Final answer:
Freezing temperatures negatively impact fresh concrete by potentially weakening its structure, reducing its impermeability, and decreasing its weather resistance due to the expansion of water upon freezing and the formation of ice within the material.
Step-by-step explanation:
The effects of freezing temperatures on fresh concrete are significant and can compromise its structural integrity. Firstly, contrary to the provided options, concrete does not gain most strength at low temperatures; in fact, the curing process of concrete is slowed down in cool conditions, and freezing temperatures can halt it completely, potentially leading to weaker concrete overall. Secondly, once concrete has frozen, it is not guaranteed to restore to its pre-frozen strength because ice formation within the concrete can lead to internal damage and compromise the structure. Therefore, frozen concrete might not regain its full strength before freezing. Thirdly, concrete that has been frozen will indeed not be as impermeable as concrete that has not undergone freezing; the expansion of water into ice can cause microcracks that lead to increased permeability, allowing more water and other substances to penetrate the concrete structure over time. Lastly, the claim that weather resistance does not change is inaccurate; concrete that has experienced freezing and thawing cycles may exhibit diminished weather resistance due to the aforementioned damage.