What actually crosses the floor
A barn floor earns its thickness from what stands and moves on it, not from how the barn looks from the road. A dry hay barn where nothing heavier than a person walks across it is a different job than a working barn where a tractor comes and goes, a loaded trailer gets parked and unhooked, and livestock cross the same alley every day.
That difference is the first thing to settle before anyone talks thickness. Tell us what the barn is actually for and what you expect to be doing in it a few years from now, not just this spring, because the floor is cheaper to build right the first time than to thicken later.
A barn that starts as hay storage and later gets a tractor parked in it is not unusual. If there is any real chance of that, it is worth building to the working floor standard from the start rather than pouring thin and living with the limits of it later.
- A tractor or skid steer working the alley or a bay
- A loaded trailer tongue set down and left in one spot
- Livestock traffic and hoof point loads on a stall floor
- Stored equipment parked in the same footprint season after season
The base gets settled before the thickness does
Concrete is strong in compression and weak in bending, so a slab sitting on soft or uneven ground will crack and drop a corner no matter how thick it is poured. Around here the ground under it is usually red clay, and clay hangs onto water and then lifts and drops on you as it freezes and thaws.
The fix is the same one we use under a shop floor. Topsoil and any soft pockets come off until we are standing on ground that does not give underfoot. The grade climbs back to elevation in crushed stone, packed a few inches at a time on the way up, because a single deep layer with one pass over the top of it reads as compacted from above and is not.
Five inches for a working floor, four for storage only
Five inches is our standard for a floor that carries real point loads, the same thickness we use on a garage or shop floor. The inch past ordinary flatwork earns its place twice over. It widens the footprint of anything concentrated, a loaded hoof or a narrow implement wheel, so nothing gets to work on one small patch of slab, and it opens up enough depth to set steel at a height where the steel is worth having.
Four inches is fine for a barn used strictly for dry storage, where the heaviest thing crossing the floor is a person carrying feed. Where one part of the barn takes something heavier than the rest, a stanchion line, a corner where a loaded trailer always sets down, we thicken that spot in the forms rather than pouring the whole floor to the worst case.
Deciding that during the pour costs very little. Deciding it after the floor is finished means saw cutting a hole where the heavy point turned out to be and patching around it, and a patch never blends in the way the rest of the floor does.
Reinforcement and the mix underneath
Five inches is deep enough that rebar or wire mesh can be carried up into the middle third of the pour, which is the only place it does anything. In a thin slab the steel usually ends up lying on the base, along for the ride. Strength gets matched to what the floor will carry, which on barn work is commonly a 4,000 PSI air entrained mix. Barn doors stand open most of the year, so the slab lives through the same Virginia winter the yard outside does.
Control joints get cut into the floor on a grid once it is placed. Concrete shrinks as it cures, and the joints decide where that shrinkage shows up, a straight line we put there on purpose instead of a crack that wanders across the middle of a stall or an alley.
Pouring inside a barn that is already standing
A good share of the barn floors we pour go in after the building is already up, a pole barn that has stood a season or two on gravel, or an old dirt floor barn finally getting concrete. A ready mix truck can only reach so far in with a chute before somebody is pushing wheelbarrows the rest of the way.
That is the job our own trailer line pump handles. We run 2.5 and 3 inch line through the door, right to where the concrete goes, so the truck stays outside on solid ground and the floor gets placed in order at one steady pace instead of racing a load that is stiffening before the far end is filled.
It matters most where a mixer truck has no shot at getting close at all, a barn up a bank behind the house, one at the end of a soft field lane, or set back past a gate. Hose goes places a chute cannot, and the floor still gets placed in one pass.