Use of Ground Penetrating Radar for Building Inspection. Seminar on Building Diagnostic and Inspection - Testing & Certification

Transcription

1 Use of Ground Penetrating Radar for Building Inspection Seminar on Building Diagnostic and Inspection - Testing & Certification Ir Dr Wallace W.L. LAI Research Fellow, Department of Civil and Structural Engineering, HK PolyU Visiting Scientist, Federal Institute of Materials Res. and Testing (BAM), Berlin, Germany

2 Contents 1. Introduction 2. MBIS, HOKLAS SC19 and local standard for concrete radar inspection 3. Diagnosis of building structure by GPR (why?) 4. GPR: from military/planetary/archaeological sciences to MBIS 5. Applications and principles 6. Data acquisition, signal processing and imaging 7. An example 8. Use of information derived in GPR analysis from structural, durability and core sampling perspectives 9. The concept NDE-structural health monitoring (NDE-SHM) 10. Challenges of NDE 11. Conclusion

3 1. Introduction Surface B-Scan C-Scan B-Scan

4 2. Mandatory Building Inspection Scheme (MBIS) Coverage: Procedural requirements; Registration as Registered Inspectors ( RIs ); Scope and standard of prescribed inspections; Detailed investigation; Prescribed repairs in respect of buildings; and Voluntary compliance MBIS does not specify what kinds of tests should be done in the inspection HOKLAS supplementary criteria no. 19 suggests 9 destructive and NDE tests.

5 2. The Hong Kong Laboratory Accreditation Scheme (HOKLAS)

6 2. Local standard for Concrete radar inspection Test methods for IR (TM1) and concrete radar (TM2) Published by Hong Kong Concrete Institute (HKCI)

7 3. Diagnosis of building structures by GPR (why?) Visual inspection: relies on surface defects to predict internal conditions of structures. Random sampling of cores for destructive tests in lab: where to core? how many cores? Is the sampling representative? NDE-CE methods are non-destructive, effective and cover a large area It serves as a screening tool before a rational coring scheme is decided for destructive tests on material properties. Visual inspection NDE inspection

8 3. Diagnosis of building structures by GPR (why?) Measured parameters related to parameters of interest (e.g. cover depth, center to center of steel bar, debond in external wall, etc.) High data resolutions (e.g. data in every 10mm) Effective data acquisition Provide detailed digital and traceable records

9 4. GPR applications in military/ planetary/archaeology science Air-borne radar MBIS Apollo 17 mission on the Moon South Polar Layered Deposits on Mars Destroyer sounding submarine Archaeology in HK

10 5. Applications and principles Ground Penetrating Radar 1. GPR is a device which emits and receives high frequency ( MHz) EM wave penetrating into materials like concrete, soil, asphalt, etc. 2. Image reconstruction of the reflected wave amplitude by signal processing and imaging techniques.

11 5. Ground penetrating radar (GPR): applications Source: Geophysical Survey Systems, Inc. (GSSI) website

12 5. Mounting GPR on vehicle or by drag

13 Building 5. Application classified by GPR frequencies GPR centre frequency MHz Planetary science Snow and ice thickness Geology, geophysics, archaeology and forensic Environmental Infrastructures (bridge, highway, tunnel, airport runway, buried utilites)

14 5. Frequencies vs sizes Source: Sensors & Software Inc., Canada

15 5. GPR Frequencies and resolution 12.5MHz 25MHz 50MHz 100MHz 200MHz higher frequency => shallower penetration depth => better resolution lower frequency => larger penetration depth => worser resolution So penetration depth and resolution are trade-off!

16 5. GPR Frequencies and resolution Lower Frequency Antennas More Penetration Lower Resolution Physically larger in size Higher Frequency Antennas Less Penetration Higher Resolution Physically smaller in size

17 6. Data acquisition 2-dimensional measurements on the surface of elements B-Scan C-Scan Surface Imaging B-Scan A-scan: 1D stationary collection of GPR waveforms B-Scan: 2D radargrams in x-z plane (compilation of A-scans) C-Scan: 2D slice view in x-y plane (signal re-construction in a particular depth z Cube view: 3D spatial re-construction in x-y-z plane

18 6. GPR signal processing and imaging Raw GPR data Processed GPR data after signal processing tasks: 1. background removal, 2. velocity estimation according to HKCI TM2 and 3. migration

19 7. An example: Mapping of internal structure of a concrete wall by GPR Steel bar matrix Details: 1.Concrete wall with size 1.6m(T)x1.5m(W)x200mm(thk) 2.28-day concrete strength 40MPa 3.Two layers of Y20 steel bars 4.Two embedded plastic pipes 5.Upper 600 mm concrete made by using salted water (Cl - ) 6.GSSI 2GHz GPR was used

20 7. Useful GPR parameters in B-scan radargram (3) (1) (4) x A B C D E (2) z Information contained in GPR data: 1. Concrete cover depth 2. Thickness of concrete wall 3. Positions and spacing of embedded objects (steel bars, plastic pipes) 4. Amplitude of the steel bar reflections C-Scan B- Scan Surface B-Scan

21 7. C-scan slice scan from 0-20cm depth Plastic pipe Salt water concrete x 0,0 y Fresh water concrete C- Scan Surface B-Scan B-Scan 2 nd layer steel bar 1 st layer steel bar C-scan made by software GSSI Radan 7

22 7. Two C-scans at two different depths First layer of steel bar and plastic pipe at depth 5+/-5cm Second layer of steel bar at depth 15+/-5cm

23 7. Three-dimensional cube view

24 8. Use of information derived in GPR analysis from STRUCTURAL perspective Info derived in GPR analysis Possible Use (1) Object positions (steel bars c/c distance, plastic pipes, etc) (2) Thickness of concrete wall/slab (3) Concrete cover depth Compliance check with design drawings

25 8. Use of information derived in GPR analysis from DURABILITY perspective Info derived in GPR analysis (1) Amplitude at the apex of the steel bar hyperbola Possible Use Interface condition between concrete and steel bar (e.g. corrosion) (2) Concrete cover depth Resistance of carbonation and Cl - ingress (3) Wave velocity measured by hyperbolic reflections from steel bars Moisture distributions up to steel bar layer

26 8. Use of information derived in GPR analysis from CORE SAMPLING perspective Info derived in GPR analysis (1) Amplitude at the apex of the steel bar hyperbola (2) Inadequate concrete cover (3) Wave velocity measured by hyperbolic reflections from steel bars Possible Use Rational sampling of cores

27 9. The concept NDE-structural health monitoring (SHM) in MBIS Yr 100 Yr 30 Refl. to DW TTT 0.0 Yr 40 DW amplitude DW peak freq Compare GPR, ultrasonics, IRT and other NDEs Material properties by destructive tests according to rational coring

28 10. Two major difficulties of NDE techniques and interpretation Object identifications and location mapping are mature. But sensor types, multi-dimensional signal processing and variation of material properties make NDE interpretation of material properties not straight-forward. Signal inversion (processing) in this context is still a big subject of research. The properties measured are not directly related to engineering properties. A example in ultrasonic pulse velocity (UPV): higher UPV is related to but not always implies high concrete strength.

29 11. Conclusion NDE techniques visualize internal structures, offer measured parameters related to interested parameters, high data resolution, effective data acquisition and provide detail digital and traceable records. In-situ NDE techniques supplement concrete evaluation from structural, durability and rational sampling perspectives. Regular NDE measurements on the same structures become a tool of structural health monitoring. MBIS and HOKLAS provides a platform for development and application of NDE techniques in HK.