I have an garage with asphalt floor and concrete footings. I plan to put a 500 lb safe in the corner, partially on the footing. That's about 125 lbs per square foot including contents, permanently.
To make a level base, I plan to dig up that patch of asphalt and pour high strength concrete.
What depth and quality of gravel do I need under the concrete pad to safely hold this weight? What depth of concrete do I need?
Here are some additional details. The corner of the garage actually has a U shaped concrete footing with asphalt in the middle. The gap between the footings is too large for the safe. The portion of the footing holding up the garage wall has been repaired and is solid. The unused/bare footing and the asphalt is known to be weak due to insufficient base.
Best Answer
Base on your description:
Concrete strength does not appear to be an issue. With a minimal 2500 psi concrete the entire safe could bear on one square inch.
The size of the footing depends on the bearing capacity of the soil. Suppose the soil has a bearing capacity of 20 pounds per square foot. Then the foundation would need to be 25 square feet. Other soil bearing capacities would require larger or smaller minimum footings.
Placing a new foundation near an existing often has substantial engineering implications. The forces overlap on a portion of the soil because the bearing capacity of a soil is a function of it's ability to distribute loads. Consultation with a professional engineer may be in order.