Mock-up wall for non-destructive testing and evaluation of thick reinforced structures in nuclear power plants

F. Al-Neshawy(1), T. Ojala(1), M. Ferreira(2), J. Puttonen(1) 1) Aalto University School of Engineering, Department of Civil Engineering, Finland 2) VTT - Technical Research Centre of Finland, Finland

Abstract

The structures in nuclear power plants (NPP) are deteriorated due to http://www.ndt.net/?id=22843 aging and environmental stressors during the lifetime of the power plant. To assess construction quality and the long-term performance of reinforced concrete structures, different non-destructive testing (NDT) and evaluation (NDE) methods are used. Some of the challenges for assessing the performance of these structures are that (i) the assessment could be performed only during the annual overhauls when testing is time- limited, (ii) the accuracy and reliability of the available NDT testing devices, (iii) uncertainty of the international uniformity of the methods used for NDT tests, and (iv) More info about this article: the creditability of results and analyses. To increase the reliability of condition evaluation, a mock-up wall representing a NPP containment concrete wall is constructed for the critical investigation of NDT&E methods and techniques. The mock-up wall contains simulated defects, which are representing typical defects in concrete structures such as dimensional errors, finishing errors, honeycombing, cracking, delamination due to structure stresses or deterioration mechanisms, embedded foreign objects in the concrete, voids adjacent to liner, and voids in grouted tendon ducts for the post-tensioned structures. This paper introduces modern NDT techniques that are suitable for assessing the condition of the mock-up wall under the real environmental conditions. The mock-up wall for NPP thick-walled reinforced concrete structures will enable the assessment of NDT&E methods and experts experience, and provides an important and very much need education environment for NDE experts.

Keywords: NDE, concrete, thick-walled, defects, deterioration, reinforcement.

1. Introduction

Non-destructive testing of concrete structures provides information about the condition of the structure including its components and materials. With modern NDT techniques, we are able to investigate relatively small details or to scan large surface areas to map reinforcing and cable ducts. For example acoustic techniques make it possible to describe the concrete structure and its mechanical properties. By combining NDT techniques, we obtain a comprehensive picture of the internal structure of the wall and the condition of large volumes of reinforced concrete in its undisturbed state. [1] The aim of this paper is discuss the use of available modern NDT methods for measuring the performance of thick-walled concrete structure. The objectives of the investigation were to test a thick-walled concrete mock-up with NDT techniques in order to: a) identify embedded simulated defects

Creative Commons CC-BY-NC licence https://creativecommons.org/licenses/by-nc/4.0/ b) investigate the possibilities of combining NDT techniques for thick-walled concrete structure The NDT methods applied were Ground Penetrating Radar (GPR) (only for the concrete exposed surface), meter, ultrasonic pulse velocity and rebound hammer. The investigation is a part of the research project "Non-destructive examination of NPP primary circuit components and concrete infrastructure (WANDA)". WANDA is a cooperation project between VTT and Aalto University and is funded by SAFIR 2018 (The Finnish Research Programme on Nuclear. Power Plant Safety 2015 – 2018).

2. Description of the thick-walled mock-up

2.1 Concrete mix design

The mix design of the used concrete for building the wall is presented in the Table 1. The compressive strength class of C40/50 was used the concrete. Furthermore, Plus (CEM II/B-M42.5N) was used as a binder. The consistency class for the concrete was S3 class (100 – 150 mm) and a Sola-Parmix polycarbosilane-based superplasticizer [2] was used to achieve the target workability of the concrete. Two types of aggregates were used: (i) 0/8 mm (sand) with moisture content of 2.5% and absorbed water content of 0.4% and (II) 8/16 mm (gravel) with moisture content of 0.1% and absorbed water content of 0.4%. The measured air content of the fresh air content was 1.5%. The mixing time of the 2.5 m³ concrete patch was 90 seconds. Table 1. Mix design of concrete used in TVO thick-wall mock-up. Amount Volume Ingredient (kg/m³) (dm³/m³) Cement 427 138 Cold water 165 165 Flushing water 2 2 Water Extra water 4 4 Aggregate moisture 19 19 (*) 0/8p sand 1028 398 8/16 gravel 733 275 Sola-Parmix (superplasticizer) 3 2 Air content 15 Total 2378 1000 *) aggregate moisture is part of the aggregates

2.2 Size and dimensions of thick-walled concrete mock-up

The thick-walled concrete mock-up represent a segment of the safety structure of the NPP reactor. The total height of the wall is 3.2 m. The wall is divided into two parts: 1) The lower part is a concrete block with 1.0 m height, 1.9 m width and average thickness of 2.55 m (max. is 3.2 m and min. is 1.9 m) 2) The upper part is two walls with height of 2.2 m and average thickness of 0.65 m. The upper part of the wall includes the open part, which represent the space for the pipes of the primary circuit.

2 The size and dimension of the thick walled concrete structure mock-up is represented in Figure 1.

Figure 1. Shape and dimensions of the cast in place thick walled concrete mock-up

2.3 Simulated concrete problems

Generally, different mechanisms and phenomena may damage the concrete. Common causes of concrete problems during the use of the structure can be classified as:  Defects: design, materials, construction  Damage: overload, fire, impact, chemical spill  Deterioration: metal corrosion, erosion, freeze/thaw, sulfate attacks, time- depended changes in concrete chemistry When a new concrete structure is taken into service there may occur damage that is attributable to unsatisfactory construction practice. The defect may have an immediate effect on the structural integrity, such as air voids in concrete structure. Poor construction usually leads to reduced durability, which will manifest itself in later years. The service life of the structure may be reduced or extensive maintenance may be required as a result of deterioration of materials, usually reinforcement steel corrosion or concrete deterioration by aggressive chemicals [3]. The most common problems of reinforced concrete structures are:  delamination and cracks  degradation of mild steel can occur as a result of corrosion, irradiation, elevated temperature, or fatigue effects  post-tensioning systems are susceptible to the same degradation mechanisms as mild steel plus time-depended loss of prestressing force, primarily due to tendon relaxation, and concrete creep and shrinkage  honeycomb and embedded items (construction defects)  voids adjacent to liner and voids in grouted tendon ducts.

3 The simulated embedded defects based on the work of Wimsatt et. al. (2012) [4] were planned to the concrete mock-up. The simulated defects were representing defects that could have occurred during the construction process or caused by the degradation of the concrete with time. As represented in Table 2 and Figure 2, types of simulated defects embedded in the thick-walled concrete mock-up were:  delamination - imitated by using 0.05-mm plastic sheets and 0.25-mm cloth squares  air-filled voids constructed by inserting foam squares 13 mm thick in vacuum- sealed plastic bags.  water-filled voids constructed by placing water-filled bags within vacuum- sealed plastic bags and carefully padding the defect with concrete  cracks and honeycombing constructed by placing a 50*200*200 mm Expanded polystyrene insulation (EPS) pieces in the concrete

Table 2. List of the simulated defects in the thick walled concrete mock-up. Defect type Defect code Description Delamination occurs in reinforced concrete Delm1 structures subject to reinforcement corrosion Delamination Delm2 (0,2 mm plastic sheets, fixed in place) Delm3 Hon01 Honeycomb Hon02 Honeycomb Hon03 (Expanded polystyrene insulation (EPS) 50*200*200 mm, fixed in vertical direction) Hon04 Construction CD01 Construction defects defects CD02 (Wooden pieces, fixed into the reinforcement) AFV01 Air-filled voids Empty air (An empty 0.5 L bottle, fixed into the voids AFV02 reinforcement) WFV01 Water-filled voids Water-filled (A water-filled 0.5 L bottle, fixed into the air voids WFV02 reinforcement)

Figure 2. Examples of the simulated defects embedded in the thick-walled concrete mock-up.

4 3. Testing results and data analysis

3.1 Compressive strength

For evaluation of the in-place compressive strength of the thick-walled mock-up, two testing methods were used: i) the rebound hammer method for measuring the surface hardness of the concrete as NDT testing technique and ii) testing of drilled concrete sample as a destructive testing technique. The rebound hammer test was performed over 13 points in different location on the wall. Three measurements were carried out at every point. The test results of the rebound hammer test are presented in Table 3. Table 3. Test results of the surface hardness of the concrete using rebound hammer. Rebound hammer readings Compressive strength, (MPa) Average compressive Point strength, 01 02 03 01 02 03 (MPa) 1 45 40 41 49 40 42 44 2 40 47 48 40 52 57 50 3 46 48 48 52 57 57 55 4 42 42 49 42 42 57 47 5 41 42 46 41 42 51 45 6 42 40 46 42 40 51 44 7 40 40 47 40 40 53 44 8 40 40 43 40 40 46 42 9 40 45 45 40 50 50 47 10 41 40 47 41 40 40 40 11 44 40 45 48 40 50 46 12 42 48 44 42 57 44 48 13 39 42 46 39 42 51 44 Average compressive strength, (MPa) 46

In-place drilled core sample were prepared as a direct method for determining the density and the compressive strength of the concrete. The test results of the density and the compressive strength for the drilled samples are represented in Table 4. Table 4. Density and compressive strength of the drilled core samples. Weight Weight Hardened Compressive in air underwater concrete density strength, Specimen (kg) (kg) (kg/m³) (MPa) 100 x 100 mm Cored cylinder 1 2,4 1,437 2492 64

2 2,435 1,416 2390 44

3 2,438 1,438 2438 52

Average 2440 53

The results show that the average compressive strength measured by Rebound Hammer is 46 MPa, which fits to the target compressive strength of the concrete mix (C40/50). The compressive strength of the drilled specimens (D/L = 100/100 = 1) was higher than the target compressive strength of the concrete mix, the reason could be the testing

5 condition of the drilled specimens. Drilled samples stored on condition chamber with relative humidity of 45±2% and temperature of 20±2°C.

3.2. Reinforcement location

Two NDT methods were used for measuring the concrete cover depth for one side of the wall: i) the concrete cover meter and ii) the Ground-Penetrating Radar (GPR). Location of the measurements is represented in Figure 3.

Figure 3. Area of the concrete wall where the cover thickness was determined. GPR lines were marked with white arrows. As shown in Figure 3, the concrete cover depth was measured using a concrete cover meter in two lines and six measurements were performed per line. The concrete meter results are presented in Table 5. The minimum value of concrete cover was 24 mm and the maximum value was 40 mm

Table 5. Concrete cover meter measurement locations and results. (The X and Y coordinates are approximated) Line 1 Line 2 Coordinates Cover depth Coordinates Cover depth X Y (mm) X Y (mm) 0.10 1.85 30 0.10 1.70 40 0.45 1.85 25 0.45 1.70 24 0.70 1.85 40 0.70 1.70 40 0.90 1.85 25 0.90 1.70 30 1.10 1.85 40 1.10 1.70 40 1.30 1.85 25 1.30 1.70 25

GPR was also used to locate the reinforcements in the concrete wall. The transmitted radar pulse is reflected from the reinforcement steel and the measured travel time of the reflected pulse is used to estimate the depth of the reinforcement steel inside the concrete wall. The area where the concrete cover meter was applied was also measured with the GPR (Figure 3). The studied area was covered with GPR lines with approximately 20 cm line spacing in two opposite directions.

6 The GPR data image was processed with the band-pass filter and the time zero correction has been applied. The travel time of reinforcement reflections were picked and saved for further calculations. Examples of the pick marks are shown with crosses in Figure 4. The travel time of each reflector was then turned into depth by assuming the velocity in concrete to be 0.1 m/ns. Estimated concrete cover thicknesses are listed in Table 6. The minimum values of the concrete cover are 25–27 mm. The coordinate information are only estimated based on the measuring wheel data and some calibration measurements on the study site as the original study plan did not include exactly locating the reinforcements.

Figure 4. Example of the interpreted reinforcements are marked with red in the GPR image.

Table 6. Results of GPR concrete cover thicknesses measurements. Line 1 Line 3 Two-way Cover Two-way Cover Coordinates Coordinates travel time depth travel time depth X Y (ns) (mm) X Y (ns) (mm) 0.10 1.85 0.95 47 0.45 1.70 0.53 27 0.30 1.85 0.93 47 0.70 1.70 1.02 51 0.45 1.85 0.52 26 0.90 1.70 0.67 34 0.70 1.85 1.14 57 1.10 1.70 1.10 55 0.90 1.85 0.61 31 1.30 1.70 0.66 33 1.10 1.85 1.01 50 1.50 1.70 1.30 65 1.30 1.85 0.60 30 1.60 1.70 0.64 32

The concrete cover depth over the reinforcement varies from 35 mm to 50 mm. The minimum cover depth measured be concrete cover meter was 24 mm and the maximum was 40 mm, which fit the design concrete cover depth of the wall. Using GPR method for measuring the concrete cover depth showed higher values at some points on the wall. The standard distribution of the concrete cover depth measured by GPR is shown in Figure 5. The results show that the average concrete cover depth is 39 mm with standard deviation of 10 mm.

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Figure 5. Using of the GPR method for concrete cover measurement. 3.3. Detection of the simulated defect and concrete surface condition

The visual interpretation of GPR data lines was carried out. The GPR signal attenuation was observed to be quite substantial, especially with the 1.6 GHz GPR system, and penetration depths were less than approximately 20 cm. The penetration depth was probably decreased because of densely located reinforcements and concrete electromagnetic properties. However, in few locations of processed GPR lines very weak reflections could be seen which might be related to the simulated construction defects. For example, water (WFV02), air-filled (AFV02) bottles and the Honeycombing defect (HON1) were detected, as presented in Figure 6, but their visual interpretation was very challenging.

Water (WFV02) and air-filled (AFV02) bottles Honeycombing defect (HON1) Figure 6. Examples of the simulated defects detected by GPR.

The used of the Ultrasonic Pulse Velocity device to investigate the condition of the concrete surface and the internal cracks or air voids of concrete failed because the device malfunctioned (battery problem). The reason could also be the low operating temperature during the measurement (the outdoors temperature was around 0°C). Some cracks were visually observed on the surface of the mock-up as shown in Figure 7. The

8 reason for these cracks could be the or early shrinkage of the concrete surface.

Figure 7. Examples of the visually observed surface crack on mock-up wall surface.

4. Conclusions

The investigation included the following steps: (i) selecting some common defects due to construction and cumulative degradation of the concrete with time, (ii) embedding of simulated defects inside the thick-walled concrete structure and (iii) using modern NDT testing methods for detecting the simulated defects. The representative defects of construction process were honeycombing, air filled voids and water filled voids. The representative cumulative ageing defects of the wall were cracks and delamination of concrete. The NDT techniques used in the investigation were Ground Penetrating Radar (GPR), Ultrasonic Pulse Velocity (UPV), rebound hammer and concrete cover meter. The results of the investigation show the importance of the combination of different NDT techniques to justify the detection of the embedded items in the wall and characterizing the properties of the concrete. For some malfunctions and limitations of the NDT devices used, the research team was not able to detect some of the simulated defects in the wall. The results of the investigation showed that the compressive strength of the concrete measured by Rebound Hammer fits to the target compressive strength of the concrete mix, while the compressive strength results of drilled samples is higher. The concrete cover depth values measures by both concrete cover meter and GPR are in the range of the target cover depth of the wall. The investigation was useful for the future research as a practical way of inducing simulated defects into concrete structures, planning the use of NDT techniques and considering the limitations of each methods. Acknowledgements

The authors would like to thank Mr. Timo Kukkola, Chief engineer, Civil Engineering, TVO Oy for his support in building the thick-walled concrete mock-up and implementing of the NDT investigation.

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