1

Study of the gammagraphy technique in the inspection of prestressed concrete structures

By Duarte M. Soares1

April, 2014

Abstract

The ageing of current structures and the rise of construction costs have substantially increased the need for rehabilitation and repair of prestressed concrete structures. With the increasing focus on sustainability and durability of constructions in civil engineering, it becomes essential to develop techniques to identify the characteristics of these constructions and their repair needs. The choice of the repairing method should only be made after the effective application of a technical inspection and diagnosis. Hence, the proper identification of the likely causes for anomalies in prestressed concrete structures will be possible, as well as the exact definition of those anomalies. The destructive techniques, which are more widely used, have impacts on the structure, unlike the non-destructive techniques. The non-destructive inspection and diagnosis techniques have known notable developments made possible by the development of recent technology. With the aim of contributing to the knowledge and development of non-destructive inspection and diagnosis techniques, experiments were developed with gamma radiation in prestressed concrete structures. The technique, called gammagraphy, presents the results in the form of images, similar to X-rays in medical applications. These images can be treated through a tomographic technique, to obtain three-dimensional profiles. The investigation presented in this paper consisted of the irradiation of two prestressed prototype beams with simulated defects, namely, (i) cracks, (ii) voids and (iii) honeycombing in concrete and (iv) voids in metallic ducts containing prestressed tendons. The results allowed to identify the heterogeneous aspect of concrete and to verify the existence of steel rebars and prestressed ducts. Moreover, it was possible to identify anomalies such as voids in concrete, honeycombing in concrete gravel and voids in prestressed ducts. Results allowed concluding that with proper development and calibration, the gammagraphy technique may be a very useful technique in the analysis of defects with a high level of accuracy and at any point of the structure. Keywords: Prestressed concrete, anomalies, non-destructive inspection and testing methods, gammagraphy, digital images

1. Introduction

It is now well-known that concrete, besides having a strong structural function in most constructions, has also a measurable (and finite) durability, which is directly linked to its maintenance, the environment to which it is exposed, as well as the level of execution quality. Given the growing number of anomalies, especially those related with durability in prestressed concrete structures, it is important to dispose of effective techniques which allow to analyse their behaviour or level of deterioration. Ideally, these techniques should be applied without damaging the concrete, which is not always possible. One of the techniques used for non-destructive evaluation (NDE) and non-destructive testing (NDT), which only recently begun to be studied for this type of application, is the gammagraphy technique. According to the electromagnetic spectrum and compared with other types of radiation, the gamma radiation has reduced wave lengths (or high frequency) which is associated to its high energy. As a result, it is

                                                            1 MSc student in Civil Engineering at Instituto Superior Técnico, Lisboa.

also much more dangerous, because it interacts with matter by way of photoelectric and Compton effect. The purpose of this paper is to analyse and discuss the potential of gammagraphy by assessing its efficiency in identifying defects in prestressed concrete structures.

2. State-of-the-art review

All elements made of prestressed concrete undergo degradation over time, which is exhibited in the form of anomalies. These anomalies can be divided into structural and non-structural and their causes are often difficult to define. The European standard EN 1504-9 defines the probable causes for those anomalies and gives guidance on the best repairing and strengthening methods to use. According to Brito & Flores [1], the characteristic of the materials that compose concrete and its reduced resistance to tension cause inevitable cracking. This anomaly can, for instance, accelerate the process of rebar corrosion and be a sign of the structure malfunction. There are other defects that also affect concrete structures, such as concrete disaggregation and delamination; voids and

2 

honeycombing in concrete; and voids in metallic ducts containing prestressed tendons, which can have the effect of diminishing the overall bending resistance of the structure. Nevertheless, according to Maierhofer [2], the most significant anomaly in concrete structures is chloride-induced and carbonation-induced corrosion of rebars and prestressed tendons. Indeed, together they are responsible for 71% of structural failures of German infrastructure buildings. The process of corrosion of steel begins with the dissolution of the protective passive film, which takes place in the presence of moisture and oxygen, reducing the pH of concrete to below 9; or when the concentration of chloride ions reaches a certain critical value. With the purpose of identifying and characterizing the anomalies described above and finding the best repairing methods, inspection techniques are often used. In most cases, destructive techniques are used with this purpose. Beyond their limitations, such techniques have major drawbacks, as they require the repair of the tested area (which sometimes needs to be completely replaced), involving the creation of dust and debris and can cause irreparable damage to structures with ornamental character or artistic value. In this context, non-destructive techniques ought to be adopted, as they allow seeing the invisible, remotely detecting features and objects hidden behind opaque surfaces without or with very few impacts on the structure. This paper focuses on an electromagnetic method of testing concrete, more specifically one that involves gamma () radiation, the gammagraphy technique. A -ray is a type of electromagnetic energy (a photon) naturally emitted by the nucleus of radioactive atoms, such as 192Ir, 137Cs and 60Co. In gammagraphy tests, the emitted gamma rays pass through the structure and then are recorded on photographic plates or special spectrometer detectors, in the form of gammagraphs, similar to radiographs. Thus, in the final gammagraphy voids within the concrete or prestressed ducts are translated by darker areas, and the rebars and prestressed tendons, denser than concrete, by lighter areas. Gamma rays can be used to examine the interior of concrete with photographic reliability and accuracy superior to that obtained with non-destructive inspection techniques used in the civil engineering field. Gamma radiation is a type of ionizing radiation and it is able to cause, directly or indirectly, the formation of ions, resulting in chemical changes in atoms or molecules of living tissues, causing cellular changes. Having this in mind, it is important to follow strict security procedures that are intended to ensure the safety of operators and prevent accidents and exposure to the population. There are some proposed measures, which consist of adopting rational methods in order to reduce exposure, such as the ALARA (As Low As Reasonable Achievable) principle, which recommends the adoption of a reasonable distance between the operator and the radiation source, the use of shielding materials that absorb radiation and the

minimization of the exposure time which will drop the radiation dose the operator is subjected to. The use of -rays for the analysis of laboratory samples of concrete was first reported by Mullins & Pearson [3] and the possibility of extracting three-dimensional information by Whiffin [4] (authors cited in [5]). Since then, numerous articles on the use of electromagnetic radiation in general (X-rays, gamma rays from radioactive sources and radiation from electron accelerators) for the analysis of concrete have been published. However, probably due to the fact that licenses are necessary and due to plausible fear of the population against the use of radiation, little interest in this non-destructive technique in civil engineering has limited the development of its full potential (McCann & Forde [6]; and Malhotra [7]; cited in [8]). Shinohara et al. [9] conducted a study with the aim of evaluating the technique of digital radiography using gamma rays. The study consisted of the detailed analysis of the degree of corrosion in steel pipes with different diameters. The results in Figure 1 show the potential and feasibility of using digital technology combined with gammagraphy to accurately detect defects.

Figure 1 – Defects in steel pipes, shown in red (adapted from [9]).

According to Frigerio et al. [10], an Argentinian company named Thasa, in collaboration with students from the University of Buenos Aires, carried out a study aimed to compare a conventional gammagraph with a digital one. An empty bottle of Coke was placed to simulate honeycombing within a block of concrete. Whereas in the projection plane perpendicular to the radiation the conventional gammagraph showed a higher spatial resolution (Figure 2), the digital gammagraph offered the possibility to manipulate the data, enabling the accurate determination of possible anomalies, as well as their representation in three-dimensional models. Concerning the study promoted by Frigerio et al. [10], it is possible to see in Figure 3 an example of an analysis of a digital gammagraph of a concrete structure and its three-dimensional representation. The same study states that the maximum concrete thickness 192Ir photons can cross is 30 cm. For higher values, holes in the concrete should be drilled and the radiation source placed inside them.

 

3

Figure 2 – Conventional (left) and digital (right)gammagraph (adapted from [10]).

Figure 3 – Digital processing of data (left) and three-dimensional reconstruction (right) (adapted from [5]).

Gammagraphy can also be used to identify corrosion in steel bars in reinforced concrete [5]. Figure 4 shows rebars with heterogeneities and evident signs of corrosion. The graph plotted in Figure 5 was obtained from a study by Mariscotti et al. [5] and shows the variation of the diameter of a bar relatively to the initial value (8 mm). It is possible to clearly identify an area (corresponding to 90 mm in the abscissa) in which the loss of useful section is significant, about 2 mm. The results demonstrated the degree of precision provided by gammagraphy, as well as the great potential of this technique in the analysis of defects with very small dimensions. In the same paper, post-tensioned beams were also inspected. Besides being able to identify the prestressed ducts, the experiment showed voids due to lack of grouting (Figure 6). A study was conducted by Oliveira Jr. et al. [11] using a 241Am gamma source with the purpose of checking the distribution of the various constituents of concrete and determining the trace of concrete (ratio between sand and binder mixture). In Figure 7 the curve representing the analytical prediction of the variation of the concrete trace with its attenuation coefficient is similar to the curve generated by experimental results. As all the samples were previously dried, the attenuation coefficient of water was not taken into account, which may explain the fact that the experimental results have a slightly lower coefficient of mass attenuation than the analytical ones.  

Figure 4 – Gammagraphs: a rebar with signs of corrosion (left) and a rebar with severe corrosion(right) (adapted from [5,10]).

Figure 5 – Analysis of corroded reinforcing bar: gammagraphy of the bar (top); edge pixel positions in terms of distance to the “mean centre” of the bar along bar length (centre); variation of the bar diameter (bottom) (adapted from [5]).

Figure 6 – Gammagraphs: voids in grouting of the ducts (left) and existence of tendons inside the duct (right) (adapted from [5]).

Conventional gammagraph Digital gammagraph

Post-tensioned tendons

4 

Figure 7 – Dependency between the trace used in theconcrete samples and the mass attenuation coefficientsof gamma ray (cm2/g): experimental data obtained from tomographic techniques using gamma radiation(dots) and theoretical values (solid line) [11].

There have been some studies with computerized tomography (CT) using X-rays which can point the future of the gammagraphy technique. Tomography consists of the full three-dimensional reconstruction of an object subjected to various projections at different angles add up until they reach one full turn. If -rays are used, then each projection can be considered a gammagraph. One of these studies aimed at evaluating the ability of CT to identify voids and the layout of gravel in the concrete and it was carried out by Mendes (2010). Cylindrical concrete samples were tested and, through software analysis, three-dimensional profiles were created. The software, using the marching cubes algorithm, was able to identify different materials, allowing to render the interface between mortar and air or to show only gravel, as shown in Figure 8.

Figure 8 – Tomographs of the samples used byMendes (adapted from [12]).

Sprague [13] conducted a study that allowed to clearly identify the surfaces of discontinuity between the concrete (cracks), the rebars and even air bubbles trapped along them (Figure 9). Air bubbles can compromise the adherence between the rebars and the concrete.

Figure 9 – Tomograph where the existence of air bubbles trapped near the bar is clear (adapted from [13]).

The studies mentioned in this literature review show results with a high level of precision. This can only be obtained using electromagnetic radiation, for instance, gamma radiation.

3. Experimental Study

Tests were conducted based on the emission of gamma rays photons produced by a radioactive material. The aim was to define an optimal procedure and equipment in order to unequivocally identify the anomalies of prestressed concrete structures. Two prestressed concrete beams with identical dimensions (Figure 10 and Figure 11) were designed. Inside them, common anomalies were simulated, such as voids in grouting of prestressed tendons, voids, honeycombing and cracks in concrete.

The tests involved (i) testing the various ways of irradiating the beam, (ii) testing different types of image acquisition equipment, and (iii) assessing the best way of processing the data.

Figure 10 – Geometry of the prototype girder: side view (left) and cross-section (right).

The voids in prestressed tendons were made by drilling a hole in the duct and inserting a steel bar in concrete which was then distanced 5 cm from the duct. As shown in Figure 12, the empty space of 5 cm length simulates a void near the duct. Figure 13 shows a simulated crack (made of a black plastic film) and a honeycomb.

 

5

Figure 11 – Three-dimensional representation of theprototypes and their simulated anomalies.

Figure 12 – Representation of voids in prestressedducts.

Figure 13 – To the left, simulation of a crack inconcrete and, to the right, simulation ofhoneycombing in concrete.

The equipment used for this type of testing has the particularity of being dangerous to health and the environment. The chosen radioactive source was Cobalt-60, which has more energy than Iridium-192 and Selenium-75, for example. Its half-life is also higher (5.27 years), so it does not require a continuous investment. However, as it emits photons with a higher intensity, it is more dangerous than other less powerful sources. According to Figure 14, the optimal range is about 30 cm of concrete thickness to about 50 cm. Digital image processing should be adopted for higher thicknesses. The source container, called projector, is installed in a wheelbarrow, as shown in Figure 15. One of its extremities is coupled to a guide tube that leads to the collimator. The collimator is responsible for dispersing the radiation (Figure 16). The other extremity of the wheelbarrow is connected to a hose, which is a control unit that triggers the movement of the source to the collimator. The

radiation that crosses the beam is then detected by an acquisition system, composed of an acquisition plate, namely an Image Plate (IP), a lead plate, to increase contrast, and a scanning equipment. The final system setup is represented in Figure 17. Finally, the data obtained was analysed with software programs, including ImageJ and Photoshop CS 6 Extended.

Figure 14 – Range of concrete thickness for different sources and techniques (adapted from [14]).

Figure 15 – Source projector (left) and wheelbarrow(right).

Figure 16 – The collimator and the dispersion of radiation in concrete.

In order to perform the tests, it was necessary to calculate the exposure time, in other words, the time that the source is irradiating outside of the projector.

Prototype 2

Prototype 1

5 cm

6 

Figure 17 – Schematic representations of a conventional assembly: shielding container (projector), collimator, guidetube, and control unit (adapted from [5]).

The exposure time depends on several factors including the following: (i) type of acquisition plate, (ii) value of the source activity, (iii) source-plate distance, and (iv) density of the materials crossed by the photons. IAEA [15] provides the following expression for calculating the exposure time:

)100.(.

60...

2

2

expRHMA

HVLe

SFDFFt   (1)

where

texp is the exposure time (in minutes), FF is the film factor, SFD is the source-to-film distance, T is the thickness of concrete, HVL is the half value layer, A is the source activity (Ci), RHM is the output of the source.

However, this expression is not suitable to estimate the exposure time when the concrete thickness is about 50 cm. In fact, IAEA [15] presented a linear formula of the exponential function Lambeert-Beer (expression 2), as it considered the concrete thickness to be only 7-11 cm and used the Iridium-192 source. The relationship between the intensity of incident and transmitted photons is:

xeII .0.   (2)

where

I is the transmitted photon intensity, I0 is the incident photon intensity, µ is the attenuation coefficient, x is the thickness of object.

The attenuation coefficient is the fraction of photons that interacts per unit of the thickness of the material crossed by the radiation. The attenuation coefficient

                                                            2 Values provided by the company Medical Consult, Lisbon.

for some materials is listed in Table 1 and it depends on the energy of the source.

Table 1 – Values of attenuation coefficients of different materials for a source with 1.17-1.33 MeV of energy2.

Material µ (cm-1) PVC 0.082

Concrete 0.134 Steel 0.421 Air 0.060

The company Thasa developed an expression to calculate the exposure times using the normalized exposure time using a source of 20 Ci of activity and irradiated material with 50 cm thick. The time (in minutes) is given by the expression [16]:

2

expexp 50

.06,1.

20'.3,1

SFD

Att   (3)

where

t’exp is the exposure time (in minutes) obtained from the logarithmic graphic provided in Figure 18, normalized for Agfa D7 conventional films,

A is the source activity (Ci), which in this study was 27.2 for the 60Co source,

SFD is the source-to-film distance.

In the above expression, the factor 1.3 is due to the application of lead filter used to optimize the contrast. The factor 1.06 represents an increase of 6% in the thickness of the irradiated material, to take into account factors such as the thickness of the acquisition plate. The company Medical Consult, responsible for calculating the exposure times, preferred to mathematize the logarithmical graphic to avoid reading errors. The t’exp can be calculated with the expression:

 

7

SFDt ..06,1.05807,0

exp 1255,0'   (4)

where

ρ is the concrete density, SFD is the source-to-film distance.

The value 0.05807 cm2/g refers to the /ρ coefficient and was calculated by Medical Consult, based on the values provided by NIST – National Institute of Science and Technology. The value 0.1255 is just a fit to the graph to guarantee consistency of the function with the values of the graphic.

Figure 18 – Logarithmical graphic used by Thasa(adapted from [16]).

Six tests were conducted, using three types of image equipment. The first system was based on results transmitted via wireless to a computer which allowed real time and on site analysis. The second and third tests were made with high-resolution IPs, which provided results that had to be revealed in laboratory. The rest of the tests were conducted using lower

resolution IPs. The characteristics of each test are described in Table 2 and the location of each gammagraph is illustrated in Figure 19.

4. Results and discussion

The first test had the purpose of finding a void in prestressed ducts. Given the fact that the source was leaning against the concrete surface, the acquisition plate was on the other face of the beam; the beam was 50 cm thick, then the SFD was 50 cm. Using the expressions (3.3) and (3.4) and knowing that A was 27.2 Ci and ρ is 2.3 g/cm3, then the calculated exposure time was 32.2 minutes. However, the maximum exposure time allowed by the equipment (from the company Vidisco) was only 3 min. Even though the system was digital and allowed the data to be collected and visualized on site and in real time, it produced low quality results, probably due to the equipment limitation. Another data acquisition system was tested. In this one the acquisition plates were of a high spatial resolution ones (50 µm) generally used in the inspection of welds. The results of the test number 2 were obtained in the format of DICOM images. The gammagraph depicted in Figure 20 had the goal of identifying a void in a prestressed duct, which was not possible. Notwithstanding, it was possible to identify rebars which were marked with blue, violet and green colours representing peaks in a grey levels’ profile and corroborating the thesis that higher attenuation coefficients, and therefore higher density objects, have a higher grey value. This grey value’s profile corresponds to the distribution of grey values along the red solid line marked in the gammagraph.

Table 2 – Characteristics of the tests conducted in the experimental study.

Test No.

Acquisition Plate Prototype

No. Anomaly

Source position

(cm)

Centre ofacquisition

plate position

(cm)

Source-to-film

distance (cm)

Exposure time (min)

Calculated Used

1 RayzorX Pro DDA

1 Void in prestressed duct

(20, 70, 0) (20, 67, 50) 50 32 3

2 XL Blue Carestream Industrex (50 µm)

(20, 70, 0) (20, 67, 50) 50 32 32

3 FLEX HR Carestream Industrex (50 µm)

(20, 70, -58) (20, 67, 50) 108 146 146

4 2 (158, 70, 9) (158, 70, 50) 41 6.5 6.5

5

Fujifilm IP Cassette Type CC (200 µm)

1 Honeycombing (20, 13, 0) (21, 17, 50) 50 32 32

6

2

Void in prestressed duct

(70, 33, 0) (71, 32, 50) 50 32 35

7 (145, 33, 0) (150, 33, 0) 50 32 35

8 Void in concrete

(150, 12, 0) (150, 17, 0) 50 32 40

9 Crack in concrete

(226, 16, 0) (226, 17, 50) 50 32 35

Equivalent thickness of concrete (cm)

Exp

osur

e tim

e (m

in)

8 

Figure 19 – Location of the gammagraphs in the prototypes.

It was also possible to verify the penumbra effect represented by a concave shape (arc represented in orange) in the graph which means it gets darker as it approaches the centre. The geometric penumbra consists of shadows at the edges of the gammagraphs or in regions with changes of geometry.

Figure 20 – Grey levels’ profile related to test 2.

This effect usually decreases when the source-to-film distance (SFD) increases and the object-to-film distance decreases. In the test 3, SFD was then increased to 108 cm. As a result, the estimated exposure time was 146 min. The result showed inferior quality compared to the gammagraph from the previous test (Figure 21). It is important to refer that the rectangular area is clearly visible due to the fact that the lead plate had smaller dimensions than the IP. Increasing the distance did not improve the gammagraphs’ quality. In fact, besides increasing the duration of the test, increasing SFD by positioning the source away from the beam created far more radiation to the environment than before, because the effect of shielding provided by the beam was no longer present. As a consequence, the photons were able to disperse in more directions. In order to assess the quality of the data acquired by high-resolution plates, a histogram was created, as shown in Figure 21. It represents the number of pixels with a determined grey value. Some grey values were not associated with any pixels. This means that many pixels were left empty, which may be due to an insufficient number of photons that crossed the

concrete and were detected or to the excess of resolution of the phosphor plate. The higher the resolution, the more photons are needed to cover all the pixels (as opposed to one with lower resolution where the same photons can fill all pixels).

Figure 21 – Grey levels histogram related to test 3.

Using the same high-resolution IPs, another test (number 4) was carried out, but this time the source was placed inside the prestressed concrete beam. The SDF value was 41 cm and the exposure time reduced significantly to 6.5 min. The results were slightly better but only inside the lead plate area (Figure 22). Acknowledging that using high-resolution IPs was not the best choice, new tests were carried out with lower resolution ones (200 µm). On the test 5 the source-to-film distance was 50 cm and the exposure time was 32 min. The goal was to identify a void in the concrete. Firstly, the gammagraph obtained was converted from DICOM to PNG format. By analysing Figure 23, despite the number of grey values dropped, apparently the histogram changed to an approximately normal distribution, resulting in an image with better contrast. The contrast was then enhanced using the Adobe Photoshop CS 6 Extended application. As illustrated in Figure 24, the void is clearly visible as well as the rebars. Histograms corresponding to different profiles of the gammagraph were created and, given the fact they are consistent with respect to each other, the method was validated.

Prototype 1 Prototype 2

2

3

4

5 6 7

89

Honeycombing Void in concrete Crack in concrete Prestressed duct

Num

ber

of p

ixel

s

Grey value

 

9

Figure 22 – Grey levels profile related to test 4.

Figure 23 – Comparison of histograms: the left isrelated to the gammagraph in DICOM format and theright to the PNG format.

Figure 24 – Grey levels profiles related to test 5.

One of the characteristics of the gammagraphy technique is the presentation of images in black and white colours. The existence of a greyscale may mask some aspects of the gammagraphs that could be useful in the analysis. As such, it was decided to test the idea of turning the map in a grey colour gradient scheme (gradient map) using primary colours (RGB: red, green and blue) and the RYB scheme (red, yellow, blue). The results presented in Figure 25 leave no doubt about the existence of a defect, which confirms the good performance of these IPs.

Figure 25 – Gammagraphs of test 5 in colours: the left in RGB and the right in RYB.

With the use of computational methods from ImageJ editing program, namely the Interactive 3D Surface Plot command, a three-dimensional model was constructed using the RGB scheme (Figure 26). It shows the honeycombing as a depression which means it has low density. This was not expected because gravels have higher density than cement. The reason may be related to the possible existence of trapped air inside the honeycombing.

Figure 26 – 3D colour model of test 5.

The sixth test was carried out with the purpose of identifying the simulated void in prestressed ducts. The exposure time was increased to 35 min to verify if it produced significantly different results. After manipulating the gammagraph to increase its contrast, the prestressed duct and the tendons were clearly visible (Figure 27). A tubular shape is depicted in the gammagraph and represents the simulated void, as Figure 12 showed previously. The gammagraph was then represented in the RYB colour scheme. The transformed image showed interesting results, which may suggest deficiencies in concreting around the duct. Another gammagraph (test 7) was taken to the prestressed ducts in a different location along its length. Besides showing another simulated void in the duct with similar characteristics, a darker area was also identified, possibly caused by lack of grouting in the duct (Figure 28). According to Table 1, air has low

Num

ber

of p

ixel

s

Grey value Grey valueNum

ber

of p

ixel

s

Rebars Honeycombing

Honeycombing

Rebars

y

x

10 

attenuation coefficient, which would correspond to a darker area.

Figure 27 – Gammagraphs of test 6, original (left) and in RYB colours (right).

Figure 28 – Gammagraphs of test 7, original (left) and in RYB colours (right).

The test 8 tried to identify a simulated void in concrete. The distribution of the grey values along the profile traced in the gammagraph in Figure 29 shows the existence of lower grey values in a specific area (marked with grey colour). It represents a void in concrete, as the mean values of grey (represented in a yellow line) are lower than the mean of the rest of the gammagraph. Voids consist of air volumes trapped inside concrete and, as they have lower attenuation coefficient than concrete, their colours appear darker. The gammagraph was then transformed into RGB and RYB colour schemes (Figure 30), confirming the conclusions. In Figure 31 a 3D modelling of the gammagraph shows the existence of the void, which assumes a green colour. As to the test 9, the equipment did not show enough sensitivity, required to distinguish the small sheet of plastic simulating the crack in concrete. As a crack always has a small thickness and as the air has a low attenuation coefficient (such as plastic), the anomaly appears invisible to the detector. The gammagraphy technique does not appear to be suitable to assess the existence of cracks in the concrete, or to measure its length, at least for the geometry of concrete members analysed in this study.

Figure 29 – Grey levels profile related to test 8.

Figure 30 – Gammagraphs of test 8 in colours: in RGB (left) and in RYB (right).

Figure 31 – 3D colour model of test 8.

Prestressed duct Simulated void

Simulated void Prestressed duct

Void in concrete

y

x

Rebars

Void in concrete

 

11

5. Summary

The existence of structures with several decades has originated frequent situations of structures needing to be repaired and/or strengthened. Due to their advanced age, in many cases the original design is not available, nor is it guaranteed that it has been followed. As such, if the status or location of the reinforcement in concrete structures is not known, it becomes essential to perform inspection techniques to determine the current condition of the structure. The inspection may include destructive techniques, the most common, and non-destructive techniques (NDT). The non-destructive ones offer advantages compared to the previous. Besides and thanks to the new technologies, they still exhibit great potential for development. One of those techniques is the application of gamma radiation to examine prestressed concrete structures in order to assess the quantity and position of rebars and prestressed ducts, as well as possible anomalies therein or in the concrete matrix. This paper described an experimental campaign, which aimed at assessing the effectiveness of the gammagraphy technique to detect different types of defects on structural elements of prestressed concrete. Therefore, this work is expected to become a practical guide for the application of this technique and provide the basis for future investigations. The experimental study included three types of equipment. The first, with wireless technology, enabled instant viewing of results. However, its incorrect calibration for concrete did not yield satisfactory results. Nevertheless, the equipment proved to be practical and relatively expeditious and therefore suited to practical situations on construction sites. The second equipment was based on high-resolution IPs, commonly used for the identification of welds in industrial engineering. These IPs promised sharp and high contrast images. However, the results were not that lightening, probably due to the existence of noise in the gammagraphs. Again, one concluded that the equipment was not prepared to analyse objects with high thickness such as the beams tested. Finally, a third acquisition device, with lower resolution plates, was used and provided better quality results. It was possible to identify honeycombing and voids in concrete and prestressing ducts. However, the results did not allow to find any cracks in concrete. The use of gamma radiation offers promising results, by obtaining images with a higher spatial resolution compared to any other technique. The fact that electromagnetic radiation related techniques are not more widely used may be related to the numerous safety measures and strict regulations that it is necessary to follow. A high investment is

needed to acquire the equipment and hire specialized staff to operate it and perform maintenance routines. 6. Acknowledgments

The author acknowledges support from the Teixeira Duarte S.A company. He also thanks the Medical Consult company for the scientific and technical assistance.

7. References

[1] Brito, J. & Flores, I.; Folhas; Diagnosis, pathology and rehabilitation of reinforced concrete buildings (in Portuguese); Construction Pathology and Rehabilitation; Instituto Superior Técnico; Lisbon; 2005.

[2] Maierhofer, C.; Reinhardt, H.W. & Dobmann, G.; Non-destrutive evaluation of reinforced concrete structures; Volume 1: Deterioration processes and standard test methods; Woodhead Publishing Limited; 2010.

[3] Mullins, L. & Pearson, H.M.; The X-ray examination of concrete; Civil Engineering and Public Works Review; pp. 256-258; London; United Kingdom; 1949.

[4] Whiffin, A.C.; Locating steel reinforcing bars in concrete slab; The Engineer; pp. 887-888; London; United Kingdom; 1954.

[5] Mariscotti, M. A. J.; Jalinoos, F.; Frigerio, T.; Ruffolo, M. & Thieberger, P.; Gamma-ray inspection for void and corrosion assessment; Safety and utility of the technology make it appropriate for field application; Concrete International; Vol. 31; No. 11; pp. 48-53; November 2009.

[6] McCann, D.M. & Forde, M.C.; Review of NDT methods in the assessment of concrete and masonry structures; NDT&E International; Vol. 34; pp. 71-84; 2001.

[7] Malhotra, V.M.; Testing hardened concrete: non-destructive methods; The Iowa State University Press and the American Concrete Institute; 1986.

[8] Mariscotti, M. A. J.; Thieberger, P.; Frigerio, T.; Mariscotti, F & Ruffolo, M; Investigations with reinforced concrete tomography; 12th International Conference of Structural Faults & Repairs; Edinburgh; 2008.

12 

[9] Shinohara, A. H.; Acioli, E. & Khoury, H. J.; Evaluation of the technique of digital radiography in gammagraphy (in Portuguese); 6the Equipment Technology Conference; Salvador; August 2002.

[10] Frigerio, T.; Mariscotti, M. A. J.; Ruffolo, M. & Thieberger, P.; Development and application of computed tomography in the inspection of reinforced concrete; Insight; Vol. 46; No. 12; December 2004.

[11] Oliveira Jr.; J. M.; Martins, A. C. G. & Milito, A.D.E; Analysis of concrete material through gamma ray computurized tomography; Brazilian Journal of Physics; Vo. 34; No. 3A; Sorocaba; September 2004.

[12] Mendes, R; The X-ray computed tomography as a non-destructive method of concrete volumetric analysis: a case study of a concrete testimony from the hydroelectric plant of Mourão (in Portuguese); Master Dissertation in Engineering and Materials Science; Universidade Federal do Paraná; Curitiba; 2010.

[13] Sprague, T. S; X-ray tomography for evaluation of damage in concrete bond; Master Dissertation in Civil Engineering; University of Washington; Washington; 2006.

[14] Redmer, B.; Weise, F. & Ewert, U.; Location of reinforcement in structures by different methods of gamma-radiography; International Symposium of Non-Destructive Testing in Civil Engineering; Germany; 2003.

[15] International Atomic Energy Agency; Guidebook on non-destructive testing of concrete structures; Training Courses Series No. 17; Vienna; 2002;

[16] Thasa; Tomography of reinforced concrete; Buenos Aires; n.d.