Identifying Strip Drains Behind Concrete Medians:

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1 CMD Civil Pty Ltd PO Box 1119 Huntingdale VIC info@cmdcivil.com Case Study: Identifying Strip Drains Behind Concrete Medians: This application note demonstrates an example of locating objects and anomalies behind or within concrete structures 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 over the eastern, northern and Western shotcrete walls, and within the bin storage area, within the basement of the property in order to determine: a. The thickness of the shotcrete walls. b. The location of the strip drains, in elevation, which are located either within or behind the shotcrete walls. c. The depth of the strip drains from the front face of the shotcrete walls over the height of the walls. d. The location of steel reinforcement in the shotcrete walls in the vicinity of the strip drains, including the distance of the reinforcement from the front face (Concrete Cover) and the distance of the reinforcement from the strip drain. Date of Investigation Works: 20 th March 2015 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. Locations of Survey: a. Testing of at least six (6) locations over the north wall b. Testing of at least four (4) locations over each of the eastern and western walls c. Testing in the vicinity of the apparent moisture in the bin storage area. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 2

3 Locations of Survey: Locations indicated are approximate only 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: A series of scans were conducted horizontally across each specified wall section at approximately 200mm - 300mm centres in order to first identify the strip drain locations. As the shotcrete walls contained reinforcing mesh (Fabric) which partially obstructs a clear view of the drains, the amount of scans conducted as necessary in order to ascertain the true central point of each drain. Once the true location of each drain in elevation could be established further survey works could then commence to verify the shotcrete wall thickness, average depth to the strip drains from the front face of each wall, the location of the steel reinforcing in relation to the strip drains including concrete cover to the reinforcing from the concrete surface. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 4

5 Results: The data collected during this site survey was able to verify the locations of the strip drains within the specified zones and the locations plotted in relation to onsite permanent fixtures. The depth recorded to the strip drains varied considerably between the locations surveyed which was crucial information required by the client, along with knowing if the strip drains themselves were positioned within the concrete walls or immediately behind would affect how the client would next proceed. A high percentage of the assumed strip drains identified were positioned behind the reinforcing mesh and therefore a clear and unobstructed image of the strip drains was generally not achievable. As a result the depths measured to the strip drains could only be measured form the limited reflective points that were visible. The steel reinforcing mesh was the first reflective material identified through scanning over the wall surface locations, clear and unobstructed images of the reinforcing mesh were obtained without other structural components affecting the data. Verifying the thickness of the shotcrete wall sections was not always achievable based on the data obtained and a clear back wall reflection was not always identified. Where data sets collected displayed partial or limited reflective sections that were deemed to be that of that of the opposing side of the shotcrete wall structure an estimated wall thickness measurement was indicated. Fig:1 Plan view of the strip footings, reinforcing mesh Fig: 2 2D GPR data image collected onsite displaying the vertical bars of the reinforcing mesh along with the position and approximate depth of the strip drains. Reinforcing bars denoted as red points with the assumed strip drains indicated in yellow. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 5

6 Fig:3 2D GPR data image depicting two strip drains obstructed by the placement of the reinforcing. Where possible data images such as this were not utilised and further scanning was undertaken. Fig:3 2D GPR data image depicting two strip drains in amongst the reinforcing mesh itself. This was collected over Location: 4 along the base of the North wall and was one of the only locations where the strip drain was positioned at such minimal concrete cover. CMD CIVIL PTY LTD GROUND PENETRATING RADAR (GPR) 6