The designation AC41/21/2.5 for concrete inserts typically refers to a specific type of anchor or fixing used in construction. Here's a breakdown of what each part likely represents:

1. AC41 – This is the model or series designation for the insert. Different manufacturers use different codes, so "AC41" could refer to a specific product line (e.g., Hilti, Fischer, or another brand).  
   - Example: Some brands use codes like "HSA" for heavy-duty anchors or "FIS" for Fischer products.  

2. 21 – Likely indicates the outer diameter (in millimeters) of the insert.  
   - A 21mm diameter suggests a medium-to-large anchor, suitable for substantial loads.  

3. 2.5 – Probably refers to the embedment depth (in centimeters or inches).  
   - If in centimeters: 2.5 cm (~1 inch) embedment.  
   - If in inches: 2.5 inches (uncommon for metric-based products, but possible).  

Common Uses:  
- Post-installed anchors (e.g., for securing steel structures, handrails, or machinery to concrete).  
- Heavy-duty applications (due to the larger diameter).  

Verification:  
To confirm exact specifications:  
- Check the manufacturer’s catalog (e.g., Hilti, Fischer, Mungo).  
- Look for load ratings (e.g., tension/shear capacity) and installation details (hole size, torque requirements).  

If you have a specific brand or context, feel free to share, and I can refine the answer!