CONDITIONAL ASSESSMENT OF CIVIL STRUCTURES BY ADVANCED NDT METHODS

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1 e-issn Volume 3 Issue 3, March 2017 pp Scientific Journal Impact Factor : CONDITIONAL ASSESSMENT OF CIVIL STRUCTURES BY ADVANCED NDT METHODS Mr.C.Pranesh 1, Mr.S.Ramesh Kumar 2, Mr.C.Karthik Department of Civil Engineering, Velalar College of Engineering and Technology, Erode. Abstract This paper gives the state of non-destructive testing as applied to civil engineering industry in the Millennium year, The principle of NDT methods are described with particular reference to the five major factors that influence the success of a survey namely depth penetration, image resolution, contrast in physical properties, signal to noise ratio and information about structure. The advanced NDT method used in investigation are discussed from literature. The integration of NDT survey into the investigation of structure is described. In this paper, advanced NDT methods like Ground Penetrating Radar and Infrared Thermography are discussed for civil structure investigation. Keywords NDT; Infrared Thermography; Ground Penetrating Radar. I. INTRODUCTION Investigation and inspection are required in conservation of structure. Prior to any structural intervention in those building, estimation of current strength and actual dimension of existing member and extensive knowledge of present condition are to be required. This can be obtained by proper inspection and investigation [1]. Investigation techniques used by modern people is grouped into two Laboratory testing method (chemical, physical and mechanical analysis) which is mostly performed test on samples collected from site. In-situ testing are two types, partially destructive technique and Non Destructive Technique All the test process has some advantages and their own limitations, apart from that NDT tests are allowed in heritage structures due to their own advantages like non-disturbing, non-damaging and also preventing aesthetical view of structure, so these NDT tests are permitted. Nondestructive testing is useful in detecting defects and hidden characteristics i.e. internal void, inclusion and flaws, characteristics of the wall section which cannot be known other than destructive tests. Also, NDT can protect the property and integrity of the structures and also the method is very simple and quick to perform and results are readily available on the site itself. There are many challenges in non destructive evaluation of the structural members as the materials used for construction are diverse and the properties of the materials are not fully known. So, the performance of NDT techniques in such structures should be well studied. The selection of investigation methods depends on the accessibility of the object and its condition [2]. There are different NDT test methods are available and they provide different information about the structure [2]. Here, we chose advanced non-destructive techniques namely Infra-Red thermography and Ground Penetrating Radar (GPR) due to its qualitative image, high depth penetration and provide quick results. II. INFRA-RED THERMOGRAPHY The thermo graphic analysis is a powerful and non-invasive means of monitoring and diagnosis the condition of buildings based on the thermal conductivity of a material using thermal All rights Reserved 127

2 The assessment is very quick and usually requires no expansive access equipment or scaffolds as most of the work can be carried out from ground level. It s acceptable to wide surface of wall it is a telemetric method and has high thermal and spatial resolution [3]. This method can identify problem early, allowing them to be documented and corrected before becoming more series and can help in following ways: Detect missing or defective insulation Source air leaks Find moisture in insulation, in roofs and walls, both in the internal and the external structure Detect mould and badly insulated areas Locate thermal bridges Locate water infiltration in ceiling Detect breaches in hot-water pipes Detect construction failures 2.1. Working principle All bodies temperature above absolute zero emit electromagnetic radiation, the wavelength depending on the absolute temperature for temperatures of practical interest in the field of conservation, the emission is in the range of thermal infrared (10Um)compared to black body, real object always show a capacity to exchange energy by irradiation (emissivity) which is lower and wavelength dependent. If the corresponding emissivity is known, the surface temperature of an object can be derived, without physical contact by simply measuring the radiant power this is the working principle of thermographic instruments [4]. Figure 1. Principle of Thermography 2.2. Instrumentation Thermal energy detection and measurement equipment comes in a large variety of forms and levels of sophistication. The basic equipment is detectors and imaging All rights Reserved 128

3 Thermal detectors-depend on a two-step process. The absorption of thermal energy in these detectors raises the temperature of the device, which in turn changes some temperaturedependent parameter, such as electrical conductivity Quantum (photon) detectors -detect photons from infrared radiation. Quantum detectors are much more sensitive but require cooling to operate properly. Thermal imaging instruments measure radiated infrared energy and converts the data to corresponding maps of temperatures. A true thermal image is a gray scale image with hot items shown in white and cold items in black. Some thermal imagers have the ability to add color, which is artificially generated by the camera's video enhancement electronics, based upon the thermal attributes seen by the camera [2]. Some instruments provide temperature data at each image pixel. Cursors can be positioned on each point, and the corresponding temperature is read out on the screen or display. A thermal imaging camera is a non-contact and unique tool which is able to scan and visualize the temperature distribution of entire surfaces quickly and accurately Procedure The thermal radiation is collected by a camera sensitive to infrared radiation; this radiation is characterized by a thermal conductivity, i.e. the capacity of the material itself of transmitting heat, and its own specific heat [4]. The result is a thermographic image in a colored or black and white scale. Each tone corresponds to a temperature range. Usually the differences of temperatures are fraction of a degree. The total flux of energy E emitted by a surface, is the sum of the energy Ec emitted by the surface by thermal excitation and the flux Et that is emitted by the surface around each point E = Ec + Et. The infrared camera, measures the energy flux E. The test is carried out at a certain distance without any physical contact with the surface. Active thermo vision can be also carried out for tests on depth. The surface of the tested wall should be heated for a certain time. In this way, the thermal conductivity of the internal part of the masonry is shown up to a certain depth. The infrared camera transforms the thermal radiation into electric signals, successively converted into images. These images can be visualized on a monitor and recorded on a computer. In the video camera, the infrared radiation that reaches the objective is transmitted by an optical system to a semiconductor element. The latter converts the radiation into a video signal, while the surveying unit signal processes the video camera signals and shows the thermographic image as shown in figure 2. Figure 2. IR Camera All rights Reserved 129

4 2.4. Calibration Images may be digitized, stored, manipulated, processed and printed out. Industry-standard image formats, such as the tagged image file format (TIFF), permit files to work with a wide array of commercially available software packages Limitation It is very sensitive to weather conditions - rain or bright sun on a wall to be investigated may prevent any successful assessment Manufacturers FLIR Systems, MSA, Mikron, Mitsubishi, Nikon III. GROUND PENETRATING RADAR (GPR) RADAR (RAdio Detection And Ranging) is an electronic device which uses radio waves to determine the range, altitude, direction or speed of objects [1]. Ground Penetrating Radar is a noninvasive electromagnetic geophysical technique for subsurface exploration, characterization and monitoring. This non-destructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum and detects the reflected signals from subsurface structures to produce high-resolution images of the subsurface typically from 0-40 m depth. In technical terms, a GPR system generates electromagnetic signals and detects the electromagnetic field interaction with the surrounding material [5] Working principle GPR works by radiating short pulses of radio energy into the ground and recording the strength and the time (two way travels) required for the return of any reflected signal. The recorded GPR signal depends on changes within the surrounding media of the GPR wave properties [6]: The radio wave phase velocity (v), Attenuation (a) Electromagnetic impedance (Z) Figure 3.Principle of All rights Reserved 130

5 Data is also saved in appropriate memory for later processing and interpretation. The thickness of a layer is given by: d i = Ct i / 2 Ɛ r, i where d i is the thickness of layer i t ; the total travel time through that layer C is the speed of light Ɛ r, i the dielectric constant of the layer 3.2. Instrumentation Antenna (Bi-static Mode) Air-couple Ground-coupled Control Unit Display device Storage device 3.3. Types of GPR Impulse (or pulse) radar-transmits numerous small pulses of short duration and limited mean signal power Continuous Wave radar (Stepped frequency radar) -Transmits multiple steps of discrete frequencies, transmit more mean power, higher level of signal processing is required, used for laboratory purposes 3.4. Procedure GPR can either be used in reflection or transmission modes. The Reflection profiling survey method is the most common and is normally conducted using two antennae (called the bistatic mode) with a separate transmitter (Tx) and receiver (Rx). These antennae are placed directly on or relatively close to the ground surface to be measured. For the higher frequency, the Tx and Rx are normally combined inside one box called as transducer [7]. The transmitting antenna radiates short pulses of the radio waves (usually polarized) into the ground. The transmitted energy is reflected from various buried objects or distinct contacts between different earth materials, across which there is a contrast in dielectric constant. Radio reflections from targets in the ground are detected by the receiver unit, amplified and displayed by the control unit. The recorded GPR signal depends on changes within the surrounding media of the GPR wave properties: the radio wave phase velocity (v), attenuation (α) and electromagnetic impedance (Z) The computer measures the time taken for a pulse to travel to and fro from the target which indicates its depth and location. Data is also saved in appropriate memory for later processing and interpretation. Various arrays of multiple offset antennae may also be used to derive layer properties such as the dielectric constant (and hence velocity and depth) in real time and also to improve subsurface resolution. The depth range of GPR is determined by the electrical conductivity of the subsurface material and the transmitting frequency. Hence Multi frequency studies are needed Data analysis When many hundreds or even thousands of reflection traces are stacked together, as they are collected along an antenna transect, a reflection profile is produced which is called Radargram. It is a graph with the two-way travel time of the reflected wave on the vertical axis (depth range) All rights Reserved 131

6 profile length on the horizontal axis and constitutes the raw data. Processing of data is necessary for better interpretation [7]. During processing of collected data, different scan are obtained namely O-scope or A scan, B scan and C scan (3D view). O scope displays the wave form amplitude of particular point in the sample. B scan shows the image in two dimensional forms i.e. in XY plan and C scan shows the radar grams in three dimensional views for C scan; data should be collected in two directions [8]. 2.6 Limitations Figure 4.Radargram Images The main problems in the data interpretation are caused by disturbances in the signals due to the breakthrough effect which commonly visible on radar records and it hides partially the wall characteristics. It is caused antenna itself reacts to the electromagnetic wave multiple echoes due to the presence of layers and joints superposition of the lateral echoes that create images of parallel reflectors. This effect could be enhanced for a wall by the lack of mortar joints and the regularity of the stones. Radar tests always need a preliminary calibration to verify the characteristics of the antenna in relation to the aim of the research. The test lines should be selected to be representative of the masonry and of the problem to investigate. The horizontal or vertical surveying lines are usually located away from the wall edges in order to avoid boundary effects or away from other sources of surface noise. There are different types of acquisition in the function of the aim of the tests. Usually echo modality is used, i.e. with the receiver and transmitter on the same side of the wall, moving the antenna along a surface [9, 10]. 2.7 Manufacturers Some of the manufactures of GPR are listed below Figure 5.GPR Radar All rights Reserved 132

7 Geographical Survey System.Inc (GSSI) include vehicle, antenna, survey and training[11] GeoScanners AB MALA GeoScience STANLAY TM IV. CONCLUSION The NDT method described here demonstrated the ability to be speedy techniques from which different material properties, defects, in homogeneity, moisture movement etc. can be detected. Some of the information gathered is common to more than one method nether less, each has proved to be an effective method and their complementary use in the investigation draw a picture of state of structure. No single NDT method will work for all flaw detection or measurement applications. Each NDT method has advantages and disadvantages when compare to other methods depending on their convenience, interface, interpretation, accuracy in post data collection phase and cost. REFERENCES [1] A primer of National workshop on Non-Destructive Testing Techniques for Historical Monuments and Heritage Structures, National Centre for Safety of Heritage Structures 15 th -17 th December [2] D M McCann, M C Forde, Review of NDT methods in the assessment of concrete and masonry structures, NDT&E International, pp , [3] Menon, Structural consvervation of historical monuments, Architecture update, 2010 [4] ASTM standard C42, Standard test methods for obtaining and testing drilling cores,west Conshohocken, PA, [5] Luigia Binda, Antonella Saisi, Luigi Zanzi, Radar Investigation And Diagnosis Of Historic Masonry,Structural Faults and Repair, [6] Luigia Binda, Dipartimento di Ingegneria Strutturale, Politecnico di Milano, Piazza Leonardo da Vinci, Test for masonry materials and structures, Non Destructive Test for masonry materials and structures, pp , [7] Bhaskar.S, Srinivasan.P, Ramanjaneyalu, Nagesh R Iyer, Impact echo & Thermographic studies on masonry test specimen, R&D02-MLP172-RR-04 July 2014, CSIR-Structural Engineering Research Centre. [8] GSSI hand book for radar inspection of concrete, Geophysical Survey System, August [9] Mercedes Solla, Henrique Lorenzo, Alexandre Novo, Belen Rivero, Evaluation of ancient structures by GPR (Ground Penetrating Radar): The arch bridges of Galicia (Spain),pp , [10] Derald G Smith, Harry M. Jol, Ground penetrating radar: antenna frequency and maximum probable depth of penetration in Quaternary sediments, Journal of Applied Geophysics, pp , [11] Bottari A, De Domenico D, Giannino F, Marino A, Teramo A, On A GPR survey to characterize the structural configuration of a masonry building, University of Messina, All rights Reserved 133