VISUALIZATION OF WATER PENETRATION INTO CONCRETE THROUGH CRACKS BY NEUTRON RADIOGRAPHY

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1 VISUALIZATION OF WATER PENETRATION INTO CONCRETE THROUGH CRACKS BY NEUTRON RADIOGRAPHY Manabu KANEMATSU Faculty of Science and Technology, Tokyo University of Science 2641 Yamasaki, Noda-shi, Chiba , Japan Ippei MARUYAMA Graduate school of Environmental Studies, Nagoya University Furo, Chikusa-ku, Nagoya , Japan Takafumi NOGUCHI Graduate School of Engineering, The University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo , Japan Hiroshi IIKURA Quantum Beam Science Directorate, Japan Atomic Energy Agency Tokai, Ibaraki , Japan ABSTRACT Neutron radiography is one of the non-destructive methods. In this research, neutron radiography was applied to visualize and quantify the water penetration into concrete through cracks. The radiographs were obtained by the TNRF (Thermal Neutron Radiography Facility) installed at a research reactor, JRR-3M, in the Japan Atomic Energy Agency (JAEA). The water content and the penetration depth in cement paste specimen were quantitatively evaluated by neutron radiography. The effects of crack and water content on the velocity and the depth of water penetration into concrete were visualized with several concrete specimens, to which surface water was supplied through the tank. Keywords: neutron radiography, crack, evaporable water behavior, visualization INTRODUCTION The crack in concrete accelerates the ingression of water and/or other chemical substances into its inside and finally devastate its structural robustness. In fact, this ingression of water is one of the major concerns of housing, other buildings and structures that require rigid water-tightness. In addition, the ingression of chemical substances such as chloride ion and oxygen induces the corrosion of reinforcement that sometimes jeopardizes the safety of structure. The crack in concrete is sometimes inevitable, and therefore, the effect of crack, e.g. the transportation of substances through crack, on service life of structure has been studied in the past. The process and mechanism of penetration of water through crack is still quantitatively unknown although it is the most basic and important one because

2 ingression of chloride ion and other substances is dependent on water behavior. Water content in concrete is commonly detected by humidity sensor which provides electrical resistance of material such as ceramic, polymer, etc. that is sensitive to moisture content. However it always affects the concrete or cement matrix system because the sensor has a limited size which becomes exogenous for the system. Additionally the sensor stirs suspicion of what is measured. This research demonstrates some experimental study with neutron radiography to visualize and quantify the water penetration into concrete through cracks. At the beginning, the detectable depth and the capability of quantitative evaluation and of water content were investigated with cement paste specimen, and then the penetrating water into concrete from the tank attached on the surface of the specimen was observed with several concrete specimens, which have artificial bending through crack and variety of water contents. ANALYSIS METHOD Basic equation for superposition principle of creep strain Neutron radiography is based upon the fact that the neutron beam is attenuated due to the interaction of neutrons with the nuclei of the atoms in the object material according to scattering or absorption. The principle of the radiography in general is the recording of the radiation passing through an object by position sensitive converter. Because the attenuation of the radiation through the object depends on material thickness and density, the image contains qualitative and quantitative information about the structure and composition of the object. The application of the neutron radiography to the cement-based material goes back to the paper of 1972 by Rijonen, H. whose trial is non-destructive monitoring of carbonation in concrete (Reijonen 1972). Recently, as the digitalization of graphic is promoted according to the development of CCD-camera, several reports have been published (Beer 2004, Hanzic 2003, Pleinert 1998), but there is no application of quantification of water in concrete and cement paste as well as visualization of water in cracked concrete. The facility used for the entire experiment was TNRF (Thermal Neutron Radiography Facility) installed at the research reactor, JRR-3M, of the Japan Atomic Energy Agency (JAEA). The neutron flux was 1.2x10 8 (n/cm 2 sec). TNRF is characterized by utilization of thermal neutron which can penetrate through various materials to large depths. Various kind of research works in not only engineering field but also agriculture and biomedical field have been carried out with this facility (Nakanishi 1999). The TNRF consists of a fluorescent converter, two quartz mirrors, one lens and one C-CCD (cooled charge coupled device) camera as shown in Fig. 1. A full transfer type C-CCD camera with an effective array of 1008 pixels x 1024 pixels of 100mm x 100mm each was used. The spatial resolution is approximately100 μm/pixcel. It takes approximately 8.0 seconds to get one image including data transfer time and the work is exposed to the neutron for 1.2 seconds. A part of the neutrons irradiated to the sample scatters with the hydrogen, and the rest penetrated the sample and reached on the fluorescent converter(6lif/zns : Ag). The neutrons falling on the converter are transformed into a visible light in

3 proportion to the flux and guided to the C-CCD camera using two quartz glass mirrors. The brightness of the image is then digitized by the image processor. Figure 2 shows a typical image of concrete specimen obtained by the TNRF. Since the neutron is interrupted especially with the hydrogen atom, the obtained image becomes the shadowgraph due to the distribution of the hydrogen atom. White spots which are pixels hit by neutrons and gamma-rays directly in a captured image occur inevitably, because the CCD camera is highly sensitive to neutrons and gamma-rays. In order to correct white spot noise in the image, an intelligent filter was applied. In addition, each figure was corrected referred to two background images. One of the background images was taken when neutrons were not irradiated and the other was the image without the sample. Therefore, at each image, dark current image subtraction and shading correction was performed. Fig.1 Schematic view of TNRF Fig.2 The transmission neutron radiograph of the concrete (w/c 50, 100x100x20mm) This processing is a general technique for a high-precision analysis with CCD camera, and generally used to research with this device. From the statistical analysis of detected intensity of the neutron flux, the relationship between the intensity of the neutron beam and the characteristics of the sample can be described as follow. I to ( Σcδ c Σ w0δ w0) = I 0e + (1) where I t0, I 0, Σ c, δ c, Σ w0 and δ w0 denote the intensity of the neutron flux with passed through the sample at the initial setup, incident intensity of the neutron flux, macroscopic cross section of water of the sample, thickness of the sample, macroscopic cross section of water at the initial setup and the thickness of the sample at the initial setup respectively. Because Σ c and Σ w0 are constant value in the thermal neutron, following equation holds between the intensity of the water Σ cδ c+ Σw0δ w0 and attenuation rate of neutron flux I to / I0. c c ( I ) Σ δ +Σ (2) w 0 δ w 0 = ln I to / 0

4 The differential intensity of the water described as equation (3) with a macroscopic cross section of water at time t ( I / I ) t0 Σ wtδ wt from time 0 to time t can be Σ wt : Σ δ = ln (3) wt wt t The visualization pictures used in this paper is represented as the gray-scale images that were converted from the intensity of water (eq. (2)) or the differential intensity of water (eq. (3)). It is widely known that back scattered radiation produced in the sample affect on the intensity of the neutron flux. This phenomenon depends on the type of sample and/or the shape and the direction of the sample, and it makes high resolution incompatible with quantification of water. In this contribution, high resolution picture is emphasized in order to grasp the behavior of water in the concrete specimen. But the variety of neutron flux intensity due to back scattered radiation is much smaller than the variety due to a distribution of aggregates in a concrete specimen. Therefore, even the high resolution images are presented, it could be noted that the data in pictures still have the potential to quantify of water in the concrete from the statistical point of view. EXPERIMENTS AND MATERIALS Visualization and quantification of the water in cement paste The penetration efficiency of the neutron is attenuated by the presence of hydrogen atom. This indicates that the cement paste specimen, which has evaporable water as well as chemically bound water, attenuate neutron flux, but states of the water in cement paste matrix are hardly distinguished (Pleinert 1998). For this reason, the range and limitation of linear sensitivity for free water in cement paste matrix should be cleared. Consequently, saturated cement paste specimen and 105 o C-dired specimen which have triangle shape in section are irradiated from the hypotenuse direction (Fig. 3). Ordinary Portland cement was used in this study. Triangle specimens of 100x100x20 mm were made of cement paste with a water-cement ratio of After 24h from the casting, the specimens submerged in water at 20. After 28 days, 100%RWC specimens were stored in water still and 0%RWC specimens were dried Fig. 3 Experiment schematic

5 Fig. 4 The relationship between intensity of the water and thickness of cross section. Fig.5 differential intensity of the water between 100%RWC and 0%RWC Table 1 Mix Proportion w/c Air Proportions (kg/m3) Super Water Cement Aggregate plasticizer (%) (%) Fine Coarse (ml) C*0.7% at 105. After 5 weeks from casting, the neutron radiography testing was conducted. Figure 4 shows the relationship between the intensity of the water and the thickness. Though it doesn't depend on the relative water content, the curves are almost linear until the curves attain the plateau and there are inflection points in the thicness of 10mm to 30mm. This figure indicates that it is possible to quantify the water content of paste within 10mm. And curve of 100%RWC indicates linear intensity as a function of thickness up to about 10.0 mm. Evaporable water per unit Fig.6 Specimens specifications volume of cement paste matrix can be evaluated the subtraction of the 100%RWC value with 0%RWC value over thickness because the value of 0%RWC corresponded with the amount of the chemically bound water. Figure 5 shows the differential intensity of the water between 100%RWC and 0%RWC. According to this figure, the evaporable water behavior in the cement paste within 10mm could be quantified, and given that the volume ratio of a usual concrete aggregate to be about 70%, the evaporable water behavior in the concrete up to the 30mm in thickness also could be monitored with this equipment. Visualization of the water behavior through the cracks Based on the discussion above, it can be quantified or qualified the water behavior in the cementitious material. In order to monitor the water behavior in a crack of concrete,

6 Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2006 W/C=50%, relative water content 0% (105 oven dry) W/C=50% relative water content 30% W/C=50% relative water content 60% Fig.7 Visualization of water behavior in a crack (water pool on the left side)

7 Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2006 the following experiments were conducted. Mixture proportion is shown in Table 1. Concrete specimens of 100x100x20mm were made with a water-cement ratio of 0.50 and relative water contents of concrete are controlled; 0% (0%RWC), 30% (30%RWC) and 60% (60%RWC) After 24 h, the specimens were cured at 20 ±2 in water. Compressive strength at 28 concrete age is 48.9N/mm2 and absorption ratio which is obtained from oven dry at 105 was 6.30%. Figure 6 shows the specimens specifications. Each specimen was cut out from a specimen of 100x100x400 mm. Artificially created horizontal crack with surface crack width of 0.05 mm was examined. Cracks of specimens are artificially produced by bending moment with high-rigidity loading machine and 2 pieces of broken specimens are fixed by adhesive aluminum tape with epoxy-bond for sealing the side on the crack. The crack width on the side surface of specimen is measured Fig.8 Quantitative measurement of water penetration into and controlled as 0.05 mm (Fig. concrete through cracks 6 left). And aluminum tank which supply water to specimen is attached by adhesive aluminum tape on the surface of specimen including crack end. After measuring an initial intensity by the neutron radiography, the aluminum tank was filled with water from the filling port. A series of images were serially taken every eight seconds for 2 h. To visualize the water penetration behavior through crack, the differential images from initial time to time t (shown in Fig. 7) were processed. Since the effect of scattered neutrons caused by the block of the water in the aluminum tank, the image just after pouring is used for the image of initial time. Figure 7 shows the results of images of water behavior in a crack. Each image shows the differential intensity of water that is converted to the gray-scale images. From these pictures, it is clearly confirmed that it makes visible the water in/near the crack with high resolution. And also it is observed that water penetrate through the crack immediately after pouring and its migration speed and distribution depends on

8 the moisture condition in the concrete. In Fig. 8 the change of the intensity of water in orthogonal oriented direction to the crack is shown. A horizontal averaged value in the area enclosed by the white-line-rectangle on the right image is plotted against the vertical coordinate. These plots illustrate the plausible results which can be predicted by former researches (Hanzic 2003, Pleinert 1998) and make it possible to quantify the water movement from the crack surface to concrete matrix as well as from crack end to inside of crack. CONCLUSION 1. With neutron radiography, the behavior of evaporable water could be quantified in cement paste with thickness of 10mm, which suggested a possible quantification in concrete up to 30mm in thickness, given that the volume ratio of aggregate is about 70% 2. The behavior of the water in/near crack could be clearly monitored with high resolution, and it was confirmed that water penetrated through the crack immediately after pouring, and its migration speed and distribution depended on the moisture condition of the concrete. References 1) F. C. de Beer, a, W. J. Strydoma and E. J. Grieselb : The drying process of concrete: a neutron radiography study, Applied Radiation and Isotopes, ) L.Hanzic and R.Ilic : Relationship between liquid sorptivity and capillarity in concrete, Cement and Concrete Research, 33, , ) T.M.Nakanishi, J. Furukawa and M. Matsubayashi : A preliminary study of CT imaging of water in a carnation flower, Nuclear Instruments and Methods in Physics Research A424, pp , ) K. Tanaka, I. Maruyama, R. Sato and Kawai : Experimental studies on salty water penetration through cracks, Int. RILEM-JCI Seminar on Concrete Durability and Service Life Planning, Ein-Bokek, Israel, pp.24-32, ) H.Pleinert, H. Sadouki and F.H.Witmann : Determination of moisture distributions in porous building materials by neutron transmission analysis, Materials and Structures, vol.31, pp , ) H.Reijonen and S.E.Pihlajavaara : On the determination by neutron radiography of the thickness of the carbonated layer of concrete based upon changes in water content, Cement and Concrete Research, vol.2, pp , 1972