Introduction:
Air entrainment is the intentional introduction of microscopic air bubbles into concrete during mixing. These air bubbles act as a lubricant, improving workability and reducing bleeding and segregation. Additionally, air entrainment enhances the freeze-thaw resistance of concrete by providing space for the expansion of water during freezing.

Effect of air entrainment in concrete:
Let's examine each of the statements and determine if they are correct:

1. Increase resistance to freezing and thawing:
This statement is correct. When water freezes, it expands by about 9%. If this expansion is restrained, it can cause internal pressure within the concrete, leading to cracking and deterioration. Air entrainment creates small voids in the concrete that provide space for the expansion of water during freezing. These voids act as pressure relief valves, reducing the risk of damage due to freezing and thawing cycles.

2. Improve workability:
This statement is correct. Air entrainment enhances the workability of concrete by reducing the surface tension of water. The microscopic air bubbles act as a lubricant, allowing for easier mixing, placing, and finishing of the concrete. This improved workability makes it easier for the concrete to flow and consolidate, resulting in a more uniform and durable structure.

3. Decrease strength:
This statement is incorrect. Air entrainment does not necessarily decrease the strength of concrete. The strength of concrete is primarily influenced by factors such as the water-cement ratio, aggregate properties, and curing conditions. While air entrainment can slightly reduce the compressive strength of concrete, it is typically within acceptable limits and does not significantly impact the overall strength or performance of the concrete.

Conclusion:
In conclusion, the correct statements regarding the effect of air entrainment in concrete are 1. Increase resistance to freezing and thawing and 2. Improve workability. Air entrainment does not decrease the strength of concrete. It is important to carefully consider the air content requirements specified in the design and construction of concrete to ensure the desired freeze-thaw resistance and workability are achieved.