When should concrete be mixed with sand and water?

The correct timing for mixing concrete with sand and water is before hardening begins. This is crucial because concrete is a composite material that needs its components to be thoroughly mixed and hydrated in order to achieve the desired strength and durability. Once the concrete begins to harden—typically within a few hours of mixing—it becomes increasingly difficult to achieve a uniform mixture. If mixed after it has dried, the concrete will not set properly, leading to a weak and potentially unusable structure.

The other options do not align with proper concrete mixing practices. Mixing during rainy weather can lead to complications, as excess water can weaken the final product. Similarly, restricting mixing to only winter does not consider factors like temperature and humidity that can significantly impact the curing process. Therefore, timing the mixing correctly, specifically before hardening starts, ensures optimal conditions for concrete performance.