Novel ground penetrating radar approach for RAAC end bearing examination
Project challenges
Reinforced Autoclaved Aerated Concrete (RAAC) has been used extensively within both public and private building estates within the UK between 1950-80. Since 2019, several high profile RAAC plank collapses in schools has raised awareness of a widespread infrastructure crisis where the structural integrity of RAAC requires re-evaluation.
Business challenge
Process Innovation
Sector
Construction
Technology or capability
Metrology & NDT
Simulation & Modelling
The Challenge
Reinforced Autoclaved Aerated Concrete (RAAC) has been used extensively within both public and private building estates within the UK between 1950-80. Since 2019, several high profile RAAC plank collapses in schools has raised awareness of a widespread infrastructure crisis where the structural integrity of RAAC requires re-evaluation.
Current RAAC surveys consider the end bearing length of RAAC planks (overlap of a plank on a ceiling support) as well as the position of transverse reinforcement over the hidden end bearing as critical to assessing structural integrity. To measure these features however currently requires intrusive surveys to be conducted which are slow, manual, disruptive to estates and can present significant health & safety risks (i.e. if required to disturb asbestos containing coatings). There is therefore a need to a rapid, non-destructive alternative to this approach to allow for a scalable inspection solution to address the size of the UK’s RAAC issues.
MTC's Solution
The MTC has developed a novel angled beam Ground Penetrating Radar (GPR) inspection methodology for RAAC examination to identify and estimate the positions of features within the hidden bearing region of RAAC roofing planks. The method required the careful design of a dielectric filled wedge, informed by simulation studies, to hold a small handheld GPR sensor at a fixed angle relative to the surface of RAAC planks to successfully interrogate the difficult-to-access end bearing region of planks, with analysis based on time-of-flight and Fermat’s principle calculations.
The approach was experimentally trialled on in-situ RAAC ceiling planks within an active hospital estate with the results validated by conventional intrusive surveying techniques, demonstrating good agreement with the GPR results. GPR was also shown to be effective in measuring the thickness of RAAC planks and detecting water ingress.
The work completed by the MTC has created an innovative solution to the UK’s RAAC challenge with a new tool to help industry to rapidly assess the structural integrity of RAAC planks accurately, non-destructively, with minimal disruption and full digital traceability.
Dr Joshua Elliott - M&NDT Technology Manager, MTC
The Outcome
- This non-destructive testing (NDT) approach takes seconds to complete a measurement on a RAAC plank, meaning that far more planks can be quantitatively assessed within a given inspection timeframe to provide more certainty in the safety of estates. The current design of the system was able to achieve a positional tolerance of 13 mm for features including the plank end bearing length and position of end bearing transverse reinforcement.
- The current design of the system was able to achieve a positional tolerance of 13 mm for features including the plank end bearing length and position of end bearing transverse reinforcement.
- GPR was shown to be able to detect water ingress within RAAC panels and is not reliant on complete permeation of the full plank thickness to be able to identify seepage as current visual techniques require.
Benefits to RAAC Estates
- The novel GPR approach is completely non-invasive, thus require no remediation works post-inspection, do not generate any dust or noise, and would be safer to deploy in environments containing asbestos.
- The inspection performance achieved can allow for rapid screening of RAAC assets within estates to quickly establish higher risk planks for further detailed intrusive assessment and a more comprehensive safety analysis.
- NDT provides an additional benefit of providing full digital outputs that are traceable and can be incorporated easily within a preventative maintenance approach for RAAC estates.
- Potential of system to be deployed from the ground (i.e. using telescopic pole) thus reducing working from height instances.
