Plant hire depot surfacing - why depot yards fail in the same three places

A yard looks like one continuous surface, but it never takes one continuous load. Traffic in a depot is channelised. Machines come off the low-loader, follow the same line to the same standing area, and go back out the same way. Loading happens in the same bay because that is where the space and the access are. Everything gets washed in the wash bay, because that is where the drainage went in.

So while the yard as a whole sees moderate use, three narrow zones absorb a hugely disproportionate share of the work. If a yard were simply old and worn you would expect even deterioration. You do not get that.

You get three concentrated failures with sound concrete in between them, which tells you the surface is not the problem.

The three failure points behind almost every depot yard breakdown

• The routes the machines drive. Not the weight, the way the load is applied. Tracks do not steer, they slew. One track drives while the other brakes, dragging steel grousers sideways across the surface, and concrete is far weaker in surface shear than it is in compression. Add telehandlers running high axle loads on comparatively small tyres, and outrigger pads concentrating several tonnes through a few hundred square centimetres, and you get fretting and spalling in narrow bands that follow the running line.
• The low-loader loading point. The most demanding square metre on the site. Every machine out and every machine back crosses the same spot, and a machine coming off a beavertail transfers weight abruptly, so the slab takes repeated impact rather than the static load it was designed for. Loading points also tend to sit at a slab edge or hard against a joint, where capacity is roughly half to two thirds of the middle of a bay (Concrete Society TR34). The heaviest loading in the yard lands on the weakest part of the slab.
• The wash bay. A Type 1 sub-base works because it is well graded, compacted and able to drain. Keep it saturated and it stops behaving like sub-base. Settlement then flattens the falls, so water that used to reach a gully sits in a depression instead and the saturation accelerates. Meanwhile silt washed off the plant blinds the gullies and clogs the separator, so the water goes across the yard rather than down the drain.

What happens underneath once the surface breaks

The damage you can see is the last stage, not the first. Two things are going on below it.

First, the sub-base starts pumping. Water gets in through a joint or a crack, and repeated loading pumps it back out again carrying fine material with it. Over months that migration of fines creates voids under the slab. The concrete has not got weaker, but it has lost its support, so it starts failing under loads it carried without complaint for years. There is a simple test for this: watch a joint as a loaded machine crosses it, and if water or grey slurry squeezes up out of it, the sub-base underneath is voided.

Second, patching makes it worse. A concrete slab is a load-spreading layer, and it only spreads load if something solid is holding it up. Break out four square metres of failed concrete, pour new concrete into the same hole, and you have replaced the part that was working and left the part that had failed. The patch is now a small isolated slab with four free edges and no load transfer to the concrete around it, which is the weakest thing you can build. That is why patches at a loading point often go faster than the original surface did.

An hour with a camera will tell you most of what you need to know. Look for:

• Water or grey slurry squeezing out of a joint as a loaded machine crosses it
• Slabs that rock or step at the joints, particularly near the loading point
• Fretting, polishing or spalling in narrow bands that follow the machine route
• Damage clustered within ten metres of where machines come off the low-loader
• A wash bay still holding water twenty minutes after the wash has finished
• Gullies and silt traps that are full, or a separator nobody can remember servicing
• Previous repairs that are failing at their own edges
• A dated photo log of all of the above

The question a proper assessment then answers is whether each defect is surface-level or structural. Damage over sound support is a repair. Damage over a failed sub-base is the start of something bigger, and patching the surface only buys the failure a few months.