This paper outlines the principles in operation, the conditions

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1 INTRODUCTION INFRARED THERMAL IMAGING CONSTRUCTION FAULT LOCATION D J Titman B.Sc., M.Sc., FFB., I.Eng., M.Inst NDT Hariy Stanger Limited Over the past few years infrared thermal imaging has been considered for a variety of engineering applications and the potential of this method is only now being realised with the range of applications still increasing (1 and 2). As a totally non-destructive method, it has the advantage of providing handsoff mformation quickly. As with other non-destructive or potentially destructive methods, however, there are particular factors which may influence the success of its application and, wherever possible, it should be used in the context of a combined testing approach (3). Because of the relative nature of data produced by a thermographic survey, the quantification of properties of a structure or the materials contained therein may require correlation by confirmatory localised 'destructive' sampling and/or inspection. This paper outlines the principles in operation, the conditions required for the survey to be successful and a range of situations where the technique has proved effective. 2. PRINCIPLES 2.1 General In a thermal imaging survey we are looking for surface temperature variations which reflect some property or feature at depth. However, in order to obtain this surface effect, it is necessary to have sorne method of thermal input into the structure under examination. There are three ways in which this is effected. 2.2 Heat Source If an object at depth under observation is hot or cold relative to the material surrounding, the surface directly over the object is correspondingly relatively hot or cold, thus indicating the object's position beneath. 274 / SPIE Vol Infrared Technology andapplications (1990)

2 2.3 Thermal Gradient If a stable thermal gradient can be produced through the body under investigation, lateral variations in the thermal conductivity of the body will lead to variations in the observed surface temperatures on either face. These lateral variations in thermal conductivity may relate to significant properties or defects within the body under investigation. 2.4 Induced Heating Application of heat (or cold) to a surface will lead to varying rates of heating or cooling of the surface, and a variation in ultimate surface temperature depending on the thermal conductivity of the material beneath the surface. Such surface temperature variations may therefore indicate material or structural variations below the surface. In any situation, more than one of the above principles may be in operation. 3. TYPES OF SITUATION 3.1 Heat Source Distinct Heating Systems Apart from the location of the buried pipes, breakdown in pipe insulation leads to hot spots at the surface. The use of this technique to locate underground sewers and associated defects has been described by Weil (4) Underfloor Pipes Central heating or cold water pipes can be accurately located (as in 3.1.1) due to their temperature difference from the surrounding medium. A hot water leakage may saturate the area around increasing the thermal conductivity so that it heats up more than dry areas adjacent to sound pipe. It is therefore possible to detect leak zones Spoil Heaps/Embankments Fires can be located at depth. Where these cannot be extinguished, it is possible by repeat surveying to monitor the development and spread of the fire Landfill Sites The generation of methane by the decay of organic matter can produce substantial heating effects. These can be detected at the surface indicating the generation site (5 and 6). SPIE Vol Infrared Technology andapplications (1990)! 275

3 3.2 Thermal Gradient Walls and Roofs Breakdown in the thermal insulation vil1 lead to hot spots developing on the cooler surface and corresponding cold spots on the warm face. This may indicate the site of water ingress saturating the wall, major interstitial condensation or missing thermal insulation Coldstores As an extension to the thermal insulation to cold stores can be susceptible to ice formation if water vapour can penetrate through the protecting wall skins. The extent of such insulation breakdown can be plotted by viewing the surface temperature variation from either face Refractory Linings The linings to chimneys, furnaces etc are checked whilst in full operation. Any areas of breakdown can be seen on the outer face in the form of hot zones Pipes The efficiency of thermal lagging to pipes can be checked. Any weak spots, gaps or saturated areas can be readily spotted Light-weight Structures Because of the variation in thermal conductivity of the various components within a wall panel, for example, it may be possible to determine the form of construction, in particular to locate internal joints, stiffeners, fixings and associated defects Cold Bridging Localised thermal conductivity variations are produced by cold bridging for example due to fixings and ties, particularly when defects allow the passage of moisture from the outer to the inner leaf in a wall. This will produce hot or cold spots on the surface Leakage Air or water leakage may occur at joints, particularly where sealants are absent or have failed in service. Such situations allow easy heat flow paths through the structure which can be detected at the surface. 276 / SPIE Vol Infrared Technology andapplications (1990)

4 3.3 Induced Heating Qeneral The most readily available source of heat is provided by solar radiation. The cooling effects on the removal of this radiation are often of even more use in thermal imaging because of the absence of reflections of the incident solar radiation from the surface Lamination/Spalling The lamination of render from its backing or spalling of concrete due to reinforcement corrosion can be detected since under solar radiation, the surface over a defective area will heat up more rapidly and to a higher ultimate temperature than over the sound material surrounding. This happens because the air gap acts as a thermal insulator. Similarly, at night time the converse should occur with, ultimately, the surface over the defective area cooling to a lower temperature than over the sound material. The results of research into the use of such techniques in bridge deck investigations have been described by Kipp and Kunz (7) and by Maser (8). In the latter case, thermography was linked with impulse radar methods. 3.3 Voids/Honeycombing Moisture Such defects, hidden behind a surface skin or render may show up by the same mechanism as in Saturated layers in built up roofing layers produce the converse effect to that in The surface above may appear relatively cool after a period of direct sunlight due to the conduction away of surface heat into the moist zone below. Again. the converse may be observed after several hours of surface cooling at night Structural Details Solar heating on a rendered surface may produce a considerable amount of information on the structure beneath due to variation in the thermal conductivities of the different materials in the structure. Particular details which may be identified include concrete columns and beams, blockwork and brickwork and variations in materials and/or workmanship in mortar joints. 4. LIMITATIONS For thermal imaging techniques to be successful in these fields it is necessary to have a ready heat source of sufficient energy input. SPIE Vol Infrared Technology andapplications (1990) / 277

In the case of enclosed spaces where walls have stabilised to the temperature behind and where there is no significant thermal gradient through the walls (e.g. in basements, tunnels) such investigations are rarely successful.

5 In the case of enclosed spaces where walls have stabilised to the temperature behind and where there is no significant thermal gradient through the walls (e.g. in basements, tunnels) such investigations are rarely successful. Some success may be achieved by the application of very powerful space heaters over a considerable period. For the assessment of details within most conventional walls and roofs (e.g. thermal insulation or water ingress investigations) a thermal gradient across the wall of the order 10 C or more is necessary. This is normally obtained by running heating inside the structure for a few hours to stabilise the inner face conditions and viewing the structure at night from either face as is appropriate. Even when viewing at night, windy conditions may have a convective surface cooling effect thus masking detail, and other factors such as water (e.g. rain) on the surface will almost certainly lead to problems. It is therefore, in theory, often preferable to survey the inner face in the dry, still air environment. However, access may be limited inside a building due to fixtures etc., and therefore these various factors have to be weighed against each other for each specific investigation before selecting the most appropriate plan of attack. Since it is required to monitor radiated heat, it is preferable to minimise reflected radiations. This is why most survey work has traditionally been conducted after sunset or before sunrise. However, it has been shown that in many instances valid surveys can be undertaken in full sunlight; but the experience of the operator is essential in recognising the effects of solar reflection. The surface emissivity is another important factor which cannot be dismissed. Variations in surface finish or material will give an apparent variation in surface temperature and again, the experience of the operator is necessary to correct for this. Finally, it is usually necessary, as with all non-destructive methods, to 'calibrate' the results obtained. This may merely involve some localised tapping (in the case of a delamination survey) or may involve taking samples and carrying out laboratory analysis (e.g. for moisture content). The combination of non-destructive methods is always advisable e.g. the use of metal detectors or impulse radar to confirm pipe locations, impulse radar and moisture meters in moisture investigations or ultrasonic pulse velocity in concrete structural investigations (3). In spite of the above limitations it is envisaged that thermal imaging will provide an increasingly powerful method in the future in engineering investigation fields. 278 / SPIE Vol Infrared Technology andapplications (1990)

Case 1 Lçing Central Heating Pipes Pipes below 80 mm screed and deep pile carpet. Layout of pipes determined with heating switched on. Hot zone (white) indicated leak zone at a junction.

6 Case 1 Lçing Central Heating Pipes Pipes below 80 mm screed and deep pile carpet. Layout of pipes determined with heating switched on. Hot zone (white) indicated leak zone at a junction. Case 2 Moisture in Roof Insulation Rain water penetration through roofing slabs into quilt beneath. When viewed after approximately 1 hour's full sun, the damp zones showed up as dark (cool) areas. SPIE Vol Infrared Technology andapplications (1990) 1 279

7 Case 3 Delarninated Render Two rendered bays viewed in full sunlight. The upper bay is severely laminated, the debond areas showing up as hot (white), whilst the lower bay is generally well bonded throughout. Case 4 Sttuctul Detai1 BehinRe1TeTT The following details showed up after a few hours full sun; vertical concrete column and horizontal beams on left half of elevation (black), landings in stair tower on right (black) and vertical reinforced concrete strips in blockwork between (grey vertical stripes). concrete stair 280 / SPIE Vol Infrared Technology andapplications (1990)

8 REFERENCES Reference Author Title Publication 1. G M Cuthbertson Thermal Imaging GEC Review Vol. 2 No D J Titman Infra-Red Thermal Imaging 3 B.S.I Testing concrete Guide to the use of non-destructive methods for hardened concrete lee Science, Education & Technology Division. Digest No. 1989/84 Conference: Medical Scanning and Imaging Techniques of Value in Non.destructive testing BS 1881 : Part 201: G J Weil Detecting the Defects Civil Engineering, September P D Johnson Remote Detection by Infra-red Techniques 6. H M Inspectorate of Pollution 7. J S Kipp and J T Kunz The Control of Lpndfill Gas Thermographic Investigation of a Bridge Deck 8. K R Maser From Guesswork to Guarantee BIOGRAPHY Symposium Methane- Facing the Problems 1989 Waste Management Paper No.27 Appendix E Public Works, September 1983 Civil Engineering, September 1989 The author and presenter of this paper is a Senior Investigation Engineer with Harry Stanger Ltd, Consulting Materials Engineers. His particular fields of activity lie in the application of non-destructive testing techniques to the investigation of structures and structural materials. He serves on the British Standard Committee for revision of BS 4408 (Non-destructive Methods for Testing Concrete). SPIE Vol Infrared Technology andapplications (1990) / 281

Part 1 General information. Chapter 1.4. Cold weather working

Part 1 General information. Chapter 1.4. Cold weather working Part 1 General information Chapter CONTENTS SCOPE DESIGN Clause Page There are no specific Design implications This Chapter gives guidance on meeting the Technical Requirements and recommendations for