| GPR Used To Aid In The Engineering Assessment Of A Building Post Formwork Failure |
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Ground Penetrating Radar (GPR) was deemed the best tool for verification of reinforcement deformation and displacement of the Post Tensioning (PT) ducts. A section of the slab was concealed from inspection below due to the design placement of dwarf support concrete block walls. The PT ducts in this section of the slab had blind ends. Thus visual inspection for end block movement was not possible. Inspection of the exposed end blocks revealed little movement of the blocks at the visable slab edge. Alpha Geoscience conducted a Ground Penetrating Radar (GPR) investigation of the slab. Due to the high moisture content and thickness of the slab a full range of antennas were utilised for the investigation. Images 1 and 2 below are of two of the antennas used in this investigation.
As the PT ducts were placed immediately beneath the reinforcement mesh, mapping the depth of cover of the mesh reinforcement would also evaluate the displacement of the ducts. Images 3 and 4 below are from the same transect collected across the slab. Image 3 is a processed data profile collected with a 1.6GHz antenna and Image 4 is the same profile collected with a 500MHz antenna. As can be seen the 1.6 GHz antenna did not penetrate sufficiently to allow mapping of the reinforcement mesh depth of cover. The images of the GPR data profile display the displacement of the anti –cracking mesh within the slab.
Mapping of the PT ducts was not completely successful. Where the higher resolution antennas could penetrate to a sufficient depth it was found that the PT ducts were just below the reinforcement mesh. Mapping of the depth of cover allowed a contour map of the depth of cover of the reinforcement to be produced, see Image 5 below. This revealed two areas of major displacement of the reinforcement mesh. Thus it was inferred that the PT ducts had also been displaced to a similar extent.
The position of the PT ducts at the two column sites were checked and confirmed. The depth of the PT ducts and the reinforcement around the columns show little displacement from planned placement. There is evidence that the PT ducts are at varying depths within the slab. The DoC of anti-cracking mesh appears to be highly variable across the slab. The depth of cover varied from 30mm to over 510mm. As the PT ducts were placed beneath the mesh the only assumption that can be drawn is that the PT ducts are also at variable depths. The PT ducts were placed below the mesh and as a consequence must also now have varying depths of cover across the slab. As the ducts contain strands that were pre – tensioned, the amount of movement of the slab must pose the question as to the location of the ‘blind’ end blocks within the slab. GPR investigation of the slab enabled the mapping of the reinforcement mesh depth of cover and consequentially the probable depth range of the PT ducts. From this assessment it was concluded that substantial movement of the blind end blocks had occurred. The use of GPR to gain this information allowed an engineering assessment to be made as to the viability of the structural integrity of the building.
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A failure in formwork and theresulting collapse of blind shoring of a ground floor slab, during the concrete pour, resulted in thickening of the slab.
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