GPR Investigation: Post Tension Cable Mapping

Transcription

1 CMD Civil Pty Ltd PO Box 1119 Huntingdale VIC info@cmdcivil.com Case Study: GPR Investigation: Post Tension Cable Mapping This application note demonstrates an example of locating post tension cables within a suspended concrete structure utilizing Ground Penetrating Radar (GPR) systems. Successful detection is a combination of experienced technicians and when available as-built plans to help assist in the verifying and interpretation of data collected. Due to our legal right of obligation, some content from this survey i.e. Client / Company name and the location this survey took place, cannot be disclosed. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 1

2 Scope: CMD was engaged to carry out non-destructive testing works utilising Ground Penetrating Radar (GPR) systems to identify the following aspects of a suspended slab structure prior to remediation works commencing. a. Post-Tension cable position in relation to drilling zones. b. Band beams locations Project Background: The premises that consisted of a large outside terrace area, restaurant and catering facilities all of which sat upon a suspended slab structure was about to undergo a multi-million dollar refurbishment. Existing as-built plans were available for the structure however the detail was poor with no exact offsets to the post-tension cables indicated. Date of Investigation Works: 15 th March 2013 CMD Site Technician / Representative: Steve Shaw Position Held within Company: NDT Business Manager / Concrete Scanning Technician Experience: 5 years onsite experience operating Mala and US Radar GPR systems for both structural and subsurface investigations. Trained by Mala GPR Australia. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 2

3 Location of Survey: Survey location approximately 625 Sq. metres CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 3

4 GPR System Utilised: The following ground penetrating radar (GPR) instrumentation was made available for the site survey. Mala CX11 Operating System. Mala 1.6GHz Shielded Antenna. The 1.6 GHz with EM antenna is a high resolution antenna used for high quality radar measurements, with medium penetration in the HF range. It is commonly used for concrete, reinforcement studies and for road mapping and quality assurance to depths of approximately 350mm 400mm. GPR Method: GPR works by sending a tiny pulse of energy into a material and recording the strength and the time required for the return of any reflected signal. A series of pulses over a single area make up what is called a scan or 2D profile. Reflections are produced whenever the energy pulse enters into a material with different electrical conduction properties or dielectric permittivity from the material it left. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants and conductivities of the two materials. This means that a pulse which moves from dry sand (low dielectric) to wet sand (higher dielectric) will produce a very strong reflection, while one moving from dry sand (low dielectric) to limestone (which has a close dielectric value to dry sand) will produce a very weak reflection. While some of the GPR energy pulse is reflected back to the antenna, energy also keeps travelling through the material until the pulse signal energy dissipates (attenuates). The rate of signal attenuation varies widely and is dependent on the dielectric properties of the material through which the pulse is passing. The resolution and investigation depth is dependent on the antenna frequency used; most often ranging from 10 MHz to 3 GHz. High frequency antennas give high resolution images but shallow depth, and low frequency antennas gives better penetration depth. Methodology: As the site was still operational and patrons were still utilising the facilities, access to the required zones was limited at times. Typically for a structure of this size the area would be cleared and scanned in grid spacing s of approximately 1000mm. As this could not be undertaken only isolated sections could be surveyed one at a time, with the positions of the post-tension cables plotted with marking tape on the carpeted or tiled slab surface. Once all of the surface areas were surveyed and all post-tension cables and band beam extents were successfully identified the offsets to the cable locations were measured off permanent fixtures and plotted into an AutoCAD plan. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 4

5 Result of Investigation: As the suspended slab structure contained minimal amounts of reinforcing the post-tension cables themselves were displayed very clearly, along with the extents of the band beams. The true centre point of each post-tension cable was plotted with the minimum allowable distance to drill away from the cable to be specified by the Structural Engineer on the project. Due to the depth and limited access over the band beam structures not all post-tension cables could be clearly identified, however as these were serious structural components of the building the Engineer did not intend to compromise them to any degree. By providing the results in an AutoCAD format the Designers and Architects were able to alter their design and position the new fixtures to be installed on the suspended slab around or away from any post-tension cables within the structure. See following plan layout of the post-tension cables on page 6. Post Tension Cables represented as green lines CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 5

6 CMD Civil Pty Ltd PO Box 1119 Huntingdale VIC ABN: DATE: LEVEL 1 SUSPENDED SLAB / POST-TENSION CABLE LAYOUT CLIENT: ABEO ARCHITECTS PROJECT ID: CMD Suite 2/8 16 Soudan Lane, Paddington NSW PROJECT: Cobram Barooga Golf Club Golf Course Road, VIC 3644 SHEET NO: 1 of 1