations where finishing characteristics are challenging due to deficient aggregate properties or gradation.
Improving finishing characteristics is another critical factor in concrete production, as it can impact the overall appearance and durability of the final product. By extending working time and reducing the rate of slump loss, it is possible to achieve a smoother finish and improve the material’s overall quality.
Furthermore, delaying the setting time of concrete can also contribute to improved performance, especially in situations where lower temperatures or higher dosages are involved. However, it should be noted that excessive dosages can lead to premature stiffening, which can negatively impact the material’s properties.
In conclusion, optimizing water requirements, reducing slump loss, extending working time, improving finishing characteristics, and delaying setting time can all contribute to enhancing the performance of concrete in construction projects. These factors should be carefully considered and balanced to achieve the desired results.
Effects on Hardened Concrete
When the water-to-cement ratio (w/cm) is decreased, there is an increase in strength, but it is important to note that this increase is not equal for both flexural and compressive strength. Specifically, the increase in flexural strength may differ from that of compressive strength.
In addition, the use of an admixture to reduce the w/cm ratio can also result in a reduction of long-term shrinkage compared to concrete without the admixture.
Furthermore, as the strength of the concrete increases due to the reduced w/cm ratio, the amount of creep in the concrete is also reduced in proportion.
Dosage
ACI 212.3R specifies that when determining the appropriate dosage of an admixture, several sources of information should be considered. One such source is the results obtained from construction works where the admixture was used under good field control. Ideally, this should be done using the same materials and conditions that are expected in the current project.
Another source of information is laboratory tests that have been conducted to evaluate the performance of the admixture. These tests can provide valuable insights into the effects of varying dosage levels on the properties of the concrete.
Lastly, technical literature and information provided by the manufacturer should also be consulted when determining the dosage of an admixture. This can help to ensure that the admixture is being used correctly and that any potential risks or issues are properly addressed.
Applications
Concrete is a widely used building material, but its properties can be affected by external factors such as temperature. In hot weather, concrete can become difficult to work with, which can lead to cracking of the final structure. To mitigate this problem, an admixture is often added to the concrete mixture. The purpose of this admixture is to offset the accelerating effect of hot weather and keep the concrete workable during the entire placing period.
Maintaining workability is especially important in complex construction projects where multiple lifts are required. If the concrete loses its workability before the next lift can be placed, a cold joint may form, which can weaken the structural integrity of the final product. By adding the appropriate admixture, the concrete can remain workable until the next lift is placed, ensuring that the final structure is strong and durable.
In some cases, such as slip-forming, it may be necessary to slow down the rate of rise of the concrete. This can be achieved through the use of a specific type of admixture. By controlling the rate of rise, the concrete can be poured and shaped according to the specific needs of the project, without compromising its structural integrity. In summary, the use of admixtures is an important aspect of modern construction, allowing for greater control and flexibility in the production of concrete structures.