# How much concrete is required to hold 125 lbs/sqft permanently?

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.

1. Concrete strength does not appear to be an issue. With a minimal 2500 psi concrete the entire safe could bear on one square inch.

2. 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.

3. 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.

• In this case the main foundation is supported by 7' pin piles because the soil wasn't sufficient to bear the load of the single story garage. How does that change the equation? Jul 25, 2014 at 21:20
• In my opinion, that takes it entirely out of a homeowner project. There is no way to do it correctly that does not involve domain specific engineering analysis due to the potential of the new loads to adversely impact the repaired foundation.
– user23752
Jul 26, 2014 at 5:02

Unless you have a particular reason to think your garage is sinking into the ground, I wouldn't worry about this much weight. Presumably the existing asphalt floor can support the weight of a car, which has more than 500 lbs of weight per tire (concentrated on a much smaller contact area than your safe).

Besides, what's the worst that could happen? If in a few years you notice that the floor is compacted a bit, you could always dig up the asphalt and pour concrete at that point.

• Yes indeed the garage had been sinking near that area. Both the concrete footings and asphalt sunk, but the footings were repaired. Jul 25, 2014 at 22:24
• @Sophit: If the soil is so poor that the entire floor is sinking, you may have to deal with that first. Even a 1' thick concrete slab will sink if the soil can't support the weight.
– Hank
Jul 26, 2014 at 4:38