What is the recommended action if air content decreases after an increase in temperature?

Choosing to use a stronger air-entraining agent is the recommended action when air content decreases after an increase in temperature because air-entraining agents are specifically designed to create and stabilize air bubbles within the concrete mix. The increase in temperature can lead to a reduction in air content due to increased evaporation rates and changes in the viscosity of the mix. By using a stronger air-entraining agent, you can ensure that the desired amount of air is maintained, which is crucial for improving the durability and resistance of the concrete to freeze-thaw cycles.

The other options do not directly address the issue of decreased air content due to temperature changes. Adding more aggregate may impact the air content negatively or not improve it effectively. Decreasing water content typically leads to a denser mix and could further decrease air content, not promote it. Reducing mixing time could also result in insufficient distribution and stability of the air bubbles, potentially worsening the air content issue. Thus, focusing on enhancing the air-entraining properties of the mix is the most effective approach in this scenario.