Air entrainment has an impact on both the workability and compressive strength of concrete. By incorporating air entrainment, the workability of concrete can be improved without significantly increasing its water-cement ratio. However, the workability of concrete and its compressive strength are inversely proportional to each other. This implies that when the workability of concrete is enhanced, its compressive strength decreases. Therefore, it is not possible to increase concrete workability to facilitate placement and compaction as it negatively affects concrete strength. To address this, air entraining admixture is used to enhance workability without increasing water content. However, the effects of air entrainment on concrete properties, specifically its strength, need to be carefully examined.
Effect of Air Entrainment on Concrete Strength
Air entrainment is a common practice in the production of concrete, particularly in cold weather conditions, to enhance the freeze-thaw durability of concrete. One of the key effects of air entrainment on concrete is its impact on compressive strength. Generally, it has been observed that the compressive strength of concrete decreases with an increase in the air content. This is mainly due to the presence of air voids that occupy space that would have otherwise been occupied by solid material, thereby reducing the overall density of the concrete.
In addition to its impact on compressive strength, air entrainment also affects the flexural strength of concrete. The presence of air voids in concrete can lead to a reduction in its flexural strength. However, this effect is not always consistent, as the impact of air entrainment on flexural strength depends on various factors such as the type and dosage of air-entraining agents used, the curing conditions, and the mix design of the concrete.
Overall, while air entrainment is an important technique to improve the durability of concrete, its effects on concrete strength cannot be ignored. To ensure optimal performance, it is essential to carefully consider the air content and other factors that can influence concrete strength and durability during the mix design process.
1. Effect of Air Entrainment on Concrete Compressive Strength
When there is a need to improve the workability of concrete without significantly reducing its compressive strength, air entraining admixture is commonly added. According to claims, the workability of air entrained concrete with a slump of 7.5 cm is superior to that of non-air entrained concrete with a slump of 12.5 cm. However, using air entraining admixture can generally result in a decrease in the compressive strength of the concrete, as indicated in Figure 1. The extent of this reduction is influenced by various factors such as the mix proportions, type and grading of the concrete, the cement used, and the specific air-entraining agent employed.
Fig. 1: Effect of air entraining admixture on concrete compressive strength
The use of air-entrained admixtures in concrete can lead to a reduction in strength of 3 to 7%. To achieve the desired compressive strength with the required amount of admixture and workability, this reduction in strength must be taken into account during the mix design process. It is recommended to conduct trial mix designs to determine the exact variation of strength with the use of air-entrained admixtures and make necessary corrections to the mix design.
For each 1% by volume of entrained air in the concrete mix, it can be assumed that there is a loss of 5% in compressive strength of the concrete. Therefore, an allowance for strength reduction should be incorporated into the mix design to estimate the water-cement ratio required for an air-entrained concrete. This includes assuming a higher target mean strength for the air-entrained mix.
In summary, when using air-entrained admixtures in concrete, it is important to account for the expected reduction in strength during the mix design process. Trial mix designs should be conducted to determine the exact strength variation, and corrections should be made to achieve the desired compressive strength. The water-cement ratio should be estimated with an allowance for strength reduction, and a higher target mean strength should be assumed for the air-entrained mix.
The given context talks about various factors affecting the compressive strength of lean concrete mix. The compressive strength is denoted by f, and it is influenced by the margin M and the percentage by volume of entrained air A.
It is further explained that the compressive strength of the lean concrete mix can be increased by ensuring maximum water reduction and using a small maximum aggregate size. By doing so, the strength of the mix can be improved.
In summary, the context highlights the importance of considering various factors in order to increase the compressive strength of lean concrete mix. It emphasizes the significance of maximizing water reduction and using small maximum aggregate size in achieving the desired strength. Additionally, the context indicates that the strength is affected by the margin and the percentage by volume of entrained air.
Fig. 2: Air Entrained Concrete
2. Effect of Air Entrainment on Flexural Strength of Concrete
Air entrainment has a less negative impact on the flexural strength of concrete compared to its compressive strength. Studies indicate that a concrete mix can have an air content of up to 4% and still achieve its maximum flexural strength. This suggests that the presence of air in the mix does not necessarily compromise its strength when subjected to bending forces.
Furthermore, research has shown that the flexural strength of lean concrete mixes can be improved by minimizing water content and using smaller maximum aggregate sizes. By reducing the amount of water in the mix, the resulting concrete is more dense and less porous, which can increase its overall strength. Using smaller maximum aggregate sizes also helps to improve the density of the concrete, which can lead to greater strength and durability in the final product. Overall, these factors can contribute to a higher flexural strength in lean concrete mixes.