What concrete mix and moisture protection does a floor safe need?

Concrete mix. Use a structural concrete of roughly strength class C30/37. A simple site mix is about 3 parts gravel or grit (0–32 mm) to 1 part Portland cement (e.g. CEM I 32.5), with only the minimum water needed for a slightly creamy, just-flowable mix — it must not be too wet.

For a steel-fibre-reinforced specification, a typical dosage per cubic metre is about 500 kg Portland cement (I-42.5R), 1000 kg aggregate 3–5 mm, 650 kg sand and 75 kg steel fibres (1 × 22 mm), again with minimum water.

Moisture protection. A cast-in floor safe can act as a thermal bridge and attract condensation. Do not install it anywhere in direct contact with water, such as gardens or other wet outdoor areas. Seal the external joints with permanently-elastic silicone and tape over any electronic keypad and battery area before pouring.

Keep a moisture absorber in the bottom of the safe, and let the installation dry out fully after casting. If condensation is a known risk for the location, add proper insulation around the safe.