NON DESTRUCTIVE EVALUATION OF DELAMINATIONS AND INTERFACES IN CONCRETE STRUCTURES

Transcription

1 NON DESTRUCTIVE EVALUATION OF DELAMINATIONS AND INTERFACES IN CONCRETE STRUCTURES Jean F. Lataste (1), Martin Krause (2), Andrzej Moczko (3), Denys Breysse (1) and Christiane Maierhofer (2) (1) Université Bordeaux 1, Ghimac, France (2) Bundesanstalt für Materialforschung und-prüfung (BAM), Germany (3) Wroclaw University of Technology, Poland Abstract Concrete is generally regarded as a continuous, homogeneous and isotropic material. An interface, as generally defined, is a physical limit between two materials. Such a limit can be viewed as an alteration of the mechanical continuity of concrete and then as a possible alteration of its properties. Engineers are interested in the knowledge of this object due to its significance: either an indicator of damage, or a parameter governing damages (e.g. by its influence on the transfer properties of concrete), or an internal geometrical characteristic. Non destructive tests are used to obtain data on this object: to detect, to identify, to locate its extension and its depth, to characterise the intensity and activity (possible evolution) of the interface. In the frame of the TC Rilem 207 INR, a state of the art is being prepared presenting techniques used today to study interfaces. Questions linked to the study of interface in concrete are treated: what is looked for? At what scale? Why it is difficult?... The ability and limits of the available test methods are discussed in this paper, in order to highlight the actual research and development needs. Keywords Interfaces, concrete, non destructive testing 473

2 1. INTRODUCTION Considering mechanical strength, tightness or other concrete function in the structure, debond is always a local property weakening. Interfaces in concrete are pertaining material behaviour. They break the material continuity, although which is the hypothesis of construction sizing. Needs are today to describe interfaces. Their assessment should allow identification of them as consequence of a damage (e.g. due to structural under sizing, or unexpected solicitation); or as the probable cause for alteration increase (by the creation of ways for aggressive fluids to flow within concrete). Signification of interfaces can be various, and are not treated identically regarding their possible impact. Anyway, such disorder which is seated at a material level, leads to consequences on the whole structure. The knowledge of the initial cause of the interface is important to be able to correctly characterize it, thought this it is not sufficient. The aims of an investigation can be the detection or the determination of objects like the location, the extension, the depth, the intensity, and the activity (possible evolution) of the interface. The consideration of interface is motivated by the fact that their study and characterization on site is a way to assess the condition of the structure at a moment of the structure life (which is the characteristic at the date of investigation), and to assess the remaining service life for the structure (what is the impact of the damage on the structure, and how could the interface evolve in time?). The RILEM Technical Committee 207-INR (Interpretation of NDT results and assessment of RC structures) aims notably at the improvement of non destructive evaluation (NDE) of interfaces in concrete. The Committee identifies today s common techniques and more special techniques in development to characterise interface and delamination. 2. DESCRIPTION, DEFINITION OF THE PROBLEM TREATED 2.1 What is looked for? At what scale? For what purpose? Concrete debonding draws interface between concretes, due to the casting process, or construction. In extreme cases, the bond can be considered as a void layer. At that shape it can appear as a delamination or de-bonding. The presence of concrete bonding can be due to a defect during construction, for instance, a defect of contact between a precast slab and the concrete casted over. Delaminations are defined as cracks parallel to the surface ( Figure 1). They are consequent to a damage of concrete or to the structure. Fire, freeze and thawing, or rebar corrosion are classic causes of this kind of damage. Extension L Depth D Thickness T Figure 1: Delamination 474

3 Whole structures or a part of them can be checked for this damage. Multilayered bonds or delaminations usually disturb surfaces about some square decimeters to square meters. Interfaces can sit very close to the surface when they are due to physico-chemical alterations from the surface (i.e. frost, fire ). Other phenomena can cause interfaces deeper in concrete (limits between precast concrete and concrete cast on site, close to rebar level or internal heat pipe level Figure 2 and Figure 3). Crack Internal heat pipe Figure 2: Interface at heat pipe level in a concrete slab [1] Figure 3: Interface at the reinforcement level due to rebar corrosion [2] The interface disturbs the homogeneity of the structure and can impair its structural behaviour. It can provide a preferential ways for fluids to flow or to store in concrete. Geometry and properties of these defects can vary, considering the filling, the origin, the evolution... Questions linked to interfaces are also risks of concrete facing falls, and so danger and aesthetic criteria. Investigation can be motivated by checking whether elements of buildings in agreement to standards or, when problem appears, to search for possible explanations; or to detect alteration of concrete (due to loading or ageing) in case of damage appearance. Investigations can be planned at the end of construction (to check the conformity), during the service life to diagnose an alteration, or to check the efficiency of repairing works like injection. 2.2 What level of interpretation? What is the required accuracy? Detection, localisation The first aim of an investigation relatively to bonding or delamination is the detection and localisation of the defect. The determination of lateral extension of the interface below the surface gives information allowing a better works planning. An example of the result with impact echo to assess surface damages is presented (Figure 4). This method leads to a quantification of damage in terms of relative surface compared to the whole surface checked. 475

4 Zone without defect Voids detected (about 10cm below the surface) Suspect measure Coring Results of investigations drawn schematically Representation on site of investigation results (zoning) Figure 4: Zoning of delamination on a concrete slab by impact echo investigation [1] Techniques based on a punctual measure (such as impact echo or electrical resistivity) allow the determination of such surfaces with the resolution depending on the measurement step. If we consider more global techniques (for instance infrared thermography) the lateral limits of interface is more difficult to estimate. Indeed the limit appears roughly clear even in a preliminary approach (Figure 5), but it is difficult to identify a definite criterion allowing the exact location and limit of the debonding: 1. The measured parameter is continuously variable, then a work on gradient should be the way for treating data, unless a threshold can be defined; 2. The result at the surface depends on the material property and the geometrical condition (how deep is the damage? What is the contrast between damaged and undamaged 476

5 concrete? Which solicitation?). These factors influence the determination of location and extension, and can lead to misunderstanding. ~1m Visible range Infrared range Figure 5: Investigation of intrados of a concrete slab of bridge by means of passive infrared thermography [2] Characterization in depth The sharp characterization of discrete interfaces in concrete is required the determination of its depth and, when it is opened, of its thickness. The characterization must be viewed as an inverse problem to solve. A first approach consists in comparison of the results pertaining to different zones. If impact echo is used, the technique is calibrated on a reference zone (known as sound), then the results on other areas are compared with this reference signal (Figure 6). A second step in interpretation consists in identifying the depth of interface from measurement, using the inverse analysis. This approach has not been validated until now. Works with impulse infrared thermography try to manage it: by the knowledge of the thermal solicitation, the measurement of material physical parameters is interpreted in terms of damage characteristics (notably the depth). One of the main difficulties to assess depth of interface with investigation from the surface is the ability to account for gradient properties. Concrete is generally variable with depth (from the surface to its core), due to skin effect but also for example to carbonation, or to water content variations (see the case of a fire damaged concrete). This variation in physical properties leads to increase the complexity of assessment of parameters in depth, since they increase the unknown variables in the inverse problem. The contrast in physical properties compared the surrounded concrete, is influenced by internal properties of concrete, and it may not be directly the characteristics of alteration which is detected. In other words, it is an open question to translate the detected parameter (corresponding to the equivalent response of anomaly, including its irregularities, its gradient ), into the real alteration profile. 477

6 Reference signal on an undamaged area Difference of the maximum signal amplitude frequency correspond to a difference of thickness Signal measured on a damaged area Figure 6: Amplitude vs. frequency of acoustic signal with the impact echo technique (this result allows detection of a delamination in the thickness of a concrete slab) 3. TECHNIQUES USED TO STUDY INTERFACES ON CONCRETE STRUCTURES Solutions are proposed today to study interfaces (detection, characterisation ) with a non destructive approach. These more or less developed methods provide various kind of information. Here we distinguish common method, which are commercially used today, from special technique which still are under development or which have just been used in some instances. We only evoke investigations performed on real structures. 3.1 Thermography Thermal methods, namely active or passive infrared thermography [3] [4], are techniques used to detect delamination and bond in concrete. The passive use of thermography is more common in this context. A limitation of the technique is the depth of alteration that can be detected: when delaminations are too deep inside, thermography is not discriminant. A standard exists which describes the use of passive infrared thermography to detect delamination in bridge decks [5]. One limitation of this procedure is that the thermal image shows only a top view. The location of the subsurface anomaly can be described only in two dimensions, on the surface of the pavement. The depth of the defect, the third dimension required to locate the exact position in three-dimensional space, is not known. A second limitation is that the climatic conditions must be such that the scanner can determine the differences in the concrete temperature. The structure must be warm enough to radiate heat in a wide enough band for the scanner to detect it [6]. The aim of investigation with thermography is to detect disorder (Figure 5), and there is no result in term of inversion (depth of bond ). Information looked for is only relative, and no calibration is needed. Concerning the active infrared thermography, the method is under development. The need to heat the sounded surface leads to practical problem concerning the homogeneity of heating, some practical problem linked to the heat apparatus (energy supply, investigations speed ). 478

7 But this approach enriches infrared thermography (the passive way) of the ability to think about inversion of results [7]. 3.2 Ultrasonic echo Acoustic techniques are sensitive to mechanical properties of material, for example to the stiffness of concrete, but also to localise damages (such as cracks and voids) and structural elements (like tendon ducts) [8]. When applying the ultrasonic echo an exact measurement of the ultrasonic speed in the material is necessary. The measurement of the wave time-of-flight (in the ultrasonic range), allows the detection of an interface and the measurement of the depth. By the measurement of the signal intensity it is also possible to assess the grade of alteration (waves being reflected on the interface). By through transmission it is also possible to check bonded and un-bonded areas in the concrete. In this case, both sides of the structural element must be accessible). Up to now ultrasonic methods in civil engineering are only standardised for through transmission [10] [11] [12]. A comparative study has been done to compare different echo methods [9]. The thickness of a concrete slab was measured at different locations having different thickness. The comparison of the values shows that the maximum uncertainty of thickness measurement is +/- 5 mm. This confirms these methods to be a reliable way to measure the depth of delaminations in concrete [9]. 3.3 Impact echo The impact echo technique [13] is quite similar to the ultrasonic method, differing by the source of the signal: in this case the mechanical input is linked to a steel ball fall. The study is done in the frequency domain, to determine which frequencies carry the signal energy reflected on interfaces (this information depends on the depth of interface). Some results (Figure 2, 4, 6) present the capacity of impact echo to detect (by zone) then to describe the location of a debond in a concrete slab. A standard outlining the methodology to determine the thickness of slab is used for characterization of debonding [14]. The depth of investigation is linked to the frequency by the relation d=v/2f with d the investigation depth, v the wave velocity, and f the peak of frequency spectrum. The depth of investigation and the resolution depend on the source (diameter of steel bar) and keep it limited. This method allows the analysis of about a few square meters per day (as a function of the investigation step, generally about a few decimetres). 3.4 Special techniques Some others ND Techniques are also developed and tested regarding the question of delamination and interface. Their results show their sensitivity to such defects but today, being their state of advancement on the subject, they cannot be recommended as standard techniques. GPR (ground Penetrating Radar) consists of emitting an electromagnetic impulse in the material, and to record the signal which has been reflected. By measuring the time between emission and reception of the signal, one can find the location of the reflector (defined as a body presenting a contrast in permittivity relatively to sound concrete) [15]. Radar is more adapted to detect larger voids than delaminations. Scott et al. (2003) [16] tested radar on site (antenna of 1.5 and 2.4 GHz) and did not obtain consistent response regarding delamination. According to them this technology is not mature enough to allow delaminations to be 479

8 detected. The main limit is the relation between the size of alteration and its depth; in other words, the measure resolution vs the depth of investigation. However, in some cases this technique succeeded. An example shows that in case of repaired delaminated areas in a prestressed concrete bridge, a clear reflection could be detected. In Figure 7, three radargrams recorded along the same trace with a length of about 2.5 m on top of a prestressed concrete bridge are displayed. Position in cm a b Time in ns a c b Depth in cm d a Figure 7: Radargrams of a trace recorded with different antennas. Top: 900 MHz antenna, polarisation perpendicular to the rebars. The backside reflection (a) as well as the tendon ducts (b) are displayed; Middle: 1.5 GHz antenna, polarisation parallel to the rebars. The backside reflection (a), the tendon ducts (b) and rebars (c) are visible. Bottom: 1.5 GHz antenna, polarisation perpendicularto the rebars. Only the reflection from the delaminations is visible (d) These radargrams were recorded with the 900 MHz antenna (top) and in two polarisations of the 1.5 GHz antenna (middle and bottom). The backside reflection, the position of the reinforcing bars as well as four tendon ducts can be detected easily. In the bottom radargram, where the polarisation of the electric field of the antenna was oriented perpendicular to the rebars, only a continuous reflection band is visible instead of rebars. This reflection band could be related to a delamination which was confirmed after opening of the area in a destructive way. The measure of electrical resistivity allows detection of anomalies in concrete. The electrical resistivity depends on the electrolytic conduction through the concrete, so it is sensitive to all parameters influencing this phenomenon: porosity, water content, salinity of water, presence of voids, cracks or interface which can appear as preferential ways for fluids to flow or on the contrary act as an insulator barrier. The sensitivity of the technique to interfaces is linked to the disturbance of the investigated volume. In the case of an interface 480

9 (between two materials with different resistivities) or of a void, the electrical current circulates differently than in a homogeneous material. Figure 8 presents apparent resistivity variations on a concrete slab [17]. Cracks and also delaminations can be localised by their effects on electrical properties: a crack, in this case, appears more conductive than sound concrete; on the contrary, a delamination increases the apparent resistivity. delaminations Size of the measurement device Map of identified damage Crack Y (cm) Apparent résistivity (Ohm.m) X (cm) Figure 8: Electrical resistivity to detect and identify delaminated zone on a concrete slab [17] 4. CONCLUSION Interfaces and delamination are alterations impacting the structure response and its service life. Technical solutions to study them exist, but they are not many and could be improved in their response to the problem. Indeed, interfaces can be described by a wide range of factors (thickness, depth, activity ) and development of new tools is in progress to better satisfy this need. Other identified needs are the better definition of sensitivity of NDT, of thresholds and limits of techniques. The inversion of results is also a serious stake. Beyond the research activity aimed at improving the practical solutions to the problem of interface assessment, the RILEM TC INR is active also in other directions. After drafting a state of the art on the subject, next group s work will be on possible combination of techniques, then on the design or recommendations for test sites. REFERENCES [1] Toussaint, P., Dondonné, E., Synthèse d un rapport de la Direction de l expertise des structures du ministère wallon de l Equipement et des Transports (MET) de Belgique, [2] Naar, S., Evaluation Non Destructive du béton par mesures de résistivité électrique et thermographie infrarouge passive, Thèse de l Université Bordeaux 1, 2006, 248p. [3] Gaussorgues, G., La thermography infrarouge : principes techniques applications, Quatrième Edition revue et argumentée, Technique et Documentation, Lavoisier, Paris (F), ISBN , 1999, 587p. 481

10 [4] Maldague, X. P. V., Non-destructive evaluation of materials by infrared thermography, London, Springer-Verlag, [5] ASTM D : Test method for detecting delaminations in bridge decks using infrared thermography, [6] Alt, D., Meggers, D., Determination of bridge deck subsurface anomalies by infrared thermography and ground penetrating radar: Polk-Quincy viaduct I-70, Topeka, Kansas, Kansas Department of Transportation, Report No. FHWA-KS-96-2, 1996, 18p. [7] Maierhofer, C., Brink, A., Röllig, M., Wiggenhauser, H., Transient thermography for structural investigation of concrete and composites in the near surface region, Infrared Physics &Technologie, 43 (2002) [8] Krause, M., Mielentz, F., Milmann, B., Müller, W. and Schmitz, V., Imaging of cracks and honeycombing in concrete elements, Arnold, W. and Hirsekorn, S. (Eds.); Proceedings of 27th International Acoustical Imaging Symposium, March, 2003, Saarbrücken, Germany, Kluwer Academic/Plenum Publishers, Dordrecht and New York, 2004, [9] Beutel, R., Reinhardt, H.-W., Grosse, Ch., Glaubitt, A., Krause, M., Maierhofer, Ch., Algernon, D., Wiggenhauser, H. and Schickert, M., Performance Demonstration of Non-Destructive Testing Methods, Proceedings of the 9th European Conference on NDT, September, Berlin, 2006 (DGZfP, BB 103-CD, Tu.3.2.2, 2006). [10] EN , Determination of ultrasonic pulse velocity, Testing concrete- Part4, 14p. [11] C597: Test method of pulse velocity through concrete. [12] BS1881, part : Recommendation for the measurement of velocity of ultrasonic pulse in concrete. [13] Sansalone, M., and Carino, N.J., Impact-Echo: A Method for Flaw Detection in Concrete Using Transient Stress Waves, NBSIR , National Bureau of Standards, September, 1986, 222 p. [14] C1383: Test method for measuring P-wave speed and thickness of concrete plates using the impact-echo method. [15] Daniels, D.J., Surface penetrating radar, Inst. Electrical Engineers, London (UK), 1996, 300p. [16] Scott, M., Rezaizadeh, A., Delahaza, A., Santos, C.G., Moore, M., Graybeal, B.,Washer, G., A comparison of non destructive evaluation methods for bridge deck assessment, NDT&E International. 26 (2003) [17] Lataste, JF., Sirieix, C., Breysse, M., Frappa, M., Electrical resistivity measurement applied to cracking assessment of reinforced concrete structures in civil engineering, NDT&E international. 36(6) (2003) (ISSN )