Holdfast Training Services Trial Blog

I was walking to my demolition site last week and noticed a peculiar pattern on a concrete slab which had been poured as part of the new retail area.

It seemed unintentional so I asked a surveyor who was there when it was poured (as it was before my time) and he told me what happened. I thought it was interesting so I thought I would share it.

The slab is constructed with top and bottom reinforcement. The mesh at the top of the slab is held in suspension by small “chairs”. The problem arose from the chairs being the wrong size and therefore pushing the top mesh higher than it should have been. As the FFL was set the mesh ate into the top cover which was only 10mm where it should have been 40mm.

When the screed beam finished the aggregate in the concrete was pushed off the mesh as it was too big to sit between the FFL and the mesh. This meant that in the exact local of the mesh, there is no aggregate and a weaker cement mixture left. This has led to local settling of the slab in the exact footprint of the mesh.

It seems that this could have been rectified by a pre pour check and if it was identified the mesh was too high then the legs could have been spread out to lower the top, the legs clipped or smaller chairs ordered.

For whatever reason this didn’t happen. The fact this has been left might mean that the floor finish might cover these sins but it will not help with the durability of this slab.