On coupling of micro-ct and nanoindentation to examine concrete microstructure

Transcription

1 On coupling of micro-ct and nanoindentation to examine concrete microstructure Damian Stefaniuk 1, Magdalena Rajczakowska 1, Adrian Różański 1 Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, Wroclaw, Aims Concrete is an important construction material. Even though it seems to be well known and its behavior is commonly understood, there is still plenty of research going on in order to investigate it on the microstructural level. X-ray microtomography was applied for concrete examination many times and there is a number of papers published on this topic [1-10]. However, micro-ct allows, in general, to measure and characterize the geometry of the material. In case of construction materials, the mechanical properties are very significant. Thus, it is crucial to look for the solutions combining geometry measurements and mechanical tests. One of the techniques allowing to investigate strength properties of concrete in the micro scale is nanoindentation. Nevertheless, there are few examples of combining the X-ray micro- Computed Tomography and nanoindentation techniques in order to find the complex information about the materials microstructure. Therfore, the aim of this paper is to verify their usefulness as well as the effectiveness of this approach on an exemplary material. Method High strength concrete extracted from the copper mine s shaft wall was chosen to be the material under investigation. Firstly, the sample was scanned with the use of SkyScan 1172 X- ray. The X-ray tube voltage was set to 100 kv and the tube s power was constant at 10 W. The scan was performed using built-in Al+Cu filter (Al 1mm and Cu 0.05mm). In addition, for each scan the flat field correction was applied. The reconstruction was performed using NRecon based on the Feldkamp algorithm. For the 3D visualization of the images, CTvox software was utilized. All the additional calculation was performed with the use of Wolfram s Mathematica software. Next, the sample was tested in terms of its mechanical parameters by means of nanoindentation technique. For this purpose, a small fragment of the material was included in the epoxy resin with the use of vacuum machine in order to remove the air from the pores. Afterwards, the surface of the sample was polished to minimize the surface roughness. The measurements were conducted with the use of CSM Nanoindentation Tester (NHT). The value of the force used for loading of the sample was 500 mn. The methodology of the measurements is presented in Figure 1. Figure 1: Nanoindentation test methodology Results In Figure 2, the results of the material imaging are presented, namely the 3D reconstruction and the 2D images of the sample. Three components of the composite are distinguishable

2 based on the differences in the absorption of the X-ray radiation: cement matrix (light grey), aggregate (dark grey) and air voids (white). It can be noticed that the aggregate has significantly higher density range then the cement matrix. Cement matrix Aggregate Figure 2: Results of the concrete sample imaging Figure 3: Image processing steps and histogram from the total dataset The images of the material s microstructure were subjected to various image processing techniques e.g. filtering (kuwarha filter) in order to acquire proper segmentation of the components. Afterwards, the initial binarization of images was performed where the structure of the material was separated from the pores and the aggregate (Fig. 3)

3 Figure 4: Attenuation profiles: a) for the image cross-sections including aggregate and pores, b) for the cement matrix only (Fig. 3) The basic measure to describe the microstructure of the material, based on the X-ray microct images, is the attenuation profile as it can be interpreted as the density relations. However, the question arises: how to calculate the average value of the attenuation in order to get the proper value for the material? In this paper, two methods are presented in order to emphasise the differences in the results. In the first method, the attenuation was calculated as the average value of all the pixels for each slice, without any prior binary operations on the images. On the contrary, in the second method, the images were preprocessed in order to obtain the pixel values of the matrix only the aggregate and the pores were subtracted from the original image (Fig. 3). The thresholds for the image segmentation were chosen based on the image histogram. The results of the attenuation profiles for both methods are shown in Figure 4. It is clearly visible that the values calculated with the use of the first method are significantly different than the values obtained with the second method. The micro-ct investigation was followed by the nanoindentation measurements. The results of the analysis are shown in Figure 5 which displays the values of the Young Modulus and Hardness of the material versus height of the sample. They correspond to the attenuation profiles presented above. Furthermore, a 3D profile of hardness was calculated. It is demonstrated in Figure 6.

4 Figure 5: Nanoindenation profiles of cement matrix Figure 6: Nanoindenation 3D map of cement matrix Conclusion The paper presented two methods of microstructural analysis of materials, namely X-ray microcomputed Tomography and nanoindentation. Both techniques are relatively new and have been used in material science for two decades. Even though they are becoming more and more popular among the scientists, there are still only several attempts to combine them in order to obtain full information about the object under investigation. The aim of the study was to prove the usefulness of such combination on a concrete sample. Summarizing, the methods presented proved to be useful when coupled together. Even though each of them has broad possibilities, applied together, they give a complete and thorough data about material s microstructure. The preprocessing of data e.g. image processing techniques in the case of microtomography are the major factor influencing the results

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