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I only use a ferrous metal detector occasionally, when things do not look right when looking at a house slab, during a building inspection. Looking at this slab floor I could see cracking across several room floor areas, more than would normally be seen and not in the normal areas. The reason for the cracking was then investigated and found to be caused by the depth of the installed concrete mesh.
Concrete reinforcement mesh should be installed with a minimum of 30mm coverage from the plastic membrane below the slab, noting that the slab depth typically is around 100mm in most rooms. The ferrous detector, can detect steel mesh in a concrete slab to a depth of 80mm showing the depth on the screen or up to 100mm with audio sound. This observation reveals that the mesh was installed incorrectly.
There should be 20mm coverage at the surface of the house slab as a minimum and 30mm from the ground below the slab. This mesh was installed as close as 20mm to the ground and 80mm from the surface. This allows the slab to flex too much when the ground around the house shrinks during the warmer seasons, resulting in the floor cracking.
The trenching for the house slab I assume was excavated too deep. The steel ligatures are pre-formed off site to the engineers specifications and can not be altered. The Steel Bar Reinforcement and Mesh was installed into the excavated trench and the formwork is set to the corrected finished floor height. This resulted in the mesh being set too deep.
With care to the surrounding house areas, to minimalise ground heave and swell, the cracking may be controlled.
This house as do most newer houses, was built on a boundary. The land adjacent the exposed slab edge may not be your property, which can result in a garden or lawn sited against this wall. This would cause greater movement of the soil and increased cracking of the house floor.