PSI - Issue 64

M. Mizuta et al. / Procedia Structural Integrity 64 (2024) 214–219 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

216

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2.2. Concrete sample To assess the applicability of neutron imaging for maintenance management of concrete structures, cylinders sampled easily from existing structures are chosen. In this study, a circular concrete sample ( φ 100 x 270 mm) is obtained from a real bridge abutment suffering from ASR degradation. Design compressive strength of the concrete is 24 N/mm 2 and mix proportion is listed in Table 1. The ASR reaction in the sample had concluded, and no visible cracks were observed on the sample surfaces. Additionally, the sample is stored indoors under dry conditions for approximately 6 years before being applied to this experiment.

Table 1. Mix proportion of concrete. G max ( mm ) W/C ( % ) s/a ( % )

Cement ( kg/m

3 ) Water ( kg/m 3 )

Sand ( kg/m 3 )

Gravel ( kg/m 3 )

25

51.7

37.0

290

150

702

1207

* G max : maximum size of gravel

2.3. Water permeation test Water is permeated unidirectionally from the surface of the concrete sample with a diameter of φ 100 mm. The sample surfaces are not covered with tape or any other coating. The water absorption amounts after 4 hours and 22 hours were 22 g and 49 g, respectively. 2.4. Image analysis method Images were captured before water absorption, after 4 hours and 22 hours of water absorption. The direction of neutron beam irradiation is perpendicular to the circular cross-section (maximum transmission distance is equal to 100 mm). The exposure time of the camera for each image is 5 minutes. In addition to the sample images, two types of images, dark images and direct beam images, are also captured. The roles of each image are as follows: • Dark images: Images captured without irradiating neutron beams, utilized for correcting dark current in the image sensor. • Direct beam images: Images captured without placing the sample in front of the detector, used for irradiating neutron beams and capturing images. Since the neutron beam intensity has a high convex intensity distribution at the center of the neutron emission port, this image is used to flatten the intensity distribution. Thus, all sample images discussed in the following section undergo correction by subtracting dark images and flattening the neutron beam intensity distribution using direct beam images. Image analysis is performed using the freely available software ImageJ.

Sample – detection surface distance

φ100mm Scattered/transmitted

Detection surface

neutrons

Neutrons

Neutrons

Sample thickness

Concrete sample

(a) Front view

(b) Top view

Fig. 3. Neutron imaging of cylinder