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Application of X-ray CT to the study of microstructure and diffusivity in cementitious materials
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| Title: | Application of X-ray CT to the study of microstructure and diffusivity in cementitious materials |
| Other Titles: | X線CTを用いたセメント硬化体の微細構造と拡散性状に関する研究 |
| Authors: | Darma, Ivan Sandi Browse this author |
| Issue Date: | 25-Mar-2014 |
| Publisher: | Hokkaido University |
| Abstract: | At the beginning of its development, the X-ray CT technique originally was developed for medical analysis. However, along with the advances in technology, the ability of X-ray CT continues to increase. Therefore, the use of X-ray CT is no longer intended for medical application but has expanded to other fields such as civil engineering especially for material science. Related to construction materials, there are many experimental methods that can be used to study the microstructure of cementitious materials such as scanning electron microscope (SEM), backscattered electron (BSE), mercury intrusion porosimetry (MIP) and permeability test. However, each of those experimental methods generally only provides a few aspects related to the microstructure of cementitious materials. On the other hand, there are many aspects which can be obtained from the microstructure of cementitious materials just through the use of X-ray CT technique. This present study describes the versatility of the application of X-ray CT technique to study the microstructure and diffusivity of cementitious materials. In this research, there are two types of X-ray CT systems used to acquire the 3D image of the internal structure of cementitious materials i.e. synchrotron X-ray CT with spatial resolution of 0.5 m and micro-focus X-ray CT with lower resolution (>10 m/pixel). The first and arguably the most important in the use of X-ray CT is the observation of the three-dimensional pore structure of the hardened cementitious materials. The application of X-ray CT certainly can be expanded through the use of different types of cementitiousmaterials as well as its curing period. In this case, cement paste specimen mixed with silica fume and fly ash (low alkali binder) was used and Ordinary Portland Cement (OPC) cement paste was prepared as a comparison. The pore structure parameters such ash total porosity, connectivity, and pore size distribution were obtained through the analysis of 3D microtomographic images. Furthermore, Random Walk Simulation (RWS) was performed to quantify the diffusion tortuosity in the pore structure. It is found that the diffusion tortuosityof percolated porosity of OPC cement paste was higher than that of low alkali cement paste at the same curing periods. On the other hand, despite having a lower resolution, the use of micro-focus X-ray CTwas intended to identify the larger void space in cementitious materials such as cracks and air voids and also allows us to use the specimens with larger size. Micro-focus X-ray CT wasemployed to observe cracked mortar specimens due to mechanical loading. The cracks in beam specimens of the size of 10 x 20 x 60 mm and cylinder specimens of the size of 20 mmin diameter and 40 mm in height were induced by flexural test and splitting tensile test,respectively. Subsequently, the 3D crack geometry was obtained from microtomographicimages. Through the analysis of 3D crack geometry, the crack geometry parameters such astortuosity and constrictivity were obtained. The tortuosity of flexural crack and splittingtensile crack in the mortar specimen with maximum size of aggregate of 1.7 mm were foundto be around 1.13 and 1.25, respectively, and it was independent to the crack opening widthand whether fly ash was added or not. Meanwhile, it is found that the type of cracks and thepresence of void play an important role on the constrictivity of the crack. The constrictivityincreased for fly ash mortar having roughly the equivalent crack opening width of OPCmortar.The scope of the observation using X-ray CT was expanded even further. Theapplication of micro-focus X-ray CT coupled with in-situ tracer diffusion test was conductedto clarify the transport mechanism in cracked mortar specimens. Diffusivity in the crack wascontrolled by following factors: the crack opening width, as well as, by its constrictivity. Thediffusive transport was also restricted due to the presence of air pockets in the crack. In thisregard, a diffusion-accessible path factor was proposed to account for the degree of wateriisaturation within the crack space. In addition, it was confirmed that the addition of fly ashcould reduce the diffusivity through the uncracked body as compared with normal OPCmortar of equivalent diffusion coefficient along the crack.Furthermore, the variations in cementitious materials were added. In this regard,deteriorated specimens were used in X-ray CT observation. There were three types ofdeterioration applied in the specimen namely leaching, mechanical loading and hightemperature exposure followed by re-curing. X-ray CT technique was used to observe thechange in microstructure in cementitious materials as the effect of deterioration. As forleaching, after the observation using synchrotron X-ray CT and image analysis, the totalporosity and percolated porosity of normal cement paste specimen significantly increasedafter leaching test as compared to that of cement paste specimen with low alkali binder.Dissolution of hydration product in normal cement paste specimen dramatically reduced thediffusion tortuosity in the percolated porosity of normal cement paste specimen. On the otherhand, although there was a reduction in the diffusion tortuosity for percolated porosity ofcement paste specimen with low alkali binder, however, it was not dramatically whencompared with that of normal cement paste specimen.In the study of deteriorated cementitious materials due to mechanical loading,synchrotron X-ray CT was combined with micro-tensile instrument and the observationduring the application of load was conducted in-situ inside synchrotron X-ray CT chamber.Although there was no definite result of this experiment, however, by improving the qualityboth sample preparation and loading instrument, the behavior of microstructure under externalload can be observed through the application of synchrotron X-ray CT combined with othermechanical tests.Using micro-focus X-ray CT, concrete specimen in three different conditions, namelybefore heating, after heating and re-curing was examined. It was observed that cracksoccurred in the concrete specimen due to heating can be categorized into three types of cracks,namely interfacial cracks between mortar and coarse aggregates, bridging cracks betweenaggregates and bridging cracks between aggregates and air voids. Therefore, the volume oftotal and percolated porosity of heated specimen increased due to the presence of these cracks.Furthermore, we observed that water re-curing is more effective in the recovery ofmicrostructure of concrete after high temperature exposure as compared with that of airre-curing.Through the application of X-ray CT technique this research could provide betterunderstanding of the microstructure change in cementitious materials. In addition, with theproposed X-ray CT technique coupled with in-situ tracer diffusion test, the transportmechanism of diffusion inside crack and through the uncracked matrix would be furtherunderstood. Therefore, this research can contribute to the clarification of durability aspect andto more accurate prediction of service life for concrete structures. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第11452号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 杉山 隆文, 教授 上田 多門, 教授 名和 豊春 |
| Degree Affiliation: | 工学院（環境フィールド工学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/55523 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （工学）
課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
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