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Identifying construction defects in London school

GPR was used to investigate construction detail and determine if precast hollow core units had reinforcement placed within them and been infilled with concrete in accordance with the specification.

Cracking had been observed in the roof slab of a London school, adjacent to a wall. There was concern that the problem was caused by a poor standard of construction.

The roof construction comprised hollow core pre-cast units, each containing six cores. The as built drawings showed the tops of three alternate cores broken out for a distance of 500mm and a reinforcement bar inserted and the core filled with concrete to that point. Every other core should therefore be filled with concrete for a distance of about 500mm from the supporting wall.

The client wished to establish by non-destructive means, the extent to which the cores had in fact been filled with concrete, in the vicinity of the cracks.

GPR survey method

Sandberg used Ground Penetrating Radar (GPR) to establish the presence and extent of in-fill concrete in the cores of nominated hollow core pre-cast units. A GSSI SIR 3000 Ground Penetrating Radar system was used with a 2.0GHz palm antenna. The antenna is very compact and ideal for scanning to within 45mm of an obstruction.

Due to the roof construction detail including a layer of insulation, all GPR scans were undertaken on the slab soffit. Single scan lines were collected within the nominated area up to 0.8 - 1.0m from the adjacent internal wall.

Survey findings

  • GPR detected the following variations within the hollow-cores:
  • Completely filled in with concrete
  • Partially filled in with concrete
  • Unfilled, no concrete

Results were presented on CAD drawings; the state of infill indicated by different colour hatch. The distribution of infilled cores was very haphazard and the extent of infill was not in accordance with the specification indicating a poor standard of construction. No reliable correlation between the infill concrete and crack positions could be established.

GPR also detected occasional linear features within the cores, likely to be the specified reinforcement. It must be noted however, that GPR will not reliably detect reinforcement within an empty or partially filled hollow core; this is because the strong reflection from the void is likely to mask the smaller, weaker reflection from the reinforcement. Consequently all rebar may not have been detected.