Can I use gravel as fill between concrete slabs?

Question

The second floor of an old 3000sq.ft. barn has shifted and sunk. There is a 15" difference between the highest and lowest point of the floor. I want to poor a new concrete layer, but I'd rather it be as thin as possible. Can I put a base of gravel first to level the ground below the concrete? Are there any special consideration in comparison to a slab poured at ground level?

Thank you for all replies

Edit following comments:

The structure is made of thick stone walls (ca. 2ft) and the second floor is supported from below by stone columns connected with arches and spaced at every 10 ft.

Because of the hilly terrain, the second floor is also at ground level on one side, so vehicles can enter the second floor directly.

The floor of the second floor is 3ft deep, made of stones packed with some type of cement (not concrete) and supports 5 ton tractors going in and out.

The sinking that took place seems to be the result of earth movements. I'm waiting for an expert report on that.

Answer 1

As folks have commented, this is absurdly wrong-headed and more likely to cause complete failure than to correct your barn floor.

There are folks that specialize in jacking barns back into shape. This is a process that seems simple, but has enough complexities that can fall down and kill someone to be considered "best hired out to an experienced firm" as opposed to a great DIY opportunity. You want someone who has done this before, many times, who has the right tools and judgement. Doing it wrong can break beams catastrophically.

In a general sense, they will provide some temporary cribbing or shoring and gradually jack up the sunken parts, then provide new foundations to hold those parts in place. Upside, it's much less likely to destroy your barn (and if it's too far gone, they'll most likely tell you that and refuse the job) and it will probably cost a lot less than your proposal to fill the second floor with concrete & gravel, as sunken (much less the probable cost of breaking your barn but good in the process, and possibly killing someone in the process as well.)

Pouring concrete on upper floors is exceedingly common in properly designed and supported steel frame structures, and is usually done on a base of corrugated steel attached to the framing. But that depends on the building framing being stable, properly supported, and designed for the load - none of which appear to apply to your barn from the limited description in the question.

Answer 2

Yes. Yes you can put gravel under concrete, even over other concrete. In fact, that's probably the right way to do it as you add some degree of movement tolerance by doing so. Note that "as thin as possible" is probably no less than 6" for your loads.

As to why you probably shouldn't, see Ecnerwal's answer. Your question updates don't ease my concerns about adding many tons of weight to an already questionable structure. 15" of slope in a fairly small barn indicates severe instability.