When must mixing begin, according to contractor responsibilities?

Mixing must begin 30 minutes after the cement is added to the aggregate to ensure that the concrete mixture achieves the desired workability and consistency. This timeframe is crucial because once the cement is mixed with water and aggregates, hydration begins. If mixing is delayed beyond this period, it can negatively impact the quality of the concrete.

Starting the mixing process after this designated interval allows for the chemical reactions involved in curing to happen correctly, reinforcing the integrity of the mix. This approach is standard practice as it balances the need for proper mixing while controlling for the setting times of the cement.

The other timeframes mentioned in the options either imply starting the process too early or are beyond the effective working time for concrete before it begins to harden. Therefore, initiating mixing immediately after adding aggregates or without adequate time for proper integration of materials could result in a suboptimal concrete mix.