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A Forensic Tool for Inspection - Imaging and Evaluation of Concrete and other areas of Civil Engineering.

Introduction

Since deterioration of RCC is natural phenomenon and has started exhibiting in large number of structures, a systematic approach is needed in dealing with such problems*.

Analyzing engineering distress to determine causes, mechanisms and routes to avoid or mitigate future problems is often relatively straightforward. A construction failure during the building process or in service generally prompts an investigation of causes. In many cases responsibilities will have to be apportioned. The systematic approach to the distress and its evaluation provides unique and valuable insight to the failure causes and aids at proper Restoration, Rehabilitation and Repair (RRR) techniques. The investigations and its outcomes provide an effective means of further process of resolving the problem. A few years ago, there were hardly any reliable diagnostic devices available to assess the magnitude of distress. Engineers by and large, relied on physical observations such as deflections distortions, cracking, discoloration, buckling etc., for assessment. Situation is changed drastically for better in the field of forensic Engineering and investigations. A few   noticeable developments and refinements in the domain of investigation of distressed structures in civil engineering industry witnessed tremendous progress in all the spheres encompassing diagnosing devices for distress assessment, suggesting materials, techniques for RRR and assessment of Performance measures is made easy*. GPR – Ground penetrating Radar, a high resolution, field portable method is one such advance technique of forensic investigations.

Ground Penetrating Radar** (GPR) is a Non- Destructive Testing (NDT) method that can be used to gather information on subsurface elements in roads, bridges, sports grounds, golf courses, cemeteries and all reinforced concrete structures. It is a geophysical method that uses radar pulses to image the subsurface. This method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum and detects the reflected signals from the surface on which it is used.

How does GPR work ?

GPR uses high-frequency (usually polarized) radio waves and transmits into the ground. When the wave hits a buried object or a boundary with different dielectric constants, the receiving antenna records variations in the reflected return signal.

The principles involved are similar to that of reflection seismology, except that electromagnetic energy is used instead of acoustic energy and reflections appear at boundaries with different dielectric constants instead of acoustic impedances.

It works by transmitting a wave into the scanned medium. The wave travels through the medium and reflects when it encounters something of different density, such as a buried concrete pipe, or a post tensioned cable in a suspended floor slab. The reflected wave is picked up by the receiver and processed by the GPR hardware and displayed as a series of images on the screen. The GPR technician then interprets the images shown to determine the location and depth of the object.

Equipment - The GPR equipment normally consists of a transmitter and receiver antenna, a radar control unit and suitable data storage and display devices. The transmitter sends out a radio frequency that reflects off objects under the surface of the scanned medium back to the receiver. The speed of the signal is adjusted to match the dielectric constant of the medium of scanning. (The dielectric constant is a measure of the resistivity of the medium of scanning).

Radar Control Unit - The radar control unit synchronizes signals to the transmitting and receiving electronics in the antennas. The synchronizing signals control the transmitter and sampling receiver electronics located in the antenna(s) in order to generate a sampled waveform of the reflected radar pulses. These waveforms may be filtered and amplified and are transmitted along with timing signals to the display and recording devices.

Filters may be used in real time to improve signal quality. Real-time signal processing for improvement of signal-to-noise ratio is available in most GPR systems. When working in areas with cultural noise and in materials causing signal attenuation, time varying gain is necessary to adjust signal amplitudes for display on monitors or plotting devices. The summing of radar signals (stacking) is used to increase effective depth of exploration by improving the signal-to-noise ratio.

Data Display - The GPR data are displayed as a continuous profile of individual radar traces. Data are commonly presented in wiggle trace display, where the intensity of the received wave at an instant of time is proportional to the amplitude of the trace the intensity of the received wave at an instant in time and is proportional to either the intensity of gray scale or to some color assignment defined according to a specified color-signal amplitude relationship.

Antennas and Control Cables - The antennas used to transmit and receive radar signals are generally electric dipoles. A single-dipole antenna can be used to both transmit and receive signals in the monostatic mode. The bi-static mode uses separate antennas for transmitting and receiving. These antennas can be housed in a single / separate enclosure where the distance between the two antennas are fixed / varied. The ability to vary the distance between the two antennas is helpful in optimizing the survey design for specific types of target detection.

Electromagnetic waves are three-dimensional vector fields where the orientation of the fields is described by the vector direction or polarization of the electrical and magnetic fields. Changing the polarization of a linearly polarized electric dipole antenna can cause maximum or minimum coupling to a scattering object.

The selection of antenna frequency depends on the depth of penetration, spatial resolution, and system portability required for the study.

The depth of the scan depends of the frequency of the antenna used. The lower the frequency the deeper the penetration of the signal, this produces a lower resolution of the detected objects. Higher frequency antennas produce much higher resolution scans but with shallower depths.

The depth range of GPR is limited by the electrical conductivity of the ground/surface, the transmitted center frequency and the radiated power. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as farer distance as lower frequencies, but give better resolution. 

Ground-penetrating radar depth section (profile) indicates very distinct hyperbolic reflections, appearing as an inverted U(yellow arrows), which are typically associated with discrete objects. Less distinct hyperbolic reflections are indicated by red arrows, dashed blue lines indicate horizontal and sloping reflectors, probably bedrock. The many smaller reflections near the surface are likely to be caused by tree roots.

GPR can be used in a variety of media, including concrete rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks. Ground - penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, GPR air launched antennas can be used above the ground.

How does this system work in concrete

A potential field is measured on the concrete surface by the use of a reference electrode, known as  half cell and a high-impendance voltmeter.

• Exposing a piece of rebar or mesh for the high-impendance voltmeter to connect to the surface requiring testing is then wet down to reduce the fluctuation of the measured values.
• The reference electrode is placed on the surface, the measured values of the half-cell potential are recorded at regular intervals.
• The data collected is transferred onto a windows based software programme.
• The programme generates a potential map of the data collected, allowing for an efficient means of interpretation of the half-cell potentials of the investigated surface area. (A half-cell is a structure that contains a conductive electrode and a surrounding conductive electrolyte).

Concrete Scanning Applications Include

• Concrete Inspection  - Locate Rebar s, Wire Mesh, Post-Tension Cables, Anchors, Electrical Conduits in Floors, Walls, Columns, Balconies and also Void in concrete/ RCC  elements,
• Determining Concrete Thickness,
• Locating Metal and Plastic Pipes,
• Locating Conduits, 
• Concrete Cover Assessment,
• Carbonation of concrete assessment,
• Rebar in structural elements like beams, columns, slabs etc.
• Post- tension cables,
• Pipes, PVC in and under slabs or in soils,
• Bridge and road inspections.

Utility Locating  in Civil Engineering

• Gas, Electric, Telecom, Fiber Optic Cables
• Water Lines, Storm and Sewer Systems
• Irrigation and Septic Field Systems
• Underground Storage Tanks and underground utilities
• Both Plastic and Metallic Pipes

Thus, GPR has the ability to provide real time results quickly with minimal hassle to the occupants of the building by avoiding floor penetrations through core drilling and anchors. The concrete imaging technology used ensures that the operations will not be interrupted by damaging existing utilities or other obstructions.

GPR produces no radiation, and is safe to work around without evacuations. GPR can be used to update older drawings, and provide a great deal of information related to Concrete thickness analysis, Rebar spacing, and placement,  void detections,  conduit locating and provides information on suspended or on-grade slabs.

Based on reading echoes of pulsed electromagnetic waves, radar measures the difference in materials by acoustic density. Figure here shows sample of GPR output on a concrete structure.

Three-dimensional imaging

GPR data can be captured in a 3D data set by setting up a grid pattern for your X,Y coordinates.  Once data is collected and processed, the data set will have Z coordinate. Individual lines of GPR data represent a sectional (profile) view of the subsurface. Multiple lines of data systematically collected over an area may be used to construct three-dimensional or tomographic images. Data may be presented as three-dimensional blocks, or as horizontal or vertical slices. With this data, the operator will be able to "slice" through the data from top to bottom in user defined measurements. Horizontal slices (known as "depth slices" or "time slices") are essentially planview maps isolating specific depths. Time-slicing has become standard practice in archaeological applications, because horizontal patterning is often the most important indicator of cultural activities 3D is very helpful in applications such as debris fields, post-tension slabs, utility locating and many others.

3D underground features can then be exported into CAD files and processed for client’s use.  If GPS is used, the GPS data can also be exported with the features.

Slicing Through3D data - The figures here are “slices" through the data from the top of the surface, penetrating into the elevated concrete slab.  This proves to be invaluable to engineers when conducting repair work or verifying rebar location / condition.  GPR – 3D can take a 3D image, and slice through layers of the data in whatever thickness required.  So it is possible to have the details of the section at any given thickness of the element where detailed view is required.

Application in Other Fields of Civil Engineering

GPR has many other applications in a number of fields of Civil Engineering. In the Earth sciences it is used to study bedrock, soils, groundwater, and ice. Engineering applications include nondestructive testing (NDT) of structures and pavements, locating buried structures and utility lines, and studying soils and bedrock.

In environmental remediation, GPR is used to define landfills, contaminant plumes, and other remediation sites, while in archaeology it is used for mapping archaeological features and cemeteries. Military uses include detection of mines, unexploded ordnance, and tunnels. Few of the applications are listed below.

A more detailed list of Civil Engineering applications are

GPR in Geology (using Ground Penetrating Radar)

• Verify soil layers and continuality
• Void detection
• Sinkhole Investigations
• Bottom of pond/lake mapping
• Organic Soils
• Caves

GPR in locating Electromagnetic  materials (Using GEONICS EM61 & GEONICS EM31)

• Locate conductive materials
• Buried drums, debris fields
• Septic Tanks and UST’s
• Groundwater contaminants
• Ferrous and non-ferrous metals

Utility Locating and Mapping

Construction professionals, business owners, and homeowners often need a reliable way to locate utilities prior to excavation and trenching, conducting site assessments, or mapping and to locate underground utilities. Utilities can be located through natural or manmade surfaces. GPR produces an image that displays the exact location and depth of both metallic and non-metallic subsurface utilities such as plastic, fiberglass, concrete, clay, and many other materials. Use of E.M. based utility locator combined with Ground-penetrating radar to locate private electrical lines, private gas lines, public lines on private property can be worked towards confirming the results previously obtained (second opinion).

• Water lines, gas, sewer, communication, electrical, etc.
• Underground storage tanks (UST's)
• Private property utility locates
• Center and depth marking,
• Electric Lines
• Phone Lines
• Cable Lines
• Oil / Gas Lines
• Water Lines
• Sewer Lines, Drain Pipes

Geophysical Investigation

Critical Lifts - To Locate Subsurface
GPR inexpensively gather data, and generate planview maps showing signs of potential subsurface failure due to heavy weights on the surface.

Moisture saturation, contamination, shear plains, sloughing and other subsurface movements can all be detected through the use of GPR.

Pre-construction Planning & Predictive Maintenance
The  jobs that  require more knowledge of the subsurface explorations ,  GPR scan can be performed before  the excavation to avoid unnecessary hassle at later stages of work.

Void Detection

In addition to critical crane lifts, other installations may require void detections beneath concrete slabs, or surrounding utilities such as fire water lines. Ground-penetrating Radar (GPR) in many situations to locate subsurface voiding. One example may be to locate which section of a charged water line may be leaking (in other countries?). Since GPR can locate the void caused by the water washout long before the void extends to the surface, it enables safe, effective, early repairs.

Contamination plume detection

Similar to void detection, Ground-penetrating Radar is capable of imaging the altered soil conditions surrounding spills and contaminations. It helps in  delineate the boundaries and help to select locations for monitoring wells, as well as guide cleanup efforts. It is  capable of working on active plant sites without the need to evacuate large areas and also  in farmer’s fields without affecting their crops. It can work in many weather conditions, at all times of the year.

Archaeological Investigation

Archaeologists worldwide use GSSI GPR and EM tools to locate areas for excavation. These tools are well suited for shallow, nondestructive investigations.

Archaeologists and remote sensing specialists around the world rely on GSSI ground penetrating radar and EM conductivity instruments as key tools for non-invasive site investigation. Whether the goal is site mapping for excavation or locating sensitive cultural resources for preservation or avoidance, GSSI's remote sensing technologies have been the tools of choice for nearly 40 years.

GPR in locating Electromagnetic  materials (Using GEONICS EM61 & GEONICS EM31)

• Locate conductive materials
• Buried drums, debris fields
• Septic Tanks and UST’s
• Groundwater contaminants
• Ferrous and non-ferrous metals